Sewerage and Sewage Treatment

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Title: Sewerage and Sewage Treatment
Author: Babbitt, Harold Eaton
Language: English
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[Illustration:

FIG. 1.—Construction of Peck’s Run Sewer, Baltimore, Maryland.

_Frontispiece._
]

SEWERAGE
AND
SEWAGE TREATMENT

HAROLD E. BABBITT, M.S.

_Assistant Professor, Municipal and Sanitary Engineering, University of
Illinois; Associate Member American Society of Civil Engineers_

NEW YORK
JOHN WILEY & SONS, INC.
LONDON: CHAPMAN & HALL, LIMITED
1922

PRESS OF
BRAUNWORTH & CO.
BOOK MANUFACTURERS
BROOKLYN, N. Y.

------------------------------------------------------------------------

PREFACE

the author for use in his classes at the University of Illinois. He has
found such notes necessary, since among the many books dealing with
sewerage and sewage treatment he has found none suitable as a text-book
designed to cover the entire subject. The need for a single book of the
character described has been expressed by engineers in practice, and by
students and teachers for use in the class-room. This book has been
prepared to meet both these needs. It is hoped that the searching
questions propounded by students in using the original notes, and the
suggestions and criticisms of engineers and teachers who have read the
manuscript, have resulted in a text which can be readily understood.

The ground covered includes an exposition of the principles and methods
for the designing, construction and maintenance of sewerage works, and
also of the treatment of sewage. In covering so wide a field the author
has deemed it necessary to include some chapters which might equally
well appear in works on other branches of engineering, such as the
chapter on Pumps and Pumping Stations. Special stress has been laid on
the fundamentals of the subject rather than the details of practice,
although illustrations have been drawn freely from practical work. The
quotation of expert opinions which may be in controversy, or the
citation of examples of different methods of accomplishing the same
thing, has been avoided when possible in order to simplify explanations
and to avoid confusing the beginner.

The work is to some extent a compilation of notes and quotations which
have been collected by the author during years of study and teaching the
subject. Credit has been given wherever due, and at the same time
references have pointed out the original sources whenever possible.
These references, which have been supplemented by brief bibliographies
at the end of certain chapters, will be useful to the student and
engineer interested in further study. Occasionally the original
reference has been lost or the phraseology of a quotation has been so
altered by class-room use, as to make it impossible to trace the
original source, so that in some few instances full credit may be
lacking.

The author is indebted to many of his friends for their criticisms and
suggestions in the preparation of the manuscript; but he desires
particularly to acknowledge the assistance of Professor A. N. Talbot,
Professor of Municipal and Sanitary Engineering at the University of
Illinois, and of Professor M. L. Enger, Professor of Mechanics and
Hydraulics at the University of Illinois, in the entire work; also that
of Mr. T. D. Pitts, Principal Assistant Engineer of the Baltimore
Sewerage Commission during the construction of the Baltimore sewers, for
his suggestions on the first half of the book; and to Mr. Paul Hansen,
consulting engineer, of Chicago, and to Mr. Langdon Pearse, Sanitary
Engineer of the Sanitary District of Chicago, for their help on the
section covering the treatment of sewage; and to Professor Edward
Bartow, Professor of Chemistry at the University of Iowa, for his review
of the chapter on Activated Sludge; in general his thanks are due to all
others who have furnished suggestions, illustrations, or quotations,
acknowledgments of which have been included in the text.

H. E. B.

URBANA, ILLINOIS, 1922.

TABLE OF CONTENTS

CHAPTER I

INTRODUCTION

PAGES
1. Sewerage and the Sanitary Engineer. 2. Historical. 3.
Methods of Collection. 4. Methods of Disposal. 5. Methods of
Treatment. 6. Definitions. 1–8

CHAPTER II

WORK PRELIMINARY TO DESIGN

7. Division of Work. 8. Preliminary. 9. Estimate of cost.
METHODS OF FINANCING. 10. Bond Issues. 11. Special
Assessment. 12. General Taxation. 13. Private Capital.
PRELIMINARY WORK. 14. Preparing for Design. 15. Underground
Surveys. 16. Borings. 9–23

CHAPTER III

QUANTITY OF SEWAGE

17. Dry Weather Flow. 18. Methods for Predicting Population.
19. Extent of Prediction. 20. Sources of Information on
Population. 21. Density of Population. 22. Changes in Area.
23. Relation between Population and Sewage Flow. 24.
Character of District. 25. Fluctuations in Rate of Sewage
Flow. 26. Effect of Ground Water. 27. Résumé of Method for
Determination of Quantity of Dry weather Sewage. QUANTITY OF
STORM WATER. 28. The Rational Method. 29. Rate of Rainfall.
30. Time of Concentration. 31. Character of Surface. 32.
Empirical Formulas. 33. Extent and Intensity of Storms. 24–50

CHAPTER IV

HYDRAULICS OF SEWERS

34. Principles. 35. Formulas. 36. Solution of Formulas. 37. Use
of Diagrams. 38. Flow in Circular Pipes Partly Full. 39.
Sections Other than Circular. 40. Non-Uniform Flow. 51–77

CHAPTER V

DESIGN OF SEWERAGE SYSTEMS

41. The Plan. 42. Preliminary Map. 43. Layout of the Separate
System. 44. Location and Numbering of Manholes. 45. Drainage
Areas. 46. Quantity of Sewage. 47. Surface Profile. 48. Slope
and Diameter of Sewers. 49. The Sewer Profile. DESIGN OF A
STORM-WATER SEWER SYSTEM. 50. Planning the System. 51.
Location of Street Inlets. 52. Drainage Areas. 53.
Computation of Flood Flow by McMath Formula. 54. Computation
of Flood Flow by Rational Method. 78–98

CHAPTER VI

APPURTENANCES

55. General. 56. Manholes. 57. Lampholes. 58. Street Inlets.
59. Catch-basins. 60. Grease Traps. 61. Flush-tanks. 62.
Siphons. 63. Regulators. 64. Junctions. 65. Outlets. 66.
Foundations. 67. Underdrains. 99–126

CHAPTER VII

PUMPS AND PUMPING STATIONS

68. Need. 69. Reliability. 70. Equipment. 71. The Building. 72.
Capacity of Pumps. 73. Capacity of Receiving Well. 74. Types
of Pumping Machinery. 75. Sizes and Descriptions of Pumps.
76. Definitions of Duties and Efficiency. 77. Details of
Centrifugal Pumps. 78. Centrifugal Pump Characteristics. 79.
Setting of Centrifugal Pumps. 80. Steam Pumps and Pumping
Engines. 81. Steam Turbines. 82. Steam Boilers. 83. Air
Ejectors. 84. Electric Motors. 85. Internal Combustion
Engines. 86. Selection of Pumping Machinery. 87. Costs of
Pumping Machinery. 88. Cost Comparisons of Different Designs.
89. Number and Capacity of Pumping Units. 127–163

CHAPTER VIII

MATERIALS FOR SEWERS

90. Materials. 91. Vitrified Clay Pipe. 92. Cement and Concrete
Pipe. 93. Proportioning of Concrete. 94. Waterproofing of
Concrete. 95. Mixing and Placing Concrete. 96. Sewer Brick.
97. Vitrified Clay Sewer Block. 98. Cast Iron, Steel, and
Wood. 164–193

CHAPTER IX

DESIGN OF THE SEWER RING

99. Stresses in Buried Pipe. 100. Design of Steel Pipe. 101.
Design of Wood Stave Pipe. 102. External Loads on Buried
Pipe. 103. Stresses in Circular Ring. 104. Analysis of Sewer
Arches. 105. Reinforced Concrete Sewer Design. 194–210

CHAPTER X

CONTRACTS AND SPECIFICATIONS

106. Importance of the Subject. 107. Scope of the Subject. 108.
Types of Contracts. 109. The Agreement. 110. The
Advertisement. 111. Information and Instructions for Bidders.
112. Proposal. 113. General Specifications. 114. Technical
Specifications. 115. Special Specifications. 116. The
Contract. 117. The Bond. 211–232

CHAPTER XI

CONSTRUCTION

118. Elements. WORK OF THE ENGINEER. 119. Duties. 120.
Inspection. 121. Interpretation of Contract. 122. Unexpected
Situations. 123. Cost Data and Estimates. 124. Progress
Reports. 125. Records. EXCAVATION. 126. Specifications. 127.
Hand Excavation. 128. Machine Excavation. 129. Types of
Machines. 130. Continuous Bucket Excavators. 131. Cableway
and Trestle Excavators. 132. Tower Cableways. 133. Steam
Shovels. 134. Drag Line and Bucket Excavators. 135.
Excavation in Quicksand. 136. Pumping and Drainage. 137.
Trench Pump. 138. Diaphragm Pump. 139. Jet Pump. 140. Steam
Vacuum Pumps. 141. Centrifugal and Reciprocating Pumps. 142.
Well Points. 143. Rock Excavation. 144. Power Drilling. 145.
Steam or Air for Power. 146. Depth of Drill Hole. 147.
Diameter of Drill Hole. 148. Spacing of Drill Holes. SHEETING
AND BRACING. 149. Purposes and Types. 150. Stay Bracing. 151.
Skeleton Sheeting. 152. Poling Boards. 153. Box Sheeting.
154. Vertical Sheeting. 155. Pulling Wood Sheeting. 156.
Earth Pressures. 157. Design of Sheeting and Bracing. 158.
Steel Sheet Piling. LINE AND GRADE. 159. Locating the Trench.
160. Final Line and Grade. 161. Transferring Grade and Line
to the Pipe. 162. Line and Grade in Tunnel. TUNNELLING. 163.
Depth. 164. Shafts. 165. Timbering. 166. Shields. 167. Tunnel
Machines. 168. Rock Tunnels. 169. Ventilation. 170.
Compressed Air. EXPLOSIVES AND BLASTING. 171. Requirements.
172. Types of Explosives. 173. Permissible Explosives. 174.
Strength. 175. Fuses and Detonators. 176. Care in Handling.
177. Priming, Loading, and Firing. 178. Quantity of
Explosive. PIPE SEWERS. 179. The Trench Bottom. 180. Laying
Pipe. 181. Joints. 182. Labor and Progress. BRICK AND BLOCK
SEWERS. 183. The Invert. 184. The Arch. 185. Block Sewers.
186. Organization. 187. Rate of Progress. CONCRETE SEWERS.
188. Construction in Open Cut. 189. Construction in Tunnels.
190. Materials for Forms. 191. Design of Forms. 192. Wooden
Forms. 193. Steel-lined Wooden Forms. 194. Steel Forms. 195.
Reinforcement. 196. Cost of Concrete Sewers. BACKFILLING.
197. Method. 233–331

CHAPTER XII

MAINTENANCE OF SEWERS

198. Work Involved. 199. Causes of Troubles. 200. Inspection.
201. Repairs. 202. Cleaning of Sewers. 203. Flushing Sewers.
204. Cleaning Catch-basins. 205. Protection of Sewers. 206.
Explosions in Sewers. 207. Valuation of Sewers. 332–351

CHAPTER XIII

COMPOSITION AND PROPERTIES OF SEWAGE

208. Physical Characteristics. 209. Chemical Composition. 210.
Significance of Chemical Constituents. 211. Sewage Bacteria.
212. Organic Life in Sewage. 213. Decomposition of Sewage.
214. The Nitrogen Cycle. 215. Plankton and Macroscopic
Organisms. 216. Variations in the Quality of Sewage. 217.
Sewage Disposal. 218. Methods of Sewage Treatment. 352–371

CHAPTER XIV

DISPOSAL BY DILUTION

219. Definition. 220. Conditions Required for Success. 221.
Self-purification of Running Streams. 222. Self-purification
of Lakes. 223. Dilution in Salt Water. 224. Quantity of
Diluting Water Needed. 225. Governmental Control. 226.
Preliminary Treatment. 227. Preliminary Investigations. 372–382

CHAPTER XV

SCREENING AND SEDIMENTATION

228. Purpose. 229. Types of Screens. 230. Sizes of Openings.
231. Design of Fixed and Movable Screens. PLAIN
SEDIMENTATION. 232. Theory of Sedimentation. 233. Types of
Sedimentation Basins. 234. Limiting Velocities. 235. Quantity
and Character of Grit. 236. Dimensions of Grit Chambers. 237.
Existing Grit Chambers. 238. Number of Grit Chambers. 239.
Quantity and Characteristics of Sludge from Plain
Sedimentation. 240. Dimensions of Sedimentation Basins.
CHEMICAL PRECIPITATION. 241. The Process. 242. Chemicals.
243. Preparation and Addition of Chemicals. 244. Results. 383–409

CHAPTER XVI

SEPTICIZATION

245. The Process. 246. The Septic Tank. 247. Results of Septic
Action. 248. Design of Septic Tanks. 249. Imhoff Tanks. 250.
Design of Imhoff Tanks. 251. Imhoff Tank Results. 252. Status
of Imhoff Tanks. 253. Operation of Imhoff Tanks. 254. Other
Tanks. 410–430

CHAPTER XVII

FILTRATION AND IRRIGATION

255. Theory. 256. The Contact Bed. 257. The Trickling Filter.
258. Intermittent Sand Filter. 259. Cost of Filtration.
IRRIGATION. 260. The Process. 261. Status. 262. Preparation
and Operation. 263. Sanitary Aspects. 264. The Crop. 431–464

CHAPTER XVIII

ACTIVATED SLUDGE

265. The Process. 266. Composition. 267. Advantages and
Disadvantages. 268. Historical. 269. Aëration Tank. 270.
Sedimentation Tank. 271. Reaëration Tank. 272. Air
Distribution. 273. Obtaining Activated Sludge. 274. Cost. 465–479

CHAPTER XIX

ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION

275. The Miles Acid Process. ELECTROLYTIC TREATMENT. 276. The
Process. DISINFECTION. 277. Disinfection of Sewage. 482–493

CHAPTER XX

SLUDGE

278. Methods of Disposal. 279. Lagooning. 280. Dilution. 281.
Burial. 282. Drying. 495–505

CHAPTER XXI

AUTOMATIC DOSING DEVICES

283. Types. 284. Operation. 285. Three Alternating Siphons.
286. Four or More Alternating Siphons. 287. Timed Siphons.
288. Multiple Alternating and Timed Siphons. 506–512

SEWERAGE AND SEWAGE TREATMENT

CHAPTER I
INTRODUCTION

=1. Sewerage and the Sanitary Engineer.=—Present day conceptions of
sanitation are based on the scientific discoveries which have resulted
so much in the increased comfort and safety of human life during the
past century, in the increase of our material possessions, and the
extent of our knowledge. The danger to health in the accumulation of
filth, the spreading of disease by various agents, the germ theory of
disease, and other important principles of sanitation can be counted
among the more recent scientific discoveries and pronouncements.
Experience has shown, and continues to show, that the increase of
population may be inhibited by accumulations of human waste in populous
districts. The removal of these wastes is therefore essential to the
existence of our modern cities.

The greatest need of a modern city is its water supply. Without it city
life would be impossible. The next most important need is the removal of
waste matters, particularly wastes containing human excreta or the germs
of disease. To exist without street lights, pavements, street cars,
telephones, and the many other attributes of modern city life might be
possible, although uncomfortable. To exist in a large city without
either water or sewerage would be impossible. The service rendered by
the sanitary engineer to the large municipality is indispensable. In
addition to the service necessary to the maintenance of life in large
cities, the sanitary engineer serves the smaller city, the rural
community, the isolated institution, and the private estate with
sanitary conveniences which make possible comfortable existence in them,
and which are frequently considered as of paramount necessity. Training
for service in municipal sanitation is training for a service which has
a more direct beneficial effect on humanity than any other engineering
work, or any other profession. W. P. Gerhard states:

_A Sanitary Engineer_ is an engineer who carries out those works
of civil engineering which have for their object:

(_a_) The promotion of the public and individual health;

(_b_) The remedying of insanitary conditions;

(_c_) The prevention of epidemic diseases.

A well-educated sanitary engineer should have a thorough knowledge
of general civil engineering, of architecture, and of sanitary
science. The practice of the sanitary engineer embraces water
supply, sewerage, and sewage and garbage disposal for cities and
for single buildings; the prevention of river pollution, the
improvement of polluted water supplies; street paving and street
cleaning, municipal sanitation, city improvement plans, the laying
out of cities, the preparation of sanitary surveys, the regulation
of noxious trades, disinfection, cremation, and the sanitation of
buildings.

The need of the work of the sanitary engineer in the provision of sewers
and drains is thrust upon us in our daily experience by the clogging of
sewers, the flooding of streets by heavy rains, filthy conditions in
unsewered districts, increased values of property and improved
conditions of living in sewered districts, and in many other ways. The
increasing demand for sewerage and the amount of money expended on sewer
construction is indicated by the information given in Table I.

=2. Historical.=—An ordinance passed by the Roman Senate in the name of
the Emperor about A.D. 80, states:

I desire that nobody shall conduct away any excess water without
having received my permission or that of my representatives; for
it is necessary that a part of the supply flowing from the
delivery tanks shall be utilized not only for cleaning our city,
but also for flushing the sewers.[1]

Neither the sewers mentioned nor the distributing pipes of the public
water supply were connected to individual residences. The contributions
to the sewers came from the ground and the street surface. The streets
were the receptacles of liquid and solid wastes and were often little
more than open sewers. A promenade after dark in an ancient, medieval,
or early modern city was accompanied not only by the underfoot dangers
of an uneven pavement or an encounter with a footpad, but with the
overhead danger from the emptying of slops into the streets from the
upper windows. Sewers were used for the collection of surface water; the
discharge of fecal matter into them was prohibited. The problem of the
collection of sewage remained unsolved until the Nineteenth Century.

TABLE 1

POPULATION TRIBUTARY TO SEWERAGE SYSTEMS

──────────────────────────────────────┬──────────┬──────────┬──────────
│ 1905[2] │ 1915[3] │ 1920[4]
──────────────────────────────────────┼──────────┼──────────┼──────────
Population discharging raw sewage into│ │ │
the sea or tidal estuaries │ 6,500,000│ 8,500,000│
Population discharging raw sewage into│ │ │
inland streams or lakes │20,400,000│26,400,000│
Population connected to systems where │ │ │
sewage is treated in some way │ 1,100,000│ 6,900,000│
Population connected with sewerage │ │ │
systems │28,000,000│41,800,000│46,300,000
──────────────────────────────────────┴──────────┴──────────┴──────────

The development of the London sewers was commenced early in the
Nineteenth Century. The sewerage system of Hamburg, Germany, was laid
out in 1842 by Lindley, an English engineer who with other English
engineers performed similar work in other German cities because of their
earlier experience in English communities. Berlin’s present system dates
from 1860. The construction of storm-water drains in Paris dates from
1663.[5] They were intended only as street drains but are now included
in the comprehensive system of the city. The first comprehensive
sewerage system in the United States was designed by E. S. Chesbrough
for the City of Chicago in 1855. Previous to this time sewers had been
installed in an indifferent manner and without definite plan. The
installation of a comprehensive sewerage system in Baltimore in 1915
marks the completion of installation of sewerage systems in all large
American cities.

In the early days of sewerage design it was considered unsafe to
discharge domestic wastes into the sewers as the concentration of so
much sewage was expected to create great nuisances and dangers to
health. That the fear that the concentration of large quantities of
sewage would create a nuisance was not ill founded is proven by the
conditions on the Thames at London in 1858–59. Dr. Budd states:[6]

For the first time in the history of man, the sewage of nearly
three millions of people had been brought to seethe and ferment
under a burning sun in one vast open _cloaca_ lying in their
midst.

The result we all know. Stench so foul we may well believe had
never before ascended to pollute this lower air. Never before at
least had a stink risen to the height of an historic event.... For
months together the topic almost monopolized the public prints....
‘India is in revolt and the Thames stinks’ were the two great
facts coupled together by a distinguished foreign writer, to mark
the climax of a national humiliation.[7]

The problem of sewage disposal followed the more or less successful
solutions of the problem of sewage collection. In England the British
Royal Commission on Sewage Disposal was appointed in 1857 and issued its
first report in 1865. The first studies in the United States were
started in 1887 by the establishment of an experiment station at
Lawrence, Massachusetts, where valuable work has been done. The station
is under the State Board of Health, which issued its first report
containing the results of the work at the station, in 1890.

Various methods of sewage treatment preparatory to disposal have been
devised from time to time. Some have fallen into disuse, such as the A.
B. C. (alum, blood and clay) process, and others have taken a permanent
place, such as the septic tank. The unsolved problems of sewage
collection, and the number of persons still unserved by sewerage and
sewage disposal opens a wide field to the study and construction of
sewerage works.

=3. Methods of Collection.=—The method of collection which involves the
removal of night soil from a privy vault, the pail system which involves
the collection of buckets of human excreta from closets and homes,
indoor chemical closets, and other makeshift methods of collection are
of extreme importance where no sewers exist, but they are not properly
considered as sewerage systems or sewerage works. These methods of
collection are generally confined to rural districts and to outlying
parts of urban communities. They require constant attention for their
proper conduct and little skill for their installation, the principal
requirements being to make the receptacles fly-proof.

The pneumatic system was introduced by Liernur, a Dutch engineer.[8] It
is used in parts of a few cities in Europe, but it is not capable of use
on a large scale. It consists of a system of air-tight pipes, connecting
water closets, kitchen sinks, etc., with a central pumping station at
which an air-tight tank is provided from which the air is partly
exhausted. As little water as possible is allowed to mix with the fecal
matter and other wastes in order not to overtax the system. Solid and
liquid wastes are drawn to the central station when the waste valve on
the plumbing fixture is opened.

The collection of sewage in a system of pipes through which it is
conducted by the buoyant effect and scouring velocity of water is known
as the water-carriage system. This is the only method of sewage
collection in general use in urban communities. In this system solid and
liquid wastes are so highly diluted with water as either to float or to
be suspended therein. The mixture resulting from this high dilution
follows the laws of hydraulics as applied to pure water, or water
containing suspended matter. It will flow freely through properly
designed conduits and will concentrate the sewage wastes at the point of
ultimate disposal.

=4. Methods of Disposal.=—Sewage is disposed of by dilution in water, by
treatment on land, or occasionally by discharging it into channels that
contain no diluting water. Some form of treatment to prepare sewage for
ultimate disposal is frequently necessary and will undoubtedly be
required in a comparatively short time for all sewage discharged into
watercourses. The solid matters removed by treatment may be buried,
burned, dumped into water, or used as a fertilizer.

If the volume of diluting water, or the area and character of land used
for disposal are not as they should be, a nuisance will be created. The
aim of all methods of sewage treatment has so far been to produce an
effluent which could be disposed of without nuisance and in certain
exceptional cases to protect public water supplies from pollution.
Financial returns have been sought only as a secondary consideration. A
few sewage farms and irrigation projects might be considered as
exceptions to this as the value of the water in the sewage as an
irrigant has been the primary incentive to the promotion of the farm.

It is to be remembered that since the aim of all sewage treatment is to
produce an effluent that can be disposed of without causing a nuisance,
the simplest process by which this result can be attained under the
conditions presented is the process to be adopted. No attempt is made to
_purify_ sewage completely, or on a practical scale to make drinking
water.

=5. Methods of Treatment.=—Screening and sedimentation are the primary
methods for the treatment of sewage. By these methods a portion of the
floating and settleable solids are removed, preventing the formation of
unsightly scum and putrefying sludge banks. Chemicals are sometimes
added to the sewage to form a heavy flocculent precipitate which hastens
sedimentation of the solid matters in the sewage. The process in these
methods is mechanical and the solid matters removed from the sewage must
be disposed of by other methods than dilution with the sewage effluent.
More complete methods of treatment are dependent on biologic action.
Under these methods of treatment complete stabilization of the effluent
is approached, and in the most complete treatment an effluent is
produced which is clear, sparkling, non-odorous, non-putrescible, and
sterile. Sterilization of sewage, usually with chlorine or some of its
compounds, has been used, not to reduce the amount of diluting water
necessary, but to reduce the number of pathogenic germs and to minimize
the danger of the transmission of disease.

=6. Definitions.=—Sewage and sewerage are not synonymous terms although
frequently confused. Sewage is the spent water supply of a community
containing the waste from domestic, industrial or commercial use, and
such surface and ground water as may enter the sewer.[9] Sewerage is the
name of the system of conduits and appurtenances designed to carry off
the sewage. It is also used to indicate anything pertaining to sewers.

A difference is made between sanitary sewage, storm sewage, and
industrial wastes. Sanitary sewage, sometimes called domestic sewage, is
the liquid wastes discharged from residences or institutions, and
contains water closet, laundry and kitchen wastes. Storm sewage is the
surface run-off which reaches the sewers during and immediately after a
storm. Industrial wastes are the liquid waste products discharged from
industrial plants.

A sewer is a conduit used for conveying sewage.

The names of the conduits through which sewage may flow are:

_Soil Stack._—A vertical pipe in a building through which waste water
containing fecal matter or urine is allowed to flow.

_Waste Pipe._—A vertical pipe in a building through which waste water
containing no fecal matter is allowed to flow.

_House Drain._—The approximately horizontal portion of a house drainage
system which conveys the drainage from the soil stack or waste pipe to
the point of discharge from the building.

_House Sewer._—The pipe which leads from the outside wall of the
building to the sewer in the street.

_Lateral Sewer._—The smallest branch in a sewerage system, exclusive of
the house sewers.

_Sub-main or Branch Sewer._—A sewer from which the sewage from two or
more laterals is discharged.[10]

_Main or Trunk Sewer._—A sewer into which the sewage from two or more
sub-main or branch sewers is discharged.[11]

_Intercepting Sewer._—A sewer generally laid transversely to a sewerage
system to intercept some portion or all of the sewage collected by the
system.

_Relief Sewer._—A sewer intended to carry a portion of the flow from a
district already provided with sewers of insufficient capacity and thus
preventing overtaxing the latter.[12]

_Outfall Sewer._—That portion of a main or trunk sewer below all
branches.

_Flushing Sewer._—A conduit through which water is conveyed for flushing
portions of a sewerage system.

_Force Main._—A conduit through which sewage is pumped under pressure.

CHAPTER II
WORK PRELIMINARY TO DESIGN

=7. Division of Work.=—Engineering work on sewerage can be divided into
four parts, namely: preliminary, design, construction and maintenance.
An engineer may be engaged during any one or all of these periods on the
same sewerage system, and should therefore be acquainted with his duties
during each period.

=8. Preliminary.=—The demand for sewerage normally follows the
installation or extension of the public water supply. It may be caused
by: a lack of drainage on some otherwise desirable tract of real estate;
from a public realization of unpleasant or unhealthful conditions in a
built-up district; or through the realization by the municipal
administration of the necessity for caring for the future. In whatever
way the demand may be created the engineer should take an active part in
the promotion of the work.

The engineer’s duties during the preliminary period are: to make a study
of the possible methods by which the demand for sewerage can be
satisfied; to present the results of this study in the form of a report
to the committee or organization responsible for the promotion of the
work; and so to familiarize himself with the conditions affecting the
installation of the proposed plans as to be able to answer all inquiries
concerning them. This work will require the general qualities of
character, judgment, efficiency and the understanding of men in
addressing interested persons individually and collectively on the
features of the proposed plans, and the exercise of engineering
technique in the survey and the drawing of the plans. The engineer
should assure himself that all legal requirements in the drawing of
petitions, advertising, permits, etc., have been complied with. This
requires some knowledge of national, state, and local laws. Although
none the less essential their description is not within the scope of
this book.

The engineer’s preliminary report should contain a section devoted to
the feasibility of one or more plans which may be explained in more or
less detail with a statement of the cost and advantages of each. A
conclusion should be reached as to the most desirable plan and a
recommendation made that this plan be installed. Other sections of the
report may be devoted to a history of the growing demand, a description
of the conditions necessitating sewerage, possible methods of financing,
and such other subjects as may be pertinent. The making of the
preliminary plan and the design of sewerage works are described in
subsequent chapters.

=9. Estimate of Cost.=—In making an estimate of cost the information
should be presented in a readable and easily comprehended manner. It is
necessary that the items be clearly defined and that all items be
included. The method of determining the costs of doubtful items such as
depreciation, interest charges, labor, etc., and the probability of the
fluctuation of the costs of certain items should be explained.

The engineer’s estimate may be divided somewhat as follows:

Labor.

Material.

Overhead. This may include construction plant, office expense,
supervision, bond, interest on borrowed capital, insurance,
transportation, etc. The amount of the item is seldom less than 15
per cent and is usually over 20 per cent of the contract price.

Contingencies. This allowance is usually 10 to 15 per cent of the
contract price.

Profit. This should be from 5 to 10 per cent of the sum of the
four preceding items.

The contract price is the sum of these items. To this may be added:

Engineering. 2 to 5 per cent of the contract price.

Extra Work. Zero to 15 per cent of the contract price; dependent
on the character of the work, the completeness of the preliminary
information, the completeness of the plans, etc.

Legal expense.

Purchase of land, rights of way, etc., etc.

The cost of the sewer may be stated as so much per linear foot for
different sizes of pipe, including all appurtenances such as manholes,
catch-basins, etc., or the items may be separated in great detail
somewhat as follows:

Earth excavation, per cu. yd.
Rock excavation, per cu. yd.
Backfill, per cu. yd.
Brick manholes, 3 feet by 4 feet, per foot of depth.
Vitrified sewer pipe with cement joints, in place,
... inches in diameter, 0 to 6 feet deep
6 to 8 feet deep
8 to 10 feet deep
Repaving, macadam per sq. yd.
asphalt per sq. yd.
Flush-tanks, ... gal. capacity, per tank.
Service pipes to flush-tanks, per linear foot., etc., etc.

These methods represent the two extremes of presenting cost estimates.
Each method, or modification thereof, may have its use, dependent on
circumstances.

Reliable cost data are difficult to obtain. Lists of prices of materials
and labor are published in certain engineering and trade periodicals.
The Handbook of Cost Data by H. P. Gillette contains lists of the amount
of material and labor used on certain specific jobs and types of
construction. The price of labor and materials on the local market can
be obtained from the local Chamber of Commerce, contractors and other
employers of labor, and dealers in the desired commodities. Contract
prices for sewerage work published in the construction news sections of
engineering periodicals may be a guide to the judgment of the probable
cost of proposed work, but are generally dangerous to rely upon as full
details are lacking in the description of the work. A wide experience in
the collection and use of cost data is the desirable qualification for
making estimates of cost. It is possessed by few and is not an
infallible aid to the judgment.

Having completed the design and summary of the bills of material and
labor necessary for each structure or portion of the sewerage system,
the product of the unit cost and the amount of each item plus an
allowance for overhead will equal the cost of the item. The total cost
will be the sum of the costs of each item. The items should be so
grouped that the cost of the different portions of the system are
separated in order that the effect on the total cost resulting from
different combinations of items or the omission of any one item may be
readily computed.

A method for estimating the approximate cost of sewers, devised by W. G.
Kirchoffer[13] depends upon the use of the diagram shown in Fig. 2. The
factors for local conditions are shown in Table 2. For example, let it
be required to find the cost of a 15–inch vitrified pipe sewer at a
depth of 9 feet, if the unit costs of labor and material and the
conditions are the same as shown in Table 3.

[Illustration:

FIG. 2.—Diagram for Estimating the Cost of Sewers.

Eng. News, Vol. 76, p. 781.
]

_Solution_

First: To find the factor depending on local conditions, enter the
diagram at the 10–inch diameter and continue down until the
intersection with the depth of trench at 8.2 feet is found. Now go
diagonally parallel to lines running from left to right upwards to
the intersection with the vertical line through a cost of 45 cents
per foot. The diagonal line running from left to right downwards
through this intersection corresponds to a factor of about 11.

TABLE 2

FACTORS FOR COSTS OF SEWERS TO BE USED WITH FIGURE 2

────────────────────────────────────────────────────────────────┬──────
Character of Material │Factor
────────────────────────────────────────────────────────────────┼──────
Clay, gravel and boulders, Medford │22–26
Mostly sand, deep trenches sheeted. Wages medium. Richland │
Center. │21–22
Sandy clay. Wages medium. Labor conditions good at Kiel. │15–20
Sand. _Sandy_ clay, some water. Labor conditions good. Pipe │
prices medium at Manston. │14–20
Gravelly clay, ⅒th laid in concrete at Burlington. │13–22
Sandy clay, some water, sheeting at La Farge. │17–23
Sand with water. │ 20
Gravel and boulders. High wages. │ 26
Clay soil. Good digging. │ 17
Sandy clay. Some water. │ 23
Clay 2 miles inland. Laborers boarded at sanitarium, Wales │ 35
Clay, gravel and boulders at Plymouth. │20–27
Sand, clay and good digging at Lake Mills. │16–19
Red clay. Machine work at North Milwaukee. │20–24
Good digging. Wages medium at West Salem. │17–19
Sandy soil, bracing only required. No water. Wages and pipe │
medium. │ 14
Red sticky clay. │ 24
Good digging in any soil. Work scarce. │ 15
Red clay. No bracing. │ 20
Work inland from railroad. Boarding laborers _and_ other │
expenses. │ 35
────────────────────────────────────────────────────────────────┴──────

Second: To find the cost of 15–inch pipe at a depth of 9.0 feet,
enter the diagram at a diameter of 15 inches and continue down
until the intersection with a depth of trench at 9 feet is found.
Now go diagonally parallel to lines running from left to right
upwards to the intersection with the diagonal line running from
left to right downwards corresponding to the factor of 11 found
above. The vertical line passing through this point shows the cost
to be 67 cents per foot.

TABLE 3

COST OF SEWER CONSTRUCTION AT ATLANTIC, IOWA

(From Gillette’s Handbook of Cost Data)

Material: Clay, not difficult to spade and requiring little or no
bracing and practically no pumping. All hand work except backfill which
was done by team and scraper. Depth of trench averaged 8.2 feet; width
30 inches. Diameter of pipe 10 inches.

───────────────────────────────────────────────────────────┬─────┬─────
Item │Wage,│Cost,
│Cents│Cents
│ per │ per
│Hour │Foot.
───────────────────────────────────────────────────────────┼─────┼─────
Pipe. │ │0.20
Hauling team and driver. │ 30│ .003
Hauling. Man helping. │ 17│ .001
Cement and sand. │ │ .006
Pipe layers. │ 22│ .014
Pipe layer’s helper. │ 17│ .014
Trenching. Top men. │ 17│ .027
Trenching. Bottom men. │ 17│ .130
Trenching. Scaffold men. │ 17│ .002
Trenching. Bracing men. │ 17│ .002
Backfilling. Shovel. │ 17│ .010
Backfilling. Team and scraper. │ 30│ .008
Backfilling. Man and scraper. │ 17│ .005
Water boy. │ 10│ .006
Foreman. │ 30│ .022
───────────────────────────────────────────────────────────┼─────┼─────
Total. │ │ .450
───────────────────────────────────────────────────────────┴─────┴─────

METHODS OF FINANCING

The construction of sewerage works may be paid for by the issue of
municipal bonds, by special assessment, by funds available from the
general taxes, or by private enterprise.

=10. Bond Issues.=—A municipal bond is a promise by the municipality to
pay the face value of the bond to the holder at a certain specified
time, with interest at a stipulated rate during the interim. The
security on the bond is the taxable property in the municipality. The
legal restrictions thrown around municipal bond issues, the value of the
taxable property in the municipality, all of which may be used as
security for municipal bonds, and the fact that a municipality can be
sued in case of default, make municipal bonds desirable and provide a
good market for their sale. The funds available from a municipal bond
issue are limited by the amount that the legal limit is in excess of the
outstanding issues. The legal limit varies in different states from
about 5 to 15 per cent of the assessed value of the property in the
municipality. In some cases the amount available from municipal bonds
has been increased by forming a municipality within a municipality such
as a sanitary district, a park district, a drainage district, etc.,
which comprises a large portion or all of an existing municipal
corporation. This case is well illustrated in some parts of the City of
Chicago where the municipal taxing powers are shared by the City
government, the Sanitary District, and Park Commissioners. The right to
create a new municipal corporation must be granted by the state
legislature. Knowledge of fixed bonds, serial bonds, life of bonds,
sinking funds, etc. is an important part of an engineer’s education.[14]

Bond issues must usually be presented to the voters for approval at an
election. If approved, and other legal procedure has been followed, the
bonds may be bought by some of the many bonding houses, or by private
individuals, and the money is immediately available for construction.
The bonds are redeemed by general taxation spread over the period of the
issue.

=11. Special Assessment.=—A special assessment is levied against
property benefited directly by the structure being paid for. Special
assessments are used for the payment for the construction of lateral
sewers which are a direct benefit to separate districts but are without
general benefit to the city. In case the construction of an outfall
sewer or the erection of a treatment plant, which may be of some general
benefit, is necessary to care for a separate district, a part of the
expense may be borne by funds available from general taxation. The legal
procedure for the raising of funds by special assessment and the purpose
to which the funds so raised may be applied are stipulated in great
detail in different states and their directions must be followed
implicitly. Illinois procedure, which is similar to that in some other
states, is as follows: a meeting of the interested property owners is
called by a committee or board of the municipal government, as the
result of a petition by interested persons or through the independent
action of the Board. At this preliminary meeting or public hearing
arguments for and against the proposed improvement are heard. The
engineer is present at this meeting to answer questions and to advise
concerning the engineering features of the plan. If approval is given by
the Board the plan and specifications are prepared complete in every
detail and incorporated in an ordinance which is presented to the
legislative branch of the city government for passage. If the project is
adopted it is taken to the county court. An assessment roll is prepared
by a commissioner appointed by the court. This roll shows the amount to
be assessed against each piece of property benefited. A hearing is then
held in the county court at which the owner of any assessed property may
voice objections to the continuation of the project. The project may be
thrown out of court for many different reasons, such as the misspelling
of a street name, an error in an elevation, an error in the description
of a pavement, but most important of all is definite proof that the
benefit is not equal to the assessment. The many minor irregularities
which may nullify the procedure in a special assessment differ in
different states and in different courts in the same state, but in
general no court can approve an assessment greater than the benefits
given. After the project has passed through the county court and the
assessment roll has been approved, bonds may be issued for the payment
of the contractor. Special assessment bonds are liens against the
property assessed and have not the same security as a general municipal
bond. For this reason a city which has reached its legal limit of
municipal bond issues can still pay for work by special assessment.

The funds available from special assessments are limited only by the
benefit to the property assessed. The amount of the benefit is difficult
to fix and may lead to much controversy. It should not exceed the amount
demanded for similar work in other localities, unless unusual and
well-understood reasons can be given.

=12. General Taxation.=—In paying for public improvements by general
taxation the money is taken from the general municipal funds which have
been apportioned for that purpose by the legislative department of the
municipal government. This method of raising funds for sewerage
construction is seldom used unless the political situation is
unfavorable to the success of a bond issue or special assessment and the
need for the improvement is great. It is usually difficult to
appropriate sufficient funds for new construction as the general tax is
apportioned to support only the operating expenses of the city, and
statutory provisions limit the amount of tax which can be levied.

=13. Private Capital.=—Private capital has been used for financing
sewerage works in some cases because of the aversion of the public in
some cities to the payment of a tax for the negative service performed
by a sewer. Sewers are buried, unseen, and frequently forgotten, but
knowledge of their necessity has spread and the number of privately
owned sewerage works is diminishing because of the better service which
can be provided by the municipality.

Franchises are granted to private companies for the construction of
sewers only after the city has exhausted other methods for the raising
of capital. The return on the private capital invested is received from
a rental paid by the city, or paid directly by the users of the system,
an initial payment usually being demanded for connection to the system.
To be successful the enterprise must be popular and must fill a great
need. This method of financing sewerage works is seldom employed as
favorable conditions are not common.

PRELIMINARY WORK

=14. Preparing for Design.=—Methods for the design of sewerage systems
are given in Chapter V. Before the design is made certain information is
essential. A survey must be made from which the preliminary map can be
prepared as described in Art. 42. Other necessary information which is
the basis of subsequent estimates of the quantity of sewage to be cared
for must be obtained by a study of rates of water consumption and the
density and growth of population, the measurement of the discharge from
existing sewers, and the compilation of rainfall and run-off data. If no
rainfall data are available estimates must be made from the nearest
available data. Observations of rainfall or run-off for periods of less
than 10 to 20 years are likely to be misleading. Methods for gathering
and using this information are explained in subsequent chapters.

Underground surveys are desirable along the lines of the proposed sewers
to learn of obstructions, difficult excavation and other conditions
which may be met. All such data are seldom gathered except for sewerage
systems involving the expenditure of a large amount of money. For
construction in small towns or small extensions to an existing system
the funds are usually insufficient for extensive preliminary
investigation. The saving in this respect is paid unknowingly to the
contractor as compensation for the risk in bidding without complete
information.

=15. Underground Surveys.=—These may be more or less extensive dependent
on the character of the district in which construction is to take place.
In built-up districts the survey should be more thorough than in
sparsely settled districts where only the character of the excavated
material is of interest and no obstructions are to be met.

Underground surveys furnish to the engineer and to prospective bidders
on contract work information on which the design and estimate of cost
and the contractor’s bid may be based and without which no intelligent
work can be done. By removing much of the uncertainty of the conditions
to be met in the construction of the sewer, the design can be made more
economical and the contractor’s bid should be markedly lower,
sufficiently so to repay more than the expense of the survey. The
information to be obtained consists of the location of the ground-water
level, and the location and sizes of water, gas, and sewer pipes,
telephone and electric conduits, street-car tracks, steam pipes, and all
other structures which may in any way interfere with subsurface
construction. These structures should be located by reference to some
permanent point on the surface. The elevation of the top of the pipes,
except sewers, rather than the depth of cover should be recorded, as the
depth of cover is subject to change. The elevation of sewers should be
given to the invert rather than to the top of the pipe.

A portion of the map of the subsurface conditions at Washington, D. C.,
is shown in Fig. 3. Many of the dimensions and notations are not shown
to avoid confusion on this small reproduction.[15] Colors are generally
used instead of different forms of cross hatching to show the different
classes of pipe and structures. In addition to a record of the
underground structures the character of the ground and the pavement
should be recorded. A comprehensive underground survey is seldom
available nor does time usually permit its being made preliminary to the
design of a sewerage system. The character of the material through which
the sewer is to pass should be determined in all cases.

[Illustration:

FIG. 3.—Record Map of Underground Structures, Washington, D. C.

Eng. Record, Vol. 74, p. 263.

The various subsurface lines are differentiated by colors as follows:
_A_—Sewers, vermilion. _B_—Water mains, blue. _C_—Potomac Electric
Power Co., carmine. _D_—Washington Railway and Electric Co.,
carmine. _E_—Capital Traction Co., violet. _F_—Chesapeake and
Potomac Telephone Co., green. _G_—Washington Gas Light Co., green.
_H_—Western Union Telegraph Co., orange. _I_—Postal Telegraph Co.,
orange. _K_—Private vaults, black. _L_—City Electric Co., yellow.
]

[Illustration:

FIG. 4.

Punch Drill.
]

Underground pipes and structures are located by excavations, which may
be quite extensive in some cases. Their position is fixed by
measurements referred to manholes and other underground structures which
are somewhat permanent in position. A city engineer should grasp every
opportunity to record underground structures when excavations are made
in the streets. The character of the material through which the sewer is
to pass is determined by borings.

=16. Borings.=—Methods used for the investigation of subsurface
conditions preliminary to sewer construction are: punch drilling, boring
with earth auger, jet boring, wash boring, percussion drilling, abrasive
drilling, and hydraulic drilling. The last three methods named are used
only for unusually deep borings or in rock.

Punch drills are of two sorts. The simplest punch drill consists of an
iron rod ⅞ of an inch to 1 inch in diameter, in sections about 4 feet
long. One section is sharpened at one end and threaded at the other so
that the next section can be screwed into it without increasing the
diameter of the rod, as shown in Fig. 4. The drill is driven by a sledge
striking upon a piece of wood held at the top of the drill to prevent
injury to the threads. The drill should be turned as it is driven to
prevent sticking. It is pulled out by a hook and lever as shown in Fig.
5. It is useful in soft ground for soundings up to 8 to 12 feet in
depth. Another form of punch drill described by A. C. Veatch[16]
consists of a cylinder of steel or iron, one to two feet long split
along one side and slightly spread. The lower portion is very slightly
expanded and tempered into a cutting edge. In use it is attached to a
rope or wooden poles and lifted and dropped in the hole by means of a
rope given a few turns about a windlass or drum. By this process the
material is forced up into the bit, slightly springs it, and so is held.
When the bit is filled it is raised to the surface and emptied. Much
deeper holes can be made with this than with the sharpened solid rod.

[Illustration:

FIG. 5.—Lever for Pulling Punch Drill.
]

[Illustration:

FIG. 6.—Earth Augers.
]

Types of earth augers about 1½-inches in diameter are shown in Fig. 6.
They are screwed on to the end of a section of the pipe or rod and as
the hole is deepened successive lengths of pipe or rod are added. The
device is operated by two men. It is pulled by straight lifting or with
the assistance of a link and lever similar to that shown in Fig. 5. The
device is suitable for soft earth or sand free from stones, and can be
used for holes 15 to 25 feet in depth. For deeper holes a block and
tackle should be used for lifting the auger from the hole. It is not
suitable for holes deeper than about 35 feet.

In the jetting method water is led into the hole through a ¾-inch or
1–inch pipe, and forced downward through the drill bit or nozzle against
the bottom of the hole. The complete equipment is shown in Fig. 7.[17]
It is not always necessary to case the hole as shown in the figure as
the muddy water and the vibration of the pipe puddle the sides so that
they will stand alone. The jet pipe may be churned in the hole by a rope
passing over a block and a revolving drum. In suitable soft materials
such as clay, sand, or gravel, holes can be bored to a depth of 100 feet
and samples collected of the material removed. An objection to the
method is the difficulty of obtaining sufficient water.

[Illustration:

FIG. 7.—Jetting Outfit.

U. S. Geological Survey, Water Supply Paper, No. 257

1. Simple Jetting Outfit. 2. Jetting Process. 3. Common Jetting Drill.
4_a_ and 4_b_. Expansion Bit or Paddy. 5. Drive Shoe.
]

Methods of drilling in rock up to depths of 20 feet are described in
Chapter XI under Rock Drilling. For deeper holes percussion, abrasive,
or hydraulic methods as used for deep well drilling must be employed.

CHAPTER III
QUANTITY OF SEWAGE

=17. Dry weather Flow.=—Estimates of the quantity of sewage flow to be
expected are ordinarily based on the population, the character of the
district, the rate of water consumption, and the probable ground-water
flow. Future conditions are estimated and provided for, as the sewers
should have sufficient capacity to care for the sewage delivered to them
during their period of usefulness.

=18. Methods for Predicting Population.=—Methods for the prediction of
future population are given in the following paragraphs.

The method of _graphical extension_. This is the quickest and most
simple of all. In this method a curve is plotted on rectangular
coordinates to any convenient scale, with population as ordinates and
years as abscissas. The curve is extended into the future by judgment of
its general tendency. An example is given of the determination of the
population of Urbana, Illinois, in 1950. Table 4 contains the population
statistics which have been plotted on line A in Fig. 8 and extended to
1950. The probable population in 1950 is shown by this line to be about
21,000.

The method of _geometrical progression_. In this method the rate of
increase during the past few years or decades is assumed to be constant
and this rate is applied to the present population to forecast the
population in the future. For example the rate of increase of population
in Urbana for the past 7 decades has varied widely, but indications are
that for the next few decades it will be about 20 per cent. Applying
this rate from 1920 to 1950 the population in 1950 is shown to be about
17,800. It is evident that this method may lead to serious error as
insufficient information is given in the table to make possible the
selection of the proper rate of increase.

TABLE 4

POPULATION STUDIES

────┬────────────────────────────
│ Urbana, Illinois
────┼──────────┬────────┬────────
Year│Population│Absolute│Per Cent
│ │Increase│Increase
│ │for Each│for Each
│ │ Decade │ Decade
────┼──────────┼────────┼────────
1850│ 210│ │
1860│ 2,038│ 1828│ 85.6
1870│ 2,277│ 239│ 10.5
1880│ 2,942│ 665│ 22.6
1890│ 3,511│ 569│ 16.2
1900│ 5,728│ 2217│ 38.7
1910│ 8,245│ 2517│ 30.5
1920│ 10,230│ 1985│ 19.4
────┴──────────┴────────┴────────

────┬───────────────────────────────────────────────────────────────
│ Population of
────┼───────┬────────┬─────────┬────────┬──────┬───────────┬────────
Year│Decatur│Danville│Champaign│Kankakee│Peoria│Bloomington│ Ann,
│ │ │ │ │ │ │ Arbor
│ │ │ │ │ │ │Michigan
│ │ │ │ │ │ │
────┼───────┼────────┼─────────┼────────┼──────┼───────────┼────────
1850│ │ 736│ │ │ 5,095│ 1,594│
1860│ 3,839│ 1,632│ 1,727│ 2,984│14,045│ 7,075│ 5,097
1870│ 7,161│ 4,751│ 4,625│ 5,189│22,849│ 14,590│ 7,368
1880│ 9,547│ 7,733│ 5,103│ 5,651│29,259│ 17,180│ 8,061
1890│ 16,841│ 11,491│ 5,839│ 9,025│41,024│ 20,484│ 9,431
1900│ 20,754│ 16,354│ 9,098│ 13,595│56,100│ 23,286│ 14,509
1910│ 31,140│ 27,871│ 12,421│ 13,986│66,950│ 25,786│ 14,817
1920│ 43,818│ 33,750│ 15,873│ 16,721│76,121│ 28,638│ 19,516
────┴───────┴────────┴─────────┴────────┴──────┴───────────┴────────

[Illustration:

FIG. 8.—Diagram Showing Methods for Estimating Future Population.
]

The method of utilizing a _decreasing rate of increase_. This method
attempts to correct the error in the assumption of a constant rate of
increase. After a certain period of growth, as the age of a city
increases its rate of increase diminishes. In applying this knowledge to
a prediction of the future population of a city the population curve is
plotted, as in the graphical method and a straight line representing a
constant rate or increase is drawn tangent to the curve at its end. The
curve is then extended at a flatter rate in accordance with the rate of
change of a similar nearby larger city. This method has not been applied
to any of the cities included in Table 4, as none has reached that
limiting period where the rate of increase has begun to diminish.

The method of utilizing an _arithmetical rate of increase_. This method
allows for the error of the geometrical progression which tends to give
too large results for old and slow-growing cities. This method generally
gives results that are too low. The absolute increase in the population
during the past decade or other period is assumed to continue throughout
the period of prediction. Applying this method to the same case, the
increase in the population during the past decade was 2,000. Adding
three times this amount to the population in 1920, the population of
Urbana in 1950 will be about 16,000.

The method involving the _graphical comparison with other cities_ with
similar characteristics. In this method population curves of a number of
cities larger than Urbana but having similar characteristics, are
plotted with years as abscissas and population as ordinates, with the
present population of Urbana as the origin of coordinates. The
population curve for Urbana is first plotted. It will lie entirely in
the third quadrant as shown by the heavy full line in Fig. 8. The
population curves of some larger cities are then plotted in such a
manner that each curve passes through the origin at the time their
population was the same as that of the present population of Urbana.
These curves lie in the first and third quadrants. The population curve
of the city in question is then extended to conform with the curves of
older cities in the most probable manner as dictated by judgment. Such a
series of plots has been made in Fig. 8. The results indicate that the
population of Urbana in 1950 will be about 25,500.

The last method described will give the most probable result as it is
the most rational. For quick approximations the geometrical progression
is used. The arithmetical progression is useful only as an approximate
estimate for old cities.

=19. Extent of Prediction.=—The period for which a sewerage system
should be designed is such that each generation bears its share of the
cost of the system. It is unfair to the present generation to build and
pay for an extensive system that will not be utilized for 25 years. It
is likewise unfair to the next generation to construct a system
sufficient to comply with present needs only, and to postpone the
payment for it by a long term bond issue. An ideal solution would be to
plan a system which would satisfy present and future needs and to
construct only those portions which would be useful during the period of
the bond issue. Unfortunately this solution is not practical, because,
1st, it is less expensive to construct portions of the system such as
the outfall, the treatment plant, etc., to care for conditions in
advance of present needs, and 2nd, the life of practically all portions
of a sewerage system is greater than the legal or customary time limit
on bond issues.

A compromise between the practical and the ideal is reached by the
design of a complete system to fulfill all probable demands, and the
construction of such portions as are needed now in accordance with this
plan. The payment should be made by bond issues with as long life as is
financially or legally practical, but which should not exceed the life
of the improvement.

The prediction of the population should therefore be made such that a
comprehensive system can be designed with intelligence. Practice has
seldom called for predictions more than 50 years in the future.

=20. Sources of Information on Population.=—The United States decennial
census furnishes the most complete information on population.
Unfortunately it becomes somewhat old towards the end of a decade. More
recent information can be obtained from local sources. Practically every
community takes an annual school census the accuracy of which is fairly
reliable. The general tendencies of the population to change can be
learned by a study of the post office records showing the amount of mail
matter handled at various periods. Local chambers of commerce and
newspapers attempt to keep records of population, but they are often
inaccurate. Another source of information is the gross receipts of
public service companies, such as street railways, water, gas,
electricity, telephone, etc. The population can be assumed to have
increased almost directly as their receipts, with proper allowance for
change in rates, character of management, and other factors.

=21. Density of Population.=—So far the study of population has been
confined to the entire city. It is frequently necessary to predict the
population of a district or small section of a city. A direct census may
be taken, or more frequently its population is determined by estimating
its density based on a comparison with similar districts of known
density, and multiplying this density by the area of the district. In
determining the density, statistics of the population of the entire city
will be helpful but are insufficient for such a problem. A special
census of the area involved would be conclusive but is generally
considered too expensive. A count of the number of buildings in the
district can be made quickly, and the density determined by
approximating the number of persons per building. Statistics of the
population of various districts together with a description of the
character of the district are given in Table 5.

[Illustration:

FIG. 9.—Density, Area, and Population, Cincinnati, Ohio. 1850 to 1950.
]

TABLE 5

DENSITIES OF POPULATION

────────────┬────────────────────────────────────────┬────────┬────────
City │ Character of District │ Area, │Density
│ │ Acres │per Acre
────────────┼────────────────────────────────────────┼────────┼────────
Philadelphia│Thomas Run. Residential. Mostly pairs of│ │
│ two and three-story houses. 1204 acres│ │
│ settled. │ 1,840│ 59
│Pine Street. Residential. Mostly solid │ │
│ four to six-story houses. 156 acres │ │
│ settled. │ 160│ 97
│Shunk Street. Residential. Mostly pairs │ │
│ of two and three-story houses. 539 │ │
│ acres settled. │ 539│ 119
│Lombard Street. Tenements and hotels, │ │
│ 145 acres settled. │ 147│ 113
│York Street. Residential and │ │
│ manufacturing. 354 acres settled. │ 358│ 94
│ │ │
New York │Residential. Three-story dwellings with │ │
City │ 18–foot frontage, and four-story flats│ │
│ with 20–foot frontage. │ │ 100
│Residential. Five-story flats. │ │520–670
│Residential. Six-story flats. │ │800–1000
│Residential. Six-story apartments. High │ │
│ class. │ │ 300
│ │ │
Chicago │1st Ward. Retail and commercial. The │ │
│ “Loop”. │ 1,440│ 20.5
│2d Ward. Commercial and low-class │ │
│ residential solidly built up. │ 800│ 53.5
│3d Ward. Low-class residential. │ 960│ 48.1
│5th Ward. Industrial. Some low-class │ │
│ residences. Not solidly built up. │ 2,240│ 25.51
│6th Ward. Residential. Four and │ │
│ five-story apartments. A few detached │ │
│ residences. │ 1,600│ 47.0
│7th Ward. Same as Ward 6. Not solidly │ │
│ built up. Contains a large park. │ 4,160│ 21.7
│8th Ward. Industrial. Sparsely settled. │ 13,624│ 4.8
│9th Ward. Industrial and low-class │ │
│ residential. Solidly built up. │ 640│ 70.0
│10th Ward. Same as Ward 9. │ 640│ 80.8
│13th Ward. Low-class residential. │ │
│ Solidly built with three and │ │
│ four-story flats. │ 6,100│ 36.7
│16th Ward. Middle-class residential. │ │
│ Some industries. Well built up. │ 800│ 81.5
│19th Ward. Industrial and commercial. │ │
│ Some low-class residences. │ 640│ 90.7
│20th Ward. Low-class residential. Some │ │
│ industries. Entirely built up. │ 800│ 77.1
│21st Ward. Industrial. Entirely built │ │
│ up. │ 960│ 49.9
│23d Ward. Industrial and residential. │ 800│ 55.4
│24th Ward. Residential apartment houses │ │
│ and middle-class residences. │ 1,120│ 46.8
│25th Ward. Residential. High-class │ │
│ apartments. Wealthy homes. Contains a │ │
│ large park. │ 4,160│ 24.0
│26th Ward. Residential. Middle-class │ │
│ homes and apartments. Fairly well │ │
│ built up. │ 4,640│ 16.1
│27th Ward. Residential. Sparsely │ │
│ settled. │ 20,480│ 5.5
│29th Ward. Low-class residential. │ │
│ Two-story frame houses. “Back of the │ │
│ Yards”. │ 6,400│ 12.8
│30th Ward. The Stock Yards. │ 1,280│ 40.1
│32d Ward. Scattered residences. │ 8,480│ 8.3
│33d Ward. Scattered residences. │ 12,944│ 5.5
│35th Ward. Scattered residences. │ 4,960│ 12.0
│ │ │
General │The most crowded conditions with │ │
average │ five-story and higher, contiguous │ │
│ buildings in poor class districts. │ │750–1000
│Five and six-story contiguous flat │ │
│ buildings. │ │500–750
│Six-story high-class apartments. │ │300–500
│Three and four-story dwellings, business│ │
│ blocks and industrial establishments. │ │
│ Closely built up. │ │100–300
│Separate residences, 50 to 75–foot │ │
│ fronts, commercial districts, │ │
│ moderately well built up. │ │ 50–100
│Sparsely settled districts and scattered│ │
│ frame dwellings for individual │ │
│ families. │ │ 0–50
────────────┴────────────────────────────────────────┴────────┴────────

The density of population in Cincinnati from 1850 to 1913 with
predictions to 1950 is given in Fig. 9.[18] This shows the densities for
the entire city and is illustrative of the manner in which future
conditions were predicted for the design of an intercepting sewer. The
data given in Table 5 are of value in estimating the densities of
population in various districts. The Committee on City Plan of the Board
of Estimate and Apportionment of New York City obtained some valuable
information on this point, especially in Manhattan. Three-story
dwellings with 18–foot frontage, or four-story flats with 20–foot
frontage, presumably contiguous, were found to hold 100 persons to the
acre. Five-story flats held 520 to 670 persons per acre. Six-story flats
held 800 to 1,000 persons per acre, and high-class six-story apartments
held less than 300 per acre.

=22. Changes in Area.=—In order to determine the probable extent of a
proposed sewerage system it is important to estimate the changes in the
area of a city as well as the changes in the population. With the same
population and an increased area the quantity of sewage will be
increased because of the larger amount of ground water which will enter
the sewers. Predictions of the area of a city are less accurate than
predictions of population because the factors affecting changes cannot
be so easily predicted. An area curve plotted against time would be
helpful in guiding the judgment, but its extension into the future based
on past occurrences would be futile. A knowledge of the city, its
political tendencies, possibilities of extension, and other factors must
be weighed and judged. The engineer, if he is ignorant of the city for
which he is making provision, is dependent upon the testimony of real
estate men, business men and others acquainted with the local situation.

=23. Relation between Population and Sewage Flow.=—The amount of sewage
discharged into a sewerage system is generally equal to the amount of
water supplied to a community, exclusive of ground water. The entire
public water supply does not reach the sewers, but the losses due to
leakage, lawn sprinkling, manufacturing processes, etc., are made up by
additions from private water supplies, surface drainage, etc. The
estimated quantity of water used but which did not reach the sewers in
Cincinnati is shown in Table 6. The amount shown represents 38 per cent
of the total consumption. Unless direct observations have been made on
existing sewers or other factors are known which will affect the
relation between water supply and sewage, the average sewage flow
exclusive of ground water, should be taken as the average rate of water
consumption. Experience has shown that water consumption increases after
the installation of sewers.

TABLE 6

ESTIMATED QUANTITY OF WATER USED BUT NOT DISCHARGED INTO THE SEWERS IN
CINCINNATI

Expressed in gallons per capita per day, and based on a total
consumption of 125 to 150 gallons per capita per day.

──────────────────────────────────────────────────────────────┬────────
Steam railroads. │6 to 7
Street sprinklers. │6 to 7
Consumers not sewered. │9 to 10½
Manufacturing and mechanical. │6 to 7
Lawn sprinklers. │3 to 3½
Leakage. │18 to 21
──────────────────────────────────────────────────────────────┴────────

The public water supply is generally installed before the sewerage
system. By collecting statistics on the rate of supply of water a fair
prediction can be made of the quantity of sewage which must be cared
for. The rate of water supply varies widely in different cities. It is
controlled by many factors such as meters, cost and availability of
water, quality of water, climate, population, etc. In American cities a
rough average of consumption is 100 gallons per capita per day. Other
factors being equal the rate of consumption after meters have been
installed will be about one-half the rate before the meters were
installed. Low cost, good quantity and good quality will increase the
rate of consumption, and the rate will increase slowly with increasing
population. Statistics of rates of water consumption are given in Table
7.

=24. Character of District.=—The various sections of a city are
classified as commercial, industrial, or residential. The residential
districts can be subdivided into sparsely populated, moderately
populated, crowded, wealthy, poor, etc. Commercial districts may be
either retail stores, office buildings, or wholesale houses. Industrial
districts may be either large factories, foundries, etc., or they may be
made up of small industries housed in loft buildings.

In cities of less than 30,000 population the refinement of such
subdivisions is generally unnecessary in the study of sewage flow, all
districts being considered the same. The data given in Tables 8 and 9
indicate the difference to be found in different districts of large
cities. The Milwaukee data are presented in a form available for
estimates on different bases. These data are shown in Table 10.

TABLE 7

RATES OF WATER CONSUMPTION

From Journals of American and New England Water Works Associations
───────────────────────────────────┬───────────┬───────────┬───────────
City │Population │ Per Cent │Consumption,
│ in │ Metered │ Gal. per
│ Thousands │ │Capita per
│ │ │ Day
───────────────────────────────────┼───────────┼───────────┼───────────
Tacoma, Wash. │ 100 │ 11.6│ 460
Buffalo, N. Y. │ 450 │ 4.9│ 310
Cheyenne, Wyo. │ 13 │ │ 270
Erie, Pa. │ 72 │ 3.0│ 198
Philadelphia, Pa. │ 1611 │ 4.6│ 180
St. Catherines, Ont. │ 17 │ 3.2│ 160
Port Arthur, Ont. │ 18 │ 14.7│ 145
Ogdensburg, N. Y. │ 18 │ 0.2│ 140
Los Angeles, Cal. │ 516 │ 77.9│ 140
Wilmington, Del. │ 92 │ 43.7│ 125
Lancaster Pa. │ 60 │ 34.6│ 120
Richmond, Va. │ 120 │ 75.2│ 115
St. Louis, Mo. │ 730 │ 6.7│ 110
Springfield, Mass. │ 100 │ 94.4│ 110
Keokuk, Ia. │ 14 │ 64.5│ 105
Jefferson City, Mo. │ 13.5│ 34.4│ 100
Muncie, Ind. │ 30 │ 23.8│ 95
Burlington, Ia. │ 24 │ 4.5│ 90
Council Bluffs, Ia. │ 32 │ 75.5│ 80
San Diego, Cal. │ 85 │ 100 │ 80
Monroe, Wis. │ 3 │ 100 │ 80
Yazoo City, Miss. │ 7 │ 84.1│ 75
Oak Park, Illinois. │ 26 │ 100 │ 70
Portsmouth, Va. │ 75 │ 8.1│ 65
New Orleans, La. │ 360 │ 99.7│ 60
Rockford, Ill. │ 53 │ 93.0│ 55
Fort Dodge, Ia. │ 20 │ 96.0│ 50
Manchester, Vt. │ 1.5│ 69.0│ 45
Woonsocket, R. I. │ 47.5│ 95.6│ 35
───────────────────────────────────┴───────────┴───────────┴───────────

Attempts have been made to express the rate of sewage flow in different
units other than in gallons per capita per day. A unit in terms of
gallons per square foot of floor area tributary has been suggested for
commercial and industrial districts. It has not been generally adopted.
The rates of flow in New York City as reported in this unit by W. S.
McGrane are given in Table 11.

The most successful way to predict the flow from commercial or
industrial districts is to study the character of the district’s
activities and to base the prediction on the quantity of water demanded
by the commerce and industry of the district affected.

=25. Fluctuations in Rate of Sewage Flow.=—The rate of flow of sewage
from any district varies with the season of the year, the day of the
week, and the hour of the day. The maximum and minimum rates of sewage
flow are the controlling factors in the design of sewers. The sewers
must be of sufficient capacity to carry the maximum load which may be
put upon them, and they must be on such a grade that deposits will not
occur during periods of minimum flow. The maximum and minimum rates of
flow are usually expressed as percentages of the average rate of flow.

TABLE 8

SEWAGE FLOW FROM DIFFERENT CLASSES OF DISTRICTS

Arranged from data by Kenneth Allen in Municipal Engineer’s Journal,
Feb., 1918.

──────────────────────────────────────────────────────┬───────┬───────
District │Gallons│Gallons
│ per │ per
│Capita │ Acre
│per Day│per Day
──────────────────────────────────────────────────────┼───────┼───────
Buffalo, N. Y. From Report of International Joint │ │
Commission on the Pollution of Boundary Waters: │ │
Industrial: Metal and automobile plants. Maximum. │ │ 13,000
Industrial: Meat packing, chemical and soap. │ │ 16,000
Commercial: Hotels, stores and office buildings. │ │ 60,000
Domestic: Average. │ 80 │
Domestic: Apartment houses. │ 147 │
Domestic: First-class dwellings. │ 129 │
Domestic: Middle-class dwellings. │ 81 │
Domestic: Lowest-class dwellings. │ 35.5│
│ │
Cincinnati, Ohio. 1913 Report on Sewerage Plan: │ │
Industrial, in addition to residential and ground │ │
water. │ │ 9,000
Commercial, in addition to residential and ground │ │
water. │ │ 40,000
Domestic. │ 135 │
│ │
Detroit, Mich.: │ │
Domestic. │ 228 │
Industrial, in addition to residential and ground │ │
water. │ │ 12,000
Commercial, in addition to residential and ground │ │
water. │ │ 50,000
│ │
Milwaukee, Wis. 1915 Report of Sewerage Commission: │ │
Industrial, maximum. │ 81 │ 16,600
Industrial, average. │ 31 │ 8,300
Commercial, maximum. │ │ 60,500
Commercial, average. │ │ 37,400
Wholesale commercial, maximum. │ │ 20,000
Wholesale commercial, average. │ │ 9,650
──────────────────────────────────────────────────────┴───────┴───────

TABLE 9

OBSERVED WATER CONSUMPTION IN DIFFERENT CLASSES OF DISTRICTS IN NEW YORK CITY

From data by Kenneth Allen in Municipal Engineers Journal, for 1918
─────────────┬─────────────┬──────────┬─────────────┬───────────┬─────────────
Hotels │ Daily Cons. │Tenements │ Daily Cons. │Office and │ Daily Cons.
│ Gals. per │ │ Gals. per │ Loft │ Gals. per
│1000 Sq. Ft. │ │1000 Sq. Ft. │ Buildings │1000 Sq. Ft.
│ Floor Area │ │ Floor Area │ │ Floor Area
─────────────┼────────┬────┼──────────┼────────┬────┼───────────┼────────┬────
Building │Max.[19]│Avg.│ Location │Max.[19]│Avg.│ Building │Max.[19]│Avg.
─────────────┼────────┼────┼──────────┼────────┼────┼───────────┼────────┼────
Hotel │ │ │78th–79th │ │ │McGraw │ │
Biltmore. │ │ │ St. and │ │ │ Bldg. │ │
│ 470 │368 │ B’way. │ 256 │192 │ │ 309 │206
Hotel │ │ │410 E. │ │ │N. Y. │ │
McAlpin. │ │ │ 65th St.│ │ │ Telephone│ │
│ 753 │694 │ │ 350 │295 │ Bldg. │ │194
Hotel Plaza. │ │ │30th St. │ │ │Met. Life │ │
│ │ │ and │ │ │ Bldg. │ │
│ │ │ Madison │ │ │ │ │
│ 630 │578 │ Ave │ 306 │188 │ │ │256
Hotel Waldorf│ │ │27 Lewis │ │ │42d St. │ │
Astoria. │ 618 │482 │ St. │ 307 │250 │ Bldg │ │271
Hotel Astor. │ │ │258 │ │ │Municipal │ │
│ │ │ Delancey│ │ │ Bldg. │ │
│ 732 │492 │ St. │ 267 │226 │ │ │118
Hotel │ │ │ │ │ │Equitable │ │
Vanderbilt.│ 604 │545 │ │ │ │ Bldg. │ 366 │268
─────────────┼────────┼────┼──────────┼────────┼────┼───────────┼────────┼────
Average │ 634 │526 │ Average │ 297 │230 │ Average │ 338 │219
─────────────┴────────┴────┴──────────┴────────┴────┴───────────┴────────┴────

TABLE 10

SEWAGE FLOW FROM DIFFERENT CLASSES OF DISTRICTS BASED ON 1915 REPORT OF
MILWAUKEE SEWERAGE COMMISSION

────────────────────────────────────────────────────────────────┬──────
Ratio of maximum to average rate for department store district. │ 1.755
Ratio of maximum to average rate for hotel district. │ 1.65
Ratio of maximum to average rate for office building district. │ 1.51
Ratio of maximum to average rate for wholesale commercial │
district. │ 2.1
│
│——————│——————
Average and maximum gallons per thousand square feet of │ │
floor area: │ Avg. │ Max.
│——————│——————
For department store district. │ 232│ 407
For office building district. │ 541│ 891
For wholesale commercial district. │ 164│ 344
For all districts except wholesale commercial. │ 381│ 618
│ │
Average and maximum gallons per day: │ │
For all districts except wholesale commercial. │17,700│29,800
For wholesale commercial district. │ 9,650│20,000
─────────────────────────────────────────────────────────┴──────┴──────

TABLE 11

RATES OF CONSUMPTION PREDICTED FOR DIFFERENT DISTRICTS IN NEW YORK CITY

────────────┬───────────┬──────┬────────┬────────┬─────────
│ Net Bldg. │ │Observed│Observed│
│Area in Sq.│ Avg. │Cons. in│Cons. in│Predicted
District │ Ft. per │Number│ g.p.d. │ g.p.d. │ Mean
│ Acre for │ of │per 1000│per 1000│ Cons.
│ Ultimate │Floors│Sq. Ft. │Sq. Ft. │
│Consumption│ │ Max. │ Avg. │
────────────┼───────────┼──────┼────────┼────────┼─────────
Hotel and │ 24,800│ 15│ 634│ 526│ 500
midtown. │ │ │ │ │
Midtown and │ 24,800│ 15│ 338│ 219│ 300
financial.│ │ │ │ │
East and │ │ │ │ │
West of │ 24,800│ 10│ 297│ 230│ 300
midtown. │ │ │ │ │
Apartment, │ │ │ │ │
59th to │ 20,400│ 7│ │ 230│ 300
155th Sts.│ │ │ │ │
Manhattan │ │ │ │ │
north of │ 20,400│ 5│ │ 230│ 300
155th St. │ │ │ │ │
────────────┴───────────┴──────┴────────┴────────┴─────────

────────────┬─────────┬─────────┬─────────┬────────┬────────
│Predicted│Predicted│Predicted│Measured│Measured
│ Mean in │ Dry │Max. Dry │Avg. Dry│Max. Dry
District │ Million │ Weather │ Weather │Weather │Weather
│Gals. per│ Flow, │ Flow, │ Flow, │ Flow,
│Acre per │ c.f.s. │ c.f.s. │ c.f.s. │ c.f.s.
│ Day │per Acre │per Acre │per Acre│per Acre
────────────┼─────────┼─────────┼─────────┼────────┼────────
Hotel and │ .20 │ .29│ .34│ 1.04 │ .146
midtown. │ │ │ │ │
Midtown and │ .12 │ .18│ .23│ .078│ .110
financial.│ │ │ │ │
East and │ │ │ │ │
West of │ .074│ .12│ .15│ .057│ .097
midtown. │ │ │ │ │
Apartment, │ │ │ │ │
59th to │ .043│ .06│ .09│ │
155th Sts.│ │ │ │ │
Manhattan │ │ │ │ │
north of │ .031│ .05│ .08│ │
155th St. │ │ │ │ │
────────────┴─────────┴─────────┴─────────┴────────┴────────

Midtown district consists of department stores, large railroad
terminals, industrial and loft buildings, and sky-scraper office
buildings.

It is difficult to set any definite figure for the percentage which the
maximum rate of flow is of the average. Fluctuations above and below the
average are greater the smaller the tributary population. This relation
can be expressed empirically as

_M_ = 500⁄_P_^⅕,

in which _M_ represents the per cent which the maximum flow is of the
average, and _P_ represents the tributary population in thousands. The
expression should not be used for populations below 1,000 nor above
1,000,000. Having determined the expected average flow of sewage by a
study of the population, water consumption, etc., the maximum quantity
of sewage is determined by multiplying the average flow by the per cent
which the maximum is of the average. In this connection W. G. Harmon[20]
offers the relation

_M_ = 1 + 14⁄(4 + √(_P_)),

which was used in the design of the Ten Mile Creek intercepting sewer at
Toledo, Ohio. For rough estimates and for comparative purposes the ratio
of the average to the minimum flow can be taken the same as the ratio of
the maximum to the average flow, unless direct gaugings or other
information show it to be otherwise.

[Illustration:

Fig. 10.—Daily and Hourly Variations of Sewage Flow.
]

1. Toledo, O.; Manufacturing average.

2. Toledo, O.; Manufacturing, Monday.

3. Toledo, O.; Manufacturing, Sunday.

4. Toledo, O.; Residential, average.

5. Toledo, O.; Residential, Monday.

6. Toledo, O.; Residential, Sunday.

7. Cincinnati, O., Industrial, average.

8. Cincinnati, O.; Residential, average.

9. Cincinnati, O.; Commercial, average.

10. Average of 7 cities.

The fluctuations of flow in commercial and industrial districts are so
different from those in residential districts that the formulas given
should not be used in the design of sewers other than those draining
residential areas. It is reasonable to suppose that fluctuations in
rates of flow from industrial districts are dependent upon the character
of the tributary industries. A study of these industries will give
valuable light on the maximum and minimum rates at which sewage will be
delivered to the sewers.

Hourly, daily, and seasonal fluctuations in rates of sewage flow are of
interest in the design of pumping stations to give knowledge of the
rates at which the pumps must operate at various periods. The
fluctuations in rates of sewage flow during various hours and days in
different cities and districts are shown in Fig. 10. Fluctuations in
rate of flow of sewage lag behind fluctuations in rate of water
consumption, the time being dependent on the distance through which the
wave of change must travel in the sewer.

=26. Effect of Ground Water.=—Sewers are seldom laid with water-tight
joints. Since they usually lie below the ground water level it is
inevitable that a certain amount of ground water will enter. Various
units have been suggested for the expression of the inflow of ground
water in an attempt to include all of the many factors. Some of these
units are: gallons per acre drained by the sewer per day, gallons per
mile of pipe per day, gallons per inch diameter per mile of pipe per
day, etc. Since the ground water enters pipe sewers at the joints, the
longer the joints the greater the probability of the entrance of ground
water. The last unit is therefore the most logical but the accuracy of
the result is scarcely worthy of such refinement and the unit usually
adopted is gallons per mile of pipe per day.

No definite figure can be given for the amount of ground water to be
expected in sewers since the character of the soil and the ground water
pressure must be considered. Relatively normal infiltration may be found
from 5,000 to 80,000 gallons per mile of pipe per day. The minimum is
seldom reached in wet ground and the maximum is frequently exceeded.
Table 12 shows the amount of ground water measured in various sewers as
given by Brooks.[21]

=27. Résumé of Method for Determination of Quantity of Dry weather
Sewage.=—The steps in the determination of the quantity of sewage are:
determine the period in the future for which the sewers are to be
designed; estimate the population and tributary area at the end of this
period; estimate the rate of water consumption and assume the sewage
flow to equal the water consumption; determine the maximum and minimum
rates of sewage flow; and finally, estimate the maximum rate of ground
water seepage and add it to the maximum rate of sewage flow to give the
total quantity of sewage to be carried by the proposed sewers.

TABLE 12

DATA ON THE INFILTRATION OF GROUND WATER INTO SEWERS

Abstracted from paper by J. N. Brooks in Transactions Am. Society of Civil
Engineers, Vol. 76, p. 1909.
───────────┬─────┬──────┬─────┬───────┬──────┬─────┬──────────────────────
Place │ │Diam- │ │ │ │ │
│ │ eter │ │ Wet │ Avg. │ │
│ │ or │ │Trench,│ Head │Char-│
│ │Dimen-│ │ Per │ of │acter│
│ │sions │ │Cent of│Ground│ of │
│ │ in │Mate-│ Total │Water,│Sub- │
│Shape│Inches│rial │Length │ Fee │grade│ Gallons per 24 Hours
───────────┼─────┼──────┼─────┼───────┼──────┼─────┼─────┬──────┬─────────
│ │ │ │ │ │ │ │ Per │
│ │ │ │ │ │ │ │ Inch │
│ │ │ │ │ │ │ │Diam- │
│ │ │ │ │ │ │ │ eter │
│ │ │ │ │ │ │ Per │ Per │
│ │ │ │ │ │ │Foot │ Mile │
│ │ │ │ │ │ │ of │ of │Per Mile
│ │ │ │ │ │ │Joint│ Pipe │ of Pipe
───────────┼─────┼──────┼─────┼───────┼──────┼─────┼─────┼──────┼─────────
Boston, │ │ 8 to │ │ │ │ │ │ │
Mass. │Circ.│ 36 │V.P. │ │ │ │ 2.6│ 1,818│ 40,000
East │ │ │ │ │ │ │ │ │
Orange, │ │ │ │ │ │ │ │ │
N. J. │ │ │ │ │ 10│ Q. │ │ │ 22,400
East │ │ │ │ │ │ │ │ │
Orange, │ │ 8 to │ │ │ │ │ │ │
N. J. │ │ 24 │V.P. │ │ │ │ 0.8│ 540│ 8,650
Joint trunk│ │ │ │ │ │ │ │ │
sewer, │ │ │ │ │ │ │ │ │
New │ │ │ │ │ │G. & │ │ │
Jersey │ │ │ │ │ │ Q. │ │ │ 25,000
Rogers │ │ │ │ │ │ │ │ │
Park, │ │ │ │ │ │ │ │ │
Ill. │ │ 6 │ │ │ │ │ 0.3│ 207│ 1,240
Altoona, │ │ │ │ │ │ │ │ │
Pa. │ │ 30 │ │ │ │ │ 5.0│ 2,890│ 86,592
Concord, │ │ │ │ │ │ │ │ │
Mass. │ │ │ │ 18│ 8│ │ │ │ 43,000
Malden, │ │ │ │ │ │ │ │ │
Mass. │Circ.│ │V.P. │ 60│ │ │ │ │ 50,000
Westboro, │ │ │ │ │ │ │ │ │
Mass. │ │ 15 │V.P. │ 100│ │ │ │88,100│1,320,300
Fond du │ │ │ │ │ │ │ │ │
Lac, Wis.│Circ.│ 24 │V.P. │ 100│ 5│ C. │ 1.5│ 1,010│ 24,370
East │ │ │ │ │ │ │ │ │
Orange, │ │10 to │ │ │ │ │ │ │
N. J. │Circ.│ 24 │V.P. │ 100│ │ │ 4.7│ 2,540│ 43,250
Ocean │ │ │ │ │ │ │ │ │
Grove, N.│ │ 4 to │ │ │ │ │ │ │
J. │Circ.│ 12 │V.P. │ 100│ 3│S.C. │ 2.7│ 1,890│ 15,126
Ocean │ │ │ │ │ │ │ │ │
Grove, N.│ │ 4 to │ │ │ │ │ │ │
J. │Circ.│ 12 │V.P. │ 100│ 4│S.C. │ 7.9│ 5,480│ 43,764
East │ │ │ │ │ │ │ │ │
Orange, │ │ 24 × │ │ │ │ │ │ │
N. J. │Rect.│ 36 │Brick│ 100│ │ │ │ │ 570,000
Westboro, │ │ │ │ │ │ │ │ │
Mass. │ │ │Brick│ │ │ │ │ │ 415,850
Altoona, │ │ 33 × │B. & │ │ │ │ │ │
Pa. │Rect.│ 44 │ C. │ │ │ │ │ 5,390│ 264,000
Columbus, │ │ 42 × │Con- │ │ │ │ │ │
Ohio. │H.S. │ 42 │crete│ │ │ │ │ 120│ 6,340
Bronx │ │ │ │ │ │ │ │ │
Valley, │ │44 to │Con- │ │ │ │ │ │
N. Y. │Circ.│ 72 │crete│ │ │ G. │ │ 123│ 7,266
Cincinnati,│ Estimated in design. Data not │ │ │ │
Ohio. │ from Brooks │ │ │ │ 67,500
Milwaukee, │ Residential districts, gals. per acre per │ │ 1460 to
Wis. │ day. Not taken from Brooks │ │ 2200
───────────┴─────────────────────────────────────────────┴──────┴─────────

Abbreviations: H.S. = horseshoe shaped; B. & C = Brick and concrete; V.P.
= vitrified pipe; G. = gravel; Q. = quicksand; S. C. = sand clay; C. =
clay.

QUANTITY OF STORM WATER

=28. The Rational Method.=—The water which falls during a storm must be
removed rapidly in order to prevent the flooding of streets and
basements, and other damages. The quantity of water to be cared for is
dependent upon: the rate of rainfall, the character and slope of the
surface, and the area to be drained. All methods for the determination
of storm-water run-off, whether rational or empirical, depend upon these
factors.

The so-called Rational Method can be expressed algebraically, as,

_Q_ = _AIR_,

in which _Q_ = rate of run-off in cubic feet per second;

_A_ = area to be drained expressed in acres;

_I_ = percentage imperviousness of the area;

_R_ = maximum average rate of rainfall over the entire
drainage area, expressed in inches per hour, which
may occur during the time of concentration.

The area to be drained is determined by a survey. A discussion of _R_
and _I_ follows in the next two sections. An example of the use of the
Rational Method is given on page 95.

=29. Rate of Rainfall.=—Rainfall observations have been made over a long
period of time by United States Weather Bureau observers and others.
Continuous records are available in a few places in this country showing
rainfall observations covering more than a century. Such records have
been the bases for a number of empirical formulas for expressing the
probable maximum rate of rainfall in inches per hour, having given the
duration of the storm. Table 13 is a collection of these formulas with a
statement as to the conditions under which each formula is applicable.
The formula most suitable to the problem in hand should be selected for
its solution.[22]

TABLE 13

RAINFALL FORMULAS

────────────┬─────────────────────────┬────────────────────────────────
Name of │ Conditions for which │ Formula
Originator │ Formula is Suitable │
────────────┼─────────────────────────┼────────────────────────────────
E. S. Dorr │ │_i_ = 150⁄(_t_ + 30)
A. N. Talbot│Maximum storms in Eastern│_i_ = 360⁄(_t_ + 30)
│ United States │
A. N. Talbot│Ordinary storms in │_i_ = 105⁄(_t_ + 15)
│ Eastern United States │
Emil │Heavy rainfall near New │_i_ = 120⁄(_t_ + 20), etc.
Kuichling │ York City │
L. J. Le │For San Francisco. See T.│
Conte │ A. S. C. E. v. 54, p. │_i_ = 7⁄_t_^½
│ 198 │
Sherman │Maximum for Boston, Mass.│_i_ = 25.12⁄_t_^{.687}
Sherman │Extraordinary for Boston,│_i_ = 18⁄_t_ ^½
│ Mass. │
Webster │Ordinary for │_i_ = 12⁄_t_^{0.6}
│ Philadelphia, Pa. │
│Ordinary storms for │
Hendrick │ Baltimore. Eng. & │_i_ = 105⁄(_t_ + 10)
│ Cont., Aug. 9. 1911 │
J. de │Ordinary storms for │_i_ = 163⁄(_t_ + 27)
Bruyn-Kops│ Savannah, Ga. │
C. D. Hill │For Chicago, Ill. │_i_ = 120⁄(_t_ + 15)
Metcalf and │Louisville, Ky. Am. Sew. │_i_ = 14⁄_t_^½
Eddy │ Prac., Vol I. │
W. W. Horner│St. Louis, Mo. Eng. News,│_i_ = 56⁄(_t_ + 5)^{.85}
│ Sept. 29, 1910 │
R. A. │For Spokane, Wash. Eng. │_i_ = 23.92⁄(_t_ + 2.15) + 0.154
Brackenbuy│ Record, Aug. 10, 1912 │
Metcalf and │New Orleans │_i_ = 19⁄_t_^½
Eddy │ │
Metcalf and │For Denver, Colo. │_i_ = 84⁄(_t_ + 4)
Eddy │ │
│Central Park, N. Y. │
Kenneth │ 51–Year Record. Eng. │_i_ = 400⁄(2_t_ + 40)[23]
Allen │ News-Record, April 7, │
│ 1921, p. 588 │
────────────┴─────────────────────────┴────────────────────────────────

=30. Time of Concentration.=—By the time of concentration is meant the
longest time without unreasonable delay that will be required for a drop
of water[24] to flow from the upper limit of a drainage area to the
outlet. Assuming a rainfall to start suddenly and to continue at a
constant rate and to be evenly distributed over a drainage area of 100
per cent imperviousness and even slope towards one point, the rate of
run-off would increase constantly until the drop of water from the upper
limit of the area reached the outlet, after which the rate of run-off
would remain constant. In nature the rate of rainfall is not constant.
The shorter the duration of a storm the greater the intensity of
rainfall. Therefore the maximum run-off during a storm will occur at the
moment when the upper limit of the area has commenced to contribute.
From that time on the rate of run-off will decrease.

The time of concentration can be measured fairly well by observing the
moment of the commencement of a rainfall, and the time of maximum
run-off from an area on which the rain is falling. A prediction of the
time of concentration is more or less guess work. As the result of
measurements some engineers assume the time of concentration on a city
block built up with impervious roofs and walks, and on a moderate slope,
is about 5 to 10 minutes. This is used as a basis for the judgment of
the time of concentration on other areas. For relatively large drainage
areas such a method cannot be used. The procedure is to measure the
length of flow through the drainage channels of the area, to assume the
velocity of the flood crest through these channels and thus to determine
the time of concentration. Table 14 shows the flood crest velocities in
various streams of the Ohio River Basin under flood conditions. The
velocity over the surface of the ground may be approximated by the use
of the formula[25]

_V_ = 2,000_I_√(_S_),

in which _V_ = the velocity of flow over the surface of the ground in
feet per minute;

_I_ = the percentage imperviousness of the ground;

_S_ = the slope of the ground.

For areas up to 100 acres where natural drainage channels are not
existent this formula will give more satisfactory results than guesses
based on the time of concentration of certain known areas.

Having determined the time of concentration, the rate of rainfall _R_ to
be used in the Rational Method is found by substitution in some one of
the rainfall formulas given in Table 13.

TABLE 14

FLOOD CREST VELOCITIES IN OHIO RIVER BASIN IN MARCH, 1913

From Table 12. U. S. G. S., Water Supply Paper. No. 334
───────────┬───────────────┬────────┬────────┬───────────────┬─────────┬───────────┬────────
River │ Stations │ │Distance│ Distance of │Velocity │ Velocity │
│ │Distance│to Mouth│ Lower Station │ between │ from │ Time
│ │between │ of │ below │Stations,│Pittsburgh,│between
│ │Stations│ River, │Starting-point,│Miles per│ Miles per │Stations
│ │in Miles│ Miles │ Miles │ Hour │ Hour │in Hours
───────────┼───────────────┼────────┼────────┼───────────────┼─────────┼───────────┼────────
Ohio │Pittsburgh, │ │ │ │ │ │
│ Pa., to │ │ │ │ │ │
│ Wheeling, W. │ │ │ │ │ │
│ Va. │ 90│ 967│ 90│ 9.0│ 9.0│ 10.0
Ohio │Wheeling, W. │ │ │ │ │ │
│ Va., to │ │ │ │ │ │
│ Marietta, │ │ │ │ │ │
│ Ohio │ 82│ 877│ 172│ 5.9│ 7.2│ 14
Ohio │Marietta, Ohio,│ │ │ │ │ │
│ to │ │ │ │ │ │
│ Parkersburg, │ │ │ │ │ │
│ W. Va. │ 12│ 795│ 184│ 0.9│ 4.8│ 14
Ohio │Parkersburg to │ │ │ │ │ │
│ Point │ │ │ │ │ │
│ Pleasant, W. │ │ │ │ │ │
│ Va. │ 80│ 783│ 264│ 6.7│ 5.3│ 12
Ohio │Point Pleasant │ │ │ │ │ │
│ to │ │ │ │ │ │
│ Huntington, │ │ │ │ │ │
│ W. Va. │ 44│ 703│ 308│ 11.0│ 5.7│ 4
Ohio │Huntington to │ │ │ │ │ │
│ Catlettsburg,│ │ │ │ │ │
│ W. Va. │ 9│ 659│ 317│ 0.8│ 4.1│ 11
Ohio │Catlettsburg, │ │ │ │ │ │
│ W. Va., to │ │ │ │ │ │
│ Portsmouth, │ │ │ │ │ │
│ Ohio │ 38│ 650│ 355│ │ 5.0│
Ohio │Portsmouth │ │ │ │ │ │
│ Ohio, to │ │ │ │ │ │
│ Maysville, │ │ │ │ │ │
│ Ky. │ 52│ 612│ 407│ 5.2│ 5.0│ 10
Ohio │Maysville, Ky.,│ │ │ │ │ │
│ to │ │ │ │ │ │
│ Cincinnati, │ │ │ │ │ │
│ Ohio │ 61│ 560│ 468│ 6.8│ 5.2│ 9
Ohio │Cincinnati, │ │ │ │ │ │
│ Ohio, to │ │ │ │ │ │
│ Louisville, │ │ │ │ │ │
│ Ky. │ 136│ 499│ 604│ 11.4│ 5.9│ 12
Ohio │Louisville, │ │ │ │ │ │
│ Ky., to │ │ │ │ │ │
│ Evansville, │ │ │ │ │ │
│ Ind. │ 183│ 363│ 787│ 1.9│ 5.3│ 96
Ohio │Evansville, │ │ │ │ │ │
│ Ind., to Mt. │ │ │ │ │ │
│ Vernon Ind. │ 36│ 180│ 823│ 9.0│ 5.3│ 4
Ohio │Mt. Vernon, │ │ │ │ │ │
│ Ind., to │ │ │ │ │ │
│ Paducah, Ky.│ 101│ 144│ 924│ 2.1│ 4.6│ 48
Ohio │Paducah, Ky. to│ │ │ │ │ │
│ Cairo, Ill. │ 43│ 43│ 967│ 2.9│ 4.2│ 15
Monongahela│Fairmont, W. │ │ │ │ │ │
│ Va., to Lock │ │ │ │ │ │
│ No. 2 Pa. │ │ │ │ │ │
│ (Upper) │ 107│ 119│ 107│ 6.7│ │ 16
Little │Creston, W. │ │ │ │ │ │
Kanawha │ Va., to Dam. │ │ │ │ │ │
│ No. 4 W. Va. │ │ │ │ │ │
│ (Upper) │ 16│ 48│ 16│ 16.0│ │ 1
New │Radford, W. │ │ │ │ │ │
│ Va., to │ │ │ │ │ │
│ Hinton, W. │ │ │ │ │ │
│ Va. │ 78│ 139│ 78│ 3.0│ │ 26
Kanawha │Kanawha Falls, │ │ │ │ │ │
│ W. Va. to │ │ │ │ │ │
│ Charleston, │ │ │ │ │ │
│ W. Va. │ 37│ 95│ 37│ 2.6│ │ 14
Scioto │Columbus, Ohio,│ │ │ │ │ │
│ to │ │ │ │ │ │
│ Chillicothe, │ │ │ │ │ │
│ Ohio │ 52│ 110│ 52│ 4.7│ │ 11
Miami │Dayton, Ohio, │ │ │ │ │ │
│ to Hamilton, │ │ │ │ │ │
│ Ohio │ 44│ 77│ 44│ 14.7│ │ 3
Kentucky │Highbridge, │ │ │ │ │ │
│ Ky., to │ │ │ │ │ │
│ Frankfort, │ │ │ │ │ │
│ Ky. │ 52│ 117│ 52│ 5.2│ │ 10
Cumberland │Celina, Tenn. │ │ │ │ │ │
│ to Nashville,│ │ │ │ │ │
│ Tenn. │ 190│ 383│ 190│ 2.9│ │ 64.5
Tennessee │Knoxville to │ │ │ │ │ │
│ Chattanooga, │ │ │ │ │ │
│ Tenn. │ 183│ 635│ 183│ 3.2│ │ 57
───────────┴───────────────┴────────┴────────┴───────────────┴─────────┴───────────┴────────
NOTE.—The velocities shown are the velocities of the crest of the flood wave and are not the
average velocity of the flow of the river. The velocity of the crest of the flood wave
should be used in determining the time of concentration. The flood crest velocity is slower
then that of the river because of the storage in the river basin.

=31. Character of Surface.=—The proportion of total rainfall which will
reach the sewers depends on the relative porosity, or imperviousness,
and the slope of the surface. Absolutely impervious surfaces such as
asphalt pavements or roofs of buildings will give nearly 100 per cent
run-off regardless of the slope, after the surfaces have become
thoroughly wet. For unpaved streets, lawns, and gardens the steeper the
slope the greater the per cent of run-off. When the ground is already
water soaked or is frozen the per cent of run-off is high, and in the
event of a warm rain on snow covered or frozen ground, the run-off may
be greater than the rainfall. The run-off during the flood of March,
1913, at Columbus, Ohio, was over 100 per cent of the rainfall. Table
15[26] shows the relative imperviousness of various types of surfaces
when dry and on low slopes. The estimates for relative imperviousness
used in the design of the Cincinnati intercepter are given in Table 16.

TABLE 15

VALUES OF RELATIVE IMPERVIOUSNESS

Roof surfaces assumed to be water-tight 0.70– 0.95
Asphalt pavements in good order .85– .90
Stone, brick, and wood-block pavements with tightly cemented
joints .75– .85
The same with open or uncemented joints .50– .70
Inferior block pavements with open joints .40– .50
Macadamized roadways .25– .60
Gravel roadways and walks .15– .30
Unpaved surfaces, railroad yards, and vacant lots .10– .30
Parks, gardens, lawns, and meadows, depending on surface
slope and character of subsoil .05– .25
Wooded areas or forest land, depending on surface slope and
character of subsoil .01– .20
Most densely populated or built up portion of a city .70– .90

TABLE 16

COEFFICIENTS OF IMPERVIOUSNESS USED IN THE DESIGN OF THE CINCINNATI SEWERS

──────────────┬─────────────────────────────┬──────────────────┬───────────
Character of │ │ │Residential,
Improvement │ │ │291.1 A. 20
│ │ │ per Acre,
│ │ │ Middle
│ │ │ Class,
│ │ │ Detached
│ │ │Dwellings,
│ │ │Yellow and
│ │Combined Tenement │ Blue Clay
│ │ and Industrial. │ Overlying
│Typical Commercial Area, 30.4│ 35.6 A., 55 per │ Beds of
│A. None Undeveloped. Sand and│ Acre. Clay, Sand │ Shale and
│ Gravel │ and Gravel │ Sandstone
──────────────┼──────┬─────┬─────┬──────────┼──────┬─────┬─────┼─────┬─────
│ Area │ │ │Equivalent│ Area │ │ │ Per │
│ in │ Per │ │Imp. Area,│ in │ Per │ │Cent │
│1000’s│Cent │ I, │ 1000’s │1000’s│Cent │ I, │ of │ I,
│Square│Total│Esti-│ Square │Square│Total│Esti-│Total│Esti-
│ Feet │Area │mated│ Feet │ Feet │Area │mated│Area │mated
──────────────┼──────┼─────┼─────┼──────────┼──────┼─────┼─────┼─────┼─────
Roofs: │ │ │ │ │ │ │ │ │
Public and │ │ │ │ │ │ │ │ │
commercial│ 881.2│ 66.5│ 0.90│ 793.0│ 66.8│ 4.3│ 0.40│ 4.8│ 0.40
Residences │ │ │ │ │ 289.2│ 18.6│ .90│ 13.1│ .90
Barns and │ │ │ │ │ │ │ │ │
sheds │ │ │ │ │ 79.2│ 5.1│ .75│ 1.4│ .75
│ │ │ │ │ │ │ │ │
Interior │ │ │ │ │ │ │ │ │
Walks: │ │ │ │ │ │ │ │ │
Brick │ 7.5│ 0.6│ .40│ 3.0│ 35.6│ 2.3│ .40│ 0.6│ .40
Cement │ 10.0│ 0.7│ .75│ 7.5│ 22.6│ 1.5│ .75│ 2.6│ .75
│ │ │ │ │ │ │ │ │
Street Walks: │ │ │ │ │ │ │ │ │
Brick │ 6.1│ 0.5│ .40│ 2.4│ 48.2│ 3.1│ .40│ 1.0│ .40
Cement │ 139.3│ 10.5│ .75│ 104.5│ 78.1│ 5.0│ .75│ 3.4│ .75
│ │ │ │ │ │ │ │ │
Street │ │ │ │ │ │ │ │ │
Pavements: │ │ │ │ │ │ │ │ │
Asphalt, │ │ │ │ │ │ │ │ │
brick, │ │ │ │ │ │ │ │ │
wood block│ 145.5│ 11.0│ .85│ 123.7│ │ │ │ 5.0│ .85
Granite │ │ │ │ │ │ │ │ │
block │ 111.4│ 8.4│ .75│ 83.6│ │ │ │ 1.0│ .75
Macadam and │ │ │ │ │ │ │ │ │
cobble │ 23.2│ 1.8│ .40│ 9.3│ 238.6│ 15.4│ .40│ 4.8│ .40
Granite and │ │ │ │ │ │ │ │ │
poor │ │ │ │ │ │ │ │ │
macadam │ │ │ │ │ │ │ │ 0.4│ .20
│ │ │ │ │ │ │ │ │
Unimproved │ │ │ │ │ │ │ │ │
yards and │ │ │ │ │ │ │ │ │
lawns: │ │ │ │ │ 692.4│ 44.7│ .15│ │
Tributary to│ │ │ │ │ │ │ │ │
paved │ │ │ │ │ │ │ │ │
gutters │ │ │ │ │ │ │ │ 57.1│ .15
Not │ │ │ │ │ │ │ │ │
tributary │ │ │ │ │ │ │ │ │
to paved │ │ │ │ │ │ │ │ │
gutters │ │ │ │ │ │ │ │ 7.9│ .10
──────────────┼──────┼─────┼─────┼──────────┼──────┼─────┼─────┼─────┼─────
Total │1324.2│100.0│ │ 1127.0│1550.7│100.0│ │100.0│
──────────────┼──────┴─────┴─────┴──────────┼──────┴─────┴─────┼─────┴─────
Impervious │ │ │
coefficient │ │ │
for the │ │ │
district │ 85.1 │ 44.4 │ 35.9
──────────────┴─────────────────────────────┴──────────────────┴───────────

C. E. Gregory[27] states that _I_, in the expression _Q_ = _AIR_ is a
function of the time of concentration or the duration of the storm. If
_t_ represents the time of concentration and _T_ represents the duration
of the storm, then when _T_ is less than _t_

_I_ = 0.175_t_^⅓,

but when _T_ is greater than _t_,

_I_ = 0.175⁄_t_(_T_^{4/3} − (_T_ − _t_)^{4/3}).

Gregory condenses Kuichling’s rules with regard to the per cent run-off,
as follows:

1. The per cent of rainfall discharged from any given drainage
area is nearly constant for heavy rains lasting equal periods of
time.

2. This per cent varies directly with the area of impervious
surface.

3. This per cent increases rapidly and directly or uniformly with
the duration of the maximum intensity of the rainfall until a
period is reached which is equal to the time required for the
concentration of the drainage waters from the entire area at the
point of observation, but if the rainfall continues at the same
intensity for a longer period this per cent will continue to
increase at a much smaller rate.

4. This per cent becomes larger when a moderate rain has
immediately preceded a heavy shower on a partially permeable
territory.

Gregory’s formulas have not been generally accepted and are not widely
used in practice. Marston stated:[28]

All that engineers are at present, warranted in doing is to make
some deduction from 100 per cent run-off ... the deduction ...
being at present left to the engineer in view of his general
knowledge and his familiarity with local conditions.

Burger states[29] in the same connection:

In its application there will usually be as many results
(differing widely from each other) as the number of men using it.

In spite of these objections the Rational Method is in more favor with
engineers than any other method.

=32. Empirical Formulas.=—The difficulty of determining run-off with
accuracy has led to the production by engineers of many empirical
formulas for their own use. Some of these formulas have attracted wide
attention and have been used extensively, in some cases under conditions
to which they are not applicable. In general these formulas are
expressions for the run-off in terms of the area drained, the relative
imperviousness, the slope of the land, and the rate of rainfall.

The Burkli-Ziegler formula, devised by a Swiss engineer for Swiss
conditions and introduced into the United States by Rudolph Hering, was
one of the earliest of the empirical formulas to attract attention in
this country. It has been used extensively in the form

_Q_ = _CiA_∜(_S_⁄_A_),

in which_Q_ = the run-off in cubic feet per second;

_i_ = the maximum rate of rainfall in inches per hour over
the entire area. This is determined only by
experience in the particular locality, and is usually
taken at from 1 to 3 inches per hour;

_S_ = the slope of the ground surface in feet per thousand,

_A_ = the area in acres;

_C_ = an expression for the character of the ground surface,
or relative imperviousness. In this form of the
expression _C_ is recommended as 0.7.

The McMath formula was developed for St. Louis conditions and was first
published in Transactions of the American Society of Civil Engineers,
Vol. 16, 1887, p. 183. Using the same notation as above, the formula is,

_Q_ = _CiA_⁵√(_S_⁄_A_),

McMath recommended the use of _C_ equal to 0.75, _i_ as 2.75 inches per
hour, and _S_ equal to 15. The formula has been extended for use with
all values of _C_, _i_, _S_, and _A_ ordinarily met in sewerage
practice. Fig. 11 is presented as an aid to the rapid solution of the
formula.

[Illustration:

FIG. 11.—Diagram for the Solution of McMath’s Formula,
_Q_ = _Aci_⁵√_S_⁄_A_.
]

Other formulas have been devised which are more applicable to drainage
areas of more than 1,000 acres.[30] Such areas are met in the design of
sewers to enclose existing stream channels draining large areas.
Kuichling’s formulas, published in 1901 in the report of the New York
State Barge Canal, were devised for areas greater than 100 square miles.
The following modification of these formulas for ordinary storms on
smaller areas was published for the first time in American Sewerage
Practice, Volume I, by Metcalf and Eddy:

_Q_ = 25,000⁄(_A_ + 125) + 15.

[Illustration:

FIG. 12.—Comparison of Empirical Run-off Formulas.
]

It is to be noted that the only factor taken into consideration is the
area of the watershed. It is obvious that other factors such as the rate
of rainfall, slope, imperviousness, etc., will have a marked effect on
the run-off.

There are other run-off formulas devised for particular conditions, some
of which are of as general applicability as those quoted. Two formulas
which are frequently quoted are: Fanning’s, _Q_ = 200_M_^⅝ and Talbot’s
_Q_ = 500_M_^¼, in which _M_ is the area of the watershed in square
miles. A comprehensive treatment of the subject is given in American
Sewerage Practice, Vol. I, by Metcalf and Eddy.

A comparison of the results obtained by the application of a few
formulas to the same conditions is shown graphically in Fig. 12. It is
to be noted that the divergence between the smallest and largest results
is over 100 per cent. As these formulas are not all applicable to the
same conditions, the differences shown are due partially to an extension
of some of them beyond the limits for which they were prepared.

=33. Extent and Intensity of Storms.=—In the design of storm sewers it
is necessary to decide how heavy a storm must be provided for. The very
heaviest storms occur infrequently. To build a sewer capable of caring
for all storms would involve a prohibitive expense over the investment
necessary to care for the ordinary heavy storms encountered annually or
once in a decade. This extra investment would lie idle for a long period
entailing a considerable interest charge for which no return is easily
seen. The alternative is to construct only for such heavy storms as are
of ordinary occurrence and to allow the sewers to overflow on
exceptional occasions. The result will be a more frequent use of the
sewerage system to its capacity, a saving in the cost of the system, and
an occasional flooding of the district in excessive storms. The amount
of damage caused by inundations must be balanced against the extra cost
of a sewerage system to avoid the damage. A municipality which does not
provide adequate storm drainage is liable, under certain circumstances,
for damages occasioned by this neglect. It is not liable if no drainage
exists, nor is it liable if the storm is of such unusual character as to
be classed legally as an act of God.

Kuichling’s studies of the probabilities of the occurrence of heavy
storms are published in Transactions of the American Society of Civil
Engineers, Vol. 54, 1905, p. 192. Information on the extent of rain
storms is given by Francis in Vol. 7, 1878, p. 224, of the same
publication. Kuichling expresses the intensity of storms which will
occur,

once in 10 years as _i_ = 105⁄(_t_ + 20),

once in 15 years as _i_ = 120⁄(_t_ + 20),

in which _i_ is the intensity of rainfall in inches per hour and _t_ is
the duration of the storm in minutes.

CHAPTER IV
THE HYDRAULICS OF SEWERS

=34. Principles.=—The hydraulics of sewers deals with the application of
the laws of hydraulics to the flow of water through conduits and open
channels. In so far as its hydraulic properties are concerned the
characteristics of sewage are so similar to those of water that the same
physical laws are applicable to both. In general it is assumed that the
energy lost due to friction between the liquid and the sides of the
channel varies as some function of the velocity, usually the square, and
that the total energy passing any section of the stream differs from the
energy passing any other section only by the loss of energy due to
friction.

The general expression for the flow of sewage would then be,

_h_ = (_f_)_V_^n,

in which _h_ is the head or energy lost between any two sections, and
_V_ is the average velocity of flow between these sections. It is to be
noted in this general expression that the quantity and rate of flow past
all sections is assumed to be constant. This condition is known as
_steady flow_. Problems are encountered in sewerage design which involve
conditions of unsteady flow, and methods of solution of them have been
developed based on modifications of this general expression. The average
velocity of flow is computed by dividing the rate (quantity) of flow
past any section by the cross-sectional area of the stream at that
section. This does not represent the true velocity at any particular
point in the stream, as the velocity near the center is faster than that
near the sides of the channel. The distribution of velocities in a
closed circular channel is somewhat in the form of a paraboloid
superimposed on a cylinder.

The laws of flow are expressed as formulas the constants of which have
been determined by experiment. It has been found that these constants
depend on the character of the material forming the channel and the
hydraulic radius. The _hydraulic radius_ is defined as the ratio of the
cross-sectional area of the stream to the length of the wetted
perimeter, or line of contact between the liquid and the channel,
exclusive of the horizontal line between the air and the liquid.

=35. Formulas.=—The loss of head due to friction caused by flow through
circular pipes flowing full as expressed by Darcy is,

_h_ = _f_(_l_⁄_d_) (_V_^2⁄2_g_),

in which _h_ is the head lost due to friction in the distance _l_, _V_
is the velocity of flow, _g_ is the acceleration due to gravity, and _f_
is a factor dependent on _d_ and the material of which the pipe is made.
A formula for _f_ expressed by Darcy as the result of experiments on
cast-iron pipe is,

_f_ = 0.0199 + 0.00166⁄_d_,

in which _d_ is the diameter in feet. In using the formula with this
factor the units used must be feet and seconds.

Another form of the same expression is known as the Chezy formula. It is
an algebraic transformation of the Darcy formula, but in the form shown
here, by the use of the hydraulic radius, it is made applicable to any
shape of conduit either full or partly full. The Chezy formula is,

_V_ = _C_√(_RS_),

in which _R_ is the hydraulic radius, _S_ the slope ratio of the
hydraulic gradient, and _C_ a factor similar to _f_ in the Darcy
formula.

Kutter’s formula was derived by the Swiss engineers, Ganguillet and
Kutter, as the result of a series of experimental observations. It was
introduced into the United States by Rudolph Hering and its derivation
is given in Hering and Trautwine’s translation of “The Flow of Water in
Open Channels by Ganguillet and Kutter.” In English units it is,

_V_ = {(1.81/_n_ + 41.67 + .0028/_S_)/(1 + (_n_/√_R_)(41.67 +
(.0028/_S_)))}√(_RS_),

in which _n_ is a factor expressing the character of the surface of the
conduit and the other notation is as in the Chezy formula. _V_ is the
velocity in feet per second, _S_ is the slope ratio, and _R_ the
hydraulic radius in feet. The values of _n_ to be used in all cases are
not agreed upon, but in general the values shown below are used in
practice.

VALUES OF _n_ IN KUTTER’S FORMULA

_n_ CHARACTER OF THE MATERIALS

0.009 Well-planed timber.

0.010 Neat cement or very smooth pipe.

0.012 Unplaned timber. Best concrete.

Smooth masonry or brickwork, or concrete sewers under ordinary
0.013 conditions.

0.015 Vitrified pipe or ordinary brickwork.

0.017 Rubble masonry or rough brickwork.

0.020 } Smooth earth.
0.035

0.030 } Rough channels overgrown with grass.
0.050

Kutter’s formula is of general application to all classes of material
and to all shapes of conduits. It is the most generally used formula in
sewerage design.

The cumbersomeness of Kutter’s formula is caused somewhat by the attempt
to allow for the effect of the low slopes of the Mississippi River
experiments on the coefficients. The correctness of these experiments
has not been well established and the slopes are so flat that the
omission of the term 0.0028⁄_S_ will have no appreciable effect on the
value of _V_ ordinarily used in sewer design. The difference between the
value of _V_ determined by the omission of this term and the value of
_V_ found by including it is less than 1 per cent for all slopes greater
than 1 in 1,000 for 8 inch pipe (_R_ = 0.167 feet). As the diameter of
the pipe or the hydraulic radius of the channel increases up to a
diameter of 13.02 feet (_R_ = 3.28 feet), the difference becomes less
and at this value of _R_ there is no difference whether the slope is
included or not. For larger pipes the difference increases slowly. For a
16 foot pipe (_R_ = 4 feet) on a slope of 1 in 1,000 the difference is
less than 0.2 per cent, and on a slope of 1 in 10,000 the difference is
approximately 1 per cent. Flatter slopes than these are seldom used in
sewer design, except for very large sewers where careful determinations
of the hydraulic slope are necessary. It is therefore safe in sewer
design to use Kutter’s formula in the modified form shown below in which
the term (.0028)⁄_S_ has been omitted.

_V_ = (1.81 + 41.67_n_)_R_√_S_/_n_(√_R_ + 41.67_n_).

Bazin’s formula is

_V_ = √(_RS_)/√(α + β/_d_)

in which α and β are constants for different classes of material. For
cast-iron pipe α is 0.00007726 and β is 0.00000647. This formula is
seldom used in sewerage design.

Exponential formulas have been developed as the result of experiments
which have demonstrated that _V_ does not vary as the one-half power of
_R_ and _S_ but that the relation should be expressed as,

_V_ = _CR_^{_p_}_S_^{_q_},

in which _p_ and _q_ are constants and _C_ is a factor dependent on the
character of the material. The various formulas coming under this
classification have been given the names of the experimenters proposing
them. Examples of these formulas are: Flamant’s, in English units, for
new cast-iron pipe, which is,

_V_ = 232_R_^{.715}_S_^{.572},

and Lampé’s for the same material which is,

_V_ = 203.3_R_^{.694}_S_^{.555}.

These formulas are useful only for the material to which they apply, but
they can be used for conduits of any shape. A. V. Saph and E. W. Schoder
have shown[31] that the general formula for all materials lies between
the limits,

_V_ = (93 to 142)_S_^{.50 to .55}_R_^{.63 to .69}.

Hazen and Williams’ formula is in the form,

_V_ = 1.31_CR_^{.63}_S_^{.54},

in which _C_ is a factor dependent on the character of the material of
the conduit. The values of _C_ as given by Hazen and Williams are,

_C_ CHARACTER OF MATERIAL
95 Steel pipe under future conditions. (Riveted steel.)
Cast iron under ordinary future conditions and brick
100 sewers in good condition.
110 New riveted steel, and cement pipe.
120 Smooth wood or masonry conduits under ordinary conditions.
Masonry conduits after some time and for very smooth pipes
such as glass, brass, lead, etc., when old, and for new
130 cast-iron pipe under ordinary conditions.

This formula is of as general application as Kutter’s formula and is
easier of solution, but being more recently in the field and because of
the ease of the solution of Kutter’s formula by diagrams it is not in
such general use. Exponential formulas are used more in waterworks than
in sewerage practice.

Manning’s formula is in the form,

_V_ = 1.486⁄_n__R_^⅔_S_^½

in which _n_ is the same as for Kutter’s formula. Charts for the
solution of Manning’s formula are given in Eng. News-Record, Vol. 85,
1920, p. 837.

=36. Solution of Formulas.=—The solution of even the simplest of these
formulas, such as Flamant’s, is laborious because of the exponents
involved. Darcy’s and Kutter’s formulas are even more cumbersome because
of the character of the coefficient. The labor involved in the solution
of these formulas has resulted in the development of a number of
diagrams and other short cuts. Since each formula involves three or more
variables it cannot be represented by a single straight line on
rectangular coordinate paper. The simplest form of diagram for the
solution of three or more variables is the nomograph, an example of
which is shown in Fig. 13 for the solution of Flamant’s formula. A
straight-edge placed on any two points of the scales of two different
vertical lines will cross the other line at a point on the scale
corresponding to its correct value in the formula. Such a diagram is in
common use for the solution of problems for the flow of water in
cast-iron pipe.

[Illustration:

FIG. 13.—Diagram for the Solution of Flamant’s Formula for the Flow of
Water in Cast-iron Pipe.
]

Fig. 14 has been prepared to simplify the solution of Hazen and
Williams’ formula. The scales of slope for different classes of material
are shown on vertical lines to the left of the slope line. For use these
scales must be projected horizontally on the slope line. The scales for
other factors are shown on independent reference lines.

For example let it be required to find the loss of head in a 12
inch pipe carrying 1 cubic foot per second when the coefficient of
roughness is 100. A straight-edge placed at 1.0 cubic feet per
second on the quantity scale, and 12 inches on the diameter scale
crosses the slope line at .00092 opposite the slope scale for _c_
= 100. It crosses the velocity line at 1.31 feet per second.

Kutter’s formula is the most commonly used for sewer design and has been
generally accepted as a standard in spite of its cumbersomeness. Fig. 15
is a graphical solution of Kutter’s formula for small pipes, and Fig. 16
for larger pipes. The diagrams are drawn on the nomographic principle
and give solutions for a wide range of materials, but they are specially
prepared for the solution of problems in which _n_ = .015. In their
preparation the effect of the slope on the coefficient has been
neglected. Fig. 17 is drawn on ordinary rectangular coordinate paper and
can be used only for the solution of problems in which _n_ = .015. Both
diagrams are given for practice in the use of the different types.

[Illustration:

FIG. 14.—Diagram for the Solution of Hazen and Williams’ Formula.
]

[Illustration:

FIG. 15.—Diagram for the Solution of Kutter’s Formula.

For values of _n_ between 0.010 and 0.020. Specially arranged for _n_
= 0.015. Values of Q from 0.1 to 10 second-feet.
]

[Illustration:

FIG. 16.—Diagram for the Solution of Kutter’s Formula.

For values of _n_ between 0.010 and 0.020. Specially arranged for _n_
= 0.015. Values of Q from 10 to 1,000 second-feet.
]

[Illustration:

FIG. 17.—Diagram for the Solution of Kutter’s Formula.
]

[Illustration:

FIG. 18.—Conversion Factors for Kutter’s Formula.
]

In Figs. 15 and 16 the diameter scales are varied for different values
of the roughness coefficient _n_. The velocity scale is shown _only for
a value of n = .015_. The velocity for other values of _n_ can be
determined by the method given in the following paragraphs.

37. =Use of Diagrams.=—There are five factors in Kutter’s formula: _n_,
_Q_, _V_, _d_ (or _R_), and _S_. If any three of these are given the
other two can be determined, except when the three given are _Q_, _V_,
and _d_. These three are related in the form _Q_ = _AV_, which is
independent of slope or the character of the material. There are only
nine different combinations possible with these five factors, which will
be met in the solution of Kutter’s formula. The solution of the problems
by means of the diagrams is simple when the data given include _n_
= .015. For other given values of _n_ the solution is more complicated.
Results of the solution of types of each of the nine problems are given
in Table 17 and the explanatory text below.

_If n is given and is equal to .015_, the solution is simple.

For example in Table 17 _case 1, example 1_; to be solved on Fig.
15. Place a straight-edge at 1.0 on the _Q_ line and at 6 inches
on the diameter line for _n_ = .015. The slope and the velocity
will be found at the intersection of the straight-edge with these
respective scales.

All problems in which _n_ is given as .015 and the solution for which
falls within the limits of Fig. 15 or 16 should be solved by placing a
straight-edge on the two known scales and reading the two unknown
results at the intersection of the straight-edge and the remaining
scales.

For example in _case 1_, _example 2_ find the intersection of the
horizontal line representing _Q_ = 100 with the sloping diameter
line representing _d_ = 48 inches. The vertical slope line passing
through this point represents _S_ = .0065 and the sloping velocity
line passing through this point represents 8.5 feet per second.

In general problems in which _n_ = .015, can be solved on Fig. 17 by
finding the intersection of the two lines representing the given data,
and reading the values of the remaining variables represented by the
other two lines passing through this point.

TABLE 17

SOLUTIONS OF PROBLEMS BY KUTTER’S FORMULA

─────┬───────┬────────────────────────────┬────────────────────────────
Case │Example│ Given │ Found
─────┼───────┼─────┬─────┬────┬────┬──────┼─────┬─────┬────┬────┬──────
│ │ _n_ │ _Q_ │_V_ │_d_ │ _S_ │ _n_ │ _Q_ │_V_ │_d_ │ _S_
─────┼───────┼─────┼─────┼────┼────┼──────┼─────┼─────┼────┼────┼──────
1 │ 1 │0.015│ 1.0│ 2.5│ 6│ │ │ │5.0 │ │0.0575
1 │ 2 │ .015│100.0│ │ │ │ │ │8.5 │ │.0065
1 │ 3 │ .020│ 1.0│ │ 6│ │ │ │5.0 │ │.13
1 │ 4 │ .020│100.0│ │ 48│ │ │ │8.5 │ │.0125
2 │ 1 │ .015│ 5.0│ │ │0.0003│ │ │1.2 │28 │
2 │ 2 │ .010│ 5.0│ │ │.0003 │ │ │1.7 │23.5│
3 │ 1 │ .015│ │ │ 18│.002 │ │ 4.0│2.25│ │
3 │ 2 │ .018│ │ │ 18│.0008 │ │ 2.0│1.1 │ │
4 │ 1 │ .015│ 2.0│ 2.5│ │ │ │ │ │12 │.00475
4 │ 2 │ .011│ 2.0│ 2.5│ │ │ │ │ │12 │.0022
5 │ 1 │ .015│ │ 5.0│ 36│ │ │ 35.0│ │ │.0038
6 │ 1 │ .018│ │ 5.0│ │.001 │ │185.0│ │80 │
7 │ 1 │ │ 3.0│ │ 18│.002 │0.019│ │1.7 │ │
7 │ 2 │ │ 50.0│ │ 36│.005 │ .012│ │7.0 │ │
8 │ 1 │ │ 6.0│ 2.5│ │.003 │ .018│ │ │21 │
9 │ 1 │ │ │ 4.2│ 66│.00059│ .011│100.0│ │ │
─────┴───────┴─────┴─────┴────┴────┴──────┴─────┴─────┴────┴────┴──────

_If n is given and is not equal to .015_ the solution is not so simple.
In Fig. 15 and 16 the diagram is so drawn that the _position_ of the
diameter scales for all values of _n_ is fixed on the vertical
“diameter” line. The _scales_ of diameter change for each value of _n_.
These scales of diameter are shown for each value of _n_ from .010
to .020 on vertical lines to the left of the “diameter” line. For use,
the proper diameter scale for any given value of _n_ must be projected
horizontally upon the vertical “diameter” line. The velocity can be
determined on Fig. 15 and 16, _only when the diameter has been
determined_ and then _only when the diameter scale for n equal .015 is
used, since the only scale shown for velocity is for n = .015._

For example, in _Case 1_, _Example 3_ there are given _n_ = .020,
_Q_, and _d_. Find the intersection of the vertical line for _n_ =
.020 with the sloping diameter line for _d_ = 6 inches. Project
the intersection horizontally to the right to the vertical
“diameter” line. Place a straight-edge at this point and at _Q_ =
1.0 on the quantity scale. The required value of _S_ is read at
the intersection of the straight-edge and the slope scale and is
equal to 0.13. The intersection of the straight-edge in this
position with the velocity scale is not the required value of the
velocity since the velocity scale is made out for _n_ = .015 and
not .020. It is necessary to change the position of the
straight-edge so that it may lie on _Q_ equal 1.0 and on _d_ equal
6 inches for _n_ equal .015. The value of _V_ is shown in this
position as 5 feet per second.

The reverse process for Fig. 15 and 16 is illustrated _by Case 4_,
_Example 2_ in which _n_ = .011 and _Q_ and _V_ are also given.
When _Q_ and _V_ are given the value of _d_ is fixed independent
of all other factors. Therefore the value of _d_ can be read from
the scale with _n_ = .015 and is found to be 12 inches. Now find
the value of _d_ = 12 inches on the scale for _n_ = .011 and
project on to the “diameter” line. Place the straight-edge at this
point and at _Q_ = 2. The required slope is read as .0022.

Fig. 17 is prepared for the solution of problems in which _n_ = .015
only. For problems in which _n_ has some other value it is necessary to
transform the data to equivalent conditions in which _n_ = .015. This is
done by means of the conversion factors shown in Fig. 18. The given
slope or velocity is multiplied by the proper factor to convert from or
to the value of _n_ = .015.

For example in _Case 1_, _Example 4_ there are given _n_ = .020,
_Q_, and _d_. With _Q_ and _d_ given the value of _V_ can be read
from Fig. 17 without conversion. The corresponding value of _S_
for _n_ = .015 is .0065. It is now necessary to use the
transformation diagram Fig. 18. The hydraulic radius of the given
pipe is one foot. On Fig. 18 at the intersection of the slope line
for _R_ = 1.0 foot and _n_ = .020 the value of the factor is read
as 1.92. Since the given _n_ is for rougher material than that
represented by _n_ = .015 the required slope must be greater than
for _n_ = .015 to give the same velocity. It is therefore
necessary to multiply .0065 × 1.92 and the required slope is
.0125.

In _Case 6_, _Example 1_ there are given _n_ = .018, _d_, and _S_.
The remaining factors are to be solved by Fig. 17. Solve first as
though _n_ = .015 in order to find an approximate value of _d_ or
_R_. In this case it is evident that _d_ is greater than 57
inches. The value of _R_ is therefore about 1.25. Referring to
Fig. 18 the conversion factor for the slope for _n_ = .018 is
about 1.52. Since the given slope for _n_ = .018 is .001, for an
equal velocity and for _n_ = .015 the slope should be less.
Therefore in reading Fig. 17 it is necessary to use a slope of
.001⁄1.52 = .00066. The diameter is found to be about 80 inches.
Since this is nearer to the correct diameter the value of the
conversion factor must be corrected for this approximation. The
hydraulic radius for an 80 inch pipe is 1.67 feet, and the
conversion factor from Fig. 18 is about 1.48. The slope for _n_ =
.015 should be therefore .001⁄1.48 = .000675 and from Fig. 17 the
required diameter and quantity are read as 80 inches and 185
second-feet, respectively.

_If n is not given_ but must be solved for, the solution on Fig. 15 and
16 is relatively simple. The desired value of _n_ is read at the
intersection of the sloping diameter line representing the known
diameter and the horizontal projection of the intersection of the
straight-edge with the vertical “diameter” line.

For example in _Case 7_, _Example 1_ there are given _Q_, _d_, and
_S_. Lay the straight-edge on the given values of _Q_ = 3 and _S_
= .002. At the point where the straight-edge crosses the vertical
“diameter” line project a horizontal line to the sloping diameter
line for _d_ = 18 inches. The vertical line passing through this
point represents a value of _n_ = .019. In order to find the value
of _V_ lay the straight-edge on _Q_ = 3 and _d_ = 18 inches for
_n_ = .015. The value of _V_ is read as 1.7.

A slightly different condition is illustrated in the solution of
_Case 8_, _Example 1_ in which _Q_, _V_ and _S_ are given.
Determine first the value of _d_ as though _n_ = .015. Then
proceed to determine _n_ as in the preceding examples.

The solution for an unknown value of _n_ on Fig. 17 is not so simple. It
must be determined by working backwards from the conversion factor.

For example in _Case 7_, _Example 2_ there are given _Q_, _d_, and
_S_. The value of _V_ is read directly as though _n_ = .015 as 7
feet per second. The value of _S_ read for _n_ = .015 is .0075.
But the given slope is .005. Since the given slope is flatter than
that for _n_ = .015 the conversion factor is less than unity and
is therefore .005⁄.0075 = 0.67. With this value of the conversion
factor and the value of _R_ given as 0.75 the value of _n_ is read
from Fig. 18 as slightly greater than .012.

=38. Flow in Circular Pipes Partly Full.=—The preceding examples have
involved the flow in circular pipes completely filled. The same methods
of solution can be used for pipes flowing partly full except that the
hydraulic radius of the wetted section is used instead of the diameter
of the pipe. Diagrams are used to save labor in finding the hydraulic
radius and the other hydraulic elements of conduits flowing partly full.

The hydraulic elements of a conduit for any depth of flow are: (_a_) The
hydraulic radius, (_b_) the area, (_c_) the velocity of flow, and (_d_)
the quantity or rate of discharge. The velocity and quantity when partly
full as expressed in terms of the velocity and quantity when full as
calculated by Kutter’s formula will vary slightly with different
diameters, slopes and coefficients of roughness. The other elements are
constant for all conditions for the same type of cross-section. The
hydraulic elements for all depths of a circular section for two
different diameters and slopes are shown in Fig. 19. The differences
between the velocity and quantity under the different conditions are
shown to be slight, and in practice allowance is seldom made for this
discrepancy.

In the solution of a problem involving part full flow in a circular
conduit the method followed is to solve the problem as though it were
for full flow conditions and then to convert to partial flow conditions
by means of Fig. 19, or to convert from partial flow conditions to full
flow conditions and solve as in the preceding section.

For example let it be required to determine the quantity of flow
in a 12–inch diameter pipe with _n_ = .015 when on a slope of .005
and the depth of flow is 3 inches. First find the quantity for
full flow. From Fig. 15 this is 2.0 cubic feet per second. The
depth of flow of 3 inches is one-fourth or 0.25 of the full depth
of 12 inches. From Fig. 19, running horizontally on the 0.25 depth
line to meet the quantity curve, the proportionate quantity at
this depth is found to be on the 0.13 vertical line, and the
quantity of flow is therefore 2 × 0.13 = 0.26 cubic feet per
second.

[Illustration:

FIG. 19.—Hydraulic Elements of Circular Sections.
]

_d_ = 12′ 0″ _s_ = .0004 _n_ = .015
_d_ = 1′ 0″ _s_ = .01 _n_ = .013

Another problem, involving the reversal of this process is illustrated
by the following example:

Let it be required to determine the diameter and full capacity of
a vitrified pipe sewer on a grade of 0.002 if the velocity of flow
is 3.0 feet per second when the sewer is discharging at 30 per
cent of its full capacity, the depth of flow being 12 inches. From
Fig. 19 the depth of flow when the sewer is carrying 30 per cent
of its full capacity is 0.38 of its full depth. Since the partial
depth is 12 inches the full diameter is 12⁄.038 = 31.6 inches. The
velocity of flow at 38 per cent depth is 86 per cent of the full
velocity. Since the velocity given is 3.0 feet per second, the
full velocity is 3.0⁄.86 = 3.5 feet per second. With a full
velocity of 3.5 feet per second and a diameter of 31.6 inches from
Fig. 16 the full capacity of the sewer is 18 cubic feet per
second.

=39. Sections Other than Circular.=—The ordinary shape used for small
sewers is circular. The difficulty of constructing large sewers in a
circular shape, special conditions of construction such as small head
room, soft foundations, etc., or widely fluctuating conditions of flow
have led to the development of other shapes. For conduits flowing full
at all times a circular section will carry more water with the same loss
of head than any other section under the same conditions. In any section
the smaller the flow the slower the velocity, an undesirable condition.
The ideal section for fluctuating flows would be one that would give the
same velocity of flow for all quantities. Such a section is yet to be
developed. Sections have been developed that will give relatively higher
velocities for small quantities of flow than are given by a circular
section. The best known of these sections is the egg shape, the
proportions and hydraulic elements of which are shown in Fig. 20. Other
shapes that have the same property, but which were not developed for the
same purpose are the rectangular, the U-shape, and the section with a
cunette. The egg-shaped section has been more widely used than any other
special section. It is, however, more difficult and expensive to build
under certain conditions, and has a smaller capacity when full than a
circular sewer of the same area of cross-section. Various sections are
illustrated in Fig. 22 and 23.

The U-shaped section is suitable where the cover is small, or close
under obstructions where a flat top is desirable and the fluctuations of
flow are so great as to make advantageous a special shape to increase
the velocity of low flows. The proportions of a U-shaped section are
shown in Fig. 23 (6). Other sections used for the same purpose are the
semicircular and special forms of the rectangular section.

The proportions and the hydraulic elements of the square-shaped section
are shown in Fig. 21. This is useful under low heads where a flat roof
is required to carry heavy loads, and the fluctuations of flow are not
large.

Sections with cunettes have not been standardized. A cunette is a small
channel in the bottom of a sewer to concentrate the low flows, as shown
in Fig. 22 (7). A cunette can be used in any shape of sewer.

[Illustration:

FIG. 20.—Hydraulic Elements of an Egg-shaped Section.

_d_ = 6′ 0″ _s_ = .00065 _n_ = .015
]

[Illustration:

FIG. 21.—Hydraulic Elements of a Square Section.

_d_ = 10′ 0″ _s_ = .0004 _n_ = .015
]

Sections developed mainly because of the greater ease of construction
under certain conditions are the basket handle, the gothic, the
catenary, and the horse shoe. Some of these shapes are shown in Fig. 22
and 23. They are suitable for large sewers on soft foundations, where it
is desirable to build the sewer in three portions, such as invert, side
walls, and arch. They are also suitable for construction in tunnels
where the shape of the sewer conforms to the shape of the timbering, or
in open cut work where the shape of the forms are easier to support.

Problems of flow in all sections can be solved by determining the
hydraulic radius involved, and substituting directly in the desired
formula, or by the use of one of the diagrams after converting to the
equivalent circular diameter. The determination of the hydraulic radius
of these special sections is laborious, and hence other less difficult
methods are followed. Problems are more commonly solved by converting
the given data into an equivalent circular sewer, solving for the
elements of this circular sewer and then reconverting into the original
terms, or by working in the other direction. The hydraulic elements of
various sections when full are given in Table 18.

TABLE 18

HYDRAULIC ELEMENTS OF SEWER SECTIONS. SEWERS FLOWING FULL.

───────────────┬─────────────┬─────────────┬─────────────┬──────────────
Section │Area in Terms│ Hydraulic │ Vert. Dia. │ Source
│ Vertical │ Radius in │_D_ in Terms │
│ Diameter │ terms of │ of Dia. _d_ │
│Squared _D_^2│Vertical Dia.│of Equivalent│
│ │ _D_ │ Circular │
│ │ │ Section │
───────────────┼─────────────┼─────────────┼─────────────┼──────────────
Circular │ 0.7854│ 0.250 │1.000 │
Egg │ 0.5150│ .1931│1.295 │Eng. Record,
│ │ │ │ Vol. 72: 608
Ovoid │ 0.5650│ .2070│1.208 │Eng. Record,
│ │ │ │ Vol. 72: 608
Semi-elliptical│ 0.8176│ .2487│1.041 │Eng. News,
│ │ │ │ Vol. 71: 552
Catenary │ 0.6625│ .2237│1.1175 │Eng. Record,
│ │ │ │ Vol. 72: 608
Horseshoe │ 0.8472│ .2536│0.985 │Eng. Record,
│ │ │ │ Vol. 72: 608
Basket handle │ 0.8313│ .2553│0.979 │Eng. Record,
│ │ │ │ Vol. 72: 608
Rectangular │ 1.3125│ .2865│0.7968 │Hydraulic
│ │ │ │ Dgms. and
│ │ │ │ Tbls.
│ │ │ │ Garrett
Square (3 sides│ 1.0000│ .333 │0.7500 │Eng. Record,
wet) │ │ │ │ Vol. 72: 608
Square (4 sides│ 1.0000│ .250 │1.0000 │Eng. Record,
wet) │ │ │ │ Vol. 72: 608
───────────────┴─────────────┴─────────────┴─────────────┴──────────────

[Illustration:

1. Standard Egg-shaped Section, North Shore Intercepter, Chicago,
Illinois.
]

[Illustration:

2. Rectangular Section, Omaha, Nebraska, Eng. Contracting, Vol. 46, p.
49.
]

[Illustration:

3. Trench in firm ground. 4. Trench in Rock.

NOTE.—Underdrains and Wedges to be used only when Ordered by the
Engineer.
]

[Illustration:

7. Brick and Concrete Sewer showing cunette.
]

[Illustration:

5. Soft Foundation. 6. Wet ground.
]

[Illustration:

8. Brick and Concrete Sewer, Evanston, Ill., Eng. Contracting, Vol.
46, p. 227.
]

FIG. 22.

[Illustration]

1. Tunnel Sections. 2. Open Cut Sections.
───────────────────────────────────────────────────────────────────────
Type A. Type B. Type C. Type D.
Where Rock Where Rock Where Rock Where Rock 16′ 6″ Where Rock
is more is more is between drops below Sewer. 25′ is above
than 16′ than 7′ Springing Springing Fill Springing
above and less Line and 7′ Line on Line
Springing than 16′ above either
Line. above Springing Side.
Springing Line on
Line on both Sides.
both Sides.

Mill Creek Sewer, St. Louis, Eng. Record, Vol. 70, pp. 434, 435.

[Illustration:

3. Circular Concrete Section in Soft and Hard Ground, Eng. Record,
Vol. 59, p. 570.
]

[Illustration:

4. Semi-Elliptical Section, Louisville, Ky., Eng. News, Vol. 62, p.
416.
]

[Illustration:

5. Reinforced Concrete Sewer, Harlem Creek, St. Louis, Eng. News, Vol.
60, p. 131.
]

[Illustration:

6. U-Shaped Section, San Francisco, Eng. News, Vol. 73, p. 310.
]

FIG. 23.

Equivalent sections are sections of the same capacity for the same slope
and coefficient of roughness. They have not necessarily the same
dimensions, shape, nor area. The diameter of the equivalent circular
section in terms of the diameter of each special section shown is given
in Table 18. The inside height of a sewer is spoken of as its diameter.

For example let it be required to determine the rate of flow in a
54–inch egg-shaped sewer on a slope of 0.001 when _n_ = .015.
First convert to the equivalent circle. From Table 18 the diameter
of the equivalent circle is 1⁄1.295 times the diameter of the
egg-shaped sewer, which becomes in this case 43 inches. From Fig.
16 the capacity of a circular sewer of this diameter with _S_ =
0.001 and _n_ = .015 is 28 cubic feet per second, which by
definition is the flow in the egg-shaped sewer.

As an example of the reverse process let it be required to find
the velocity of flow in an egg-shaped sewer flowing full and
equivalent to a 48–inch circular sewer. Both sewers are on a slope
of 0.005 and have a roughness coefficient of _n_ = .015. It is
first necessary to find the quantity of flow in the circular
sewer, which by definition is the quantity of flow in the
equivalent egg-shaped sewer. The velocity of flow in the
egg-shaped sewer is found by dividing this quantity by the area of
the egg-shaped section. As read from the diagram the quantity of
flow is 90 cubic feet per second. From Table 18 the area of the
egg-shaped sewer is 0.51_D_^2 where _D_ is the diameter of the
egg-shaped sewer, and _D_ = 1.295_d_ where _d_ is the diameter of
the equivalent circular sewer. Therefore the area equals (0.51) ×
(1.295 × 4)^2 = 13.5 square feet and the velocity of flow is
90⁄13.5 = 6.7 feet per second. This is slightly less than the
velocity in the circular section.

Some lines for egg-shaped sewers have been shown on Fig. 17 by which
solutions can be made directly. For other shapes, and for sizes of
egg-shaped sewers not found on Fig. 17 the preceding method or the
original formula must be used for solution. Problems in partial flow in
special sections are solved similarly to partial flow in circular
sections, by converting first to the conditions of full flow or by
working in the opposite direction.

=40. Non-uniform Flow.=—In the preceding articles it is assumed that the
mean velocity and the rate of flow past all sections are constant. This
condition is known as steady, uniform flow. In this article it will be
assumed that conditions of steady non-uniform flow exist, that is, the
rate of flow past all sections is constant, but the velocity of flow
past these sections is different for different sections. Under such
conditions the surface of the stream is not parallel to the invert of
the channel. If the velocity of flow is increasing down stream the
surface curve is known as the drop-down curve. If the velocity of flow
is decreasing down stream the surface curve is known as the backwater
curve. The hydraulic jump represents a condition of non-uniform flow in
which the velocity of flow decreases down stream in such a manner that
the surface of the stream stands normal to the invert of the channel at
the point where the change in velocity occurs. Above and below this
point conditions of uniform flow may exist.

Conditions of non-uniform flow exist at the outlet of all sewers, except
under the unusual conditions where the depth of flow in the sewer under
conditions of steady, uniform flow with the given rate of discharge
would raise the surface of water in the sewer, at the point of
discharge, to the same elevation as the surface of the body of water
into which discharge is taking place. By an application of the
principles of non-uniform flow to the design of outfall sewers, smaller
sewers, steeper grades, greater depth of cover, and other advantages can
be obtained.

The backwater curve is caused by an obstruction in the sewer, by a
flattening of the slope of the invert, or by allowing the sewer to
discharge into a body of water whose surface elevation would be above
the surface of the water in the sewer, at the point of discharge, under
conditions of steady, uniform flow with the given rate of discharge.

The drop-down curve is caused by a sudden steepening of the slope of the
invert; by allowing a free discharge; or by allowing a discharge into a
body of water whose surface elevation would be below the surface of the
water in the sewer, at the point of discharge, under conditions of
steady, uniform flow with the given rate of discharge. The last
described condition is common at the outlet of many sewers, hence the
common occurrence of the drop-down curve.

The hydraulic jump is a phenomenon which is seldom considered in sewer
design. If not guarded against it may cause trouble at overflow weirs
and at other control devices, in grit chambers, and at unexpected
places. The causes of the hydraulic jump are sufficiently well
understood to permit designs that will avoid its occurrence, but if it
is allowed to occur the exact place of the occurrence of the jump and
its height are difficult, if not impossible, to determine under the
present state of knowledge concerning them. The hydraulic jump will
occur when a high velocity of flow is interrupted by an obstruction in
the channel, by a change in grade of the invert, or the approach of the
velocity to the “critical” velocity. The “critical” velocity is equal to
√(_gh_), where _h_ is the depth of flow and _g_ is the acceleration due
to gravity. The velocity in the channel above the jump must be greater
than √(_gh__{1}), where _h__{1} is the depth of flow in the channel
above the jump. The velocity in the channel below the jump must be
greater than √(_gh__{2}), where _h__{2} is the depth of flow below the
jump. The jump will not take place unless the slope of the invert of the
channel is greater than _g_⁄_C_^2,in which _C_ is the coefficient in the
Chezy formula. With this information it is possible to avoid the jump by
slowing down the velocity by the installation of drop manholes, flight
sewers, or by other expedients.

The shape of the drop-down curve can be expressed, in some cases, by
mathematical formulas of more or less simplicity, dependent on the shape
of the conduit. The formula for a circular conduit is complicated. Due
to the assumptions which must be made in the deduction of these
formulas, the results obtained by their use are of no greater value than
those obtained by approximate methods. A method for the determination of
the drop-down curve is given by C. D. Hill.[32] In this method it is
necessary that the rate of flow past all sections shall be the same;
that the depth of submergence at the outlet shall be known; and that the
depth of flow at some unknown distance up the stream shall be assumed.
The shape and material of construction of the sewer and the slope of the
invert should also be known. The problem is then to determine the
distance between cross-sections, one where the depth of flow is known,
and the other where the depth of flow has been assumed. This distance
can be expressed as follows:

_L_ = ((_d__{2} − _d__{1}) − (_H__{1} − _H__{2}))⁄(_S_ − S_{1}) = (_d_′
− _H_′)⁄_S_′,

in which _L_ = the distance between cross-sections;

_d__{1} = the depth of flow at the lower section;

_d__{2} = the depth of flow at the upper section;

_H__{1} = the velocity head at the lower section;

_H__{2} = the velocity head at the upper section;

_S_ = the hydraulic slope of the stream surface;

_S__{1} = the slope of the invert of the sewer.

In order to solve such problems with a satisfactory degree of accuracy
the difference between _d__{1} and _d__{2} should be taken sufficiently
small to divide the entire length of the sewer to be investigated into a
large number of sections. The solution of the problem requires the
determination of the wetted area, the hydraulic radius, and other
hydraulic elements at many sections. The labor involved can be
simplified by the use of diagrams, such as Fig. 19, or by specially
prepared diagrams such as those accompanying the original article by C.
D. Hill. The solution of the problem can be simplified by tabulating the
computations as follows:

DROP-DOWN CURVE COMPUTATION SHEET

Uniform discharge. Varying depth

┌───────────────────────────────────────────────────────────────────────┐
│ _D_ = _Q_ = _A_ = _V_ = │
│ _Q_⁄_A_ = _S__{1} = _L_ = (_d__{1} − _H__{1})⁄_S__{1} │
├───┬───┬───────┬───┬───┬───────┬───────┬───┬───┬───────┬───┬─────┬─────┤
│ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 10 │11 │ 12 │ 13 │
├───┴───┴───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┴─────┤
│ Depth │_R_│_H_│_H__{1}│_d__{1}│_V_│_S_│_S__{1}│_L_│ Elevation │
│ │ │ │ │ − │ │ │ │ │ │
│ │ │ │ │_H__{1}│ │ │ │ │ │
├───┬───┬───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┬─────┤
│_D_│_d_│_d__{1}│ │ │ │ │ │ │ │ │Sewer│W. L.│
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │
├───┼───┼───────┼───┼───┼───────┼───────┼───┼───┼───────┼───┼─────┼─────┤
│ │ │ │ │ │ │ │ │ │ │ │ │ │

At the head of the computation sheet should be recorded the diameter of
the sewer in feet, the assumed volume of flow, the area of the full
cross-section of the sewer, the velocity of the assumed volume flowing
through the full bore of the sewer, and the gradient or slope of the
invert. In the 1st column enter the assumed depth in decimal parts of
the diameter for each cross-section; in the 2nd column enter the same
depth in feet; in the 3rd column enter the difference in feet between
the successive cross-sections; in the 4th column enter the hydraulic
radius corresponding to the depth at each cross-section; in the 8th
column enter the velocity, equal to the volume divided by the wetted
area, for each cross-section; in the 5th column enter the corresponding
velocity head; in the 6th column enter the difference between the
velocity heads at successive cross-sections; in the 7th column enter the
difference between the quantities in the third and in the sixth columns;
in the 9th column enter the hydraulic slope corresponding to the
velocity and hydraulic radius of each cross-section; in the 10th column
enter the difference between the hydraulic slope and the slope or
gradient of the sewer; in the 11th column enter the computed distance
between successive cross-sections; in the 12th column enter the
elevation of the bottom of the sewer at each cross-section; and in the
13th column enter the corresponding elevation of the surface of the
water.

The table should be filled in until the distance to the required section
is determined, or if the distance is known, it should be filled in until
the depth of flow with the assumed rate of discharge has been checked.

If only the depth of flow at some section is known and it is required to
know the maximum rate of flow with a free discharge, or a discharge with
a submergence at the outlet less than the depth of flow with the maximum
rate of discharge, it is necessary to make a preliminary estimate of the
maximum rate of flow in order to fill in the quantity _Q_ at the head of
the table. The procedure should be as follows:

1st. Assume a depth of flow at the outlet.

2nd. Compute the area (_A_) and the hydraulic radius (_R_) at the known
section and at the outlet.

3rd. Determine the area and the hydraulic radius half way between these
two sections as the mean of the areas and the hydraulic radii of
the two sections.

4th. Determine the rate of flow through the sewer from the condition
that the difference in head at the two sections is the head lost
due to friction caused by the average velocity of flow between
the sections (equals (_lV_^2)⁄(_C_^2_R_)) plus the gain in
velocity head (equals _V__{2}^2 − (_V__{1}^2)⁄(2_g_)), which
then combined and transposed result in the expression:

_Q_ = _AA__{1}_A__{2} √(2_Rgh_⁄(2_A__{1}^2_A__{2}^2_gl_ + (_A__{1}
− _A__{2})(_A_^2_C_^2_R_)))

in which _Q_ = rate of flow;

_A_ = the area determined in the 3rd step;

_A__{1} = the area at the upper cross-section;

_A__{2} = the area at the lower cross-section;

_C_ = the coefficient in the Chezy formula;

_g_ = the acceleration due to gravity;

_h_ = the difference in elevation of the surface of the
stream at the two cross-sections;

_l_ = the distance between the cross-sections;

_R_ = the hydraulic radius determined in the third step.

5th. Continue this process by assuming different depths at the outlet
until the maximum rate of discharge has been found by trial.

With this rate of discharge and depth of flow at the outlet, the depth
of flow at the known section can be checked. If appreciably in error a
correction should be made by the assumption of a different depth of flow
at the outlet. The approximate character of the method is scarcely
worthy of the refinement in the results which will be obtained by
checking back for the depth of flow at the known section. It will be
sufficiently accurate to assume the rate of flow obtained by trial from
the preceding expression, as the maximum rate of discharge from the
sewer.

CHAPTER V
DESIGN OF SEWERAGE SYSTEMS

=41. The Plan.=—Good practice demands that a comprehensive plan for a
sewerage system be provided for the needs of a community for the entire
extent of its probable future growth, and that sewers be constructed as
needed in accordance with this plan.

Sewerage systems may be laid out on any one of three systems: separate,
storm, or combined. A separate system of sewers is one in which only
sanitary sewage or industrial wastes or both are allowed to flow. Storm
sewers carry only surface drainage, exclusive of sanitary sewage.
Combined sewers carry both sanitary and storm sewage. The use of a
combined or a separate system of sewerage is a question of expediency.
Portions of the same system may be either separate, combined, or storm
sewers.

Some conditions favorable to the adoption of the separate system are
where:

_a._ The sanitary sewage must be concentrated at one outlet, such
as at a treatment plant, and other outlets are available for the
storm drainage.

_b._ The topography is flat necessitating deep excavation and
steeper grades for the larger combined sewers.

_c._ The sanitary sewers must be placed materially deeper than the
necessary depth for the storm-water drains.

_d._ The sewers are to be laid in rock, necessitating more
difficult excavation for the larger combined sewers.

_e._ An existing sewerage system can be used to convey the dry
weather flow, but is not large enough for the storm sewage.

_f._ The city finances are such that the greater cost of the
combined system cannot be met and sanitary drainage is imperative.

_g._ The district to be sewered is an old residential section
where property values are not increasing and the assessment must
be kept down.

Some additional points given in a report by Alvord and Burdick to the
city of Billings, Montana, are:

The separate system of sewerage should be used, where:

1st. Storm water does not require extensive underground removal,
or where it can be concentrated in a few shallow underground
channels.

2nd. Drainage areas are short and steep facilitating rapid flow of
water over street surfaces to the natural water courses.

3rd. The sanitary sewage must be pumped.

4th. Sewers are being built in advance of the city’s development
to encourage its growth.

5th. The existing sewer is laid at grades unsuitable for sanitary
sewage, it can be used as a storm sewer.

A combined system must be relatively larger than a separate storm
sewer as the latter may overflow on exceptional occasions, but the
former never.

A combined system of sewerage should be used where:

1st. It is evident that storm and sanitary sewerage must be
provided soon.

2nd. Both sanitary and storm sewage must be pumped.

3rd. The district is densely built up.

=42. Preliminary Map.=—The first step in the design of a sewerage system
is the preparation of a map of the district to be served within the
limits of its probable growth. The map should be on a scale of at least
200 feet to the inch in the built up sections or other areas where it is
anticipated that sewers may be built, and where much detail is to be
shown a scale as large as 40 feet to the inch may have to be used. The
adoption of so large a scale will usually necessitate the division of
the city or sewer district into sections. A key map should be drawn to
such a scale that the various sections represented by separate drawings
can all be shown upon it. In preparing the enlarged portions of the map
it is not necessary to include these portions of the city in which it is
improbable that sewers will be constructed, such as parks and
cemeteries.

The contour interval should depend on the character of the district and
the slope of the land. In those sections drawn to a scale of 200 feet to
the inch for slopes over 5 per cent, the contour interval need not be
closer than 10 feet. For slopes between 1 and 5 per cent the contour
interval should be 5 feet. For flatter slopes the interval should not
exceed 2 feet, and a one foot interval is sometimes desirable. In
general the horizontal distances between contours should not exceed 400
feet and they should be close enough to show important features of the
natural drainage. Elevations should also be given at street
intersections, and at abrupt changes in grade. For portions of the map
on a smaller scale the contours need be sufficiently close to show only
the drainage lines and the general slope of the land.

The following may be shown on the preliminary map: the elevation of lots
and cellars; the character of the built up districts, whether cheap
frame residences, flat-roof buildings, manufacturing plants, etc.;
property lines; width of streets between property lines and between curb
lines; the width and character of the sidewalks and pavements; street
car and railroad tracks; existing underground structures such as sewers,
water pipes, telephone conduits, etc.; the location of important
structures which may have a bearing on the design of the sewers such as
bridges, railroad tunnels, deep cuts, culverts, etc.; and the location
of possible sewer outlets and the sites for sewage disposal plants.

Fig. 24 shows a preliminary map for a section of a city, on which the
necessary information has been entered. The map is made from survey
notes. All streets are paved with brick. The alleys are unpaved. The
entire section is built up with high-class detached residences averaging
one to each lot. The lots vary from 1 to 3 feet above the elevation of
the street.

=43. Layout of the Separate System.=—Upon completion of the preliminary
map a tentative plan of the system is laid out. The lines of the sewer
pipe are drawn in pencil, usually along the center line of the street or
alley in such a manner that a sewer will be provided within 50 feet or
less of every lot. The location of the sewers should be such as to give
the most desirable combination of low cost, short house connections,
proper depth for cellar drainage, and avoidance of paved streets. Some
dispute arises among engineers as to the advisability of placing pipes
in alleys, although there is less opposition to so placing sewers than
any other utility conduit. The principal advantage in placing sewers in
alleys is to avoid disturbing the pavement of the street, but if both
street and alley are paved it is usually more economical to place the
sewer in the street as the house connections will be shorter. On
boulevards and other wide streets such as Meridian Avenue in Fig. 24,
the sewers are placed in the parking on each side of the street, rather
than to disturb the pavement and lay long house connections to the
center of the street.

All pipes should be made to slope, where possible, in the direction of
the natural slope of the ground. The preliminary layout of the system is
shown in Fig. 24. The lowest point in the portion of the system shown is
in the alley between Alabama and Tennessee Streets. The flow in all
pipes is towards this point, and only one pipe drains away from any
junction, except that more than one pipe may drain from a terminal
manhole on a summit.

=44. Location and Numbering of Manholes.=—Manholes are next located on
the pipes of this tentative layout. Good practice calls for the location
of a manhole at every change in direction, grade, elevation, or size of
pipe, except in sewers 60 inches in diameter or larger. The manholes
should not be more than 300 to 500 feet apart, and preferably as close
as 200 to 300 feet. In sewers too small for a man to enter the distance
is fixed by the length of sewer rods which can be worked successfully.
In the larger sewers the distances are sometimes made greater but
inadvisedly so, since quick means of escape should be provided for
workmen from a sudden rise of water in the sewer, or the effect of an
asphyxiating gas. In the preliminary layout the manholes are located at
pipe intersections, changes in direction, and not over 300 to 500 feet
apart on long straight runs at convenient points such as opposite street
intersections where other sewers may enter.

No standard system of manhole numbering has been adopted. A system which
avoids confusion and is subject to unlimited extension is to number the
manholes consecutively upwards from the outlet, beginning a new series
of numbers prefixed by some index number or letter for each branch or
lateral. This system has been followed with the manholes on Fig. 24.

[Illustration:

FIG. 24.—Typical Map Used in the Design of a Separate Sewer System.
]

[Illustration:

FIG. 25.—Typical Map Used in the Design of a Storm Sewer System.
]

=45. Drainage Areas.=—The quantity of dry weather sewage is determined
by the population rather than the topography. Lot lines and street
intersections or other artificial lines marking the boundaries between
districts are therefore taken as watershed lines for sanitary sewers.
The quantity of sewage to be carried and the available slope are the
determining factors in fixing the diameter of the sewer. Since there may
be no change in diameter or slope between manholes the quantity of
sewage delivered by a sewer into any manhole will determine the diameter
of the sewer between it and the next manhole above. In order to
determine the additional amount contributed between manholes a line is
drawn around the drainage area tributary to each manhole. This line
generally follows property lines and the center lines of streets or
alleys, its position being such that it includes all the area draining
into one manhole, and excludes all areas draining elsewhere. An entire
lot is usually assumed to lie within the drainage area into which the
building on the lot drains. In laying out these areas it is best to
commence at the upper end of a lateral and work down to a junction. Then
start again at the upper end of another lateral entering this junction,
and continue thus until the map has been covered.

The areas are given the same numbers as the manholes into which they
drain. The dividing lines for the drainage areas on Fig. 24 are shown as
dot and dash lines, and the areas enclosed are appropriately numbered.
If more than one sewer drains into the same manhole the area should be
subdivided so that each subdivision encloses only the area contributing
through one sewer. Such a condition is shown at manhole _C_2. The areas
are designated by subletters or symbols corresponding to the symbol used
for the sewer into which they drain. For example, the two areas
contributing to manhole _C_2 are lettered _C_2_{_K_} and _C_2_{_D_}. The
sewer from manhole _C_3 to _C_2 receives no addition, it being assumed
that all the lots adjacent to it drain into the sewer on the alley.
There is therefore no area _C_2. Likewise there is no area _A_1_{_C_}.

=46. Quantity of Sewage.=—The remaining work in the computation of the
quantity of sewage is best kept in order by a tabulation. Table 19 shows
the computations for the sewers discharging from the east into manhole
No. 142. The computation should begin at the upper end of a lateral,
continue to a junction, and then start again at the upper end of another
lateral entering this junction. Each line in the table should be filled
in completely from left to right before proceeding with the computations
on the next line. In the illustrative solution in Table 19, computations
for quantity have not been made between manholes where it was apparent
that there would be an insufficient additional quantity to necessitate a
change in the size of the pipe.

In making these computations the assumptions of quantity and other
factors given below indicate the sort of assumptions which must be made,
based on such studies as are given in Chapter III. The density of
population was taken as 20 persons per acre, the assumption being based
on the census and the character of the district. The average sanitary
sewage flow was taken as 100 gallons per capita per day. The per cent
which the maximum dry weather flow is of the average was taken as _M_ =
500⁄_P_^⅕, in which _P_ is the population in thousands. The per cent is
not to exceed 500 nor to be less than 150. The rate of infiltration of
ground water was assumed as 50,000 gallons per mile of pipe per day.

In the first line of Table 19, the entries in columns (1) to (6) are
self-explanatory. There are no entries in columns (7) to (10), as no
additional sewage is contributed between manholes 3.5 and 3.4. In column
(11), 2250 persons are recorded as the number tributary to manhole No.
3.5 in the district to the north and west. These people contribute an
average of 100 gallons per person per day, or a total of 0.346 second
foot. This quantity is entered in column (13). The figure in column (14)
is obtained from the expression _M_ = (500)⁄_P_^⅕. Column (15) is .01 of
the product of columns (13) and (14). Column (16) is the product of the
length of pipe between manholes 3.5 and 3.4, and the ground water unit
reduced to cubic feet per second. Column (17) is the sum of column (16),
and all of the ground water tributary to manhole 3.5, which is not
recorded in the table. Column (18) is the sum of columns (15) and (17).

No new principle is represented in the second and third lines.

In the fourth line the first 10 columns need no further explanation. The
(11th) column is the sum of the (10th) column, and the (11th) column in
the third line. It represents the total number of persons tributary to
manhole 3.4 on lateral No. 8. Column (13) in the fourth line is the sum
of column (13) in the third line and the (12th) column in the fourth
line, and the (15th) column in the fourth line is the product of the 2
preceding columns in the fourth line. Note that in no case is the figure
in column (15) the sum of any previous figures in column (15). With this
introduction the student should be able to check the remaining figures
in the table, and should compute the quantity of sewage entering manhole
No. 142 from the west, making reasonable assumptions for the tributary
quantities from beyond the limits of the map.

TABLE 19

COMPUTATIONS FOR QUANTITY OF SEWAGE FOR A SEPARATE SEWERAGE SYSTEM

──────────┬──────────┬──────────┬───────┬───────┬──────┬───────┬─────
On Street │ From │To Street │ From │ To │Length│Mark of│Area,
│ Street │ │Manhole│Manhole│ Feet │ Added │Acres
│ │ │ │ │ │ Areas │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼─────
Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ │
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 8.2│ 2.7
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 8.1│ 3.41
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│3.4_{8}│ 2.68
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ │
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
│ │ │ │ │ │ 7.1│
Alley S. │Railroad │Nebraska │ 7.2│ 3.3│ 800│3.3_{7}│ 7.14
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ │
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
│ │ │ │ │ │ 6.1│
Alley S. │Railroad │Nebraska │ 6.2│ 3.2│ 609│3.2_{6}│ 3.82
of Smith│ │ St. │ │ │ │ │
Ave. │ │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ │
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│3.1_{4}│ 3.10
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│3.1_{5}│ 2.69
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ │
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 148│ 1.53
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 148│ 147│ 492│ │
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 147│ 146│ 430│ │
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 146│ 0.81
│ St. │ │ │ │ │ │
│ │ │ │ │ │ 2.1│
Long St. │Harris St.│Tennessee │ 145│ 143│ 725│143–145│ 6.6
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴─────

──────────┬──────────┬──────────┬──────────┬───────┬─────────┬─────────
On Street │ From │To Street │Population│Number │ Total │ Avg.
│ Street │ │ per Acre │ of │ Persons │Sanitary
│ │ │ │Persons│Tributary│ Flow,
│ │ │ │ │ │ C.F.S.
│ │ │ │ │ │
──────────┼──────────┼──────────┼──────────┼───────┼─────────┼─────────
Nebraska │Map margin│Alley S. │ │ │ 2250│ 0.0000
St. │ │ Grant │ │ │ │
│ │ St. │ │ │ │
Alley S. │Railroad │E. of │ 20│ 54│ 54│ .0084
of Grant│ │ Missouri│ │ │ │
St. │ │ St. │ │ │ │
Alley S. │E. of │E. of │ 20│ 68│ 122│ .0106
of Grant│ Missouri│ Kansas │ │ │ │
St. │ St. │ St. │ │ │ │
Alley S. │E. of │Nebraska │ 20│ 54│ 176│ .0084
of Grant│ Kansas │ St. │ │ │ │
St. │ St. │ │ │ │ │
Nebraska │Alley S. │Alley S. │ │ │ 2428│ .0000
St. │ of Grant│ of │ │ │ │
│ St. │ Meridian│ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ 20│ 142│ 142│ .0221
of │ │ St. │ │ │ │
Meridian│ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ │ │ 2568│ .0000
St. │ of │ of Smith│ │ │ │
│ Meridian│ Av. │ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ 20│ 76│ 76│ .0119
of Smith│ │ St. │ │ │ │
Ave. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ │ │ 2644│ .0000
St. │ of Smith│ Cordovez│ │ │ │
│ Ave. │ St. │ │ │ │
S. of │Railroad │Nebraska │ 20│ 62│ 62│ .0096
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
S. of │Map margin│Nebraska │ 20│ 54│ 54│ .0084
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
Nebraska │S. of │Long St. │ │ │ 2760│ .0000
St. │ Cordovez│ │ │ │ │
│ St. │ │ │ │ │
Long St. │Map margin│Nebraska │ 20│ 31│ 31│ .0048
│ │ St. │ │ │ │
Long St. │Nebraska │N. │ │ │ 2791│ .0000
│ St. │ Carolina│ │ │ │
│ │ St. │ │ │ │
Long St. │N. │Georgia │ │ │ 2791│1.000[33]
│ Carolina│ St. │ │ │ │
│ St. │ │ │ │ │
Long St. │Georgia │Harris St.│ 20│ 16│ 2807│ .0025
│ St. │ │ │ │ │
│ │ │ │ │ │
Long St. │Harris St.│Tennessee │ 20│ 132│ 2936│ .0205
│ │ St. │ │ │ │
│ │ │ │ │ │
Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12)
(1) │ │ │ │ │ │
──────────┴──────────┴──────────┴──────────┴───────┴─────────┴─────────

──────────┬──────────┬──────────┬──────────┬────────┬─────────
On Street │ From │To Street │Cumulative│Per cent│ Total
│ Street │ │ Avg. │ Max. │ Max.
│ │ │ Sanitary │Sanitary│Sanitary,
│ │ │ Flow, │ is of │ C.F.S.
│ │ │ C.F.S. │Average │
──────────┼──────────┼──────────┼──────────┼────────┼─────────
Nebraska │Map margin│Alley S. │ 0.346│ 425│ 1.47
St. │ │ Grant │ │ │
│ │ St. │ │ │
Alley S. │Railroad │E. of │ .0084│ 500│ 0.041
of Grant│ │ Missouri│ │ │
St. │ │ St. │ │ │
Alley S. │E. of │E. of │ .0190│ 500│ 0.095
of Grant│ Missouri│ Kansas │ │ │
St. │ St. │ St. │ │ │
Alley S. │E. of │Nebraska │ .0274│ 500│ 0.137
of Grant│ Kansas │ St. │ │ │
St. │ St. │ │ │ │
Nebraska │Alley S. │Alley S. │ .373│ 423│ 1.58
St. │ of Grant│ of │ │ │
│ St. │ Meridian│ │ │
│ │ │ │ │
Alley S. │Railroad │Nebraska │ .0221│ 500│ 0.111
of │ │ St. │ │ │
Meridian│ │ │ │ │
Nebraska │Alley S. │Alley S. │ .395│ 414│ 1.63
St. │ of │ of Smith│ │ │
│ Meridian│ Av. │ │ │
│ │ │ │ │
Alley S. │Railroad │Nebraska │ .0119│ 500│ 0.060
of Smith│ │ St. │ │ │
Ave. │ │ │ │ │
Nebraska │Alley S. │S. of │ .407│ 414│ 1.68
St. │ of Smith│ Cordovez│ │ │
│ Ave. │ St. │ │ │
S. of │Railroad │Nebraska │ .0096│ 500│ 0.048
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
S. of │Map margin│Nebraska │ .0084│ 500│ 0.042
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
Nebraska │S. of │Long St. │ .425│ 409│ 1.74
St. │ Cordovez│ │ │ │
│ St. │ │ │ │
Long St. │Map margin│Nebraska │ .0048│ 500│ 0.024
│ │ St. │ │ │
Long St. │Nebraska │N. │ .430│ 409│ 1.76
│ St. │ Carolina│ │ │
│ │ St. │ │ │
Long St. │N. │Georgia │ .430│ 409│ 1.76
│ Carolina│ St. │ │ │
│ St. │ │ │ │
Long St. │Georgia │Harris St.│ .433│ 407│ 1.76
│ St. │ │ │ │
│ │ │ │ │
Long St. │Harris St.│Tennessee │ .454│ 403│ 1.83
│ │ St. │ │ │
│ │ │ │ │
Column No.│ (2) │ (3) │ (13) │ (14) │ (15)
(1) │ │ │ │ │
──────────┴──────────┴──────────┴──────────┴────────┴─────────

──────────┬──────────┬──────────┬─────────┬──────────┬──────┬──────
On Street │ From │To Street │Increment│Cumulative│Total │ Line
│ Street │ │of Ground│ Ground │Flow, │Number
│ │ │ Water, │ Water, │C.F.S.│
│ │ │ C.F.S. │ C.F.S. │ │
│ │ │ │ │ │
──────────┼──────────┼──────────┼─────────┼──────────┼──────┼──────
Nebraska │Map margin│Alley S. │ 0.005│ 0.0187│ 1.66│ 1
St. │ │ Grant │ │ │ │
│ │ St. │ │ │ │
Alley S. │Railroad │E. of │ .0048│ .0048│ 0.046│ 2
of Grant│ │ Missouri│ │ │ │
St. │ │ St. │ │ │ │
Alley S. │E. of │E. of │ .0052│ .010│ 0.105│ 3
of Grant│ Missouri│ Kansas │ │ │ │
St. │ St. │ St. │ │ │ │
Alley S. │E. of │Nebraska │ .0050│ .015│ 0.152│ 4
of Grant│ Kansas │ St. │ │ │ │
St. │ St. │ │ │ │ │
Nebraska │Alley S. │Alley S. │ .0058│ .208│ 1.79│ 5
St. │ of Grant│ of │ │ │ │
│ St. │ Meridian│ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ .0117│ .0117│ 0.123│ 6
of │ │ St. │ │ │ │
Meridian│ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ .0045│ .224│ 1.85│ 7
St. │ of │ of Smith│ │ │ │
│ Meridian│ Av. │ │ │ │
│ │ │ │ │ │
Alley S. │Railroad │Nebraska │ .0089│ .0089│ 0.069│ 8
of Smith│ │ St. │ │ │ │
Ave. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ .0044│ .237│ 1.92│ 9
St. │ of Smith│ Cordovez│ │ │ │
│ Ave. │ St. │ │ │ │
S. of │Railroad │Nebraska │ .006│ .006│ 0.054│ 10
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
S. of │Map margin│Nebraska │ .0056│ .0056│ 0.048│ 11
Cordovez│ │ St. │ │ │ │
St. │ │ │ │ │ │
Nebraska │S. of │Long St. │ .0025│ .251│ 1.99│ 12
St. │ Cordovez│ │ │ │ │
│ St. │ │ │ │ │
Long St. │Map margin│Nebraska │ .0056│ .0056│ 0.030│ 13
│ │ St. │ │ │ │
Long St. │Nebraska │N. │ .0072│ .264│ 2.02│ 14
│ St. │ Carolina│ │ │ │
│ │ St. │ │ │ │
Long St. │N. │Georgia │ .0064│ 1.27│ 3.03│ 15
│ Carolina│ St. │ │ │ │
│ St. │ │ │ │ │
Long St. │Georgia │Harris St.│ .0061│ 1.28│ 3.04│ 16
│ St. │ │ │ │ │
│ │ │ │ │ │
Long St. │Harris St.│Tennessee │ .024│ 1.30│ 3.13│ 17
│ │ St. │ │ │ │
│ │ │ │ │ │
Column No.│ (2) │ (3) │ (16) │ (17) │ (18) │
(1) │ │ │ │ │ │
──────────┴──────────┴──────────┴─────────┴──────────┴──────┴──────
TABLE 20

COMPUTATIONS FOR SLOPE AND DIAMETER OF PIPES FOR A SEPARATE SEWERAGE
SYSTEM

──────────┬──────────┬──────────┬───────┬───────┬──────┬───────────────
On Street │ From │To Street │ From │ To │Length│El. of Surface
│ Street │ │Manhole│Manhole│ Feet │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┬───────
│ │ │ │ │ │ Upper │ Lower
│ │ │ │ │ │Manhole│Manhole
──────────┼──────────┼──────────┼───────┼───────┼──────┼───────┼───────
Nebraska │Map margin│Alley S. │ 3.5│ 3.4│ 338│ 105.8│ 102.4
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 8.3│ 8.2│ 328│ 113.5│ 112.0
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 8.2│ 8.1│ 355│ 112.0│ 107.7
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 8.1│ 3.4│ 340│ 107.7│ 102.4
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.4│ 3.3│ 380│ 102.4│ 100.7
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
Alley S. │Railroad │Kansas St.│ 7.2│ 7.1│ 400│ 111.8│ 107.0
of │ │ │ │ │ │ │
Meridian│ │ │ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 7.1│ 3.3│ 400│ 107.0│ 100.7
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 3.3│ 3.2│ 304│ 100.7│ 99.3
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
Alley S. │Railroad │East of │ 6.2│ 6.1│ 305│ 109.3│ 105.3
of Smith│ │ Kansas │ │ │ │ │
Ave. │ │ St. │ │ │ │ │
Alley S. │East of │Nebraska │ 6.1│ 3.2│ 304│ 105.3│ 99.3
of Smith│ Kansas │ St. │ │ │ │ │
Ave. │ St. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 3.2│ 3.1│ 300│ 99.3│ 101.1
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ 4.1│ 3.1│ 410│ 100.8│ 101.1
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 5.1│ 3.1│ 380│ 104.6│ 101.1
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 3.1│ 148│ 172│ 101.1│ 98.7
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 149│ 148│ 380│ 103.8│ 98.7
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 148│ 147│ 492│ 98.7│ 103.8
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 147│ 146│ 430│ 103.8│ 99.1
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 146│ 145│ 419│ 99.1│ 96.9
│ St. │ │ │ │ │ │
Alley S. │End of │Harris St.│ 2.2│ 2.1│ 350│ 105.2│ 98.1
of Janis│ Janis │ │ │ │ │ │
St. │ St. │ │ │ │ │ │
Harris St.│Alley N. │Long St. │ 2.1│ 145│ 135│ 98.1│ 96.9
│ of Janis│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Harris St.│Kentucky │ 145│ 144│ 258│ 96.9│ 94.4
│ │ St. │ │ │ │ │
Long St. │Kentucky │Tennessee │ 144│ 143│ 282│ 94.4│ 93.6
│ St. │ St. │ │ │ │ │
Tarbell │Harris St.│Long St. │ 1.1│ 143│ 417│ 98.7│ 92.6
Ave. │ │ │ │ │ │ │
Long St. │Tennessee │Alley W. │ 143│ 142│ 185│ 92.6│ 92.3
│ St. │ of Tenn.│ │ │ │ │
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (4) │ (5) │ (6) │ (7) │ (8)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────┴───────┴───────

──────────┬──────────┬──────────┬──────┬──────┬──────┬────────┬────────
On Street │ From │To Street │Total │Slope │ Dia. │Velocity│Capacity
│ Street │ │Flow, │ │ of │ when │ when
│ │ │C.F.S.│ │Pipe, │ Full, │ Full,
│ │ │ │ │Inches│Ft. per │ Second
│ │ │ │ │ │ Second │ Feet
──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼────────
│ │ │ │ │ │ │
│ │ │ │ │ │ │
──────────┼──────────┼──────────┼──────┼──────┼──────┼────────┼────────
Nebraska │Map margin│Alley S. │ 1.66│0.0108│ 10│ 3.25│ 1.78
St. │ │ Grant │ │ │ │ │
│ │ St. │ │ │ │ │
Alley S. │Railroad │E. of │ 0.046│.00575│ 8│ 2.00│ 0.71
of Grant│ │ Missouri│ │ │ │ │
St. │ │ St. │ │ │ │ │
Alley S. │E. of │E. of │ 0.105│ .0110│ 8│ 2.78│ 0.98
of Grant│ Missouri│ Kansas │ │ │ │ │
St. │ St. │ St. │ │ │ │ │
Alley S. │E. of │Nebraska │ 0.152│ .0156│ 8│ 3.27│ 1.18
of Grant│ Kansas │ St. │ │ │ │ │
St. │ St. │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 1.79│.00385│ 12│ 2.28│ 1.79
St. │ of Grant│ of │ │ │ │ │
│ St. │ Meridian│ │ │ │ │
Alley S. │Railroad │Kansas St.│ │ .0120│ 8│ 2.90│ 1.03
of │ │ │ │ │ │ │
Meridian│ │ │ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 0.123│ .0157│ 8│ 3.28│ 1.18
of │ │ St. │ │ │ │ │
Meridian│ │ │ │ │ │ │
Nebraska │Alley S. │Alley S. │ 1.85│ .0042│ 12│ 2.36│ 1.85
St. │ of │ of Smith│ │ │ │ │
│ Meridian│ Av. │ │ │ │ │
Alley S. │Railroad │East of │ │ .0131│ 8│ 3.00│ 1.08
of Smith│ │ Kansas │ │ │ │ │
Ave. │ │ St. │ │ │ │ │
Alley S. │East of │Nebraska │ 0.069│ .0197│ 8│ 3.70│ 1.32
of Smith│ Kansas │ St. │ │ │ │ │
Ave. │ St. │ │ │ │ │ │
Nebraska │Alley S. │S. of │ 1.92│.00213│ 15│ 2.00│ 2.45
St. │ of Smith│ Cordovez│ │ │ │ │
│ Ave. │ St. │ │ │ │ │
S. of │Railroad │Nebraska │ │.00574│ 8│ 2.00│ 0.71
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
S. of │Map margin│Nebraska │ 0.054│.00854│ 8│ 2.46│ 0.87
Cordovez│ │ St. │ │ │ │ │
St. │ │ │ │ │ │ │
Nebraska │S. of │Long St. │ 1.99│.00213│ 15│ 2.00│ 2.45
St. │ Cordovez│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Map margin│Nebraska │ 0.030│ .0134│ 8│ 3.04│ 1.08
│ │ St. │ │ │ │ │
Long St. │Nebraska │N. │ 2.02│.00213│ 15│ 2.00│ 2.45
│ St. │ Carolina│ │ │ │ │
│ │ St. │ │ │ │ │
Long St. │N. │Georgia │ 3.03│ .0016│ 18│ 2.00│ 3.50
│ Carolina│ St. │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Georgia │Harris St.│ 3.04│ .0016│ 18│ 2.00│ 3.50
│ St. │ │ │ │ │ │
Alley S. │End of │Harris St.│ │ .0203│ 8│ 3.78│ 1.35
of Janis│ Janis │ │ │ │ │ │
St. │ St. │ │ │ │ │ │
Harris St.│Alley N. │Long St. │ │ .0088│ 8│ 2.53│ 0.89
│ of Janis│ │ │ │ │ │
│ St. │ │ │ │ │ │
Long St. │Harris St.│Kentucky │ │.00353│ 18│ 2.98│ 5.20
│ │ St. │ │ │ │ │
Long St. │Kentucky │Tennessee │ │.00635│ 18│ 4.00│ 7.00
│ St. │ St. │ │ │ │ │
Tarbell │Harris St.│Long St. │ │ .0146│ 8│ 3.18│ 1.14
Ave. │ │ │ │ │ │ │
Long St. │Tennessee │Alley W. │ 3.13│ .0016│ 18│ 2.00│ 3.50
│ St. │ of Tenn.│ │ │ │ │
│ │ St. │ │ │ │ │
│ │ │ │ │ │ │
Column No.│ (2) │ (3) │ (9) │ (10) │ (11) │ (12) │ (13)
(1) │ │ │ │ │ │ │
──────────┴──────────┴──────────┴──────┴──────┴──────┴────────┴────────

──────────┬──────────┬──────────┬───────────────┬──────
On Street │ From │To Street │ El. of Invert │ Line
│ Street │ │ │Number
│ │ │ │
│ │ │ │
│ │ │ │
──────────┼──────────┼──────────┼───────┬───────┼──────
│ │ │ Upper │ Lower │
│ │ │Manhole│Manhole│
──────────┼──────────┼──────────┼───────┼───────┼──────
Nebraska │Map margin│Alley S. │ 97.80│ 94.40│ 1
St. │ │ Grant │ │ │
│ │ St. │ │ │
Alley S. │Railroad │E. of │ 105.50│ 103.62│ 2
of Grant│ │ Missouri│ │ │
St. │ │ St. │ │ │
Alley S. │E. of │E. of │ 103.61│ 99.70│ 3
of Grant│ Missouri│ Kansas │ │ │
St. │ St. │ St. │ │ │
Alley S. │E. of │Nebraska │ 99.69│ 94.40│ 4
of Grant│ Kansas │ St. │ │ │
St. │ St. │ │ │ │
Nebraska │Alley S. │Alley S. │ 94.07│ 92.61│ 5
St. │ of Grant│ of │ │ │
│ St. │ Meridian│ │ │
Alley S. │Railroad │Kansas St.│ 103.80│ 99.00│ 6
of │ │ │ │ │
Meridian│ │ │ │ │
Alley S. │Kansas St.│Nebraska │ 98.99│ 92.70│ 7
of │ │ St. │ │ │
Meridian│ │ │ │ │
Nebraska │Alley S. │Alley S. │ 92.37│ 91.09│ 8
St. │ of │ of Smith│ │ │
│ Meridian│ Av. │ │ │
Alley S. │Railroad │East of │ 101.30│ 97.30│ 9
of Smith│ │ Kansas │ │ │
Ave. │ │ St. │ │ │
Alley S. │East of │Nebraska │ 97.29│ 91.30│ 10
of Smith│ Kansas │ St. │ │ │
Ave. │ St. │ │ │ │
Nebraska │Alley S. │S. of │ 90.84│ 90.20│ 11
St. │ of Smith│ Cordovez│ │ │
│ Ave. │ St. │ │ │
S. of │Railroad │Nebraska │ 92.80│ 90.62│ 12
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
S. of │Map margin│Nebraska │ 96.60│ 93.10│ 13
Cordovez│ │ St. │ │ │
St. │ │ │ │ │
Nebraska │S. of │Long St. │ 90.04│ 89.87│ 14
St. │ Cordovez│ │ │ │
│ St. │ │ │ │
Long St. │Map margin│Nebraska │ 95.80│ 90.70│ 15
│ │ St. │ │ │
Long St. │Nebraska │N. │ 89.86│ 88.94│ 16
│ St. │ Carolina│ │ │
│ │ St. │ │ │
Long St. │N. │Georgia │ 88.69│ 88.00│ 17
│ Carolina│ St. │ │ │
│ St. │ │ │ │
Long St. │Georgia │Harris St.│ 87.99│ 87.32│ 18
│ St. │ │ │ │
Alley S. │End of │Harris St.│ 97.20│ 90.10│ 19
of Janis│ Janis │ │ │ │
St. │ St. │ │ │ │
Harris St.│Alley N. │Long St. │ 90.09│ 88.90│ 20
│ of Janis│ │ │ │
│ St. │ │ │ │
Long St. │Harris St.│Kentucky │ 87.31│ 86.40│ 21
│ │ St. │ │ │
Long St. │Kentucky │Tennessee │ 86.39│ 84.60│ 22
│ St. │ St. │ │ │
Tarbell │Harris St.│Long St. │ 90.70│ 84.60│ 23
Ave. │ │ │ │ │
Long St. │Tennessee │Alley W. │ 83.77│ 83.47│ 24
│ St. │ of Tenn.│ │ │
│ │ St. │ │ │
│ │ │ │ │
Column No.│ (2) │ (3) │ (14) │ (15) │
(1) │ │ │ │ │
──────────┴──────────┴──────────┴───────┴───────┴──────

=47. Surface Profile.=—A profile of the surface of the ground along the
proposed lines of the sewers should be drawn after the completion of the
computations for quantity. An example of a profile is shown in Fig. 26
for the line between manholes No. 3.5 and No. 147. The vertical scale
should be at least 10 times the horizontal. A horizontal scale of 1 inch
to 200 feet can be used where not much detail is to be shown, but a
scale of one 1 to 100 feet is more common and more satisfactory and even
one inch to 10 feet has been used. The information to be given and the
method of showing it are illustrated on Fig. 26. The profile should show
the character of the material to be passed through and the location of
underground obstacles which may be encountered. The method of obtaining
this information is taken up in Chapter II. The collection of the
information should be completed as far as possible previous to design,
and borings and other investigations made as soon as the tentative
routes for the sewers have been selected.

=48. Slope and Diameter of Sewers.=—After the quantity of sewage to be
carried has been determined, and the profile of the ground surface has
been drawn, it is possible to determine the slope and diameter of the
sewer. A table such as No. 20 is made up somewhat similar to No. 19, or
which may be an extension of Table 19 since the first 6 columns in both
tables are the same. The elevation of the surface at the upper and lower
manholes is read from the profile.

The depth of the sewer below the ground surface is first determined.
Sewers should be sufficiently deep to drain cellars of ordinary depth.
In residential districts cellars are seldom more than 5 feet below the
ground surface. To this depth must be added the drop necessary for the
grade of the house sewer. Six-inch pipe laid on a minimum grade of 1.67
per cent is a common size and slope restriction for house drains or
sewers. An additional 12 inches should be allowed for the bends in the
pipe and the depth of the pipe under the cellar floor. Where the
elevation of the street and lots is about the same, and the street is
not over 80 feet in width between property lines, a minimum depth of 8
feet to the invert of sewers, 24 inches or less in diameter is
satisfactory. This is on the assumption that the axes of the house drain
and the sewer intersect. For larger pipes the depth should be increased
so that when the street sewer is flowing full, sewage will not back up
into the cellars or for any great distance into the tributary pipes.

[Illustration:

FIG. 26.—Typical Profile Used in the Design of a Separate Sewer
System.
]

The grade or slope at which a sewer shall be may be fixed by: the slope
of the ground surface; the minimum permissible self-cleansing velocity;
a combination of diameter, velocity, and quantity; or the maximum
permissible velocity of flow. Sewers are laid either parallel to the
ground surface where the slope is sufficient or where possible without
coming too near the surface they are laid on a flatter grade to avoid
unnecessary excavation. The minimum permissible slope is fixed by the
minimum permissible velocity.

The velocity of flow in a sewer should be sufficient to prevent the
sedimentation of sludge and light mineral matter. Such a velocity is in
the neighborhood of 1 foot per second. Since sewers seldom flow full
this velocity should be available under ordinary conditions of dry
weather flow. The minimum velocity when full should therefore be about 2
feet per second. Under this condition, the velocity of 1 foot per second
is not reached until the sewer is less than 18 per cent full. The
velocity in small sewers should be made somewhat faster than in large
sewers since the velocity of flow for small depths in small pipes is
less than for the same proportionate depth in large pipes. The maximum
permissible velocity of flow is fixed at about 10 feet per second in
order to avoid excessive erosion of the invert. If the sewer is
carefully laid this limit may be exceeded in sanitary sewers.

The method for determining the grade and diameter of sewers is best
explained through an illustrative problem which is worked out in Table
20 for the profile shown on Fig. 26. The figures are inserted in the
table from left to right in each line, one line being completed before
the next one is commenced. The headings in the first 6 columns are
self-explanatory. The elevations of the surface at the upper and lower
manholes are read from the profile. The total flow is read from column
(18) in Table 19. The slope of the ground surface is then computed, and
with the quantity, slope, and coefficient of roughness, the diameter of
the pipe and the velocity of flow are read from Fig. 15.

The following conditions may arise:

(1) The diameter required is less than 8 inches. Use a diameter of
8 inches as experience has shown that the use of smaller diameters
is unsatisfactory.

(2) The velocity of flow when the sewer is full is less than 2
feet per second. Increase the slope until the velocity when full
is 2 feet per second.

(3) The diameter of the pipe required is not one of the commercial
sizes shown in Fig. 15. Use the next largest commercial size.

(4) The slope of the ground surface is steeper than necessary to
maintain the required minimum velocity and the upper end of the
sewer is deeper than the required minimum depth. Place the sewer
on the minimum permissible grade, or upon such a grade that its
lower end will be at the minimum permissible depth.

(5) The slope of the ground surface is so steep as to make the
velocity of flow greater than the maximum rate permissible. Reduce
the grade by deepening the sewer at the upper manhole and using a
drop manhole at this point.

It is not permissible to use a pipe larger than that called for by the
above conditions. This is attempted sometimes in order to reduce the
grade and thereby save excavation, under the rule of a minimum velocity
of 2 feet per second when full. It is better to use the smaller pipe on
the flat grade as the quantity of sewage is insufficient to fill the
larger sewer and the minimum permissible velocity is more quickly
reached.

Having determined the slope, the diameter, and the capacity of the pipe
to be used, these values are entered in the table. The elevations of the
invert of the pipe at the upper and lower manholes are next computed and
entered in the table. This method is followed until all of the
diameters, slopes, and elevations have been determined.

The slopes are computed from center to center of manholes, but an extra
allowance of 0.01 of a foot is allowed by some designers for the
increased loss in head in passing through the manhole. When it becomes
necessary to increase the diameter of the sewer the top of the outgoing
sewer is placed at the same elevation or below the top of the lowest
incoming sewer. No extra allowance is made to compensate for loss in
head in the manhole in this case. This case is illustrated in columns
(14) and (15) in lines (16) and (17) of Table 20. All of the conditions
listed above are illustrated in Table 20, except the condition for a
velocity greater than 10 feet per second.

The first condition is met at the head of practically every lateral, and
is illustrated in the second line.

The second condition is also illustrated in the second line. The slope
of the ground surface is 0.0046, which gives a velocity of only 1.8 feet
per second in an 8–inch pipe. The slope is therefore increased to
0.00575, on which the full velocity is 2 feet per second.

The third condition is met in the first line. The diameter called for to
carry 1.66 cubic feet per second on a slope of 0.0108 is slightly less
than 10 inches. A 10–inch pipe is therefore used and its full capacity
and velocity are recorded.

The fourth condition is illustrated in the fourteenth line. The cut at
manhole No. 3.1 is 11.1 feet. The slope of the ground is 0.014, much
steeper than is necessary to maintain the minimum velocity in a 15–inch
pipe. The pipe is therefore placed on the minimum permissible slope, and
excavation is saved. The student should check the figures in Table 20
and be sure that they are understood before an attempt is made to make a
design independently.

=49. The Sewer Profile.=—The profile is next completed as shown in Fig.
26, the pipe line being drawn in as the computations are made. The cut
is recorded to the nearest ⅒th of a foot at each manhole, or change in
grade. It should not be given elsewhere as it invites controversy with
the contractor. The cut is the difference of the elevation of the invert
of the lowest pipe in the trench at the point in question, and the
surface of the ground.

The stationing should be shown to the nearest ⅒th of a foot. It should
commence at 0 + 00 at the outlet and increase up the sewer. The station
of any point on the sewer may show the distance from it to the outlet,
or a new system of stationing may be commenced at important junctions or
at each junction.

Elevations of the surface of the ground should be shown to the nearest
⅒th of a foot, and the invert elevation to the nearest 1/100th of a
foot.

Only the main line sewer is shown in profile in Fig. 26. The profiles of
the laterals computed in Table 20, have not been shown. The approximate
location of all house inlets are shown on the profile and located
exactly, and are made a matter of record during construction.

DESIGN OF A STORM WATER SEWER SYSTEM

=50. Planning the System.=—Storm sewer systems are seldom as extensive
as separate or combined sewer systems, since storm sewage can be
discharged into the nearest suitable point in a flowing stream or other
drainage channel, whereas dry weather or combined sewage must be
conducted to some point where its discharge will be inoffensive. The
need of a comprehensive general plan of a storm sewer system is quite as
great, however, as for a separate system. The haphazard construction of
sewers at the points most needed for the moment results in the
duplication of forgotten drains, expense in increasing the capacity of
inadequate sewers, and difficult construction due to underground
structures thoughtlessly located. A comprehensive plan permits the
construction of sewers where they are needed as they are required, and
enables all probable future needs to be cared for at a minimum of
expense.

The same preliminary survey, map, and underground information are
necessary for the design of a storm sewer system as for a separate sewer
system. The map shown on Fig. 25 has been used for the design of a
storm-water sewer system.

The steps in the design of a storm-water sewer system are:

1st. Note the most advantageous points to locate the inlets and lay out
the system to drain these inlets. 2nd. Determine the required capacity
of the sewers by a study of the run-off from the different drainage
areas. 3rd. Draw the profile and compute the diameter and slope of the
pipes required.

=51. Location of Street Inlets.=—The location of storm sewers is
determined mainly by the desirable location of the street inlets. The
inlets must therefore be located before the system can be planned. In
general the inlets should be located so that no water will flow across a
street or sidewalk, in order to reach the sewer. This requires that
inlets be placed on the high corners at street intersections, in
depressions between street intersections, and at sufficiently frequent
intervals that the gutters may not be overloaded. City blocks are seldom
so long as to necessitate the location of inlets between crossings
solely on account of inadequate gutter capacity. The capacity of a
gutter can be computed approximately by the application of Kutter’s
formula. Inlet capacities are discussed in Chapter VI. When the area
drained is sufficiently large to tax the capacity of the gutter or
inlet, an inlet should be installed regardless of the location of the
street intersections.

The street inlets are located on the map as shown in Fig. 25. The sewer
lines are then located so as to make the length of pipe to pass near to
all inlets a minimum. Storm sewers are seldom placed near the center of
a street because of the frequent crowded condition on this line.

=52. Drainage Areas.=—The outline of a drainage area is drawn so that
all water falling within the area outlined will enter the same inlet,
and water falling on any point beyond the outline will enter some other
inlet. This requires that the outline follow true drainage lines rather
than the artificial land divisions used in locating the drainage lines
in the design of sanitary sewers. The drainage lines are determined by
pavement slopes, location of downspouts, paved or unpaved yards, grading
of lawns and the many other features of the natural drainage which are
altered by the building up of a city. The location of the drainage lines
is fixed as the result of a study of local conditions.

The watershed or drainage lines are shown on Fig. 25 by means of dot and
dash lines. A drainage line passes down the middle of each street
because the crown of the street throws the water to either side and
directs it to different inlets. A watershed line is drawn about 50 feet
west of such streets as Kentucky St., Florida St., etc., because the
downspouts from the houses on those streets discharge or will discharge
into the street on which they face. The location of any watershed line
within 20 feet more or less is, in most cases, a matter of judgment
rather than exactness. Each area is given an identifying number or mark
which is useful only in design. It usually corresponds to the inlet
number.

=53. Computation of Flood Flow by McMath Formula.=—McMath’s Formula is
used as an example of the method pursued when an empirical formula is
adopted for the computation of run-off, and because of its frequent use
in practice. Other formulas may be more satisfactory under favorable
conditions.

Computations should be kept in order by a tabulation such as is shown in
Table 21, in which the quantity of storm flow discharged from the sewer
at the foot of Tennessee St., on Fig. 25, has been computed by means of
the McMath Formula, using the constants suggested for St. Louis
conditions, _i_ = 2.75, and _c_ = 0.75. The solutions of the formula
have been made by means of Fig. 11. The column headings in the Table are
explanatory of the figures as recorded. The computation should begin at
the upper end of a lateral, proceed to the first junction and then
return to the head of another lateral tributary to this junction. They
should be continued in the same manner until all tributary areas have
been covered. Special computations will be necessary for the
determination of the maximum quantity of storm water entering each inlet
to avoid the flooding of an inlet or gutter. These computations have not
been shown as they are so easily made by the application of McMath’s
Formula to each area concerned.

The determination of the average slope ratio is a matter of judgment,
based on the average natural slope of the surface of the ground and an
estimate of the probable future conditions.

=54. Computation of Flood Flow by Rational Method.=—The rational method
for the computation of storm-water run-off is described in Chapter III.
An example of its application to storm sewer design is given here for
the district shown in Fig. 25.[34] The computations are shown in Table
21. As in the preceding designs the table has been filled in from left
to right and line by line. Computations have started at the upper end of
laterals tributary to each junction. The column headed _I_ represents
the imperviousness factor in the expression _Q_ = _AIR_. It is based on
judgment guided by the constants given in Chapter III concerning
imperviousness. The column headed “Equivalent 100 per cent _I_ acres” is
the product of the two preceding columns. It reduces all areas to the
same terms so that they can be added for entry in the column headed
“Total 100 per cent _I_ acres.” It may be necessary to record the values
for this column on several lines where the imperviousnesses of the
tributary areas are different. This condition is illustrated in the last
line of the table, for the length of sewer nearest the outlet. In the
preceding lines the imperviousness recorded represents an average for
all the tributary areas.

TABLE 21

COMPUTATIONS FOR THE QUANTITY OF STORM SEWAGE AT THE FOOT OF TENNESSEE
STREET ON FIGURE 25

─────────┬──────────┬──────────┬───────────┬─────────────────────────────────
On Street│ From │To Street │Identifying│ By McMath’s Formula
│ Street │ │ Number of │
│ │ │ Acres │
│ │ │ Drained │
─────────┼──────────┼──────────┼───────────┼──────────┬───────┬───────┬──────
│ │ │ │Additional│ Total │ Slope │ Run
│ │ │ │ Acres │ Acres │ of │Off in
│ │ │ │ Drained │Drained│Surface│C.F.S.
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┼──────────┼──────────┼───────────┼──────────┼───────┼───────┼──────
State │N. │S. │ 91 and 92 │2.35 │ 2.35│ 0.005│ 5.5
│ Carolina│ Carolina│ │ │ │ │
State │S. │Georgia │88, 89 and │3.0 │ 5.35│ .005│ 10.8
│ Carolina│ │ 90 │ │ │ │
State │Georgia │Florida │85, 86 and │3.0 │ 8.35│ .007│ 16.5
│ │ │ 87 │ │ │ │
State │Florida │Kentucky │81, 83 and │3.0 │ 11.35│ .009│ 22.0
│ │ │ 84 │ │ │ │
State │Kentucky │Tennessee │79, 80 and │3.0 │ 14.35│ .010│ 28.0
│ │ │ 82 │ │ │ │
State │Texas │Louisiana │ 76 and │3.8 │ 3.8│ .005│ 8.3
│ │ │ others │ │ │ │
State │Louisiana │Alabama │73, 74 and │3.7 │ 7.5│ .007│ 15.0
│ │ │ 75 │ │ │ │
State │Alabama │Tennessee │70, 71 and │3.0 │ 10.5│ .006│ 19.0
│ │ │ 72 │ │ │ │
Tennessee│State │Talon │68, 69, 77 │4.3 │ 29.15│ .15│ 52
│ │ │ and 78 │ │ │ │
Talon │Albemarle │Tennessee │65, 66 and │2.8 │ 2.8│ .018│ 8.4
│ │ │ 67 │ │ │ │
Tennessee│Talon │Burnside │ 64 and │0.7 │ 29.85│ .15│ 55
│ │ │ 64_a_ │ │ │ │
Burnside │N. │S. │57, 58 and │2.84 │ 2.84│ .008│ 7.2
│ Carolina│ Carolina│ 59 │ │ │ │
Burnside │S. │Georgia │54, 55 and │3.88 │ 6.72│ .010│ 14.9
│ Carolina│ │ 56 │ │ │ │
Burnside │Georgia │Florida │50, 52 and │3.88 │ 10.60│ .012│ 22
│ │ │ 53 │ │ │ │
Burnside │Florida │Kentucky │47, 48 and │3.88 │ 14.48│ .013│ 30
│ │ │ 51 │ │ │ │
Burnside │Kentucky │Tennessee │44, 45 and │3.88 │ 18.36│ .013│ 36
│ │ │ 46 │ │ │ │
Tennessee│Burnside │Elm │ 42 and 43 │2.84 │ 51.05│ .015│ 82
Elm │Above │Chetwood │ Included in next line below │ │
│ Chetwood│ │ │ │
Elm │Chetwood │Albemarle │31, 32 and │2.75 │ 2.75│ .007│ 7.0
│ │ │ 33 │ │ │ │
Elm │Albemarle │Tennessee │27, 28, 29 │5.75 │ 8.50│ .016│ 20
│ │ │ and 30 │ │ │ │
Tennessee│Elm │Varennes │25, 26 and │2.62 │ 62.17│ .017│ 100
│ │ │ 41 │ │ │ │
Varennes │S. │Georgia │17, 18 and │3.17 │ 3.17│ .010│ 8.3
│ Carolina│ │ 19 │ │ │ │
Varennes │Georgia │Florida │14, 15 and │3.17 │ 6.34│ .011│ 14.5
│ │ │ 16 │ │ │ │
Varennes │Florida │Kentucky │11, 12 and │3.17 │ 9.51│ .013│ 21
│ │ │ 13 │ │ │ │
Varennes │Kentucky │Tennessee │8, 9 and 10│3.17 │ 12.68│ .013│ 26
Tennessee│Varennes │Boulevard │ 6 and 7 │2.32 │ 77.17│ .017│ 120
Tennessee│Boulevard │Outlet │1, 2, 3, 4,│4.72 │ 81.89│ .017│ 122
│ │ │ and 5 │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┴──────────┴──────────┴───────────┴──────────┴───────┴───────┴──────

─────────┬──────────┬──────────┬───────────────────────────────────────────
On Street│ From │To Street │ By Rational Method
│ Street │ │
│ │ │
│ │ │
─────────┼──────────┼──────────┼─────┬─────┬──────────┬─────┬──────────────
│ │ │Area,│ _I_ │Equivalent│Total│ Time of
│ │ │Acres│ │ 100 Per │ 100 │Concentration,
│ │ │ │ │ Cent _I_ │ Per │ Minutes
│ │ │ │ │ Acres │Cent │
│ │ │ │ │ │ _I_ │
│ │ │ │ │ │Acres│
─────────┼──────────┼──────────┼─────┼─────┼──────────┼─────┼──────────────
State │N. │S. │ 2.35│ 0.50│ 1.17│ 1.17│ 7.0
│ Carolina│ Carolina│ │ │ │ │
State │S. │Georgia │ 3.00│ .50│ 1.50│ 2.67│ 8.1
│ Carolina│ │ │ │ │ │
State │Georgia │Florida │ 3.00│ .50│ 1.50│ 4.17│ 9.0
│ │ │ │ │ │ │
State │Florida │Kentucky │ 3.00│ .50│ 1.50│ 5.67│ 9.9
│ │ │ │ │ │ │
State │Kentucky │Tennessee │ 3.00│ .50│ 1.50│ 7.17│ 10.7
│ │ │ │ │ │ │
State │Texas │Louisiana │ 3.80│ .35│ 1.33│ 1.33│ 10.0
│ │ │ │ │ │ │
State │Louisiana │Alabama │ 3.70│ .40│ 1.48│ 2.81│ 11.9
│ │ │ │ │ │ │
State │Alabama │Tennessee │ 3.00│ .45│ 1.35│ 4.16│ 12.9
│ │ │ │ │ │ │
Tennessee│State │Talon │ 4.30│ .50│ 2.15│13.48│ 14.5
│ │ │ │ │ │ │
Talon │Albemarle │Tennessee │ 2.80│ .40│ 1.12│ 1.12│ 8.0
│ │ │ │ │ │ │
Tennessee│Talon │Burnside │ 0.70│ .20│ 0.14│14.74│ 15.3
│ │ │ │ │ │ │
Burnside │N. │S. │ 2.84│ .55│ 1.56│ 1.56│ 10.0
│ Carolina│ Carolina│ │ │ │ │
Burnside │S. │Georgia │ 3.88│ .55│ 2.13│ 3.69│ 11.1
│ Carolina│ │ │ │ │ │
Burnside │Georgia │Florida │ 3.88│ .55│ 2.13│ 5.82│ 12.2
│ │ │ │ │ │ │
Burnside │Florida │Kentucky │ 3.88│ .55│ 2.13│ 7.95│ 13.1
│ │ │ │ │ │ │
Burnside │Kentucky │Tennessee │ 3.88│ .55│ 2.13│10.08│ 13.8
│ │ │ │ │ │ │
Tennessee│Burnside │Elm │ 2.84│ .45│ 2.28│26.10│ 15.7
Elm │Above │Chetwood │ │ │ │ │
│ Chetwood│ │ │ │ │ │
Elm │Chetwood │Albemarle │ 2.75│ .40│ 1.10│ 1.10│ 8.0
│ │ │ │ │ │ │
Elm │Albemarle │Tennessee │ 5.75│ .45│ 2.59│ 3.69│ 9.5
│ │ │ │ │ │ │
Tennessee│Elm │Varennes │ 2.62│ .50│ 1.31│30.00│ 16.2
│ │ │ │ │ │ │
Varennes │S. │Georgia │ 3.17│ .55│ 1.74│ 1.74│ 9.0
│ Carolina│ │ │ │ │ │
Varennes │Georgia │Florida │ 3.17│ .55│ 1.74│ 3.48│ 9.9
│ │ │ │ │ │ │
Varennes │Florida │Kentucky │ 3.17│ .55│ 1.74│ 5.22│ 10.8
│ │ │ │ │ │ │
Varennes │Kentucky │Tennessee │ 3.17│ .55│ 1.74│ 6.96│ 11.4
Tennessee│Varennes │Boulevard │ 2.32│ .55│ 1.28│32.84│ 16.5
Tennessee│Boulevard │Outlet │ 0.18│ .80│ 0.14│ Area No. 1
│ │ │ │ │ │
│ │ │ 1.38│ .50│ 0.69│ Area No. 2
│ │ │ 2.80│ .55│ 1.54│ Areas No. 3 and 4
│ │ │ 0.36│ .75│ 0.27│35.48│ 16.9
│ │ │ │ │ │ │
─────────┴──────────┴──────────┴─────┴─────┴──────────┴─────┴──────────────

─────────┬──────────┬──────────┬─────────────────────────────────┬──────
On Street│ From │To Street │ By Rational Method │ Line
│ Street │ │ │Number
│ │ │ │
│ │ │ │
─────────┼──────────┼──────────┼───┬────┬─────┬────┬───────┬─────┼──────
│ │ │_R_│_Q_ │ _S_ │_V_ │ Sewer │Time │
│ │ │ │ │ │ │Length,│ in │
│ │ │ │ │ │ │ Feet │Sewer│
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │
─────────┼──────────┼──────────┼───┼────┼─────┼────┼───────┼─────┼──────
State │N. │S. │4.8│ 5.6│0.011│ 4.6│ 300│ 1.1│ 1
│ Carolina│ Carolina│ │ │ │ │ │ │
State │S. │Georgia │4.6│12.2│ .010│ 5.5│ 300│ 0.9│ 2
│ Carolina│ │ │ │ │ │ │ │
State │Georgia │Florida │4.4│18.3│ .012│ 5.8│ 300│ 0.9│ 3
│ │ │ │ │ │ │ │ │
State │Florida │Kentucky │4.2│23.9│ .009│ 6.0│ 300│ 0.8│ 4
│ │ │ │ │ │ │ │ │
State │Kentucky │Tennessee │4.1│29.3│ .009│ 6.2│ 300│ 0.8│ 5
│ │ │ │ │ │ │ │ │
State │Texas │Louisiana │4.2│ 5.6│ .009│ 3.2│ 370│ 1.9│ 6
│ │ │ │ │ │ │ │ │
State │Louisiana │Alabama │3.9│11.0│ .011│ 5.2│ 300│ 1.0│ 7
│ │ │ │ │ │ │ │ │
State │Alabama │Tennessee │3.8│15.8│ .002│ 3.2│ 300│ 1.6│ 8
│ │ │ │ │ │ │ │ │
Tennessee│State │Talon │3.6│48.5│ .019│ 9.8│ 450│ 0.8│ 9
│ │ │ │ │ │ │ │ │
Talon │Albemarle │Tennessee │4.6│ 5.2│ .004│ 3.0│ 210│ 1.2│ 10
│ │ │ │ │ │ │ │ │
Tennessee│Talon │Burnside │3.5│51.5│ .006│ 5.0│ 120│ 0.4│ 11
│ │ │ │ │ │ │ │ │
Burnside │N. │S. │4.2│ 6.5│ .008│ 4.5│ 300│ 1.1│ 12
│ Carolina│ Carolina│ │ │ │ │ │ │
Burnside │S. │Georgia │4.0│14.8│ .007│ 4.7│ 300│ 1.1│ 13
│ Carolina│ │ │ │ │ │ │ │
Burnside │Georgia │Florida │3.9│22.7│ .011│ 5.8│ 300│ 0.9│ 14
│ │ │ │ │ │ │ │ │
Burnside │Florida │Kentucky │3.7│29.4│ .016│ 7.5│ 300│ 0.7│ 15
│ │ │ │ │ │ │ │ │
Burnside │Kentucky │Tennessee │3.7│37.3│ .019│ 9.2│ 300│ 0.5│ 16
│ │ │ │ │ │ │ │ │
Tennessee│Burnside │Elm │3.4│88.8│ .015│10.2│ 280.│ 0.5│ 17
Elm │Above │Chetwood │ │ │ │ │ │ │ 18
│ Chetwood│ │ │ │ │ │ │ │
Elm │Chetwood │Albemarle │4.6│ 5.1│ .020│ 5.3│ 480│ 1.5│ 19
│ │ │ │ │ │ │ │ │
Elm │Albemarle │Tennessee │4.3│15.8│ .012│ 6.1│ 410│ 1.1│ 20
│ │ │ │ │ │ │ │ │
Tennessee│Elm │Varennes │3.4│ 102│ .012│10.2│ 180│ 0.3│ 21
│ │ │ │ │ │ │ │ │
Varennes │S. │Georgia │4.4│ 7.7│ .012│ 5.2│ 270│ 0.9│ 22
│ Carolina│ │ │ │ │ │ │ │
Varennes │Georgia │Florida │4.2│14.5│ .010│ 5.7│ 300│ 0.9│ 23
│ │ │ │ │ │ │ │ │
Varennes │Florida │Kentucky │4.1│21.4│ .017│ 7.7│ 300│ 0.6│ 24
│ │ │ │ │ │ │ │ │
Varennes │Kentucky │Tennessee │4.0│27.8│ .015│ 7.8│ 300│ 0.6│ 25
Tennessee│Varennes │Boulevard │3.3│ 108│ .012│10.2│ 230│ 0.4│ 26
Tennessee│Boulevard │Outlet │ │ │ │ │ │ │ 27
│ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │ 28
│ │ │ │ │ │ │ │ 29
│ │ │3.3│ 117│Areas No. 1–5 inclusive │ 30
│ │ │ │ │ │ │ │ │
─────────┴──────────┴──────────┴───┴────┴─────┴────┴───────┴─────┴──────

The time of concentration in minutes is assumed by judgment for the
first area. For all subsequent areas it is the sum of the time of
concentration for the area or areas tributary to the inlet next above
and the time of flow in the sewer from the inlet next above to the inlet
in question. For example, in line 2 the time 8.1 minutes is the sum of
7.0 minutes time of concentration to the inlet at the corner of State
and North Carolina St., and the time of flow of 1.1 minute in the sewer
on State St. from North Carolina St. to South Carolina St. Where two
sewers are converging as at the corner of Varennes Road and Tennessee
St. the longest time is taken. For example, the time of concentration
down Varennes Road to Tennessee St. is shown in line 25 as 11.4 + 0.6 =
12.0 minutes. The time to the same point down Tennessee St. is shown in
line 21 as 16.2 + 0.3 = 16.5 minutes. This time is therefore used in
line 26.

_R_, the rate of rainfall in inches per hour is determined by Talbot’s
formula.

_Q_, is in cubic feet per second and is the product of the 8th and 10th
columns. Since the 8th column is the sum of the products of the 5th and
the 6th columns for the lines representing tributary areas, then the
11th column is the product of _A_, _I_, and _R_.

_S_, is the slope on which it is assumed that the sewer will be laid. It
is usually assumed as parallel to the ground surface unless the velocity
for this slope becomes less than 2 feet per second. In such a case the
slope is taken as one which will cause this velocity.

_V_, the velocity in feet per second, is computed from diagrams for the
solution of Kutter’s formula. The length in feet is scaled from the map
as the distance between inlets or groups of inlets, and the time is the
length in feet divided by the velocity in feet per minute.

Having computed the quantity of flow to be carried in the sewer, the
design is completed by drawing the profile and computing the diameters
and slopes by the same method as used in the design of separate sewers.

CHAPTER VI
APPURTENANCES

=55. General.=—The appurtenances to a sewerage system are those devices
which, in addition to the pipes and conduits, are essential to or are of
assistance in the operation of the system. Under this heading are
included such structures and devices as: manholes, lampholes,
flush-tanks, catch-basins, street inlets, regulators, siphons,
junctions, outlets, grease traps, foundations and underdrains.

=56. Manholes.=—A manhole is an opening constructed in a sewer, of
sufficient size to permit a man to gain access to the sewer. Manholes
are the most common appurtenances to sewerage systems and are used to
permit inspection and the removal of obstructions from the pipes. The
details of the Baltimore standard manholes are shown in Fig. 27 and a
manhole on a large sewer in Omaha is shown in Fig. 28. The features of
these designs which should be noted are the size of the opening and
working space, and the strength of the structure. Manhole openings are
seldom made less than 20 inches in diameter and openings 24 inches in
diameter are preferable. A man can pass through any opening that he can
get his hips through provided he can bend his knees and twist his
shoulders immediately on passing the hole. For this reason the manhole
should widen out rapidly immediately below the opening, as shown in Fig.
27 and 38.

The walls of the manhole may be built either of brick or of concrete.
Brick is more commonly used, as the forms necessary for concrete make
the work more expensive unless they can be used a number of times. The
walls of the manhole should be at least 8 inches thick. Greater
thicknesses are used in treacherous soils and for deep manholes, or to
exclude moisture. A rough expression for the thickness of the walls of a
brick manhole more than 12 feet deep in ordinary firm material is _t_ =
_d_⁄2 + 2, in which _t_ is the thickness in inches and _d_ is the depth
in feet. The thickness of brick walls may be changed every 5 to 10 feet
or so. Concrete walls may be built thinner than brick walls.

[Illustration:

FIG. 27.—Baltimore Standard Manhole Details.
]

The bottoms of brick manholes are frequently made of concrete as shown
in Fig. 27. The floor slopes towards the center and is constructed so
that the sewage flows in a half round or U-shaped channel of greater
capacity than the tributary sewers. The sides of the channel should be
high enough to prevent the overflow of sewage onto the sloping floor,
which should have a pitch of about one vertical to 10 or 12 horizontal.
In manholes where two or more sewers join at approximately the same
level the channels in the bottom should join with smooth easy curves.
Where the inlet and outlet pipes are not of the same diameter the tops
of the pipes should ordinarily be placed at the same elevation to
prevent back flow in the smaller pipes when the larger pipes are flowing
full.

The dimensions of the manhole should not be less than 3 feet wide by 4
feet long for a height of at least 4 feet, when built in the form of an
ellipse, or 4 feet in diameter when built circular. No standard method
for the reduction of the diameter of the manhole near the top is
observed, the rate being more or less dependent on the depth of the
manhole. The use of sloping sides above the frost line is desirable as
such a form is more resistant to heaving by frost action.

For sewers up to 48 inches in diameter the manhole is usually centered
over the intersection of the pipes and has a special foundation. For
larger sewers the manhole walls spring from the walls of the sewer as
shown in Fig. 28.

[Illustration:

FIG. 28.—Details of a Manhole and a Well Hole.
]

In the case of a decided drop in the elevation of a sewer, or of a
tributary sewer appreciably higher than an outlet in any manhole, the
sewage is allowed to drop vertically at the manhole, hence the name drop
manhole. The Baltimore standard drop manhole is shown in Fig. 27. A well
hole is an unusually deep drop manhole in which the force of the
vertical drop of sewage is broken by a series of baffle plates, or by a
sump at the bottom of the well hole. Fig. 28 shows a well hole at St.
Paul, Minn. The use of drop manholes can be avoided in large sewers by
the construction of a flight of steps or flight sewer as shown in Fig.
29, which allows the use of a steep grade and serves to break the
velocity of the sewage.

The specifications of the Sanitary District of Chicago, covering the
construction of manhole covers and frames are:

All castings shall be of tough, close grained, gray iron, free
from blow holes, shrinkage and cold shuts, and sound, smooth,
clean and free from blisters and all defects.

All castings shall be made accurately to dimensions to be
furnished and shall be planed where marked or where otherwise
necessary to secure perfectly flat and true surfaces. Allowance
shall be made in the patterns so that the specified thickness
shall not be reduced.

All castings shall be thoroughly cleaned and painted before
rusting begins and before leaving the shop with two coats of high
grade asphaltum or any other varnish that the Engineer may direct.
After the castings have been placed in a satisfactory manner, all
foreign adhering substances shall be removed and the castings
given one additional coat of asphaltum. No castings shall be
accepted the weight of which shall be less than that due to its
dimensions by more than 5 per cent.

[Illustration:

FIG. 29.—Flight Sewer at Baltimore.

Eng. Record, Vol. 59, p. 161.
]

[Illustration:

FIG. 30.—Baltimore Standard Manhole Frame and Cover.
]

The weights of frames and covers in use vary from 200 to 600 pounds, the
weight of the frame being about 5 times that of the cover. The lightest
weights are used where no traffic other than an occasional pedestrian
will pass over the manhole. Frames and covers weighing about 400 pounds
are commonly used on residential streets, whereas 600 pound frames and
covers are desirable in streets on which the traffic is heavy. The
frames should be so designed that the pavement will rest firmly against
it and wear at the same rate as the surrounding street surface.
Experience has shown that vertical sides should be used for the outside
of the frame to approach this condition, and that the frame should not
be less than 8 inches high. The cover should be roughened in some
desirable pattern as shown in Fig. 30. Smooth covers become dangerously
slippery. Where the ventilation of the sewers is not satisfactory the
manhole covers are sometimes perforated. This is undesirable from other
points of view as the rising odors and vapors are obnoxious at the
surface and the entering dirt and water are detrimental to the operation
of the sewer. The stealing and destruction of manhole covers and the
unauthorized entering of sewers has occasionally required the locking of
the covers to the frame when in place. The locks commonly used consist
of a tumbler which falls into place when the manhole is closed, and
which can be opened only by a special wrench or hook. Adjustable frames
are sometimes used where the street grade is settling, or may be raised
in order that the elevation of the top of the cover may be made to
conform to that of the street surface, without reconstructing the top of
the manhole. One type of adjustable cover is shown in Fig. 31. Manhole
covers should be so marked that the sanitary sewer can be distinguished
from the storm-water sewer, and both from the telephone conduit, etc.

[Illustration:

FIG. 31.—Adjustable Manhole Frame and Cover.
]

Iron steps are set into the walls of the manhole about 15 inches apart
vertically to allow entrance and exit to and from the manhole.
Galvanized iron is preferable to unprotected metal as the action of rust
is particularly rapid in the moist air of the sewer. One type of these
manhole steps is shown in Fig. 27. The steps should have a firm grip in
the wall as a loose step is a source of danger.

[Illustration:

FIG. 32.—Baltimore Standard Lamphole.
]

=57. Lampholes.=—A lamphole is an opening from the surface of the ground
into a sewer, large enough to permit the lowering of a lantern into the
sewer. Lampholes are used in the place of manholes to permit the
inspection or the flushing of sewers, and to avoid the expense of a
manhole. They are located from 300 to 400 feet from the nearest manhole
in such a manner that a lamp lowered in the lamp hole can be seen from
the two nearest manholes.

Lampholes should be constructed of 8– to 12–inch tile or cast-iron pipe.
The lower section should be a cast-iron T on a firm foundation, but if
constructed of tile it should be reinforced with concrete to take up the
weight of the shaft. The details of the Baltimore standard lamphole are
shown in Fig. 32. Lampholes are not commonly used on sewerage systems on
account of their lack of real usefulness and the troubles encountered by
breaking of the pipe below the shaft.

=58. Street Inlets.=—A street inlet is an opening in the gutter through
which storm water gains access to the sewer. The types used in different
cities vary widely. Details of an inlet in successful use are shown in
Fig. 33. The figure shows also a common form of connection to the sewer.
A water-seal trap is sometimes used to prevent the escape of odors from
the sewer. Care must be taken in design that such traps do not freeze in
winter nor dry out in summer, although it is not always possible to
prevent these contingencies.

[Illustration:

FIG. 33.—Details of an Untrapped Street Inlet, without Catch-Basin.
]

The important features to be observed in the design of a street inlet
are: height and length of opening, character of grating, and location.
The general location of inlets is discussed in Chapter V. The clear
height of opening commonly used is from 5 to 6 inches, with a clear
length of 24 to 30 inches or longer. Inlets of this size have given
satisfaction on paved streets with moderate slopes, where the drainage
area is not greater than 10,000 to 12,000 square feet of pavement. W. W.
Horner states:[35]

The St. Louis type of inlet “old” style was a vertical opening in
the curb 8 inches high and 4 feet in length with a horizontal bar
making the net opening about 5 inches. It has a broad sill
extending under the sidewalk. The “new” style inlet is 4½ feet
long with a clear opening of 6 inches and no bar. The sill is done
away with and the opening drops down directly from the curb. Tests
were made of the capacity of this inlet on pavements on different
slopes with sumps of depths varying from 0 to 6 inches in front of
the inlet, extending out 3 feet from the gutter, and returning to
the elevation of the gutter at a slope of 3 inches to the foot.
The results of these tests are shown in Table 22. The capacity of
the inlet is expressed as the amount of water entering just before
some water begins to lap past. If a large amount of water is
allowed to flow past much more water will enter the inlet thus
furnishing a factor of safety for large storms. It was noted that
by beginning the rise in the pavement about opposite the middle of
the inlet the capacity of the inlet was increased from 20 to 50
per cent.

TABLE 22

CAPACITIES OF ST. LOUIS STREET INLETS

From tests by W. W. Horner. Cubic feet per second
─────────┬───────────────────┬───────────────────
Slope in │ 0.42 │ 1.5
Per Ct. │ │
─────────┼────┬────┬────┬────┼────┬────┬────┬────
Depth of │ 0.0│ 2│ 4│ 6│ 0│ 2│ 4│ 6
Sump, │ │ │ │ │ │ │ │
Inches │ │ │ │ │ │ │ │
Capacity,│ │ │1.27│ │ │ │ │
old │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
Capacity,│ 0.1│ 0.5│ 1.5│ 2.5│0.08│ 0.4│ 1.1│ 2.1
new │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
─────────┴────┴────┴────┴────┴────┴────┴────┴────

─────────┬───────────────────┬───────────────────
Slope in │ 2.85 │ 4.5
Per Ct. │ │
─────────┼────┬────┬────┬────┼────┬────┬────┬────
Depth of │ 0│ 2│ 4│ 6│ 0│ 2│ 4│ 6
Sump, │ │ │ │ │ │ │ │
Inches │ │ │ │ │ │ │ │
Capacity,│0.03│0.25│0.78│1.49│ │ │ │
old │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
Capacity,│0.03│0.28│0.87│1.62│0.02│0.15│0.45│ 1.0
new │ │ │ │ │ │ │ │
style │ │ │ │ │ │ │ │
─────────┴────┴────┴────┴────┴────┴────┴────┴────

Gratings with horizontal bars will admit more water than gratings with
vertical bars, but they will also admit more rubbish such as sticks,
papers, leaves, etc., which serve to clog the sewers. Vertical barred
gratings and gratings in the bottom of the gutter clog more quickly than
other types. In the selection of the type of grating to be used a
decision must be made as to whether it is more desirable to clean the
sewer or catch-basin, or to flood the street as a result of clogged
inlets. Where catch-basins are used or the sewers are large, horizontal
bars are more satisfactory. The openings between bars should be small
enough to prevent the entrance of a horse’s hoof or objects of
sufficient size to clog the sewer. Four inches in the clear for vertical
openings and 6 inches for horizontal openings are reasonable sizes.

The location of the inlets at the intersection of the two curb lines at
a corner results in a lower first cost but on heavily traveled streets
this may result in a higher maintenance cost than for other locations
because of the concentration of traffic at street corners, hammering the
inlet casting out of shape or position. Vehicles making short turns will
tend to climb the curb and will intensify the wear upon the inlet. These
objections can be overcome by the use of two inlets at each corner, set
back far enough from the curb intersection to avoid interference with
the cross-walks. This also makes it possible to raise the cross-walks
without the use of gutters under them.

The size of the pipe from the inlet to the catch-basin or sewer should
be large enough to care for all of the water which may enter the inlet.
As the capacity of the inlet is seldom known with accuracy and the
capacity of the pipe is difficult of determination, it has become
customary to use a 10–inch or a 12–inch connecting pipe for each
ordinary independent inlet.

=59. Catch-basins.=—Catch-basins are used to interrupt the velocity of
sewage before entering the sewer, causing the deposition of suspended
grit and sludge and the detention of floating rubbish which might enter
and clog the sewer. A separate catch-basin may be used for each inlet,
or to save expense, the pipes from several inlets may discharge into one
catch-basin.

[Illustration:

FIG. 34.—Catch-basin.

Outlets are not always trapped.
]

The types in successful use are extremely varied, but the distinguishing
feature of all is an outlet located above the floor of the basin. A
common form of catch-basin is shown in Fig. 34. It is constructed
similar to a manhole with a diameter of about 4 or 4½ feet and a depth
of retained water from 3 to 4 feet. Catch-basins of this size will care
for the sewage from the inlets at the four corners of a street
intersection, each draining a city block. In unusual situations it may
be necessary to install a larger basin, but too large a catch-basin is
less desirable than one which is too small, as the former stinks and the
latter is useless. Traps are sometimes used to prevent the escape of
odors from the sewer into the street, but odors are often created in the
catch-basins themselves. Some engineers arrange the trap so that it can
be opened for observation down the sewer as in Fig. 34, thus combining
the advantages of a manhole with the catch-basin.

The use of catch-basins is objectionable because: they furnish a
breeding place for mosquitoes and other flying insects; the septic
action in them produces offensive odors; if on a combined sewer they
permit the escape of offensive odors in dry weather when the water seal
in the trap has evaporated; and the freezing of the water seal in the
trap prevents the entrance of water to the sewer. The sole advantage
lies in the prevention of the clogging of the sewers, but this may be
sufficient to overbalance all of the disadvantages. In general
catch-basins should be provided on paved streets which are cleaned by
flushing the material into the sewers, or where the drainage is from an
unimproved or macadamized street, sandy country, or into sewers in which
the velocity of flow is less than 2 feet per second.

[Illustration:

FIG. 35.—Diagrammatic Section through a Grease Trap.
]

=60. Grease Traps.=—The presence of grease in sewers results in the
formation of incrustations which are difficult to remove and which cause
a material loss in the capacity of the sewer. The presence of oil and
gasoline has resulted in violent and destructive explosions as is
described in Chapter XII. A type of grease trap used on the drains from
hotels, restaurants, or other large grease producing industries is shown
in Fig. 35. The trap is similar to a catch-basin except that it is too
small for a man to enter, and the outlet is necessarily trapped in order
to prevent the escape of grease. The details of a gasoline and oil
separator approved by the New York City Fire Department are shown in
Fig. 36.[36]

[Illustration:

FIG. 36.—Gasoline and Oil Separator.
]

=61. Flush-Tanks.=—These are devices to hold water used in flushing
sewers. They are required only on sanitary and combined sewers. Their
use tends to prevent the clogging of sewers laid on flat grades and
permits flatter grades in construction than could otherwise be adopted.
Flush-tanks may be operated either by hand or automatically. Automatic
operation is more common than hand operation. The hand-operated tanks
are similar to manholes so arranged that the inlet and outlet sewers can
be plugged while the manhole or tank is being filled with water either
from a hose or a special service connection. When sufficient water has
been accumulated the outlet is opened and the sewer is flushed by the
rush of water. A sluice gate, flap valve, or a specially fitted board is
sufficient to fit over the mouth of the inlet and outlet during the
filling of the tank. Such an arrangement has the advantage of being
cheap, simple, and satisfactory, though somewhat crude. In some cases
water is run into the tank at the same rate that it is discharged
through the open outlet, maintaining a depth of 4 or 5 feet in the tank
until the water passing the manhole below runs clean. The volume of
water required by this method is large. Flushing water under a
relatively high head is sometimes obtained by the use of tank wagons
which are quickly emptied into the sewer through a canvas pipe dropped
down a manhole. In all such cases if not well constructed the manhole is
subject to caving due to the rush of water around the outlet.
Precautions should be taken to minimize this danger by limiting the
depth of water which may be accumulated. This can be done by
constructing an overflow at a height of 4 or 5 feet above the bottom of
the manhole, discharging into the sewer through an outside drain.

Automatic flush-tanks are constructed similar to a manhole, but special
care should be taken to make them water-tight. The apparatus for
providing the automatic discharge may operate either with or without
moving parts, the latter being preferable as they require less attention
and are not so liable to get out of order. An automatic flush-tank of
the Miller type is shown in Fig. 37. It is a patented device
manufactured by the Pacific Flush Tank Company. The small pipe at the
left is a service connection to the water main. Water is allowed to flow
continuously into the tank at such a rate as to fill it in the required
interval between discharges. The tanks are discharged as nearly once a
day as it is practicable to regulate them. The rate of flow into the
tank is determined by trial and varies to some extent with the water
pressure. The regulator shown in the figure is desirable as the
continuous flow through the ordinary cock soon wears it away. Some
waters will cause deposits to form in the small passages of the cocks or
regulators, thus cutting off the flow.

[Illustration:

FIG. 37.—Automatic Flush-Tank.

Pacific Flush Tank Co.
]

The tank operates as follows: when the water rising in the tank reaches
the bottom of the bell, air is trapped in the bell and prevented from
escaping through the main trap by the water at _A_. As the water
continues to rise in the tank the air in the bell is compressed, the
water level at _A_ is driven down and water trickles from the siphon at
_C_. The height of the water in the tank above the level of the water in
the bell is equal at all times to the height of _C_ above the lowering
position of _A_. When _A_ reaches the position of _B_ a small amount of
air is released through the short leg of the trap and a corresponding
volume of water enters the bell. The head of water above the bell then
becomes greater than the head of water in the short leg of the trap,
which results in the discharge of all of the air in the long leg of the
trap and the rapid discharge of the water in the tank through the
siphon. The discharge is continued until the siphonic action is broken
by the admission of air when the water level in the tank is lowered to
the bottom of the bell. The size of the siphons is fixed by the diameter
of the leg of the siphon. Table 23 shows the capacity and size of sewers
for which the different sizes of siphons are recommended by the
manufacturers.[37]

TABLE 23

SIZES OF SIPHONS TO BE USED WITH AUTOMATIC FLUSH-TANKS

──────────┬───────────┬───────────┬───────────┬───────────┬────────────
│ │ │ │ │ Height of
│Diameter of│ Total │ │ │ the
│Tank at the│ Discharge │ Average │ │ Discharge
Diameter │ Discharge │ for One │ Rate of │Diameter of│ Line above
of Siphon │ Line in │ Flush in │ Discharge │ Sewer in │the Edge of
in Inches │ Feet │ Gallons │in Sec.-ft.│ Inches │ the Bell
──────────┼───────────┼───────────┼───────────┼───────────┼────────────
4│ 3│ 60│ 0.35│ 4 to 6│1 ft. 2 in.
5│ 3│ 100│ 0.73│ 6 to 8│1 ft. 11 in.
6│ 4│ 240│ 1.06│ 8 to 10│2 ft. 6 in.
8│ 4│ 280│ 2.12│ 12 to 15│2 ft. 11 in.
──────────┴───────────┴───────────┴───────────┴───────────┴────────────

When flush-tanks are placed at the upper end of laterals provision
should be made for inspecting and cleaning the sewer by the construction
of a separate manhole, or by combining the features of a manhole and a
flush-tank in the same structure. Such a combination is shown in Fig. 38
from a design by Alexander Potter.

Except under unusual conditions flush-tanks are used only on separate
sewers. They should be placed at the upper end of laterals in which the
velocity of flow when full is less than 2 to 4 feet per second. The
capacity of the tank or the volume of the dose is dependent on the
diameter and slope of the sewer. The most effective flush is obtained by
a volume of water traveling at a high velocity and completely filling
the sewer. A large volume allowed to run slowly through the sewer will
have but little if any flushing action. Data on the quantity of flushing
water needed are given in Table 24.[38] As the result of a series of
experiments conducted by Prof. H. N. Ogden on the flushing of
sewers,[39] the conclusion was reached that the effect of a flush of
about 300 gallons in an 8–inch sewer on a grade less than 1 per cent
would not be effective beyond 800 to 1,000 feet, but that on steeper
grades much smaller quantities of water would produce equally good
results.

[Illustration:

FIG. 38.—Automatic Flush-Tank and Manhole.

Miller-Potter Design. Pacific Flush Tank Co.
]

TABLE 24

GALLONS OF WATER NEEDED FOR FLUSHING SEWERS

─────────────────┬─────────────────────────────────────────────────────
Slope │ Diameter of Sewer in Inches
─────────────────┼─────────────────┬─────────────────┬─────────────────
│ 8│ 10│ 12
─────────────────┼─────────────────┼─────────────────┼─────────────────
0.005 │ 80│ 90│ 100
.0075 │ 55│ 65│ 80
.01 │ 45│ 55│ 70
.02 │ 20│ 30│ 35
.03 │ 15│ 20│ 24
─────────────────┴─────────────────┴─────────────────┴─────────────────

Engineers do not agree upon the advisability of the use of automatic
flush-tanks, some believing that they are a needless expense that can be
avoided by hand flushing, and others feeling that a flush-tank should be
placed at the upper end of every lateral. These diverse opinions are the
result of different experiences in different cities.

=62. Siphons.=—There are two forms of siphons used in sewerage practice,
a true siphon and an inverted siphon. A true siphon is a bent tube
through which liquid will flow at a pressure less than atmospheric,
first upwards and then downwards, entering and leaving at atmospheric
pressure. An inverted siphon is a bent tube through which liquid will
flow at a pressure greater than atmospheric first downwards and then
upwards, entering and leaving at atmospheric pressure.

In sewerage practice the word siphon refers to an inverted siphon unless
otherwise qualified. Siphons, both true and inverted, are used in
sewerage systems to pass above or below obstacles. True siphons are
seldom used as they must be kept constantly filled with liquid.[40]
Accumulated gas must be removed in order to prevent the breaking of the
siphon which results in the cessation of flow. By the breaking of a true
siphon is meant the stoppage of siphonic action due to the accumulation
of air or gas at the peak of the siphon. Since the rate of flow of
sewage fluctuates widely it is extremely difficult to control the flow
so that a true siphon may be completely filled with liquid at all times.

In the design of inverted siphons care must be taken to prevent
sedimentation, and to permit inspection and cleaning. Sedimentation is
prevented by maintaining a velocity greater than a fixed minimum,
usually taken at about 2 feet per second. This minimum is attained by
providing a number of channels. The smallest channel is designed to
convey the least expected flow at the minimum velocity. Each of the
other channels is made as small as possible, within the limits of
economy and simplicity, in order that the minimum velocity shall be
exceeded quickly after flow has commenced in them. The last channel or
channels to be filled are made somewhat larger, because the sewage
conveyed in them contains less settleable matter than is contained in
the more concentrated dry weather flow. The type of siphon used in New
York to pass under the subway is shown in Fig. 39. Note should be taken
of the clean-out manhole provided on the 14–inch pipe. The other pipes
are large enough for a man to enter and clean.

[Illustration:

FIG. 39.—Sewer Siphon under New York Subway.

Eng. News Vol. 76, p. 443.
]

The computations involved in the design of a siphon are illustrated in
the following example, in which it is desired to construct a siphon to
pass under the railway cut shown in Fig. 40. The first step is to
determine the limiting diameter and slope of the smallest pipe in the
siphon. One-sixth of the capacity of the 6–foot approach sewer or 19
cubic feet per second will be assumed as the minimum flow. The diameter
of the pipe necessary to carry 19 cubic feet per second at a velocity of
2 feet per second is 42 inches. The hydraulic gradient should have a
slope of 0.0005 if the material used has a roughness coefficient
of .015. This is the minimum permissible slope of the siphon. The
selection of a steeper slope will necessitate the laying of the sewer at
a greater depth, and will permit the use of smaller pipes for the
siphon. The selection of the exact slope must then be based on judgment
with the minimum limitation above placed. The slope will be arbitrarily
selected as 0.001, the same as that of the approach sewer. The diameter
of the dry weather pipe will therefore be 36 inches, with a capacity of
18 second-feet, which is approximately the assumed dry weather flow. The
velocity of flow will be 2.5 feet per second. The length of flow along
the siphon is 150 feet.

[Illustration:

FIG. 40.—Diagram for the Design of an Inverted Siphon.
]

The next step should be the determination of the elevation at the lower
end of the 36–inch pipe. This is done by multiplying the assumed grade
by the equivalent length of straight pipe, and subtracting the product
from the elevation at the upper end. The length of straight pipe which
will give the same loss of head as the siphon is called the equivalent
pipe. It is determined as follows:

First, determine the head loss at entrance. This will vary between
nothing and one velocity head, dependent on the arrangement at the
entrance.[41] The length of straight pipe which will give this same loss
can be computed from the expression _l_ = _h_⁄_S_, using for _S_ the
assumed slope of the hydraulic gradient.

Second, determine the head loss due to the bends, This is determined
from the expression

_h_ = (_fl_)⁄(_d_) (_V_^2)⁄(2_g_)

in which _h_ = the head loss in the bend;

_l_ = the length of the bend;

_d_ = the diameter of the pipe;

_v_ = the average velocity of flow;

_g_ = the acceleration due to gravity;

_f_ = a factor dependent on the radius (_R_) of the bend and
_d_.

The relation between _f_, _R_, and _d_, for 90° bends is shown as
follows:[42]

_R_⁄_d_ 24 16 10 6 4 2.4

_f_ 0.036 0.037 0.047 0.060 0.062 0.072

After the head loss has been determined, the equivalent length of
straight pipe is determined as above.

Third. The equivalent length of pipe will be the sum of the actual
length of pipe and the equivalent lengths as found above.

In the problem in hand the head lost at the entrance will be assumed as
one-third of a velocity head, or 0.0324 foot. With the assumed slope of
0.001 this is equivalent to 32 feet of pipe. The radius of the bend is
about 20 feet and the length for a 45° central angle is about 16 feet.
The head loss for this angle will probably be a little more than
one-half that for a 90° angle. The expression will therefore be taken as
about 0.2(_V_^2)⁄(2_g_) and for two bends is equivalent to about 40 feet
of pipe. The equivalent length of pipe is therefore 150 + 32 + 40 = 222
feet. The elevation at the lower end should therefore be: the elevation
at the upper end, 92.07 − 222 × .001 = 91.85.

The diameters of the remaining pipes in the siphon are determined so
that the sewage in the approach sewer is backed up as little as is
consistent with good judgment before each pipe comes into action. This
is done satisfactorily by a method of cut and try. Let it be assumed
that the siphon will be composed of three pipes: the dry weather pipe
taking 18 second-feet, the second pipe taking 28 second-feet, and the
third pipe taking the remaining 70 second-feet. The diameters of the two
larger pipes on the assumed slope of 0.001 will therefore be 42 inches
and 60 inches respectively. Other combinations might be used which would
be equally satisfactory. There are many methods by which the sewage can
be diverted into the different channels of the siphon. For example, the
openings into the different pipes may be placed at the same elevation,
and the sewage allowed to enter them in turn through automatically or
hand-controlled gates, or in another method of control the openings may
be placed at such elevations that when the capacity of one pipe has been
exceeded the sewage will flow into the next largest pipe as shown in
Fig. 40. The outlets from the different pipes are ordinarily placed at
the same elevation, thus leaving each pipe standing full of sewage. Stop
planks should be provided at the outlet in order that the pipes may be
pumped out for cleaning. The objection to this arrangement is that the
larger pipes may operate at a velocity less than 2 feet per second, and
they will be standing full of sewage which might become septic. However,
as they will take nothing but the storm flow near the top of the sewer
no difficulty should be encountered from sedimentation in them, and all
are large enough for a man to enter for inspection or cleaning.

[Illustration:

FIG. 41.—Coffin Sewer Regulator.
]

=63. Regulators.=—Regulators are commonly used to divert the direction
of flow of sewage in order to prevent the overcharging of a sewer or to
regulate the flow to a treatment plant. Sewer regulators are of two
types, those with moving parts and those without moving parts. An
example of the moving part type is shown in Fig. 41. In this type as the
sewage rises the float closes the gate to the inlet sewer, thus
preventing the entrance of sewage under head from the larger sewer.
There are many variations in the details of float controlled regulators,
but the principle of operation is similar in all. These regulators can
be adjusted to fix the maximum rate of flow to a relief channel or
sewage treatment plant, or during times of storm to cut off the outlet
to the dry weather channel. Another form of the moving part type is
shown in Fig. 42.[43] It has been used extensively by the Milwaukee
Sewerage Commission. In its operation the dry weather flow is diverted
by the dam into the intercepter. It passes under the movable gate on its
way to the treatment plant. As the flow increases the dam is overtopped
and flood waters are discharged down the storm channel. The movable gate
is hung on a pivot placed below center. As the water rises in the
intercepter, the pressure against the upper portion of the gate becomes
greater than that against the lower portion, and the gate closes. An
opening is left at the bottom to allow an amount of sewage equal to the
dry weather flow to escape beneath the gate to prevent clogging or
sedimentation in the intercepter channel.

Objections to all moving part regulators are their need of attention and
liability to become clogged.

[Illustration:

FIG. 42.—Moving Part Regulator without Float.
]

[Illustration:

FIG. 43.—Leaping Weir at Danville, Illinois.
]

[Illustration:

FIG. 44.—Overflow Weir at San Francisco.

Eng. News, Vol. 73, p. 307.
]

[Illustration:

FIG. 45.—Overflow Weir in Action.

Shadow of steel knife edge which forms the lip of the weir can be seen
through the falling sewage.
]

The overflow weir and the leaping weir have no moving parts and are used
for the regulation of the flow in sewers. A leaping weir is formed by a
gap in the invert of a sewer through which the dry weather flow will
fall and over which a portion or all of the storm flow will leap. One
form of leaping weir is shown in Fig. 43. An overflow weir is formed by
an opening in the side of a sewer high enough to permit the discharge of
excess flow into a relief channel. A weir at San Francisco is shown in
Fig. 44. A series of tests were run on leaping weirs and overflow weirs
in the hydraulic laboratory of the University of Illinois. The type of
leaping weir tested was formed by the smooth spigot end of a standard
vitrified sewer pipe. The overflow weirs were formed by a steel knife
edge in the side of the pipe parallel to its axis as shown in Fig. 45.
Tests were made in 18–inch and 24–inch pipes on various slopes from
0.018 to 0.005, for both leaping weirs and overflow weirs. The overflow
weirs were varied in length from 16 inches to 42 inches and were placed
at various heights from 25 per cent to 50 per cent of the diameter above
the invert of the sewer. As the result of the observations the following
formulas were developed. For the leaping weir the expressions for the
coordinates of the curve of the surfaces of the falling stream, are:

For the outside surface _x_ = 0.53_V_^⅔ + _y_^{4⁄7}

For the inside surface _x_ = 0.30_V_^{4⁄7} + _y_^¾

in which _x_ and _y_ are the coordinates. The origin is in the upper
surface of the stream vertically above the end of the invert of the
pipe. The ordinate _y_ is measured vertically downwards. _V_ is the
velocity of approach in feet per second. These expressions are
applicable to any diameter of sewer up to 10 or 15 feet. They should
_not_ be used for depths of flow greater than about 14 inches; nor for
slopes of more than 25 per 1,000; nor for velocities less than 1 foot
per second nor more than 10 feet per second. The expression for the
ordinate of the inside curve is not good for less than 6 inches nor more
than 5 feet. The expression for the ordinate of the outside curve is
limited to values between the origin and 5 feet below it.

The expression for the length of an overflow weir of the type shown in
Fig. 45, necessary to discharge a given quantity, is in the form,

_l_ = 2.3_Vd_ log _h__{1}⁄_h__{2}

in which _l_ = the length of the weir in feet;

_V_ = the velocity of approach in feet per second;

_d_ = the diameter of the pipe in feet;

_h__{1} = the head of water on the upper end of the weir;

_h__{2} = the head of water on the lower end of the weir.

In the design of an overflow weir by this formula the height of the weir
above the invert of the sewer and the flow over the weir should be
determined arbitrarily. The height should be subtracted from the
computed depth of water above the weir to determine the value of
_h__{1}. The difference between the flow over the weir and the flow
above the weir will represent the rate of flow in the sewer below the
weir. The value of _h__{2} can then be computed as the difference in the
depth of flow below the weir and the height of the weir above the
invert. The value of _V_ is computed from Kutter’s formula. The length
of the weir is determined by substituting these values in the formula.

=64. Junctions.=—At the junction of two or more sewers the elevation of
the inverts should be such that the normal flow lines are at the same
elevation in all sewers. The sewers should approach the junction on a
steep grade to prevent sewage backing up in one when the other is
flowing full. The velocity of flow at the junction should not be
decreased and turbulence should be avoided in order to prevent
sedimentation and loss of head. The junction should be effected on
smooth easy curves with radii at least five times the diameter of the
sewer where possible. Curves with short radii cause backing up of sewage
thus reducing the capacity of the sewers.

The terms bellmouth or trumpet arch are sometimes applied to the
junction of sewers large enough to be entered by a man. In small sewers
the Y branches and special junctions are manufactured so that the center
lines of the pipes intersect, and the junctions of mains and laterals
are made in manholes. In the construction of a bellmouth the arch is
carried over all the sewers. A manhole should be constructed at these
junctions as clogging frequently occurs there, due to swirling and back
eddies which cannot be avoided.

=65. Outlets.=—The outlets to a sewerage system discharging into a
swiftly running stream must be protected against wash and floating
debris. In a stream or other body of water subject to wide variations in
elevation the backing up of the sewage during high water should be
avoided. Where tidal flats or low ground about the outlet may be
alternately submerged and uncovered the discharge should always be into
swiftly running water. In quiescent bodies of water such as lakes and
harbors, and in rivers where the dilution is low, and in many other
cases, the sewer outlet should be submerged.

[Illustration:

FIG. 46.—Tide Gate.
]

Outlets are protected against wash and the impact of debris by the
construction of deep foundations and heavy protecting walls. Although
the construction of an outlet in a slow current or a back eddy would
avoid danger from wash and debris, the discharge of the sewage into the
most rapid current possible aids in the prevention of a local nuisance.
A row of batter piles on the upstream or exposed side of the sewer is
desirable, or it may be necessary to construct a break-water to prevent
the wash of the current from dislodging the pipe. These break-waters are
low dams of wood or broken stone, more or less loosely thrown together.
The backing up of water into the sewer can be prevented by constructing
the sewer above the outlet on a steep grade. Where this is not possible
the use of tide gates will be helpful. A tide gate, one form of which is
shown in Fig. 46, is a special form of check valve placed on the end of
the sewer.

[Illustration:

FIG. 47.—Sewer Outlet on a Trestle.

Eng. News, Vol. 49, p. 9.
]

Sewer outlets are sometimes constructed on long trestles in order to
reach deep or running water. Such a trestle is shown in Fig. 47. In
Boston the outlet sewers are submerged under the harbor and discharge
through outlets well out in the tidal currents. The sewage is discharged
under pressure and the pumps are operated at some of the stations only
at such times as the tidal currents will carry the sewage away from the
harbor. It is not always necessary in a combined sewerage system to
carry the storm flow to a distant submerged outlet. A double outlet can
be constructed as shown in Fig. 48 in which the dry weather flow is
carried to the channel in a submerged sewer and the storm flow is
discharged on the bank.[44] Cast-iron pipe should be used for submerged
outlets as the sewer is subject to disturbance by the currents, anchors,
ice, and other causes.

[Illustration:

FIG. 48.—Dry Weather and Storm Sewer Outlet at Minneapolis, Minnesota.

Eng. Record, Vol. 63, p. 383.
]

=66. Foundations.=—Sewers constructed in firm dry soil require no
special foundation to distribute the weight over the supporting medium.
In soft materials the lower half of the sewer ring may be spread as
shown in Fig. 22, and in rock the pressures on sewer pipes are evenly
distributed by a cushion of sand. In wet ground such as quicksand, mud,
swamp land, etc., a foundation must be constructed if the water cannot
be drained off.

The permissible intensities of pressure on foundations in various
classes of material allowed by the building codes in different cities
are given in Table 25. These figures are based on the assumption that
the material is restrained laterally, which is generally the condition
in sewer construction. In the softer materials it becomes necessary to
spread the foundations not only to reduce the intensity of pressure, but
also to care for the thrust of the sewer arch. The arch thrust may be
found by one of the methods of arch analysis, and the haunches spread to
care for this, or the sewer invert may be transversally reinforced to
assist in caring for this action. Some sewer sections in hard and soft
material are shown in Fig. 22 and 23.

TABLE 25

ALLOWABLE BEARING VALUE ON SOILS IN VARIOUS CITIES

From Proc. Am. Soc. Civil Engrs., Vol. 46, 1920, p. 906

─────────────────────────┬─────────────────────────────────────────────
Quicksand and alluvial │½ to 1 ton per sq. ft. for Providence, R. I.,
soil │ ½ ton per sq. ft. for 6 cities
─────────────────────────┼─────────────────────────────────────────────
Soft clay │1 ton per sq. ft. for 27 cities, ¾ ton per
│ sq. ft. for New Orleans, 2 to 3 tons for
│ Providence, R. I.
─────────────────────────┼─────────────────────────────────────────────
Moderately dry clay and │2 tons for 7 cities, 1¾ to 2¼ for Chicago, 2½
fine sand, clean and │ tons for Louisville, 2 to 4 tons for
dry │ Providence, 3 tons for Grand Rapids and Los
│ Angeles
─────────────────────────┼─────────────────────────────────────────────
Clay and sand in │2 tons for 19 cities, 1¾ to 2¼ for Chicago, 3
alternate layers │ to 5 tons for Providence
─────────────────────────┼─────────────────────────────────────────────
Firm and dry loam or │3 tons for 24 cities, 2½ tons for 2 cities, 2
clay, or hard dry clay │ to 3 tons for Atlanta, 3½ tons for
or fine sand │ Philadelphia, 4 tons for 6 cities
─────────────────────────┼─────────────────────────────────────────────
Compact coarse sand, │4 tons for 25 cities, 3½ tons for Buffalo, 3
stiff gravel or natural│ to 4 tons for Atlanta, 4 to 5 tons for
earth │ Cincinnati, 5 tons for Denver, 4 to 6 tons
│ for 3 cities, 6 tons for Rochester, N. Y.
─────────────────────────┼─────────────────────────────────────────────
Coarse gravel, stratified│6 tons for 3 cities, 5 tons for 2 cities, 8
stone and clay, or rock│ tons for 1 city
inferior to best brick │
masonry │
─────────────────────────┼─────────────────────────────────────────────
Gravel and sand well │8 tons for 5 cities, 6 tons for 2 cities, 8
cemented │ to 10 tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Good hard pan or hard │10 tons for 4 cities, 6 tons for 2 cities, 8
shale │ tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Good hard pan or hard │8 tons for 1 city, 10 to 15 tons for 1 city,
shale unexposed to air,│ 12 to 18 tons for 1 city
frost or water │
─────────────────────────┼─────────────────────────────────────────────
Very hard native bed rock│20 tons for 5 cities, 15 tons for 1 city, 10
│ tons for 1 city, 25 to 50 tons for 1 city
─────────────────────────┼─────────────────────────────────────────────
Rock under caisson │24 tons for Baltimore, 25 tons for Cleveland
─────────────────────────┴─────────────────────────────────────────────

On soft foundations such as swamps or for outfalls on the muck bottom of
rivers the sewer may be carried on a platform. For small sewers 2–inch
planks, 2 to 4 feet longer than the diameter of the pipe are laid across
the trench, and the sewer rests directly upon them. For large sewers
imposing a heavy concentrated load, a pile foundation should be
constructed. The foundation may consist of piles alone, pile bents, or a
wooden platform supported on pile bents. The load which can be carried
by a pile is determined with accuracy only by driving a test pile and
placing a load on it. Where piles do not penetrate to a firm stratum the
load they will support can be determined by any one of the various
formulas, either theoretical or empirical, which have been devised.
Probably the best known of these formulas are the so-called Engineering
News formulas one of which is:

_P_ = 2_Wh_⁄(_S_ + 1) for a pile driven by a drop hammer,

in which _P_ = the safe load on the pile in pounds. The factor of
safety is 6;

_W_ = the weight of the hammer in pounds;

_h_ = the fall of the hammer in feet;

_S_ = the penetration of the pile in inches at the last
driving blow. The blow is assumed to be driven on
sound wood without rebound of the hammer.

Reference should be made to engineering handbooks for other forms of
pile formulas. The accuracy of all of these formulas is not of a high
degree.

The piles are driven at about 2 to 4 feet centers, to a depth of from 8
to 20 feet, unless hard bottom is struck at a lesser depth. The butt
diameter of the piles used for the smallest sewers is about 6 to 8
inches. If bents are used, 2 or 3 piles are driven in a row across the
line of the sewer and are capped with a timber. For brick, block, pipe,
and some concrete sewers, a wooden platform must be constructed between
the pile bents for the support of the sewer.

=67. Underdrains.=—The construction of special foundations can sometimes
be avoided by laying drains under the sewers, thus removing the water
held in the soil. The laying of the underdrains facilitates the
construction of the sewer and reduces the amount of ground water
entering the sewer. The underdrains usually consist of 6– or 8–inch
vitrified tile laid with open joints from 1 to 2 feet below the bottom
of the sewer as shown in Fig. 1. If the sewers are large, parallel lines
of drains may be laid beneath them. An observation hole should be
constructed from the bottom of the manhole to each underdrain. This hole
usually consists of a 6– or 8–inch pipe, embedded in concrete, connected
to the drain and open at the top. It is too small to permit effective
cleaning of the underdrains, which are usually neglected after
construction, and which as a result clog and cease to function. Since
the principal period of usefulness of the drains is during construction,
their stoppage after the work is completed is not serious. The hollow
tile used in vitrified block sewers serve as underdrains after
construction, but are of little or no assistance to the draining of the
trench during construction.

CHAPTER VII
PUMPS AND PUMPING STATIONS

=68. Need.=—In the design of a sewerage system it is occasionally
necessary to concentrate the sewage of a low-lying district at some
convenient point from which it must be lifted by pumps. In the
construction of sewers in flat topography the grade required to cause
proper velocity of sewage flow necessitates deep excavation. It is
sometimes less expensive to raise the sewage by pumping than to continue
the construction of the sewers with deep excavation.

In the operation of a sewage-treatment plant a certain amount of head is
necessary. If the sewage is delivered to the plant at a depth too great
to make possible the utilization of gravity for the required head, pumps
must be installed to lift the sewage. In the construction of large
office buildings, business blocks, etc., the sub-basements are
frequently constructed below the sewer level. The sewage and other
drainage from the low portion of the building must therefore be removed
by pumping. Because pumps are often an essential part of a sewerage
system, their details should be understood by the engineer who must
write the specifications under which they are purchased and installed.

=69. Reliability.=—If the only outlet from a sewerage system is through
a pumping station, the inability of the pumps to handle all of the
sewage delivered to them may so back up the sewage as to flood streets
and basements, endangering lives and health and destroying property.
Such an occurrence should be guarded against by providing sufficient
pumping capacity and machinery of the greatest reliability.

=70. Equipment.=—The equipment of a sewage pumping station, in addition
to pumping machinery, may include a grit chamber, a screen, and a
receiving well. The grit chamber and screen are necessary to protect the
pumps from wear and clogging. Grit chambers are not necessary in sewage
devoid of gritty matter, such as the average domestic sewage, unless
reciprocating pumps are used. Sufficient gritty matter is found in
average domestic sewage to have an undesirable effect on reciprocating
pumps. Receiving wells are used in small pumping stations where the
capacity of the pumps is greater than the average rate of sewage flow.
The pumps are then operated intermittently, the pumps standing idle
during the time that the receiving well is filling.

Except for a few types of pumps of which the valve openings are
unsuitable, any machine capable of pumping water is capable of pumping
sewage which has been properly screened. The principles of sewage pumps
are then similar to principles of water pumps. The conditions under
which these principles are applied differ but slightly in the character
of the liquid, and a smaller range of discharge pressures. Pumps with
large passages, discharging under low heads are more commonly found
among sewage pumps.

[Illustration:

FIG. 49.—Calumet Sewage Pumping Station, Chicago, Illinois.
]

=71. The Building.=—The pumping station should, if possible, be of
pleasing design and should be surrounded by attractive grounds. The
Calumet Sewage Pumping Station in Chicago is shown in Fig. 49. Its
architecture is pleasing particularly in contrast with its location and
immediate surroundings. Such structures tend to remove the popular
prejudice from sewerage and to arouse interest in sewerage questions.
Service to the public is of value. It can be rendered more easily by
arousing public interest and cooperation by the erection of attractive
structures, than by feeding popular prejudice by the construction of
miserable eyesores.

=72. Capacity of Pumps.=—The capacity of the pumping equipment should be
sufficient to care for the maximum quantity of sewage delivered to it,
with the largest pumping unit shut down, and the provision of such
additional capacity as, in the opinion of the designer, will provide the
necessary factor of safety.

Pumps can usually be operated under more or less overload. Power pumps
and centrifugal pumps driven by constant speed electric motors have no
overload capacity. A power pump or a centrifugal pump may be overloaded
up to the maximum horse-power of any variable speed motor or steam
engine driving it, provided the pump has been designed to permit it.
Direct-acting steam pumps which are designed for proper piston speed and
valve action at normal loads, can carry a 50 per cent overload for short
periods, although the strain on the pump is great. They will carry a 20
to 25 per cent overload for about eight hours with less vibration and
strain. The use of pumps capable of working at an appreciable overload
is somewhat of an additional factor of safety, but the overload factor
should not be taken into consideration in determining the capacity of
the pumping equipment.

The load on a pumping station consists of the quantity of sewage to be
pumped and the height it must be lifted. Variations in the quantity are
discussed in Chapter III. The head against which the pumps must operate
fluctuates with the level in the tributary sewer or pump well, and in
the discharge conduit. For a free discharge or discharge into a short
force main the greater the rate of sewage flow the smaller the lift, as
the depth of flow in the tributary sewer increases more rapidly than
that in the discharge conduit. If the discharge is into a large body of
water or under other conditions where the discharge head is
approximately constant, the fluctuations in total head should not exceed
the diameter of the tributary sewer. Such fluctuations are of minor
importance in the operation of direct-acting steam pumps, but may be of
great importance in the operation of centrifugal pumps, as is brought
out in Art. 78.

=73. Capacity of Receiving Well.=—The use of receiving wells is
restricted to small installations which require, in addition to the
standby unit, only one pump, the capacity of which is equal to the
maximum rate of sewage flow. When the receiving well has been pumped dry
the pump stops, allowing the well to fill again. Although the use of a
large receiving well, or an equalizing reservoir, for a large pumping
station would permit the operation of the pumps under more economical
conditions, the storage of sewage for the length of time required would
not be feasible. The sewage would probably become septic, creating odors
and corroding the pumps. The extra cost of the reservoir might not
compensate for the saving in the capacity and operation of the pumps.

The capacity of the receiving well should be so designed that the pump
when operating will be working at its maximum capacity, and the period
of rest during conditions of average rate of flow should be in the
neighborhood of 15 to 20 minutes. For example, assume an average rate of
flow of 2 cubic feet per second, with a maximum rate of double this
amount. The pump should have a capacity of 4 cubic feet per second, and
if the receiving well is to be filled in 15 minutes by the average rate
of sewage flow its capacity should be 15 × 5 × 60 × 7.5 or 14,000
gallons. Under these circumstances the pump will operate 15 minutes and
rest 15 minutes, during average conditions of flow.

=74. Types of Pumping Machinery.=—The two principal types of pumping
machines for lifting sewage are centrifugal pumps and reciprocating
pumps. A centrifugal pump is, in general, any pump which raises a liquid
by the centrifugal force created by a wheel, called the impeller,
revolving in a tight casing, as shown in Fig. 50. A reciprocating pump
is one in which there is a periodic reversal of motion of the parts of
the pump.

Centrifugal pumps are sometimes classified as volute pumps and turbine
pumps. A volute pump is a centrifugal pump with a spiral casing into
which the water is discharged from the impeller with the same velocity
at all points around the circumference, as shown in Fig. 51. A turbine
pump is a centrifugal pump in which the water is discharged from the
impeller through guide passages into a collecting chamber, in such a
manner as to prevent loss of energy in changing from kinetic head to
pressure head. A turbine pump is shown in section in Fig. 51.
Centrifugal pumps are sometimes classified as single stage and
multi-stage. A centrifugal pump from which the water is discharged at
the pressure created by a single impeller is called a single-stage pump.
If the water in the pump is discharged from one impeller into the
suction of another impeller the pump is known as a multi-stage pump. The
number of impellers operating at different pressures determines the
number of stages of the pump. A three-stage pump is shown in Fig. 52.

[Illustration:

FIG. 50.—Section through de Laval Single-Stage, Double Suction
Centrifugal Pump.
]

375 Lubricating ring.

380 Oil hole cap.

383 Oil drain tube.

404 Shaft sleeve lock nut.

440 Driving coupling.

441 Driven coupling.

443 Coupling check nut.

450 Coupling bolt.

451 Coupling bolt nut.

452 Coupling rubber.

453 Coupling rubber steel tube.

500 Pump case.

550 Bearing bracket cap.

551 Bearing.

552 Shaft.

553 Shaft sleeve, right hand thread.

PW Impeller.

554 Shaft sleeve, left hand thread.

555 Shaft stop collar, inner.

555–1 Shaft stop collar, outer.

556 Guide ring.

560 Packing gland.

563 Bearing.

567R Impeller protecting ring, right hand thread.

567L Impeller protecting ring, left hand thread.

583 Pump case protecting ring.

567 Labyrinth packing.

583 Labyrinth packing.

600 Pump case cover.

692 Impeller key.

815 Bearing bracket, outer.

815–1 Bearing bracket, inner.

[Illustration:

FIG. 51.—Types of Centrifugal Pumps.
]

[Illustration:

FIG. 52.—Section of a Multi-Stage Centrifugal Pump.

Courtesy DeLaval Steam Turbine Co.
]

Reciprocating pumps are generally driven by steam and are either
direct-acting, or of the crank-and-fly-wheel type. Power pumps are
reciprocating machines which may be driven by any form of motor, to
which they are connected by belt, chain or shaft. A Deming triplex
power pump is shown in Fig. 53. Power pumps can be used only where the
character of the sewage will not clog the valves nor corrode the pump.
A direct-acting steam pump is one in which the steam and water
cylinders are in the same straight line and the steam is used at full
boiler pressure throughout the full length of the stroke. The
crank-and-fly-wheel type of pumping engine permits the use of steam
expansively during a part of the stroke, the energy stored in the
flywheel carrying the machine through the remainder of the stroke.
Reciprocating pumps are sometimes classified as plunger pumps and
piston pumps. In the action of a plunger pump the water is expelled
from the water cylinder, by the action of a plunger which only partly
fills the water cylinder, as shown in Figs. 54 and 55. In a piston
pump the water is expelled from the water cylinder by the action of a
piston which completely fills the water cylinder, as shown in Fig. 63,
which illustrates a direct-acting piston pump.

[Illustration:

FIG. 53.—Triplex Power Pump.

Courtesy, The Deming Co.
]

Plungers are better than pistons for pumping sewage as the wear between
the pistons and the inside face of the cylinder soon reduces the
efficiency of the pump. Outside packed plungers are better than the
inside packed type because the packing can be taken up without stopping
the pump and the leakage from the pump is visible at all times. Outside
packed pumps are more expensive in first cost, but are easier to
maintain and have a longer life than piston pumps.

[Illustration:

FIG. 54.—Water End of Inside Center-Packed Plunger Pump.
]

In selecting a pump to perform certain work the size of the water
cylinder and the speed of the travel of the piston should be
investigated to insure proper capacity. The average linear travel of the
piston for slow speed pumps is estimated at about 100 feet per minute,
dependent on the length of stroke and the valve area. For short strokes
and small valve areas the speed may be as low as 40 feet per minute, and
for long stroke fire pumps with large valves the piston can be operated
at a speed of 200 feet per minute.[45] Vertical, triple-expansion,
crank-and-fly-wheel, outside packed plunger pumps with flap valves can
be operated at speeds of 200 feet per minute when lifting sewage, and
when equipped with mechanically operated valves and lifting water they
can be run at speeds of 400 to 500 feet per minute. The speed of travel
multiplied by the volume of piston or plunger displacement, with proper
allowance for slippage, will give the capacity of the pump. The slippage
allowance may be from 3 to 8 per cent for the best pumps, and for pumps
in poor conditions it may be a high as 30 to 40 per cent.

[Illustration:

FIG. 55—Water End of Outside Center-Packed Plunger Pump.

Courtesy Allis-Chalmers Co.
]

There are two types of ejector pumps used for lifting sewage. One of
these depends on the vacuum created by the velocity of a stream of water
or steam passing through a small nozzle. The operation of this pump is
described in Art. 139 and it is illustrated in Fig. 97. The other type
of ejector pump is known as the compressed-air ejector. It is operated
by means of compressed air which is turned into a receptacle containing
sewage. The details of this type are explained in Art. 83 and are
illustrated in Fig. 68.

=75. Sizes and Description of Pumps.=—The size of a centrifugal pump is
expressed as the diameter of the discharge pipe in inches. It has
nothing to do with the head for which the pump is suited. On the
assumption of a velocity of flow of 10 feet per second through the
discharge pipe the capacity of the pump can be approximated.

The size of a reciprocating pump involves the expression of the
diameters of the steam cylinders, the water cylinder, and the length of
the stroke in inches, in the order named, beginning with the steam
cylinder with the highest pressure. A complete description of a steam
pumping engine might be; compound, duplex, horizontal, condensing,
crank-and-fly-wheel, outside-center-packed, 12″ × 24″ × 18″ × 24″ pump.
The word compound means that there are a high-pressure and a
low-pressure steam cylinder; the word duplex means that there are two of
each of these cylinders; the word horizontal means that the axes of
these cylinders are in a horizontal plane; the word condensing means
that the steam is discharged from the low-pressure cylinders into a
condenser; the name crank-and-fly-wheel is self-explanatory; the name
outside-center-packed means that the water cylinder is divided into two
portions between which the plunger is exposed to the atmosphere, and
that the packing rings are on the outside of the two portions of the
cylinder as shown in Fig. 55; the figures shown mean that the
high-pressure steam cylinder is 12 inches in diameter, the low-pressure
24 inches in diameter, the water cylinder is 18 inches in diameter, and
the stroke of the pump is 24 inches.

=76. Definitions of Duty and Efficiency.=—The duty of a pump is the
number of foot-pounds of work done by the pump per million B.T.U., per
thousand pounds of steam, or per hundred pounds of coal, consumed in
performing the work. These units are only approximately equal as 100
pounds of coal or 1,000 pounds of steam do not always contain the same
number of B.T.U. and may only approximately equal 1,000,000 B.T.U.

Since 1,000,000 B.T.U. are equal to 778,000,000 foot-pounds of work, a
pump with a duty of 778,000,000 will have an efficiency of 100 per cent.
The efficiency of a pump is therefore its duty based on B.T.U. divided
by 778,000,000. The efficiencies or duties of various types of pumps are
given in Table 26.[46]

TABLE 26

APPROXIMATE DUTIES OF STEAM PUMPS

Small duplex, non-condensing 10,000,000
Large duplex, non-condensing 25,000,000
Small simple, flywheel, condensing 50,000,000
Large simple, flywheel, condensing 65,000,000
Small compound, flywheel, condensing 65,000,000
Large compound, flywheel, condensing 120,000,000
Small triple, flywheel, condensing 150,000,000
Large triple, flywheel, condensing 165,000,000

=77. Details of Centrifugal Pumps.=—A section of a centrifugal pump with
the names of the parts marked thereon is shown in Fig. 50. Among the
important parts which require the attention of the purchaser are: the
impeller (_PW_), the impeller packing rings (567 _R_ & _L_), the
bearings (551, 563), the thrust bearings (555–1), the shaft (552), and
the shaft coupling (440).

The impeller should be of bronze, gun metal, or other alloy, because
there is no rusting or roughening of the surface, and the efficiency
does not fall with age. Normal fresh sewage is not corrosive, but septic
sewage and sludge are usually so corrosive that iron parts cannot be
used with success in contact with them. The impeller should be machined
and polished to reduce the friction with the liquid. Impellers are made
either closed or open, i.e., either with or without plates on the sides
connecting the blades to avoid the friction of the liquid against the
side of the casing. The closed type of impeller is shown in Fig. 50.
Closed impellers are slightly more expensive, but generally give better
service and higher efficiencies than the open type. Single impeller
pumps should have an inlet on each side of the impeller to aid in
balancing the machine, unless the plane of the impeller is to be
horizontal when operating. Multi-impeller pumps usually have single
inlet openings for each impeller. Vibration in the pump is sometimes
caused by an unbalanced impeller. The moving parts may be balanced at
one speed and unbalanced at another. To determine if the moving parts
are balanced the pump should be run free at different speeds and the
amount of vibration observed. If the impeller is removed from the pump
its balance when at rest can be studied by resting it on horizontal
knife edges. If there is a tendency to rotate in any direction from any
position the impeller is not perfectly balanced.

Packing rings are used to prevent the escape of water from the discharge
chamber back into the suction chamber. These rings should be made of the
same material as the impeller. Labyrinth type rings, as shown in Fig.
50, are sometimes used as the long tortuous passages are efficient in
preventing leakage.

The bearings must be carefully made because of the high speed of the
pump. They are usually made of cast iron with babbitt lining. They
should be placed on the ends of the shaft on the outside of the pump
casing, as shown in Fig. 50, and should be split horizontally so as to
be easily renewed. Exterior bearings are oil lubricated by means of ring
or chain oilers with deep oil wells. Where interior bearings are
necessary, because of the length of the shaft, they should be made of
hard brass and should be water lubricated.

[Illustration:

FIG. 56.—Marine Type Thrust Bearing.

Courtesy, DeLaval Steam Turbine Co.
]

Thrust bearings or thrust balancing devices are used to take up the end
thrust which occurs in even the best designed pumps. To overcome this
pumps are designed with double suction, opposed impellers, or two pumps
with their impellers opposed may be placed on the same shaft. Due to
inequalities in wear, workmanship or other conditions, end thrust will
occur and must be cared for. Various types of thrust bearings are in
successful use, such as: the piston, ball, roller or marine types. The
marine type thrust bearing is shown in Fig. 56. The piston type of
hydraulic balancing device is shown in Fig. 57. In the figure _A_
represents the impeller, and _B_ a piston fixed to the shaft and
revolving with it. There is a passage for water through the openings
(1), (2), and (3) leading from the impeller chamber to the atmosphere or
to the suction of the pump. If the impeller tends to move to the right
opening (1) is closed resulting in pressure on the right of the impeller
forcing it to the left. If the impeller moves to the left (1) is opened
thus transmitting pressure to the piston _B_ forcing the impeller to the
right. The flange _C_ is not essential, but is advantageous in pumps
handling gritty matter. As the channel (2) wears larger the pressure
against the piston decreases allowing it to move to the left. This
partially closes (3) building up the pressure again.

[Illustration:

FIG. 57.—Piston Type of Thrust Balancing Device.
]

Flexible shaft couplings should be used if the shaft of the driving
motor and the pump are in the same line, as direct alignment is
difficult to obtain or to maintain. Where connected to steam turbines,
reduction gearing and rigid couplings are usually used on sewage pumps
to obtain slow speed and permit large passages. Flexible couplings are
of various types, one of which is shown in Fig. 50. A rigid coupling
would be formed by bolting the flanges firmly together. Shaft couplings
are usually not necessary where the pump is driven by belt connection to
the engine or motor, or where the pump and pulley rest on only two
bearings.

The stuffing box shown in Fig. 50 is packed loosely with two layers of
hemp between which is a lantern gland, in order to permit a small amount
of leakage. A drip box is placed below this gland to catch the leakage
and return it to the pump. The leakage is permitted as it aids in
lubrication and the tightening of the gland will cause binding of the
shaft. The gland on the suction side of the pump should be connected by
a small pipe to the discharge chamber in order to keep a constant supply
of water for lubrication and to prevent the entrance of air to the
suction end of the pump.

=78. Centrifugal Pump Characteristics.=—The capacity of a centrifugal
pump is fixed by the size and type of its impeller and by the speed of
revolution. Roughly, the capacity of a pump, for maximum efficiency,
varies directly as the speed of revolution, the discharge pressure
varies as the square of the speed, and the power varies as the cube of
the speed. These relations are found not to hold exactly in tests
because of internal hydraulic friction in the pump.

The characteristic curves for a centrifugal pump, or the so-called pump
characteristics, are represented graphically by the relation between
quantity and efficiency, quantity and power necessary to drive, and
quantity and head, all at the same speed. The quantities are plotted as
abscissas in every case. The curve whose ordinates are head and whose
abscissas are quantities is known as “the characteristic.” The curve
showing the relation between quantities and speeds is sometimes included
among the characteristics. Characteristics of pumps with different
styles of impellers are shown in Fig. 58. Fig. 59 shows the
characteristics of a pump run at different speeds, the efficiencies at
these speeds when pumping at different rates, and the maximum efficiency
at different speeds. It is to be noted that the information given in
this figure is more extensive than that in Fig. 58. The operating
conditions under any head, rate of discharge, and speed are given. The
curves of constant speed are parallel, and their distances apart vary as
the square of the speed. The line of maximum efficiency is approximately
a parabola.

[Illustration:

FIG. 58.—Characteristics of Centrifugal Pumps with Different Styles of
Impellers at Constant Speed.
]

A study of the characteristics of any particular pump should be made
with a view to its selection for the load and conditions under which it
is to be used. Among the important things to be considered in the
selection of a centrifugal pump for the expected conditions of load are:
the capacity required, the maximum and minimum total head to be pumped
against, the maximum variations in suction and discharge heads, and the
nature of the drive. For example, the pump, whose characteristics are
shown in Fig. 59, should be operated at about 800 revolutions per
minute. Under total heads between 40 and 50 feet, the discharge for the
best efficiency will vary between 600 and 670 gallons per minute.

[Illustration:

FIG. 59.—Efficiency and Characteristic Curves of a Centrifugal Pump at
Different Speeds.
]

[Illustration:

FIG. 60.—Efficiencies of Centrifugal Pumps.
]

The efficiencies of centrifugal pumps increase with their capacities as
is shown approximately in Fig. 60.

=79. Setting of Centrifugal Pumps.=—In setting a centrifugal pump, care
should be taken to provide a firm foundation to hold the shafts of the
pump and the electric motor or the reduction gearing in good alignment,
or to prevent the pump from being displaced by the pull of a belt. It is
desirable that the foundation be level. Centrifugal pumps should be set
submerged for small pumping stations automatically controlled. Sludge
pumps must be set submerged as otherwise they will not prime
successfully. Provision should be made by which the pump can be lifted
from the sewage, or sludge, for inspection and repair. In many cases the
pump can be made self-priming by setting it in a dry, water-tight vault
below the low level of sewage flow. Where possible it is desirable not
to set the pump submerged as it will receive better care when easily
accessible.

[Illustration:

FIG. 61.—Centrifugal Pump in Manhole at Duluth, Minn.

Eng. Contracting, Vol. 43, 1915, p. 310.
]

The suction pipe should be free from vertical bends where air might
collect and should be straight for at least 18 to 24 inches from the
pump casing. An elbow on the suction pipe, attached directly to the
casing of the pump gives a lower efficiency than a suction pipe with a
short straight run. Centrifugal pumps will operate with as high a
suction lift as reciprocating pumps, but at the start they must be
primed and some provision must be made for priming them. The suction
pipe should be equipped with foot valves to hold the priming, or some
method may be provided for exhausting the air from the suction pipe. The
foot valves should be so installed as to form no appreciable obstruction
to the flow of water. They should have an area of opening at least 50
per cent greater than the cross-section of the suction pipe. A strainer
on the suction pipe is undesirable as it becomes clogged and is usually
in an inaccessible position for cleaning. A screen should be placed at
the entrance to the suction well to prevent the entrance of objects that
are likely to clog the pump. A gate-valve and a check-valve should be
provided on the discharge pipe, the former to assist in controlling the
rate of discharge and the latter to prevent back flow into the pump when
it is not operating.

Centrifugal pumps are well adapted to service in either large or small
units. An installation in a manhole at Park Point, Duluth, is shown in
Fig. 61. This station is controlled by an automatic electric device
which is operated by a float in the suction pit. Such automatic control
is an added advantage of the use of electrically driven centrifugal
pumps. The Calumet Pumping Station in Chicago, shown in Fig. 49, has a
capacity of approximately 1,000 cubic feet per second. The simplicity of
the layout of this station is shown in Fig. 62.

[Illustration:

FIG. 62.—Interior Arrangement of the Calumet Sewage Pumping Station,
Chicago.

Eng. News-Record, Vol. 85, 1920, p. 872.
]

=80. Steam Pumps and Pumping Engines.=—The direct-acting steam pump, one
type of which is shown in Fig. 63, is adapted to pumping sewage the
character of which will not corrode or clog the valves. In this form of
pump it is necessary to utilize the steam at full pressure throughout
the entire length of the stroke, which results in high steam
consumption. A flywheel permits the use of steam expansively during a
part of the stroke, thus increasing the economy of operation. Other
devices used for the same purpose are known as compensators. They are
not in general use.

Steam engines are classified in many different ways, for example;
according to the type of valve gear, as, plain slide valve, Corliss,
Lentz, etc.; or according to the number of steam expansions, as, simple,
compound, triple-expansion, etc.; or according to the efficiency of the
machine as low duty or high duty; or as

[Illustration:

FIG. 63.—Section of Duplex Piston Steam Pump.

Courtesy, The John H. McGowan Co.
]

STEAM END

2 Steam cylinder and housing combined.

8 Steam piston head.

9 Steam piston follower.

10 Steam piston inside ring.

11 Steam piston outside ring (2).

12 Steam cylinder head.

14 Steam chest.

16 Steam chest cover.

17 Steam slide valve.

18 Steam valve rod.

20 Steam valve rod, pin and nut.

22 Steam valve rod, collar and set screw.

23 Steam valve rod, stuffing box.

24 Steam valve rod, stuffing box, nut and gland.

38 Piston rod.

47 Piston rod stuffing box.

48 Piston rod, stuffing box, nut and gland.

49 Valve gear stand.

51 Long valve crank and shaft.

52 Short valve crank and shaft.

PUMP END

115 Pump body.

127 Brass liner.

129 Water piston head.

130 Water piston follower.

137 Cylinder head.

139 Valve plate.

140 Cap.

152 Suction flange.

161 Discharge flange.

162 Valve seat, suction or discharge.

163 Valve, suction or discharge.

164 Suction valve spring.

167 Discharge valve spring.

168 Valve plate, suction or discharge.

169 Valve stem, suction or discharge.

STEAM END

55 Crank pin.

56 Valve rod link.

61 Long rocker arm.

62 Short rocker arm.

63 Rocker arm wiper.

69 Cross head.

condensing or non-condensing, etc. Throttling engines or automatic
engines refer to the method of control of the steam by the governor. In
throttling engines the governor controls the amount of opening of the
throttle valve, in automatic engines the governor controls the position
of the cut-off.

The simple slide valve, low-duty, non-condensing, throttling engine, is
the lowest in first cost and the most expensive in the consumption of
fuel. The triple-expansion Corliss, or the non-releasing Corliss,
high-duty pumping engine is the most expensive in first cost but
consumes less steam for the power delivered than any other form of
reciprocating engine. For pumps of very small capacity the cost of fuel
is not so important an item as the first cost of the machine. For this
reason and because of the lower cost of attendance low-duty pumps are
more frequently found in small pumping stations.

[Illustration:

FIG. 64.—Diagram Showing Rates of Steam Consumption for Different Size
Units under Different Loads.
]

TABLE 27

WATER RATES OF PRIME MOVERS AT FULL AND PART LOADS

───────────────────────────────┬──────┬─────────────────────────┬──────
Type of Engine │ │ │Boiler
│Power,│ │Press.
│ K.W. │ Per Cent of Full Load │ Lbs.
───────────────────────────────┼──────┼────┬────┬────┬─────┬────┼──────
│ │ 25 │ 50 │ 75 │ 100 │125 │
───────────────────────────────┼──────┼────┼────┼────┼─────┼────┼──────
Single cylinder, high speed, │ │ │ │ │ │ │100 to
non-condensing │ 25│ 33│ 27│26.3│ 27.0│27.5│ 150
│ 250│ 42│37.5│ 35│ 34.0│34.0│
│ │ │ │ │ │ │
Automatic, flat four valve, │ │ │ │ │ │ │100 to
high speed │ 150│ │ 32│ 30│ 26.5│29.0│ 125
│ 250│ │ 33│ 31│ 28│30.0│
│ │ │ │ │ │ │
Tandem compound condensing, │ │ │ │ │ │ │100 to
high speed │ 125│ │ 23│ 19│ 17│ 18│ 150
│ │ │ 25│ 20│ 19.5│ 21│
│ │ │ │ │ │ │
Cross compound, condensing, │ │ │ │ │ │ │ 125
high speed │ │ 30│ 26│ 24│ 23│23.5│
│ │ │ │ │ │ │
Cross compound, non-condensing,│ │ │ │ │ │ │ 125
high speed │ │ 39│ 31│ 27│ 26│27.5│
│ │ │ │ │ │ │
Single cylinder Corliss, │ │ │ │ │ │ │ 100
condensing │ 120│23.7│20.4│ 19│ 18.5│19.0│
│ 500│26.3│22.8│21.3│ 20.8│21.3│ 125
│ │ │ │ │ │ │
Compound Corliss, condensing │ │16.5│ 14│12.5│ 12.1│12.5│ 100
│ │22.2│ 19│17.0│ 16.5│17.0│ 150
│ │ │ │ │ │ │
Single cylinder, rotary four │ │ │ │ │ │ │ 100
valve, non-condensing │ 75│26.2│22.3│21.3│ 21.6│22.8│
│ 400│35.0│27.2│26.4│ 26.0│26.8│ 180
│ │ │ │ │ │ │
Rotary four valve, tandem │ │ │ │ │ │ │ 100
compound non-condensing │ 125│32.0│22.0│ 20│18.25│18.5│
│ 600│40.0│28.3│23.2│ 22.5│22.7│ 150
│ │ │ │ │ │ │
Cross compound, non-condensing │ │ │ │ │ │ │ 100
rotary four valve │ 125│ 25│ 21│19.1│ 18.5│19.0│
│ 600│39.4│ 28│22.3│ 20.6│20.7│ 150
│ │ │ │ │ │ │
Single cylinder, poppett valve,│ │ │ │ │ │ │ 100
non-condensing │ 120│22.7│20.5│19.7│ 19.1│20.1│
│ 600│28.5│26.0│25.0│ 24.3│25.5│ 150
│ │ │ │ │ │ │
Single cylinder, poppett valve,│ │ │ │ │ │ │ 100
condensing │ 120│18.5│16.7│16.1│ 15.6│16.4│
│ 600│24.6│22.3│21.4│ 20.8│21.9│ 150
│ │ │ │ │ │ │
Compound condensing, poppett │ │ │ │ │ │ │ 100
valve │ 200│14.2│13.0│12.5│ 12.2│12.9│
│ 1200│18.4│16.9│16.3│ 15.9│16.8│ 150
│ │ │ │ │ │ │
Uniflow │ 125│14.6│13.7│13.4│ 13.4│13.3│ 150
│ 600│15.0│14.3│13.7│ 13.5│14.0│
│ │ │ │ │ │ │
Steam turbines, condensing, │ │ │ │ │ │ │ 125
Allis-Chalmers │ 300│ │ 24│ 17│ 160│16.5│
│ 2000│ │31.9│26.3│ 23.8│23.0│ 175
│ │ │ │ │ │ │
Steam turbines, condensing, │ │ │ │ │ │ │ 125
Westinghouse │ 300│ │13.7│12.8│ 12.2│12.6│
│ 2000│ │18.2│16.9│ 16.2│16.8│ 175
│ │ │ │ │ │ │
Steam turbines, high pressure, │ │ │ │ │ │ │
non-con., 12″ to 36″ wheel, │4 to 8│ │ │ │ │ │
1000 to 3600 R.P.M. │stages│ │ │ │ 28 5│ │
│ │ │ │ │116.5│ │
│ │ │ │ │ │ │
Ditto. Condensing, 26–inch │ │ │ │ │ 17 3│ │
│ │ │ │ │112.0│ │
│ │ │ │ │ │ │
───────────────────────────────┴──────┴────┴────┴────┴─────┴────┴──────

The steam consumption per indicated horse-power, better known as the
water rate of the engine, for various types of engines at full and at
part load is shown in Fig. 64. The steam consumption of other types at
full load is shown in Table 27. The indicated horse-power (I.H.P.) of a
steam engine is the product of the mean effective pressure (M.E.P.), the
area of the steam pistons, the length of the stroke, and the number of
strokes per unit of time. A common form of this expression is,

I.H.P = _PLAN_⁄33,000,

in which _P_ = the M.E.P. in pounds per square inch;

_L_ = the length of the stroke in inches;

_A_ = the sum of the areas of the pistons in square inches;

_N_ = the number of revolutions per minute.

The I.H.P. multiplied by the mechanical efficiency of the machine will
give the brake or water horse-power, that is, the horse-power delivered
by the machine. The product of the M.E.P., the sum of the areas of the
steam pistons and the mechanical efficiency of the machine, should equal
the product of the total head of water pumped against expressed in
pounds per square inch and the sum of the areas of the water pistons or
plungers. The M.E.P. is determined from indicator cards taken from the
steam cylinders during operation. These cards show the steam pressure on
the head and crank ends of each cylinder at all points during the
stroke.

=81. Steam Turbines.=—Among the advantages in the use of steam turbines
as compared with reciprocating steam engines for driving centrifugal
pumps are their simplicity of operation, the small floor space needed,
their freedom from vibration requiring a relatively light foundation,
and their ability to operate successfully and economically either
condensing or non-condensing under varying steam pressure. They can be
operated with steam at atmospheric or low pressure, thus taking the
exhaust from other engines. The greatest economy of operation for the
turbine alone will be obtained by operating with high pressure,
superheated steam and with a vacuum of 28 inches. In large units the
economy of operation of steam turbines is equal to that of the best type
of reciprocating engines. In order to develop the highest economy
turbines are operated at speeds from about 3,600 to 10,000 r.p.m. or
greater, the smaller turbines operating at the higher speeds. As these
speeds are usually too great for the operation of centrifugal pumps for
lifting sewage, reduction gears must be introduced between the turbine
and the pump. Although the best form of spiral-cut reduction gears may
obtain efficiencies of 95 to 98 per cent, or even higher, their use,
particularly in small units, is an undesirable feature of the steam
turbine for driving pumps.

The steam consumption of DeLaval turbines of different powers, and the
steam consumption of a 450 horse-power DeLaval turbine at different
loads are shown in Fig. 64. Some steam consumptions of other turbines
are recorded in Table 27. It is to be noted that the steam consumption
of the 450 horse-power turbine at part loads is not markedly greater
than that at full loads. This is an advantage of steam turbines as
compared with reciprocating engines. The steam consumption of any
turbine is dependent on the conditions of operation and is lower the
higher the vacuum into which the exhaust takes place.

[Illustration:

FIG. 65.—The DeLaval Trade Mark, Illustrating the Principle of the
DeLaval Steam Turbine.

Courtesy, DeLaval Steam Turbine Co.
]

There are two types of turbines in general use, the single stage or
impulse machines, and the compound or reaction type. The DeLaval is a
well-known make of the single stage or impulse type. The principle of
its operation is indicated in Fig. 65, which is the trade mark of the
DeLaval Steam Turbine Co. The energy of the steam is transmitted to the
wheel due to the high velocity of the steam impinging against the vanes.
In the compound or reaction type of machine the steam expands from one
stage to the next imparting its energy to the wheel by virtue of its
expansion in the passages of the turbine. For this reason the
single-stage or impulse type is operated at higher speeds than the
compound or reaction machines.

=82. Steam Boilers.=—Among the important points to be considered in the
selection of a steam boiler for a sewage pumping station are: the
necessary power; the quality of the feed water; the available floor
space; the steam pressure to be carried; and the quality and character
of the fuel. Tubular boilers of the type shown in Fig. 66, are lower in
first cost than other types of boilers. They are not ordinarily built in
units larger than 250 to 300 horse-power and where more power is desired
a number of units must be used. They are objectionable because of the
relatively large floor space required, and because of their relatively
poor economy of operation. The efficiencies of water-tube boilers of
different types are given in Table 28. Large power units of the
water-tube type, as shown in Fig. 67, although more expensive in first
cost, require less floor space. Almost any desired steam pressure can be
obtained from either type but water-tube boilers are more commonly used
for high pressures. The grate or stoker can be arranged to burn almost
any kind of fuel under either water-tube or fire-tube boilers. The use
of poor quality of water in water-tube boilers is undesirable as the
tubes are more likely to become clogged than the larger passages of the
fire-tube boilers. If necessary, a feed-water purification plant should
be installed, as it is usually cheaper to take the impurities out of the
water than to take the scale out of the boiler.

[Illustration:

FIG. 66.—Horizontal Fire-tube Boiler.
]

[Illustration:

FIG. 67.—Babcock and Wilcox Water-tube Boiler.
]

Not less than two boiler units should be used in any power station,
regardless of the demands for power, and if the feed water is bad, three
or even four units should be provided, as two units may be down at any
time. An appreciable factor of safety is provided by the ability of a
boiler to be operated at 30 to 50 per cent overload, if sufficient draft
is available, but with resulting reduction in the economy of operation.
The number of units provided should be such that the maximum load on the
pumping station can be carried with at least one in every 6 units or
less, out of service for repairs or other cause.

TABLE 28

EFFICIENCIES OF STEAM BOILERS

From Marks’ Mechanical Engineer’s Handbook
────────┬───────────┬──────────┬──────┬────────┬──────┬──────┬──────────
Type │Horse-power│ Furnace │ │ │ │Evap. │
│ │ │ │ │ │ from │
│ │ │ │ │ │and at│
│ │ │ │ │B.T.U.│ 212° │ Combined
│ │ │ Sq. │Per Cent│ per │ per │Efficiency
│ │ │ Ft. │of Rated│ Lb. │ Lb. │of Boiler
│ │ │Grate │Capacity│ Dry │ Dry │ and
│ │ │ Area │D’v’l’d │ Coal │ Coal │ Furnace
────────┼───────────┼──────────┼──────┼────────┼──────┼──────┼──────────
Babcock │ 300│Hand-fired│ │ │ │ │
& │ │ │ │ │ │ │
Wilcox│ │ │ 84│ 118.7│11,912│ 8.81│ 71.8
Babcock │ 640│Hand-fired│ │ │ │ │
& │ │ │ │ │ │ │
Wilcox│ │ │ 118│ 121.5│14,602│ 10.83│ 72.0
Stirling│ 1128│B. & W. │ │ │ │ │
│ │ chain │ │ │ │ │
│ │ grate │ 187│ 198.3│12,130│ 9.51│ 76.1
Rust │ 335│Hand-fired│ 68│ 210.5│13,202│ 9.42│ 68.9
Heine │ 400│Green │ │ │ │ │
│ │ chain │ │ │ │ │
│ │ grate │ 83.5│ 123.8│11,608│ 8.79│ 73.5
Maximum efficiency recorded │ │ │ │ │ 83
───────────────────────────────┴──────┴────────┴──────┴──────┴──────────

The steam delivered by a boiler is the basis of the measurement of its
capacity or power. A boiler horse-power is the delivery of 33,320 B.T.U.
per hour. It is approximately equal to the raising of 30 pounds of water
per hour from a temperature of 100° Fahrenheit, to steam at a pressure
of 70 pounds per square inch, or to 34 pounds of water per hour changed
to steam from and at 212° Fahrenheit, at atmospheric pressure. The
horse-power of a boiler is sometimes approximated by the area of its
grate or heating surface. Such a method of measuring has a low degree of
accuracy on account of the variations in the quality of the fuel, and
the rate of combustion. For example, the rate of combustion under a
locomotive boiler is high and there is less than ⅒th of a square foot of
grate area and about 4.5 square feet of heating surface per boiler
horse-power. The Scotch Marine type of boiler used on steam ships, has
slightly more grate area and slightly less heating surface than the
locomotive type of boiler, because the rate of combustion is lower.
Stationary water-tube boilers may have 2 to 3 times as much grate area
and heating surface per horse-power as is found in locomotive boilers.
If a poor type of fuel is to be used the area of the grate should be
increased about inversely as the heat content of the fuel. The
approximate heat content of various types of fuels is shown in Table 29.

TABLE 29

APPROXIMATE HEAT VALUE OF FUELS

─────────────────────────────────────┬────────────────┬────────────────
Fuel │ │Pounds of Water
│ │Evaporated from
│ │ and at 212° F.
│ │ All heat
│B.T.U. per Pound│ utilized
─────────────────────────────────────┼────────────────┼────────────────
Anthracite │ 13,500│ 14.0
Semi-bituminous, Pennsylvania │ 15,000│ 15.5
Semi-bituminous, best, West Virginia │ 15,000│ 15.8
Bituminous, best, Pennsylvania │ 14,450│ 15.0
Bituminous, poor, Illinois │ 10,500│ 10.9
Lignite, best, Utah │ 11,000│ 11.4
Lignite, poor, Oregon │ 8,500│ 8.8
Wood, best oak │ 9,300│ 9.6
Wood, poor ash │ 8,500│ 8.8
─────────────────────────────────────┴────────────────┴────────────────

=83. Air Ejectors.=—The Ansonia compressed-air sewage ejector is shown
in Fig. 68. In its operation, sewage enters the reservoir through the
inlet pipe at the right, the air displaced being expelled slowly through
the air valve marked B. The rising sewage lifts the float which actuates
the balanced piston valve in the pipe above the reservoir when the
reservoir fills. The lifting of the valve admits compressed air to the
reservoir. The air pressure closes valve A and the inlet valve at the
right, and ejects the sewage through the discharge pipe at the left. As
the float drops with the descending sewage it shuts off the air supply
and opens the air exhaust through the small pipe at the top center.
Sewage is prevented from flowing back into the reservoir by the check
valve in the discharge pipe. Other ejectors operating on a similar
principle are the Ellis, the Pacific, the Priestmann and the Shone.

=84. Electric Motors.=—The most common form of alternating current
electric motor used for driving sewage pumps where continuous operation
and steady loads are met is the squirrel-cage polyphase induction motor.
These motors operate at a nearly constant speed which should be selected
to develop the maximum efficiency of the pump and motor set. While Fig.
59 shows the best efficiency under varying heads to be obtained with
variable speed, the advantages of cost, attention, and availability make
the use of a constant speed motor common.[47] This type of motor is
undesirable where stopping and starting are frequent because it has a
relatively small starting torque and it requires a large starting
current. Such motors can be constructed in small sizes for high starting
torques by increasing the resistance of the rotor, but at the expense of
the efficiency of operation.

[Illustration:

FIG. 68.—Ansonia Compressed-Air Sewage Ejector.
]

Alternating current motors are more generally used than direct-current
motors because of the greater economy of transmission of alternating
current, but where direct current is available constant speed shunt
wound motors should be adopted.

In the selection of a motor to drive a centrifugal pump it is important
that the motor have not only the requisite power, but that its speed
will develop the maximum efficiency from the pump and motor combined. If
the pump and motor operate on the same shaft the speed of the two
machines must be the same. If the two are belt connected, the size of
the pulleys may be selected so as to give the required speed. If the
motor is to be connected to a power pump an adequate automatic pressure
relief valve should be provided on the discharge pipe from the pump, to
prevent the overloading of the motor or bursting of the pump in case of
a sudden stoppage in the pipe. The motor must be selected to suit the
conditions of voltage, cycle, and phase on the line. Transformers are
available to step the voltage up or down to practically any value.
Rotary converters are used to change direct to alternating current or
vice versa.

=85. Internal Combustion Engines.=—Internal combustion engines are used
for driving pumps. Units are available in size from fractions of 1
horse-power to 2,000 horse-power or more, although the use of the larger
sizes is exceptional. These engines are not commonly used for sewage
pumping but when used they are ordinarily belt connected to a
centrifugal pump, or to an electric generator which in turn drives
electric motors which operate centrifugal pumps. This type of engine is
more commonly adapted to small loads, although not entirely confined to
this field, as they serve admirably as emergency units to supplement an
electrically equipped pumping station. The fuel efficiency of internal
combustion engines is higher than for steam engines as is indicated in
Table 30, but the fuel is more expensive.

The four-cycle gas engine shown in Fig. 69 is the type most commonly
used. Its horse-power is the product of: the mean effective pressure,
the length of the stroke, the area of the piston, and the number of
explosions per second divided by 550. The M.E.P. is dependent on the
character of the fuel used and the compression of the gas before
ignition. Producer gas will furnish mean effective pressures between 60
and 70 pounds per square inch, natural gas and gasoline, 85 to 90 pounds
per square inch, and alcohol from 95 to 110 pounds per square inch.

TABLE 30

COMPARATIVE FUEL COSTS FOR PRIME MOVERS

───────────────────────────────────────┬───────────────┬───────────────
Type of Engine │ Quantity of │Cost of Fuel in
│ Fuel per H.P. │ Cents per
│ Hour │ Horse-power
│ │ Hour
───────────────────────────────────────┼───────────────┼───────────────
Reciprocating steam engines, simple, │ 21 to 8 lb. │ 4.2 to 1.6
non-condensing, 25 to 200 H.P. │ coal │
Triple condensing, 2000 to 10,000 │2.3 to 1.9 lb. │ 0.46 to 0.37
H.P. │ coal │
───────────────────────────────────────┼───────────────┼───────────────
Steam turbines, high pressure, │ │
non-condensing, │ │
200 to 500 K.W. │6.5 to 4.2 lb. │ 1.3 to 0.86
│ coal │
500 to 3000 K.W. │2.6 to 1.9 lb. │ 0.52 to 0.37
│ coal │
Condensing 5000 to 20,000 K.W. │1.8 to 1.43 lb.│ 0.36 to 0.28
│ coal │
───────────────────────────────────────┼───────────────┼───────────────
Gas engines │ │
Natural gas, 50 to 200 H.P. │ 19 to 11 cu. │
│ ft. │
Producer gas, 50 to 200 H.P. │ 2 to 1.5 cu. │
│ ft. │
Illuminating gas, 10 to 75 H.P. │ 26 to 19 cu. │ 2.1 to 1.5
│ ft. │
Gasoline, 10 to 75 H.P. │ 1.5 to 0.8 │ 5.6 to 3.0
│ pints │
───────────────────────────────────────┼───────────────┼───────────────
Oil engines, 100 to 500 H.P. │1.1 to 0.75 lb.│
│ oil │
───────────────────────────────────────┴───────────────┴───────────────
NOTE.—Coal assumed at $4.00 per ton, illuminating gas at 80 cents per
thousand cubic feet, and gasoline at 30 cents per gallon.

[Illustration:

FIG. 69.—Bessemer Oil Engine. Twin Cylinder, Valve Side.
]

The Diesel Engine is the most efficient of internal combustion engines.
The original aim of the inventor, Dr. Rudolph Diesel, was to avoid the
explosive effect of the ordinary internal combustion engine by injecting
a fuel into air so highly compressed that its heat would ignite the
fuel, causing slow combustion of the fuel thus utilizing its energy to a
greater extent. The fuel and air were to be so proportioned as to
require no cooling. Although the ideal condition has not been attained,
the heat efficiency of Diesel engines is high. They will consume from
0.3 to 0.5 of a pound of oil (containing 18,000 B.T.U. per pound) per
brake horse-power hour, giving an effective heat efficiency of 25 to 30
per cent. Although not now in extensive use in the United States it is
probable that this engine will be more generally adopted for conditions
suitable for internal combustion engines.

=86. Selection of Pumping Machinery.=—Centrifugal pumps are particularly
adapted to the lifting of sewage because of their large passages, and
their lack of valves. The low lifts, nearly constant head, and the
possibility of equalizing the load by means of reservoirs are
particularly suited to efficient operation of centrifugal pumps. They
require less floor space than reciprocating pumps of the same capacity,
and because of their freedom from vibration they do not demand so heavy
a foundation. The discharge from the pump is continuous thus relieving
the piping from vibration. In case of emergency the discharge valve can
be shut off without shutting down the pump, an important point in “fool
proof” operation.

Volute pumps are better adapted to pumping sewage as their passages are
more free and they are better suited to the low lifts met. Gritty and
solid matter will cause wear on the diffusion vanes of turbine pumps in
spite of the most careful design. Although turbine pumps can possibly be
built with higher efficiency than volute pumps, their efficiency at part
load falls rapidly and the fluctuations of sewage flow are sufficient to
affect the economy of operation. Turbine pumps are more expensive and
heavier than volute pumps on account of the increased size necessitated
by the diffusion vanes.

Multi-stage pumps are used for high lifts and are seldom if ever
required in sewage pumping. As ordinarily manufactured, each stage is
good for an additional 40 to 100 pounds pressure, but wide variations in
the limiting pressures between stages are to be found.

Reciprocating plunger pumps are sometimes used for sewage pumping where
the character of the sewage is such that the valves will not be clogged
nor parts of the pump corroded. These pumps are seldom used in small
installations or for low lifts. They are not adapted to automatic or
long distance control as are electrically driven centrifugal pumps. The
use of reciprocating pumps for sewage pumping is practically restricted
to very large pumping stations with capacities in the neighborhood of
50,000,000 gallons per day or more. Steam-driven pumps are the most
common of the reciprocating type, but power pumps are sometimes used in
special cases for small installations and may be driven by either a
steam or gas engine or an electric motor.

Compressed air ejectors, as described in Art. 83 are used for lifting
sewage and other drainage from the basement of buildings below the sewer
level.

Centrifugal pumps electrically driven are, as a rule, the most
satisfactory for sewage pumping. Electric drive lends itself to control
by automatic devices, which are particularly convenient in small pumping
stations. The control can be arranged so that the pump is operated only
at full load and high efficiency, and when not operating no power is
being consumed, as is not the case with a steam pump where steam
pressure must be maintained at all times. The electric driven pump is
thrown into operation by a float controlled switch which is closed when
the reservoir fills, and opens when the pump has emptied the reservoir.
The choice between steam and electric power for large pumping stations
is a matter of relative reliability and economy.

The selection of the proper type of pump, whether reciprocating or
otherwise, requires some experience in the consideration of the factors
involved. Fig. 70 is of some assistance. In discussing this figure,
Chester states:

“Fig. 70 attempts to represent graphically, the writer’s ideas
under general conditions, of the machines that should be selected
for certain capacities for both principal engine and alternate and
the station duty they may be expected to produce, but you must
realize that this intends the principal engine doing at least 90
per cent of the work and that the head, the cost of coal, the load
factor, the cost of real estate ... the boiler pressure, and the
space available, and finally ... the funds available, are factors
which may shift both the horizontal and curved lines. In the field
of low service pumps of 10,000,000 capacity or over, the
centrifugal pump reigns supreme, and for constant low heads of
20,000,000 capacity or over the turbine driven centrifugal usurps
the field.”

A reciprocating pump of any type would have to be specially built for
pumping sewage not carefully screened or otherwise treated, as the
valves, ordinarily used in such pumps for lifting water, would clog. The
vertical triple-expansion pumping engine with special valves and for
large installations, and the centrifugal pump for large or small
installations are the only suitable types for pumping sewage. With steam
turbine or electric drive the centrifugal has the field to itself.

[Illustration:

FIG. 70.—Expectancy Curves for Pumping Engines Working against a
Pressure of 100 Pounds per Square Inch.

J. N. Chester, Journal Am. Water Works Ass’n, Vol. 3, 1916, p. 493.
]

=87. Costs of Pumping Machinery.=—The cost of pumping machinery can not
be stated accurately as the many factors involved vary with the
fluctuations in the prices of raw materials, transportation, labor, etc.
The actual purchase price of machinery can be found accurately only from
the seller. The costs given in this chapter are useful principally for
comparative purposes and for exercise in the making of estimates. The
costs of complete pumping stations are shown in Table 31.[48] These
figures represent costs in 1911.

TABLE 31

COSTS OF COMPLETE PUMPING STATIONS

These costs include the best type of triple-expansion engines,
high-pressure boilers, brick or inexpensive stone building with slate
roof, chimney and intake. Cost of land is not included.
─────────────────┬─────────────────┬─────────────────┬─────────────────
Discharge │ Horse-power per │ │
Pressure, Lbs. │ Million Gals. │Cost, Dollars per│Cost, Dollars per
per Sq. In. │ Pumped │ Horse-power │ Million Gallons
─────────────────┼─────────────────┼─────────────────┼─────────────────
30│ 12│ 562│ 6,750
40│ 16│ 438│ 7,000
50│ 20│ 362│ 7,250
60│ 24│ 312│ 7,500
70│ 28│ 277│ 7,750
80│ 32│ 250│ 8,000
90│ 36│ 229│ 8,250
100│ 40│ 213│ 8,500
110│ 44│ 200│ 8,750
120│ 48│ 187│ 9,000
130│ 52│ 192│ 10,000
│ │ │
─────────────────┴─────────────────┴─────────────────┴─────────────────

=88. Cost Comparisons of Different Designs.=—In the design of a pumping
station and its equipment the relative costs of different designs should
be compared, and the least expensive design selected, due consideration
being given to serviceability, reliability, and other factors without
definite financial value. In comparing the costs of different types of
machinery, all items in connection with the pumping station should be
considered. For example, the cost of an electrically driven centrifugal
pump and equipment may be less than the total cost of a steam driven
reciprocating pump and equipment because of the saving in the cost of
boilers, boiler house, etc., but a comparison of the capitalized cost of
the two might show in favor of the reciprocating steam pump because of
the lower cost of operation.

The total cost of a plant, or any portion thereof, may be considered as
made up of three parts: (1) The first cost, (2) operation and
maintenance and, (3) renewal. The total cost S can be expressed as

_S_ = _C_ + _O_⁄_r_ + _R_,

in which _C_ = the first cost;

_O_ = the annual expenditure for operation and maintenance;

_R_ = the amount set aside to cover renewal;

_r_ = the rate of interest.

_S_ is called the capitalized cost of a plant. The annual payment
necessary to perpetuate a plant is

_A_ = _Sr_ = _Cr_ + _O_ + _Rr_.

The value of _R_ is useful when expressed in terms of the life of the
plant or machine and the current rate of interest. It is sometimes
called the depreciation factor or capitalized depreciation. If it is
borne in mind that _R_ is the amount to be set aside at compound
interest for the life of the plant, at the end of which time the accrued
interest should be sufficient to renew the plant, it is evident that

_R_(1 + _R_)^n − _R_ = _C_

or _R_ = _C_⁄((1+_r_)^n − 1)

in which _n_ is the period of usefulness, or life of the plant,
expressed in years, no allowance being made for scrap value.

A comparison of the annual expense of three different plants is shown in
Table 32. It is evident from this comparison that the machinery with the
least first cost is not always the least expensive when all items are
considered.

A sinking fund is a sum of money to which additions are made annually
for the purpose of renewing a plant at the expiration of its period of
usefulness. The annual payment into the sinking fund is equivalent to
the term _Rr_ in the expression for annual cost, or in terms of _C_,
_r_, and _n_, the annual payment is

_Cr_⁄((1 + _r_)^n − 1).

It is the same as the capitalized depreciation multiplied by the rate of
interest. The expression _r_⁄((1 + _r_)^n − 1) is sometimes called the
rate of depreciation.

The present worth of a machine is the difference between its first cost
and the present value of the sinking fund. If _m_ represents the present
age of a plant in years, then the present worth is

_P_ = _C_(1 – ((1 + _r_)^n − 1)⁄((1 + _r_)^m − 1)).

TABLE 32

COMPARISON OF COSTS OF THREE DIFFERENT PUMPING STATIONS. NOMINAL
CAPACITY THIRTY MILLION GALLONS PER DAY RAISED THIRTY FEET

────────────────┬──────────────────────────────────
Equipment │ Plant A
────────────────┼──────────────────────────────────
│One Acre of Land. Brick Building,
│ Steel Trussed Roof, Slate
│ Covered. Cross Compound
│ Condensing Horizontal Pumping
│ Engine
────────────────┼───────┬──────────┬───────┬───────
│Annual │ Years of │Sinking│ Total
│Payment│Usefulness│ Fund │
│ on │ │Payment│
│ First │ │ │
│ Cost │ │ │
────────────────┼───────┼──────────┼───────┼───────
Land │ 100│ │ 0│ 100
Permanent │ 1188│ 50│ 1080│ 2,260
Structures[49]│ │ │ │
Pumps and │ 440│ 15│ 435│ 875
Machinery │ │ │ │
Boilers │ 280│ 10│ 446│ 726
Labor │ │ │ │ 14,000
Fuel │ │ │ │ 5,500
Repairs, etc. │ │ │ │ 480
────────────────┼───────┼──────────┼───────┼───────
Total │ │ │ │ 23,941
────────────────┴───────┴──────────┴───────┴───────

────────────────┬──────────────────────────────────
Equipment │ Plant B
────────────────┼──────────────────────────────────
│One Acre of Land. Brick Building.
│ Steel Trussed Roof, Slate
│ Covered. Compound Condensing Low
│ Duty Horizontal Pumping Engine
│
────────────────┼───────┬──────────┬───────┬───────
│Annual │ Years of │Sinking│ Total
│Payment│Usefulness│ Fund │
│ on │ │Payment│
│ First │ │ │
│ Cost │ │ │
────────────────┼───────┼──────────┼───────┼───────
Land │ 100│ │ 0│ 100
Permanent │ 1180│ 50│ 1080│ 2,260
Structures[49]│ │ │ │
Pumps and │ 390│ 15│ 395│ 785
Machinery │ │ │ │
Boilers │ 252│ 10│ 400│ 652
Labor │ │ │ │ 14,000
Fuel │ │ │ │ 7,200
Repairs, etc. │ │ │ │ 400
────────────────┼───────┼──────────┼───────┼───────
Total │ │ │ │ 25,497
────────────────┴───────┴──────────┴───────┴───────

────────────────┬──────────────────────────────────
Equipment │ Plant C
────────────────┼──────────────────────────────────
│One Acre of Land. Frame Building,
│ Shingle Roof. Compound Duplex
│ Non-Condensing Pumping Engine.
│
│
────────────────┼───────┬──────────┬───────┬───────
│Annual │ Years of │Sinking│ Total
│Payment│Usefulness│ Fund │
│ on │ │Payment│
│ First │ │ │
│ Cost │ │ │
────────────────┼───────┼──────────┼───────┼───────
Land │ 100│ │ 0│ 100
Permanent │ 810│ 50│ 775│ 1,585
Structures[49]│ │ │ │
Pumps and │ 360│ 15│ 352│ 712
Machinery │ │ │ │
Boilers │ 308│ 10│ 490│ 798
Labor │ │ │ │ 14,000
Fuel │ │ │ │ 8,200
Repairs, etc. │ │ │ │ 550
────────────────┼───────┼──────────┼───────┼───────
Total │ │ │ │ 25,945
────────────────┴───────┴──────────┴───────┴───────

Where straight-line depreciation is spoken of it is assumed that the
worth of a machine depreciates an equal part of its first cost each
year. For example, if the life of a plant is assumed to be 20 years,
straight-line depreciation will assume that the plant loses 1/20 of its
original value annually. The present worth of a plant under this
assumption would be the product of its first cost and the ratio between
its remaining life and its total life. This method of estimating
depreciation and worth is frequently used, particularly for short-lived
plants and for simplicity in bookkeeping, but it is less logical than
the method given above.

=89. Number and Capacity of Pumping Units.=—In order to select the
number and capacity of pumping units for the best economy, a comparison
of the costs of different combinations of units should be made and the
most economical combination determined by trial. The principles outlined
in the preceding articles should be observed in making these
comparisons. In a steam pumping station, when the number of units
operating is less than the average daily maximum for the period, steam
must nevertheless be kept on a sufficient number of boilers to operate
the maximum number of pumps. This, and corresponding standby losses must
not be overlooked, as they may show that a smaller number of larger
units is ultimately more economical.

TABLE 33

SUMMARY OF FLUCTUATIONS OF SEWAGE FLOW AT A PROPOSED PUMPING STATION

─────────────────┬─────────────────┬─────────────────┬─────────────────
Number of Days │Flow in Thousand │ │
Loads Occurred in│ Gallons per │ │
One Year │ Minute │ Lift in Feet │ Horse-power
─────────────────┼─────────────────┼─────────────────┼─────────────────
1│ 293│ 6.0│ 450
8│ 163│ 8.6│ 354
15│ 119│ 10.0│ 300
18│ 106│ 10.6│ 284
23│ 88│ 11.2│ 249
31│ 69│ 12.2│ 211
32│ 65│ 12.4│ 204
45│ 51│ 13.4│ 173
41│ 50│ 13.5│ 169
30│ 45│ 13.8│ 158
28│ 44│ 13.9│ 154
23│ 40│ 14.2│ 143
21│ 38│ 14.4│ 137
18│ 35│ 14.6│ 129
12│ 29│ 15.0│ 111
8│ 24│ 15.6│ 95
5│ 20│ 16.0│ 79
3│ 16│ 16.5│ 65
2│ 14│ 16.8│ 58
1│ 6.5│ 18.0│ 29
─────────────────┴─────────────────┴─────────────────┴─────────────────
Total horse-power days for one year, 102,000.
Average load in horse-power, 280.

TABLE 34

POSSIBLE COMBINATIONS OF FIVE PUMPING UNITS TO CARE FOR THE LOADS SHOWN
IN TABLE 33[50]

──────────────────────────────────┬───────────────
40 Horse-power │ Load
Type 1[51] │
────────┬──────┬───────────┬──────┼───────┬───────
Per Cent│Pounds│ Load in │Pounds│Number │ Total
of Rated│Steam │Horse-power│Steam,│of Days│ Load
Capacity│ per │ │Units │Load is│Carried
│ H.P. │ │10,000│Carried│ on
│ Hour │ │Pounds│in Year│ these
│ │ │ │ │Days in
│ │ │ │ │ H.P.
────────┼──────┼───────────┼──────┼───────┼───────
151│ 45│ 60.4│ 6.5│ 1│ 681
120│ 44│ 48│ 40.5│ 8│ 542
102│ 45│ 40.8│ 66.1│ 15│ 458
96│ 45│ 38.4│ 74.8│ 18│ 434
98│ 45│ 39.2│ 97.5│ 23│ 381
│ │ │ │ 31│ 322
│ │ │ │ 32│ 312
│ │ │ │ 45│ 264
│ │ │ │ 41│ 258
101│ 45│ 40.4│ 131│ 30│ 242
98│ 45│ 39.2│ 119│ 28│ 235
│ │ │ │ 23│ 218
│ │ │ │ 21│ 210
│ │ │ │ 18│ 198
│ │ │ │ 12│ 170
104│ 45│ 41.6│ 20.9│ 8│ 145
│ │ │ │ 5│ 121
│ │ │ │ 3│ 100
99│ 45│ 39.6│ 8.5│ 2│ 89
113│ 44│ 45.2│ 4.8│ 1│ 45
│ │ │ ————│ │
Sub-total │ 596.6│ │
Grand total in pounds, 65,700,000
──────────────────────────────────────────────────

──────────────────────────────────┬───────────────
50 Horse-power │ Load
Type 1[51] │
────────┬──────┬───────────┬──────┼───────┬───────
Per Cent│Pounds│ Load in │Pounds│Number │ Total
of Rated│Steam │Horse-power│Steam,│of Days│ Load
Capacity│ per │ │Units │Load is│Carried
│ H.P. │ │10,000│Carried│ on
│ Hour │ │Pounds│in Year│ these
│ │ │ │ │Days in
│ │ │ │ │ H.P.
────────┼──────┼───────────┼──────┼───────┼───────
151│ 45│ 75.5│ 8.2│ 1│ 681
120│ 44│ 60.0│ 50.7│ 8│ 542
102│ 45│ 51.0│ 82.7│ 15│ 458
90│ 45│ 48.0│ 93.5│ 18│ 434
98│ 45│ 49.0│ 122.0│ 23│ 381
104│ 45│ 52.0│ 174.5│ 31│ 322
101│ 45│ 50.5│ 174.8│ 32│ 312
│ │ │ │ 45│ 264
103│ 45│ 51.5│ 228│ 41│ 258
│ │ │ │ 30│ 242
│ │ │ │ 28│ 235
│ │ │ │ 23│ 218
│ │ │ │ 21│ 210
│ │ │ │ 18│ 198
│ │ │ │ 12│ 170
│ │ │ │ 8│ 145
109│ 44│ 54.5│ 28.8│ 5│ 121
│ │ │ │ 3│ 100
99│ 45│ 49.5│ 10.7│ 2│ 89
│ │ │ │ 1│ 45
│ │ │ ————│ │
│ 973.9│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────
60 Horse-power │ Load
Type 1[51] │
────────┬──────┬───────────┬──────┼───────┬───────
Per Cent│Pounds│ Load in │Pounds│Number │ Total
of Rated│Steam │Horse-power│Steam,│of Days│ Load
Capacity│ per │ │Units │Load is│Carried
│ H.P. │ │10,000│Carried│ on
│ Hour │ │Pounds│in Year│ these
│ │ │ │ │Days in
│ │ │ │ │ H.P.
────────┼──────┼───────────┼──────┼───────┼───────
151│ 45│ 90.6│ 9.8│ 1│ 681
120│ 44│ 72.0│ 60.8│ 8│ 542
102│ 45│ 61.2│ 99.2│ 15│ 458
96│ 45│ 57.6│ 112│ 18│ 434
│ │ │ │ 23│ 381
104│ 45│ 62.4│ 209.0│ 31│ 322
101│ 45│ 60.6│ 210│ 32│ 312
102│ 45│ 61.2│ 325│ 45│ 264
│ │ │ │ 41│ 258
│ │ │ │ 30│ 242
│ │ │ │ 28│ 235
│ │ │ │ 23│ 218
│ │ │ │ 21│ 210
│ │ │ │ 18│ 198
106│ 45│ 63.6│ 137│ 12│ 170
│ │ │ │ 8│ 145
109│ 44│ 65.4│ 34.5│ 5│ 121
│ │ │ │ 3│ 100
│ │ │ │ 2│ 89
│ │ │ │ 1│ 45
│ │ │ ————│ │
│1197.3│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────
100 Horse-power │ Load
Type 4[51] │
────────┬──────┬───────────┬──────┼───────┬───────
Per Cent│Pounds│ Load in │Pounds│Number │ Total
of Rated│Steam │Horse-power│Steam,│of Days│ Load
Capacity│ per │ │Units │Load is│Carried
│ H.P. │ │10,000│Carried│ on
│ Hour │ │Pounds│in Year│ these
│ │ │ │ │Days in
│ │ │ │ │ H.P.
────────┼──────┼───────────┼──────┼───────┼───────
151│ 28│ 151│ 10.2│ 1│ 681
120│ 25│ 120│ 57.5│ 8│ 542
102│ 25│ 102│ 62.5│ 15│ 458
96│ 25│ 96│ 103.8│ 18│ 434
98│ 25│ 98│ 135.1│ 23│ 381
│ │ │ │ 31│ 322
│ │ │ │ 32│ 312
│ │ │ │ 45│ 264
│ │ │ │ 41│ 258
│ │ │ │ 30│ 242
│ │ │ │ 28│ 235
│ │ │ │ 23│ 218
│ │ │ │ 21│ 210
│ │ │ │ 18│ 198
106│ 25│ 106│ 76.5│ 12│ 170
104│ 25│ 104│ 29.1│ 8│ 145
│ │ │ │ 5│ 121
100│ 25│ 100│ 32.4│ 3│ 100
│ │ │ │ 2│ 89
│ │ │ │ 1│ 45
│ │ │ ————│ │
│ 507.1│ │

──────────────────────────────────────────────────

──────────────────────────────────┬───────────────
200 Horse-power │ Load
Type 5[51] │
────────┬──────┬───────────┬──────┼───────┬───────
Per Cent│Pounds│ Load in │Pounds│Number │ Total
of Rated│Steam │Horse-power│Steam,│of Days│ Load
Capacity│ per │ │Units │Load is│Carried
│ H.P. │ │10,000│Carried│ on
│ Hour │ │Pounds│in Year│ these
│ │ │ │ │Days in
│ │ │ │ │ H.P.
────────┼──────┼───────────┼──────┼───────┼───────
151│ 23│ 302│ 16.7│ 1│ 681
120│ 20│ 240│ 92.0│ 8│ 542
102│ 20│ 204│ 147│ 15│ 458
96│ 20│ 192│ 166│ 18│ 434
98│ 20│ 196│ 216│ 23│ 381
104│ 20│ 208│ 309.5│ 31│ 322
101│ 20│ 202│ 310│ 32│ 312
102│ 20│ 204│ 481│ 45│ 264
103│ 20│ 206│ 405│ 41│ 258
101│ 20│ 202│ 291│ 30│ 242
98│ 20│ 196│ 264│ 28│ 235
109│ 20│ 218│ 241│ 23│ 218
105│ 20│ 210│ 212│ 21│ 210
99│ 20│ 198│ 171│ 18│ 198
│ │ │ │ 12│ 170
│ │ │ │ 8│ 145
│ │ │ │ 5│ 121
│ │ │ │ 3│ 100
│ │ │ │ 2│ 89
│ │ │ │ 1│ 45
│ │ │ ————│ │
│3322.2│ │

──────────────────────────────────────────────────

TABLE 35

FINANCIAL COMPARISON OF PUMPING EQUIPMENTS

The loads to be cared for are shown in Table 34. An emergency unit is
supplied to bring the overload capacity of the plant, less the largest
unit, equal to the maximum load on the plant. No unit will be
overloaded more than fifty per cent of its rated capacity.

───────────┬───────────┬───────────┬───────────┬───────────┬───────────
Number of │ │ │ │ │
Units │ │ │ │ │
Exclusive │ │ │ │ │
of │ │ │ │ │
Emergency │ │ │ │ │
Unit │ 5 │ 4 │ 3 │ 2 │ 1
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
Capacity │ 40 h.p.,│ │ │ │
and Type of│ Type 1│ │ │ │
Units │ 50 h.p.,│ 50 h.p.,│ │ │
│ Type 1│ Type 1│ │ │
│ 60 h.p.,│ 100 h.p.,│ 50 h.p.,│ │
│ Type 1│ Type 4│ Type 1│ │
│ 100 h.p.,│ 125 h.p.,│ 150 h.p.,│ 200 h.p.,│
│ Type 4│ Type 4│ Type 5│ Type 5│
│ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p.,
│ Type 5│ Type 5│ Type 6│ Type 6│ Type 7
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
Emergency │ │ │ │ │
Unit, │ │ │ │ │
Capacity │ 200 h.p.,│ 175 h.p.,│ 250 h.p.,│ 250 h.p.,│ 450 h.p.,
and Type │ Type 5│ Type 5│ Type 6│ Type 6│ Type 7
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
Annual │ │ │ │ │
payments,│ │ │ │ │
Dollars │ │ │ │ │
First │ │ │ │ │
cost of│ │ │ │ │
pumps │ 1,560│ 1,660│ 1,480│ 1,440│ 1,500
Renewal │ │ │ │ │
of │ │ │ │ │
pumps │ 1,340│ 1,430│ 1,270│ 1,240│ 1,290
First │ │ │ │ │
cost, │ │ │ │ │
boilers│ 1,024│ 1,089│ 1,125│ 1,115│ 1,410
Renewal, │ │ │ │ │
boilers│ 800│ 935│ 966│ 958│ 1,210
Fuel │ 13,140│ 11,860│ 10,490│ 9,420│ 9,400
Repairs, │ │ │ │ │
oil, │ │ │ │ │
etc. │ 2,000│ 1,800│ 1,500│ 1,300│ 1,200
Labor │ 35,000│ 31,500│ 29,500│ 27,000│ 27,000
Emergency│ │ │ │ │
unit. │ │ │ │ │
First │ │ │ │ │
cost │ 640│ 560│ 800│ 800│ 1,500
Emergency│ │ │ │ │
unit. │ │ │ │ │
Renewal│ 550│ 480│ 690│ 690│ 1,290
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
Total │ 56,134│ 51,314│ 47,821│ 43,963│ 45,800
───────────┴───────────┴───────────┴───────────┴───────────┴───────────

Type 1. Simple duplex, non-condensing, horizontal.

Type 4. Compound condensing low duty horizontal.

Type 5. Low duty, triple, condensing, horizontal.

Type 6. Cross compound, condensing, horizontal.

Type 7. High duty, triple, condensing, vertical.

For example, the sewage flow expected at a proposed pumping station is
shown in Table 33. The steps involved in the selection of the number and
capacity of pumping units to care for these quantities are as follows:
(1) Determine the rated capacity of the equipment to be provided. In
this case the capacity will be taken as 450 horse-power, which is the
maximum load to be placed on the pumps. (2) Select any number of units
of such different types and capacities as are available for comparison,
and arrange them in different combinations so that each unit will
operate as nearly as possible at its rated capacity. The work involved
in such a study for 5 units is shown in Table 34. The weight of steam
consumed per indicated horse-power hour corresponding to the per cent of
the rated capacity at which the unit is operating is read from Fig. 64
or other data. (3) Repeat this step for other numbers and types of
units. (4) Prepare a table showing the annual costs of combinations of
different numbers and types of units as shown for this example in Table
35. The figures in Table 35 show that the least expensive of the
combinations of the units studied is one 200 horse-power unit, and one
250 horse-power unit, with a 250 horse-power unit in reserve. It is to
be noted that a reserve unit has been provided in each combination, the
capacity of which is equal to that of the largest unit of the
combination.

CHAPTER VIII
MATERIALS FOR SEWERS

=90. Materials.=—The materials most commonly used for the manufacture of
sewer pipe are vitrified clay and concrete. Cast iron, steel, and wood
are also used, but only under special conditions. For pipes built in the
trench, concrete, concrete blocks, brick, and vitrified clay blocks are
used. Concrete is being used to-day more than bricks or blocks because
it is cheaper. A decade or more ago all large sewers were built of
bricks. Vitrified clay and concrete are used for manufactured pipe 42
inches and less in diameter. Concrete is used almost exclusively for
larger sizes of pipe, particularly for pipe constructed in place,
although a brick invert lining is advisable when high velocities of flow
are expected.

The character of the external load, the velocity of flow and the quality
of sewage are important factors in determining the material to be used
in the construction of sewers. Reinforced concrete should be used for
large sewers near the surface subjected to heavy moving loads. A high
velocity of flow with erosive suspended matter demand a brick wearing
surface on the invert. Many engineers consider concrete less suitable
than vitrified clay or brick for conveying septic sewage or acid
industrial wastes, as concrete deteriorates more rapidly under such
conditions. Concrete should be used on soft yielding foundations,
whereas a hard compact earth, which can be cut to the form of the sewer,
is suitable to the use of brick or concrete.

Cast-iron pipe with lead joints is used for sewers flowing under
pressure, or where movements of the soil are to be expected. If the
sewage is not flowing under pressure, cement joints are sometimes used
in the cast-iron pipe. Movements of the soil are to be expected on side
hills, under railroad tracks, etc. Steel pipe is used on long outfalls
or under other conditions where external loads are light and the cost is
less than for other materials. Because of the thin plates used and the
liability to corrosion steel is not frequently used. It should never be
deeply buried nor externally loaded because of its weakness in resisting
such forces. Like wood pipe, its lightness is favorable to use on
bridges, but the greater heat conductivity of steel than wood
necessitates protection against freezing in exposed positions. Wood is
preferable only where the economy of its use is pronounced and the pipe
is running full at all times. It is desirable that the wood pipe should
be always submerged as the life of alternately wet and dry wood is
short.

Corrugated galvanized iron and unglazed tile have been used for sewers,
but usually only in emergencies or as a makeshift. Corrugated iron is
not suitable on account of its roughness and liability to corrosion, and
unglazed tile because of its lack of strength.

[Illustration:

FIG. 71.—Diagrammatic Section through Clay-pipe Press.
]

=91. Vitrified Clay Pipe.=—In general the physical and chemical
qualities of clays before burning are not sufficient to cause their
condemnation or approval by the engineer, as their behavior in the
furnace is quite individual and depends greatly on the manner in which
they are fired. The engineer is interested in the result and writes his
specifications accordingly.

In the manufacture of clay pipe, the clay as excavated is taken to a
mill and ground while dry, to as fine a condition as possible. It is
then sent to storage bins from which it is taken for wet grinding and
tempering. In this process the clay is mixed with water to the proper
degree of plasticity. A variation of 1 to 1½ per cent in the moisture
content will mean failure. Too wet a mixture will not have sufficient
strength to maintain its shape in the kiln. Too dry a mixture will show
laminations as it is pressed through the discs.

A press used in the manufacture of clay pipe is shown in cross-section
in Fig. 71. With the piston heads in the steam and mud cylinders at
their extreme upward positions, the mud cylinder is filled with clay of
the proper consistency. Steam is then turned into the steam cylinder
under pressure and the clay is squeezed into the space between the inner
and outer shells of the die and mandrel to form the hub of the pipe. The
pressure on the clay may be from 250 to 600 pounds per square inch. When
clay appears at the holes, marked _hh_ at the bottom of the mud
cylinder, the bottom plate and the center portion of the die are removed
and the remainder or straight portion of the pipe is formed by squeezing
the clay between the mandrel and the outer wall of the die. A completely
formed pipe can be seen issuing from the press in Fig. 72. Any sized
pipe that is desired can be formed from the same press by changing the
size of the dies and mandrel.

[Illustration:

FIG. 72.—Clay-pipe Press.

Courtesy, Blackmer and Post Manufacturing Co.
]

Curved pipes are made in two ways—by bending directly as they issue from
the press, or by shaping by hand in plaster of paris molds. Junctions
are made by cutting the branch pipe to the shape of the outside of the
main pipe, fastening the branch in place with soft clay and then cutting
out the wall of the main pipe the size of the branch. Special fittings
are usually made by hand in plaster molds.

After being pressed into shape the pipes are taken to a steam-heated
drying room where a constant temperature is maintained in order to
prevent cracking of the pipes. They remain in the drying room from 3 to
10 days until dry, when they are taken to the kilns. If taken to the
kilns when moist blisters will be produced.

The dried pipes are piled carefully in the kiln so that heat and weight
may be as evenly distributed as possible, and the fire is then started
in the kiln. The process of burning can be roughly divided into five
stages:

1st. Water smoking, which lasts about 72 hours during which the
temperature is raised gradually to 350 degrees Fahrenheit.

2nd. Heating, during which the temperature is raised to 800 degrees
Fahrenheit in 24 hours.

3rd. Oxidation, during which the temperature is raised to 1,400 degrees
Fahrenheit in 84 hours.

4th. Vitrification, in which the temperature is raised to 2,100 degrees
Fahrenheit in 48 hours, and finally,

5th. Glazing, during which the temperature is unchanged but salt (NaCl)
is thrown in and allowed to burn.

Oxidation must be complete before vitrification is started as otherwise
blisters will be raised due to imprisoned carbon dioxide. The important
points in vitrification are to make the required temperature within a
reasonable time and to maintain a uniform distribution of heat
throughout the kiln. When vitrification is complete as shown by a glassy
fracture of a broken sample taken from the kiln, glazing is accomplished
by throwing a shovelful of salt on the hottest part of the fire. About
five to six applications of salt from two to three hours apart may be
needed. The kiln is then allowed to cool and the manufacture of the pipe
is complete. The completeness of vitrification is indicated by the
amount of water that the finished pipe will absorb. Completely vitrified
pipe will absorb no moisture. Soft-burned pipe may absorb as much as 15
per cent moisture.

Vitrified clay blocks are made of the same material and in the same
manner as vitrified clay pipe.

The following data on vitrified pipe have been abstracted from the
specifications for vitrified pipe adopted by the American Society for
Testing Materials.

Pipes shall be subject to rejection on account of the following:

(_a_) Variation in any dimension exceeding the permissible
variations given in Table 36.

(_b_) Fracture or cracks passing through the shell or hub, except
that a single crack at either end of a pipe not exceeding 2 inches
in length or a single fracture in the hub not exceeding 3 inches
in width nor 2 inches in length will not be deemed cause for
rejection unless these defects exist in more than 5 per cent of
the entire shipment or delivery.

(_c_) Blisters or where the glazing is broken or which exceed 3
inches in diameter, or which project more than ⅛ inch above the
surface.

(_d_) Laminations which indicate extended voids in the pipe
material.

(_e_) Fire cracks or hair cracks sufficient to impair the
strength, durability or serviceability of the pipe.

(_f_) Variations of more than ⅛ inch per linear foot in alignment
of a pipe intended to be straight.

(_g_) Glaze which does not fully cover and protect all parts of
the shell and ends except those exempted in Sect. 31. Also glaze
which is not equal to best salt glaze.

(_h_) Failure to give a clear ringing sound when placed on end and
dry tapped with a light hammer.

(_i_) Insecure attachment of branches or spurs.

_Workmanship and Finish_

(29) Pipes shall be substantially free from fractures, large or
deep cracks and blisters, laminations and surface roughness.

(31) The glaze shall consist of a continuous layer of bright or
semi-bright glass substantially free from coarse blisters and
pimples.... Not more than 10 per cent of the inner surface of any
pipe barrel shall be bare of glaze except the hub, where it may be
entirely absent. Glazing will not be required on the outer surface
of the barrel at the spigot end for a distance from the end equal
to ⅔ the specified depth of the socket for the corresponding size
of pipe. Where glazing is required there shall be absence of any
well defined network of crazing lines or hair cracks.

(32) The ends of the pipe shall be square with their longitudinal
axis.

(33) Special shapes shall have a plain spigot end and a hub end
corresponding in all respects with the dimensions specified for
pipes of the corresponding internal diameter.

TABLE 36

PROPERTIES OF CLAY SEWER PIPE

Abstracts from Tentative Specifications of the American Society for
Testing Materials

─────────┬─────────┬───────────┬───────┬────────┬──────┬──────┬─────────
Internal │ Minimum │ Maximum │Laying │Diameter│Depth │Taper │ Minimum
Diameter,│Crushing │Absorption,│length,│ of │ of │ of │Thickness
Inches │Strength,│ Per Cent │ Feet │ Inside │Socket│Socket│ of
│ Pounds │ │ │ of │Inches│ │ Barrel.
│ per │ │ │Socket, │ │ │ Inches
│ Linear │ │ │ Inches │ │ │
│ Foot. │ │ │ │ │ │
│See Note │ │ │ │ │ │
│ 2 │ │ │ │ │ │
─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────
│ │ │ │ │ │ │
│ │ │ │ │ │ │
─────────┼─────────┼───────────┼───────┼────────┼──────┼──────┼─────────
6 │ 1430 │ 5 │2, 2½, │ 8¼ │ 2 │1 : 20│ ⅝
│ │ │ 3 │ │ │ │
8 │ 1430 │ 5 │2, 2½, │ 10¾ │ 2¼ │1 : 20│ ¾
│ │ │ 3 │ │ │ │
10 │ 1570 │ 5 │2, 2½, │ 13 │ 2½ │1 : 20│ ⅞
│ │ │ 3 │ │ │ │
12 │ 1710 │ 5 │2, 2½, │ 15¼ │ 2½ │1 : 20│ 1
│ │ │ 3 │ │ │ │
15 │ 1960 │ 5 │2, 2½, │ 18¾ │ 2½ │1 : 20│ 1¼
│ │ │ 3 │ │ │ │
18 │ 2200 │ 5 │2, 2½, │ 22¼ │ 3 │1 : 20│ 1½
│ │ │ 3 │ │ │ │
21 │ 2590 │ 5 │2, 2½, │ 26 │ 3 │1 : 20│ 1¾
│ │ │ 3 │ │ │ │
24 │ 3070 │ 5 │2, 2½, │ 29½ │ 3 │1 : 20│ 2
│ │ │ 3 │ │ │ │
27 │ 3370 │ 5 │ 3 │ 33¼ │ 3½ │1 : 20│ 2¼
30 │ 3690 │ 5 │ 3 │ 37 │ 3½ │1 : 20│ 2½
33 │ 3930 │ 5 │ 3 │ 40¼ │ 4 │1 : 20│ 2⅝
36 │ 4400 │ 5 │ 3 │ 44 │ 4 │1 : 20│ 2¾
39 │ 4710 │ 5 │ 3 │ 47¼ │ 4 │1 : 20│ 2⅞
42 │ 5030 │ 5 │ 3 │ 51 │ 4 │1 : 20│ 3
─────────┴─────────┴───────────┴───────┴────────┴──────┴──────┴─────────

─────────┬────────────────────────────────────────────────┬────────
Internal │ Permissible Variations │ Number
Diameter,│ │ of
Inches │ │Scorings
│ │ on
│ │ Spigot
│ │ and
│ │Socket ⅛
│ │ Inch
│ │ Deep
─────────┼───────┬─────────────┬────────┬───────┬─────────┼────────
│Length,│ Internal │ Length │ Depth │Thickness│
│ Inches│ Diameter, │ of Two │ of │ of │
│ (-), │ Inches │Opposite│Socket,│ Barrel, │
│ per │ │ Sides, │Inches │ Inches │
│ Foot │ │ Inches │ (-) │ (-) │
─────────┼───────┼──────┬──────┼────────┼───────┼─────────┼────────
│ │Spigot│Socket│ │ │ │
│ │ (±) │ (±) │ │ │ │
─────────┼───────┼──────┼──────┼────────┼───────┼─────────┼────────
6 │ ¼ │ 3/16 │ ¼ │ ⅛ │ ¼ │ 1/16 │ 2
│ │ │ │ │ │ │
8 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2
│ │ │ │ │ │ │
10 │ ¼ │ ¼ │ 5/16 │ ⅛ │ ¼ │ 1/16 │ 2
│ │ │ │ │ │ │
12 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 1/16 │ 2
│ │ │ │ │ │ │
15 │ ¼ │ 5/16 │ ⅜ │ ⅛ │ ¼ │ 3/32 │ 3
│ │ │ │ │ │ │
18 │ ¼ │ ⅜ │ 7/16 │ 3/16 │ ¼ │ 3/32 │ 3
│ │ │ │ │ │ │
21 │ ¼ │ 7/16 │ ½ │ 3/16 │ ¼ │ ⅛ │ 3
│ │ │ │ │ │ │
24 │ ⅜ │ ½ │ 9/16 │ ¼ │ ¼ │ ⅛ │ 4
│ │ │ │ │ │ │
27 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4
30 │ ⅜ │ ⅝ │11/16 │ ¼ │ ¼ │ ⅛ │ 4
33 │ ⅜ │ ¾ │13/16 │ ¼ │ ¼ │ 3/16 │ 5
36 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5
39 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5
42 │ ⅜ │ ¾ │13/16 │ ⅜ │ ¼ │ 3/16 │ 5
─────────┴───────┴──────┴──────┴────────┴───────┴─────────┴────────

NOTE 1. For methods of making tests see Proc. Am. Soc. for Testing
Materials.

NOTE 2. Concentrated load at end of vertical diameter.

(_a_) Slants shall have their spigot ends cut at an angle of
approximately 45 degrees with the longitudinal axis.

(_b_) Curves shall be at angles of 90, 45, 22½, and 11¼ degrees as
required. They shall conform substantially to the curvature
specified.

(_c_) ... All branches shall terminate in sockets.

[Illustration:

FIG. 73.—Standard Clay Pipe Specials.

Courtesy, Blackmer and Post Manufacturing Co.
]

In Fig. 73 are shown the various forms of vitrified pipe and specials
which are ordinarily available on the market.

The life of vitrified clay sewers and some observations on the results
of the inspection of the sewers in Manhattan are discussed in Chapter
XII. The strength of vitrified sewer pipes is shown in Table 37.

TABLE 37

STRENGTH OF SEWER PIPE

Strength in pounds per linear foot to carry loads from ditch filling
material such as ordinary sand and thoroughly wet clay, with the under
side of the pipe bedded 60° to 90° by ordinary good methods. From Proc.
Am. Society for Testing Materials, Vol. 20, 1920, page 604.
────────┬──────────────────────────────────────────────────────────────
Height │
of Fill │
Above │
Top of │
Pipe, │
Feet │ Breadth of the Ditch a Little Below the Top of the Pipe
────────┼───────────┬───────────┬───────────┬────────────┬─────────────
│ 1 Foot │ 2 Feet │ 3 Feet │ 4 Feet │ 5 Feet
────────┼───────────┴───────────┴───────────┴────────────┴─────────────
│ Ditch Filling Material
────────┼─────┬─────┬─────┬─────┬─────┬─────┬─────┬──────┬──────┬──────
│sand │clay │sand │clay │sand │clay │sand │ clay │ sand │ clay
────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼──────┼──────┼──────
2│ 265│ 280│ 615│ 635│ 970│ 990│ 1330│ 1,350│ 1,690│ 1,710
4│ 400│ 450│ 1055│ 1125│ 1745│ 1825│ 2455│ 2,535│ 3,165│ 3,250
6│ 470│ 545│ 1370│ 1500│ 2370│ 2525│ 3405│ 3,575│ 4,460│ 4,740
8│ 505│ 605│ 1600│ 1790│ 2875│ 3115│ 4215│ 4,495│ 5,595│ 5,890
10│ 525│ 640│ 1765│ 2015│ 3275│ 3610│ 4900│ 5,295│ 6,590│ 7,020
12│ 535│ 660│ 1880│ 2185│ 3600│ 4030│ 5485│ 6,000│ 7,460│ 8,035
14│ 540│ 675│ 1965│ 2320│ 3855│ 4380│ 5975│ 6,620│ 8,225│ 8,950
16│ 545│ 680│ 2025│ 2425│ 4065│ 4675│ 6395│ 7,165│ 8,890│ 9,775
18│ 545│ 685│ 2070│ 2505│ 4230│ 4920│ 6750│ 7,630│ 9,480│10,520
20│ 545│ 690│ 2100│ 2565│ 4365│ 5130│ 7050│ 8,060│ 9,995│11,190
22│ 545│ 690│ 2125│ 2610│ 4470│ 5305│ 7305│ 8,425│10,445│11,795
24│ 545│ 690│ 2140│ 2645│ 4560│ 5445│ 7525│ 8,750│10,840│12,340
26│ 545│ 690│ 2150│ 2675│ 4630│ 5575│ 7705│ 9,035│11,185│12,830
28│ 545│ 690│ 2160│ 2695│ 4685│ 5680│ 7860│ 9,280│11,490│13,270
30│ 545│ 690│ 2165│ 2715│ 4725│ 5765│ 7990│ 9,500│11,755│13,670
Very │ │ │ │ │ │ │ │ │ │
great │ 545│ 690│ 2180│ 2770│ 4910│ 6230│ 8725│11,075│13,635│17,305
────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴──────┴──────┴──────

=92. Cement and Concrete Pipe.=—Although there is no general recognition
of a difference between cement and concrete pipe, there is a tendency to
term manufactured pipe of small diameter cement pipe, and large pipes or
pipes constructed in place, concrete pipe. Cement, unlike clay, is used
in the manufacture of pipe in the field or by more or less unskilled
operators in “one man” plants. Great care should be used in the
selection of cement, aggregate, and reinforcement for precast cement
pipe since the shocks to which it is subjected in transit are more
liable to rupture it than the heavier but steadier loads imposed on it
in the trench.

The United States Government, various scientific and engineering
societies, and other interested organizations have collaborated in the
preparation of specifications for cement and cement tests. These
specifications can be found in Trans. Am. Soc. Civil Engineers, Vol. 82,
1918, p. 166, and in other publications.

The following abstracts have been taken from the proposed tentative
specifications for Concrete Aggregates, of the Am. Society for Testing
Materials, issued June 21, 1921:

1. Fine aggregate shall consist of sand, stone screenings, or
other inert materials with similar characteristics, or a
combination thereof, having clean, hard, strong, durable uncoated
grains, free from injurious amounts of dust, lumps, soft or flaky
particles, shale, alkali, organic matter, loam or other
deleterious substances.

2. Fine aggregates shall preferably be graded from fine to coarse,
with the coarser particles predominating, within the following
limits:

Passing No. 4 sieve 100 per cent
Passing No. 50 sieve, not more than 50 per cent
Weight removed by elutriation test, not more than 3 per cent

Sieves shall conform to the U. S. Bureau of Standards
specifications for sieves.

3. The fine aggregate shall be tested in combination with the
coarse aggregate and the cement with which it is to be used and in
the proportions, including water, in which they are to be used on
the work, in accordance with the requirements specified in Section
6....

7. Coarse aggregate shall consist of crushed stone, gravel or
other approved inert materials with similar characteristics, or a
combination thereof, having clean, hard, strong, durable, uncoated
pieces free from injurious amounts of soft, friable, thin,
elongated or laminated pieces, alkali, organic or other
deleterious matter.

* * * * *

The following Table indicates desirable gradings, in percentages,
for coarse aggregate for certain maximum sizes.

GRADINGS OF COARSE AGGREGATES

─────────┬─────────────────────────────────────────┬───────────────────
Maximum │ Circular Openings, Inches │ Passing Screen
Size of │ │ Having Circular
Aggregate│ │Openings ¼ Inch in
Inches │ │diameter, not more
│ │ than
─────────┼───┬───┬───┬─────┬─────┬─────┬─────┬─────┼───────────────────
│ 3 │2½ │ 2 │ 1½ │ 1¼ │ 1 │ ¾ │ ½ │
─────────┼───┼───┼───┼─────┼─────┼─────┼─────┼─────┼───────────────────
3 │100│ │ │40–75│ │ │ │ │ 15 per cent
2½ │ │100│ │ │40–75│ │ │ │ 15 per cent
2 │ │ │100│ │ │40–75│ │ │ 15 per cent
1½ │ │ │ │ 100 │ │ │40–75│ │ 15 per cent
1¼ │ │ │ │ │ 100 │ │ │35–70│ 15 per cent
1 │ │ │ │ │ │ 100 │ │40–75│ 15 per cent
¾ │ │ │ │ │ │ │ 100 │ │ 15 per cent
─────────┴───┴───┴───┴─────┴─────┴─────┴─────┴─────┴───────────────────

The manufacture of small size cement pipe requires relatively more skill
than equipment. As a result great care must be observed in the
inspection of cement pipe and in the enforcement of specifications. For
large size concrete pipe and reinforced concrete pipe the difficulty of
holding the pipe together during transportation and lowering into the
trench aid in insuring a good product.

Cement pipe is made by ramming a mixture of cement, sand, and water into
a cylindrical mold and allowing it to stand until set. The mold is then
removed and the pipe stands for a further period of time to become
cured. The selection and proportion of materials, the amount of water,
the method of ramming, the period of setting, the length of time of
curing, and the control of moisture and temperature during this period
are of great importance in the resulting product. E. S. Hanson[52]
states that the most conservative engineers recommend a mixture of one
sack of cement to 2½ cubic feet of aggregate measured as loosely thrown
into the measuring box. In making up the aggregate, clean gravel or
broken stone up to ¼ inch in size is used. The American Concrete
Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent
a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent
a No. 30, and 20 per cent a No. 40. The materials should be carefully
graded by experiment and not guessed at, as the behavior of all
aggregates is not the same. Too coarse an aggregate is difficult to
handle in manufacturing. It causes loss of pipe when the jacket or mold
is removed and results in rough pipe, stone pockets, and pin holes
through which water spurts when pressure tests are applied. Too fine an
aggregate causes loss of strength and with ordinary mixtures tends to
produce a pipe which will show seepage under internal pressure tests.
The amount of water in the mixture will vary, from 15 to 20 per cent.
The mixture should appear dry but should ball in the hand under some
pressure.

[Illustration:

FIG. 74.—Details of 24–Inch Concrete Pipe Form.
]

The mixture can be rammed into the molds by hand or machine. A
machine-made pipe is preferable as it produces a more even and stronger
product. There are two types of machines for this purpose. One type
consists of a number of tamping feet which deliver about 200 blows to
the minute with a pressure of about 800 pounds per square inch of area
exposed. In the other type a revolving core is drawn through the pipe,
packing and polishing the concrete as it is pulled through, with special
provision for packing the bell of the pipe. The tamping machines can
make 1,500 feet of small size pipe to 300 feet of 24–inch pipe in a day.
Machines of the second type can make 750 feet of 8–inch to 200 feet of
30–inch pipe in 30–inch lengths in 9 hours. The inside and outside forms
for a 24–inch pipe are shown in Fig. 74 as used with the tamping
machines. The forms are swabbed with oil before being filled in order to
facilitate their removal. In making a Y-branch or other special, a hole
is cut in the pipe or mold the size of the joining pipe which is then
set in place and the joint wiped smooth with cement.

TABLE 38

PROPERTIES OF CEMENT CONCRETE SEWER PIPE

1917 Specifications of American Society for Testing Materials, with
Subsequent Revisions

─────────┬───────┬────────┬───────┬───────┬──────┬─────────
│Laying │Diameter│Normal │ Depth │Taper │ Minimum
│Length,│ at │Annular│ of │ of │Thickness
│ Feet │ Inside │Space, │Socket,│Socket│ of
│ │ of │Inches │Inches │ │ Barrel,
│ │Socket, │ │ │ │ Inches
│ │ Inches │ │ │ │
Internal │ │ │ │ │ │
Diameter,│ │ │ │ │ │
Inches │ │ │ │ │ │
─────────┼───────┼────────┼───────┼───────┼──────┼─────────
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
─────────┼───────┼────────┼───────┼───────┼──────┼─────────
│ │ │ │ │ │
│ │ │ │ │ │
─────────┼───────┼────────┼───────┼───────┼──────┼─────────
│2, 2½, │ 8¼ │ ½ │ 2 │1 : 20│ ⅝
6 │ 3 │ │ │ │ │
│2, 2½, │ 11 │ ⅝ │ 2¼ │1 : 20│ ¾
8 │ 3 │ │ │ │ │
│2, 2½, │ 13¼ │ ⅝ │ 2½ │1 : 20│ ⅞
10 │ 3 │ │ │ │ │
│2, 2½, │ 15⅝ │ ⅝ │ 2½ │1 : 20│ 1
12 │ 3 │ │ │ │ │
│2, 2½, │ 19¼ │ ⅝ │ 2½ │1 : 20│ 1¼
15 │ 3 │ │ │ │ │
│2, 2½, │ 22¾ │ ⅝ │ 2¾ │1 : 20│ 1½
18 │ 3 │ │ │ │ │
│2, 2½, │ 26½ │ ¾ │ 2¾ │1 : 20│ 1¾
21 │ 3 │ │ │ │ │
│2, 2½, │ 30¼ │ ¾ │ 3 │1 : 20│ 2⅛
24 │ 3 │ │ │ │ │
27 │ 3 │ 34 │ ⅞ │ 3¼ │1 : 20│ 2¼
30 │ 3 │ 38 │ 1 │ 3½ │1 : 20│ 2½
33 │ 3 │ 41½ │ 1 │ 4 │1 : 20│ 2¾
36 │ 3 │ 45½ │ 1¼ │ 4 │1 : 20│ 3
39 │ 3 │ 49 │ 1¼ │ 4 │1 : 20│ 3¼
42 │ 3 │ 53 │ 1½ │ 4 │1 : 20│ 3½
─────────┴───────┴────────┴───────┴───────┴──────┴─────────

─────────┬──────────────────────────────────────┬─────────┬───────────
│ Limits of Permissible Variations │ Minimum │ Maximum
│ │Crushing │Absorption,
│ │Strength,│ Per Cent
│ │ Pounds │
│ │ per │
│ │ Linear │
Internal │ │ Foot at │
Diameter,│ │ End of │
Inches │ │Diameter │
─────────┼───────┬─────────────┬──────┬─────────┼─────────┼───────────
│Length,│ Internal │Depth │Thickness│ │
│ Inch │ Diameter, │of Hub│of Barrel│ │
│ per │ Inches │ (-) │ (-) │ │
│ Foot │ │Inches│ Inches │ │
│ (-) │ │ │ │ │
─────────┼───────┼──────┬──────┼──────┼─────────┼─────────┼───────────
│ │Spigot│Socket│ │ │ │
│ │ (±) │ (±) │ │ │ │
─────────┼───────┼──────┼──────┼──────┼─────────┼─────────┼───────────
│ ¼ │ 3/16 │ 3/16 │ ¼ │ 1/16 │ 1430 │ 8
6 │ │ │ │ │ │ │
│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1430 │ 8
8 │ │ │ │ │ │ │
│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1570 │ 8
10 │ │ │ │ │ │ │
│ ¼ │ ¼ │ ¼ │ ¼ │ 1/16 │ 1910 │ 8
12 │ │ │ │ │ │ │
│ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 1960 │ 8
15 │ │ │ │ │ │ │
│ ¼ │ ¼ │ ¼ │ ¼ │ 3/32 │ 2200 │ 8
18 │ │ │ │ │ │ │
│ ¼ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 2590 │ 8
21 │ │ │ │ │ │ │
│ ⅜ │ 5/16 │ 5/16 │ ¼ │ ⅛ │ 3070 │ 8
24 │ │ │ │ │ │ │
27 │ ⅜ │ 5/16 │ ⅜ │ ¼ │ ⅛ │ 3370 │ 8
30 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ ⅛ │ 3690 │ 8
33 │ ⅜ │ ⅜ │ ⅜ │ ¼ │ 3/16 │ 3930 │ 8
36 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4400 │ 8
39 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 4710 │ 8
42 │ ⅜ │ ½ │ ½ │ ¼ │ 3/16 │ 5030 │ 8
─────────┴───────┴──────┴──────┴──────┴─────────┴─────────┴───────────

After the removal of the mold the pipe may be cured by the water or the
steam process. Hanson states:

By the former the pipe are simply set on the floor of the plant
and as soon as they are sufficiently strong so that they can be
sprinkled with water without falling down; sprinkling is commenced
and continued at such intervals for 6 or 7 days that the pipe will
be moist at all times. This is a slower process than steam curing.
It is also less uniform and less subject to control than where the
product is cured by steam.

In the steam process the pipe is exposed to low-pressure steam with
plenty of moisture in a closed receptacle for 24 hours, or until
hardened. It has been found by tests that pipes sprinkled for 28 days
are as strong as steam-cured pipes.

The dimensions of cement concrete sewer pipe as recommended by the Am.
Society for Testing Materials are shown in Table 38.

The following has been abstracted from the description of the
manufacture of one form of concrete pipe by G. C. Bartram.[53] All pipe
are manufactured in 4–foot lengths near the site at which they are to be
installed because of their great weight, for example, 36–inch pipe
weighs one ton. The plant for the manufacture of the pipe consists of
cast-iron bottom and top rings for each size to be used on the job, and
inside and outside steel casings. There are three bases for each steel
casing as the pipes stand on the bases for 72 hours and the steel casing
remains on for only 24 hours after the concrete has been poured. The
pipes are then lifted off the bases and stored for aging. The pipes are
cast with the spigot end up.

The concrete is ordinarily mixed in the proportions of 1 : 2 : 4. The
materials are placed in the mixer in the following order: first, the
stone, then the sand, then the cement, and finally the water. Sufficient
water is added to make the concrete flow freely. In cold weather or for
a hurry-up job the molds are covered with canvas and are steamed for 2
or 3 hours immediately after the concrete is poured. The molds are then
removed but the pipe should be steamed before use. Otherwise they are
allowed to stand 72 hours, as explained above. In cold weather the steam
is used to prevent freezing and not to hasten the completion of the
pipe.

[Illustration:

FIG. 75.—Triangle Mesh Reinforced Concrete Pipe.

As made by the Am. Concrete Pipe and Pile Co., Chicago.
]

[Illustration:

FIG. 76.—Methods of Joining and Reinforcing Concrete Pipe.
]

One layer or ring of reinforcement is used for sizes from 24 to 48
inches and two layers or rings for larger pipe. A type of reinforcement
sometimes used is the American Steel and Wire Company’s Triangular Mesh,
an illustration of which is shown in Fig. 75. The wire mesh is cut to
fit and is placed in a slot in the cast-iron base. The slot is then
filled with sand so that the concrete cannot enter, thus leaving a
portion of the reinforcement exposed. The inside reinforcement extends
through and out of the spigot of the completed pipe. In the trench the
two reinforcements overlap in the key-shaped space left on the inside of
the pipe by the design of the bell and spigot. This space is shown in
Fig. 76 A. When the pipe is placed in the trench the key-shaped space is
plastered with mortar and a piece is knocked out of the bell to receive
the grout with which the joint is closed. A spring steel band is then
put on the outside of the joint and grout poured into the hole at the
top. The band is removed as soon as the joint materials have set.

The rules for the reinforcement of concrete pipe recommended in Volume
XV, 1919, of the Transactions of the Concrete Institute are as follows:

No reinforcement is approved for pipe between 30 and 60 inches in
diameter or in rock or hard soils. For pipe 36 inches in diameter
or less the minimum thickness of shell shall be 5 inches. For
60–inch pipe the minimum thickness shall be 7 inches with
intermediate sizes in proportion. Reinforcement for circular pipe
shall consist of one or two rings of circular wire fabric or rods
of the areas shown in Table 39. All sewers near the surface and
subject to vibration should be reinforced. For sewers 6 feet or
less in diameter the reinforcement should consist of at least ½ of
1 per cent of the area of the concrete. It should be placed near
the inside at the crown and near the outside at the haunches. If
large horizontal pressures are expected the pipe should be
reinforced for these reverse stresses, which involves placing the
reinforcement near the outside at the crown and near the inside at
the haunches. The minimum thickness of the walls of sewers greater
than 6 feet in diameter with flat bottom and arch, with or without
side walls, should be 8 inches.

TABLE 39

REINFORCEMENT FOR CIRCULAR CONCRETE SEWER PIPE

(See Vol. XV, Proceedings Am. Concrete Institute)
─────────────────┬─────────────────┬─────────────────┬─────────────────
Diameter in │Minimum Thickness│ Number of Rings │ Cross Sectional
Inches │ of Shell in │ │Area of Each Ring
│ Inches │ │
─────────────────┼─────────────────┼─────────────────┼─────────────────
24 │3 │1 │.058
27 │3 │1 │.068
30 │3½ │1 │.080
33 │4 │1 │.107
36 │4 │1 │.146
39 │4 │1 │.146
42 │4½ │1 │.153
48 │5 │2 │.107
54 │5½ │2 │.123
60 │6 │2 │.146
66 │6½ │2 │.168
72 │7 │2 │.180
84 │8 │2 │.208
96 │9 │2 │.245
─────────────────┴─────────────────┴─────────────────┴─────────────────

Three methods for the reinforcement of concrete sewers are shown in Fig.
76 B.

=93. Proportioning of Concrete.=—In the proportioning of concrete
questions of strength, of permeability, and of workability[54] may need
consideration. All of these qualities are affected by the amount of
cement, the nature and gradation and relative proportions of the fine
and the coarse aggregate, and the amount of mixing water used.

Other things being equal the strength varies with the amount of cement
put into the concrete. For the same amount of cement and the same
consistency of the mixture, the strength increases with increased
density of concrete (that is, with decreased voids), and the effort
should be made so to proportion the fine and coarse aggregates as to
produce the densest concrete (least voids) with the aggregates
available. For the same consistency, the strength then will vary with
the ratio of the amount of cement to the amount of the voids.

So far as the mixing water is concerned, the greatest strength in the
concrete will be attained at a rather dry mix; that which produces the
least volume of concrete. The addition of more water results in a
concrete of less strength; 40 per cent more water may give a concrete of
less than half the normal strength. The reduction in strength is then
very marked for the wetter mixes, and the water content used is a
feature of considerable importance in the design of concrete mixtures.

Permeability is affected by the same elements as strength, but the size
and discontinuity of the pores have a greater influence.

Workability is an important quality; in some respects it will have to be
obtained at the expense of strength. Increasing the amount of mixing
water increases the workability of the mixtures, with a resulting
decrease in strength which may have to be accepted or else overcome by
increasing the cement in the mix.

An excess of water is often used unnecessarily through ignorance of the
injurious results. A high proportion of coarse aggregate, up to a
certain limit, will give concrete of high strength, but the mixture will
be harsh-working and not easy to place. Lower proportions of coarse
aggregate will give greater workability and better uniformity of
product, the latter being an important matter. It is apparent that the
degree of workability of the mixture needed will depend upon the nature
of the construction—for a pavement where the concrete will receive
substantial tamping or working the water content may be much less than
that which may need to be used in placing concrete around reinforcement
in narrow members, or where little tamping or spading can be done. The
nature of the work will affect the standard of consistency to be
specified.

The proportioning of the concrete should then be dependent upon the
needs of the structure and the manner of placing the concrete. The
proportions selected should be carefully adhered to and especially
should care be taken to see that the right quantity of mixing water is
used.

The materials are commonly measured volumetrically (by bulk). Because of
the variations which are introduced by volumetric measurement of the
materials by the presence of varying degrees of moisture, measurements
by weight would be more accurate, but these would also be affected by
differences in the specific gravity of the materials. The methods of
measuring, the allowance for moisture, as well as the proportions of the
materials, should be specified.

The methods for proportioning concrete are:

(1) Arbitrarily selected proportions.

(2) Proportions based on minimum voids.

(3) Proportions based on trial mixtures.

(4) Proportions based on a sieve analysis curve.

(5) Proportions based on the surface area of the aggregates.

(6) Proportions based on the water-cement ratio and the fineness
modulus.

(7) Proportions based on mortar-voids and cement-voids ratio.

Arbitrarily selected proportions are in quite general use; they are
intended to apply to the materials most commonly used in the vicinity of
the work. The most common practice is to use twice as great a volume of
coarse aggregate as fine aggregate, as for instance 1 part cement, 2
parts fine aggregate, and 4 parts coarse aggregate. Decreasing the ratio
of coarse aggregate to fine aggregate may give a more easily worked mix
or require relatively less water for a given workability, and in some
cases it will be proper to increase this ratio and thus secure an
increase of strength. Judgment and experience with given materials may
warrant changes from a stated ratio. The proportions are now frequently
given as one part cement to a certain number of parts of the mixed
aggregate, leaving the proportions of the fine to coarse to be
determined otherwise, since small variations in the relation of these
will not greatly affect the strength. Proportions in common use are:[55]

Mortar for
Laying brick and stone masonry from 1 : 0 to 1 : 3
Filling joints in sewer pipe 1 : 0 to 1 : 2
Surfaces, floors, sidewalks, pavements 1 : 0 to 1 : 2
Waterproof linings 1 : 0 to 1 : 2
Cement, bricks, and blocks 1 : 2½ to 1 : 4
Concrete for
Gravity retaining walls, heavy
foundations, structures needing mass
more than strength from 1 : 3 : 6 to 1 : 4 : 8
Retaining walls, piers, sewers,
pavements, foundations, and work
requiring strength. (Compressive
strength in 28 days, 1,500 to 2,000
pounds per square inch) from 1 : 2 : 4 to 1 : 3 : 6
Floors, beams, pavements, reinforced
concrete, arch bridges, low-pressure
tanks. (Compressive strength in 28
days, 2,000 to 3,000 pounds per
square inch) from 1 : 1½ : 3 to 1 : 2½ : 4½
Reinforced concrete columns, conduit
pipe, impervious concrete.
(Compressive strength in 28 days,
3,000 to 4,000 pounds per square
inch) from 1 : 1 : 2 to 1 : 1½ : 3

The usual method of proportioning based on minimum voids is to assume
that the particles of fine aggregate should fill the voids in the coarse
aggregate and that the particles of the cement will fill the voids in
the fine aggregate. About 5 to 10 per cent additional fine aggregate is
generally added to push the particles of the coarse aggregate apart and
thus give a more easily worked concrete and one freer from void spaces.
This method is inaccurate, principally because of the effect of the
moisture on the volume of the voids, and because the effect on the
volume by the addition of water is unknown.

Trial mixtures may be made by carefully weighing each of the ingredients
and then combining them to give a workable concrete. Using a given
amount of cement, the proportion of ingredients, of the same total
weight, which will give the least volume and therefore the densest
concrete is adopted. When making the comparison the consistency of the
mixes must be maintained constant.

Proportioning may be based on an ideal sieve analysis curve of the mixed
cement and aggregates. The sieve analysis of the aggregates is made by
screening a predetermined weight of the sample through a series of 5 to
8 sieves graded in size from slightly below the size of the largest
particle to slightly above the smallest particle of the aggregate. The
analysis is then expressed in the form of a curve. The ideal curve,
according to Fuller,[56] is shown in Fig. 77.

[Illustration:

FIG. 77.—Gravel Analysis.

The dotted line indicates the ideal combination of the coarse and fine
portions. The heavy full line indicates the combination attained.
]

The method of proportioning concrete by surface areas is based on the
theory that the strength of a concrete depends on the amount of cement
used in proportion to the surface area of the aggregates.[57]

The proportioning of concrete on the basis of a water-cement ratio and a
fineness modulus was introduced by Prof. D. A. Abrams.[58] It is based
on the theory that with fixed conditions of aggregate, moisture, etc.,
the ratio of water to cement determines the strength of the concrete.

A method of proportioning concrete by determining experimentally the
voids in mortars made up with a given amount of sand and definite
proportions of cement, and then calculating the voids in the concrete
made up by adding a definite amount of coarse aggregate to the mixture,
has been developed.[59] The method is based on the theory that the
strength of the concrete is a known function of the ratio of the volume
of cement to the volume of the voids in the concrete. The effect of
varying the proportion of the ingredients, including an increase in the
amount of mixing water beyond that required to give the densest mixture,
may be found by the method, and a comparison may be made of results
obtainable with different classes of fine and coarse aggregates.

Arbitrarily selected proportions, proportions based on voids, and
proportions based on trial mixtures are usually satisfactory for small
jobs where the amount of materials involved is not large. Where the
saving in materials will permit, more accurate methods should be used.
The methods can be studied more fully by reference to the original
articles quoted in the footnotes, or to the following texts:

Materials of Construction, Johnson, 5th Edition, 1918.
Materials of Engineering, H. F. Moore, 2d Edition, 1920.
Masonry Construction, I. O. Baker, 10th Edition, 1912.
Concrete Engineer’s Handbook, Hool and Johnson, 1918.
Concrete, Plain and Reinforced, Taylor and Thompson, 1916.

=94. Waterproofing Concrete.=—The waterproofing of concrete is most
satisfactorily done by making dense mixtures. In practice such
substances as hydrated lime, clay, alum and soap, and proprietary
compounds such as Ceresit, Medusa, etc., are frequently mixed with the
concrete under the theory that these very fine substances will fill any
remaining voids and render the concrete impervious. The specifications
of the Joint Committee issued on June 4, 1921, are much briefer and
contain less detailed instruction than those issued earlier.[60] The
earlier instructions follow.

Many expedients have been resorted to for making concrete
impervious to water. Experience shows, however, that when mortar
or concrete is proportioned to obtain the greatest practicable
density and is mixed to the proper consistency, the resulting
mortar or concrete is impervious under moderate pressure.

On the other hand concrete of dry consistency is more or less
pervious to water, and, though compounds of various kinds have
been mixed with the concrete or applied as a wash to the surface,
in an effort to offset this defect, these expedients have
generally been disappointing, for the reason that many of these
compounds have at best but temporary value, and in time lose their
power of imparting impermeability to the concrete.

In the case of subways, long retaining walls, and reservoirs,
provided the concrete itself is impervious, cracks may be so
reduced, by horizontal and vertical reinforcement properly
proportioned and located, that they will be too minute to permit
leakage, or will be closed by infiltration of silt.

Asphaltic or coal tar preparations applied either as a mastic or
as a coating on felt cloth or fabric, are used for waterproofing,
and should be proof against injury by liquids or gases.

For retaining and similar walls in direct contact with the earth,
the application of one or two coatings of hot coal tar pitch,
following a painting with a thin wash of coal tar dissolved in
benzol, to the thoroughly dried surface of concrete is an
efficient method of preventing the penetration of moisture from
the earth.

Tar paper and asphaltic compounds are not often used in sewer work as
absolute imperviousness is seldom necessary.

=95. Mixing and Placing Concrete.=—Careful workmanship is desirable in
the mixing and placing of concrete in sewers since water-tight
construction is desired. Because of the difficulty of inspecting
concrete in wet, dark and crowded excavations, and the careless habits
of workmen experienced in concrete sewer construction, the highest class
of concrete work cannot be expected. The situation is met by designing
thick walls as shown in the sections illustrated in Fig. 22 and 23.

In the report of the Joint Committee on Concrete and Reinforced Concrete
in Transactions of the American Society of Civil Engineers for 1917, on
page 1101 the recommendation is made concerning the mixing and placing
of concrete as follows:[61]

The mixing of concrete should be thorough and should continue
until the mass is uniform in color and is homogeneous. As the
maximum density and greatest strength of a given mixture depends
largely on thorough and complete mixing, it is essential that this
part of the work should receive special attention and care.

Inasmuch as it is difficult to determine by visual inspection
whether the concrete is uniformly mixed, especially where
aggregates having the color of cement are used, it is essential
that the mixing should occupy a definite period of time. The
minimum time will depend on whether the mixing is done by machine
or hand.

(_a_) Measuring Ingredients: Methods of measurement of the various
ingredients should be used which will secure at all times separate
and uniform measurements of cement, fine aggregate, coarse
aggregate and water.

(_b_) Machine Mixing: The mixing should be done in a batch machine
mixer of a type which will insure the uniform distribution of the
materials throughout the mass, and should continue for the minimum
time of 1½ minutes after all the ingredients are assembled in the
mixer. For mixers of 2 or more cubic yards capacity, the minimum
time of mixing should be 2 minutes. Since the strength of the
concrete is dependent on thorough mixing, a longer time than this
minimum is preferable. It is desirable to have the mixer equipped
with an attachment for automatically locking the discharging
device so as to prevent the emptying of the mixer until all the
materials have been mixed together for the minimum time required
after they are assembled in the mixer. Means should be provided to
prevent aggregates being added after the mixing has commenced. The
mixer should also be equipped with water storage, and an automatic
measuring device which can be locked if desired. It is also
desirable to equip the mixer with a device recording the
revolutions of the drum. The number of revolutions should be so
regulated as to give at the periphery of the drum a uniform speed.
About 200 feet per minute seems to be the best speed in the
present state of the art.

(_c_) Hand Mixing: Hand mixing should be done on a water-tight
platform and especial precautions taken after the water has been
added, to turn all the ingredients together at least 6 times, and
until the mass is homogeneous in appearance and color.

(_d_) Consistency: The materials should be mixed wet enough to
produce a concrete of such a consistency as will flow sluggishly
into the forms and about the metal reinforcement when used, and
which at the same time can be conveyed from the mixer to the forms
without separation of the coarse aggregate from the mortar. The
quantity of water is of the greatest importance in securing
concrete of maximum strength and density; too much water is as
objectionable as too little.

(_e_) Retempering: The remixing of concrete and mortar that has
partly reset should not be permitted.

_Placing Concrete_

(_a_) Methods: Concrete after the completion of the mixing should
be conveyed rapidly to the place of final deposit; under no
circumstances should concrete be used that has partly set.

Concrete should be deposited in such a manner as will permit the
most thorough compacting such as can be obtained by working with a
straight shovel or slicing tool kept moving up and down until all
the ingredients are in their proper place. Special care should be
exercised to prevent the formation of laitance; where laitance has
formed it should be removed, since it lacks strength and prevents
a proper bond in the concrete.

Care should be taken that the forms are substantial and thoroughly
wetted (except in freezing weather) or oiled, and that the space
to be occupied by the concrete is free from all debris. When the
placing of concrete is suspended, all necessary grooves for
joining future work should be made before the concrete has set.

When work is resumed concrete previously placed should be
roughened, cleansed of foreign material and laitance, thoroughly
wetted and then slushed with a mortar consisting of one part
Portland cement and not more than 2 parts of fine aggregate.

The surfaces of concrete exposed to premature drying should be
kept covered and wet for at least 7 days.

Where concrete is conveyed by spouting, the plant should be of
such a size and design as to insure a practically continuous
stream in the spout. The angle of the spout with the horizontal
should be such as to allow the concrete to flow without separation
of the ingredients; in general an angle of about 27 degrees or 1
vertical to 2 horizontal is good practice. The spout should be
thoroughly flushed with water before and after each run. The
delivery from the spout should be as close as possible from the
point of deposit. Where the discharge must be intermittent, a
hopper should be provided at the bottom. Spouting through a
vertical pipe is satisfactory when the flow is continuous; when it
is checked and discontinuous it is highly objectionable unless the
flow is checked by baffle plates.

(_b_) Freezing Weather: Concrete should not be mixed or deposited
at a freezing temperature, unless special precautions are taken to
prevent the use of materials covered with ice crystals or
containing frost, and to prevent the concrete from freezing before
it has set and sufficiently hardened.

As the coarse aggregate forms the greater portion of the concrete,
it is particularly important that this material be warmed to well
above the freezing point.

The enclosing of a structure and the warming of a space inside the
enclosure is recommended, but the use of salt to lower the
freezing point is not recommended.

(_c_) Rubble Concrete: Where the concrete is to be deposited in
massive work, its value may be improved and its cost materially
reduced by the use of clean stones saturated with water,
thoroughly embedded in and completely surrounded by concrete.

(_d_) Under Water: In placing concrete under water, it is
essential to maintain still water at the place of deposit. With
careful inspection the use of tremies, properly designed and
operated, is a satisfactory method of placing concrete through
water. The concrete should be mixed very wet (more so than is
ordinarily permissible) so that it will flow readily through the
tremie and into place with practically a level surface.

The coarse aggregate should be smaller than ordinarily used and
never more than one inch in diameter. The use of gravel
facilitates the mixing and assists the flow. The mouth of the
tremie should be buried in the concrete so that it is at all times
entirely sealed and the surrounding water prevented from forcing
itself into the tremie. The concrete will then discharge without
coming in contact with the water. The tremie should be suspended
so that it can be lowered quickly when it is necessary either to
choke off or to prevent too rapid flow. The lateral flow
preferably should not be over 15 feet.

The flow should be continuous in order to produce a monolithic
mass and to prevent the formation of laitance in the interior.

In case the flow is interrupted it is important that all laitance
be removed before proceeding with the work.

In large structures it may be necessary to divide the mass of
concrete into several small compartments or units to permit the
continuous filling of each one. With proper care it is possible in
this manner to obtain as good results under water as in the air.

A less desirable method is the use of the drop bottom bucket.
Where this method is used the bottom of the bucket should be
released when in contact with the surface of the place of deposit.

Concrete sewers should be constructed in longitudinal sections in a
continuous operation without interruption for the entire invert, side
walls, or arch. In pouring the concrete it should be kept level in the
forms and should rise evenly on each side of the sewer. All rough places
in the concrete should be finished smooth by brushing with a grout of
neat cement and water and honeycombs should be filled with neat cement
or a one-to-one mortar.

=96. Sewer Brick.=—The quality of brick used in sewers is seldom
specified with the minute care that is taken in the specifications for
concrete, iron, and certain other materials of construction, as inferior
materials in brick are more easily detected. The specifications of the
Baltimore Sewerage Commission for sewer brick are:

Sewer brick shall be whole, new bricks of the best quality, of
uniform standard size, with straight and parallel edges and square
corners: they shall be of compact texture, burned hard and
entirely through, free from injurious cracks and flaws, tough and
strong, and shall have a clear ring when struck together. The
sides, ends and faces of all bricks shall be plane surfaces at
right angles and parallel to each other. Bricks of any one make
shall not vary more than 1/16th of an inch in thickness, nor more
than 1⅛th of an inch in width or length, from the average of the
samples submitted for approval.

The truest bricks shall be used in the face of the masonry and the
exposed surfaces shall be true and smooth planes.

All bricks delivered for use shall be culled by the Contractor
when required. No brick thrown out in the culling shall be used in
any work done under any contract of the Sewerage Commission,
except that the best of the culls may be used in manholes, above
the level of the top of the sewer, if permitted by the Engineer.

The average amount of water absorbed by the bricks, after being
thoroughly dried and then immersed for 24 hours, shall not exceed
6 per cent. All bricks shall be uniform in quality and percentage
of absorption.

Whenever vitrified bricks are required in the invert of the sewer,
they shall be smooth, hard, tough, and of such durability as will
fit them for this use. They shall be of standard size, well and
uniformly burned, thoroughly vitrified throughout, and free from
warps, cracks, and other defects. The surfaces and edges shall be
true and straight and the corners sharp and square. They shall be
in every respect satisfactory to the Engineer, and in all respects
equal to the sample in the office of the Engineer.

The remaining paragraphs of the specifications deal with the manner in
which samples shall be submitted and the necessity for conformity
between the samples submitted and the bricks used.

A common size of brick in use for sewers is 2¼ × 4 × 8¼ inches, but the
variations in size are many. The bricks in use on any one job should be
as near the same size as possible as the extra mortar filling necessary
to make up for small brick detracts from the strength of the sewer.
Small brick are undesirable as the cost of laying small and large bricks
is the same, but the thickness of the finished sewer is less. Sewer
brick should not absorb more than 10 to 20 per cent moisture by volume,
in 24 hours; except the special paving brick used to prevent erosion at
the invert which should absorb less than 5 per cent moisture.

=97. Vitrified Sewer Block.=—Blocks and bricks are manufactured in a
manner similar to the manufacture of vitrified sewer pipe described in
Art. 91. J. M. Egan describes two types of sewer blocks[62] as follows:

There are on the market two designs of blocks, one being a
single-ring block and the other a double-ring block. The former
has a ship-lap joint on the ends and a tongue-and-groove joint on
the sides. In the double block the laps and joints are made in the
construction of the sewer and the blocks are placed one on top of
the other as in a two ring brick sewer. The blocks are hollow
longitudinally with web braces. They are made for sewers from 30
inches to 108 inches in diameter and weigh from 40 to 120 pounds.
They are 18 inches to 24 inches long, 9 to 15 inches wide, and 5
to 10 inches thick. Short lengths are made for convenience in
construction and for use on sharp curves. Special blocks are made
for connections and junctions.

A special block is also made for inverts, which has occasionally been
used with brick sewers to avoid the difficulty of constructing with
brick at this point. Such blocks are objectionable, as they leave a line
of weakness along the longitudinal joint so formed. They are not used
frequently in present day practice.

Vitrified blocks are generally cheaper than bricks, but they do not make
so strong a structure. In some cases it is possible to lay vitrified
block without the expense of high-priced bricklayers, thus saving on the
cost of the sewer and obtaining a conduit with a smoother interior
finish.

=98. Cast Iron, Steel, and Wood.=—Cast iron, steel, and wood pipe belong
more to the field of waterworks than of sewerage, as they are not
extensively used in the construction of sewers. There are, however, some
special conditions under which these materials may be serviceable.

The iron used in cast-iron pipe for sewers, and in castings for manhole
covers, inlet frames, etc., is seldom carefully or definitely specified.
The standard specifications of the American Water Works Association with
regard to the quality of iron for water pipe are:

All pipe and special castings shall be made of cast iron of good
quality and of such character as shall make the metal of the
castings strong, tough, and of even grain and soft enough to
satisfactorily admit of drilling and cutting. The metal shall be
made without the admixture of cinder iron or other inferior metal,
and shall be remelted in a cupola or air furnace.

The specifications of the Sanitary District of Chicago for the quality
of iron to be used in manhole covers, etc., are given on page 101.

Although sewer pipes are not ordinarily subjected to internal pressure,
cast-iron pipe for sewers should be as heavy or heavier than water pipe
to resist the corrosive action of the sewage and the external stresses
that are to be imposed upon it. The sizes and details of standard
cast-iron pipe used for both water works and sewerage can be found in
specification of the American and New England Water Works Associations.

The quality of steel used for reinforcing concrete should be carefully
specified because of the possibility of the substitution of inferior
material. The specifications for “Billet Steel Concrete Reinforcement
Bars,” of the American Society for Testing Materials[63] are the
standard for engineering practice, or the following specifications may
be used:

All reinforcement shall be free from excessive rust, scale, paint,
or coatings of any character which will tend to destroy the bond.
The bars shall be rolled from new billets. No rerolled material
will be accepted. All reinforcement bars shall develop an ultimate
tensile strength of not less than 70,000 pounds per square inch.
The test specimen shall bend cold around a pin, whose diameter is
two times the thickness of the bar, 180 degrees without cracking
on the outside portion. The reinforcing bars shall in all respects
fulfill the requirements of the standard specifications of the
American Society for Testing Materials for Billet Steel Concrete
Reinforcing Bars serial designation A 15–14.

The steel used in pipe should be a soft, open-hearth steel with an
ultimate tensile strength of 60,000 pounds per square inch, an elastic
limit of 30,000 pounds per square inch, an elongation in 8 inches before
fracture between 22 and 25 per cent, and a reduction in area before
fracture of 50 per cent. The working strength of the steel is taken at
16,000 to 20,000 pounds per square inch in tension, 10,000 to 12,000
pounds per square inch in shear, and 20,000 to 24,000 pounds per square
inch in bearing. A liberal allowance should be made for corrosion. The
standard specifications for Open-Hearth Boiler Plate and Rivet Steel of
the American Society for Testing Materials, Aug. 16, 1919, include
“flange steel,” which is suitable for the manufacture of plates, and
extra soft steel which is suitable for rivets.

Steel pipe should be coated both inside and out to protect it against
corrosion. The various proprietary coatings are mainly coal tar pitches,
or mixtures of coal tar pitch and asphalt. A coal tar pitch is a
distillate of coal tar from which the naphtha has been removed and to
which about one per cent of heavy linseed oil has been added. The
coating is applied to the pipe at a temperature of about 300 degrees
Fahrenheit, by dipping hot pipe in the heated coating material. The pipe
should be carefully cleaned and all rust and scale removed before it is
dipped. In some cases the steel is pickled before dipping. This consists
in rolling the cold plates to a short radius to loosen the scale,
heating them to about 125 degrees, and dipping them in a warm 5 per cent
acid solution for about 3 minutes, and finally rinsing in a weakly basic
wash water.

The woods commonly used for the manufacture of wood pipe are spruce,
Oregon fir, Douglas fir, and California redwood. Wood pipe lines have
been constructed of other kinds of lumber but only in more or less
unusual conditions. The following has been abstracted from the
specifications for California redwood given by J. F. Partridge.[64]

The staves shall be of clear, air-dried, California redwood,
seasoned at least one year in the open air, and shall be free from
knots (except small knots appearing on one face only), sap, dry
rot, wind shakes, pitch, pitch seams, pitch pockets, or other
defects which would materially impair their strength or
durability. The sides of the staves shall be milled to conform to
the inside and outside radii of the pipe; and the edges shall be
beveled to true radial planes. The staves shall be milled from
stock sizes of lumber, the net finished thickness of the stave,
for the various diameters of pipe, shall be as given in Table 40.
The ends shall be cut square and slotted to receive the metallic
tongues which form the butt joints. The slots shall appear in the
same position on each stave, and shall be cut to make a tight fit
with the tongues in all directions. The staves shall have an
average length of at least 15 ft. 6 in. and not more than one per
cent shall have a length of less than 9 ft. 6 in. Staves shorter
than 8 ft. will not be accepted.

The bands shall be spaced on the pipe with a factor of safety of
at least four, and shall consist of round, mild steel rods,
connected with malleable iron shoes. Either open-hearth or
Bessemer steel may be used.... The ultimate strength shall be from
55,000 to 65,000 lb. per sq. in.

The original reference should be consulted for complete details and for
specifications for various kinds of wood and classes of pipe. The
discussion following the specifications is of value.

Machine-made wood pipe is superior to stave pipe put together in the
field. It is seldom manufactured in sizes large enough for use in
sewers, which results in the almost exclusive use of field constructed
stave pipe. The steel bands used to hold the staves together should be
coated similarly to steel plates. All lumber, except California redwood
should receive a preservative coating of creosote[65] or other material.
One of the best methods of preserving the wood is to keep it submerged
and to maintain the pipe under internal pressure.

TABLE 40

DETAILS OF DESIGN FOR CONTINUOUS STAVE WOOD PIPE

CLASSES A, B, AND C

(By J. F. Partridge, Trans. A. S. C. E., Vol. 82, page 461)
───────────┬───────────┬───────────┬───────────┬───────────┬───────────
│ Stave │Stock Size │ Size of │ Top Width │Spacing of
│Thickness, │of Lumber, │ Band, │of Staves, │ Bands for
Diameter, │ Standard, │ Inches │ Inches │ Standard, │ 100 Feet
Inches │ Inches │ │ │ Inches │ Head
───────────┼───────────┼───────────┼───────────┼───────────┼───────────
12│ 1⅜ │ 2 × 4 │ ⅜ │ 3.56 │ 6.38
18│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.66 │ 5.76
24│ 1–7/16 │ 2 × 4 │ 7/16 │ 3.70 │ 4.34
30│ 1½ │ 2 × 6 │ ½ │ 5.48 │ 4.53
36│ 1–9/16 │ 2 × 6 │ ½ │ 5.62 │ 3.77
42│ 1⅝ │ 2 × 6 │ ½ │ 5.51 │ 3.23
│ 1⅝ │ 2 × 6 │ ½ or ⅝ │ 5.60 │ 2.84 or
48│ │ │ │ │ 4.41
60│ 2½ │ 3 × 6 │ ⅝ │ 5.56 │ 3.54
│ 3½ │ 4 × 6 │ ⅝ or ¾ │ 5.69 │ 2.95 or
72│ │ │ │ │ 4.24
84│ 3½ │ 4 × 6 │ ¾ │ 5.65 │ 3.63
120│ 3⅝ │ 4 × 6 │ ¾ │ 5.68 │ 2.54
│ 3⅝ │ 4 × 6 │ ¾ or ⅞ │ 5.64 │ 2.12 or
144│ │ │ │ │ 2.89
───────────┴───────────┴───────────┴───────────┴───────────┴───────────

CHAPTER IX
DESIGN OF THE SEWER RING

=99. Stresses in Buried Pipe.=—The stresses which sewer pipe should be
designed to resist are: internal bursting pressure, for sewers flowing
under pressure; stresses due to handling, for precast pipe; temperature
stresses; and external loads. The latter is by far the most important
and frequently is the only stress considered in design.

The thickness of a pipe to resist internal stress should be

(_PR_)⁄_f__{_t_},

in which _P_ = the intensity of internal pressure;

_R_ = the radius of the inside of the pipe, and

_f__{_t_} = the unit-strength of the material in tension

The derivation of this expression is simple. The stresses due to
handling cannot be computed and are cared for by a thickness of material
dictated by experience. These thicknesses are given for vitrified clay
and cement pipe in the specifications in the preceding chapter.
Temperature stresses are not allowed for in the design of the pipe ring,
but allowance must be made for them in long rigid pipe lines exposed to
wide variations in temperature. Such a condition seldom exists in
sewerage works.

The external forces are ordinarily the controlling features in the
design of sewer rings. The simplest problems arise in the design of a
circular pipe. If the external loading is uniform about the
circumference of the pipe the internal stresses will all be compression.
Almost all other forms of loading will cause bending moments resulting
in tension and compression in different parts of the pipe. The maximum
bending is caused by two concentrated loads diametrically opposed. As
such a condition is extreme it is not cared for in ordinary design, but
a loading between this condition and perfect distribution is assumed, as
explained in Art. 103.

=100. Design of Steel Pipe.=—The stresses which may occur in steel sewer
pipes are commonly caused by the internal or bursting pressure of the
contained liquid. Occasionally a steel pipe may be used as a bridge or
as a stressed member of a bridge, but steel pipes should not be used to
withstand compression normal to the axis. In order to avoid such
stresses the bursting tensile stresses should exceed the external
compressive stresses. Such a condition in design requires that buried
pipes shall never be emptied, a condition that cannot always be
fulfilled. Precaution should be taken, by the installation of proper
valves, to prevent the emptying of the pipe at so rapid a rate that a
vacuum is created resulting in the collapse of the pipe.

Steel pipes are ordinarily made of plates curved to the proper diameter,
the edges being held together by rivets. The design of the pipe consists
in the determination of the thickness of the plate and the design of the
riveted joint. The longitudinal joint and the thickness of the plate are
first designed. The design of the joint consists in determining the
diameter and pitch of the rivets and the thickness of the plate so that
the full strength of the uncut metal shall be developed as nearly as
possible under bearing, tearing, and shearing. This is done by making
the efficiency of the joint the same under all stresses. The efficiency
of the joint is the ratio of the strength of the joint under any kind of
stress to the strength in tension of the unpunched plate. Properties of
riveted joints are given in Table 41.

The diameter of the rivet holes should be computed as 1/16 of an inch
larger than the diameter of the rivets. Rivets and plates should be
designed for the nearest or next largest commercial size, and a generous
allowance for corrosion should be made in determining the thickness of
the plate. The distance from the edge of the plate to the side of the
rivet should not be less than 1½ times the diameter of the rivet. The
unit-strengths of the metal are given in the preceding chapter.

The transverse joint must be designed empirically as the stresses in it
are indeterminate. The common form of joint for pipes less than 48
inches in diameter is a single-riveted lap joint, and for larger pipes
or for pipes exposed to unusual stresses, a double riveted lap joint is
used. The same size rivets are used as in the longitudinal joint. The
maximum permissible distance between rivets should be used in the
transverse joint.

TABLE 41

PROPERTIES OF RIVETED JOINTS

(Chicago Bridge and Iron Works)
──────────────────────┬─────────┬────────┬────────┬──────────┬─────────
Type of Joint │Thickness│Diameter│ Pitch, │Efficiency│Thickness
│ Plate, │ of │ Inches │of Joint, │ Butt
│ Inch │ Rivet, │ │ Per Cent │ Plate,
│ │ Inch │ │ │ Inches
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Single-riveted lap │ ¼ │ ⅝ │ 1.88 │ 49 │
│ ¼ │ ¾ │ 2.25 │ 50 │
│ 5/16 │ ⅞ │ 2.63 │ 50 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Double riveted lap │ ¼ │ ⅝ │ 2.50 │ 70 │
│ 5/16 │ ¾ │ 3.00 │ 71 │
│ ⅜ │ ⅞ │ 3.40 │ 71 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Triple riveted lap │ ¼ │ ½ │ 2.39 │ 74 │
│ 5/16 │ ⅝ │ 2.96 │ 74 │
│ ⅜ │ ¾ │ 3.53 │ 75 │
│ 7/16 │ ⅞ │ 4.09 │ 76 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Quadruple riveted lap │ ⅜ │ ⅝ │ 3.20 │ 77 │
│ 7/16 │ ¾ │ 3.90 │ 78 │
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Double riveted butt │ ½ │ ⅞ │ 3.62 │ 72 │ ⅜
│ 9/16 │ ⅞ │ 3.62 │ 72 │ ⅜
│ ⅝ │ ⅞ │ 3.62 │ 72 │ ⅜
│ 11/16 │ ⅞ │ 3.62 │ 72 │ 7/16
│ ¾ │ 1 │ 4.12 │ 73 │ 7/16
│ ⅞ │ 1 │ 3.82 │ 71 │ ½
│ 1 │ 1 │ 3.48 │ 68 │ 9/16
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Triple riveted butt │ ⅝ │ ⅞ │ 4.94 │ 80 │ ½
│ ¾ │ 1 │ 5.62 │ 80 │ 9/16
│ ⅞ │ 1 │ 5.16 │ 78 │ 9/16
│ 1 │ 1 │ 4.66 │ 76 │ 9/16
──────────────────────┼─────────┼────────┼────────┼──────────┼─────────
Quadruple riveted butt│ ¾ │ 1 │ 7.13 │ 84 │ ¾
│ ⅞ │ 1 │ 6.51 │ 83 │ 11/16
│ 1 │ 1 │ 5.84 │ 81 │ ⅝
──────────────────────┴─────────┴────────┴────────┴──────────┴─────────

Pipes used as compression members of a bridge are stiffened by riveting
standard rolled steel sections longitudinally on the pipe.

[Illustration:

FIG. 78.—Lock Bar Pipe.
]

Lock Bar Pipe is a steel pipe with a special form of joint made by the
East Jersey Pipe Corporation. It is arranged as shown in Fig. 78 and has
the advantage of developing the full strength of the plate. It is
equivalent to a joint with 100 per cent efficiency, which permits the
use of thinner plates.

=101. Design of Wood Stave Pipe.=—In the design of wood stave pipe[66]
the entire bursting pressure is taken up by steel bands wrapped around
the outside of wood staves which make up the shell of the pipe. The pipe
is not designed to resist external loads except those which may be
overcome by the internal pressure in the pipe. The thickness of the
staves is fixed by experience. The sizes of staves and bands recommended
by J. F. Partridge[67] are given in Table 40. The size of the steel
bands can be determined from the expression;

_S_ = _Cr_(_R_ + _t_)

in which _S_ = the total stress in the band;

_R_ = the radius of the inside of the pipe;

_t_ = the thickness of the stave;

_r_ = the area of bearing per unit length of the band on the
wood. For circular bands it is assumed as the radius
of the band;

_C_ = the crushing strength of wood, usually taken at 650
pounds per sq. in.

The preceding expression can be derived easily by the application of the
laws of mechanics, and from it the expression for the distance between
bands follows logically. It is,

_p_ = _S_⁄(_PR_ + _kt_)

in which _S_ = the strength of the band;

_p_ = the distance between bands;

_P_ = the intensity of bursting pressure in the pipe;

_R_ = the radius of the inside of the pipe;

_t_ = the thickness of the staves;

_k_ = the swelling strength of wood, usually taken at 100
pounds per sq. in.

[Illustration:

FIG. 79.—Shoe for Wood Stave Pipe.
]

Transverse joints between staves are closed by inserting metal strips
between them, or by shaping the edges irregularly so that they fit
closely together with an irregular joint. Transverse joints between all
staves at any one point are avoided by splitting the joints between
staves. Longitudinal joints between staves are usually made smooth and
are closed by steel bands which are drawn tight about the pipe by
inserting the ends in coupling shoes as shown in Fig. 79.

[Illustration:

FIG. 80.—_B_ in Formula _W_ = _CwB_^2
]

=102. External Loads on Buried Pipe.=—Prof. Anston Marston and H. C.
Anderson published[68] the results of a series of experiments on the
loads on buried pipes which are of extreme value in the design of sewer
pipe. The load on the pipe is given by the empirical expression _W_ =
_CwB_^2, in which _w_ is the weight of the backfilling material in
pounds per cubic foot, _B_ is the width of the trench in feet at the
elevation of the end of a radius making an angle of 45 degrees upwards
with the horizontal diameter of the pipe as illustrated in Fig. 80, and
_C_ is a coefficient dependent on the character of the backfill and the
ratio of the width to the depth of the trench. Values of _C_ are given
in Table 42. The weights of various classes of backfilling are given in
Table 43.

TABLE 42

APPROXIMATE SAFE WORKING VALUES OF _C_ IN THE EXPRESSION _W_ = _CwB_^2

From Bulletin No. 31 of the Engineering Experiment Station, Iowa State
College of Agriculture.
──────────────┬────────────────────────────────────────────────────────
Ratio of Depth│ Approximate Values of _C_
to Width │
──────────────┼──────────────┬─────────────┬─────────────┬─────────────
│Damp Top Soil │Saturated Top│ Damp Yellow │ Saturated
│ and Dry and │ Soil │ Clay │ Yellow Clay
│ Wet Sand │ │ │
──────────────┼──────────────┼─────────────┼─────────────┼─────────────
0.5 │ 0.46 │ 0.47 │ 0.47 │ 0.48
1.0 │ 0.35 │ 0.86 │ 0.88 │ 0.90
1.6 │ 1.16 │ 1.21 │ 1.25 │ 1.27
3.0 │ 1.47 │ 1.51 │ 1.56 │ 1.62
2.6 │ 1.70 │ 1.77 │ 1.83 │ 1.91
3.0 │ 1.90 │ 1.99 │ 2.08 │ 2.19
3.6 │ 2.08 │ 2.18 │ 2.28 │ 2.43
4.0 │ 2.22 │ 2.35 │ 2.47 │ 2.65
4.6 │ 2.34 │ 2.49 │ 2.63 │ 2.85
6.0 │ 2.45 │ 2.61 │ 2.78 │ 3.02
6.5 │ 2.54 │ 2.72 │ 2.90 │ 3.18
6.0 │ 2.61 │ 2.81 │ 3.01 │ 3.32
6.6 │ 2.68 │ 2.89 │ 3.11 │ 3.44
7.0 │ 2.73 │ 2.95 │ 3.19 │ 3.55
7.5 │ 2.78 │ 3.01 │ 3.27 │ 3.66
8.0 │ 2.82 │ 3.06 │ 3.33 │ 3.74
8.5 │ 2.85 │ 3.10 │ 3.39 │ 3.82
9.0 │ 2.88 │ 3.14 │ 3.44 │ 3.89
9.5 │ 2.90 │ 3.18 │ 3.48 │ 3.96
10.0 │ 2.92 │ 3.20 │ 3.52 │ 4.01
11.0 │ 2.95 │ 3.25 │ 3.58 │ 4.11
12.0 │ 2.97 │ 3.28 │ 3.63 │ 4.19
13.0 │ 2.99 │ 3.31 │ 3.67 │ 4.25
14.0 │ 3.00 │ 3.33 │ 3.70 │ 4.30
15.0 │ 3.01 │ 3.34 │ 3.72 │ 4.34
∞ │ 3.03 │ 3.38 │ 3.79 │ 4.50
──────────────┴──────────────┴─────────────┴─────────────┴─────────────

TABLE 43

APPROXIMATE WEIGHTS OF DITCH FILLING MATERIAL TO BE USED IN THE
EXPRESSION _W_ = _CwB_^2[69]

───────────────────────────────────┬───────────────────────────────────
Ditch Filling │ Pounds per Cubic Foot
───────────────────────────────────┼───────────────────────────────────
Partly compacted top soil (damp) │ 90
Saturated top soil │ 110
Partly compacted damp yellow clay │ 100
Saturated yellow clay │ 130
Dry sand │ 100
Wet sand │ 120
───────────────────────────────────┴───────────────────────────────────

Where surface loads are to be carried on the sewer trench the proper
proportion of the load to be carried by the sewer is determined by the
expression _L__{_p_} = _CL_, in which _L__{_p_} is the equivalent
backfill load per unit length of the trench, _L_ is the surface load per
unit length of the trench, and _C_ is a coefficient in which allowance
is made for the character of the backfilling, the ratio of depth to
width of trench, and the character of the load, whether long or short. A
long load is a load extending along the length of the trench such as a
pile of building material. A short load is one extending across the
trench and for only a short distance along it, such as that caused by a
street car or road roller crossing the trench. Values of _C_ are given
in Table 44 for long loads, and in Table 45 for short loads. Values of
long and short loads occasionally met in practice are given in Tables 46
and 47 respectively.

TABLE 44

RATIO OF LOAD ON PIPE TO LONG LOAD ON TRENCH[70]

──────────────┬──────────────┬──────────────┬──────────────┬──────────────
Ratio of Depth│Sand and Damp │Saturated Top │ Damp Yellow │ Saturated
to Width │ Top Soil │ Soil │ Clay │ Yellow Clay
──────────────┼──────────────┼──────────────┼──────────────┼──────────────
0.0│ 1.00│ 1.00│ 1.00│ 1.00
0.5│ 0.85│ 0.86│ 0.88│ 0.89
1.0│ 0.72│ 0.75│ 0.77│ 0.80
1.5│ 0.61│ 0.64│ 0.67│ 0.72
2.0│ 0.52│ 0.53│ 0.59│ 0.64
2.5│ 0.44│ 0.48│ 0.52│ 0.57
3.0│ 0.37│ 0.41│ 0.45│ 0.51
4.0│ 0.27│ 0.31│ 0.35│ 0.41
5.0│ 0.19│ 0.23│ 0.27│ 0.33
6.0│ 0.14│ 0.17│ 0.20│ 0.26
8.0│ 0.07│ 0.09│ 0.12│ 0.17
10.0│ 0.04│ 0.05│ 0.07│ 0.11
──────────────┴──────────────┴──────────────┴──────────────┴──────────────

For example, let it be desired to determine the load on a 72–inch
concrete sewer with a 9–inch shell under the following conditions:
depth of backfill over the top of the pipe, 15 feet; character of
backfill, saturated yellow clay; superimposed load, pile of
building brick 6 feet high. The ratio of the depth of backfill to
the width of the trench is 15 ÷ 9 or 1.67. The coefficient in the
expression _CwB_^2 is 1.39, from Table 42. The weight of saturated
yellow clay is 130 pounds per cubic foot, from Table 43. Therefore
the load per foot length of the sewer due to the backfill is:

_W_ = _CwB_^2 = 1.39 × 130 × 81 = 14,600 pounds.

TABLE 45

RATIO OF LOAD ON PIPE TO SHORT LOAD ON TRENCH[71]

───────┬───────────────┬───────────────┬───────────────┬───────────────
Ratio │ │ │ │
of │ │ │ │
Height │ │ │ │
to │ │ │ │
Width │ │ │ │
of │ Sand and Damp │ Saturated Top │ Damp Yellow │ Saturated
Trench │ Top Soil │ Soil │ Clay │ Yellow Clay
───────┼───────────────┴───────────────┴───────────────┴───────────────
│ Length of Load Equal to
───────┼───────┬───────┬───────┬───────┬───────┬───────┬───────┬───────
│ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width│ Width │⅒ Width
│ of │ of │ of │ of │ of │ of │ of │ of
│Trench │Trench │Trench │Trench │Trench │Trench │Trench │Trench
───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────┼───────
0.0│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00│ 1.00
0.5│ 0.77│ 0.12│ 0.78│ 0.13│ 0.79│ 0.13│ 0.81│ 0.13
1.0│ 0.59│ 0.02│ 0.61│ 0.02│ 0.63│ 0.02│ 0.66│ 0.02
1.5│ 0.46│ │ 0.48│ │ 0.51│ │ 0.54│
2.0│ 0.35│ │ 0.38│ │ 0.40│ │ 0.44│
2.5│ 0.27│ │ 0.29│ │ 0.32│ │ 0.35│
3.0│ 0.21│ │ 0.23│ │ 0.25│ │ 0.29│
4.0│ 0 12│ │ 0.12│ │ 0.16│ │ 0.19│
5.0│ 0.07│ │ 0.09│ │ 0.10│ │ 0.13│
6.0│ 0.04│ │ 0.05│ │ 0.06│ │ 0.08│
8.0│ 0.02│ │ 0.02│ │ 0.03│ │ 0.04│
10.0│ 0.01│ │ 0.01│ │ 0.01│ │ 0.02│
───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────┴───────

TABLE 46

WEIGHTS OR COMMON BUILDING MATERIAL WHEN PILED FOR STORAGE. POUNDS PER
CUBIC FOOT

────────────────────────────────────────┬──────────────────────────────
Brick │ 120
Cement │ 90
Sand │ 90
Broken stone │ 150
Lumber │ 35
Granite paving │ 160
Coal │ 50
Pig iron │ 400
────────────────────────────────────────┴──────────────────────────────

The pressure of the pile of brick per square foot of trench area
is, from Table 46, 120 × 6 = 720 pounds per square foot. The value
of _C_ from Table 44, is about 0.70. Therefore _L_{p}_ is 0.7 × 9
× 720 = 4536 pounds. The equivalent depth of backfill weighing 130
pounds per cubic foot is (4536)⁄130 × 9 = 3.88 foot. The total
equivalent depth of backfill is therefore 3.88 + 15 = 18.88 feet.
The ratio of depth to width is 18.88⁄9 = 2.98. The coefficient _C_
in the expression _W_ = _CwB_^2 is 2.17. The total load per foot
length of sewer is therefore _W_ = 2.17 × 130 × 81 = 22,800
pounds.

TABLE 47

WEIGHTS OF SHORT LOADS ON SEWER TRENCHES

(Adapted from Specifications of the American Bridge Company for
Bridges)
──────────────────────────────┬────────────────────────────────────────
Street railways, heavy │A load of 24 tons on 2 axles on 10 foot
│ centers.
Street railways, light │A load of 18 tons on 2 axles on 10 foot
│ centers.
For city streets, heavy │A load of 24 tons on 2 axles 10 feet
traffic │ apart and 5 foot gage.
For city streets, moderate │A load of 12 tons on 2 axles 10 feet
traffic │ apart and 5 foot gage.
For city streets, light │A load of 6 tons on 2 axles 10 feet
traffic or country roads │ apart and 5 foot gage.
│
Road rollers │Total weight 30,000 pounds. Weight on
│ front wheel, 12,000 pounds, and on
│ each of two rear wheels, 9,000 pounds.
│ Width of front wheel, 4 feet and of
│ each of two rear wheels 20 inches.
│ Distance between front and rear axles
│ 11 feet. Gage of rear wheels, 5 feet,
│ c. to c.
──────────────────────────────┴────────────────────────────────────────

=103. Stresses in Circular Ring=—In Fig. 81_a_ the loads shown indicate
the distribution ordinarily assumed in sewer design, the forces being
uniformly distributed across the diameter. To find the bending moment in
the pipe caused by this loading, let _ab_ in Fig. 81_b_ represent a
section of a pipe loaded with equally distributed horizontal and
vertical forces. Then the vertical component on a strip of differential
length _ds_ is _wds_ cos Θ and the horizontal component is _wds_ sin Θ
and resolving, the resultant normal to the surface is _wds_, in which
_w_ is the intensity per unit length of the horizontal and vertical
forces and Θ is the angle which the tangent to _ds_ makes with the
horizontal. Thus the loading of the nature shown in Fig. 81_b_ is
equivalent to a loading of equally distributed normal forces which give
no moment in the ring.

[Illustration:

FIG. 81.—Distribution of Stresses on Buried Pipe.
]

Considering a ring subjected to vertical forces only, the moments will
be as shown in Fig. 81_c_ and if loaded with horizontal forces only, the
moments will be as shown in Fig. 81_d_. Because of the symmetry of the
figure, moment (1) equals moment (4) but is opposite in direction and
moment (2) equals moment (3) but is opposite in direction. When the
horizontal and vertical forces are combined on the same ring as in Fig.
81_b_ these moments cancel each other as has been proven. Therefore
moment (1) equals moment (2) and moment (3) equals moment (4). Then in
Fig. 81_e_, _M_{a}_ = _M_{b}_. Now ∑_M_ = _O_ for conditions of
equilibrium, therefore _M_{a}_ + _M_{b}_ + (_W_⁄2)(_d_⁄4) = _O_ and
solving _M_{a}_ = (_Wd_)⁄16. This moment occurs at the ends of the
horizontal and vertical diameters and causes tension on the inside of
the pipe at the top and on the outside at the ends of the horizontal
diameter. There will also be compression at each end of the horizontal
diameter equal to one-half of the total load on the pipe. If the
material of the pipe is homogeneous, the maximum fiber stress _f_ can be
found through the expression _f_ = (_My_)⁄_I_ ± _P_⁄_A_ in which _M_ is
the bending moment, _y_ is the distance from the neutral axis to the
extreme fiber of a cross-section of the shell of the pipe of unit
length, _I_ is the moment of inertia of this cross-section about its
neutral axis, _P_ is one-half the total load on the pipe, and _A_ is the
area of the cross-section. For reinforced concrete, the standard
formulas should be used with this expression for _M_. The stresses in a
circular ring subjected to other distributions of loads are shown in
Table 48. An exhaustive study of the stresses in circular rings was
published by Prof. A. N. Talbot in Bulletin No. 22 of the Engineering
Experiment Station at the University of Illinois, 1908.

TABLE 48

MAXIMUM STRESS IN FLEXIBLE RINGS DUE TO DIFFERENT LOADINGS

(From Marston)
──────────────────┬───────────────┬───────────────┬───────────
Symmetrical │Moment at Crown│ Moment at End │Compressive
Vertical Loadings │ of Sewer │ of Horizontal │ Thrust at
│ │ Diameter │ Crown
│ │ │
│ │ │
────────────┬─────┼───────────────┼───────────────┼───────────
Character │Width│ │ │
────────────┼─────┼───────────────┼───────────────┼───────────
Concentrated│ 0°│+ .318_R__W_/12│- .182_R__W_/12│ 0.000
Uniform │ 60°│+ .207_R__W_/12│- .168_R__W_/12│ 0.000
Uniform │ 90°│+ .169_R__W_/12│- .154_R__W_/12│ 0.000
Uniform │ 180°│+ .125_R__W_/12│- .125_R__W_/12│ 0.000
────────────┴─────┴───────────────┴───────────────┴───────────

──────────────────┬────────────┬───────────┬──────────
Symmetrical │Compressive │ Shear at │ Shear at
Vertical Loadings │ Thrust at │ Crown │ End of
│ End of │ │Horizontal
│ Horizontal │ │ Diameter
│ Diameter │ │
────────────┬─────┼────────────┼───────────┼──────────
Character │Width│ │ │
────────────┼─────┼────────────┼───────────┼──────────
Concentrated│ 0°│+ .500_W_/12│0.500_W_/12│ 0.000
Uniform │ 60°│+ .500_W_/12│0.000_W_/12│ 0.000
Uniform │ 90°│+ .500_W_/12│0.000_W_/12│ 0.000
Uniform │ 180°│+ .500_W_/12│0.000_W_/12│ 0.000
────────────┴─────┴────────────┴───────────┴──────────

_R_ = the radius of the pipe, _W_ = total weight of ditch filling and
superimposed load plus ⅝ of the weight of the pipe itself (usually
neglected), expressed in pounds per foot length of pipe. Moments are
inch-pounds per inch length of pipe. Shears and thrusts are in pounds
per inch length of pipe.

=104. Analysis of Sewer Arches.=—The preceding method for the
determination of the stresses in a sewer ring has referred only to a
circular pipe uniformly loaded. Other methods must be used if the pipe
is not circular or the load is not uniformly distributed. The simplest
method, is the static or so-called vouissoir method. In this method the
arch is assumed to be fixed at both ends, presumably at the springing
line or line of intersection between the inside face of the arch and the
abutment, and it is so designed that the resultant of all the forces
acting on any section shall lie within the middle third of that section.

[Illustration:

FIG. 82.—Voussoir Arch Analysis.
]

[Illustration:

FIG. 83.—Force Polygon for Voussoir Arch Analysis.
]

To design an unreinforced sewer arch by the vouissoir method, a desired
arch is drawn to scale in apparently good proportions for the loadings
anticipated. The arch is then divided into any number of sections of
equal or approximately equal length called vouissoirs, and the line of
action of the resultant load, including the weight of the vouissoir is
drawn above each vouissoir as shown in Fig. 82. The forces are assumed
to act as shown in the figure. In symmetrically loaded sewer arches
there is no vertical reaction at the crown. The resultant _R_ is assumed
to act at the lower middle third of the skewback, which is the inclined
joint between the arch and the abutment. The upper horizontal force _H_
is assumed to act at the upper middle third of the middle or crown
section. The magnitude of _H_ is computed by equating the sum of the
moments of all forces about the point of application of _R_ at the
skewback to zero, and solving. The force polygon is then drawn as shown
in Fig. 83, and the equilibrium polygon is completed in Fig. 82 with its
rays parallel to the corresponding strings drawn from the end of _H_ as
origin in Fig. 83. If the equilibrium polygon line, called the
resistance line, lies wholly within the middle third of each vouissoir,
the arch is satisfactory to support the assumed load without
reinforcement. If any portion of the resistance line lies outside of the
middle third, an attempt should be made to find a resistance line which
lies wholly within the middle third. The true resistance line is that
which deviates the least from the neutral axis of the arch. To
approximate more nearly the true resistance line find two points at
which the resistance line already drawn deviates the most from the
neutral axis of the arch. Select points _M_ and _N_ on these joints, _M_
being nearer the crown than _N_. Then let _W_{1}_ and _W_{2}_ be the sum
of all the loads between the crown and _M_ and _N_ respectively, _y_
represent the vertical distance from the crown to _N_, and _y′_
represent the vertical distance between _M_ and _N_, and _x_{1}_ and
_x_{2}_ represent the horizontal distance from _W_{1}_ and _W_{2}_ to
_M_ and _N_ respectively. Then the horizontal thrust, _H_, and _a_, the
distance from the crown to the point of application of _H_, are,

_H_ = ((_W_{2}x_{2}_ − _W_{1}x_{1})_)⁄_y′_,

_a_ = _y_ − (_W_{2}x_{2}_)⁄_H_.[72]

A resistance line should be drawn with this new horizontal thrust. If no
resistance line can be found lying wholly within the middle third, new
sections should be designed until a resistance line can be drawn lying
wholly within the middle third—unless the arch is to be reinforced. A
number of satisfactory arches should be designed and the easiest one to
build should be selected. This method is limited in its application to
sewer arches with rigid side walls and it cannot be extended to include
the invert. Although an approximate method it is accurate within less
than 10 per cent of the true stresses and is usually quite close.

[Illustration:

FIG. 84.—Method for Dividing Arch into Proportion _I_⁄_S_.
]

The elastic method for the design of arches locates the true line of
resistance without approximations and is more accurate though not so
simple to apply as the static or vouissoir method. In this method a
desired form of arch is drawn as in the static method and subdivided
into vouissoirs so that the distance _S_ along the neutral axis between
joints is such that the ratio _I_⁄_S_ shall be the same for all
vouissoirs. _I_ is the average of the moments of inertia of the surfaces
of the two limiting joints about the neutral axis. If the thickness of
the arch is constant the distance between joints will be the same. The
method for dividing the arch into sections such that the ratio _I_⁄_S_
shall be a constant[73] is as follows: divide the half arch axis into
any number of equal parts; measure the radial depth at each point of
division; lay off the length of the arch axis to scale on a straight
line; divide this line into the same number of equal parts as the half
arch, as shown in Fig. 84; at each point erect a perpendicular equal in
length by scale to the moment of inertia at the corresponding point on
the arch section; draw a smooth curve through the tops of these lines;
draw a line _ab_ at any slope from the center of the original straight
line to the curve, and then a line _bc_ back to the straight line to
form an isosceles triangle _abc_; continue forming these triangles in a
similar manner thus dividing the original straight line in the required
ratio. The distance between joints is represented by the bases of the
triangles. By construction the altitude of the triangle represents the
average moment of inertia between the two limiting joints. The base of
each isosceles triangle is _S_, and _I_⁄_S_ = ½ tan α in which α is the
base angle of all the isosceles triangles.

[Illustration:

FIG. 85.—Elastic Arch Analysis.
]

The following steps in the procedure are taken from the second edition
of the American Civil Engineers Pocket Book, p. 634:

In Fig. 85 let the middle points of the joints be marked 1, 2, 3,
etc. and the coordinates _x_ and _y_ from the crown be found for
each by computation or measurement. For a load _W_ placed at one
of these points, let _z_ denote the distance from it, toward the
nearest skewback, to another middle point. Let ∑_zx_ be the sum of
the products of all the values of _z_ by the corresponding _x_,
and ∑_zy_ be the sum of all the products of _z_ by the
corresponding _y_; that is, each _z_ in the last two summations is
multiplied by the _x_ or _y_ of the point back of _W_ which
corresponds to _z_.

For a single load _W_ on the left semi-arch of Fig. 85 the
following formulas are deduced from the elastic theory, _n_ being
the number of parts into which the semi-arch is divided.

Horizontal thrust, _H_ = (_W_⁄2)(_n_∑_zy_ − ∑_y_·∑_z_)⁄(_n_∑_y_^2
− (∑_y_)^2) (1)

Moment at Crown, _M__{0} = (½_W_∑_z_ − _H_∑_y_)⁄_n_ (2)

Shear at Crown, _V__{0} = (½_W_∑_zx_)⁄∑_x_^2 (3)

For symmetrical loading such as _W_ on the left and _W_ on the
right the horizontal thrust and crown moment due to both loads are
double those found by the above formulas, while the crown shear
_V__{0} is zero. For several loads unsymmetrically placed the
formulas are to be applied to each in succession and the results
added algebraically, the value of _V__{0} being taken as negative
for the left semi-arch and positive for the right semi-arch.

For any joint whose middle point is at a distance _x_ from the
crown

_M_ = _M__{0} + _Hy_ + _V__{0}_x_ − ∑_Wz_,

_V_ = _V__{0} − ∑_W_,

where ∑_W_ is the sum of all the loads between the joint and the
crown and ∑_Wz_ is the sum of the moments of those loads with
respect to the middle of the joint. The components of the
resultant thrust normal and parallel to the joints are,

_N_ = _H_ cos θ − _V_ sin θ,

_F_ = _H_ sin θ + _V_ cos θ,

in which θ is the angle which the plane of the joint makes with
the vertical.

The distances from the neutral axis to the resistance line are,

at the crown, _e__{0} = _M__{0}⁄_H_,

at the joint, _e_ = _M_⁄_N_.

The resistance line should be located as in the vouissoir method and if
not within the middle third a new design should be studied.

=105. Reinforced Concrete Sewer Design.=—The method to be followed in
the design of reinforced concrete arches is similar except that the
moment of inertia should include both the concrete and the steel, that
is,

_I_ = _I_{c}_ + _nI_{s}_,

in which _I_ is the moment of inertia to be employed, _I_{c}_ is the
moment of inertia of the concrete, _I_{s}_ is the moment of inertia of
the steel, and _n_ is the ratio of their moduli of elasticity, generally
taken as 15. All of the moments of inertia are referred to the neutral
axis of the beam. The reinforcement called for in precast circular pipes
is given in Table 39. Sewers cast in place are ordinarily designed to
avoid reinforcement, except where the depth of cover is small and the
sewer may be subjected to superimposed loads.

Concrete sewers are sometimes reinforced longitudinally, with expansion
joints from 30 to 50 feet apart. This reinforcement is to reduce the
size of expansion and contraction cracks by distributing them over the
length of a section. The pipe is divided into sections to concentrate
motion due to expansion or contraction at definite points where it can
be cared for.

The amount of longitudinal reinforcement to be used is a matter of
judgment. It varies in practice from 0.1 to 0.4 per cent of the area of
the section. Since the coefficients of expansion of concrete and of
steel are nearly the same, movements of the structure are as important
as the stresses due to changes in temperature.

Because of the uncertain and difficult conditions under which concrete
sewers are frequently constructed it is advisable to specify the best
grade of concrete and not to stress the concrete over 450 pounds per
square inch in compression, with no allowable stress in tension. The
concrete covering of reinforcing steel should be thicker than is
ordinarily used for concrete building design, because of the possibility
of poor concrete allowing the sewage to gain access to the steel,
resulting in more rapid deterioration than would be caused by exposure
to the atmosphere. A minimum covering of about 2 inches is advisable,
except in very thin sections not in contact with the sewage. A minimum
thickness of concrete of about 9 inches is frequently used in design,
although crown thicknesses of 4½ inches have been used with success.
Greater thicknesses should be used near the surface, particularly in
locations subjected to heavy or moving loads.

Brick linings are often provided for the invert where moderately high
velocities of about 10 feet per second when flowing full are to be
expected. For velocities in the neighborhood of 20 feet per second the
invert should be lined with the best quality vitrified brick. Although
concrete may erode no faster than brick under the same conditions, brick
linings are more easily replaced and at a smaller expense.

CHAPTER X
CONTRACTS AND SPECIFICATIONS

=106. Importance of the Subject.=—Sewers may be constructed by day labor
or by contract. Under the day labor plan a city official or commission
is charged with the purchase of material, the hiring and firing of
employees, and the management of the work. Under the contract system a
private individual or company contracts to supply all the material and
labor necessary for the completion of the work.

Under the day labor plan all persons engaged are “working for the City.”
There is not the same sense of individual responsibility, the same
incentive to economize, the same feeling of loyalty that is inspired by
work under the personality of a contractor. Under either the day labor
or contract plan unscrupulous politics are likely to enter into the
relations of the employees of the city and the city officials or between
the contractor and the city officials. Neither the day labor nor the
contract plan offer a sure cure for unscrupulous political misdealings.
Under the contract plan the contractor is led to keep his bid as low as
possible, realizing the competition of other bidders, and during
construction he will obtain greater efficiency from his labor because of
their realization of the different conditions under which they are
working. In some states and cities it is illegal for the municipality to
do sewer construction except under the contract method.

The contract method is therefore used in the majority of cases, and it
is to the interest of the engineer that he be acquainted with the
essentials of contracts and specifications necessary for the proper
prosecution of sewer construction.

=107. Scope of Subject.=—The making of a contract is one of the most
common episodes of every day life. The contract may be an informal
verbal agreement to meet at a certain place at a certain time, or it may
be a formal document hedged about by confusing legal phraseology and
bearing varieties of penalties and dire consequences in the event of its
breach. The purpose of this chapter is to explain only those general
features of an engineering contract which have particular bearing upon
sewerage construction. Only the most essential points can be touched in
the limited space available to this subject, it being presumed that the
engineer is previously grounded in the principles of business law.[74]

=108. Types of Contracts.=—Contracts are known as lump sum, cost-plus,
unit-price, and by other titles indicating the method of payment.

A lump sum contract is one in which a stated amount is fixed upon,
before the execution of the contract, to be paid for all the work to be
done and materials to be furnished under the contract. Such an
arrangement is not advisable for a sewer contract, as the cautious
contractor will bid high enough to protect himself in the event of any
probable emergency. The principal must therefore pay whether the
emergency or unforeseen difficulty is met or not. The advantage of this
type of payment is that the principal knows exactly the cost of the work
to him before construction is commenced.

Cost-plus contracts are those in which the cost of the work to the
contractor is to be paid by the principal, plus, (_a_) a fixed sum of
money, (_b_) a percentage of the cost of the work, (_c_) a percentage of
the cost of the work but with a fixed limit, (_d_) a percentage of the
difference between the cost of the work and some fixed sum, or other
variations of this principle. Such contracts have the advantage that the
principal assumes all the risk in construction and therefore pays for
only those contingencies which actually arise. Except for the last named
form, they have the disadvantage that there is little or no incentive
for the contractor to keep the cost of the work down. They are most
successful where the contractor can be selected by the principal, but
where it is necessary to let contracts to the lowest bidder, the
“cost-plus” contract is not easily managed. In most states a
municipality cannot make a cost-plus contract.

A unit-price contract is one in which the amount to be paid is fixed in
proportion to the amount of work done or materials supplied. This type
of contract is the most suitable for sewer construction for a
municipality where the contract must be let to the lowest bidder. The
contractor is protected in the event of many unforeseen emergencies and
the principal is protected against a raise in bids to cover such
emergencies and against increase in the cost of the work in order to
increase the profits under a “cost-plus” contract.

It is sometimes desirable for the principal to furnish a portion of the
materials, the bidders being notified beforehand that this material will
be furnished. In this manner the quality of material is assured,
contractors with the necessary skill but small capital may be attracted
to bid, and uncertainties in the procuring of materials is eliminated.

=109. The Agreement.=—A contract is an agreement between two or more
interested parties to do a certain thing. A contract for the
construction of a sewer is an agreement between a municipality or
individual desiring sewerage facilities and a company or individual
engaged in the construction of sewers. The latter promises to construct
a sewer in return for which the former promises to pay a certain amount
of money.

The various portions of the agreement which are bound together as the
complete contract are: I. The Advertisement, II. Information and
Instructions for Bidders, III. Proposal, IV. General Specifications, V.
Technical Specifications, VI. Special Specifications, VII. Contract,
VIII. Bond, and IX. Contract Drawings. These should be fastened together
in pamphlet form and constitute the complete instrument called the
contract. No binding contract and specifications can be drawn upon
logical deductions alone as legal precedent and tried methods must be
followed to insure success. To draw up an original contract requires the
combined knowledge of an engineer and a lawyer. The engineer of to-day
writes his specifications by copying copiously from specifications used
on work which has been completed successfully. In order that selections
may be made with judgment and discrimination some examples have been
selected from existing published specifications and contracts.

=110. The Advertisement.=—This should contain: (1) A heading indicating
the type of work, (2) A statement as to when, where and how bids will be
received and opened, (3) A brief description of the character and amount
of work to be done, (4) The method of payment, (5) The conditions under
which further information can be obtained, (6) A statement as to the
amount of money which must be deposited with the bid, and (7) Any other
pertinent facts concerning the work.[75] An example of an advertisement
follows;

Sewer Construction

Construction Turkey Creek Sewer

Kansas City, Missouri.

Bids for the construction of the Turkey Creek Sewer, two sewage
pumping stations to be used in connection therewith, and certain
laterals and extensions of existing sewers thereto, for Kansas
City, Missouri, will be received up to 2 p.m. August 19, 1919, at
the office of the Board of Public Works, City Hall, Kansas City,
Missouri.

The main sewer will be about one and one-fifth miles long, and the
laterals and extensions about three and one-half miles: the main
sewer will be constructed of reinforced concrete, the laterals and
extensions will consist of concrete, segment blocks, and clay
pipe.

This work is estimated to cost from $1,500,000 to $1,750,000.
Payment for the work will be made in four year special tax bills,
bearing 7 per cent interest, payable one-fourth each year. Time
600 working days, barring strikes, bad weather, etc.

Bidders are required to deposit $15,000 in cash or a certified
check with bid, to insure signing of contract when let. Same to be
returned on execution of the contract or rejection of bid.

Complete plans and specifications for the work may be had and all
information obtained by seeing or writing to A. D. Ludlow,
Engineer of Sewers, City Hall, Kansas City, Missouri. Twenty-five
($25.00) Dollars will be required to be deposited for a set of the
plans, but $20.00 thereof will be refunded upon return of the
plans in good condition.

BOARD OF PUBLIC WORKS,

Kansas City, Missouri,

by F. E. McCabe, Secretary.

There are usually legal restrictions which require that the
advertisement be inserted a certain number of times in specified
newspapers or other advertising mediums before the opening of bids. If
the contract is of sufficient size to attract outside contractors, the
advertisement should be inserted in engineering and contracting journals
of wide circulation. Although the advertisement appears separately from
the other portions of the contract, a copy is usually bound in as the
first page of the pamphlet containing the contract and specifications
and is made an integral part thereof.

=111. Information and Instructions for Bidders.=—This is somewhat on the
order of an introduction to the pamphlet in which the specifications,
contract, and contract drawings are published. As examples of the type
of information and instructions given to prospective bidders the
abstracts below have been taken from the “Contract, Specifications,
Bond, and Proposal for the North Shore Sanitary Intercepting Sewer” by
the Sanitary District of Chicago. The information and instructions to
bidders can be divided into the following sections: 1st. Examination of
Site, 2nd. Character and Quantity of Work, 3rd. Qualification for
Bidding, 4th. Instructions for Making out Proposal, 5th. Certified
Check, and 6th. Rejection of Bids.

REQUIREMENTS FOR BIDDING AND INSTRUCTIONS TO BIDDERS

Bidders are required to submit their bids upon the following express
conditions:

Bidders must carefully examine the entire sites of the work
and the adjacent premises, and the various means of approach
to the sites, and shall make all necessary investigations to
inform themselves thoroughly as to the facilities for
delivering and handling materials at the sites and to inform
themselves thoroughly as to all the difficulties that may be
involved in the complete execution of all work under the
attached contract in accordance with the specifications hereto
attached.

Bidders are also required to examine all maps, plans, and data
mentioned in the specifications, contract or proposal as being
on file in the office of the Chief Engineer, for examination
by bidders. No plea of ignorance of conditions that exist or
that may hereafter exist or of conditions or difficulties that
may be encountered in the execution of the work under this
contract, as the result of a failure to make the necessary
examinations and investigations, will be accepted as an excuse
for any failure or omission on the part of the Contractor to
fulfill in every detail all of the requirements of said
contract, specifications and plans, or will be accepted as a
basis for any claims whatsoever for extra compensation. Upon
application all information in the possession of the Chief
Engineer will be shown to bidders, but the correctness of such
information will not be guaranteed by the Sanitary District.

The following schedule of quantities, although stated with as
much accuracy as is possible in advance, is approximate only,
and is assumed solely for the purpose of comparing bids.

Then follows an itemized schedule of the quantity of work to be done
after which comes the following:

Bidders must determine for themselves the quantities of work that
will be required, by such means as they may prefer, and shall
assume all risks as to variations in the quantities of the
different classes of work actually furnished under the contract.
Bidders shall not at any time after the submission of this
proposal, dispute or complain of the aforesaid schedules of
quantities or assert that there was any misunderstanding in regard
to the amount or the character of the work to be done, and shall
not make any claims for damages or for loss of profits because of
a difference between the quantities of the various classes of work
assumed for comparison of bids and the quantities of work actually
performed.

Proposals that contain any omissions, erasures, or alterations,
conditions or items not called for in the contract and plans
attached hereto, or that contain irregularities of any kind, will
be rejected as informal.

Bids manifestly unbalanced will not be considered in awarding the
contract.[76]

No bid will be accepted unless the party making it shall furnish
evidence satisfactory to the Board of Trustees of the Sanitary
District of Chicago of his experience and familiarity with work of
the character specified and of his financial ability to
successfully and properly prosecute the proposed work to
completion within the specified time.

Each bid shall be accompanied by a certified check, or cash, to
the amount of ten (10) per cent of the total amount of said bid
figured on the quantities given herewith, the lowest alternative
total being allowed. Said amounts deposited with bids, shall be
held until all of the bids have been canvassed and the contract
awarded and signed. The return of said check or cash to the bidder
to whom the contract for said work is awarded will be conditioned
upon his appearing and executing a contract for the work so
awarded and giving bond satisfactory to said Board of Trustees,
for the fulfillment of each contract in the amount of fifty (50)
per cent of the amount of each contract.

The said Board of Trustees reserves the right to reject any or all
bids.

Accompanying the contract form are plans which, together with the
specifications, show the work on which said tenders are to be
made.

The proposal must not be detached herefrom or from the contract by
any bidder when submitting a bid.

=112. Proposal.=—The proposal is a blank printed form on which the
bidder is required to enter the prices for which he proposes to do the
work. The proposal blank is necessary in order that the bids may be
sufficiently uniform for proper comparison. Sewers are often paid for,
particularly for small sizes, per foot of completed sewer as measured
along the center line of the pipe parallel to the surface of the ground
with the exterior length of manholes and other structures deducted.
Sometimes, under other conditions, a different rate is allowed for each
additional two feet of depth of sewer, and special structures, such as
manholes, catch-basins, flush-tanks, etc., are paid for at a unit price
according to the depth. Water connections to flush-tanks are paid for
per foot of length of service pipe laid. In especially large or
difficult work, materials are paid for at a unit-price, for example, per
cubic yard of excavation, per cubic yard of concrete, per thousand feet
board measure of lumber, etc.

The following example is taken from the contract for the North Shore
Intercepting Sewer previously quoted, to indicate the type of Proposal
used:

PROPOSAL

FOR THE CONSTRUCTION OF THE NORTH SHORE INTERCEPTING SEWER

To the Honorable, the President and the Board of Trustees of the
Sanitary District of Chicago:

Gentlemen:__The undersigned hereby certi____ that ____ ha____
examined the specifications and form of contract and the
accompanying plans for the construction of the North Shore
Intercepting sewer, and ha____ also examined the premises at and
adjacent to the sites of the proposed work, as herein described,
and the means of approach to the said sites.

The undersigned ha____ also examined the foregoing “Requirements
for Bidding and Instructions to Bidders” and propose ____ to do
all the work called for in said specifications and contract, and
shown on said plans, and to furnish all materials, tools, labor
and all appliances and appurtenances necessary to the full
completion of said work at the rates and prices for said work as
follows, to_wit:

(1_a_) For six (6) by nine (9) foot concrete sewer, complete in
place, as specified, the sum of ____ Dollars and ____ cents ($
____ ) per linear foot.

(6_a_) For manholes, concrete, complete in place, as specified the
sum of ____ Dollars and ____ cents ($ ____ ) each.

The following plans showing the work to be performed in accordance
with the attached specifications, have been examined by the
undersigned in preparing the foregoing proposal, to-wit: ____ ____
In accordance with the requirements set forth in the attached
Information and Instructions for Bidders, there is deposited
herewith the sum of ____ ____ Dollars and ____ cents ($ ____ )
which under the terms therein mentioned entitle ____ to bid on
said work, the said sum to be refunded to ____ ____ upon the
faithful performance of all conditions set forth in the
Information and Instructions for Bidders.

Name ____
Address ____

Blanks are provided for each item. No place is left at the end for a
summary. The proposal ends with an acknowledgment that the contract has
been examined completely and all preliminary directions therein have
been complied with. A blank is prepared for inserting the amount of the
required certified check, and finally for the signature of the bidders.

=113. General Specifications.=—The specifications, both general and
technical, are occasionally incorporated in the contract form, but more
frequently they are printed separately and are bound in the pamphlet
preceding the contract. The general specifications relate to the
conditions under which all work must be performed and are as applicable
to the construction of a pumping station as to the smallest lateral,
unless otherwise specified. It is not possible to include a complete set
of General Specifications in the limited space of this text, but the
more important specifications will be emphasized by examples taken from
specifications in use.[77]

The subjects covered in General Specifications are:

(1) Definitions of doubtful terms.

(2) The Engineer to settle disputes.

(3) Duties of the Engineer.

(4) Duties of the Contractor.

(5) Hours and days of work.

(6) No work to be done in the absence of an inspector.

(7) Contractor to be represented at all times.

(8) Time of commencing and completing the work.

(9) Liquidated damages for delay in completion.

(10) The City may change the plans.

(11) The City may increase the amount of the work.

(12) Inspection and its conduct.

(13) The Contractor to be acquainted with laws relating to the work.

(14) Contractor responsible for damages to persons or property.

(15) City to be protected against patent claims.

(16) Abandonment of contract and its remedy.

(17) Estimates of work done and moneys due.

(18) Payments for extra work.

(19) Character of workmen to be employed.

(20) City may reserve a sum for repairs during stipulated term after
completion.

(21) City may use money due Contractor to pay claims for labor or
materials used on the work and not paid for by the Contractor.

(22) The Contractor shall have no claim for damages on account of delay
or unforeseen difficulties.

(23) The Contractor may not assign nor sublet the contract without the
City’s consent.

(24) Cleaning up after completion.

(25) The Contractor’s relations to other contractors.

(26) The portions composing the contract.

The following examples cover the subjects named in the preceding titles:

1. Definitions. The word Engineer whenever not qualified shall
mean the Chief Engineer of the Commission, acting either directly
or through his properly authorized agents, such agents acting
severally within the scope of the particular duties entrusted to
them.

This article may include words that may be in dispute or ambiguous such
as: Board of Trustees, Elevation, City, Contractor, Rock, Earth, etc.,
etc.

2. Disputes. To prevent disputes and litigations, the Engineer
shall in all cases determine the amount, quality, and
acceptability of the work which is to be paid for under the
contract; shall decide all questions in relation to said work and
the performance thereof, and shall in all cases decide every
question which may arise relative to the fulfillment of the
contract on the part of the Contractor. His determination,
decision and estimate shall be final and conclusive, and in case
any question shall arise between the parties touching the
contract, such determination, decision, and estimate shall be a
condition precedent to the right of the Contractor to receive any
moneys under the contract.

3. Duties of the Engineer. The Engineer shall make all necessary
explanations as to the meaning and intentions of the
specifications and shall give all orders and directions, either
contemplated therein or thereby, or in every case in which a
difficulty or unforeseen condition shall arise in the performance
of the work. Should there be any discrepancies in or between, or
should any misunderstanding arise as to the import of anything
contained in the plans and specifications, the decision of the
Engineer shall be final and binding. Any errors or omissions in
plans and specifications may be corrected by the Engineer, when
such corrections are necessary for the proper fulfillment of their
intentions as construed by him.

4. Duties of the Contractor. The Contractor shall do all the work
and furnish all the labor, materials, tools and appliances
necessary or proper for performing and completing the work
required by the contract, in the manner called for by the
specifications, and within the contract time. He shall complete
the entire work at the prices agreed upon and fixed therefor to
the satisfaction of the Commission and its Chief Engineer and in
accordance with the specifications, the drawings, and such
detailed drawings as may be furnished from time to time, together
with such extra work as may be required for the performance of
which written orders may be given and received as hereinafter
provided.

The Contractor shall place sufficient lights on or near the work
and keep them burning from twilight to sunrise; shall erect
suitable railings, fences or other protections about all open
trenches, and provide all watchmen on the work, by day or night,
that may be necessary for the public safety. The Contractor shall,
upon notice from the Engineer that he has not satisfactorily
complied with the foregoing requirements, immediately take such
methods and provide such means and labor to comply therewith as
the Engineer may direct, but the Contractor shall not be relieved
of this obligation under the contract by any such notice or
directions given by the Engineer, or by neglect, failure, or
refusal on the part of the Engineer to give such notice and
directions.

The Contractor shall furnish such stakes and the necessary labor
for driving them as may be required by the Engineer. He shall
maintain the stakes when set, with reasonable diligence, and
stakes misplaced due to the carelessness of the Contractor or his
workmen shall be reset under the direction of the Engineer, at the
Contractor’s expense.

5. Night, Sunday, and Holiday Work:[78] No night, Sunday, nor
holiday work requiring the presence of an engineer or inspector
will be permitted except in case of emergency, and then only to
such an extent as is absolutely necessary and with the written
permission of the Engineer; provided that this clause shall not
operate in the case of a gang organized for regular and continuous
night, Sunday, or holiday work.

6. Absence of Engineer or Inspector. Any work done without lines,
levels, and instructions having been given by the Engineer or
without the supervision of an assistant or inspector, will not be
estimated or paid for except when such work is authorized by the
Engineer in writing. Work so done may be ordered removed and
replaced at the Contractor’s sole cost and expense.

7. Absence of Contractor. During the absence of the Contractor he
shall at all times have a duly authorized representative on the
work. The Contractor shall give written notice to the Commission
of the name and address of said representative and shall state
where and how such representative can be reached, at any and all
hours, whether by day or night.

Whenever the Contractor or his representative is not present at
any place on the work where it may be necessary to give orders or
directions, such orders or directions will be given by the
Engineer and they shall be received and promptly obeyed by the
superintendent or foreman who may have immediate charge of the
particular work in relation to which the order may be given.

8. Commencing Work. The Contractor agrees to begin the work
covered by this contract within —— days of the execution of the
contract and to prosecute the same with all due diligence and to
entirely complete the work within —— days.

It is understood and agreed that time is of the essence of this
contract, and that a failure on the part of the Contractor to
complete the work herein specified within the time specified will
result in great loss and damage to said Sanitary District and that
on account of the peculiar nature of such loss it is difficult, if
not impossible, to accurately ascertain and definitely determine
the amount thereof.

9. Liquidated Damages. It is therefore covenanted and agreed that
in case the said Contractor shall fail or neglect to complete the
work herein specified on or before the date hereinbefore fixed for
completion, the said Contractor shall and will pay the said
Sanitary District the sum of —— Dollars for each and every day the
Contractor shall be in default in the time of completion of this
contract.

Said sum of —— Dollars per day is hereby agreed upon, fixed and
determined by the parties hereto as the liquidated damages which
said Sanitary District will suffer by reason of such defaults, and
not by way of a penalty.

10. Changes in Plans. The Board reserves the right to change the
alignment, grade, form, length, dimensions or materials of the
sewers or any of their appurtenances, whenever any condition or
obstructions are met that render such changes desirable or
necessary. In case the alterations thus ordered make the work less
expensive to the Contractor a proper deduction shall be made from
the contract prices and the Contractor shall have no claim on this
account for damages or for anticipated profits on the work that
may be dispensed with. In case such alterations make the work more
expensive, a proper addition shall be made to the contract prices.
Any deduction or addition as aforesaid shall be determined and
fixed by the Engineer.

11. Extensions and Additions. In the event that any material
alterations or additions are made as herein specified which in the
opinion of the Engineer will require additional time for execution
of all the work under this contract, then, in that case the time
of completion of the work shall be extended by such a period or
periods of time as may be fixed by said Engineer and his decision
shall be final and binding upon both parties hereto, provided that
in such case the Contractor, within four (4) days after being
notified in writing of such alterations and additions, shall
request in writing an extension of time, but the provisions of
this paragraph shall not otherwise alter the provisions of this
contract with reference to _liquidated damages_, and the said
Contractor shall not be entitled to any damages or compensation
from the said Sanitary District on account of such additional time
required for the execution of the work.

12. Inspection. All materials of whatsoever kind to be used in the
work shall be subject to the inspection and approval of the
Engineer and shall be subject to constant inspection before
acceptance. Any imperfect work that may be discovered before its
final acceptance shall be corrected immediately, and any
unsatisfactory materials used in the work or delivered at the site
shall be rejected and removed on the requirement of the Engineer.
The inspection of any work shall not relieve the Contractor of any
of his obligations to perform proper and satisfactory work as
herein specified, and all work which, during the progress and
before the final acceptance, may become damaged from any cause,
shall be removed and replaced by good and satisfactory work
without extra charge therefor. The Engineer and his assistants
shall have at all times free access to every part of the work and
to all points where material to be used in the work is
manufactured, procured or stored and shall be allowed to examine
any material furnished for use in the work under this contract.

All inspection of any and all material furnished for use in work
to be performed under this contract shall be made at the site of
the work after the delivery of the material, provided, that, if
requested by the Contractor the Engineer may at his option
perform, or have performed, inspection of materials at points
other than the site of the work. In any such case the Contractor
shall pay the Sanitary District the extra cost of such inspection,
including the necessary expenses of the inspector for the extra
time expended in performing any such inspection at said other
points.

13. Legal Requirements. The Contractor shall keep himself fully
informed of all existing and future national and state laws and
local ordinances and regulations in any manner affecting those
engaged or employed in the work, or the materials used in the
work, or of all such orders and degrees of bodies or tribunals
having any jurisdiction or authority over the same, and shall
protect and indemnify the party of the first part against any
claim or liability arising from or based on the violation of such
law, ordinance, regulation, order or decree, whether by himself or
his employees.

14. Damages. If any damage shall be done by the Contractor or by
any person or persons in his employ to the owner or occupants of
any land or to any real or personal property adjoining, or in the
vicinity of the work herein contracted to be done or to the
property of a neighboring contractor the Engineer shall have the
right to estimate the amount of said damage and to cause the
Sanitary District to pay the same to the said owner, occupant, or
contractor, and the amount so paid shall be deducted from the
money due said Contractor under this contract. Said Contractor
covenants and agrees to pay all damages for any personal injury
sustained by any person growing out of any act or doing of himself
or his employees that is in the nature of a legal liability, and
he hereby agrees to indemnify and save the Sanitary District
harmless against all suits or actions of every name and
description brought against said Sanitary District, for or on
account of any such injuries, or such damages received or
sustained by any person or persons; and the said Contractor
further agrees that so much of the money due to him under this
contract, as shall be considered necessary by the Board of
Trustees of said Sanitary District, may be retained by the
Sanitary District until such suit or claim for damages shall have
been settled, and evidence to that effect shall have been
furnished to the satisfaction of said Board of Trustees.

15. Patents. It is further agreed that the Contractor shall
indemnify, keep and save harmless said Sanitary District from all
liabilities, judgments, costs, damages and expenses which may in
any wise come against said Sanitary District, or which may be the
result of an infringement of any patent by reason of the use of
any materials, machinery, devices, apparatus, or process furnished
or used in the performance of this contract, or by reason of the
use of designs furnished by the Contractor and accepted by the
Sanitary District, and in the event of any claim or suit or action
at law or in equity of any kind whatsoever being made or brought
against said Sanitary District, then the Sanitary District shall
have the right to retain a sufficient amount of money in the same
manner and upon the conditions as hereinafter specified.

16. Abandonment of Contract. If the work to be done under the
contract shall be abandoned by the Contractor, or if at any time
the Engineer shall be of the opinion, and shall so certify, in
writing, to the Commission, that the performance of the contract
is unnecessarily or unreasonably delayed, or that the Contractor
is willfully violating any of the conditions of the
specifications, or is executing the same in bad faith, or not in
accordance with the terms thereof, or if the work be not fully
completed within the time named in the contract for its
completion, the Commission may notify the Contractor to
discontinue all work thereunder, or any part thereof, by a written
notice served upon the Contractor, as herein provided; and
thereupon the Contractor shall discontinue the work, or such part
thereof, and the Commission shall thereupon have the power to
contract for the completion of said work in the manner prescribed
by law, or to procure and furnish all necessary materials,
animals, machinery, tools and appliances, and to place such and so
many persons as it may deem advisable to work at and complete the
work described in the specifications, or such part thereof, and to
charge the entire cost and expense thereof to the Contractor. And
for such completion of the work or any part thereof, the
Commission may for itself or its contractors, take possession of
and use or cause to be used any or all such materials, animals,
machinery, tools and implements of every description as may be
found on the line of the said work. The cost and expense so
charged shall be deducted from, and paid by the City out of such
moneys as may be due or may become due to the Contractor, under
and by virtue of the contract. In case such expense shall exceed
the amount which would have been payable under the contract, if
the same had been completed by the Contractor, he shall pay the
amount of such excess to the City. When any particular part of the
work is being carried on by the Commission, by contract or
otherwise, under the provisions of this clause of the contract,
the Contractor shall continue the remainder of the work in
conformity with the terms of his contract, and in such manner as
in no wise to hinder or interfere with the persons or workmen
employed by the Commission by contract or otherwise as above
provided, to do any part of the work or to complete the same under
the provisions hereof.

17. Estimates. The Engineer shall from time to time as the work
progresses, on or about the last day of each month, make in
writing an estimate, such as he shall believe to be just and fair,
of the amount and value of the work done and the materials
incorporated into the work by the Contractor under the
specifications, provided however that no such estimate shall be
required to be made when, in the judgment of the Engineer the
total value of the work done and the materials incorporated into
the work since the last preceding estimate is less than ——
dollars. Such estimates shall not be required to be made by strict
measurements, but they may be approximate only.

The Contractor shall not be entitled to demand from the Commission
as a right, a detailed statement of the measurements or quantities
entering into the several items of the monthly estimates, but he
will be given such opportunities and facilities to verify the
estimates as may be deemed reasonable by the Commission.

When in the opinion of the Engineer, the Contractor shall have
completely performed the contract on his part, the Engineer shall
make a final estimate, based on actual measurements, of the whole
amount of the work under and according to the terms of the
contract, and shall certify to the Commission in writing, the
amount of the final estimate at the completion of the work. After
the completion of the work the City shall pay to the Contractor
the amount remaining after deducting from the total amount or
value of the work, as stated in the final estimate, all such sums
as have theretofore been paid to the Contractor under any of the
provisions of the contract, except such sums as may have been paid
for extra work, and also any sum or all sums of money which by the
terms thereof the City is or may be authorized to reserve or
retain; provided that nothing therein contained shall affect the
right of the City, hereby reserved, to reject the whole or any
portion of the aforesaid work, should the said certificate be
found or known to be inconsistent with the terms of the contract
or otherwise improperly given. All monthly estimates upon which
partial payments have been made, being merely estimates, shall be
subject to correction in the final estimate, which final estimate
may be made without notice thereof to the Contractor, or of the
measurements upon which it is based.

18. Extra Work. The Contractor shall do any work not herein
otherwise provided for, when and as ordered in writing by the
Engineer or his agents specially authorized thereto in writing,
and shall when requested by the Engineer so to do, furnish
itemized statements of the cost of the work ordered and give the
Engineer access to accounts, bills, vouchers, etc. relating
thereto. If the Contractor claims compensation for extra work not
ordered as aforesaid, or for any damages sustained, he shall
within one week after the beginning of any such work or the
sustaining of any such damage, make a written statement of the
nature of the work performed or the damage sustained, to the
Engineer, and shall, on or before the fifteenth day of the month
succeeding that in which any such extra work shall have been done
or any such damage shall have been sustained, file with the
Engineer an itemized statement of the details and amount of any
such work or damage; and unless such statement shall be made as so
required, his claim for compensation shall be forfeited and he
shall not be entitled to payment on account of any such work or
damage.

For all such extra work the Contractor shall receive the
reasonable cost of said work, plus fifteen (15) per cent of said
cost.

19. Competent Employees. The Contractor shall employ only
competent skillful men to do the work; and whenever the Engineer
shall notify the Contractor, in writing, that any man employed on
the work is, in his opinion unsatisfactory, such man shall be
discharged from the work and shall not again be employed on it,
except with the consent of the Engineer.

20. Money Retained. Upon the completion of the work and its
acceptance by the City, the City shall reserve and retain five (5)
per cent of the total value of the work done under the contract as
shown by the final estimate, over and above any and all other
reservations which the city by the terms thereof is entitled or
required to retain and shall hold the said five (5) per cent for a
period of nine (9) months from and after the date of completion
and acceptance, and the City shall be authorized to apply such
part of said five (5) per cent so retained to any and all costs of
repairs and renewals as may become necessary during such period of
nine (9) months, due to improper work done or materials furnished
by the Contractor, if the Contractor shall fail to make such
repairs or renewals within twenty-four (24) hours after receiving
notice from the City so to do.

Upon the expiration of said nine (9) months from and after the
completion and acceptance of the work, the City shall pay to the
Contractor the said five (5) per cent hereby retained, less such
sums as may have been retained hereunder.

21. Unpaid Claims against Contractor. The Contractor shall furnish
the City with satisfactory evidence that all persons who have done
work or furnished materials under the contract, and have given
written notices to the City, before and within ten (10) days after
the final completion and acceptance of the whole work under the
contract, that any balance for such work or materials is due and
unpaid, have been fully paid or satisfactorily secured. And in
case such evidence is not furnished as aforesaid, such amount as
may be necessary to meet the claims of the persons aforesaid shall
be fully discharged or such notices withdrawn.

22. Delays and Difficulties. The Contractor shall not be entitled
to any claims for damages on account of postponement or delay in
the work occasioned by forces beyond the control of the City, nor
for postponement or delay in the work where ten (10) days written
notice has been given the Contractor of such postponement or
delay, nor where unforeseen difficulties are encountered in the
prosecution of the work. In the event of a postponement or delay
ordered in writing by the City the time of completion of the
contract shall be extended a number of days equal to the number of
days that the work has been postponed or delayed.

23. Assignment of Contract. The Contractor shall not assign by
power of attorney or otherwise, nor sublet the work or any part
thereof, without the previous written consent of the party of the
first part, and shall not either legally nor equitably assign any
of the moneys payable under this agreement or his claim thereto
unless by and with the consent of the party of the first part.

24. Cleaning Up. On or before the completion of the work, the
Contractor shall, without charge therefor, tear down and remove
all buildings and other structures built by him, shall remove all
rubbish of all kinds from any grounds which he has occupied, and
shall leave the line of the work in a clean and neat condition.

25. Access to Work and Other Contractors. The Commission and its
engineers, agents and employees may at any time and for any
purpose enter upon the work and the premises used by the
Contractor, and the Contractor shall provide proper and safe
facilities therefor. Other contractors of the Commission may also
when so authorized by the Engineer, enter upon the work and the
premises used by the Contractor for all the purposes which may be
required by their contracts. Any differences or conflicts which
may arise between this Contractor and other contractors of the
Commission in regard to their work shall be adjusted and
determined by the Engineer.

26. The Contract. It is understood and agreed by the City and the
Contractor that the terms of this contract are embodied and
included in the Advertisement, Information and Instructions to
Bidders, Proposal, Specifications of every nature, the Bond and
the contract drawings hereto attached.

These few articles have been given as examples of some of the essential
subjects to be treated in general specifications. It is to be understood
that these examples do not represent a complete set of general
specifications and items have been omitted the absence of which in a
complete contract might be injurious to the successful completion of the
work.

=114. Technical Specifications.=—These ordinarily follow the general
specifications and have to do with the quality of materials, the manner
of putting them together, and the method of doing the work. The subject
headings in the Technical Specifications on the Baltimore Sewerage
Commission are:

Excavation
Tunneling
Rock Excavation
Sheeting
Sheet Piling
Sheeting and Bracing
Piles
Blasting
Pumping and Drainage
Foundations
Refilling
Repaving
Underdrains
Buildings
Inlets and Catch-Basins
Cement
Mortar
Concrete
Brick
Masonry
Reinforced Concrete
Vitrified Pipe
Concrete and Brick Sewers
Vitrified Pipe Sewers and Drains
Manholes
Iron Castings
House Connections
Obstructions
Fences
Flush-Tanks

Each of these subjects is treated in the appropriate section of this
book.

An important part of each section of the technical specifications is the
clause providing for the method of payment for the work specified. This
is usually the last clause in the section. For example, the last clause
in the Baltimore Specifications relating to Rock Excavation, is:

“Payment will be made for the number of cubic yards of rock
measured and allowed as above specified at the price of four
dollars and fifty cents ($4.50) per cu. yd., measured in place.
Payment for rock excavation will be made in addition to the prices
bid for excavation.”

=115. Special Specifications.=—These have to do with problems, methods
of construction, or materials peculiar to certain contracts or certain
portions of the work. It frequently occurs that the construction of
sewerage works will be let out under a number of contracts, or bids will
be called for on different alternatives to which the entire
Advertisement, Information and Instructions for Bidders, Proposal, and
General Specifications are applicable. The special specifications will
apply only to the contract in question, e.g., in some work done under
the direction of the author, the sewer on one contract came within
twelve inches of the surface of a highway. The special specification
relating to this piece of construction, was:

“Where crossing under the Chicago Road the pipe sewer shall be
embedded in concrete as shown on the contract drawings. The
concrete for this purpose shall be mixed in the proportions of one
(1) part cement, three (3) parts fine aggregate, and six (6) parts
coarse aggregate. Payment for the concrete so used will be made at
the unit price stated in the accompanying Proposal.”

In order to avoid confusion the special specifications are either
incorporated directly in the Contract form, or follow the Technical
Specifications and are grouped according to the contracts to which they
apply.

=116. The Contract.=—The contract is a brief instrument which includes a
simple statement of the obligations of each party involved. The
following is an example of a form in successful use:

CONTRACT

This agreement made and entered into this ____ day of ____ in the
year one thousand nine hundred and ____ by and between the City of
____ by its duly constituted or elected authorities herein acting
for the City of ____ without personal liability to themselves,
party of the first part, hereinafter designated as the City, and
____ party of the second part hereinafter designated as the
Contractor.

WITNESSETH, that the parties to these presents each in
consideration of the undertakings, promises and agreements on the
part of the other herein contained, have undertaken, promised and
agreed, and do hereby undertake, promise and agree, the party of
the first part for itself, its successors and assigns, and the
part ____ of the second part for ____ and ____ heirs, executors,
administrators and assigns as follows, to-wit:

Art. I. To be bounden by all the articles of the General,
Technical, and Special Specifications applicable, and by the terms
of the Advertisement, Information and Instructions for Bidders,
Proposal and Contract Drawings hereto attached, and which are
understood and acknowledged to be an integral part of this
contract.

Art. II. The work to be completed under this contract is ____

Art. III. The City shall pay and the Contractor shall receive as
full compensation for everything furnished and done by the
Contractor under this contract, including all work required but
not specifically mentioned in the following items, and also for
all loss or damage arising from the nature of the work aforesaid,
or from the action of the elements, or from any unforeseen
obstruction or difficulty encountered in the prosecution of the
work and for well and faithfully completing the work as herein
provided, as follows:

Then follows a copy of the Proposal with the prices bid. The contract
closes with the final clause:

In witness whereof the said City of ____, party of the first part
have hereunto set their hands and seals, and the Contractor has
also hereunto set his hand and seal and the party of the first
part and the Contractor have executed this agreement in duplicate,
one part to remain with the party of the first part and one to be
delivered to the Contractor this ____ day of ____ in the year one
thousand nine hundred and ____

City of ____
____
____

Contractor ____
____
____

=117. The Bond.=—The bond called for in the Information and Instructions
for Bidders is bound in the pamphlet following the Contract. No uniform
practice is followed in the amount of the bond required. It varies from
50 to 100 per cent of the contract price and may be stated as a lump sum
before the contract price is known. There is a possibility that the
Contractor may fail before he has commenced work and the City may be
unable to procure another contractor to take up the work. The City
should then be protected by a 100 per cent bond. Such a contingency is
remote. The Contractor seldom fails until work is well under way, and
other contractors are usually available, although the failure of one
contractor tends to increase the bids of other contractors for the same
work. In fixing the amount of the bond the judgment of the Engineer is
called into play in order that the amount may be as low as possible in
fairness to the Contractor, and high enough to protect the interests to
the City. By reducing the amount of the bond the expense to the City is
also reduced as the City ultimately must pay its cost.

Upon the acceptance of the bond and the execution of the Contract, the
Engineer’s duties take him out of the designing office and into the
construction field.

CHAPTER XI
CONSTRUCTION

=118. Elements.=—The principal elements in construction are: labor,
materials, tools, and transportation. The lack of or inadequateness of
any one of these detracts from the effectiveness of the others. The
engineer should assure himself of the completeness of his plans or those
of the contractor on each of these points. The disposition of labor and
the handling of materials to obtain the largest amount of good with the
least expenditure of money and effort are problems which must be solved
by the engineer or the contractor during construction.

WORK OF THE ENGINEER

=119. Duties.=—The duties of the engineer during construction consist in
giving lines and grades; inspecting materials; interpreting the
contract, specifications and drawings; making decisions when unexpected
conditions are encountered; making estimates of work done; collecting
cost data; making progress reports; keeping records; and in guarding the
interests of the City.

=120. Inspection.=—In the inspection of workmanship and materials, the
engineer is assisted by a corps of inspectors and assistants who act
under his direction. The duties of the inspector are to be present at
all times that work is in progress and to act for the engineer in
enforcing the terms of the contract, the details of the drawings, and
the tests applicable to the workmanship and materials that he is
delegated to inspect. He should have a copy of the contract, or that
portion of it which pertains to his work, available at all times. He
should examine all materials as they are delivered on the job and see
that rejected materials are removed at once. An ordinary recourse of
some foremen will be to place rejected material to one side until a
brief absence of the inspector will present the opportunity for the use
of the rejected material. The methods to be followed in the inspection
of materials and workmanship should be such as to discover discrepancies
between the specifications and the materials delivered or the work done.
Other duties of the inspector are: to record the location of house
connections or to drive a stake over them for subsequent location by the
engineer; to see that plugs are put in the branches left for future
house connections; to inspect the workmanship in the making of joints in
pipe sewers; to protect the line and grade stakes from displacement; to
check the size, depth, and grade of sewers and elevations of special
structures, etc.

Dishonest and unscrupulous workmen have many tricks to get by the
inspector. These tricks are best learned by experience as no academic
list can impress them properly on the memory. The position of the
inspector is not always enviable. He must hold the respect of the
workmen, of the contractor, and of the engineer. To do this he must not
be unreasonable or arbitrary in his decisions, but when a decision is
once made he must be firm in following up its enforcement. He must be
careful not to give directions whose fulfillment he cannot enforce, nor
for which he cannot give adequate reason to his superiors. His integrity
must never be questioned. He must not allow himself to become under
obligations to the contractor by the acceptance of favors he cannot
return except at the expense of his employer, yet at the same time he
must not appear priggish by the refusal of all favors or social
invitations. In brief he must be friendly without being intimate,
independent without being aloof, and firm without being arbitrary.

The engineer must support his inspectors in their decisions or discharge
them if he cannot.

=121. Interpretation of Contract.=—In interpreting the contract,
specifications and drawings, the engineer is supposedly an impartial
arbiter between the interests of the city and the contractor. His
decisions, as to the meaning of the contract, must be founded on his
engineering judgment, and should aim to produce the best results without
demanding more from the contractor than, in his honest opinion, it is
the intention of the contract to demand. However conscientiously he may
attempt to remain impartial, and in spite of the honesty of the
contractor, his position, as an employee of the city will almost
invariably cause him to favor the city in his decisions on close points.
The experienced contractor knows this and fixes his bid accordingly, the
personality of the engineer sometimes acting as an important factor in
the amount of the bid. The situation arises through the character of the
contract, and not through a lack of moral integrity on the part of
anyone concerned.

=122. Unexpected Situations.=—When unexpected or uncertain conditions
are encountered in construction the engineer should visit the spot at
once and should advise or direct, according to the terms of the
contract, the procedure to be followed. Such conditions may be the
encountering of other pipes, quicksand, rock, etc. Each case is a
problem in itself. Water, gas, telephone and electric wire conduits can
be moved above or below the sewer being constructed with comparative
ease. Other sewers, if smaller, may be permitted to flow temporarily
across the line of the sewer under construction and finally discharge
into the completed sewer, or one sewer must be made to pass under the
other, either as an inverted siphon or by changing the grade of one of
the sewers. Rock, or other material for which a special rate of payment
is allowed, must be measured as soon as uncovered in order to avoid
delaying the work or losing the record of the amount removed. When
quicksand is met special precautions must be taken to safeguard the
sewer foundation and to insure that the sewer will remain in place until
after the backfilling is completed. These precautions are described in
Art. 135.

=123. Cost Data and Estimates.=—Cost account keeping and the making of
monthly or other estimates are closely connected. Cost accounts are of
value in estimating the amount of work done to date, and in making
preliminary estimates of the cost of similar work. Although the engineer
is not always required to keep such accounts, they are usually of
sufficient value to pay for the labor of keeping them. Under some
contracts the contractor’s accounts are open to examination by the
engineer. Usually, however, he must depend on reports from the
inspectors for information concerning the man-hours required on
different pieces of work, and on his own measurements of materials used
and his knowledge of their unit costs, in order to make up an estimate
of total cost.

The measurement of a completed structure and a summary of the materials
used in its construction may act as a check on the use of proper
materials as called for in the contract. For example, if it is known
that 2,000 bricks are required for the construction of a manhole and if
only 15,000 have been used in the construction of ten manholes, it is
probable that some or all of the manholes have been skimped. Similar
conditions may show in the proportions of concrete, backfilling in
tunnels, sheeting to be left in place, etc.

The statement of a few principles of cost accounting, and the
illustration of a few blanks in use should be sufficiently suggestive to
lead a resourceful engineer in the right direction.[79] Costs should be
divided into four general classifications: labor, materials, equipment,
and overhead. Labor should be subdivided under its several different
classifications arranged in accordance with rates of pay. The number of
laborers under each classification and the amount of work done per day
should be recorded. Fig. 86 is an example of a form which may be used
for such a purpose.

[Illustration:

FIG. 86.—Foreman’s Daily Payroll Report.

From Engineering and Contracting, 1907.
]

Materials may be recorded as they are delivered on the job, as they are
used, or in both cases. Measurements are usually easier to make at the
time of delivery, but records made at the time materials are used are
more serviceable. For example, 100 barrels of cement may be delivered on
a job in November, 50 of them are used before the job freezes up and the
other 50 are held over until spring. It would be misleading to charge
100 barrels used in November. Fig. 87 is a form in use for an
inspector’s report on materials. The total cost must be made up in the
office from these records and a knowledge of unit costs.

[Illustration:

FIG. 87.—Foreman’s Daily Material Report.

From Engineering and Contracting, 1907.
]

Equipment consists of tools, animals, machinery, and apparatus used in
construction. Only equipment that is actually used should be charged to
the job and a credit should be made at the completion of the job for the
fair value of the equipment remaining after the completion of the work.

Overhead charges include the expense of the office force,
superintendence, and miscellaneous items such as insurance, rent,
transportation, etc., which cannot be charged to any particular portion
of the work but are equally applicable to all portions. It happens
frequently that many jobs are handled in the same main office. The
division of overhead becomes more difficult and is frequently arranged
on an arbitrary basis, e.g., each job may be charged the proportion of
overhead that its contract price bears to the total contract prices
being performed under that office. This rule may be modified when it
becomes evident that some job is taking distinctly more than its share
of the overhead.

Estimates of work done in any period can be made with the above data in
hand by subtracting the total costs of the work up to the beginning of
the period from the total costs up to the end of the period. Fig. 88
shows a sample blank from the final estimate sheets used at Scarsdale,
N. Y.

=124. Progress Reports.=[80]—These are kept by the engineer in order
that he may see that the work is progressing as called for in the
contract, and any portion which is lagging behind without reason may be
pushed. Such reports are most useful when the information is expressed
graphically, as the eye quickly catches points where the work is falling
behind schedule.

=125. Records.=—The contract drawings are supposed to show exactly where
and how construction is to be done. Due to unexpected contingencies
changes occur, of which a record should be made and preserved. These
records may be kept in a form similar to the contract drawings, or if
the changes are not extensive, they can be recorded on the original
contract drawings. The location of house and other connections should be
recorded in a separate note book available for immediate consultation.
The engineer should keep a diary of the work in which are recorded
events of ordinary routine as well as those of special interest and
importance. This diary should be illustrated by photographs showing the
condition of the streets before and after construction, methods of
construction, accidents, etc. Such accounts are of great value in
defending subsequent litigation and their existence sometimes prevents
litigation. A contractor may wait a year or so after the completion of a
piece of work until the engineer and other city officials have broken
their connection with the city. Suit is then brought against the city
and unless good records are available the administration may be forced
to buy the claimant off or may elect to enter court, only to be beaten.

[Illustration:

FIG. 88.—Samples of Cost Record Forms.

From Engineering and Contracting, 1909.
]

Excavation

=126. Specifications.=—The following abstracts have been taken from the
specifications on Excavation by the Baltimore Sewerage Commission as
illustrative of good practice. In conducting the work the contractor
shall:

... remove all paving, or grub and clear the surface over the
trench, whenever it may be necessary and shall remove all surface
materials of whatever nature or kind. He shall properly classify
the materials removed, separating them as required by the
Engineer; and shall properly store, guard, and preserve such as
may be required for future use in backfilling, surfacing, repaving
or otherwise. All macadam material removed shall be separated and
graded into such sizes as the Engineer may direct and materials of
different sizes shall be kept separate from each other and from
any and all other materials.

All the curb, gutter, and flag-stones and all paving material
which may be removed, together with all rock, earth and sand taken
from the trenches shall be stored in such parts of the carriageway
or such other suitable place, and in such manner as the Engineer
may approve. The Contractor shall be responsible for the loss of
or damage to curb, gutter and flag-stones and to paving material
because of careless removal or wasteful storage, disposal, or use
of the same.

... When so directed by the Engineer the bottom of the trench
shall be excavated to the exact form of the lower half of the
sewer or of the foundation under the sewer.

The bottom width of the trench for a brick or concrete sewer shall
be ... not less in any case than the overall width of the sewer,
as shown on the plans. In case the trench is sheeted this minimum
width will be measured between the interior faces of the sheeting
as driven, but in no case shall bracing, stringers, or waling
strips be left within any portion of the masonry of the sewer
except by permission of the Engineer; and such braces, stringers
and waling strips shall not, in any case, be allowed to remain
within the neat lines of the masonry as shown on the plans. In
case that the distance between faces of the sheeting is less than
that called for by the width of the sewer to be laid in the
trench, the Engineer may direct the sheeting to be drawn and
redriven, or otherwise changed and altered; or he may direct that
the sewer be reinforced in such manner and to such an extent as he
may deem necessary without compensation to the Contractor, even
though such narrower trench was not caused by negligence or other
fault on the part of the Contractor.

Trenches for vitrified pipe shall be at all points at least six
inches wider in the clear on each side than the greatest external
width of the sewer, measured over the hubs of the pipe.... Bell
holes shall be excavated in the bottoms of trenches for vitrified
pipe sewers wherever necessary.

Not more than three hundred feet of trench shall be opened at any
one time or place in advance of the completed building of the
sewer, unless by written permission of the Engineer and for a
distance therein specified....

The excavation of the trench shall be fully completed at least
twenty feet in advance of the construction of the invert, unless
otherwise ordered.

During the progress of construction the Contractor will be
required to preserve from obstruction all fire hydrants and the
carriageway on each side of the line of the work.

The streets, cross-walks, and sidewalks shall be kept clean,
clear, and free for the passage of carts, wagons, carriages and
street or steam railway cars, or pedestrians, unless otherwise
authorized by special permission in writing from the Engineer. In
all cases a straight and continuous passageway on the sidewalks
and over the cross walks of not less than three feet in width
shall be preserved free from all obstruction.

Where any cross walk is cut by the trench it shall be temporarily
replaced by a timber bridge at least three feet wide, with side
railings, at the Contractor’s expense. The placing of planks
across the trench without proper means of connection or
fastenings, or pipe or other material, or the using of any other
makeshift in place of properly constructed bridges, will not be
permitted.

This is equally applicable to certain wagon bridges to be fixed upon by
the Engineer, on the basis of traffic requirements.

In streets that are important thoroughfares or in narrow streets
the material excavated from the first one hundred feet of any
opening or from such additional length as may be required, shall
upon the order of the Engineer, be removed by the Contractor, as
soon as excavated. The material subsequently excavated shall be
used to refill the trench where the sewer has been built.

The preceding specifications are applicable to open-trench excavation.
Rigid restrictions are placed about tunneling because of the greater
difficulty of doing good work, the greater danger to life and property
and the possibility of later surface subsidence if the backfilling is
done improperly. A common clause in specifications is:

All excavations for sewers and their appurtenances shall be made
in open trenches unless written permission to excavate in tunnel
shall be given by the Engineer.

=127. Hand Excavation.=—Earth excavation by pick and shovel is the
simplest and most primitive mode of excavation. Only small jobs are
handled in this manner in order to save the investment necessary in
machines or the expense of hiring and moving one to the work. The tools
used in the hand excavation of trenches are: picks, pickaxes,
long-handled and short-handled pointed shovels, square-edged long- and
short-handled shovels, scoop shovels, axes, crowbars, rock drills,
mauls, sledges, etc. The excavating gangs are divided up into units of
20 to 50 men under one foreman or straw boss, and among the men may be a
few higher priced laborers who set the pace for the others. Each laborer
on excavation should be provided with a shovel, the style being
dependent on the character of the material being excavated and the depth
of the trench. In stiff material and deep trenches requiring the lifting
of the material in the shovel, long-handled pointed shovels should be
used. In loose sandy material loaded directly into buckets
short-handled, square pointed shovels are satisfactory. Picks are used
in cemented gravels or where hard obstructions prevent cutting down with
the edge of the shovel. Very stiff but not hard material can be cut out
in chunks with a pickaxe and thrown from the trench or into a bucket
with a scoop shovel. Scoop shovels are also useful in wet running
quicksand. The number of picks, axes, crowbars, and other tools must be
proportioned according to the material being excavated. Under the worst
conditions of excavation in a hard cemented gravel it may be necessary
to provide each man with a pick as well as a shovel, whereas in sand
only a shovel is necessary. Two or three crowbars, axes, a length of
chain, two or three screw jacks, etc., are provided per gang in case of
an unexpected encounter with an obstruction in the trench, such as a
boulder, a tree stump, a length of pipe, etc.

In laying out the work the foreman marks the outlines of the trench on
the ground by means of a scratch made with a pick, chalk marks, tape, or
other devices. These marks are measured from offset or center stakes set
by the engineer. Center stakes are less conducive to error but are more
likely to be disturbed before use than are offset stakes, but careless
foremen make more errors with offset than with center stakes. The
inspector should assist or be present at the laying out of the trench.
After the trench has been laid out each laborer should be given a
certain specific portion of it to dig and this portion is marked out on
the ground. In this way a check can be kept upon the performance of each
laborer and the knowledge of this fact tends to a uniformly better
performance. The amount of work that can be performed by one man with a
pick and shovel is as shown in Table 49. Some men may exceed these
rates, many will not attain them. The allotted task must be gaged on the
character of the ground in order that the tasks may be equal and a
spirit of competition fostered. The hard worker will set the pace for
the lazy man. Some contractors have adopted the expedient of dismissing
laborers for the day as soon as the allotted task is done.

TABLE 49

AMOUNT OF MATERIAL MOVED BY ONE MAN WITH A PICK AND SHOVEL

(From H. P. Gillette)
────────────────────────────────────────┬──────────────────────────────
Material │ Cubic Yard per hour
────────────────────────────────────────┼──────────────────────────────
Hardpan │ 0.33
Common earth │ 0.8 to 1.2
Stiff clay │ 0.85
Clay │ 1.00
Sand │ 1.25
Sandy soil │ 0.8 to 1.2
Clayey earth │ 1.3
Sandy soil (frozen) │ 0.75
────────────────────────────────────────┴──────────────────────────────

The opening of the trench may be facilitated by breaking ground with a
plow. In hard ground or on paved roads it may be necessary to cut
through the surface crust with a hammer and drill, although in some
cases a plow can be used successfully. Frozen ground can be thawed by
building fires along the line of the trench, or greater economy may be
achieved by placing steam pipes along the surface with perforations
about every 18 inches and either boxing them on the top and sides or
burying them in the frozen earth with a covering of sand. Another
arrangement is to blow steam into a line of bottomless boxes in which
each box is about 8 feet long. Holes are left in the top of the boxes
into which the pipe is shoved, and after its withdrawal the holes are
covered. Blasting of frozen earth is sometimes successful but cannot be
resorted to in built up districts where it is unsafe unless properly
controlled. Once the frost crust is broken through it can be attacked
from below and frequently broken down by undermining.

A laborer cannot dig and raise the earth much more than to the height of
his head, and preferably not quite so high, without tiring quickly.
After the trench has passed a depth of 4 feet he cannot throw the earth
clear of the trench. An additional laborer is needed then at the surface
to throw the earth back. He should shovel the earth from a board
platform placed at the edge of the trench as a protection to the bank.
When the trench passes the 6–foot depth a staging is put in about 4 feet
from the top on which the lowest laborer piles his materials. It is then
passed up to the surface by a second laborer on the staging, and a third
laborer on the surface throws the material back clear of the trench.
Stagings are put in about every 5 or 6 feet for the full depth of the
trench.

When the trench has come within half the diameter of the pipe of the
final grade, if the material is sufficiently firm, the remainder of the
trench should be cut to conform to the shape of the lower half of the
outside of the pipe, with proper enlargements for each bell.

=128. Machine Excavation.=—On work of moderately large magnitude
excavation by machine is cheaper than by pick and shovel alone. In
comparing the cost of excavation by the two methods all items such as
sheeting, pipe laying, backfilling, etc., should be included, since
these items will be affected by the method of excavation. The cost of
setting up and reshipping the machine must be included as this is
frequently the item on which the use of the machine depends. Because of
the cost of setting up and shipping, which must be distributed over the
total number of yards excavated, the cost per cubic yard of excavating
by machine varies with the number of cubic yards excavated. The point of
economy in the use of a machine is reached when the cost by hand and by
machine are equal. For all work of greater magnitude, excavation by
machine will prove cheaper.[81] Items favoring the use of machinery
which may cause its adoption for small jobs are: its greater speed,
reliability, ease in handling, economy in sheeting, economy in labor,
and small amount of space needed making it useful in crowded streets.
Continuous bucket machines, drag lines, and occasionally steam shovels
are not adapted to conditions where rocks, pipes and other underground
obstacles are frequently met.

The following problem is an example of the work necessary in making a
comparison of the relative economy of machine and hand excavation:

It is assumed that a man can excavate 15 feet of trench 30 inches
wide and 8 feet deep in 10 hours. He receives 55 cents per hour
for his work. A machine costing $10,000 has a life of 6 years. It
can be kept busy 150 days in the year. When operating it costs
$1.25 per hour for the operator, fuel and repairs. It will
excavate 800 linear feet of 30 inch trench to a depth of 8 feet in
10 hours. It is assumed that capital is worth 10 per cent on such
a venture and that the sinking fund will draw 10 per cent. If the
cost of moving and setting up the machine is $1,800, how many
cubic yards of excavation must there be to make excavation by
machine economical? Costs of sheeting, pumping, etc., are assumed
to be the same for machine or hand work.

_Solution._—For hand work the man excavated 1.11 cubic yard per
hour at 55 cents. The relative cost of hand excavation is then 50
cents per cubic yard.

The cost of machine work will be divided into: interest on first
cost; operation and repairs; and sinking fund for renewal. The
interest on the first cost of $10,000 at 10 per cent is $1,000 per
year. The machine works 1,500 hours in the year. Therefore the
cost per hour is $0.67.

The sinking fund payment, as found from sinking fund tables or the
accumulation of $10,000 in. 6 years, is $1,300 per year or per
hour for 1,500 hours is $0.87.

The cost of operation per hour is given as $1.25.

The total cost per hour is therefore $2.79.

The machine excavated 59.3 cubic yards per hour which makes the
cost, exclusive of moving, equal to $0.47 per cubic yard. In order
to equalize the cost of machine and hand excavation the cost of
moving the machine must be divided among a sufficient number of
cubic yards so that the cost per cubic yard shall be 3 cents. The
cost of moving is given as $1,800. This amount divided among
60,000 cubic yards equals 3 cents per cubic yard. Therefore the
job must provide at least 60,000 cubic yards of excavation in
order that the use of the machine shall be justifiable from the
viewpoint of economy alone.

=129. Types of Machines.=—Machines particularly adapted to the
excavation of sewer and water pipe trenches are of four types: (1)
continuous bucket excavators; (2) overhead cableway or track excavators;
(3) steam shovels; and (4) boom and bucket excavators. Other types of
excavating machinery can be used for sewer trenches under special
conditions. Machines are ordinarily limited to a minimum width of trench
of 22 inches. Between widths of 22 inches and 36 inches the limit of
depth for the first class of machines is about 25 feet. For other types
of machines there is no definite limit, though the economical depth for
open cut work seldom exceeds 40 feet.

=130. Continuous Bucket Excavators.=—Continuous bucket excavators are of
the types shown in Figs. 89 and 90. The buckets which do the digging and
raising of the earth may be supported on a wheel as in Fig. 89 or on an
endless chain as in Fig. 90. The support of the wheel or endless chain
can be raised or lowered at the will of the operator so as to keep the
trench as close to grade as can be done by hand work. In some machines
the shape of the buckets can be made such as to cut the bottom of the
trench, in suitable material, to the shape of the sewer invert. In
operation, the buckets are at the rear of the machine and revolve so
that at the lowest point in their path they are traveling forward. The
excavated material is dropped on to a continuous belt which throws it on
the ground clear of the trench, into dump wagons, or on to another
continuous belt running parallel with the trench to the backfiller, by
means of which the excavated material is thrown directly into the
backfill without rehandling. The body of the machine supporting the
engine travels on wheels ahead of the excavation and is kept in line by
means of the pivoted front axle. When obstacles are encountered the
excavating wheel or chain is raised to pass over the obstacle, and
allowed to dig itself in on the other side.

[Illustration:

FIG. 89.—Buckeye Wheel Excavator.

Courtesy, Buckeye Traction Ditcher Co.
]

[Illustration:

FIG. 90.—Buckeye Endless-chain Excavator.

Courtesy, Buckeye Traction Ditcher Co.
]

[Illustration:

FIG. 91.—Movable Sheeting Fastened to Traction Ditcher.

From Eng. News-Record, Vol. 82, 1919, p. 740.
]

Wheel excavators are not adapted to the excavation of sewer trenches
over 3 to 4 feet in width and 6 to 8 feet in depth. The endless-chain
excavators are suitable for depths of 25 feet with widths from 22 to 72
inches, and due to the arrangement permitting buckets to be moved
sideways they will cut trenches of different widths with the same size
buckets. This is an advantage where there are to be irregularities in
the width of the trench such as for manholes or changes in size of pipe.
With excavating machines pipe can be laid within 3 feet of the moving
buckets and the trench backfilled immediately, thus making an
appreciable saving in the amount of sheeting. In the construction of
trenches for drain tile at Garden Prairie, Illinois, the sheeting was
built in the form of a box or shield fastened to the rear of the machine
and pulled along after it as is shown in Fig. 91.

The performance of this type of excavating machine under suitable
conditions is large. A remarkable record was made by Ryan and Co. in
Chicago,[82] with an excavating machine. 1338 feet of 32–inch trench
were excavated to an average depth of 8½ feet in 7 hours, or an average
of 160 cubic yards per hour. More could have been accomplished if it had
not been for delays in supplies. Another crew at Greeley, Colorado,[83]
with a Buckeye endless-chain ditcher weighing 17 tons and costing $5200,
averaged 232 cubic yards per day for 300 days, and the cost was 10.7
cents per cubic yard. A 15–ton Austin excavator can be expected to
remove 300 to 500 cubic yards per day.

The cost of operation of the machines is made up of items listed in
Table 50. The figures given are merely suggestive.

TABLE 50

COST OF OPERATING DITCHING MACHINE

─────────────────────────────────────────────────────────┬──────┬──────
│ Per │
│ Day │Total
─────────────────────────────────────────────────────────┼──────┼──────
Labor: │ │
1 Operator at $150 per month │ $6.00│
1 Assistant Operator at $120 per month │ 4.00│
4 laborers at 4.00 per day │ 16.00│
│——————│
│ │$26.00
│ │
Fuel: │ │
20 Gallons of gasoline at 28 cents │ 5.60│ 5.60
│ │
Miscellaneous: │ │
Oil, waste, etc. │ 1.20│
Repairs and maintenance │ 10.00│
Interest, 6 per cent on $10,000 for 150 days │ 4.00│
Depreciation, 200 working days per year and an 8 year │ │
life │ 11.11│ 26.31
│——————│——————
Total cost per day │ │$57.91
─────────────────────────────────────────────────────────┴──────┴──────

TABLE 51

COMPARISON OF COST OF HAND EXCAVATION AND MACHINE EXCAVATION WITH
CONTINUOUS-BUCKET EXCAVATOR

───────────────────────────┬───────┬───────────────────────────┬───────
Hand Work │ Per │ Machine Work │ Per
│ Day, │ │ Day,
│Dollars│ │Dollars
───────────────────────────┼───────┼───────────────────────────┼───────
Foreman │ 4.00│Engineer │ 4.00
Timberman │ 3.00│Fireman │ 2.50
Helper │ 2.50│Coal │ 5.00
4 Laborers at $2.00 │ 80.00│Team │ 4.00
│ │Foreman │ 4.00
│ │Pipe layer │ 3.00
│ │Helper │ 2.50
│ │2 Teams backfilling │ 8.00
│ │2 Helpers │ 4.00
│ │Interest, depreciation and │
│ │ repairs │ 10.00
│ ——————│ │ ——————
Total │ 95.00│ Total │ 54.50
───────────────────────────┴───────┴───────────────────────────┴───────

In making a comparison of the cost of hand and machine excavation the
figures given in Table 51 are from “Excavating Machinery” by McDaniel,
who quotes the cost of machine excavation from the manufacturers of the
Parsons machine issued as the result of several years’ experience with
their excavator. In the comparison the hand crew is assumed to dig 315
linear feet of trench 28 inches wide by 12 feet deep in a day of 10
hours. This assumes that each man will excavate 7 cubic yards per day.
The machine is assumed to excavate 250 feet of the same trench. The
comparison indicates that an excavator will work at about 50 per cent of
the cost of hand excavation, if the cost of moving the machine is not
included.

[Illustration:

FIG. 92.—Carson Excavating Machine on Trench Excavation in South
Milwaukee.

Courtesy, Mr. C. F. Henning.
]

=131. Cableway and Trestle Excavators.=—Cableway and trestle excavators
are most suitable for deep trenches and crowded conditions. They should
not be used for trenches much less than 8 feet in depth. They differ
from the continuous bucket excavators in that the actual dislodgment of
the material is done by pick and shovel, the excavated material being
thrown by hand into the buckets of the machine. A machine of the Carson
type is shown in Fig. 92. The machine consists of a series of
demountable frames held together by cross braces and struts to form a
semirigid structure. An I beam or channel extending the length of the
machine is hung closely below the top of the struts. The lower flange of
this beam serves as a track for the carriages which carry the buckets.
All the carriages are attached to each other and to an endless cable
leading to a drum on the engine. This cable serves to move the buckets
along the trench. The buckets are attached to another cable which is
wound around another drum on the engine and serves to lower or raise all
the buckets at the same time. In operation there are always at least two
buckets for each carriage, one in the trench being filled and the other
on the machine being dumped. There should be a surplus of buckets to
replace those needing repairs.

The machines may be from 200 to 350 feet in length, and the number of
buckets which can be lifted at one time varies from one to a dozen or
more. On trenches over 5 to 6 feet in width a double line of buckets is
sometimes used. The entire machine rests on rollers and straddles the
trench. It is moved along the trench by its own power, either by gearing
or chains attached to the wheels, or by a cable attached to a dead-man
ahead.

The Potter trench machine differs from the Carson in that only 2 buckets
are used at a time and these are carried on a car which travels on a
track on top of the trestle. The movement of the buckets and the car are
controlled by 2 dump men who ride on the car and who can raise or lower
the buckets independently.

The organization needed to operate these machines is: a lockman who
locks and unlocks the buckets on the cable, a dumper, as many shovelers
as there are buckets on the machine, and an engineman who is usually his
own fireman. From 50 to 400 cubic yards of material can be excavated in
a day with one of these machines, dependent on the character of the
material and the depth of the trench. H. P. Gillette in his Handbook of
Cost Data reports that about 190 cubic yards were excavated per day with
a Potter machine. The machine was 370 feet long. Six ¾-yard buckets were
used, 4 in the trench and 2 on the carrier. The trench was 10½ feet wide
and 18 feet deep in wet sand and soft blue clay. The organization
consisted of an engineman, a fireman, 2 dumpmen on the carrier, and from
17 to 21 excavating laborers depending on the kind and the amount of the
excavation. In general the capacity of such machines is limited by the
amount of material which can be shoveled into them by hand.

=132. Tower Cableways.=—These are essentially of the same class as the
trestle cableway machines. They differ in that the carriage supporting
the buckets travels on a cable suspended between 2 towers instead of on
a track supported on a trestle. As a rule only one bucket is handled in
the machine at a time. They are used in sewer work only in exceptional
cases as the towers must be taken down and re-erected each time that
there is an advance in the trench greater than the distance between the
towers.

=133. Steam Shovels.=—The use of steam shovels for the excavation of
sewer trenches is becoming more prevalent because of their growing
dependability and durability as compared with other machines, their
adaptability for small trenches, and the relatively large number of
widely different uses to which they can be put. In excavating a trench
the shovel straddles the trench and runs on tractors, wheels, or rollers
on either side of it. The shovel cuts the trench ahead of it. As a
result it is difficult to set sheeting and bracing close to the end of
the trench while the shovel is operating. Steam shovels are therefore
not suitable for excavation in unstable material, unless the sheeting is
driven ahead of the excavation. It is only in the softest ground that
ordinary wood sheeting can be driven ahead of the excavation. Steel
sheet piling is more suitable for such use. Fig. 93[84] shows a shovel
at work on a trench in Evanston, Illinois.

Shovels are equipped with extra long dipper handles to adapt them to
trench excavation. The dipper handle in the picture is longer than the
standard for this type of machine. The method of supporting the shovel
can be seen in the picture under the machine and the method of bracing
and of finishing the trench by hand work are also shown. The excavated
material is taken out in the shovel and dropped on the bank or into
wagons.

The limiting depth to which trenches can be excavated by steam shovels
is about 20 to 25 feet, where the trench is too narrow for the shovel to
enter. Wider trenches are cut in steps of about 15 feet, the shovel
working in the trench for additional depths. Shovels are now made to cut
trenches as narrow as a man can enter to lay pipe. The greatest width
that can be cut from one position of the shovel is from 15 to 40 feet,
dependent on the size of the shovel. Occasionally a combination of a
drag line and a steam shovel can be used, as on the construction of the
Calumet sewer in Chicago. On this work the first step was cut by a steam
shovel. It was followed by a drag line resting on the step thus
prepared, and excavating the remaining distance to grade. The depth of
the trench in this work averaged about 25 to 30 feet.

[Illustration:

FIG. 93.—Steam Shovel at Work on Sewer Trench for North Shore
Intercepting Sewer, Evanston, Illinois.
]

Steam shovels are rated according to their tonnage and the capacity of
the dipper in cubic yards. Both are necessary as the size of the dipper
is varied for the same weight of machine, dependent on the character of
the material being excavated. For rock the dipper is made smaller than
for sand. Gillette in his Hand Book of Cost Data gives the coal and
water consumption of steam shovels as shown in Table 52. The performance
of steam shovels is recorded in Table 53. The conditions of the work
have a marked effect on the output of the shovel. A shovel in a thorough
cut, i.e., in a trench just wide enough for the shovel to turn 180
degrees but too narrow to run cars or wagons along side of it, will
perform less than one-half of the work that it can perform in a side
cut, i.e., where the cars can be run along side the shovel which turns
less than 90 degrees.

TABLE 52

COAL AND WATER CONSUMPTION BY STEAM SHOVELS

(From Handbook of Cost Data, by H. P. Gillette)
───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬─────
Weight in tons │ 35 │ 45 │ 55 │ 65 │ 75 │ 90
Dipper, cubic yards │ 1¼ │ 1½ │ 1¾ │ 2 │ 2½ │ 3
Coal, tons per 10 hour day │ ¾ │ 1 │ 1¼ │ 1½ │ 2 │ 2¼
Water, gallons per 10 hour day │1500 │2000 │2500 │3000 │4000 │4500
───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────

TABLE 53

PERFORMANCE BY STEAM SHOVELS

──────┬──────┬──────┬───────┬───────────┬──────┬──────────────┬────────
Weight│Dipper│Depth │ Width │ 10–Hour │ Cost │ Authority │Remarks
in │Cubic │ of │of Cut │Performance│ in │ │
Tons │Yards │ Cut, │ │ │Cents,│ │
│ │ Feet │ │ │ per │ │
│ │ │ │ │Cubic │ │
│ │ │ │ │ Yard │ │
──────┼──────┼──────┼───────┼───────────┼──────┼──────────────┼────────
25 │ 1 │ 9 │36 in. │ 85 │ 22.6 │R. T. Dana │ 1
│ │ │ │ │ │ Eng. Rec., │
│ │ │ │ │ │ 69:581 │
25 │ 1 │ 8 │35 in. │ 96 │ 23.5 │ do. │ 2
70 │ 2 │ 26 │16 ft. │ 569 │ 6.7 │ do. │ 3
30 │ 1 │15–18 │60 in. │ 300 │ │A. B. McDaniel│ 4
│ │ │ │ │ │ Excavating │
│ │ │ │ │ │ Machinery │
15 │ ⅝ │ 14 │134 ft.│ 400 │ │Eng. Cont’r, │ 5
│ │ │ │ │ │ 8–25–09 │
│ 8 │ 36 │ Very │16 yd. cars│ │Marion Steam │ 6
│ │ │ wide │ │ │ Shovel Co. │
55 │ │ │ │ 296 │ │H. P. │ 7
│ │ │ │ │ │ Gillette’s │
│ │ │ │ │ │ Cost Data │
65 │ 2¼ │ │ │ 280 │ │ do. │
│ │ │Greater│ 700 │ 30.6 │G. C. D. │ 8
│ │ │than 78│ │ │ Lenth, Eng. │
│ │ │ in. │ │ │ News-Record,│
│ │ │ │ │ │ 85:22 │
──────┴──────┴──────┴───────┴───────────┴──────┴──────────────┴────────

Remarks:

1. One runner at $5.00, one fireman at $2.31, two laborers
at $1.70 each, supplies at $4.50, and interest and
depreciation on 200 days per year, $4.00. Total per
day, $19.21. Material, clay and gravel.

2. Average of 11 jobs with the same shovel.

3. Cost per day, one runner at $5.00, one crane-man at $3.60,
one fireman at $2.00, 7 roller men at $1.50 each, supplies
$9.00 and interest and depreciation on $9000 at 200 days
per year $8.00. Total, $38.10.

4. Hard clay.

5. Stiff clay for the basement of a building in Chicago.

6. Stripping ore. This is a maximum record. The average was
about three hundred and twenty 16 cubic yard cars per day.

7. Blasted mica-schist.

8. General average.

=134. Drag Line and Bucket Excavators.=—A drag line excavator is shown
in Fig. 94. The back of the bucket is attached to a drum on the engine
by means of a cable passing over the wheel in the end of the long boom.
The front of the bucket is attached by another cable directly to another
drum on the engine. In operation the bucket is raised by its rear end
and dropped out to the extremity of the boom. It is then dragged over
the ground towards the machine, digging itself in at the same time. When
filled the bucket is raised by tightening up on the two cables, swung to
one side by means of the movable boom, and dumped.

[Illustration:

FIG. 94.—Drag Line at Work on Trench for Drain Tile.
]

Drag line excavators will perform as much work as steam shovels under
favorable conditions. They are less expensive in first cost and
operation, and are equally reliable but they are not adapted to the more
difficult situations where steam shovels can be used to advantage. Drag
lines are suitable only for relatively wide trenches in material
requiring no bracing, and in a locality where relatively long stretches
of trench can be opened at one time.

The bucket excavator differs from the drag line in that the bucket can
be lifted vertically only and the types of buckets used in the two types
of machine are different. The bucket may be self filling of the
orange-peel or clam-shell type, or a cylindrical container which must be
filled by hand. A drag line can be easily converted into a boom and
bucket excavator. Boom and bucket excavators are well adapted to use in
deep, closely braced trenches and shafts.

=135. Excavation in Quicksand.=[85]—A sand or other granular material in
which there is sufficient upward flow of ground water to lift it, is
known as quicksand. Its most important property, from the viewpoint of
sewer construction, is its inability to support any weight unless the
sand is so confined as to prevent flowing of the sand, or unless the
water is removed from the sand.

Excavation in quicksand is troublesome and expensive and is frequently
dangerous. The material will flow sluggishly as a liquid, it cannot be
pumped easily, and its excavation causes the sides of the trench to fall
in or the bottom to rise. The foundations of nearby structures may be
undermined, causing collapse and serious damage. These conditions may
arise even after the backfilling has been placed unless proper care has
been taken. The greatest safeguard against such dangers is not only to
exercise care in the backfilling to see that it is compactly tamped and
placed, but to leave all sheeting in position after the completion of
the work.

The ordinary method of combating quicksand and in conducting work in wet
trenches is to drive water-tight sheeting 2 or 3 feet below the bottom
of the trench, and to dewater the sand by pumping. When dry it can be
excavated relatively easily. A more primitive but equally successful
method is to throw straw, brickbats, ashes, or other filling material
into the trench in order to hold the excavation once made, or this may
supplement the attempts at pumping, or the wet sand may be bailed out in
buckets. Successful excavation in quicksand requires experience,
resourcefulness. and a careful watch for unexpected developments. The
well points described in Art. 142 are used for dewatering quicksand.

=136. Pumping and Drainage.=—Ground water is to be expected in nearly
all sewer construction and provision should be made for its care. Where
geological conditions are well known or where previous excavations have
been made and it is known that no ground water exists it may be safe to
make no provision for encountering ground water. Where ground water is
to be expected the amount must remain uncertain within certain rather
wide limits until actually encountered.

In order to avoid the necessity for pumping, or working in wet trenches
it is sometimes possible to build the sewer from the low end upwards and
to drain the trench into the new sewer. The wettest trenches are the
most difficult to drain in this manner as the material is usually soft
and the water so laden with sediment as to threaten the clogging of the
sewer. It is undesirable to run water through the pipes until the cement
in the joints has set. This necessitates damming up the trench for a
period which may be so long as to flood the trench or delay the progress
of the work. If it is not possible to drain the trench through the sewer
already constructed the amount of water to be pumped can be reduced by
the use of tight sheeting.

[Illustration:

FIG. 95. Improvised Trench Pump.
]

Pumps for dewatering trenches must be proof against injury by sand, mud,
and other solids in the water. For this purpose pumps with wide passages
and without valves or packed joints are desirable. The types of pumps
used are: simple flap valve pumps improvised on the job, diaphragm
pumps, jet pumps, steam vacuum pumps, centrifugal pumps, and
reciprocating pumps. All are of the simplest of their type and little
attention is paid to the economy of operation because of the temporary
nature of their service.

=137. Trench Pump.=—A simple pump which can be improvised on the job is
shown in section in Fig. 95. Its capacity is about 20 gallons per minute
but its operation is backaching work. It is inexpensive, quickly put
together and may be a help in an emergency. It is to be noted that the
passages are large and straight, that there are no packed joints, and
that the velocity of flow is so small that it is not liable to clogging
by picking up small objects.

[Illustration:

FIG. 96.—Diaphragm Pump

Courtesy, Edson Manufacturing Co.
]

=138. Diaphragm Pump.=—The type of pump shown in Fig. 96 is the most
common in use for draining small quantities of water from excavations.
It is known as the diaphragm pump from the large rubber diaphragm on
which the operation depends. The pump is made of a short cast-iron
cylinder, divided by the rubber diaphragm or disk to the center of which
the handle is connected. The valve is shown at the center of the disk.
As the diaphragm is lifted the valve remains closed, creating a partial
vacuum in the suction pipe and at the same time discharging the water
which passed through the valve on the previous down stroke. When the
valve is lowered the foot valve on the suction pipe closes, holding the
water in place, and the valve in the pump opens allowing the water to
flow out on top of the disk to be discharged on the next up stroke.
Table 54 shows the capacities of some diaphragm pumps as rated by the
manufacturers. The smaller sizes are the more frequently used and are
equipped with a 3–inch suction hose with strainer and foot valve. They
are not adapted to suction lifts over 10 to 12 feet. Where greater lifts
are necessary one pump may discharge into a tub in which the foot valve
of a higher pump is submerged.

TABLE 54

CAPACITIES OF DIAPHRAGM PUMPS

─────────────────┬─────────────────┬─────────────────┬─────────────────
Diameter of │ Diameter of │Length of Stroke │ Capacity per
Cylinder, Inches │ Suction, Inches │ in Inches │ Stroke, Gallons
─────────────────┼─────────────────┼─────────────────┼─────────────────
6│ 3│ 4│ 0.49
8½│ 4│ 6│ 1.47
9[86]│ 2½│ │ 0.75
12½[86]│ 3│ │ 1.25
│ Power driven by 1 horse-power │
12½[86]│ engine │ 0.58[87]
─────────────────┴───────────────────────────────────┴─────────────────

[Illustration:

FIG. 97.—McGowan Steam Jet Pump.

Courtesy, The John H. McGowan Co.
]

=139. Jet Pump.=—The simplicity of the parts of the jet pump is shown in
Fig. 97. It has a distinct advantage over pumps containing valves and
moving parts in that there are no obstructions offered to the passage of
solids as well as liquids through the pump. It is not economical in the
use of steam, however. It operates by means of a steam jet entering a
pipe at high velocity through a nozzle. This action causes a vacuum
which will lift water from 6 to 10 feet. The lower the suction lift,
however, the greater the efficiency of the work. The sizes and
capacities of jet pumps as manufactured by the J. H. McGowan Co. are
shown in Table 55.

TABLE 55

CAPACITIES OF JET PUMPS

(J. H. McGowan Co.)
──────────────┬─────────────┬─────────────┬─────────────┬──────────────
Size of Pump │ │ │ Capacity, │ Approximate
and Suction │ Discharge │ Steam Pipe, │ Gallons per │ Horse-power
Pipe, Inches │Pipe, Inches │ Inches │ Minute │ Required
──────────────┼─────────────┼─────────────┼─────────────┼──────────────
¾│ ½│ ⅜│ 8│ 2
1│ ¾│ ½│ 15│ 3
1¼│ 1│ ½│ 20│ 4
1½│ 1¼│ ¾│ 30│ 6
2│ 1½│ ¾│ 40│ 8
2½│ 2│ 1│ 50│ 10
3│ 2½│ 1│ 60│ 15
4│ 3½│ 1¼│ 85│ 25
──────────────┴─────────────┴─────────────┴─────────────┴──────────────

=140. Steam Vacuum Pumps.=—This type of pump depends on the condensation
of steam in a closed chamber to create a vacuum which lifts water into
the chamber previously occupied by the steam and from which the water is
ejected by the admission of more steam. The best known pumps of this
type are the Pulsometer, manufactured by the Pulsometer Steam Pump Co.,
the Emerson, manufactured by the Emerson Pump and Valve Co., and the Nye
Pump, manufactured by the Nye Steam Pump and Machinery Co.

[Illustration:

FIG. 98.—Pulsometer Steam Vacuum Pump.
]

A section of a Pulsometer is shown in Fig. 98. It consists of two
bottle-shaped chambers _A_ and _B_ with their necks communicating at the
top and each opening into the outlet chamber _O_ through a check valve.
Steam is admitted at the top and enters chamber _A_ or _B_ according to
the position of the steam valve _C_ as shown. This steam valve is a ball
which is free to roll either to the right or left and forms a
steam-tight joint with whichever seat it rests upon. In normal operation
chamber _A_ would be filled with water as the steam enters the cylinder.
At the same time a check valve at the top opens to admit a small
quantity of air which forms a cushion insulating the steam from the
water, reduces the condensation of the steam, and serves as a cushion
for the incoming water on the opposite stroke. The pressure of the steam
depresses the surface of the water without agitation and forces the
water through the check valve _F_ into the discharge chamber _O_. When
the water falls to the level of the discharge chamber the even surface
is broken up and the intimate contact of the steam and water condenses
the former instantaneously. This forms a vacuum in chamber _A_ which,
assisted by a slight upward pressure in chamber _B_ caused by the
incoming water, immediately pulls the ball _C_ over to the other seat
and directs the steam into chamber _B_. The vacuum in chamber _A_ now
draws up a new charge of water through the suction pipe into the
chamber.

[Illustration:

FIG. 99.—Emerson Steam Vacuum Pump.
]

A section of the Emerson pump is shown in Fig. 99. The pump consists of
two vertical cylinders _B_ and _C_. Each chamber has a suction valve _L_
at the bottom, opening upward from a common chamber from which the
discharge pipe _U_ extends. On the top of each chamber is a baffle plate
_G_ which operates to distribute the steam evenly to the two chambers
and to prevent it from agitating the surface of the water in the
chambers. A condenser nozzle _F_ is connected with the bottom of the
opposite chamber by a pipe into which a check valve opens upward. As the
pressure in the chamber alternates water will be injected through _F_
into the opposite chamber and condense the steam therein, promptly
forming a vacuum. An air valve _P_ admits a small quantity of air while
the chamber is filling with water, the air acting as an insulating
cushion as in the Pulsometer. Valve _O_, just above the top connection
_S_ is used to regulate the amount of steam that enters the pump. The
top connection _S_ has two ports, one leading to each chamber. An
oscillating valve enclosed in it admits the steam through these ports to
the two chambers alternately. This valve is driven by a small
three-cylinder engine, the crank shaft of which extends into the top
connection in the center of the bearing on which the valve oscillates. A
positive geared connection is made between the valve and the engine and
so arranged that the engine will run faster than the valve.

The action of these pumps consists of alternately filling and emptying
the two chambers. They will continue operation without attention or
lubrication so long as the steam is turned on. In view of the simplicity
of their operation and make-up, their ability to handle liquids heavily
charged with solids, and their reasonable steam consumption these pumps
are widely used for pumping water in construction work. They have an
added advantage that no foundation or setting is required for them as
they can be hung by a chain from any available support.

These pumps are manufactured in sizes varying from 25 to 2500 gallons
per minute at a 25–foot head, and with a steam consumption of about 150
pounds per horse-power hour. They reduce about 4 per cent in capacity
for each 10 feet of additional lift. They will operate satisfactorily
between heads of 5 to 150 feet, with a suction lift not to exceed 15
feet. Lower suction lifts are desirable and the best operation is
obtained when the pump is partly submerged. The steam pressure should be
balanced against the total head. It varies from 50 to 75 pounds for
lifts up to 50 feet, and increases proportionally for higher lifts. The
dryer the steam the lower the necessary boiler pressure.

=141. Centrifugal and Reciprocating Pumps.=—The details of these pumps,
their adaptability to various conditions, and their capacities are given
in Chapter VII. The centrifugal is better adapted to trench pumping as
it is not so affected by water containing sand and grit, but for clear
water, high suction lifts and fairly permanent installations,
reciprocating pumps can be used with satisfaction.

=142. Well Points.=—In dewatering quicksand a method frequently attended
with success is to drive a number of well points into the sand and
connect them all to a single pump. Figure 100 shows a well point system
used on sewer work in Indiana. The well points are 3 feet apart and are
connected to a 2½-inch header which in turn is connected to six Nye
pumps, each with a capacity of 200 gallons per minute for a lift of 50
feet. The number and size of well points and pumps to use will depend on
conditions as met on the job. On a piece of work in Atlantic City[88]
the equipment consisted of two complete outfits each comprising one
hundred 1½ inch by 36–inch No. 60 well points, one hundred 6–foot
lengths of rubber hose, about 600 feet of suction main, one hundred
valved T connections, and a 7 × 8–inch Gould Triplex Pump with a
capacity of 200 gallons per minute, belted to a 7½ horse-power motor.

[Illustration:

FIG. 100.—Well Points Pumped by Nye Steam Vacuum Pump.
]

=143. Rock Excavation.=—A common definition of rock used in
specifications is: whenever the word Rock is used as the name of an
excavated material it shall mean the ledge material removed or to be
removed properly by channeling, wedging, barring, or blasting; boulders
having a volume of 9 (this volume may be varied) cubic feet or more, and
any excavated masonry. No soft disintegrated rock which can be removed
with a pick, nor loose shale, nor previously blasted material, nor
material which may have fallen into the trench will be measured or
allowed as rock.

Channeling consists in cutting long narrow channels in the rock to free
the sides of large blocks of stone. The block is then loosened by
driving in wedges or it is pried loose with bars. It is a method used
more frequently in quarrying than in trench excavation where it is not
necessary to preserve the stone intact. In blasting, a hole is drilled
in the rock, and is loaded with an explosive which when fired shatters
the rock and loosens it from its position.

[Illustration:

FIG. 101.—Plug and Feathers for Splitting Rock.
]

In drilling rock by hand the drill is manipulated by one man who holds
it and turns it in the hole with one hand while striking it with a
hammer weighing about 4 pounds held in the other hand, or one man may
hold and turn the drill while one or two others strike it with heavier
hammers. In churn drilling a heavy drill is raised and dropped in the
hole, the force of the blow developing from the weight of the falling
drill. Hand drills are steel bars of a length suitable for the depth of
the hole, with the cutting edge widened and sharpened to an angle as
sharp as can be used without breaking. The drill bar is usually about
⅛th of an inch smaller than the diameter of the face of the drill.

Wedges used are called plugs and feathers. They are shown in Fig. 101
which shows also the method of their use. The feathers are wedges with
one round and one flat face on which the flat faces of the plug slide.

=144. Power Drilling.=—In power drilling the drill is driven by a
reciprocating machine which either strikes and turns the drill in the
hole, or lifts and turns it as in churn drilling, or the drill may be
driven by a rotary machine which is revolved by compressed air, steam,
or electricity. There are many different types of machines suitable for
drilling in the different classes of material encountered and for
utilizing the various forms of power available.

A jack hammer drill is shown in Fig. 102. In its lightest form the drill
weighs about 20 pounds and is capable of drilling ⅞-inch holes to a
depth of 4 feet. Heavier machines are available for drilling larger and
deeper holes. The same machine can be adapted to the use of steam or
compressed air. When in use the point of the drill is placed against the
rock and a pressure on the handle opens a valve admitting air or steam.
The piston is caused to reciprocate in the cylinder, striking the head
of the drill at each stroke. The drill is revolved in the hole by hand
or by a mechanism in the machine. A hollow drill can be used by means of
which the operator admits air or steam to the hole, thus blowing it out
and keeping it clean. These machines have the advantage of small size,
portability and simplicity. They can be easily and quickly set up and
the drills can be changed rapidly. Their undesirable features are the
vibration transmitted to the operator and the dust raised in the trench.

[Illustration:

FIG. 102.—Jack Hammer Rock Drill.
]

[Illustration:

FIG. 103.—Tripod Drill.
]

A type of drill heavier and larger than the jack hammer drill is shown
in Fig. 103. It requires some form of support such as a tripod, or in
tunnel work it can be braced against the roof or sides. Some data on
steam and air drills are given in Table 56. The effect of the length of
the transmission pipe, temperature of the outside air, pressure at the
boiler or compressor, etc., will have a marked effect on the amount of
steam or air to be delivered to the drill. Compressed air is affected
more than steam by these outside factors, but it has an advantage in
that as it loses in pressure it increases in volume so that the loss of
power is not so marked. Gillette states:

We may assume that a cubic foot of steam will do practically the
same work in a drill as a cubic foot of compressed air at the same
pressure, because neither the steam nor the air acts expansively
to any great extent in a drill cylinder, due to the late cut-off.
This being so ... one pound of steam is equivalent to nearly 30
cubic feet of free air ... all at the same pressure of 75 pounds
per square inch. If a drill consumes at the rate of 100 cubic feet
of free air per minute ... it would therefore consume 240 pounds
of steam (at 75 pounds pressure) per hour.... Where not more than
three or four drills are to be operated, probably no power can
equal compressed air generated by gasoline. It will require 12
horse-power to compress air for each drill, hence 1½ gallons of
gasoline will be required per hour per drill while actually
drilling.

TABLE 56

DATA ON ROCK DRILLS

(From H. P. Gillette)
───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬─────
Diameter of cylinder in inches │ 2¼│ 2½│ 2¾│ 3⅛│ 3¼│ 3⅜
Length of stroke in inches │ 5│ 6│ 6½│ 6⅝│ 6⅝│ 7¼
Length of drill from end of crank │ │ │ │ │ │
to end of piston │ 36│ 43│ 50│ 50│ 50│ 52
Depth of hole drilled without │ │ │ │ │ │
change of bit, inches │ 15│ 20│ 24│ 24│ 24│ 24
Diameter of supply inlet. Standard │ │ │ │ │ │
pipe, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Approximate strokes per minute with│ │ │ │ │ │
60 pound pressure at the drill │ 500│ 450│ 375│ 350│ 325│ 300
Depth of vertical hole each machine│ │ │ │ │ │
will drill easily, feet │ 6│ 8│ 10│ 14│ 16│ 20
Diameter of holes drilled, inches │ ¾ to 1½ as desired
Diameter of octagon steel, inches │ ¾ to│ ⅞ to│ 1 to│1⅛ to│1⅛ to│1¼ to
│ ⅞│ 1│ 1⅛│ 1¼│ 1¼│ 1⅜
Best size of boiler to give plenty │ │ │ │ │ │
of steam at high pressure, │ │ │ │ │ │
horse-power │ 6│ 8│ 8│ 9│ 10│ 12
Best size of supply pipe to carry │ │ │ │ │ │
steam 100 to 200 feet, inches │ ¾│ ¾│ ¾│ 1│ 1│ 1¼
Weight of drill unmounted, with │ │ │ │ │ │
wrenches and fittings, hot boxed,│ │ │ │ │ │
pounds │ 128│ 190│ 265│ 315│ 385│ 390
Weight of tripod, without weights, │ │ │ │ │ │
not boxed, pounds │ 80│ 160│ 160│ 160│ 210│ 275
Weight of holding down weights, not│ │ │ │ │ │
boxed, pounds │ 120│ 270│ 270│ 285│ 330│ 375
Cubic feet of free air per minute │ │ │ │ │ │
required to run one drill at 100 │ │ │ │ │ │
pounds │ 92│ 104│ 126│ 146│ 154│ 160
───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴─────
For more than one drill, multiply the value in the above line by the
following factors: For 2 drills, 1.8; 5 by 4.1; 10 by 7.1; 15 by 9.5;
20 by 11.7; 30 by 15.8; 40 by 21.4; 70 by 33.2.

Since gasoline air compressors are self regulating, when the drill
is not using air very little gasoline is burned by the gasoline
engine driving the compressor. A gasoline compressor possesses
other very important economic advantages over a small steam-driven
plant. First, there is the saving in wages of firemen and second,
there is the saving in hauling and pumping of water and the
hauling of fuel. The cost of gasoline is often less than the cost
of coal for operating a small plant.

An electric drill[89] operated on the principle of the solenoid does
away with motor, valves, pipes, vapor, freezing, and other difficulties
attendant on the use of steam or air.

The rates of drilling in different classes of rock are shown in Table
57. Frequent changes of drills and relocation of tripods will materially
reduce the performance of a drill, for as much as 45 minutes may be lost
in making a new set up. In this the jack hammer drills show their
advantage as no time is lost in a set up.

TABLE 57

RATES OF ROCK DRILLING

Rates in Feet per Ten-hour Shift. Vertical Holes 10–20 Feet Deep.
(From Gillette)

Hard Adirondack granite 48
Maine and Massachusetts granite 45–50
Mica-schist of New York City. Possible 60–70
Mica-schist of New York City. Average 40–50
Hard, Hudson River trap rock 40
Soft red sand stone of Northern New Jersey 90
Hard limestone near Rochester, N. Y 70
Limestone of Chicago Drainage Canal 70–80
Douglass, Indiana, syenite. Difficult set ups 36
Canadian granite on Grand Trunk R. R 30
Windmill point, Ontario limestone:
3⅝-inch drills 75
2¾-inch drills 60
2¼-inch drills 37

=145. Steam or Air for Power=.—The choice between steam or air is
dependent on the conditions of the work. Steam is undesirable in tunnels
on account of the heat produced. In open cut work it is at a
disadvantage because of the loss of power due to radiation from the hose
or pipe. The life of the hose is not so long as when air is used,
escaping steam causes clouds of vapor which obscure the work, and
serious burns may occur due to hot water thrown from the exhaust. It is
advantageous since leaks may be easily discovered and remedied, it
requires less machinery than air, and it is sometimes less expensive.
With compressed air, gasoline or electric motors can be used for
operating the compressors.

TABLE 58

ROCK BLASTING

(From Gillette)
────────────────────┬────────────────────┬─────────┬─────────┬─────────
Character of │Powder Used per Hole│ │Distance │Distance
Material │ │Depth of │ Back of │ Hole to
│ │ Hole, │ Face, │ Hole,
│ │ Feet │ feet │ feet
────────────────────┼────────────────────┼─────────┼─────────┼─────────
Limestone of Chicago│40 per cent dynamite│ │ │
Drainage Canal │ │ 12│ 8│ 8
Sandstone │200 pounds black │ │ │
│ powder │ 20│ 18│ 14
Granite │2 pounds 60 per cent│ │ │
│ dynamite │ 12│ 1½│ 4½ to 5
Pit mining, │ │ │ │
Treadwell, Mine, │ │ │ │
Alaska │ │ 12│ 2½│ 6
────────────────────┴────────────────────┴─────────┴─────────┴─────────

=146. Depth of Drill Hole.=—The depth of the hole is dependent on the
character of the work. The deepest holes can be used in open cut work
where the shattered rock is to be removed by steam shovel. The face can
be made 10 to 15 feet high. The depth of the hole in center cut tunnel
facings are from 6 to 10 or even 12 feet. In the bench the depth is
equal to the height of the bench. In narrow trenches where the rock is
to be removed by derrick or thrown into a bucket by hand, the hole
should be sufficiently deep to shatter the rock to a depth of at least 6
inches below the finished sewer. Frequently shooting to this depth at
one shot cannot be done due to the built up condition of the
neighborhood or other local factors. The depth of the hole in trench
work should not much exceed the distance between holes. Deep holes are
usually desirable as a matter of economy in saving frequent set ups, but
the holes cannot be made much over 20 feet in depth without increasing
the friction on the drill to a prohibitive amount.

=147. Diameter of Drill Hole.=—The diameter of the hole should be such
as to take the desired size of explosive cartridge. The common sizes of
dynamite cartridges are from ⅞ inch to 2 inches in diameter. In
drilling, the diameter of the hole is reduced about one-eighth of an
inch at a time as the drill begins to stick. This reduction should be
allowed for, and experience is the best guide for the size of the hole
at the start. In general the softer or more faulty or seamy the rock,
the more frequent the necessary reductions in size of bit.[90] For hard
homogeneous rock the holes can be drilled 10 feet or more without
changing the size of the drill bit.

=148. Spacing of Drill Holes.=—The spacing of holes in open cut
excavation is commonly equal to the depth of the hole. The character of
the material being excavated has much to do with the spacing of the
holes. The spacing, diameter and depth of holes used on some jobs is
shown in Table 58. Gillette states:

It is obviously impossible to lay down any hard and fast rule for
drill holes. In stratified rock that is friable, and in traps that
are full of natural joints and seams, it is often possible to
space the holes a distance apart somewhat greater than their
depth, and still break the rock to comparatively small sizes upon
blasting. In tough granite, gneiss, syenite, and in trap where
joints are few and far between, the holes may have to be spaced 3
to 8 feet apart regardless of their depth for with wider spacing
the blocks thrown down will be too large to handle with ordinary
appliances. Since in shallow excavations the holes can seldom be
much further apart than one to one and one-half times their depth
we see that the cost of drilling per cubic yard increases very
rapidly the shallower the excavation. Furthermore the cost of
drilling a foot of hole is much increased where frequent shifting
of the drill tripod is necessary.

The common practice in placing drill holes is to put down holes in
pairs, one hole on each side of the proposed trench; and if the
trench is wide one or more holes are drilled between these two
side holes[91] but in narrow trench work, such as for a 12–inch
pipe, one hole in the middle of the trench will usually prove
sufficient.

The holes are spaced about 3 feet apart longitudinally. After the holes
have been completed they should be plugged to keep out dirt and water.

SHEETING AND BRACING

=149. Purposes and Types.=—Sheeting and bracing are used in trenching to
prevent caving of the banks and to prevent or retard the entrance of
ground water. The different methods of placing wooden sheeting are
called stay bracing, skeleton sheeting, poling boards, box sheeting, and
vertical sheeting. Steel sheeting is usually driven to secure
water-tightness and if braced the bracing is similar to the form used
for vertical wooden sheeting.

=150. Stay Bracing.=—This consists of boards placed vertically against
the sides of the trench and held in position by cross braces which are
wedged in place. The purpose of the board against the side of the trench
is to prevent the cross brace from sinking into the earth. The boards
should be from 1½ × 4 inches to 2 × 6 inches and 3 to 4 feet long. The
cross braces should not be less than 2 × 4 inches for the narrowest
trenches and larger sizes should be used for wider trenches. The spacing
between the cross braces is dependent on the character of the trench and
the judgment of the foreman. Stay bracing is used as a precautionary
measure in relatively shallow trenches with sides of stiff clay or other
cohesive material. It should not be used where a tendency towards caving
is pronounced. Stay bracing is dangerous in trenches where sliding has
commenced as it gives a false sense of security. The boards and cross
braces are placed in position after the trench has been excavated.

=151. Skeleton Sheeting.=—This consists of rangers and braces with a
piece of vertical sheeting behind each brace. A section of skeleton
sheeting is shown in Fig. 104 with the names of the different pieces
marked on them. This form of sheeting is used in uncertain soils which
apparently require only slight support, but may show a tendency to cave
with but little warning. When the warning is given vertical sheeting can
be quickly driven behind the rangers and additional braces placed if
necessary. The sizes of pieces, spacing and method of placing should be
the same as for complete vertical sheeting in order that this may be
placed if necessary.

=152. Poling Boards.=—These are planks placed vertically against the
sides of the trench and held in place by rangers and braces. They differ
from vertical sheeting in that the poling board is about 3 or 4 feet
long. It is placed after the trench has been excavated; not driven down
with the excavation like vertical sheeting. An arrangement of poling
boards is shown in Fig. 105. This type of support is used in material
that will stand unsupported for from 3 to 4 feet in height. Its
advantages lie in that no driving is necessary, thus saving the trench
from jarring; no sheeting is sticking above the sides of the trench to
interfere with the excavation; and only short planks are necessary.

[Illustration:

FIG. 104.—Skeleton Sheeting.
]

[Illustration:

FIG. 105.—Poling Boards.

Showing Different Types of Cross Bracing.
]

The method of placing poling boards is as follows: Excavate the trench
as far as the cohesion of the bank will permit. Poling boards, 1½ inch
to 2 inch planks, 6 inches or more in width, are then stood on end at
the desired intervals along each side of the trench for the length of
one ranger. The poling boards may be held in place by one or two
rangers. Two are safer than one but may not always be necessary. If one
ranger is to be used it is placed at the center of the poling board.
After the poling boards are in position the rangers are laid in the
trench and the cross braces are cut to fit. If wedges are to be used for
tightening the cross braces, the cross braces are cut about 2 inches
short. If jacks are to be used the braces are cut short enough to
accommodate the jacks when closed, or adjustable trench braces may be
used as shown in Fig. 106. The use of extension braces saves the labor
of fitting wooden braces. With everything in readiness in the trench,
the cross brace is pressed against the ranger which is thus held in
place. The wedge or jack is then tightened holding the poling boards and
cross brace in position.

[Illustration:

FIG. 106.—Box Sheeting.

Showing Different Types of Cross Bracing.
]

=153. Box Sheeting.=—Box sheeting is composed of horizontal planks held
in position against the sides of the trench by vertical pieces supported
by braces extending across the trench. The arrangement of planks and
braces for box sheeting is shown in Fig. 106. This type of sheeting is
used in material not sufficiently cohesive to permit the use of poling
boards, and under such conditions that it is inadvisable to use vertical
sheeting which protrudes above the sides of the trench while being
driven. This sheeting is put in position as the trench is excavated. No
more of the excavation than the width of three or four planks need be
unsupported at any one time. In placing the sheeting the trench is
excavated for a depth of 12 to 24 inches. Three or four planks are then
placed against the sides of the trench and are caught in position by a
vertical brace which is in turn supported by a horizontal cross brace.

[Illustration:

FIG. 107.—Vertical Sheeting.
]

=154. Vertical Sheeting.=—This is the most complete and the strongest of
the methods for sheeting a trench. It consists of a system of rangers
and cross braces so arranged as to support a solid wall of vertical
planks against the sides of the trench. An arrangement of complete
vertical sheeting is shown in Fig. 107. This type can be made nearly
water-tight by the use of matched boards, Wakefield piling, steel
piling, etc. Wakefield piling is made up of three planks of the same
width and usually the same thickness. They are nailed together so that
the two outside planks protrude beyond the inside one on one side, and
the inside one protrudes beyond the two outside ones on the other side
as shown in Fig. 108. The protruding inside plank forms a tongue which
fits into the groove formed by the protruding outside planks of the
adjacent pile.

[Illustration:

FIG. 108.—Wakefield Sheet Piling.
]

[Illustration:

FIG. 109. Section through Malleable Steel Driving Cap.
]

In placing vertical sheeting the trench is excavated as far as it is
safe below the surface. Blocks of the same thickness as the sheeting are
then placed against the bank at the middle and at the ends of two
rangers on opposite sides of the trench. The ranger rest against blocks,
and are held away from the sides of the trench by them. Cross braces are
next tightened into position opposite the blocks to hold the rangers in
place. After the skeleton sheeting is in place the planks forming the
vertical sheeting are put in position with a chisel edge cut on the
lower end of the plank, with the flat side against the bank. The planks
should be driven with a maul, the edge of the plank following closely
behind the excavation. In relatively dry work the driving of the plank
is facilitated by excavating beneath the edge as it is driven. The upper
end of the sheeting should be protected by a malleable steel or iron cap
to prevent brooming of the lumber. A cap is shown in Fig. 109. A sledge
hammer may be used for driving when the lumber is protected. If the
sheeting is to start at the surface and is to be driven by hand, the
first length should not exceed 4 feet unless a platform is erected for
the driver. Succeeding lengths may be longer, the driver standing on
planks supported on the cross braces in the trench. Steam hammers and
pile drivers are sometimes used for driving sheeting.

The framework of the sheeting should be placed with a cross brace for
each end of each ranger and a cross brace for the middle of each ranger.
If the ends of two rangers rest on the same cross brace an accident
displacing one ranger will be passed on to the next and might cause a
progressive collapse of a length of trench, whereas the movement of an
independently supported ranger should have no effect on another ranger.
The cross braces should have horizontal cleats nailed on top of them as
shown in Fig. 107 to prevent the braces from being knocked out of place
by falling objects. In driving vertical sheeting a vacant place will be
left behind each cross brace corresponding to the original block placed
to hold the ranger away from the bank. This is an undesirable feature in
the use of vertical sheeting. It is ordinarily remedied by slipping in
planks the width of the slot and wedging or nailing them against the
convenient cross bracing. In extremely wet trenches, after all other
pieces of vertical sheeting are in place, the original cleat behind the
cross brace can be knocked out and a piece of sheeting slipped into this
opening and driven. Care must be taken in this event not to drive the
rangers down when driving the sheeting. If the bracing begins to drop,
it should be supported by vertical pieces between the rangers and
resting on a sill at the bottom of the trench.

[Illustration:

FIG. 110.—Steel Clamp for Pulling Wood Sheeting.
]

=155. Pulling Wood Sheeting.=—Wood sheeting is pulled after the
completion of the trench by a device shown in Fig. 110. In wet trenches
where the removal of the sheeting would permit a movement of the banks,
resulting in danger to the sewer or other structures, the sheeting
should be left in place in the trench. If sufficient saving can be made
the sheeting is cut off in the trench immediately above the danger line,
usually the ground water line. The cutting is done with an axe or by a
power driven saw devised for the purpose.

=156. Earth Pressures.[92]=—The various theories of earth pressure are
so conflicting in their conclusions as to be confusing. Rankine’s
theory, the most frequently used, assumes that the pressure increases
with the depth, whereas Meem’s theory[93] leads to an opposite
conclusion. The discussion following Meem’s article is very
illuminating. It indicates that no matter how good the theory, practical
experience together with the use of generous sizes and close spacing are
the best guides for bracing trenches and coffer dams. All are not
possessed with the desired practical experience and some basis on which
to commence work is essential. Another factor affecting computations of
sizes based on theory is the tendency in practice to use the same size
material for rangers and braces on any one job for all except very deep
trenches and other special cases. Occasionally where there is an
independent brace for each end of each ranger, the brace is made
thinner, but is of the same depth as the ranger.

The application of Rankine’s theory of earth pressure to the computation
of the sizes of rangers and braces will be shown. His formula for the
active earth pressure against a retaining wall is:

_P_ = _wh_ cosθ (cos θ − √(cos^2 θ − cos^2 φ))⁄(cos θ + √(cos^2 θ −
cos^2 φ))

in which _w_ = the weight of earth in pounds per cubic foot;

_h_ = depth in feet at point at which pressure is to be
determined;

θ = the angle of surcharge, or the angle which the surface
makes with the horizontal;

φ = the angle of repose of the earth. Usually taken as
33°–41′ = 1½ horizontal to 1 vertical;

_P_ = the intensity of pressure in pounds per square foot on
a vertical plane in a direction parallel to the
surface of the ground.

In studying the pressures for trenches the surface of the ground will be
assumed as horizontal and the formula reduces to

_P_ = (1 − sin φ)⁄(1 + sin φ)_wh_.

=157. Design of Sheeting and Bracing=.—The trench shown in Fig. 111 is
assumed to be constructed in moist sand weighing 110 pounds per cubic
foot, with an angle of repose of 30 degrees. The material used for
sheeting and bracing is yellow pine. The steps taken in the design of
the sheeting and bracing for this trench are as follows:

[Illustration:

FIG. 111.—Diagram for the Design of Wood Sheeting.
]

1. _Earth Pressure._—Substituting the units given in the data, in
Rankine’s formula for earth pressures,

_P_ = 36.7_h_.

Because the earth has been freshly cut and will not be kept open long
enough to break up the cohesiveness of the banks it is customary to
reduce the assumed pressure by dividing by 2, 3, or 4, according to the
natural cohesiveness of the material. The cohesiveness of sand is not
great, therefore the pressure will be assumed as one-half of the amount
given by the formula, or

_p_ = 18_h_.

2. _Thickness of Sheeting and Spacing of Rangers._—It is desirable to
use the same thickness of sheeting throughout the depth of the trench.
Computations should therefore be commenced at the bottom of the trench
where the pressures are the greatest and the thickest sheeting will be
required. It is necessary to determine by trial a spacing for the
rangers and a thickness of sheeting so that the sheeting is stressed to
its full working strength. Having determined the thickness of the
sheeting at the bottom, the remainder of the computations consists in
determining the spacing of the rangers.

In the example the lower ranger will be assumed as 3 feet from the
bottom of the trench and the distance to the next ranger as 4 feet.

The intensity of pressure at 22 feet 9 inches is 409.5 pounds per
square foot.

The intensity of pressure at 26 feet 9 inches is 481.5 pounds per
square foot.

The distribution of pressures is shown by the diagram on Fig. 111. The
maximum bending moment is slightly below the point midway between the
rangers and for a 12–inch strip is 10,500 inch-pounds.

Assuming 3 inch sheeting the maximum fiber stress is:

_f_ = _Mc_⁄_I_ = (10,400 × 1.5 × 12)⁄12 × 27 = 568 pounds per square
inch.

The working strength of yellow pine as given in Table 59, is 1200 pounds
per square inch. Thinner sheeting should therefore be used.

TABLE 59

WORKING UNIT STRESSES FOR TIMBER

The most used value in the Building Codes of Baltimore, Boston,
Cincinnati, Chicago, District of Columbia, and New York City
─────────────┬────────┬───────────┬───────────┬──────────┬──────┬──────
Wood │ │ │ │ │Shear │Shear
│ │ │ │ │ With │Across
│ │ │Compression│Transverse│Grain,│Grain,
│Tension,│Compression│ Across │ Bending, │ lb. │ lb.
│lb. sq. │With Grain,│Grain, lb. │ lb. sq. │ sq. │ sq.
│ in. │lb. sq. in.│ sq. in. │ in. │ in. │ in.
─────────────┼────────┼───────────┼───────────┼──────────┼──────┼──────
Yellow pine │ 1200│ 1000│ 600│ 1200│ 70│ 500
White pine │ 800│ 800│ 400│ 800│ 40│ 250
Spruce and │ │ │ │ │ │
Va. pine. │ 800│ 800│ 400│ 800│ 50│ 320
Oak │ 1000│ 900│ 800│ 1000│ 100│ 600
Hemlock │ 600│ 500│ 500│ 600│ 40│ 275
Chestnut │ 600│ 500│ 1000│ 800│ │ 150
Locust │ │ 1200│ 1000│ 1200│ 100│ 720
─────────────┴────────┴───────────┴───────────┴──────────┴──────┴──────
As published in American Civil Engineers Pocket Book.

Assuming 2–inch sheeting, the fiber stress is 1,300 pounds per square
inch. This stress is too large. By reducing the ranger spacing slightly
the stress can be brought within the required limits.

Assuming a ranger spacing of 3 feet 9 inches the depth to the upper
ranger is changed to 23 feet and the maximum stress in the 2–inch
sheeting becomes 1,140 pounds per square inch, a satisfactory result.
The results for the computations for the other ranger spacings are shown
in Table 60. The spacing of the rangers at the sheeting junctions is
controlled by convenience and is not computed so long as it is obviously
safe.

3. _Size of Rangers._—The rangers will be assumed as 16 feet long with
two end cross braces and one intermediate cross brace for each ranger.
Starting as before at the bottom of the trench.

The area of the panel below the ranger and between cross
braces is 24 square feet.

The average intensity of pressure is 28.25 × 18 = 508.5
pounds per square inch.

The load transmitted to the ranger is 6,000 pounds.

Similarly the load transmitted to the ranger from the
panel above is 6,890 pounds.

The total distributed load on the ranger is 12,890 pounds.

If _b_ is the vertical dimension of the ranger and _d_ is the horizontal
dimension in inches, then from the beam theory, using _f_ as 1,200
pounds per square inch, _bd_^2 = _M_⁄200, in which _M_ is expressed in
inch-pounds. The maximum bending moment is

(_Wl_)⁄8 = 12,200 × 8 × 12⁄8 = 155,000 inch-pounds

Therefore, _bd_^2 = 775.

An 8 × 10 inch beam will fulfill the conditions closely. Substituting
these dimensions in the beam formula

_f_ = (_Mc_)⁄_I_ = (155,000 × 5 × 12)⁄8 × 1000

= 1,160 pounds per square inch tension in outer fiber. The results of
the computations for other rangers are shown in Table 60.

4. _Size of Cross Braces._—The cross braces act as columns. The
dimensions of the cross braces are determined by trial in such a manner
that the vertical dimension of the brace is equal to the vertical
dimension of the ranger and the compressive stress in pounds per square
inch is computed from the expression,

_S_ ⪙ _S__{1}(1 − _l_⁄(60_d_)),[94]

TABLE 60

COMPUTATIONS FOR SHEETING AND BRACING FOR TRENCH SHOWN IN FIG. 111

Material is moist sand weighing 110 pounds per cubic foot, with an angle of
repose of 30°. Lumber is yellow pine, with working stress as given in Table
59. Working stresses for columns given as _S_(1 − _l_⁄(60_d_)).
──────────────────────────────┬───────────────────────────────────────────────
Sheeting 2 inches × 12 Inches │ Cross Braces
──────────┬───────────┬───────┼───────────┬──────┬──────┬──────────┬──────────
│ │Maximum│ │ │ │ │
│ │ Fiber │ │ │ │ │
│ │Stress,│ │ │ │ Actual │Allowable
│ Maximum │Pounds │ │ │ │Intensity,│Intensity,
│ Bending │ per │ │Total │ │Pounds per│Pounds per
│ Moment, │Square │ Depth and │Load, │Size, │ Square │ Square
Depth │Inch-Pounds│ Inch │Description│Pounds│Inches│ Inch │ Inch
──────────┼───────────┼───────┼───────────┼──────┼──────┼──────────┼──────────
│ │ │end at 26′│ │ │ │
23′–26.75′│ 9100│ 1140│ 9″│ 6,445│ 4 × 8│ 202│ 784
│ │ │int. at 26′│ │ │ │
19′–23′│ 8800│ 1100│ 9″│12,890│ 4 × 8│ 403│ 784
│ │ │end at 23′│ │ │ │
13′–17.5′│ 8550│ 1070│ 0″│ 6,393│ 4 × 8│ 200│ 784
│ │ │int. at 23′│ │ │ │
8′–13′│ 7160│ 900│ 0″│12,785│ 4 × 8│ 400│ 784
│ │ │end at 19′│ │ │ │
0′–6′│ 3000│ 375│ 0″│ 3,930│ 4 × 8│ 123│ 784
│ │ │int. at 19′│ │ │ │
│ │ │ 0″│ 7,860│ 4 × 8│ 240│ 784
│ │ │end at 17′│ │ │ │
│ │ │ 6″│ 3,566│ 4 × 8│ 112│ 684
│ │ │int. at 17′│ │ │ │
│ │ │ 6″│ 7,132│ 4 × 8│ 224│ 684
│ │ │end at 13′│ │ │ │
│ │ │ 0″│ 4,385│ 4 × 8│ 137│ 684
│ │ │int. at 13′│ │ │ │
│ │ │ 0″│ 8,770│ 4 × 8│ 274│ 684
│ │ │end at 8′│ │ │ │
│ │ │ 0″│ 2,270│ 4 × 6│ 96│ 687
│ │ │int. at 8′│ │ │ │
│ │ │ 0″│ 4,540│ 4 × 6│ 189│ 667
│ │ │end at 6′│ │ │ │
│ │ │ 0″│ 1,344│ 4 × 6│ 60│ 584
│ │ │int. at 6′│ │ │ │
│ │ │ 0″│ 2,687│ 4 × 6│ 112│ 584
│ │ │end at 0′│ │ │ │
│ │ │ 0″│ 432│ 4 × 6│ 18│ 584
│ │ │int. at 0′│ │ │ │
│ │ │ 0″│ 863│ 4 × 6│ 36│ 584
──────────┴───────────┴───────┴───────────┴──────┴──────┴──────────┴──────────

Rangers
──────┬──────┬─────────┬──────┬────────────────────┬──────┬───────────┬───────
│ Area │ │ │ │ │ │
│ of │Intensity│ │ │ │ │
│Panel │ of │ │ │ │ │Maximum
│Below │Pressure,│ │ │ │ Maximum │Stress
│ this │ Pounds │Total │ │ │ Bending │Pounds
│Depth,│ per │ Load │ │ │ Moment in │ per
│Square│ Square │ in │Load Transmitted to │Size, │ Thousand │Square
Depth │ Feet │ Inch │Pounds│the Ranger from the │Inches│Inch-Pounds│ Inch
──────┼──────┼─────────┼──────┼──────┬──────┬──────┼──────┼───────────┼───────
│ │ │ │Panel │Panel │ Both │ │ │
│ │ │ │Below │Above │Panels│ │ │
──────┼──────┼─────────┼──────┼──────┼──────┼──────┼──────┼───────────┼───────
26′ 9″│ 24│ 508.5│12,200│ 6000│ 6890│12,890│8 × 10│ 155│ 1160
23′ 0″│ 30│ 448│13,440│ 6545│ 6240│12,785│8 × 10│ 153│ 1150
19′ 0″│ 32│ 378│12,100│ 5860│ 2000│ 7,860│8 × 10│ 94.3│ 708
17′ 6″│ 12│ 328.5│ 3,942│ 1942│ 5190│ 7,132│8 × 10│ 85.6│ 636
13′ 0″│ 36│ 274.5│ 9,880│ 4690│ 4080│ 8,770│8 × 10│ 105│ 790
8′ 0″│ 40│ 189│ 7,560│ 3480│ 1060│ 4,540│6 × 8│ 54.4│ 850
6′ 0″│ 16│ 126│ 2,020│ 960│ 1727│ 2,687│6 × 8│ 32.2│ 503
0′ 0″│ 48│ 54│ 2,590│ 863│ 0│ 863│6 × 8│ 10.4│ 161
──────┴──────┴─────────┴──────┴──────┴──────┴──────┴──────┴───────────┴───────

in which _S_ = permissible crushing across the grain in a column whose
length is greater than 15 diameters;

_S__{1} = unit working compressive strength of wood;

_l_ = length of the column;

_d_ = smallest dimension of the column;

_l_ and _d_ are in the same units.

The lower intermediate cross brace supports a length of 8 feet of the
lower ranger on which the load has been found to be 12,890 pounds. The
load on the end cross brace for the same ranger is one-half of this or
6,445 pounds. The length of each brace is 4 feet 4 inches. From Table
59, _S__{1} is 1,000 pounds per square inch. From the column formula,
_S_ is 784 pounds per square inch.

A 4 × 8 inch cross brace is the smallest that is feasible. This is
stressed only 12,890 pounds or 403 pounds per square inch, which is well
within the permissible limits. The results of the other computations for
cross braces are shown in Table 60.

=158. Steel Sheet Piling.=—This is coming into more general use with the
increased cost of lumber and better acquaintance with its superiority
over wood under many conditions. Although its first cost is higher than
that of wood, the fact that with proper care it can be used almost an
indefinite number of times renders it economical to contractors who may
have an opportunity to make repeated use of it. The life of good yellow
pine sheeting with the best of care may be as much as three or four
seasons. With no particular care it will be destroyed at the first
using. Fig. 112 shows various sections of steel piling used for trench
sheeting. These forms are practically water-tight and aid materially in
maintaining dry trenches. The piling can be made water tight by slipping
a piece of soft wood between the steel sections when they are being
driven, or by pouring in between the piles some dry material which will
swell when wet. The piling is generally driven by a steam hammer and is
pulled by attaching a ring through a bolt hole in the pile, or by
grasping the pile with a clutch that tightens its grasp as the pull
increases. An inverted steam hammer attached to the pile is sometimes
used in pulling it. The impulses of the hammer together with a steady
pull on the cable serve to drag out the most stubborn piece of piling.

[Illustration:

FIG. 112.—Sections of Lackawanna Steel Sheet Piling.
]

LINE AND GRADE

=159. Locating the Trench.=—In order to locate a trench a line of stakes
should be driven at about 50–foot intervals along the center line of the
proposed sewer before excavation is commenced. Reference stakes or
reference points to this line are located at some fixed offset or easily
described point, or the stakes marking the center line of the trench may
be driven at some constant offset distance one side of the trench, in
order to avoid danger of loss or disturbance of the stakes. Grade or cut
is seldom marked on the line of preliminary stakes, although the
approximate cut may be indicated.

For hand excavation the foreman lays out the trench from these stakes.
In machine work the operator guides the machine so as to follow the line
of the stakes.

=160. Final Line and Grade.=—After the excavation of the trench has
proceeded to within a foot or two of the final depth, the grade and line
are transferred to markers supported over the center of the trench. The
markers are horizontal boards spanning the trench and held in position
either by nails driven into stakes at the side of the trench, by nails
driven into the sheeting, or by weights holding the boards on the
ground. Two stakes driven in the ground at the side of the trench as
shown in Fig. 113 are the common method of support. If the banks are too
weak to stand under the jarring of the driving of the stakes, or
pavement or other causes prevent their use the horizontal cross piece
may be weighted down by bricks or a bank of earth. The cross pieces are
located about every 25 feet along the trench and at any convenient
distance above the surface of the ground. The nearer the ground the
stronger the support but the greater the interference with work in the
trench. The center line of the sewer is marked on the cross pieces after
they are set, and vertical struts are nailed on them with one edge of
the strut straight, vertical, and on the center line as shown in Fig. 1.
The corresponding edge should be used on all struts in order to avoid
confusion. The edge is placed in a vertical position by means of a plumb
bob or carpenter’s level.

[Illustration:

FIG. 113.—Methods for the Support of the Grade Line.
]

The cut to the invert of the sewer is recorded to an even number of feet
where practicable by driving a nail in the upright strut so that the top
edge of the nail is at the desired elevation above the sewer, or the
upright is nailed with its top at the proper number of feet above the
sewer invert. The cut is marked on the upright in feet, tenths, and
hundredths from the recorded point to the elevation of the invert.

The inspector should watch these grade markers with care by sighting
back along them to see that they are in line and have not moved. In
quicksand or caving material the marks may move during the setting of
the pipes and the levelman should be on the job constantly.

When excavation is being done by machine the depth of the excavation is
controlled by the operator who maintains a sighting rod on the machine
in line with the grade marks on the uprights.

[Illustration:

FIG. 114.—Diagram Showing the Use of the Grade Rod for Fixing the
Elevation of a Sewer.
]

=161. Transferring Grade and Line to the Pipe.=—In transferring grade
and line to the sewer a light strong string is stretched tightly from
nail to nail on the uprights marking the line and grade. A rod with a
right angle projection at the lower end, as shown in Fig. 114, is marked
with chalk or a notch at such a distance from the end that when the mark
is held on the grade cord the lower portion of the rod which projects
into the pipe will rest on the invert. The pipe is placed in line by
hanging a plumb bob so that the plumb bob string touches the grade and
center line cord. These marks are taken only as frequently as may be
necessary to keep the sewer in line. An experienced workman can maintain
the line by eye for considerable distances. Measurements should never be
taken to the top of the pipe in order to determine position and grade as
the variations in the diameter of the pipe may cause appreciable errors.

The position and elevation of the forms for brick, concrete, and unit
block sewers are located by reference to the grade line, or they may be
placed under the immediate direction of the survey party, or by
specially located stakes. For large sewers requiring deep and wide
excavation the grade and line stakes are driven in the bottom of the
trench about a foot above the finished grade. This requires the constant
presence of an engineer who is usually available on work of such
magnitude.

=162. Line and Grade in Tunnel.=—In tunnels, line and grade are given by
nails driven in the roof, the progress of excavation or the shield being
followed by eye and the forms set by direct measurement to the nails.

TUNNELING

=163. Depth.=—The depth at which it becomes economical to tunnel depends
mainly upon the character of the material to be excavated and on the
surface conditions. In soft dry material with unobstructed working space
at the surface, open cut may be desirable to depths as great as 35 or 40
feet. Tunnels are cut in rock at depths of 15 feet or less. In some very
wet and running quicksand encountered in the construction of sewers for
the Sanitary District of Chicago it was found economical to tunnel at
depths of 20 feet and less. Crowded conditions on the surface, expensive
pavements, or extensive underground structures near the surface may make
it advantageous to tunnel at shallower depths than would otherwise be
economical. Winter is the best season for tunneling as the workmen are
protected from the elements and labor is more plentiful.

=164. Shafts.=—In sinking a shaft in soft material, the excavation is
usually done by hand, the material being thrown into a bucket which is
hoisted to the surface and dumped. The size of the shaft is independent
of the size of the sewer and depends principally on the machinery which
it is necessary to lower into the tunnel. Ordinarily a shaft 6 feet in
the clear is satisfactory. A method of timbering a shaft is shown in
Fig. 115. Because of the timbering the shaft must be started
sufficiently large at the top to finish with the desired dimensions at
the bottom. This excess size is sometimes obviated by driving the
sheeting at an angle to maintain the same size of shaft from top to
bottom.

In timbering a shaft as shown in Fig. 115 the upper frame is staked
securely in position at the surface of the ground. This frame is
composed of timbers fastened together in the form of a square with the
ends of the timbers extending about 12 inches on all sides. The
protruding ends are used to hold the frame in position. Excavation is
begun inside the frame, and sheeting is driven around the outside of it
as excavation progresses. Only two or three men can work advantageously
at one time in these small shafts. The second frame is made up of the
same size timbers, but all are cut off flush with the outside of the
square. The outside dimensions of this frame are such as to allow
sheeting to be slipped in between it and the sheeting already driven.
The frame is lowered into position and supported from the upper frame by
vertical struts nailed to it. The lower end of the sheeting already
driven is held out from the lower frame by blocks of the thickness of
the next length of sheeting. These blocks are removed as the next length
of sheeting is placed and driven. The driving of the sheeting is
facilitated by excavating beneath it as it descends.

[Illustration:

FIG. 115.—Section of Shaft Timbering.

Abbot, Journal Western Society of Engineers, Vol. 22.
]

The sizes of sheeting and timbering should be computed on the same basis
as that for trench sheeting except that for depths greater than 30 to 35
feet Rankine’s Theory is not applicable and judgment must be relied on
for computing the sizes for deep shafts. In stiff dry material the
pressures will change very little as the depth increases. Sheeting is
needed in shaft excavation in rock only to protect the workmen from
falling fragments, but in sand, particularly in quicksand and in wet
ground, the pressures increase directly with the depth and the sheeting
should be computed accordingly. Care must be taken to prevent the
formation of cavities behind the sheeting, to fill them if formed, and
to see that all pieces of the sheeting and bracing have a firm bearing.
It is difficult to prevent the collapse of the shaft once the movement
of earth against the sheeting has commenced.

Shafts are also sunk in soft ground by constructing a concrete or metal
shell resting on a cutting shoe on the surface. The material inside is
dug out and the shell sinks of its own or added weight. The first
section of the shell may be from 5 to 10 feet long. As this section
sinks other sections are added. This is called the caisson method. It is
advantageous in wet ground and when the shafts are to be left as a
permanent manhole. If a permanent shaft is to be left in an excavation
being braced with wood, the permanent lining should follow within 20 to
30 feet of the shaft excavation. This is done to avoid the difficulty of
maintaining a great length of temporary wood shaft with the danger of
collapse, or of blocks or other objects falling on the workers below.

The distance between shafts is controlled by the depth and size of the
tunnel, surface conditions, and the character of the material being
tunneled. Except where surface conditions are crowded the shallower the
cover to the tunnel the more frequent the shafts. The advantage of
frequent shafts lies in the possibility of removing excavated material
from the tunnel promptly, and in making ventilation of the tunnel
easier. The saving made by the construction of numerous shafts must be
balanced against the extra cost of the shafts. For the shallowest
tunnels the shafts are seldom placed closer than every 500 feet.

=165. Timbering.=—After the shaft has been excavated to the proper grade
the tunnel is struck out either by cutting through the wooden sheeting
or by removing portions of the caisson lining. Practically all tunnels
except those in solid rock must be framed to some extent. Some of the
types of frames used in tunnel construction are shown in Fig. 116.
Different combinations of these may be used in different classes of
materials. In solid rock which remains firm on exposure no timbering is
necessary. Where the roof only need be supported and the sides are
strong enough to be used for support, a timber “hitch” or frame
supported on the sides of the tunnel may be used. This is suitable for
loose rock roofs with solid rock sides. Timbering such as is shown in
the lower left hand corner of Fig. 116 becomes necessary in extremely
soft, wet, or swelling material, where the bottom and sides as well as
the roof tend to push in. The remaining frame in Fig. 116 shows a form
frequently used and lying between the two extremes indicated. In wet
tunnels a channel may be cut in the bottom below the sill for drainage
purposes as shown in this form. The needle beam method of timbering is
also shown in Fig. 116. This method of timbering is used mainly near the
heading because of the speed and ease with which it can be installed,
but it is undesirable because of the space occupied.

The distance between frames is dependent on the size of the tunnel and
the character of the material. It is seldom greater than 6 feet and the
frames are sometimes placed touching each other. The size of the
timbering is a matter of experience and is generally determined by the
judgment of the responsible person in charge of the construction as the
result of observation during the progress of the work.

The sheeting between frames is called poling boards, or spiling or
lagging according as it is sharpened and driven ahead of the excavation
or placed after the excavation has progressed. The horizontal strips
placed between the frames to keep them apart are called wales.

[Illustration:

FIG. 116.—Types of Frames and Timbering for Tunnels.
]

In cutting out from the shaft in soft materials requiring support, where
the width of the tunnel is the same or smaller than that of the shaft, a
frame with a maximum width four thicknesses of sheeting less than the
width of the tunnel is set up against the lining of the shaft. The
vertical side pieces of the tunnel frame rest on the bottom frame of the
shaft as a sill and are securely wedged into position. As the lining of
the shaft at the top is cut away the top poling boards of the tunnel are
slipped in between the cap of the first tunnel frame and the shaft frame
immediately above it. The poling boards are driven with an upward pitch
so that there may be room to slip the second length of boards between
the next tunnel frame and the first length of boards. The placing of the
side sheeting follows in a similar manner. Excavation is then started
and the poling boards driven to keep pace with it. The next frame is
placed in position and the previous sheeting or boards wedged out a
sufficient distance to allow the advance lining to be slipped in when
the wedges are removed. Waling pieces are nailed firmly between the
frames to hold them in position. The various phases in the driving of a
12–foot sewer tunnel in Seattle are shown in Fig. 117.

[Illustration:

FIG. 117.—Stages of Sewer Tunneling.

Eng. Record, Vol. 69, 1914, p. 195.
]

In soft or running material it may be necessary to protect the face of
the tunnel by horizontal boards, called breast boards, wedged back to
the last frame placed. The excavation is performed by removing one board
at a time, excavating behind it and then replacing it in the advance
position. The advance is made from the top downwards. This represents
the method pursued in the most difficult material where wooden sheeting
without a shield is used. The timbering during the advance may be
modified in any manner that the character of the material will permit.
The timbering may lag behind the excavation a distance of two or more
frames, or it may be omitted altogether. Heavier timbering may be
necessary in soft, slipping or shattered rock.

[Illustration:

FIG. 118.—Shield for Driving Milwaukee Sewer Tunnel.

Eng. News-Record, Vol. 80, 1918, p. 669.
]

=166. Shields.=—Shields are used in tunneling in soft wet material and
are particularly suitable for work under air pressure. They are used in
rock tunnels where water is anticipated or air pressure is used. The
shields often save the expense and difficulty of timbering as the
masonry of the sewer follows closely behind the shield. Fig. 118 shows
the arrangement for a shield for tunneling in soft material in the
construction of the Milwaukee sewers. The shield has an exterior
diameter of 9 feet 4 inches and an overall length of 9 feet 8⅛ inches.
The cutting edge section is 20 inches long. The shell is made of one
inch plate to the back of the jack chambers and one-half inch plate in
the tail. The shield is driven by ten 60–ton hydraulic jacks. The jacks
are shown in position in the figure. These jacks rest against the
finished tunnel lining and serve to consolidate it at the same time that
they push the shield into the material to be excavated. The face of the
tunnel is cut with a pick and shovel while the jacks are removed one at
a time and a new ring of lining is put in place. The lining may be
temporary timbering to receive the thrust of the jacks, but it is
usually desirable that the permanent lining follow immediately behind
the shield. Since the shield is larger than the outside of the lining
the space left by its passage should be grouted immediately after it has
passed.

=167. Tunnel Machines.=—Tunnel machines have been used successfully on
sewer tunnels in soft materials, but not in rock.[95] The machines are
of different types, but in general consist of a revolving cutting head,
equipped with knives, and driven by an electric motor. The bearing on
which the shaft for the cutting head rests is supported against the
sides of the tunnel. The muck is carried away by means of a conveyor and
dumped into muck cars without rehandling. Rapid progress can be made
with these machines in suitable conditions.

[Illustration:

FIG. 119.—Method of Drilling and Loading Rock Tunnel Face.

Courtesy, Aetna Power Co.
]

=168. Rock Tunnels.=—Tunnels in rock are advanced by drilling into the
face as shown in diagrammatic form in Fig. 119. The holes near the
center are driven in at an angle towards the center and to depths from 6
to 15 feet. The harder the rock the greater the angle with the tunnel.
This is called the center cut. Other holes are driven near the outer
edge of the tunnel and parallel to its axis. When fired, the wedge of
rock between the center cut holes is thrown back into the tunnel and a
delayed explosion then throws the sides into the hole thus made. A final
delay thrusting shot throws the muck so formed away from the face of the
tunnel. For tunnels up to 6 or 8 feet in height the entire bore is cut
out in this fashion. For larger tunnels, the upper portion called the
heading, is taken out in this way, and the remainder, called the bench,
is taken out by drilling and blowing holes normal to the axis of the
tunnel. The amount of powder necessary in the bench holes is much less
than that required in the heading.

=169. Ventilation.=—No tunnel more than 50 feet long should be built
without ventilation. A fair amount of air for ordinary conditions is 75
cubic feet of free air per minute per person in the tunnel, and double
this amount for each animal. Where explosive gases are met, or under
conditions where the tunnel is hot, five or six times as much air may be
needed in order to cool the tunnel or to dilute the gases. In order that
the air may be fresh and cool at the face of the tunnel where work is
going on it should be conducted to the tunnel face in a pipe and blown
out into the tunnel. Immediately following a blast at the face the
current should be reversed so as to draw the poisonous gases out of the
tunnel through the duct. The high pressure air line leading to the
drills should be opened at the same time to create a current towards the
face in order to accelerate the clearing of the air at the heading. The
capacity of the air machines should be sufficient to exhaust four times
the volume of the gases created by the explosion, in 15 minutes. This
will ordinarily call for a capacity of about 4,000 cubic feet of free
air per minute. If the same blower is to be used for exhausting the
gases as for ventilation while work is going on, it should have a high
overload capacity to care for this situation. The air line should be
arranged to allow for reversal of flow.

The diameter of the air pipe should be determined by a study of the
saving of the cost and operation of the air equipment compared to the
increased cost of a larger pipe line. Other factors affecting the size
of the pipe line to be used are: the available space in the tunnel, the
temporary character of the installation, the use of the exhaust from
high-pressure air machines for the purpose of ventilation, etc.
Cast-iron, spiral-riveted galvanized sheet iron, and canvas pipes have
been used for conducting low-pressure ventilating air.

Ventilation in tunnels working under air pressure is supplied from the
compressors, and the air is delivered near the face of the heading,
except that being used in the locks. In tunnels using air drills, the
air for the drills is conducted through a separate pipe as it is not
economical to compress the ventilating air to the pressure necessary to
operate the drills.

=170. Compressed Air.=—Compressed air is used in tunnel work to prevent
the entrance of water into the tunnel and to keep the work dry. The
pressure of air used is closely that of the pressure of the ground water
but in a large tunnel or a tunnel with a weak roof the pressure may be
somewhat lower on account of the danger of blowing through the roof. It
is evident that the water pressure cannot be balanced at the top and the
bottom of the tunnel. To balance it at the bottom makes a blow out near
the top more probable. To balance the pressure at the top may leave the
bottom wet. Judgment and care must be exercised during construction and
if the pressure is balanced at or near the bottom the roof must be
carefully guarded by grouting and puddling with clay, or the surface,
particularly if under water, may be covered with a clay bank. If the
cavities in the tunnel lining are large, sawdust can be mixed with the
grout to advantage, the mixture being pumped through holes in the roof
by hand or power operated force pumps. “Blows” must be carefully guarded
against as they endanger the lives of the workmen and threaten the loss
of the tunnel. The pressure and volume of air supplied for some large
subaqueous tunnels is shown in Table 61.

Labor under compressed air is arduous and dangerous with the best of
safeguards.[96] Pressure more than about 43 pounds per square inch
cannot be used and at this high pressure men cannot work more than four
hours at a time. Little or no distress is noted at pressures less than
15 pounds.

TABLE 61

VOLUME AND PRESSURE OF COMPRESSED AIR IN TUNNELS

(American Civil Engineers Pocket Book)
──────────┬────────┬───────┬─────────┬─────────┬───────────────────────
Tunnel │ │ │ Maximum │ Average │ Conditions and Cubic
│Maximum │ │ Air │ Air │ Feet of Free Air per
│Distance│ │Pressure,│Pressure,│ Minute
│ High │ │ Pounds │ Pounds │
│Water to│Minimum│ per │ per │
│Invert, │ Cover │ Square │ Square │
│ Feet │in Feet│ Inch │ Inch │
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
City and │ │ │ │ │In water bearing-sand.
South │ │ │ │ │ 1660 cubic feet per
London │ │ │ │ │ minute per face. When
│ │ │ │ │ grouted 1000 to 1300
│ │ │ │ │ cubic feet per minute
│ 34│ 42│ 15│ │ per face
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Blackwall │ │ │ │ │10,000 cubic feet per
│ │ │ │ │ minute per face in
│ │ │ │ │ open ballast for some
│ 80│ 5│ 37│ 35│ time
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Baker St. │ │ │ │ │In gravel, 3300 cubic
and │ │ │ │ │ feet of air per
Waterloo│ │ │ │ │ minute per face.
│ │ │ │ │ Parallel tunnel 1650
│ │ │ │ │ cubic feet per min.
│ 70│ 18│ 35│ 28│ per face
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Greenwich │ │ │ │ │Average 83.5 per man
│ │ │ │ │ per minute. Never
│ 70│ 30│ 28│ 20│ less than 66.7
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
Battery, │ │ │ │ │In sand. Two working
East │ │ │ │ │ faces. Maximum 32,000
River. │ │ │ │ │
N. Y. │ 94│ 12│ 42│ 26│
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
East │ │ │ │ │Maximum for one face
River, │ │ │ │ │ 25,000 cubic feet per
N. Y., │ │ │ │ │ minute for 24 hours.
Penn. │ │ │ │ │ Capacity of plant for
R.R. │ │ │ │ │ 8 faces, 80,400 cubic
│ 93│ 8│ 42│ 27│ feet per minute
──────────┼────────┼───────┼─────────┼─────────┼───────────────────────
North │ │ │ │ │Maximum in gravel
River, │ │ │ │ │ 10,000 cubic feet per
N. Y., │ │ │ │ │ man per hour.
Penn. │ │ │ │ │ Generally ranged
R.R. │ 98│ 20│ 37│ 26│ between 1500 and 5000
──────────┴────────┴───────┴─────────┴─────────┴───────────────────────

Entrance and exit to the tunnel are gained through air locks. These are
sheet iron cylinders concreted into the lining of the tunnel or shaft.
Air-tight iron doors are provided at both ends, which open inwards
towards the tunnel. On entering the lock from the outside the door to
the tunnel is found tightly closed. The outside door is then closed by
hand, the air valve is opened and air is admitted to the lock until the
pressure on the lock side of the tunnel door equalizes that on the
tunnel side and the tunnel door is swung open by hand. When the lock is
open to the tunnel the pressure in the tunnel keeps the outside door
closed. In order to leave the tunnel the process is reversed. Materials
are passed through the lock by the lock tender or tenders who pass
through the lock with the material if the pressure is low, or who
manipulate the air outside of the lock if the pressure is high. If
pressures of 30 to 40 pounds are being used, two or even three locks may
be necessary.

EXPLOSIVES AND BLASTING[97]

=171. Requirements.=—The desirable features in an explosive to be used
in trenching and tunneling in rock are: (1) stability in make up so as
not to deteriorate in strength or to become dangerous during storage,
(2) imperviousness to ordinary variations in temperature and moisture,
(3) insensibility to ordinary shocks received in transportation and
handling, (4) not too difficult of detonation, (5) convenient form for
transportation and loading and for making up charges of different
weights, (6) the non-formation of poisonous gases when fired, (7)
imperviousness to water and usefulness in wet holes, (8) power without
bulk, etc.

=172. Types of Explosives.=—Explosives which fill some or all these
requirements can be divided into two classes, deflagrating and
detonating. A deflagration is an explosion transmitted progressively
from grain to grain. A detonation is a sudden disruption caused by
synchronous vibrations of a wave-like character. The deflagrating
explosives are represented by gun-powders and contractors’ powders. They
must be carefully tamped in the hole to develop their full power and
they must be ignited by a fuse or flame. They are valueless in water or
moist holes. These powders are used mainly for loosening frozen earth,
soft sandstone, cemented gravels and similar materials where a thrusting
action rather than a disruption is desired. The detonating explosives
are most commonly represented by the dynamites. These are exploded by a
shock usually caused by another explosive which has been ignited by a
fuse or electric spark, and which is known as the “detonator.”
Detonating explosives are more powerful than deflagrating explosives and
are used in all but the softest materials.

_Gunpowder._—This is a mechanical mixture of sulphur, charcoal, and
saltpeter generally in the proportions of 10 parts sulphur, 15 parts
charcoal, and 75 parts saltpeter (sodium nitrate). It weighs about 62½
pounds per cubic foot and produces about 280 times its own volume in gas
at a pressure of 4.68 tons per square inch at a temperature of 32
degrees F., which amounts to a pressure of approximately 38 tons per
square inch at the temperature of explosion of 4,000 degrees F.

_Blasting Powder._—This is a mixture of 19 parts sulphur, 15 parts
charcoal, and 66 parts saltpeter. These powders are made in different
size angular polished grains, from the size of a pin head to sizes just
passing a ⅜ to ½ inch hole. The larger the grains the slower the action
of the powder.

_Nitro-Substitution Compounds._—These compounds are formed by the action
of nitric acid on hydrocarbons. Triton, T.N.T., or trinitrotoluene, made
famous during the war, is an example of these compounds. It is made by
the successive nitration of toluene, a coal tar derivative. It melts at
80 degrees C., is very stable, and is of great explosive strength. It is
manufactured in a convenient form, being compressed into blocks about 2
inches square by about 4 inches long with a specific gravity of about
1.5. The blocks are usually copper plated to protect the T.N.T. from
moisture. The more dense it is the less its sensitiveness. It is also
put up in crystalline form in cartridges like dynamite, in which
condition it is practically equal to 40 per cent dynamite. It can be cut
with a knife, pounded with a hammer, and will burn freely and slowly in
small quantities in the open air without exploding. It is suitable for
all but the hardest rocks. It creates poisonous gases on detonation
which are quickly dissipated in the open air but which render it
unsuitable for use in tunnel work.

_Nitro-glycerine._—This is formed by the action of nitric and sulphuric
acids on animal compounds such as gelatine or glycerine. Nitro-glycerine
is a yellowish, oily, highly unstable explosive liquid with a specific
gravity of about 1.6. It will burn quietly when ignited in the open air.
It will freeze at 41 degrees F., and will explode at 388 degrees F., or
on concussion at a lower temperature. It develops about 1,500 times its
volume in gas, which due to the heat of combustion is increased to about
10,000 times its volume. It is a very dangerous explosive to handle, and
is unsuitable for use in the liquid form.

_Blasting Gelatine._—This is made by soaking guncotton in
nitro-glycerine. Gelatine dynamite is a combination of blasting gelatine
and an absorbent. Forcite is a gelatine dynamite in which the blasting
gelatine, forming 50 per cent of the compound, contains 90 per cent
nitro-glycerine and 2 per cent guncotton; and the absorbent, forming the
other 50 per cent of the compound, contains 76 per cent of sodium
nitrate, 3 per cent sulphur, 20 per cent of wood tar, and 1 per cent of
wood pulp.

Blasting gelatine is packed in a jelly-like mass in metal lined wooden
boxes. It is less sensitive than straight dynamite and is one of the
most powerful explosives known. It can be made up to equal 100 per cent
dynamite. It is suitable for use in the hardest rocks and for subaqueous
work as it is not affected by moisture. It is suitable for use in
tunnels as the amount of carbon monoxide, peroxide of nitrogen, hydrogen
sulphide and other dangerous gases is comparatively low when fully
detonated. Gelatine dynamite[98] is sold as 30 per cent to 70 per cent
dynamite, the actual percentage of nitro-glycerine being less than the
nominal quantity given.

_Dynamite._—The dynamites are made by soaking nitro-glycerine in some
absorbent. If the absorbent is some neutral substance such as infusorial
earth the combination is known as a true dynamite. The false or active
dynamites are those in which the absorbent is also an explosive
compound. The false dynamites form the best known contractors’
explosives. Among the materials mixed with the nitro-glycerine are:
magnesium carbonate, sulphur, wood meal, wood pulp, wood fiber, wood
tar, nut galls, kieselguhr, sawdust, resin, pitch, sugar, charcoal, and
guncotton. The strength of dynamites is noted by the per cent of
nitro-glycerine and nitro substitutes contained. Dualin and Hercules
powder both contain 40 per cent nitro-glycerine. Dualin contains 30 per
cent sawdust and 30 per cent potassium nitrate, but the Hercules powder,
which is stronger, contains 16 per cent sugar, 3 per cent potassium
chlorate, 31 per cent potassium nitrate, and 10 per cent magnesium
carbonate.

Dynamite is the most common explosive used on construction work. It is
supplied in cylindrical sticks wrapped in paper, the diameter of the
sticks varying between ⅞ and 2 inches. They are about 8 inches long.
Forty per cent dynamite is the common strength found on the market. It
is suitable for ordinary work in all but very hard rocks or very soft
material. Direct contact with water separates the nitro-glycerine from
the base and is dangerous when the explosive is used in wet places
unless it is fired immediately after the hole is loaded. It freezes at
about 42 degrees F., or at even higher temperatures and in the frozen
state it is highly dangerous, requiring powerful detonators for firing,
but exploding spontaneously from a slight jar, or the breaking of the
stick. Special low-freezing dynamites are made that will not freeze
above 35 degrees F.

_Ammonia Compounds._—Ammonia dynamite is a combination of
nitro-glycerine, ammonium nitrate and such other ingredients as sodium
nitrate, calcium carbonate and combustible material. This form of
explosive is advantageous for underground work because, like gelatine
dynamite, its explosion does not create large quantities of poisonous
gases. It has a low freezing point and is relatively low in cost. It is
seriously affected by moisture, however, and can not be used in wet
places. Ammonium nitrate explosives which do not contain nitro-glycerine
include 70 per cent to 95 per cent ammonium nitrate and some combustible
material. Ammonal is a special type of this class formed by a mixture of
ammonium nitrate, aluminum, and triton. All of these explosives are
deliquescent, insensitive to shock, and are cheaper than the dynamites.

=173. Permissible Explosives.=—As specified by the United States Bureau
of Mines explosives whose rapidity, detonation, and temperature of
explosion will not ignite explosive mixtures of pit gases and air are
known as permissible explosives. They include nitrate explosives,
ammonia dynamite, and others.

Gunpowder, triton, picric acid, blasting gelatine, dynamite, guncotton,
etc., are not classed as permissible explosives.

=174. Strength.=—The relative weights for equal strength of various
explosives are given in Table 62.

TABLE 62

RELATIVE WEIGHTS OF EXPLOSIVES WITH THE SAME STRENGTH AS A UNIT WEIGHT
OF 40 PER CENT DYNAMITE

───────────────────────────────────────────────────────┬───────────────
Explosive │Relative Weight
───────────────────────────────────────────────────────┼───────────────
Picric acid │ 0.86
Gun powder (well tamped) │ 3.10
Straight dynamite, 15% │ 1.45
Straight dynamite, 20 │ 1.33
Straight dynamite, 25 │ 1.28
Straight dynamite, 30 │ 1.18
Straight dynamite, 35 │ 1.07
Straight dynamite, 40 │ 1.00
Straight dynamite, 45 │ 0.93
Straight dynamite, 50 │ 0.86
Straight dynamite, 55 │ 0.83
Straight dynamite, 60 │ 0.78
│
Low-freezing dynamites are the same as straight │
dynamites │
Smokeless powder, well tamped │ 0.74
│
Triton │ 0.86
Blasting gelatine │ 0.43
Gelatine dynamite, 30% │ 1.28
Gelatine dynamite, 35 │ 1.21
Gelatine dynamite, 40 │ 1.14
Gelatine dynamite, 50 │ 1.04
Gelatine dynamite, 55 │ 0.97
Gelatine dynamite, 60 │ 0.90
Gelatine dynamite, 70 │ 0.83
│
Ammonia dynamites are the same as gelatine dynamites. │
Chlorates (sprengle) Rack-a-rock │ 1.33
Guncotton │ 0.72
───────────────────────────────────────────────────────┴───────────────

=175. Fuses and Detonators.=—The explosion of gunpowder and other
deflagrating explosives is caused by the direct application of a flame
led to the charge by a powder fuse, or they may be fired by a blasting
cap which is itself exploded by the heat from a fuse or an electric
spark. The powder fuse is a cord made up of a train of powder securely
wrapped in a number of thicknesses of woven cotton or linen threads and
usually made waterproof. Ordinary fuse burns at about 2 feet per minute
but there may be wide variations from this rate due to the quality of
the fuse, moisture, temperature, or pressure. Moisture tends to retard
the rate, pressure to increase it. Instantaneous fuse will burn at about
120 feet per second. It is distinguished from the ordinary safety fuse
both by eye and touch due to the rough red braid with which it is
covered. It is used in firing a number of charges simultaneously. Powder
fuses are lighted by the application of a flame or smoldering torch to
the freshly cut or opened end exposing the powder grains. Cordeau
Bickford is lead tubing filled with triton, in which the flame travels
at about 17,000 feet per second. This is also used for igniting charges
simultaneously.

The detonation of an explosive is caused by the shock or heat of the
explosion of a more sensitive substance which has been exploded by a
powder fuse or electric spark. The common method of detonating explosive
charges is by the firing of a blasting cap. These caps are copper
cylinders, closed at one end, about 1½ inches long and ¼ to ⅜ of an inch
in diameter, or larger. They contain a mixture of about 85 per cent
fulminate of mercury and 15 per cent potassium chlorate held in place by
a wad of shellac, collodion, or paper. The strength of detonators is
based on the weight of fulminate of mercury and is designated as shown
in Table 63.

TABLE 63

STRENGTH OF BLASTING CAPS

────────────────────────────────────────┬──────────────────────────────
Blasting Cap, Commercial Grade │ Grains Fulminate of Mercury
────────────────────────────────────────┼──────────────────────────────
3X or Triple │ 8.3
4X or Quadruple │ 10.0
5X or Quintuple │ 12.3
6X or Sextuple │ 15.4
7X or Number 20 │ 23.1
8X or Number 30 │ 30.9
Single strength │ 12.3
Double strength │ 15.4
Triple strength │ 23.1
Quadruple strength │ 30.9
────────────────────────────────────────┴──────────────────────────────

The force of the explosion is markedly affected by the strength of the
caps, the effect being greater for low-grade powders. For 40 per cent
dynamite the explosion caused by a 5X cap is 15 per cent stronger than
that caused by a 3X cap. For 60 per cent dynamite the difference is only
6 per cent. The deterioration of the caps will reduce the strength of an
explosion noticeably. With straight dynamite, 3X caps are generally
used, but with gelatine dynamite 6X or heavier caps must be used. Caps
may be tested by exploding them in a confined space and noting the
report and the effect on the shell. A full strength cap will tear the
shell into minute pieces, while a deteriorated cap will merely tear it
into three or four large pieces. An ordinary blasting cap is shown in
Fig. 120 together with other equipment for blasting.

Firing by electricity is generally safer and more satisfactory than by
the use of ordinary caps and powder fuses. The explosion is more certain
and its exact time is under the control of the operator. Fig. 121 shows
a section through an electric blasting cap or detonator, commonly called
an electric fuse. Delayed action electric detonators are made by
inserting a slow-burning substance between the platinum bridge and the
detonating substance. The time of delay is controlled by the depth of
the slow-burning substance. Delayed action detonators are useful in
tunnel work where it is desired to explode the charge in three or four
stages in order that the debris from one charge may be out of the way of
the following, and that the forces of the explosions may not serve to
nullify each other.

[Illustration:

FIG. 120.—Blasting Supplies.

Courtesy, Aetna Powder Co.
]

=176. Care in Handling.=—Some of the don’ts in the handling of
explosives recommended by the U. S. Army Engineer Field Manual are: in
the use of nitro-glycerine explosives of all kinds—

(_a_) Don’t store detonators with explosives. Detonators should be
kept by themselves.

(_b_) Don’t open packages of explosives in a store house.

(_c_) Don’t open packages of explosives with a nail puller, pick
or chisel. Packages should be opened with a hard wood wedge and
mallet, outside of the magazine and at some distance from it.

(_d_) Don’t store explosives in a hot or damp place. All
explosives spoil rapidly if so stored.

(_e_) Don’t store explosives containing nitro-glycerine so that
the cartridges stand on end. The nitro-glycerine is more likely to
leak from the cartridges when they stand on end than it is when
they lie on their sides.

(_f_) Don’t use explosives that are frozen or partly frozen. The
charge may not explode completely and serious accidents may
result. If the explosion is not complete the full strength of the
charge is not exerted and larger quantities of harmful gases are
given off.

[Illustration:

FIG. 121.—Electric Fuse.

Full size.
]

(_g_) Don’t thaw frozen explosives in front of an open fire, nor
in a stove, nor over a lamp, nor near a boiler, nor near steam
pipes, nor by placing cartridges in hot water. Use a commercial or
improvised thawer.

(_h_) Don’t put hot water or steam pipes in a magazine for thawing
purposes.

(_i_) Don’t carry detonators and explosives in the same package.
Detonators are extremely sensitive to heat, friction, or blows of
any kind.

(_j_) Don’t handle detonators or explosives near an open flame.

(_k_) Don’t expose detonators or explosives to direct sunlight for
any length of time. Such exposure may increase the danger in their
use.

(_l_) Don’t open a package of explosives until ready to use the
explosive, then use it promptly.

(_m_) Don’t handle explosives carelessly. They are all sensitive
to blows, friction, and fire.

(_n_) Don’t crimp a detonator (blasting cap) around a fuse with
the teeth. Use a cap crimper, which is supplied for this purpose.

(_o_) Don’t economize by using a short length of fuse.

(_p_) Don’t return to a charge for at least one-half hour after a
miss fire. Hang fires are likely to happen.

(_q_) Don’t attempt to draw nor to dig out the charge in case of a
miss fire.

Some of the positive rules in connection with the handling of explosives
are: build the magazine on an earth foundation remote from any other
structures, protect it with earth embankments that will direct the force
of the explosion upwards, and build it of materials that will supply as
few missiles as possible. Hollow tile brick, double-walled galvanized
iron filled with sand, and similar constructions are satisfactory. The
magazine may be heated by steam or hot-water pipes so located that
explosives cannot come in contact with them, or by a cluster of
incandescent bulbs, but if the explosives become frozen they must not be
thawed out by turning on the steam or hot water. If powder or
nitro-glycerine is dropped on the floor the magazine should be emptied,
washed out with a hose and spots of nitro-glycerine scrubbed with a
brush and a mixture of ½ gallon of wood alcohol, ½ gallon of water and 2
pounds of sodium sulphide. Frozen explosives may be thawed by spreading
out on special shelves in a warm thaw house—not in the magazine proper,
by burying in a manure pile so that the explosive may not become
moistened, or more commonly by heating slowly in a water bath. This is a
dry kettle in which the explosives are placed and covered. The kettle is
then put in another containing water which is heated gently to about 120
degrees F. It should not be boiled.

In case of a miss fire, instead of digging out the old charge put a new
charge on top of the old and fire the two simultaneously.

=177. Priming, Loading, and Firing.=—Priming is the act of placing the
cap or detonator in the cartridge of explosive. The primer is either the
cap or the cap and cartridge which are to be detonated by the fuse. If a
cap and safety fuse are to be used the paper at the upper end of the
cartridge is opened, a hole is poked in the explosive with the finger or
a piece of wood, the cap and the attached fuse are pushed into the hole
and gently embedded in the explosive so that the end of the cap is
exposed sufficiently to prevent the fuse from igniting the dynamite
directly. The paper is then folded up and tied firmly around the fuse
with a piece of string. The result is shown in Fig. 122.

[Illustration:

FIG. 122.—Dynamite Cartridge, Safety Fuse, and Cap.
]

In placing the fuse in the cap the end of the fuse is cut off square,
and inserted in the open end of the cap, care being taken not to spill
the loose grains of powder or to grind the fuse down on top of the cap.
When the fuse is shoved firmly into place the upper portion of the
copper cap is pressed or crimped with the cap crimpers shown in Fig.
120.

The number of primers to be used is dependent on the size and location
of the charge, but in practically all sewer work only one primer is used
to each hole. In bulky charges the primer should be placed near the
center of the charge and the fuse so protected that it will not ignite
the charge prematurely. In drill holes the primer is put in last with
the cap end down.

In loading a hole, it is first pumped and cleaned out. This can be done
satisfactorily with the end of a stick frayed out into a broom.
Cartridges which very nearly fill the hole are dropped in one at a time
and are pressed firmly together, with a light wooden tamping bar. They
should not be pounded. After the primer is placed, a wad of clay or
similar material is pressed gently into the hole against it and the hole
is then filled with well-tamped clay. In tunnel work tamping is not so
essential as an overcharge of powder is usually used and the time of
tamping, which is worth more than two or three sticks of dynamite, is
saved. In handling bulk explosives, such as gunpowder, they are poured
into the hole, the fuse is set in the upper portion and the remainder of
the hole is tamped with clay as for dynamite cartridges.

[Illustration:

FIG. 123.—Methods for Cutting Safety Fuse for Splicing.
]

If a large number of charges are to be fired simultaneously with a
safety fuse, the length of the fuse to each charge should be made equal
or a safety fuse used to a common center and approximately equal lengths
of instantaneous fuse or Cordeau Bickford used from there to the charge.
In splicing the fuses for such connections they are cut diagonally as
shown in Fig. 123 and bound together firmly with tape. Electric
connections are particularly advantageous under such conditions as they
avoid the dangers incidental to spliced fuses and are less expensive. In
tunnel work simultaneous electric detonation is not desirable as the
holes should be fired progressively: 1st, the cuts; 2nd, the relievers;
3rd, the backs; 4th, the sides; and 5th, the lifters. Different lengths
of safety fuse, or delayed action electric fuses can be used for these
delay shots.

In igniting a safety fuse an open flame such as that furnished by a
match or candle is the most satisfactory. For electric fuses the current
is generated by a magneto shown in Fig. 120. Pressing vigorously down on
the handle closes the circuit and generates an electric current which
heats the platinum bridges and explodes the charges. For the small
number of charges used in ordinary construction they are connected in
series so that if there is a broken connection anywhere no charge will
be exploded. If many charges are to be fired and a line circuit is to be
used, the final connection should not be made until just before the
charge is to be fired in order to obviate the danger of stray currents
firing the charge prematurely. Care should be taken to see that all
connections are good and that there are no broken wires on the line.

=178. Quantity of Explosive.=—The quantity of explosive to be used can
be determined satisfactorily only by experience on the job in question,
as the factors affecting the necessary quantity are so diverse. The
figures in Table 64 indicate the relative amounts needed under different
conditions.

TABLE 64

QUANTITIES OF EXPLOSIVES

───────────┬──────┬────────┬─────────┬─────────────┬─────────┬──────────
Kind of │Drift │Feet[99]│Black[99]│Dynamite[99],│Grade of │
Rock │ in │of Hole │ Powder, │ Pounds │Dynamite,│ Remarks
│ Feet │ │ Pounds │ │Per Cent │
───────────┼──────┼────────┼─────────┼─────────────┼─────────┼──────────
Limestone, │ │ │ │ │ │
Chicago │ 12│ 0.40│ │ 0.75│ 40│Gillette
Drainage │ │ │ │ │ │
Canal │ │ │ │ │ │
Limestone │ │ │ │ │ │
for │ 6│ 1.00│ │ 0.70│ 40│Gillette
crushing │ │ │ │ │ │
Limestone │ │ │ │ │ │
for │ 20│ │ │ 0.37│ 50│Gillette
cement │ │ │ │ │ │
Limestone, │ │ │ │ │ │
holes │ 15│ 0.40│ │ 0.26│ 50│Gillette
sprung │ │ │ │ │ │
Sandstone, │ 20│ 0.10│ 1.0│ 0.10│ 40│Gillette
side cut │ │ │ │ │ │
Sandstone, │ │ │ │ │ │
thorough │ 20│ 0.20│ 2.0│ 0.20│ 40│Gillette
cut │ │ │ │ │ │
Shale, soft│ 24│ 0.08│ 0.7│ 0.03│ 40│Gillette.
side cut │ │ │ │ │ │ Open cut
Shale, hard│ │ │ │ │ │
thorough │ 24│ 0.20│ 1.5│ 0.10│ 40│Gillette
cut │ │ │ │ │ │
Granite for│ 16│ 1.36│ │ 0.20│ 60│Gillette
rubble │ │ │ │ │ │
Gneiss, New│ 12│ 1.33│ │ 0.60│ 40│Gillette
York City│ │ │ │ │ │
Gneiss, New│ 14│ 0.63│ │ 0.50│ 40│Gillette
York City│ │ │ │ │ │
Syenite, │ │ │ │ │ │
Treadwell│ 12│ 1.70│ │ 0.67│ 40│Gillette
Mine │ │ │ │ │ │
Magnetic │ 12½│ 0.32│ │ 0.44│ 52│Gillette
iron ore │ │ │ │ │ │
Trap, seamy│ 14│ 0.35│ │ 0.20│ 75│Gillette
Trap, │ 17│ 1.00│ │ 0.70│ 40│Gillette
massive │ │ │ │ │ │
│ │ │ │ │ │50%
Granite, │ │ │ │ │ │ dynamite
Grand │ 25│ 0.10│ │ 0.80│ 50│ used to
Trunk │ │ │ │ │ │ spring
│ │ │ │ │ │ holes
Clay, rock │ │ │ │ │ │
and │Tunnel│ │ │ 1.00│ │
Gypsum │ │ │ │ │ │
│ │ │ │ │Grade │
│ │ │ │ │ varied │
│ │ │ │ │ ⅗ at │
Hard shale │Tunnel│ │ │ 2.07│ 45%, ⅕ │
│ │ │ │ │ at 60%,│
│ │ │ │ │ some at│
│ │ │ │ │ 100% │
Hard rocky │Tunnel│ 1.60│ │ 3.57│ │
slate │ │ │ │ │ │
Hard rocky │Tunnel│ 1.46│ │ 3.57│ │
slate │ │ │ │ │ │
Mill Creek │ │ │ │ │ │Mun.
sewer, │Tunnel│ │ │ 4.00│ 60│ Eng’g.
St. Louis│ │ │ │ │ │ Vol. 52,
│ │ │ │ │ │ p. 14
───────────┴──────┴────────┴─────────┴─────────────┴─────────┴──────────

PIPE SEWERS

=179. The Trench Bottom.=—It is customary to dig the bottom of the
trench to conform to the shape of the lower 45 degrees to 90 degrees of
the sewer if the character of the material will allow such construction.
In soft material which will not hold its shape the sewer may be encased
in concrete or a concrete cradle may be prepared for the pipe. In rock
the trench is excavated to about 6 inches below grade and refilled with
well-tamped earth so as to form a cradle giving bearing to 60 to 90
degrees of the pipe circumference. For large sewers to be constructed in
the trench special foundations are sometimes built.

=180. Laying Pipe.=—Before the pipe is lowered into the trench the
sections which are to be adjacent should be fitted together on the
surface and the relative positions marked by chalk so that the same
position can be obtained in the trench.

Small pipes are lowered into the trench and swung into position on a
hook as shown in Fig. 124. Pipes up to 15 or 18 inches in diameter can
be handled by the pipe layer and helper in the trench without
assistance. Heavier pipes may be lowered into the trench by passing
ropes around each end of the pipe. One end of the rope is fastened at
the surface and the ropes are paid out by the men at the surface as the
pipe is lowered. If the pipes have been fitted together and marked at
the surface it is undesirable to use this method of lowering as the
position in which the pipes arrive in the bottom of the trench can not
be easily predicted. A cradle may be used for shoving the pipe into
position as is shown in Fig. 125.

[Illustration:

FIG. 124.—Hook for Lowering and Placing Sewer Pipe.
]

[Illustration:

FIG. 125.—Cradle for Placing Sewer Pipe.
]

Pipes above 24 to 27 inches in diameter are too large to be handled from
the side of the trench. A hook as shown in Fig. 124 is placed in the
pipe so that it will be in the proper position when lowered. It is
raised by a rope passing through a block at the peak of a stiff-legged
derrick which spans the trench, or by a crane. If a derrick is used the
rope passes to a windlass on the opposite side of the trench from the
pipe. Mechanical power may be used for raising pipes too heavy to be
raised by hand. The pipe is then lowered and swung into position while
supported from the derrick. Excessive swinging is prevented by holding
back on the guide rope as the pipe is raised and lowered.

Pipes are usually laid with the bell end up grade as it is easier to fit
the succeeding pipe into the bell so laid and to make the joint,
particularly on steep grades. The Baltimore specifications state:

The ends of the pipe shall abut against each other in such a
manner that there shall be no shoulder or unevenness of any kind
along the inside of the bottom half of the sewer or drain. Special
care should be taken that the pipe are well bedded on a solid
foundation.... The trenches where pipe laying is in progress shall
be kept dry, and no pipe shall be laid in water or upon a wet bed
unless especially allowed in writing by the Engineer. As the pipe
are laid throughout the work they must be thoroughly cleaned and
protected from dirt and water, no water being allowed to flow in
them in any case during the construction except such as may be
permitted in writing by the Engineer. No length of pipe shall be
laid until the preceding length has been thoroughly embedded and
secured in place, so as to prevent any movement or disturbance of
the finished joint.

The mouth of the pipe shall be provided with a board or stopper,
carefully fitted to the pipe, to prevent all earth and any other
substances from washing in.

=181. Joints.=—Pipes may be laid with open joints, mortar joints, cement
joints, or poured joints. Open joints are used for storm sewers in dry
ground close to the surface. Mortar and cement joints are commonly used
on all sewers except in special cases. Cement joints are more carefully
made than mortar joints and result in a greater percentage of
water-tight joints. Poured joints are used in wet trenches where it is
necessary to exclude ground water from the sewer.

A specification used in some cities for open joints is:

Pipes laid with open joints are to be laid with their inverts in
the same straight line and shall be firmly bedded throughout their
length on the bottom of the trench. No cement or mortar is to be
used in the joints. Not more than ⅛ inch shall be left between the
spigot end of the pipe and the shoulder of the hub of the pipe
into which it fits. The joints shall be surrounded with cheese
cloth, burlap, broken pipe, gravel or broken stone.

The purpose of the cheese cloth, etc., is to prevent fine earth from
sifting into the pipe until the cheese cloth or other material has
rotted away, by which time the earth has become arched over the opening.

Mortar joints are specified by Metcalf and Eddy as follows:

Before a pipe is laid the lower part of the bell of the preceding
pipe shall be plastered on the inside with stiff mortar of equal
parts of Portland cement and sand, of sufficient thickness to
bring the inner bottoms of the abutting pipe flush and even. After
the pipe is laid the remainder of the bell shall be thoroughly
filled with similar mortar and the joint wiped inside and finished
to a smooth bevel outside.

In some work a wood block or a stone is embedded in the mortar at the
bottom of the joint to bring the spigot in place concentric with the
next pipe.

Cement joints are specified in the Baltimore specifications as follows:

Cement joints shall be made with a narrow gasket of hemp or jute
and cement mortar, and special care shall be taken to secure tight
joints. The gasket shall be soaked in Portland cement grout and
then carefully inserted between the bell and the spigot, and well
calked with suitable hardwood or iron calking tools. It shall be
in one continuous piece for each joint, and of such thickness as
to bring the inverts of the two pipes smooth and even. The
remainder of the joint shall be filled with cement mortar all
around, on the bottom, top and sides, applied by hand with rubber
mittens, well pressed into the annular space and beveled off from
the outer edge of the bell to a distance of two inches therefrom,
or to an angle of 45 degrees. The inside of each joint shall be
thoroughly cleansed of all surplus mortar that may squeeze out in
making the joint; and to accomplish this some suitable scraper or
follower, or form shall be provided and always used immediately
after each joint is finished.

Cement joints so made, form the most satisfactory joint for ordinary
conditions and are the most frequently used. They are not always
water-tight and can be penetrated by roots. Some roots are able to
penetrate holes of almost microscopic size and to form growths in the
sewer or to split the joints.

Poured joints are made by pouring some jointing compound, while in a
fluid state, into the joint in which it hardens, thus sealing the joint.
Water-tightness in sewer lines to exclude ground water has also been
attempted by using the ordinary cement joint and surrounding the pipe
with a layer of cement or concrete. This has not always been successful
as it is difficult to obtain the proper class of workmanship in wet
sewer trenches.

The requisite qualities of a poured jointing material are:

(1) It should make a joint proof against the entrance of water and
roots.

(2) It should be inexpensive.

(3) It should have a long life.

(4) It should not deteriorate in sewage which may be either acid
or alkaline.

(5) It should adhere to the surface of the pipe.

(6) It should run at a temperature below about 400° F., as too
high temperatures will crack the pipe.

(7) It should neither melt nor soften at temperatures below 250°
F. in order to maintain the joint if hot liquids are poured into
the sewer.

(8) It should be elastic enough to permit slight movements of the
pipes.

(9) It should not require great skill in using as it must be
handled ordinarily by unskilled workers.

The materials used for poured joints are: cement grout; sulphur and
sand; and asphalt or some bituminous compound made of vulcanized linseed
oil, clay, and other substances the resulting mixture having the
appearance of vulcanized rubber or coal tar. The bituminous materials
most nearly approach the ideal conditions.

Cement grout is made up of pure cement and water mixed into a soupy
consistency. Its main advantages are its cheapness and ease in handling
in wet trenches or difficult situations. The result is no better than a
well made cement joint. There is no elasticity to the joint and a
movement of the pipe will break it.

Sulphur and sand are inexpensive, comparatively easy to handle, and make
an absolutely water-tight and rigid joint which is stronger than the
pipe itself. It frequently results in the cracking of the pipe and is
objected to by some engineers on that account. In making the mixture,
powdered sulphur and very fine sand are mixed in equal proportions. It
is essential that the sand be fine so that it will mix well with the
sulphur and not precipitate out when the sulphur is melted. Ninety per
cent of the sand should pass a No. 100 sieve and 50 per cent should pass
a No. 200 sieve. The mixture melts at about 260° F. and does not soften
at lower temperatures. For making a joint in an 8 inch pipe about 1½
pounds of sulphur, 1½ pounds of sand, ½ pound of jute, and 0.4 pound of
pitch are used. The pitch is used to paint the surface of the joint
while still hot in order to close up any possible cracks.

Among the better known of the bituminous joint compounds are: “G.K.”
Compound made by the Atlas Company, Mertztown, Pa., Jointite and
Filtite, manufactured by the Pacific Flush Tank Co., Chicago and New
York, and some of the products of the Warren Brothers Co., Boston. These
compounds fill nearly all of the ideal conditions except as to cost and
ease in handling. They are somewhat expensive and if overheated or
heated too long become carbonized and brittle. In cold weather they do
not stick to the pipe well unless the pipe is heated before the joint is
poured. On some work joints have been poured under water with these
compounds, but success is doubtful without skillful handling. An
overheated compound will make steam in the joint causing explosions
which will blow the joint clean, and an underheated compound will harden
before the joint is completed.

The materials should be heated in an iron kettle over a gasoline furnace
or other controllable fire, until they just commence to bubble and are
of the consistency of a thin sirup. Only a sufficient quantity of
material for immediate use should be prepared and it should be used
within 10 to 15 minutes after it has become properly heated. The ladle
used should be large enough to pour the entire joint without refilling.
There are other important points to be considered in pouring joints
which can be learned best by experience.

The quantity of material necessary for making these joints, as announced
by the manufacturers, is shown in Table 65.

TABLE 65

QUANTITY OF COMPOUND NEEDED FOR POURED JOINTS

───────────┬───────────────────────────────────────────────────────────
Diameter of│
Pipe, in │ Quantity of Material in Pounds per Joint
Inches │
───────────┼─────────────────────────────┬─────────────────────────────
│ Standard Socket │ Deep and Wide Socket
───────────┼─────────┬─────────┬─────────┼─────────┬─────────┬─────────
│Jointite │ Filtite │ G. K. │Jointite │ Filtite │ G. K.
───────────┼─────────┼─────────┼─────────┼─────────┼─────────┼─────────
6│ 0.82│ 0.72│ 0.42│ 1.46│ 1.28│ 0.72
8│ 1.06│ 0.95│ 0.73│ 1.82│ 1.60│ 1.25
10│ 1.30│ 1.15│ 0.89│ 2.26│ 1.98│ 1.52
12│ 2.08│ 1.82│ 1.42│ 2.65│ 2.32│ 1.80
15│ 2.52│ 2.20│ 1.74│ 3.20│ 2.80│ 2.20
18│ 3.02│ 2.64│ 2.58│ 3.75│ 3.29│ 3.25
20│ 3.44│ 3.00│ 2.86│ 4.30│ 3.78│ 3 60
22│ 3.62│ 3.16│ 3.13│ 4.62│ 4.07│ 3.97
24│ 4.03│ 3.50│ 3.41│ 4.91│ 4.31│ 4.27
───────────┴─────────┴─────────┴─────────┴─────────┴─────────┴─────────

In making a poured joint the pipes are first lined up in position. A
hemp or oakum gasket is forced into the joint to fill a space of about ¾
of an inch. An asbestos or other non-combustible gasket such as a rubber
hose smeared with clay is forced about ½ inch into the opening between
the bell and the spigot and the compound is poured down one side of the
pipe through a hole broken in the bell, until it appears on the other
side, and the hole is filled. Occasionally the non-combustible gasket is
wrapped tightly around the spigot of the pipe and pressed or tied firmly
to the bell. In pouring cement grout joints a paper gasket is used which
is held to the bell and spigot by draw strings. Greater speed in
construction and economy in the use of materials are obtained by joining
two or three lengths of pipe on the bank and lowering them into the
trench as a unit. The pipes are set in a vertical position on the bank
with the bell end up, one length resting in the other. The joint is
calked with hemp and poured without the use of the gasket. The joint
should always be poured immediately after being calked so that the hemp
can not become water soaked. The asbestos gasket should be removed as
soon as possible after the joint is poured in order to prevent sticking
with resultant danger of breaking of the joint when attempting to pull
the gasket free.

One man can pour about 33 eight-inch joints, and two men can complete
about 26 twelve-inch joints per hour on the bank where conditions are
more or less fixed.

=182. Labor and Progress.=—The labor required for the laying of pipe
sewers, exclusive of excavation, bracing and backfilling, consists of
pipe layers and helpers. For pipes 24 to 27 inches in diameter or
smaller one pipe layer and one or more helpers are necessary, dependent
on the size of the pipe and the depth of the trench. For larger pipes
two pipe layers can work economically each working on one-half of the
pipe and making half of the joint. The speed of pipe laying is
ordinarily limited by the speed of the excavation, but on a job in
Topeka, Kan.,[100] where the average day’s progress with a machine
excavator was 200 to 500 feet of trench per day, the pace was limited by
the speed of the pipe laying gang. This gang consisted of two pipe
layers in the trench and two helpers on the surface. The sizes of pipes
handled were from 8 to 27 inches.

BRICK AND BLOCK SEWERS

=183. The Invert.=—In good firm ground the excavation is cut to the
shape of the sewer and the bricks are laid directly on the ground, being
embedded in a thick layer of mortar. After the foundation has been
prepared and before the bricks are laid, two wooden templates, called
profiles, are prepared, similar to that shown in Fig. 126, to conform to
the shape of the inside and outside of the sewer. Each course of bricks
is represented by a row of nails in the profile and each nail
corresponds to a joint in the row. The two profiles are set true to line
and grade. A cord is stretched tightly between the two lowest nails on
opposite templates and a row of bricks is laid. The bricks are laid
radially and on edge with their long dimension parallel to the axis of
the sewer and with one edge just touching the string. As each one or two
or three rows are completed the guide line is moved up to the next
nails. When the bricks are laid on the ground all but large depressions
are filled in with tamped sand or mortar by the masons. Approximately
the same number of rows of bricks is kept completed on either side of
the center line. The succeeding courses follow within three to five rows
of each other, the only bond between courses being the mortar joint.
This is called row lock bond and with few exceptions has been used on
all brick sewers in the United States. As the sides of the sewer become
higher during the construction, platforms must be built for the masons.
These platforms are built of wood and rest directly on the green
brickwork. They should be designed to spread the load as much as
possible. The brickwork of the invert is continued up in this way to the
springing line. As soon as one section is completed one profile is moved
10 to 20 feet ahead along the trench according to the standard length of
sections, and set in position. The line is then strung from it to nails
driven or pushed into the cement joints of the last completed section.
Between work done on separate days the bricks are racked back in courses
to provide a satisfactory bond.

[Illustration:

FIG. 126.—Profile for Brick Sewers.
]

In ground too soft to support the brickwork directly a cradle is
prepared by placing profiles in position in the sewer and nailing 2–inch
planks to these profiles, first firmly tamping earth under the planks.
The bricks are laid in this cradle in a manner similar to that explained
for sewers with a firm foundation. In still softer ground it may be
necessary to construct a concrete cradle to support the bricks.

=184. The Arch.=—The arch centering consists of a wooden form made up of
wooden ribs as shown in Fig. 127. The center conforms to the shape of
the inside of the arch with allowance for the thickness of the lagging.
The lagging is nailed on the ribs in straight strips parallel to the
axis of the sewer. The center is supported on triangular struts resting
against the sides and on the bottom of the sewer and is lifted into
position by wedges driven between it and the support. The centers may be
placed immediately after the completion of the invert, or a day or two
may be allowed to pass to give the invert an opportunity to set. After
the centers are fixed in place the arch brick are carried up evenly on
each side and are pounded firmly into place. The center is usually, but
not always “struck” immediately, and the arch brick are cleaned and
pointed up from the inside. The outside is covered with a layer of ¼ to
¾ of an inch of cement mortar and may be backfilled to the top of the
arch in order to maintain the moisture of the mortar during setting and
to press the bricks of the arch together firmly. The centers are
sometimes made collapsible so that they can be carried or rolled through
the finished brickwork to the advanced position. In “striking” the
centers the wedges are removed and the wings folded in.

[Illustration:

FIG. 127.—Centering for Brick Sewer.
]

In tunneling, the invert of the sewer is constructed in the same fashion
as for open cut work. The arch centering is made in short sections and
the bricks are put in position by reaching in over the end of the
centering. All of the timbering of the tunnel is removed except the
poling boards or lagging against which the bricks or mortar are tightly
pressed, the boards being bricked in permanently.

=185. Block Sewers.=—Sewers made of unit blocks of concrete or vitrified
clay are constructed in a similar manner to brick sewers. Fig. 128 shows
the construction of a block sewer at Clinton, Iowa. In this sewer there
are two rings; an inside one of solid blocks and an outside one of
hollow blocks. Block sewers do not demand the skill in construction that
is demanded by brick sewers, as the blocks are so cast that the joints
are radial, whereas only experienced masons can lay bricks radially.

[Illustration:

FIG. 128.—Segmental Block Sewer at Clinton, Iowa.
]

=186. Organization.=—The number of men employed on a brick or block
sewer is proportioned according to the size of the sewer and the working
conditions. The number of men working on different tasks usually bears
the same ratio to the number of masons employed, regardless of the size
of the work. These proportions are shown for different jobs, in Table
66.

TABLE 66

ORGANIZATIONS FOR THE CONSTRUCTION OF BRICK AND BLOCK SEWERS

─────────────┬──────────────┬────────┬────────┬────────┬────────┬──────────
│ │ │ │ │ 84– to │
│General Ratio │15–foot,│66–inch │84–inch │108–inch│ 42–inch
Type of Work │ on Basis of │ 5–ring │Circular│Circular│ Sewer │Lock-Joint
│ Four Brick │ Brick, │ Brick, │ Brick, │Brick in│Tile Block
│ Layers │Chicago │ Gary │ Gary │Detroit │
│ │ │ │ │ Tunnel │
─────────────┼──────────────┼────────┼────────┼────────┼────────┼──────────
Foreman │ 1│ 1│ 1│ 1│ 1│ 1
Brick layers │ 4│ 12│ 6│ 6│ 5│ 2
Helpers │ 2│ 11│ 3│ 3│ │ 1
Scaffold men │ 2│ 21│ 3│ │ │
Brick tossers│ 2│ 7│ │ 15│ │ 2
Brick │ 2│ 2│ │ │ │ 2
carriers │ │ │ │ │ │
Cement mixers│ 2│ 6│ 6│ 5│ │ 1
Cement │ 2│ 10│ │ 8│ │
carriers │ │ │ │ │ │
Form setters │ 1│ │ 3│ 3│ │
Laborers │ 1│ 8│ 19│ 3│ 14│ 7
│ Municipal │
Source of │ Engineering, │ H. P. Gillette, Handbook of Cost Data
Information│ Vol. 54, p. │
│ 228 │
─────────────┴──────────────┴──────────────────────────────────────────────

=187. Rate of Progress.=—In a general way it can be assumed that the
laying of 1,000 bricks will require 3⅓ hours of the time of one mason,
10 man-hours for helpers and laborers, 2 barrels of cement, 0.6 cubic
yard of sand, and about 10 feet board measure of centering. One thousand
bricks will make about 2 cubic yards of brickwork. To the costs, as
estimated on the basis of materials and labor, must be added about 15
per cent for overhead and an additional amount for the contractor’s
profit. The number of bricks required in various size sewers is shown in
Table 67. A mason can lay more bricks per hour in a large sewer than in
a small one as there is a smaller percentage of face work, there is more
room to work, and it is easier to lay the bricks radially. The number of
bricks laid and the rate of progress on various jobs are shown in Table
68.

TABLE 67

BRICK MASONRY IN CIRCULAR SEWERS. CUBIC YARDS PER LINEAR FOOT

(From H. P. Gillette)
─────────────────┬─────────────────┬─────────────────┬─────────────────
Diameter, │ One Ring │ Two Ring │ Three ring
Feet and Inches │ (4½ Inches) │ (9 Inches) │ (13½ Inches)
─────────────────┼─────────────────┼─────────────────┼─────────────────
2 0│ 0.103│ 0.240│
2 6│ 0.125│ 0.280│
3 0│ 0.147│ 0.327│
3 6│ 0.169│ 0.371│
4 0│ 0.191│ 0.415│
4 6│ 0.213│ 0.458│
5 0│ 0.234│ 0.501│ 0.802
5 6│ 0.256│ 0.545│ 0.867
6 0│ 0.278│ 0.589│ 0.933
6 6│ │ 0.633│ 1.000
7 0│ │ 0.677│ 1.063
7 6│ │ 0.720│ 1.128
8 0│ │ 0.763│ 1.193
8 6│ │ 0.807│ 1.260
9 0│ │ 0.851│ 1.325
9 6│ │ 0.895│ 1.390
10 0│ │ 0.938│ 1.456
─────────────────┴─────────────────┴─────────────────┴─────────────────

CONCRETE SEWERS

=188. Construction in Open Cut.=—In the construction of sewer pipe of
cement and concrete one of two methods may be employed; 1st, to
manufacture the pipe in a plant at some distance from the place of final
use, or 2nd, to manufacture the pipe in place. The methods of the
manufacture of cement and concrete pipe which are to be transported to
the place of use are treated in Chapter VIII. The process of
constructing the pipes in place is ordinarily used for pipes 48 inches
or more in diameter. For smaller sizes, brick, vitrified clay, and
precast cement pipes are usually more economical.

The preparation of the foundation of a concrete sewer is similar to that
for a brick sewer. If the ground is suitable the trench is shaped to the
outside form of the sewer and the concrete poured directly on it. In
soft material which would give poor support to a sewer with a rounded
exterior, the bottom of the trench is cut horizontal and a concrete
cradle of poorer quality than that in the finished sewer is poured on
the soft ground, on a board platform, on piles, or on cribbing supported
on piles.

If the invert of the sewer is so flat that the concrete will stand
without an inside form the shape of the invert is obtained by a screed
or straight-edge which is passed over the surface of the concrete and
guided on two centers, or on one center and the face of the finished
work. The construction of a flat invert sewer at Baltimore is shown in
Fig. 1. The center for the concrete is shown in the foreground. When the
concrete for the next section is poured it will be smoothed to shape by
a screed or straight-edge resting on the face of the finished concrete
and the center. The center is shaped to conform to that of the finished
concrete. It is firmly staked in position and acts as a bulkhead for the
concrete as it is poured, as well as a guide for the screed.

TABLE 68

RATE OF PROGRESS ON BRICK SEWER CONSTRUCTION

(Based on 8–hour day)
────────┬────────┬──────┬──────┬──────┬────────
│ │ │ │Bricks│
Diameter│ │Number│Number│ per │ Number
of Sewer│ Shape │Rings,│Masons│Mason │Laborers
│ │Brick │ │ per │
│ │ │ │ Day │
────────┼────────┼──────┼──────┼──────┼────────
7′ 0″│Circular│ │ │ │
8′ 11″│ and │ 2½ │ 6│ 4710│ 39
│ Oval │ │ │ │
│ │ │ │ │
│ │ │ │ │
4′ 0″│Circular│ 2 │ 3│ 2500│
│ │ │ │ │
│ │3 arch│ │ │
6′ 8″│Circular│ 1 │ 18│ │ 62
│ │invert│ │ │
│ │ │ │ │
│ │1 arch│ │ │
2′ 9″│Egg │ 2 │ 2│ │ 3
│ │invert│ │ │
│ │ │ │ │
5′ 6″│Circular│ 2 │ 6│ 4570│ 35
6′ 6″│Circular│ │ 4│ 4800│
│ │ │ │ │
│ │ │ │ │
2′ 9″│Circular│ 2 │ 2│ 2080│ 5
│ │ │ │ │
16′ 0″│Circular│ 5 │ 8│ 5 cu.│
│ │ │ │ yd.│
16′ 0″│Circular│ 5 │ 12│ │ 70–75
3′ 6″│Egg │ │ │ 2300│
9′ 6″│Circular│ │ │ 3000│
│ │ │ │ │
│ │ │ │ │
3′ 6″│Circular│blocks│ 2│ │ 13
│ │ │ │ │
│ │ │ │ │
────────┴────────┴──────┴──────┴──────┴────────

────────┬────────┬────────────┬─────────┬───────────
│ │ │ │
Diameter│ Feet │ │ │
of Sewer│Progress│ Location │Authority│ Remarks
│per Day │ │ │
│ │ │ │
────────┼────────┼────────────┼─────────┼───────────
7′ 0″│ │ │ │
8′ 11″│ 60│Gary │Gillette │9–hour day
│ │ │ │
│ │ │ │
│ │ │Metcalf │General
4′ 0″│ 36│ │ and │ average
│ │ │ Eddy │
│ │ │ │Concrete
6′ 8″│ │Denver │Gillette │ invert
│ │ │ │
│ │ │Eng. │
│ │Springfield,│ Con., │
2′ 9″│ │ Mass. │ Jan. │
│ │ │ 16, │
│ │ │ 1907 │
5′ 6″│ 110│Gary │Gillette │
6′ 6″│ │ │Gillette │Exceptional
│ │ │ │ speed
│ │ │ │Tunnel
2′ 9″│ 13.9│Syracuse │Gillette │ 12–hour
│ │ │ │ day
16′ 0″│ 22│Chicago │Gillette │First year
│ │ │ │
16′ 0″│ 35│Chicago │Gillette │Second year
3′ 6″│ │St. Louis │Gillette │
9′ 6″│ 12.5│Chicago │H. R. │
│ │ │ Abbott │
│ │ │ │Lock joint
3′ 6″│ 30│ │ │ and tile.
│ │ │ │ 10–hour
│ │ │ │ day
────────┴────────┴────────────┴─────────┴───────────

If inside forms are to be used they are made as units in lengths of 12
or 16 feet for wooden forms, and 5 feet for steel forms. The inside form
is supported by precast concrete blocks placed under it and which are
concreted into the sewer. It is held in position by cleats nailed to the
outside form, to the sheeting, or wedged against the outside of the
trench. In some cases, particularly where steel forms are used, the
inside form is hung by chains from braces across the trench as is shown
in Fig. 129. The form is easily brought to proper grade by adjustment of
the turnbuckles and is then wedged into position to prevent movement
either sideways or upwards during the pouring of the concrete. It may be
necessary to weight the forms down to prevent flotation. Cross bracing
in the trench which interferes with the placing of the form is removed
and the braces are placed against the form until the concrete is poured.
They are removed immediately in advance of the rising concrete.

[Illustration:

FIG. 129.—Blaw Standard Half Round Sewer Form, Suspended from Overhead
Support.

Courtesy, Blaw Steel Form Co.
]

The sewer section may be built as a monolith, in two parts, or in three
parts. In casting the sewer as a monolith the complete full round inside
form is fixed in place by concrete blocks and wires. The full round
outside form is completed as far as possible without interfering too
much with the placing and tamping of the concrete. The concrete is
poured from the top, being kept at the same height on each side of the
form, and tamped while being poured. The remaining panels of the outside
form are placed in position as the concrete rises to them. An opening is
left at the top of the outside arch forms which is of such a width that
the concrete will stand without support. The casting of sewers as a
monolith is difficult and is usually undesirable because of the
uncertainty of the quality of the work. It has the advantage, however,
of eliminating longitudinal working joints in the sewers which may allow
the entrance of water or act as a line of weakness.

[Illustration:

FIG. 130.—Construction Joints for Concrete Sewers.
]

If the sewer is to be cast in two sections the invert is poured to the
springing line or higher. A triangular or rectangular timber is set in
the top of the wet concrete as shown in Fig. 130. When the concrete has
set the timber is removed and the groove thus left forms a working joint
with the arch. After the invert concrete has set, the arch centering is
placed and the arch is completed. This is the most common method for the
construction of medium-sized circular sewers.

Large sewers with relatively flat bottoms are poured in two or three
sections. First the invert is poured without forms and is shaped with a
screed. About 6 inches of vertical wall is poured at the same time. This
acts as a support for the side-wall forms. The side walls reach to the
springing line of the arch and are poured after the invert has set. At
the third pouring the arch is completed. The sewer shown in Fig. 1 is
being poured in two steps, as the side walls are so low that they are
poured at the same time as the invert. A transverse working joint
similar to one of the types used in Fig. 130 is set between each day’s
work.

The length of the form used and the capacity of the plant should be
adjusted so that one complete unit of invert, side wall, or arch can be
poured in one operation. The forms are left in place until the concrete
has set. Invert and side-wall forms are generally left in position for
at least two days, and in cold weather longer. The arch forms are left
in place for double this time. For example if 20 feet of invert and arch
can be poured in a day, 60 feet of invert form and 100 feet of arch form
will be required. As the forms are released they must be moved forward
through those in place. For this reason collapsible or demountable forms
are necessary and steel forms are advantageous. Wooden arch forms are
sometimes dismantled and carried forward in sections, but are preferably
designed to collapse as shown in Fig. 131, so that they can be pulled
through on rollers or a carriage.

=189. Construction in Tunnels.=—In tunnels the invert and side walls are
constructed in the same manner as for open cut work. The tunneling,
which acts as the outside form, is concreted permanently in place. The
concreting of a tunnel by hand is shown in Fig. 132. If the work is to
be done by hand the concrete is thrown in between the ribs of the arch
centering and behind the plates or lagging, which are set in advance of
the rising concrete. The lagging plates are 5 feet long which makes it
possible to throw the concrete in place at the arch, and to tamp it in
place from the end. A bulkhead and a well-greased joint timber are
placed in position as the concrete rises.

[Illustration:

FIG. 131.—Section through a Collapsible Wood Form.
]

Pneumatic transmission of concrete is also used for filling the arch
forms as well as the side walls and invert forms. In using this method
the mixer may be placed at the surface or at the bottom of the shaft or
other convenient permanent location which may be some distance from the
form. The mixture is discharged into a pipe line through which it is
blown by air to the forms. The starting pressure of about 80 pounds per
square inch can be reduced after flow has commenced. In constructing the
St. Louis Water Works tunnel the compressor equipment for moving the
concrete had a capacity of 1,600 cubic feet per minute at a pressure of
110 pounds. The tunnel is horseshoe shaped, 8 feet in height and with
walls varying from 9 to 20 inches in thickness. The extreme travel of
the concrete was 1,100 feet in an 8 inch pipe. The amount of air
consumed at 110 pounds varied from 1.2 to 1.7 cubic feet of free air per
linear foot of pipe. By the time the batch had been discharged the
pressure had reduced to 25 to 40 pounds, depending on the length of the
pipe. It is reported that a 6–inch pipe line would probably have given
better results.

[Illustration:

FIG. 132.—Ogier’s Run Intercepting Storm-Water Drain, Baltimore,
Maryland.

Placing concrete in Arch. The steel lagging of the forms is carried up
in sections as the concrete is deposited. The drain is horseshoe
shaped, and is 12 feet 3 inches high and 12 feet 3 inches wide.
]

The end of the concrete conveying pipe is provided with a flexible joint
the simplest form of which can be made by slipping a section of pipe of
larger diameter over the end of the transmission line. The concrete is
deposited directly on the invert or into the side-wall forms and can be
blown into the arch forms for 20 to 25 feet.

=190. Materials for Forms.=—The materials used in forms for concrete
sewers are: wood, wood with steel lining, and steel alone. The first
cost of wood forms is lower than that of steel but their life is
relatively short. If the forms are to be used a number of times steel is
more economical. With proper care and repairs steel forms will outlast
any other material. Because of the increasing price of lumber and
improvements in steel forms, wood forms are not frequently used. A
common type of specification under which forms are used is:

The material of the forms shall be of sufficient thickness and the
frames holding the forms shall be of sufficient strength so that
the forms shall be unyielding during the process of filling. The
face of the form next to the concrete shall be smooth. If wooden
forms are used the planking forming the lining shall invariably be
fastened to the studding in horizontal lines, the ends of these
planks shall be neatly butted against each other, and the inner
surface of the form shall be as nearly as possible perfectly
smooth, without crevices or offsets between the ends of adjacent
planks. Where forms are used a second time, they shall be freshly
jointed so as to make a perfectly smooth finish to the concrete.
All forms shall be water-tight and shall be wetted before using.

Any material in contact with wet concrete should be oiled or greased
beforehand in order to prevent adherence to the concrete.

=191. Design of Forms.=—The design of forms for reinforced concrete work
requires some knowledge of the strength of materials and the theories of
beams, columns, and arches. Forms can be constructed without such
knowledge but that they will be both economical and adequate is an
improbability. The ordinary beam and column formulas are applicable to
the design of forms. The maximum bending moment for sheeting and ribs is
taken as (_wl_^2)⁄8, where _w_ is the load per unit length, and _l_ is
the length between supports. Sanford Thompson recommends that the
deflection be calculated as (_wl_^3)⁄(128_EI_), in which _E_ is the
modulus of elasticity of the material, and _I_ is the moment of inertia
of the cross-section referred to the neutral axis. The horizontal
pressure of the concrete against the forms has been expressed
empirically by E. B. Smith,[101] as

_P_ = _H_^{0.2}_R_^{0.3} + 120_C_ − 0.3_S_

in which _P_ = lateral pressure in pounds per square inch;

_R_ = rate of filling forms in feet per hour;

_H_ = head of fill. Ordinarily taken as ½_R_, but in cold
weather or when continuously agitated it may be as
high as ¾_R_;

_C_ = ratio, by volume, of cement to aggregate;

_S_ = consistency in inches of slump.

Earlier investigators have usually concluded that the pressures were
equal to those caused by a liquid weighing 144 pounds per cubic foot,
but the tests of the United States Bureau of Public Roads, from which
the above formula was devised, show the pressures to be decidedly below
this amount under certain conditions.

[Illustration:

FIG. 133.—Centering for Large Forms.
]

With these units and formulas the design of the lagging becomes a matter
of substitution in, and the solution of, the equations produced.[102]
The forces acting on the ribs are indeterminate. No more satisfactory
design can be made for the ribs than to follow successful practice, or
what is seldom done, to determine the stresses in the forms by the
application of one of the theories for the solution of arch stresses.
The sizes of the lumber used in the ribs varies from 1½ × 6 inches to 2
× 10 inches, depending on the size of the sewer. If vertical posts are
used at the ends to support the arch forms they are computed as columns
taking the full weight of the arch. If the span is so wide that radial
supports are used as shown in Fig. 133 the load at the center is assumed
as one-fourth of the weight of the arch.

=192. Wooden Forms.=—Norway and Southern pine, spruce, and fir are
satisfactory for form construction. White pine is satisfactory but is
generally too expensive. The hard woods are too difficult to work. The
lumber should be only partly dried as kiln-dried lumber swells too much
when it is moistened, warping the forms out of shape or crushing the
lagging at the joints. Green lumber must be kept moist constantly to
prevent warping before use and when it is used it does not swell enough
to close the cracks. The lumber should be dressed on the face next to
the concrete and at the ends. Either beveled or matched lumber may be
used for lagging. The joint made by beveled lumber shown in Fig. 134 is
cheaper but less satisfactory than a tongued and grooved joint.

[Illustration:

FIG. 134.—Beveled Joint for Wood Fords.
]

[Illustration:

FIG. 135.—Collapsible Wooden Invert Form for Concrete Sewers.
]

[Illustration:

FIG. 136.—Support for Arch Centering.
]

[Illustration:

FIG. 137.—Wooden Forms Used in Tunnel, North Shore Sewer, Sanitary
District of Chicago.

Journal Western Society of Engineers, Vol. 22, p. 385.
]

Types of wooden forms are shown in Figs. 135 and 136 for use in sewers
to be built as monoliths or in two portions. Fig. 137 shows the details
of a built-up wooden form used in tunnel work for a 42½ inch egg-shaped
sewer.

=193. Steel-lined Wooden Forms.=—Sheet metal linings are sometimes used
on wooden forms. They permit the use of cheaper undressed lumber, demand
less care in the joining of the lagging, and when in good condition give
a smooth surface to the finished concrete. Their use has frequently been
found unsatisfactory and more expensive than well-constructed wooden
forms because of the difficulty of preventing warping and crinkling of
the metal lining and in keeping the ends fastened down so that they will
not curl. Sheet steel or iron of No. 18 or 20 gage (0.05 to 0.0375 of an
inch) weighing 2 to 1½ pounds per square foot is ordinarily used for the
lining.

[Illustration:

FIG. 138.—Blaw Standard Full Round Telescopic Sewer Forms, Showing
Knocked-Down Sections Loaded on a Truck.

Courtesy, Blaw Steel Form Co.
]

=194. Steel Forms.=—These are simple, light, durable, and easy to
handle. The engineer is seldom called upon to design these forms as the
types most frequently used are manufactured by the patentees and are
furnished to the contractor at a fixed rental per foot of form,
exclusive of freight and hauling from the point of manufacture. The
forms can be made in any shape desired, the ordinary stock shapes such
as the circular forms being the least expensive. The smaller circular
forms are adjustable within about 3 inches to different diameters so
that the same form can be used for two sizes of sewers. The same form
can be used for arch and invert in circular sewers. Fig. 138 shows the
collapsible circular forms and the manner in which they are pulled
through those still in position. Fig. 129 shows a half round steel form
swung in position by chains and turnbuckles from the trench bracing, and
Fig. 139 shows the free unobstructed working space in the interior of
some large steel forms.

[Illustration:

FIG. 139.—Interior of Steel Forms for Calumet Sewer, Chicago.

Sewer is 16 feet wide. Note absence of obstructions. Courtesy,
Hydraulic Steelcraft Co.
]

=195. Reinforcement.=—It is essential that the reinforcement be held
firmly in place during the pouring of the concrete. A section of
reinforcement misplaced during construction may serve no useful purpose
and result in the collapse of the sewer. In sewer construction a few
longitudinal bars may be laid in order that the transverse bars may be
wired to them and held in position by notches in the centering and in
fastenings to bars protruding from the finished work. This construction
is shown in Fig. 1. The network of reinforcement is held up from the
bottom of the trench by notched boards which are removed as the concrete
reaches them, or better by stones or concrete blocks which are concreted
in. Sometimes the reinforcement is laid on top of the freshly poured
portion of the concrete the surface of which is at the proper distance
from the finished face of the work. This method has the advantage of not
requiring any special support for the reinforcement, but it is
undesirable because of the resulting irregularity in the reinforcement
spacing and position.

In the side walls the position of the reinforcement is fixed by wires or
metal strips which are fastened to the outside forms or to stakes driven
into the ground. Wires are then fastened to the reinforcement bars and
are drawn through holes in the forms and twisted tight. When the forms
are removed the wires or strips are cut leaving a short portion
protruding from the face of the wall. The reinforcing steel from the
invert should protrude into the arch or the side walls for a distance of
about 40 diameters in order to provide good bond between the sections.
The protruding ends are used as fastenings for the new reinforcement.
The arch steel may be supported above the forms by specially designed
metal supports, by small stones or concrete blocks which are concreted
into the finished work; or by notched strips of wood which are removed
as the concrete approaches them. Strips of wood are not satisfactory
because they are sometimes carelessly left in place in the concrete
resulting in a line of weakness in the structure. Metal chairs are the
most secure supports. They are fastened to the forms and the bars are
wired to the chairs. In some instances the entire reinforcement has been
formed of one or two bars which are fastened into position as a complete
ring. This results in a better bond in the reinforcement, requires less
fastening and trouble in handling, but is in the way during the pouring
of the concrete and interferes with the handling of the forms.

=196. Costs of Concrete Sewers.=—Under present day conditions a general
statement of the costs of an engineering structure can not be given with
accuracy. Only the items of labor, materials, and transportation that go
to make up the cost can be estimated quantitively, and the total cost
computed by multiplying the amount of each item by its proper unit cost
obtained from the market quotations.

A summary of some of the items that go to make up the cost of a concrete
sewer and the relative amount of these items on different jobs is given
in Tables 69 and 70.

TABLE 69

DIVISION OF LABOR COSTS FOR THE CONSTRUCTION OF 96–INCH CIRCULAR
CONCRETE SEWER

─────────────────────────────────────────╥─────────────────────────────
Classification of Labor ║ Classification of Work
─────────────────┬───────────┬───────────╫─────────────────┬───────────
Task or Title │ │ Total ║ Type of Work │
│ Number of │dollars per║ │Dollars per
│ men │ day ║ │ foot
─────────────────┼───────────┼───────────╫─────────────────┼───────────
Superintendent │ 1│ 6.00║Excavation │ 1.80
Engineman │ │ ║Sheeting and │
│ 1│ 3.50║ bracing │ 0.58
Hoister │ │ ║Bottom plank │
(engineman) │ 1│ 2.00║ │ 0.17
Tag-men │ 2│ 3.30║Pulling sheeting │ 0.45
Earth diggers │ 10│ 16.50║Backfilling │ 0.33
On dump cars │ │ ║Making and │
│ 2│ 3.30║ placing invert │ 1.17
Carpenter on │ │ ║Making and │
bracing │ 2│ 3.00║ placing arch │ 1.54
Carpenters’ │ │ ║Laying brick in │
helpers │ 2│ 3.30║ invert │ 0.29
Laying bottom │ 2│ 3.30║ │
Moving pumps, │ │ ║Bending and │
etc. │ │ ║ placing steel │
│ 2│ 3.30║ in arch │ 0.20
Pulling sheeting │ 3│ 5.25║ │
Mixing and │ │ ║Bending and │
placing │ │ ║ placing steel │
concrete │ 16│ 26.40║ in invert │ 0.09
On steel forms │ │ ║Moving forms and │
│ 3│ 5.25║ centers │ 0.62
Water boy │ │ ║Watchmen, water │
│ 1│ 1.00║ boy, etc. │ 0.62
Coal and oil │ │ 5.00║ │
│ │ —————║ │ —————
Total │ │ 90.40║ Total │ 7.86
─────────────────┴───────────┴───────────╨─────────────────┴───────────
NOTES.—Trench was 12½ feet wide and of various depths. At depth of 12
feet the cost of excavation was $1.61 per foot. From Engineering and
Contracting, Vol. 47, p. 157.

BACKFILLING

=197. Methods.=—Careful backfilling is necessary to prevent the
displacement of the newly laid pipe and to avoid subsequent settlement
at the surface resulting in uneven street surfaces and dangers to
foundations and other structures.

The backfilling should commence as soon as the cement in the joints or
in the sewer has obtained its initial set. Clay, sand, rock dust, or
other fine compactible material is then packed by hand under and around
the pipe and rammed with a shovel and light tamper. This method of
filling is continued up to the top of the pipe. The backfill should rise
evenly on both sides of the pipe and tamping should be continuous during
the placing of the backfill. For the next 2 feet of depth the backfill
should be placed with a shovel so as not to disturb the pipe, and should
be tamped while being placed, but no tamping should be done within 6
inches of the crown of the sewer. The tamping should become
progressively heavier as the depth of the backfill increases. Generally
one man tamping is provided for each man shoveling.

TABLE 70

DIVISION OF COSTS FOR THE CONSTRUCTION OF CONCRETE SEWERS

Gillette’s Handbook of Cost Data.
──────────────┬────────────────────────────────────────────────────────
Item │ Location
──────────────┼─────────┬────────┬──────────┬──────────────────────────
│ Fond du │ South │ │
│ Lac │ Bend │Wilmington│ Richmond, Indiana
──────────────┼─────────┼────────┼──────────┼────────┬────────┬────────
Diameter in │ │ │ │ │ │
inches │ 30 │ 66 │ 53 │ 54 │ 48 │ 42
Shape │circular │circular│horseshoe │circular│circular│circular
Plain or │ │ │ │ │ │
reinforced │ plain │ rein. │ rein. │ rein. │ rein. │ rein.
Cubic yards │ │ │ │ │ │
per foot │ 0.11 │ 0.594 │ 0.37 │5″ shell│5″ shell│4″ shell
Daily │ │ │ │ │ │
progress, │ │ │ │ │ │
feet │ 47 │24 to 36│ │ │ │
Cost per foot,│ │ │ │ │ │
dollars │ 1.20 │ 4.40 │ 2.97 │ 1.35 │ 1.08 │ 0.91
Per cent of │ │ │ │
total cost: │ │ │ │
Labor │39.0[103]│ 33.5 │ 33.0 │ =17.1=
Tools │ 1.5 │ 11.5 │ │
Sand and │ │ │ │
gravel │ 12.4 │ 15.5 │ 18.9 │ =19.3=
Lumber │ 0.9 │ │ │
Water │ 0.7 │ 11.5 │ │
Reinforcing │ 0.0 │ │ 14.5 │ =22.3=
Cement │ 23.0 │ 20.0 │ 27.5 │ =32.0[104]=
Frost │ │ │ │
prevention│ 2.0 │ │ │
Forms │ 12.5 │ 8.0 │ 6.1 │ =9.3=
Engineering │ 8.0 │ │ │
Length of day,│ │ │ │
hours │ 8 │ 10 │ │
Year of │ │ │
construction│ 1908 │ 1906 │ Pre-war conditions
──────────────┴─────────┴────────┴─────────────────────────────────────

Above a point 2 feet above the top of the sewer the method pursued and
the care observed in backfilling will depend on the character of the
backfilling material and the location of the sewer. If the sewer is in a
paved street the backfill is spread in layers 6 inches thick and tamped
with rammers weighing about 40 pounds with a surface of about 30 square
inches. One man tamping for each man shoveling is frequently specified.
If no pavement is to be laid but it is required that the finished
surface shall be smooth, slightly less care need be taken and only one
man tamping is specified for each two men shoveling. On paved streets a
reinforced concrete slab with a bearing of at least 12 inches on the
undisturbed sides of the trench may be designed to support the pavement
and its loads. This is of great help in preventing the unsightly
appearance and roughness due to an improperly backfilled trench. On
unpaved streets the backfill is crowned over the trench to a depth of
about 6 inches and then rolled smooth by a road roller. In open fields,
in side ditches, or in locations where obstruction to traffic or
unsightliness need not be considered, after the first 2 feet of backfill
have been placed with proper care, the remainder is scraped or thrown
into the trench by hand or machine, care being taken not to drop the
material so far as to disturb the sewer.

If the top of the sewer, manhole, or other structure comes close to or
above the surface of the ground, an earth embankment should be built at
least 3 feet thick over and around the structure. The embankment should
have side slopes of at least 1½ on 1 and should be tamped to a smooth
and even finish.

If sheeting is to be withdrawn from the trench it should be withdrawn
immediately ahead of the backfilling, and in trenches subject to caving
it may be pulled as the backfilling rises.

Puddling is a process of backfilling in which the trench is filled with
water before the filling material is thrown in. It avoids the necessity
for tamping and can be used satisfactorily with materials that will
drain well and will not shrink on drying. Sand and gravel are suitable
materials for puddling, heavy clay is unsatisfactory. Puddling should
not be resorted to before the first 2 feet of backfill has been
carefully placed. More compact work can be obtained by tamping than with
puddling.

Frozen earth, rubbish, old lumber, and similar materials should not be
used where a permanent finished surface is desired as these will
decompose or soften resulting in settlement. Rocks may be thrown in the
backfill if not dropped too far and the earth is carefully tamped around
and over them. In rock trenches fine materials such as loam, clay, sand,
etc., must be provided for the backfilling of the first portion of the
trench for 2 feet over the top of the pipe. More clay can generally be
packed in an excavation than was taken out of it, but sand and gravel
occupy more space than originally even when carefully tamped.

Tamping machines have not come into general use. One type of machine
sometimes used consists of a gasoline engine which raises and drops a
weighted rod. The rod can be swung back and forth across the trench
while the apparatus is being pushed along. It is claimed that two men
operating the machine can do the work of six to ten men tamping by hand.
The machine delivers 50 to 60 blows per minute, with a 2 foot drop of
the 80 to 90 pound tamping head.

Backfilling in tunnels is usually difficult because of the small space
available in which to work. Ordinarily the timbering is left in place
and concrete is thrown in from the end of the pipe between the outside
of the pipe and the tunnel walls and roof. If vitrified pipe is used in
the tunnel, the backfilling is done with selected clayey material which
is packed into place around the pipe by workmen with long tamping tools.
The backfilling should be done with care under the supervision of a
vigilant inspector in order that subsequent settlement of the surface
may be prevented.

CHAPTER XII
MAINTENANCE OF SEWERS

=198. Work Involved.=—The principal effort in maintaining sewers is to
keep them clean and unobstructed. A sewerage system, although buried,
cannot be forgotten as it will not care for itself, but becoming clogged
will force itself on the attention of the community. Besides the
cleaning and repairing of sewers and the making of inspections for
determining the necessity for this work, ordinances should be prepared
and enforced for the purpose of protecting the sewers from abuse.
Inspections to determine the amount of the depreciation of sewers with a
view towards possible renewal, or to determine the capacity of a sewer
in relation to the load imposed upon it are sometimes necessary. The
valuation of the sewerage system as an item in the inventory of city
property may be assigned to the engineer in charge of sewer maintenance.

The work involved in the inspection and cleaning of sewers in New York
City for the year ending May, 1914, included the removal of 22,687 cubic
yards of material from catch-basins, and 14,826 catch-basin cleanings.
This made an average of two and one-half cleanings per catch-basin per
year, or 1½ cubic yards removed at each cleaning. The 6,432 catch-basins
were inspected 71,890 times. There were 4,112 cubic yards of material
removed from 517 miles of sewers, or about 8 cubic yards per mile.
Inspection of 194 miles of brick sewers were made, 4.4 miles were
flushed, and 27 miles were cleaned. Inspections of 198 miles of pipe
sewers were made, 80 miles were examined more closely, 37 miles were
flushed, and 91 miles were cleaned. The field organization for this work
consisted of 17 foremen, 8 assistant foremen, 29 laborers, 71 cleaners,
13 mechanics, 7 inspectors of construction, 3 inspectors of sewer
connections, 13 horses and wagons, and 28 horses and carts.[105]

=199. Causes of Troubles.=—The complaints most frequently received about
sewers are caused by clogging, breakage of pipes, and bad odors. Sewers
become clogged by the deposition of sand and other detritus which
results in the formation of pools in which organic matter deposits,
aggravating the clogged condition of the sewers and causing the odors
complained of. Grease is a prolific cause of trouble. It is discharged
into the sewer in hot wastes, and becoming cooled, deposits in thick
layers which may effectively block the sewer if not removed. It can be
prevented from entering the sewers by the installation of grease traps
as described in Chapter VI. The periodic cleaning of these traps is as
important as their installation.

Tree roots are troublesome, particularly in small pipe sewers in
residential districts. Roots of the North Carolina poplar, silver leaf
poplar, willow, elm, and other trees will enter the sewer through minute
holes and may fill the sewer barrel completely if not cut away in time.
Fungus growths occasionally cause trouble in sewers by forming a network
of tendrils that catches floating objects and builds a barricade across
the sewer. Difficulties from fungus growths are not common, but constant
attention must be given to the removal of grit, grease, and roots. Tarry
deposits from gas-manufacturing plants are occasionally a cause of
trouble, as they cement the detritus already deposited into a tough and
gummy mass that clings tenaciously to the sewer.

Broken sewers are caused by excessive superimposed loads, undermining,
and progressive deterioration. The changing character of a district may
result in a change of street grade, an increase in the weight of
traffic, or in the construction of other structures causing loads upon
the sewer for which it was not designed. The presence of corrosive acids
or gases may cause the deterioration of the material of the sewer.

=200. Inspection.=—The maintenance of a sewerage system is usually
placed under the direction of a sewer department. In the organization of
the work of this department no regular routine of inspection of all
sewers need be followed ordinarily. Attention should be given regularly
to those sewers that are known to give trouble, whereas the less
troublesome sewers need not be inspected more frequently than once a
year, preferably during the winter when labor is easier to obtain.

The routine inspection of sewers too small to enter is made by an
examination at the manhole. If the water is running as freely at one
manhole as at the next manhole above, it is assumed that the sewer
between the manholes is clean and no further inspection need be given
unless there is some other reason to suspect clogging between manholes.
If the sewage is backed up in a manhole it indicates that there is an
obstruction in the sewer below. If the sewage in a manhole is flowing
sluggishly and is covered with scum it is an indication of clogging,
slow velocity and septic action in the sewer. Sludge banks on the
sloping bottom of the manhole or signs of sewage high upon the walls
indicate an occasional flooding of the sewer due to inadequate capacity
or clogging.

[Illustration:

FIG. 140.—Inspecting Sewers with Reflected Sunlight.
]

If any of the signs observed indicate that the sewer is clogged, the
manhole should be entered and the sewer more carefully inspected. Such
inspection may be made with the aid of mirrors as shown in Fig. 140 or
with a periscope device as shown in Fig. 141. Sunlight is more brilliant
than the electric lamp shown in Fig. 141, but the mirror in the manhole
directs the sunlight into the eyes of the observer, dazzling him and
preventing a good view of the sides of the sewer. The observers’ eyes
can be protected against the direct rays of the electric light, which
can be projected against the sides of the pipe by proper shades and
reflectors. It is possible with this device to locate house connection,
stoppages, breaks of the pipe, and to determine fairly accurately the
condition of the sewer without discomfort to the observers.

Sewers that are large enough to enter should be inspected by walking
through them where possible. The inspection should be conducted by
cleaning off the sewer surface in spots with a small broom, and
examining the brick wall for loose bricks, loose cement or cement lost
from the joints, open joints, broken bond, eroded invert, and such other
items as may cause trouble. An inspection in storm sewers is sometimes
of value in detecting the presence of forbidden house connections.

[Illustration:

FIG. 141.—Inspecting Sewers with Periscope and Electric Light. The G-K
System.
]

Certain precautions should be taken before entering sewers or manholes.
If a distinct odor of gasoline is evident the sewer should be ventilated
as well as possible by leaving a number of manhole covers open along the
line until the odor of gasoline has disappeared. The strength of
gasoline odor above which it is unsafe to enter a sewer is a matter of
experience possessed by few. A slight odor of gasoline is evident in
many sewers and indicates no special danger. A discussion of the amount
of gasoline necessary to create explosive conditions is given in Art.
206. In making observations of the odor it should also be noted whether
air is entering or leaving the manhole. The presence of gasoline cannot
be detected at a manhole into which air is entering.

As soon as it is considered that the odors from a sewer indicate the
absence of an explosive mixture, a lighted lantern or other open flame
should be lowered into the manhole to test the presence of oxygen.
Carbon monoxide or other asphyxiating gases may accumulate in the sewer,
and if present will extinguish the flame. If the flame burns brilliantly
the sewer is probably safe to enter, but if conditions are unknown or
uncertain, the man entering should wear a life belt attached to a rope
and tended by a man at the surface. Asphyxiating or explosive gases are
sometimes run into without warning due to their lack of odor, or the
presence of stronger odors in the sewer. Breathing masks and electric
lamps are precautions against these dangers, the masks being ready for
use only when actually needed. More deaths have occurred in sewers due
to asphyxiating gases than by explosions, as the average sewer explosion
is of insufficient violence to do great damage, although on occasion,
extremely violent explosions have occurred. During inspections of sewers
there should always be at least one man at the surface to call help in
case of accident and the inspecting party should consist of at least two
men.

It must not be felt that entering sewers is fraught with great danger,
as it is perfectly safe to enter the average sewer. The air is not
unpleasant and no discomfort is felt, but conditions are such that
unexpected situations may arise for which the man in the sewer should be
prepared. It is therefore wise to take certain precautions. These may
indicate to the uninitiated, a greater danger than actually exists.

The inspection of sewers should include the inspection of the
flush-tanks, control devices, grit chambers, and other appurtenances. A
common difficulty found with flush-tanks is that the tank is “drooling,”
that is to say the water is trickling out of the siphon as fast as it is
entering the tank, and the intermittency of the discharge has ceased.
If, when the tank is first inspected the water is about at the level of
the top of the bell it is probable that the siphon is drooling. A mark
should be made at the elevation of the water surface and the tank
inspected again in the course of an hour or more. If the water level is
unchanged the siphon is drooling. This may be caused by the clogging of
the snift hole or by a rag or other obstacle hanging over the siphon
which permits water to pass before the air has been exhausted, or a
misplacement of the cap over the siphon, or other difficulty which may
be recognized when the principle on which the siphon operates is
understood. Occasionally it is discovered that an over zealous water
department has shut off the service.

Control devices, such as leaping or overflow weirs, automatic valves,
etc., may become clogged and cease to operate satisfactorily. They
should be inspected frequently, dependent upon their importance and the
frequency with which they have been found to be inoperative. An
inspection will reveal the obstacle which should be removed. Floats
should be examined for loss of buoyancy or leaks rendering them useless.
Grit and screen chambers should be examined for sludge deposits.

Catch-basins on storm sewers are a frequent cause of trouble and need
more or less frequent cleaning. Cleanings are more important than
inspections for catch-basins for if they are operating properly they are
usually in need of cleaning after every storm of any magnitude, and a
regular schedule of cleaning should be maintained.

A record should be kept of all inspections made. It should include an
account of the inspection, its date, the conditions found, by whom made
and the remedies taken to effect repairs.

=201. Repairs.=—Common repairs to sewerage systems consist in replacing
street inlets or catch-basin covers broken by traffic; raising or
lowering catch-basin or manhole heads to compensate for the sinking of
the manhole or the wear of the pavement; replacing of broken pipes,
loosened bricks or mortar which has dropped out; and other miscellaneous
repairs as the necessity may arise. Connections from private drains are
a source of trouble because either the sewer or the drain has broken due
to careless work or the settlement of the foundation or the backfill.

=202. Cleaning Sewers.=—Sewers too small to enter are cleaned by
thrusting rods or by dragging through them some one of the various
instruments available. The common sewer rod shown in Fig. 142 is a
hickory stick, or light metal rod, 3 or 4 feet long, on the end of which
is a coupling which cannot come undone in the sewer. Sections of the rod
are joined in the manhole and pushed down the sewer until the
obstruction is reached and dislodged. Occasionally pieces of pipe
screwed together are used with success. The end section may be fitted
with a special cutting shoe for dislodging obstructions. In extreme
cases these rods may be pushed 400 to 500 feet, but are more effective
at shorter distances. Obstructions may be dislodged by shoving a fire
hose, which is discharging water under high pressure through a small
nozzle, down the sewer toward the obstruction. The water pressure
stiffens the hose, which, together with the support from the sides of
the conduit, make it possible to push the hose in for effective work 100
feet or more from the manhole. A strip of flexible steel about ½ inch
thick and 1½ to 2 inches wide is useful for “rodding” a short length of
crooked sewer.

[Illustration:

FIG. 142.—Sewer Rods
]

Sewers are seldom so clogged that no channel whatever remains. As a
sewer becomes more and more clogged, the passage becomes smaller,
thereby increasing the velocity of flow of the sewage around the
obstruction and maintaining a passageway by erosion. This phenomenon has
been taken advantage of in the cleaning of sewers by “pills.” These
consist of a series of light hollow balls varying in size. One of the
smaller balls is put into the sewer at a manhole. When the ball strikes
an obstruction it is caught and jammed against the roof of the sewer.
The sewage is backed up and seeks an outlet around the ball, thus
clearing a channel and washing the ball along with it. The ball is
caught at the next manhole below. A net should be placed for catching
the ball and a small dam to prevent the dislodged detritus from passing
down into the next length of pipe. The feeding of the balls into the
sewer is continued, using larger and larger sizes, until the sewer is
clean. This method is particularly useful for the removal of sludge
deposits, but it is not effective against roots and grease. The balls
should be sufficiently light to float. Hollow metal balls are better
than heavier wooden ones.

[Illustration:

FIG. 143.—Cable and Windlass Method of Cleaning Sewers.

The cable is held to the bottom of the sewer by bracing a 2 x 4
upright in the sewer, with a snatch block attached. A trailer is
attached to the scoop to prevent loss of material.
]

Plows and other scraping instruments are dragged through pipe sewers to
loosen banks of sludge and detritus and to cut roots or dislodge
obstructions. One form of plow consists of a scoop[106] similar to a
grocer’s sugar scoop, which is pushed or dragged up a sewer against the
direction of flow. As fast as the scoop is filled it is drawn back and
emptied. The method of dragging this through a sewer is indicated in
Fig. 143. At Atlantic City the crew operating the scoop comprises five
men, two are at work in each manhole and one on the surface to warn
traffic and wait on the men in the manholes. The outfit of tools is
contained in a hand-drawn tool box and includes sewer rods, metal scoops
for all sizes of sewers, picks, shovels, hatchets, chisels, lanterns,
grease and root cutters, etc., and two winches with from 400 to 600 feet
of ⅜-inch wire cable.

[Illustration:

FIG. 144.—Sewer Cleaning Device.

Eng. News, Vol. 42, 1899, p. 328.
]

[Illustration:

FIG. 145.—Tools for Cleaning Sewers.
]

[Illustration:

FIG. 146.—Turbine Sewer Machine Connected to Forcing Jack.

The forcing jack is used when windlass and cable cannot be used.

Courtesy, The Turbine Sewer Machine Co.
]

Another form of plow or drag consists of a set of hooks or teeth hinged
to a central bar as shown in Fig. 144. A root cutter and grease scraper
in the form of a spiral spring with sharpened edges, and other tools for
cleaning sewers are shown in Fig. 145. A turbine sewer cleaner shown in
Fig. 146 consists of a set of cutting blades which are revolved by a
hydraulic motor of about 3 horse-power under an operating pressure of
about 60 pounds per square inch. The turbine is attached to a standard
fire hose and is pushed through the sewer by utilizing the stiffness of
the hose, or by rods attached to a pushing jack as shown in the figure.
This machine was invented and patented by W. A. Stevenson in 1914. Its
performance is excellent. The blades revolve at about 600 R.P.M.,
cutting roots and grease. The revolving blades and the escaping water
also serve to loosen and stir up the deposits and the forward helical
motion imparted to the water is useful in pushing the material ahead of
the machine and in scrubbing the walls of the sewer. In Milwaukee four
men with the machine cleaned 319 feet of 12–inch sewer in 16 hours, and
in Kansas City 7,801 feet of sewers were cleaned in 14 days.

Sewers large enough to enter may be cleaned by hand. The materials to be
removed are shoveled into buckets which are carried or floated to
manholes, raised to the surface and dumped. In very large sewers
temporary tracks have been laid and small cars pushed to the manhole for
the removal of the material. Hydraulic sand ejectors may also be used
for the removal of deposits, similar to the steam ejector pump shown in
Fig. 97. The water enters the apparatus at high velocity, under a
pressure of about 60 pounds per square inch, leaps a gap in the machine
from a nozzle to a funnel-shaped guide leading to the discharge pipe.
The suction pipe of the machine leads to the chamber in which the leap
is made. In leaping this gap the water creates a vacuum that is
sufficient to remove the uncemented detritus large enough to pass
through the machine, and will lift small stones to a height of 10 to 12
feet. Occasionally barricades of logs, tree branches, rope, leaves, and
other obstructions which have piled up against some inward projecting
portion of the sewer, must be removed by hand either by cutting with an
axe or by pulling them out. Projections from the sides of sewers are
objectionable because of their tendency to catch obstacles and form
barricades.

Little authentic information on the cost of cleaning sewers is
available. A permanent sewer organization is maintained by many cities.
The division of their time between repairs, cleaning, and other duties
is seldom made a matter of record. From data published in Public
Works[107] it is probable that the cost varies from $3 to $15 per cubic
yard of material removed. From the information in Vol. II of “American
Sewerage Practice” by Metcalf and Eddy the combined cost of cleaning and
flushing will vary between $10 and $40 per mile; the expense of either
flushing or cleaning alone being about one-half of this.

=203. Flushing Sewers.=—Sewers can sometimes be cleaned or kept clean by
flushing. Flushing may be automatic and frequent, or hand flushing may
be resorted to at intervals to remove accumulated deposits. Automatic
flush-tanks, flushing manholes, a fire hose, a connection to a water
main, a temporary fixed dam, a moving dam, and other methods are used in
flushing sewers. The design, operation, and results obtained from the
use of automatic flush-tanks and flushing manholes are discussed in
Chapter VI.

The method in use for cleaning a sewer by thrusting a fire hose down it
can also be used for flushing sewers. It is an inexpensive and fairly
satisfactory method. There is, however, some danger of displacing the
sewer pipe because of the high velocity of the water. An easier and
safer but less effective method is to allow water to enter at the
manhole and flow down the sewer by gravity. Direct connections to the
water mains are sometimes opened for the same purpose.

Sewers are sometimes flushed by the construction of a temporary dam
across the sewer, causing the sewage to back up. When the sewer is half
to three-quarters full the dam is suddenly removed and the accumulated
sewage allowed to rush down the sewer, thus flushing it out. The dam may
be made of sand bags, boards fitted to the sewer, or a combination of
boards and bags. The expense of equipment for flushing by this method is
less than that by any other method, but the results obtained are not
always desirable. Below the dam the results compare favorably with those
obtained by other methods, but above the dam the stoppage of the flow of
the sewage may cause depositions of greater quantities of material than
have been flushed out below. A time should be chosen for the application
of this method when the sewage is comparatively weak and free from
suspended matter. The most convenient place for the construction of a
dam is at a manhole in order that the operator may be clear of the rush
of sewage when the dam is removed.

Movable dams or scrapers are useful in cleaning sewers of a moderate
size, but are of little value in small sewers. The scraper fits loosely
against the sides of the sewer and is pushed forward by the pressure of
the sewage accumulated behind it. The iron-shod sides of the dam serve
to scrape grease and growths attached to the sewer and to stir up sand
and sludge deposited on the bottom. The high velocity of the sewage
escaping around the sides of the dam aids in cleaning and scrubbing the
sewer.

A natural watercourse may be diverted into the sewer if topographical
conditions permit, or where sewers discharge into the sea below high
tide a gate may be closed during the flood and held closed until the
ebb. The rush of sewage on the opening of the gate serves to flush the
sewers and stir up the sludge deposited during high tide. Other methods
of flushing sewers may be used dependent on the local conditions and the
ingenuity of the engineer or foreman in charge.

In some sewers it is not necessary to remove the clogging material from
the sewer. It is sufficient to flush and push it along until it is
picked up and carried away by higher velocities caused by steeper grades
or larger amounts of sewage.

=204. Cleaning Catch-basins.=[108]—Catch-basins have no reason for
existence if they are not kept clean. Their purpose is to catch
undesirable settling solids and to prevent them from entering the
sewers, on the theory that it is cheaper to clean a catch-basin than it
is to clean a sewer. If the cleaning of storm sewers below some inlet to
which no catch-basin is attached becomes burdensome, the engineer in
charge of maintenance should install an adequate catch-basin and keep it
clean. Catch-basins are cleaned by hand, suction pumps, and grab
buckets. In cleaning by hand the accumulated water and sludge are
removed by a bucket or dipper and dumped into a wagon from which the
surplus settled water is allowed to run back into the sewer. The grit at
the bottom of the catch-basin is removed by shoveling it into buckets
which are then hoisted to the surface and emptied.

Suction pumps in use for cleaning catch-basins are of the hydraulic
eductor type. The eductor works on the principle of the steam pump shown
in Fig. 97, except that water is used instead of steam. The material
removed may be discharged into settling basins constructed in the
street, or may be discharged directly into wagons.[109] In Chicago a
special motor-driven apparatus is used. This consists of a 5–yard body
on a 5–ton truck, and a centrifugal pump driven by the truck motor. In
use, the truck, about half filled with water, drives up to the
catch-basin, the eductor pipe is lowered and water pumped from the truck
into the eductor and back into the truck again, together with the
contents of the catch-basin. The surplus water drains back into the
sewer. The Chicago Bureau of Sewers reports a truck so equipped to have
cleaned 1013 catch-basins, removing 1763 cubic yards of material, and
running 1380 miles, during the months of August, September and October,
1917. The cost, including all items of depreciation, wages, repairs,
etc., was $1,393.89. Orange-peel buckets, about 20 inches in diameter,
operated by hand or by the motor of a 3½ to 5–ton truck with a
water-tight body, are used for cleaning catch-basins in some cities.

Catch-basins in unpaved streets and on steep sandy slopes should be
cleaned after every storm of consequence. Basins which serve to catch
only the grit from pavement washings require cleaning about two or three
times per year, and from one to three cubic yards of material are
removed at each cleaning. The cost of cleaning ordinary catch-basins by
hand may vary from $15 to $25, but with the use of eductors or
orange-peel buckets the cost is somewhat lower. In Seattle the cost of
cleaning large detritus basins by hand is said[110] to vary from $45 to
$60. With the use of eductors this cost has been reduced to one-third or
one-fifth the cost of cleaning by hand.

=205. Protection of Sewers.=[111]—City ordinances should be wisely drawn
and strictly enforced for the protection of sewers against abuse and
destruction. The requirements of some city ordinances are given in the
following paragraphs.

Washington, D. C.,[112] sewer ordinances provide that:

No person shall make or maintain any connection with any public
sewer or appurtenance thereof whereby there may be conveyed into
the same any hot, suffocating, corrosive, inflammable or explosive
liquid, gas, vapor, substance or material of any kind ... provided
that the provisions of this act shall not apply to water from
ordinary hot water boilers or residences.

The following extracts from the ordinances of Indianapolis are typical
of those from many cities:

2950. No connection shall be made with any public sewer without
the written permission of the Committee on Sewers and the Sewerage
Engineer.

2953. No person shall be authorized to do the work of making
connections until he has furnished a satisfactory certificate that
he is qualified for the duties. He shall also file bond for not
less than $1,000 that he will indemnify the City from all loss or
damage that may result from his work and that he will do the work
in conformity to the rules and regulations established by the City
Council.

2955. It shall be unlawful for any person to allow premises
connected to the sewers or drains to remain without good fixtures
so attached as to allow a sufficiency of water to be applied to
keep the same unobstructed.

2956. No butcher’s offal or garbage, or dead animals, or
obstructions of any kind shall be thrown in any receiving basin or
sewer in penalty not greater than $100. Any person injuring,
breaking, or removing any portion of any receiving basin, manhole
cover, etc., shall be fined not more than $100.

2962. No person shall drain the contents of any cesspool or privy
vault into any sewer without the permission of the Common Council.

The Cleveland ordinances are similar and contain the following in
addition:

1251. Rule 4. All connections with the main or branch sewers shall
be made at the regular connections or junctions built into the
same, except by special permit.

Rule 16. No steam pipe, nor the exhaust, nor the blow off from any
steam engine shall be connected with any sewer.

Evanston, Illinois, protects its sewers against the additions of grease
and other undesirable substances as follows:

1444. It is unlawful for any person to use any sewer or
appurtenance to the sewerage system in any manner contrary to the
orders of the Commissioner of Public Works.

1446. Wastes from any kitchen sinks, floor drains, or other
fixtures likely to contain greasy matter from hotels, certain
apartment houses, boarding houses, restaurants, butcher shops,
packing houses, lard rendering establishments, bakeries,
laundries, cleaning establishments, garages, stables, yard and
floor drains, and drains from gravel roofs shall be made through
intervening receiving basins constructed as prescribed in par.
VIII of this code.

Receiving basins suitable for the work required in the code are
illustrated in Chapter VI.

=206. Explosions in Sewers.=—Disastrous explosions in sewers were first
recorded about 1886.[113] Up to about 1905 explosions were infrequent
and were considered as unavoidable accidents and so rare as to be
unworthy of study. For a decade or more after 1905 explosions occurred
with increasing violence and frequency causing destruction of property,
but by some freakish chance, but little loss of life. A violent and
destructive explosion occurred in Pittsburgh on Nov. 25, 1913,[114] and
another on March 12, 1916. The property damage amounted to $300,000 to
$500,000 on each occasion, but there was no loss of life. Two miles of
pavement were ripped up, gas, water, and other sewer pipes were broken,
buildings collapsed and the streets were flooded. The streets were
rendered unserviceable for long periods during the expensive repairs
that were necessary. In recent years the number of explosions in sewers
has been smaller, due probably to the gain in knowledge of the causes
and intelligent methods of prevention.

The three principal causes of explosions in sewers are: gasoline vapor,
illuminating gas, and calcium carbide. It is probable that gasoline
vapor is by far the most troublesome. Explosions caused by these gases
are not so violent as those caused by dynamite or other high explosives,
as the volume of gas and the temperature generated are much less. The
violence of sewer explosions may be increased somewhat by the sudden
pressures that are put upon them.

Gasoline finds its way into sewers from garages and cleaning
establishments. A mixture of 1½ per cent gasoline vapor and air may be
explosive. It needs only the stray spark of an electric current, a
lighted match, or a cigar thrown into the sewer to cause the explosion.
As the result of a series of experiments on 2,706 feet of 8–foot sewer,
Burrell and Boyd conclude.[115]

One gallon of gasoline if entirely vaporized produces about 32
cubic feet of vapor at ordinary temperature and pressure. If 1½
per cent be adopted as the low explosive limit of mixtures of
gasoline vapor and air, 55 gallons or a barrel of gasoline would
produce enough vapor to render explosive the mixture in 1,900 feet
of 9 foot sewer provided the gasoline and the air were perfectly
mixed. Many different factors, however, govern explosibility, such
as: size of the sewer, velocity of the sewage, temperature of the
sewer, volatility and rate of inflow of the gasoline. Only under
identical conditions of tests would duplicate results be obtained.
A large amount of gasoline poured in at one time is less dangerous
than the same amount allowed to run in slowly. With a velocity of
flow of about 6½ feet per second it was evident that 55 gallons of
gasoline poured all at once into a manhole rendered the air
explosive only a few minutes (less than 10) at any particular
point. With the same amount of gasoline run in at the rate of 5
gallons per minute, an explosive flame would have swept along the
sewer if ignited 15 minutes after the gasoline had been dumped.
With a slow velocity of flow and a submerged outlet the gasoline
vapor being heavier than air accumulated at one point and
extremely explosive conditions could result from a small amount of
gasoline. Comparatively rich explosive mixtures were found 5 hours
after the gasoline had been discharged. High-test gasoline is much
more dangerous than the naphtha used in cleaning establishments,
yet on account of the large quantity of waste naphtha the sewage
from cleaning establishments may be very dangerous.

Illuminating gas is not so dangerous as gasoline vapor as it is lighter
than air and it is more likely to escape from the sewer than to
accumulate in it. It requires about one part of illuminating gas to
seven parts of air to produce an explosive mixture.

Calcium carbide is dangerous because it is self igniting. The heat of
the generation of gas is sufficient to ignite the explosive mixture. The
gases are highly explosive and cause a relatively powerful explosion.
Fortunately large amounts of this material seldom reach a sewer, the gas
being generated in garage drains or traps and escaping in the
atmosphere.

A hydrocarbon oil used by railroads in preventing the freezing of
switches, if allowed to reach the sewers, may cause explosions
therein.[116] The oil crystallizes and in this form it is soluble in
water. It will thus pass traps and on volatilization will produce
explosive mixtures.

Methane, generated by the decomposition of organic matter, is a feebly
explosive gas occasionally found in sewers. Its presence may add to the
strength of other explosive mixtures.

Sewer explosions may be prevented by the building of proper forms of
intercepting basins to prevent the entrance of gasoline and calcium
carbide gases, and by ventilation to dilute the explosive mixtures which
may be made up in the sewer. There are no practical means to predict
when an explosion is about to occur, and after an explosion has occurred
it is difficult to determine the cause as all evidence is usually
destroyed.

=207. Valuation of Sewers.=—The necessity for the valuation of a
sewerage system may arise from the legal provisions in some states
limiting the amount of outstanding bonds which may be issued by a
municipality to a certain percentage of the present worth of municipal
property. The investment in the sewerage system is usually great and
forms a large portion of the City’s tangible property. It may be
desirable also to determine the depreciation of the sewers with a view
towards their renewal.

The most valuable work on the valuation of sewers has been done in New
York City[117] by the engineers of the Sewer Department. The committee
of engineers appointed to do the work recommended: (1) that the original
cost be made the basis of valuation, and that (2), in fixing this cost
the cost of pavement should be omitted or at most the cost of a cheap
(cobblestone) pavement should be included. Trenches previously excavated
in rock were considered as undepreciated assets.

The present worth of sewers depends on many factors aside from the
effects of age, such as the care exercised in the original construction,
the material used, the kind and quantity of sewage carried, the care
taken in maintenance, and finally the injury caused by the careless
building of adjoining substructures. During the progress of the
inspections the examination of brick sewers, due to their accessibility,
yielded better results than the examination of pipe sewers. The routine
of the examination of the brick sewers consisted in cleaning off the
bricks with a short broom, tapping the brick with a light hammer to
determine solidity, and testing the cement joints by scraping with a
chisel. In addition, measurements of height and width were taken every
30 feet. The bricks in the invert at and below the flow line were
examined for wear.

A study of the reports of these examinations disclosed that the
following defects were noticeable:

1. Cement partly out at water line.

2. Cement partly out above water line.

3. Depressed arch and sewer slightly spread.

4. Large open joints.

5. Loose brick.

6. Bond of brick broken.

7. Distorted sides, uneven bottom, joints out of line.

[Illustration:

FIG. 147.—Diagrams used in Estimating Depreciation of Brick Sewers Due
to Age, Manhattan Borough, New York City.

_a._ Proportionate deterioration from various causes.

_b._ Percentage of depreciation based on examination of sewers, use of
deterioration curve (Fig. a), and age of sewers examined.

Eng. News, Vol. 71, p. 84.
]

Inspection of pipe sewers from manholes, the pipe being illuminated by
floating candles, was found to be unsatisfactory. Reliance was placed on
the reports of men experienced in making connections and repairs to the
sewers. Early pipe sewers in New York were laid directly on the bottom
of the trench. Under these circumstances a small leak at a joint was
sufficient to wash the earth away and to drop the pipe, causing serious
conditions along the line. No wear or deterioration of pipe sewers were
noted, the only defects being cracking of the pipes at the center line
due to poor foundation and to defects in the pipe itself.

[Illustration:

FIG. 148.—Diagram Showing Rate of Depreciation of Pipe Sewers.

Eng. News, Vol. 71, p. 86.
]

The depreciation of brick sewers as studied in New York, is shown
graphically in Fig. 147. At zero the sewer is in good condition and at
100 it is in such a state of dilapidation as to require instant
rebuilding. Repairs are not considered economical in this condition. In
the preparation of this diagram each condition on the list above was
given a certain number of points, which when added together represented
the state of depreciation of the sewer. These sums were plotted as
ordinates and the corresponding ages of the sewer were plotted as
abscissas. The various points were taken cumulatively, and where the
bond of the brickwork was broken (given a value of 72) plus other
defects gave a total of 164 the sewer was considered as valueless and
not worth repair. The scale of 164 was later reduced to a percentage
basis as shown on the right of the figure. Fig. 148 shows a similar
diagram for the depreciation of pipe sewers.

It was concluded that the life of a brick sewer in New York is 64 years.
Some of the sewers examined were over 200 years old. The total original
cost of 483 miles of brick, pipe and wood sewers was figured as
$23,880,000 with a present worth of $18,665,000 and an average annual
depreciation of 2.2 per cent. In figuring these amounts no account was
taken of obsolescence. The deterioration of catch-basins proceeded at
about the same rate as for brick sewers.

CHAPTER XIII
COMPOSITION AND PROPERTIES OF SEWAGE

=208. Physical Characteristics.=—Sewage is the spent water supply of a
community containing the wastes from domestic, industrial, or commercial
use, and such surface and ground water as may enter the sewer.[118]
Sewages are classed as: domestic sewage, industrial waste, storm water,
surface water, street wash, and ground water. Domestic sewage is the
liquid discharged from residences or institutions and contains water
closet, laundry, and kitchen wastes. It is sometimes called sanitary
sewage. Industrial sewage is the liquid waste resulting from processes
employed in industrial establishments. Storm water is that part of the
rainfall which runs over the surface of the ground during a storm and
for such a short period following a storm as the flow exceeds the normal
and ordinary run-off. Surface water is that part of the rainfall which
runs over the surface of the ground some time after a storm. Street wash
is the liquid flowing on or from the street surface. Ground water is
water standing in or flowing through the ground below its surface.

Ordinary fresh sewage is gray in color, somewhat of the appearance of
soapy dish water. It contains particles of suspended matter which are
visible to the naked eye. If the sewage is fresh the character of some
of the suspended matter can be distinguished as: matches, bits of paper,
fecal matter, rags, etc. The amount of suspended matter in sewage is
small, so small as to have no practical effect on the specific gravity
of the liquid nor to necessitate the modification of hydraulic formulas
developed for application to the flow of water. The total suspended
matter in a normal strong domestic sewage is about 500 parts per
1,000,000. It is represented graphically in Fig. 149. The quantity of
organic or volatile suspended matter is about 200 parts per 1,000,000.
It is shown graphically in the smaller cube in Fig. 149.

[Illustration:

FIG. 149.—Graphical Representation of Relative Volumes of Liquids and
Solids in Sewage.
]

The odor of fresh sewage is faint and not necessarily unpleasant. It has
a slightly pungent odor, somewhat like a damp unventilated cellar.
Occasionally the odor of gasoline, or some other predominating waste
matter may hide all other odors. Stale sewage is black and gives off
nauseating odors of hydrogen sulphide and other gases. If the sewage is
so stale as to become septic, bubbles of gas will be seen breaking the
surface and a black or gray scum may be present. Before the South Branch
of the Chicago River was cleaned up and flushed this scum became so
thick in places, particularly in that portion of the Stock Yards where
the river became known as Bubbly Creek, that it is said that weeds and
small bushes sprouted in it, and chickens and small animals ran across
its surface.

A physical analysis of sewage should include an observation of its
appearance, and a determination of its temperature, turbidity, color,
and odor, both hot and cold. The temperature is useful in indicating
certain of the antecedents of the sewage, its effect on certain forms of
bacterial life, and its effect on the possible content of dissolved
gases. Temperatures higher than normal are indicative of the presence of
trades wastes discharged while hot into the sewers. A low temperature
may indicate the presence of ground water. If the temperature is much
over 40° C. bacterial action will be inhibited and the content of
dissolved gases will be reduced. Turbidity, color, and odor
determinations may be of value in the control of treatment devices, or
to indicate the presence of certain trades wastes, which give typical
reactions. Since all normal sewages are high in color and turbidity, the
relative amounts of these two constituents in two different sewages has
little significance regarding the relative strengths of the two sewages
or the proper method of treating them. A fresh domestic sewage should
have no highly offensive odor. The presence of certain trades wastes can
be detected sometimes in fresh sewages, and a stale sewage may sometimes
be recognized by its odor.

Sewage is a liability to the community producing it. Although some
substances of value can be obtained from sewage[119] the cost of the
processes usually exceed the value of the substances obtained. Where it
becomes necessary to treat sewage the value of these substances may be
helpful in defraying the cost of treatment.

=209. Chemical Composition.=—Sewage is composed of mineral and organic
compounds which are either in solution or are suspended in water. In
making a standard chemical analysis of sewage only those chemical
radicals and elements are determined which are indicative of certain
important constituents. Neither a complete qualitative nor quantitative
analysis is made. A sewage analysis will not show, therefore, the number
of grams of sodium chloride present or any other constituent. A complete
standard sanitary chemical analysis will report the constituents as
named in the first column of Table 71. The quantities of these materials
found in average strong, medium and weak sewages are also shown in this
table. These values are not intended as fixed boundaries between sewages
of different strengths. They are presented merely as a guide to the
interpretation of sewage analyses.

The principal objects of a chemical analysis of sewage are to determine
its strength and its state of decomposition. The influents and effluents
of a sewage treatment device are analyzed to aid in the control of the
device and to gain information concerning the effect of the treatment.
Chemical and other analyses, in connection with the desired conditions
after disposal, will indicate the extent of treatment which may be
required. The standard methods of water and sewage analysis adopted by
the American Public Health Association have been generally accepted by
sanitarians. These uniform methods make possible comparisons of the
results obtained by laboratories working according to these standards.

CHEMICAL ANALYSIS OF SEWAGES

(Parts per million)

From Report on Industrial Wastes from the Stock Yards and Packingtown,
Chicago by the Sanitary District of Chicago in 1921, page 231.

──────────┬──────────────────┬───────┬────────┬──────────
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │Waterbury,
│ │Boston │Columbus│ Conn.,
│ Typical Analyses │1905–7 │ 1904–5 │ 1905–6
──────────┼──────┬──────┬────┼───────┼────────┼──────────
│Strong│Medium│Weak│ │ │
──────────┼──────┼──────┼────┼───────┼────────┼──────────
Nitrogen │ │ │ │ │ │
as │ │ │ │ │ │
Organic │ │ │ │ │ │
Nitrogen│ 35│ 20│ 10│ 9.1│ 9.0│ 14.8
Free │ │ │ │ │ │
Ammonia │ 50│ 30│ 15│ 13.9│ 11.0│ 7.8
Nitrites │ 0.10│ 0.05│ 0.0│ 0.0│ 0.09│ 0.14
Nitrates │ 0.40│ 0.20│ 0.1│ 0.20│ 0.20│ 1.52
Oxygen │ │ │ │ │ │
consumed│ 75│ 50│ 30│56[120]│ 51[121]│ 46[120]
Oxygen │ │ │ │ │ │
demand │ 300│ 200│ 100│ │ │
Chlorine │ 175│ 100│ 15│ 2300│ 65│ 48
Suspended │ │ │ │ │ │
matter │ 500│ 300│ 150│ 135│ 209│ 165
Volatile│ │ │ │ 91│ 79│ 115
Fixed │ │ │ │ 44│ 130│ 50
Alkalinity│ 200│ 100│ 50│ 125│ 350│ 41
Fats │ 40│ 20│ │ │ 25│ 26
──────────┴──────┴──────┴────┴───────┴────────┴──────────

──────────┬─────────────┬──────────┬───────────┬───────────
│ │ │ │ Chicago,
│ │ │ │ Center
│ │ │ Chicago, │ Avenue.
│Gloversville,│Worcester,│ 39th St. │Industrial.
│ N. Y. │ Mass. │Residential│Day Sewage
│ 1908–9 │ 1908 │ 1909–12 │ 1913
──────────┼─────────────┼──────────┼───────────┼───────────
│ │ │ │
──────────┼─────────────┼──────────┼───────────┼───────────
Nitrogen │ │ │ │
as │ │ │ │
Organic │ │ │ │
Nitrogen│ 23.0│ │ 7.8│ 79
Free │ │ │ │
Ammonia │ 12.0│ 22.2│ 9.1│ 22
Nitrites │ 0.38│ │ 0.10│ 0.49
Nitrates │ 0.88│ │ 0.33│ 3.04
Oxygen │ │ │ │
consumed│ 95[120]│ 117│ 43│ 268
Oxygen │ │ │ │
demand │ │ │ │
Chlorine │ 158│ 57│ 40│ 1100
Suspended │ │ │ │
matter │ 406│ 258│ 144│ 605
Volatile│ 229│ 166│ 90│ 46
Fixed │ 177│ 92│ 54│ 144
Alkalinity│ 233│ │ 212│ 291
Fats │ 48│ │ 23[122]│ 198[123]
──────────┴─────────────┴──────────┴───────────┴───────────

=210. Significance of Chemical Constituents.=—Organic nitrogen and free
ammonia taken together are an index of the organic matter in the sewage.
Organic nitrogen includes all of the nitrogen present with the exception
of that in the form of ammonia, nitrites, and nitrates. Free ammonia or
ammonia nitrogen is the result of bacterial decomposition of organic
matter. A fresh cold sewage should be relatively high in organic
nitrogen and low in free ammonia. A stale warm sewage should be
relatively high in free ammonia and low in organic nitrogen. The sum of
the two should be unchanged in the same sewage.

Nitrites (RNO_{2}) and nitrates (RNO_{3})[124] are found in fresh
sewages only in concentrations of less than one part per million. In
well-oxidized effluents from treatment plants the concentration will
probably be much higher. Nitrates contain one more atom of oxygen than
nitrites. They represent the most stable form of nitrogenous matter in
sewage. Nitrites are not stable and are reduced to ammonias or are
oxidized to nitrates. Their presence indicates a process of change. They
are not found in large quantities in raw sewage because their formation
requires oxygen which must be absorbed from some other source than the
sewage. In an ordinary sewer or sluggishly flowing open stream this
absorption cannot take place from the atmosphere with sufficient
rapidity to supply the necessary oxygen.

Oxygen consumed is an index of the amount of carbonaceous matter readily
oxidizable by potassium permanganate. It does not indicate the total
quantity of any particular constituent, but it is the most useful index
of carbonaceous matter. Carbonaceous matter is usually difficult of
treatment and a high oxygen consumed is indicative of a sewage difficult
to care for. The amount of oxygen consumed, as expressed in the
analysis, is dependent on the amount of oxidizable carbonaceous matter
present, the oxidizing agent used, and the time and temperature of
contact of the sewage and the oxidizing agent. It is essential therefore
that the test be conducted according to some standard method, since the
results are of value only as compared with results obtained under
similar conditions.

Total solids (residue on evaporation) are an index of the strength of
the sewage. They are made up of organic and inorganic substances. The
inorganic substances include sand, clay, and oxides of iron and
aluminum, which are usually insoluble, and chlorides, carbonates,
sulphates and phosphates, which are usually soluble. The insoluble
inorganic substances are undesirable in sewage because of their sediment
forming properties which result in the clogging of sewers, treatment
plants, pumps, and stream beds. The soluble inorganic substances are
generally harmless and cause no nuisance, except that the presence of
sulphur may permit the formation of hydrogen sulphide, which has a
highly offensive odor. The organic substances are: carbohydrates, fats,
and soaps, which are carbonaceous and are difficult of removal by
biological processes; and the nitrogenous substances such as urea,
proteins, amines, and amino acids. The inorganic and organic substances
may be either in solution or suspension or in a colloidal condition.

Volatile solids are used as an index of the organic matter present, as
it is assumed that the organic matter is more easily volatilized than
the inorganic matter. The amount of volatile inorganic matter present is
usually so small as to be negligible.

Fixed solids are reported as the difference between the total and
volatile solids. They are therefore representative of the amount of
inorganic matter present.

Suspended matter is the undissolved portion of the total solids. High
volatile suspended matter is an indication of offensive qualities in the
nature of putrefying organic matter, whereas fixed suspended matter is
indicative of inoffensive inorganic matter. It is difficult to obtain a
sample of sewage which will represent the amount of suspended matter in
the sewage, since a sample taken from near the surface will contain less
inorganic matter and grit than a sample taken near the bottom.

Settling solids are indicative of the sludge forming properties of the
sewage and of the probable degree of success of treatment by plain
sedimentation. Volatile settling solids indicate the property of the
formation of offensive putrefying sludge banks. There is no chemical
test which will indicate the scum-forming properties of sewage. Fixed
settling solids indicate the presence of inorganic matter, probably
gritty material such as sand, clay, iron oxide, etc.

Colloidal matter is material which is too finely divided to be removed
by filtration or sedimentation, yet is not held in solution. It can
sometimes be removed by violent agitation in the presence of a
flocculent precipitate, as in the treatment with activated sludge, or by
the flocculent precipitate alone, as in chemical precipitation, or by
the acidulation of the sewage so as to precipitate the colloids.
Colloidal matter is probably the result of the constant abrasion of
finely divided suspended matter while flowing through the sewer or other
channel. High colloidal matter may therefore indicate a stale sewage, or
the presence of a particular trades waste. Colloids are difficult of
removal. For this reason, where sewage is to be treated, turbulence in
the tributary channels should be avoided.

Alkalinity may indicate the possibility of success of the biologic
treatment of sewage, since bacterial life flourishes better in a
slightly alkaline than in a slightly acid sewage. Within the normal
limits of the amount of alkalinity in sewage the exact amount has little
significance in sewage analyses. Sewages are normally slightly alkaline.
An abnormal alkalinity or acidity may indicate the presence of certain
trades wastes necessitating special methods of treatment. A method of
sewage treatment may be successful without changing the amount of
alkalinity in the sewage since the amount of alkalinity is not
inherently an objection.

Chlorine, in the form of sodium chloride, is an inorganic substance
found in the urine of man and animals. The amount of chlorine above the
normal chlorine content of pure waters in the district is used as an
index of the strength of the sewage. The chlorine content may be
affected by certain trades wastes such as ice-cream factories,
meat-salting plants, etc., which will increase the amount of chlorine
materially. Since chlorine is an inorganic substance which is in
solution it is not affected by biological processes nor sedimentation.
Its diminution in a treatment plant or in a flowing stream is indicative
of dilution and the reduction of chlorine will be approximately
proportional to the amount of dilution.

Fats have a recoverable market value when present in sufficient quantity
to be skimmed off the surface of the sewage. Ordinarily fats are an
undesirable constituent of sewage as they precipitate on and clog the
interstices in filtering material, they form objectionable scum in tanks
and streams, and they are acted on very slowly by biological processes
of sewage treatment. Although fats are carbonaceous matter they are not
indicated by the oxygen consumed test because they are not easily
oxidized. They are therefore determined in another manner; by
evaporation of the liquid and extracting the fats from the residue by
dissolving them in ether.

Relative stability and bio-chemical oxygen demand are the most important
tests indicating the putrefying characteristics of sewage. Since
stability and putrescibility have opposite meanings the relative
stability test is sometimes called the putrescibility test. The relative
stability of a sewage is an expression for the amount of oxygen present
in terms of the amount required for complete stability.

A relative stability of 75 signifies that the sample examined
contains a supply of available oxygen equal to 75 per cent of the
amount of oxygen which it requires in order to become perfectly
stable. The available oxygen is approximately equivalent to the
dissolved oxygen plus the available oxygen of nitrate and
nitrite.[125]

TABLE 72

RELATIVE STABILITY NUMBERS

──────────────────────────────────┬──────────────────────────────────
Time Required for Decolorization │
at 20° C. Days │ Relative Stability Number
──────────────────────────────────┼──────────────────────────────────
0.5│ 11
1.0│ 21
1.5│ 30
2.0│ 37
2.5│ 44
3.0│ 50
4.0[126]│ 60
5.0│ 68
6.0│ 75
7.0│ 80
8.0│ 84
9.0│ 87
10.0│ 90
11.0│ 92
12.0│ 94
13.0│ 95
14.0│ 96
16.0│ 97
18.0│ 98
20.0│ 90
──────────────────────────────────┴──────────────────────────────────

The relative stability numbers, given in Table 72, are computed from the
expression, _S_ = 100(1 − 0.794_t_) in which _S_ is the stability number
and _t_ is the time in days that the sample has been incubated at 20° C.
The bio-chemical oxygen demand is more directly an index of the
consumption of available oxygen by the biological and chemical changes
which take place in the decomposition of sewage or polluted water. As
such it is a more valuable, though less easily performed test than the
test of relative stability.

The methods for the determination of the relative stability and the
bio-chemical oxygen demand are given to show more clearly what these
tests represent. The procedure in the relative stability test is to add
0.4 c.c. of a standard solution of methylene blue to 150 c.c. of the
sample. The mixture is then allowed to stand in a completely filled and
tightly stoppered bottle at 20° C. for 20 days or until the blue fades
out due to the exhaustion of the available oxygen. There are three
methods in use for the determination of the bio-chemical oxygen
demand;[127] the relative stability method, the excess nitrate method,
and the excess oxygen method. In the relative stability method the
sample to be treated should have a relative stability of at least 50. If
it is lower than this the sample should be diluted with water containing
oxygen until the relative stability has been raised to or above this
point. The oxygen demand in parts per million is then expressed as

_O′_ = ((1 − _P_)_O_)⁄_RP_,[128]

in which _O′_ is the oxygen demand, _O_ is the initial oxygen in parts
per million (p.p.m.) in the diluting water or sewage, _P_ is the
proportion of sewage in the mixture expressed as a ratio, and _R_ is the
relative stability of the mixture expressed as a decimal. For the
effluents from sewage treatment plants, polluted waters, and similar
liquids, the total available oxygen expressed as the sum of the
dissolved oxygen, nitrites, and nitrates, divided by the relative
stability expressed as a decimal will give the bio-chemical oxygen
demand. The excess nitrate method requires the determination of the
total oxygen available as dissolved oxygen, nitrites, and nitrates and
the addition of a sufficient amount of oxygen in the form of sodium
nitrate to prevent the exhaustion of oxygen during a 10–day period of
incubation. At the end of the period the total available oxygen is again
determined. The difference between the original and the final oxygen
content represents the bio-chemical oxygen demand. The excess oxygen
test requires the determination of the total available oxygen as before,
and the addition of a sufficient amount of oxygen, in the form of
dissolved oxygen in the diluting water, to prevent exhaustion of the
oxygen in a 10–day period of incubation. The difference between the
original and final oxygen content represents the bio-chemical oxygen
demand. Theriault concludes as a result of his tests, that the relative
stability and excess nitrate methods are open to objections but that the
excess oxygen method yields very accurate and consistent results with as
little or less labor than is required by other methods.

Dissolved oxygen represents what its name implies, the amount of oxygen
(_O__{2}) which is dissolved in the liquid. Normal sewage contains no
dissolved oxygen unless it is unusually fresh. It is well, if possible,
to treat a sewage before the original dissolved oxygen has been
exhausted. Normal pure surface water contains all of the oxygen which it
is capable of dissolving, as shown in Table 73. The presence of a
smaller amount of oxygen than is shown in this table indicates the
presence of organic matter in the process of oxidation, which may be in
such quantities as ultimately to reduce the oxygen content to zero.
Normal pure ground waters may be deficient in dissolved oxygen because
of the absence of available oxygen for solution. The presence of certain
oxygen-producing organisms in polluted or otherwise potable surface
waters may cause a supersaturation with oxygen however.

The dissolved-oxygen test for polluted water is probably the most
significant of all tests. If dissolved oxygen is found in a polluted
water it means that putrefactive odors will not occur, since
putrefaction cannot begin in the presence of oxygen. It is possible for
different strata in a body of water to have different quantities of
dissolved oxygen, and putrefaction may be proceeding in the lower strata
before the oxygen is exhausted from the upper strata. The oxygen content
of a river water will indicate the ability of the river to receive
sewage without resulting in a nuisance.

TABLE 73

SOLUBILITY OF OXYGEN IN WATER

Under an atmospheric pressure of 760 mm. of mercury, the atmosphere
containing 20.9 per cent of oxygen.
───────────────────────────────────┬───────────────────────────────────
Temperature, degrees C │ Oxygen in parts per million
───────────────────────────────────┼───────────────────────────────────
0│ 14.62
1│ 14.23
2│ 13.84
3│ 13.48
4│ 13.13
5│ 12.8
6│ 12.48
7│ 12.17
8│ 11.87
9│ 11.59
10│ 11.33
11│ 11.08
12│ 10.83
13│ 10.6
14│ 10.37
15│ 10.15
16│ 9.95
17│ 9.74
18│ 9.54
19│ 9.35
20│ 9.17
21│ 8.99
22│ 8.83
23│ 8.68
24│ 8.53
25│ 8.38
26│ 8.22
27│ 8.07
28│ 7.92
29│ 7.77
30│ 7.63
───────────────────────────────────┴───────────────────────────────────

=211. Sewage Bacteria.=—A slight knowledge of the nature of bacteria is
necessary in order that the biological changes which occur in the
treatment of sewage may be understood. Bacteria are living organisms
which are so small that it is difficult or impossible to study them
either with the eye alone or with the aid of powerful microscopes. They
are studied by means of cultures, stains, and certain characteristic
phenomena such as the production of a gas, the production of a red
colony on litmus lactose agar, etc. Bacteria occur in three forms:
spherical, called coccus; cylindrical, called bacillus; and spiral,
called spirillum. In size they vary from the largest at about 1⁄10,000
of an inch to sizes so small as to be invisible under the most powerful
microscope. An ordinary size is 1⁄25,000 of an inch. The cylindrical or
rod bacteria are about four times as long as they are wide. Some
bacteria possess the power of motion due to the presence of flagella or
hairs which can be moved and cause the cell to progress at a rate as
high as 18 cm. per hour, but usually the rate is very much less than
this. The composition of the bacterial cell has never been definitely
determined.

Bacteria are unicellular plants. They possess no digestive organs and
apparently obtain their food by absorption from the surrounding media.
Reproduction is by the division of the cell into two approximately equal
portions. This reproduction may occur as frequently as once every half
hour and if unchecked would quickly mount to unimaginable numbers. The
natural cause limiting the growth of bacteria is the generation by the
bacterium of certain substances such as the amino acids, which are
injurious to cell life. The exhaustion of the food supply is not
considered as an important cause of inhibition of multiplication. The
products of growth of one species of bacteria may be helpful or harmful
to other forms. Where the products are helpful the effect is known as
symbiosis, and where harmful the effect is known as antibiosis. In
sewage the presence of both aërobic and anaërobic bacteria is usually
mutually helpful and the condition is an example of symbiosis. The
aërobes, sometimes called obligatory aërobes, are bacteria which demand
available oxygen for their growth. The anaërobes, or obligatory
anaërobes, can grow only in the absence of oxygen. There are other forms
that are known as facultative anaërobes (or aërobes) whose growth is
independent of the presence or absence of oxygen.

Spores are formed by some bacteria when they are subjected to an
unfavorable environment such as high temperatures, the absence of food,
the absence of moisture, etc. Spores are cells in which growth and
animation are suspended but the life of the cell is carried on through
the unsuitable period, somewhat similar to the condition in a plant
seed.

=212. Organic Life in Sewage.=—Living organisms, both plants and
animals, exist in sewage. Bacteria are the smallest of these organisms.
Others, which can be studied easily under the microscope or can be seen
with difficulty by the naked eye but which do not require special
cultures for their study, are classed as microscopic organisms or
plankton. Organisms which are large enough to be studied without the aid
of a microscope or special cultures are classed as macroscopic. The part
taken in the biolysis of sewage by macroscopic organisms belonging to
the animal kingdom, such as birds, fish, insects, rodents, etc., which
feed upon substances in the sewage is so inconsequential as to be of no
importance. Both plants and animals are found among the macroscopic
organisms.

Organisms in sewage may be either harmful, harmless, or beneficial. From
the viewpoint of mankind the harmful organisms are the pathogenic
bacteria. Their condition of life in sewage is not normal and in general
their existence therein is of short duration. It may be of sufficient
length, however, to permit the transmission of disease. The diseases
which can be transmitted by sewage are only those that are contracted
through the alimentary canal, such as typhoid fever, dysentery, cholera,
etc. Diseases are not commonly contracted by contact of sewage with the
skin nor by breathing the air of sewers. It is safe to work in and
around sewage so long as the sewage is kept out of the mouth, and
asphyxiating or toxic gases are avoided.

The beneficial organisms in sewage are those on which dependence is
placed for the success of certain methods of treatment. These organisms
have not all been isolated or identified.

The total number of bacteria in a sample of sewage has little or no
significance. In a normal sewage the number may be between 2,000,000 and
20,000,000 per c.c. and because of the extreme rapidity of
multiplication of bacteria a sample showing a count of 1,000,000 per
c.c. on the first analysis may show 4 to 5 times as many 3 or 4 hours
later. A bacterial analysis of sewage is ordinarily of little or no
value, since pathogenic organisms are practically certain to be present,
there is no interest in the harmless organisms, and the helpful
nitrifying and aërobic bacteria will not grow on ordinary laboratory
media. Occasionally the presence of certain bacteria may indicate the
presence of certain trades wastes. In general, the total bacterial
count, as sometimes reported, represents only the number of bacteria
which have grown under the conditions provided. It bears no relation to
the total number of bacteria in the sample.

The presence of bacteria in sewage is of great importance however, as
practically all methods of treatment depend on bacterial action, and all
sewages which do not contain deleterious trades wastes, contain or will
support the necessary bacteria for their successful treatment, if
properly developed.

=213. Decomposition of Sewage.=—If a glass container be filled with
sewage and allowed to stand, open to the air, a black sediment will
appear after a short time, a greasy scum may rise to the surface, and
offensive odors will be given off. This condition will persist for
several weeks, after which the liquid will become clear and odorless.
The sewage has been decomposed and is now in a stable condition. The
decomposition of sewage is brought about by bacterial action the exact
nature of which is uncertain.

It[129] is well established that many of the chemical effects
wrought by bacteria, as by other living cells, are due, not to the
direct action of the protoplasm, but to the intervention of
soluble ferments or enzymes.

Enzymes are soluble ferments produced by the growth of the bacterial
cell.

In[130] many cases the enzymes diffuse out from the cell and exert
their effort on the ambient substances ... in others the enzyme
action occurs within the cell and the products pass out, (for
example) ... the alcohol-producing enzymes of the yeast cell act
upon sugar within the cell, the resulting alcohol and carbon
dioxide being ejected.

Other chemical effects may be brought about by the direct action of the
living cells, but this has never been well established.

Metabolism is the life process of living cells by which they absorb
their food and convert it into energy and other products. It is the
metabolism of bacterial growth that in itself or by the production of
enzymes hastens the putrefactive or oxidizing stages of the organic
cycles in sewage treatment. Bacteria can assimilate only liquid food
since they have no digestive tract through which solid food can enter.
The surrounding solids are dissolved by the action of the enzymes, the
resulting solution diffusing through the chromatin or outer skin, and
being digested throughout the interior cytoplasm.

Bacteria are sometimes classified as parasites and saprophytes. The
parasites live only on the growing cells of other plant or animal life.
The saprophytes obtain their food only from the life products of living
organisms and do not exist at the expense of the organisms themselves.
Facultative saprophytes (or parasites) may exist on either living or
dead tissue.

The decomposition of sewage may be divided into anaërobic and aërobic
stages. These conditions are usually, but not always, distinctly
separate. The growth of certain forms of bacteria is concurrent, while
the growth of other forms is dependent on the results of the life
processes of other bacteria in the early stages of decomposition.

When sewage is very fresh it contains some oxygen. This oxygen is
quickly exhausted so that the first important step in the decomposition
of sewage is carried on under anaërobic conditions. This is accompanied
by the creation of foul odors of organic substances, ammonia, hydrogen
sulphide, etc.; other odorless gases such as carbon dioxide, hydrogen,
and marsh gas, the latter being inflammable and explosive; and other
complicated compounds. An exception to the rule that putrefaction takes
place only in the absence of oxygen is the production of other
foul-smelling substances by the putrefactive activity of obligatory and
facultative aërobes. Hydrogen sulphide may be produced apparently in the
presence of oxygen the action which takes place not being thoroughly
understood.

The biolysis of sewage is the term applied to the changes through which
its organic constituents pass due to the metabolism of bacterial life.
Organic matter is composed almost exclusively of the four elements:
carbon, oxygen, hydrogen, and nitrogen (COHN) and sometimes in addition
sulphur and phosphorus. The organic constituents of sewage can be
divided into the proteins, carbohydrates, and fats. The proteins are
principally constituents of animal tissue, but they are also found in
the seeds of plants. The principal distinguishing characteristic of the
proteins is the possession of between 15 and 16 per cent of nitrogen. To
this group belong the albumens and casein. The carbohydrates are organic
compounds in which the ratio of hydrogen to oxygen is the same as in
water, and the number of carbon atoms is 6 or a multiple of 6. To this
group belong the sugars, starches and celluloses. The fats are salts
formed, together with water, by the combination of the fatty acids with
the tri-acid base glycerol. The more common fats are _stearin_,
_palmatin_, _olein_, and _butyrine_. The soaps are mineral salts of the
fatty acids formed by replacing the weak base glycerol with some of the
stronger alkalies.

The first state in the biolysis of sewage is marked by the rapid
disappearance of the available oxygen present in the water mixed with
organic matter to form sewage. In this state the urea, ammonia, and
other products of digestive or putrefactive decomposition are partially
oxidized and in this oxidation the available oxygen present is rapidly
consumed, the conditions in the sewage becoming anaërobic. The second
state is putrefaction in which the action is under anaërobic conditions.
The proteins are broken down to form urea, ammonia, the foul-smelling
mercaptans, hydrogen sulphide, etc., and fatty and aromatic acids. The
carbohydrates are broken down into their original fatty acid, water,
carbon dioxide, hydrogen, methane, and other substances. Cellulose is
also broken down but much more slowly. The fats and soaps are affected
somewhat similarly to the hydrocarbons and are broken down to form the
original acids of their make up together with carbon dioxide, hydrogen,
methane, etc. The bacterial action on fats and soaps is much slower than
on the proteins, and the active biological agents in the biolysis of the
hydrocarbons, fats, and soaps are not so closely confined to anaërobes
as in the biolysis of the proteins. The third state in the biolysis of
sewage is the oxidation or nitrification of the products of
decomposition resulting from the putrefactive state. The products of
decomposition are converted to nitrites and nitrates, which are in a
stable condition and are available for plant food. It must be understood
that the various states may be coexistent but that the conditions of the
different states predominate approximately in the order stated. In the
biolysis of sewage there is no destruction of matter. The same elements
exist in the same amount as at the start of the biolytic action.

=214. The Nitrogen Cycle.=—Nitrogen is an element that is found in all
organic compounds. Its presence is necessary to all plant and animal
life. The nitrogenous compounds are most readily attacked by bacterial
action in sewage treatment. The non-nitrogenous substances such as soaps
and fats, and the inorganic compounds are more slowly affected by
bacterial action alone. The element nitrogen passes through a course of
events from life to death and back to life again that is known as the
Nitrogen Cycle. It is typical of the cycles through which all of the
organic elements pass.

Upon the death of a plant or animal, decomposition sets in accompanied
by the formation of urea which is broken down into ammonia. This is
known as the _putrefactive stage_ of the Nitrogen Cycle. The next state
is _nitrification_ in which the compounds of ammonia are oxidized to
nitrites and nitrates, and are thus prepared for plant food. In the
state of _plant life_ the nitrites and nitrates are denitrified so as to
be available as a plant or animal food. The highest state of the
Nitrogen Cycle is _animal life_, in which nitrogen is a part of the
living animal substance or is charged from protein to urea, ammonia,
etc., by the functions of life in the animal. Upon the death of this
animal organism the cycle is repeated. The Nitrogen Cycle, like the
cycle of Life and Death, is purely an ideal condition as in nature there
are many short circuits and back currents which prevent the continuous
progression of the cycle. The conception of this cycle is an aid,
however, in understanding the processes of sewage treatment.

=215. Plankton and Macroscopic Organisms.=—In general the part played by
these organisms in the biolysis of sewage is not sufficiently well
understood to aid in the selection of methods of sewage treatment
involving their activities. The presence in bodies of water receiving
sewage, of certain plankton which are known to exist only when
putrefaction is not imminent, indicates that the body of water into
which the discharge of sewage is occurring is not being overtaxed. The
control of sewage treatment plant effluents so as to avoid the poisoning
of fish life or the contamination of shell fish is likewise important.
The study of plankton and macroscopic life in the treatment of sewage is
an open field for research.

=216. Variations in the Quality of Sewage.=—The quality of sewage varies
with the hour of the day and the season of the year. Some of the causes
of these variations are: changes in the amount of diluting water due to
the inflow of storm water or flushing of the streets or sewers;
variations in domestic activities such as the suspension of
contributions of organic wastes during the night, Monday’s wash, etc.;
characteristics of different industries which discharge different kinds
of wastes according to the stage of the manufacturing process, etc. In
general night sewage is markedly weaker than day sewage in both domestic
and industrial wastes, but in specific cases the varying strength
depends entirely upon the characteristics of the district. Some analyses
are given in Table 74, which emphasize these points.

TABLE 74

SEWAGE ANALYSES SHOWING HOURLY, DAILY, AND SEASONAL VARIATIONS IN
QUALITY
────────────┬────────────┬─────┬────────┬─────────┬──────────┬─────────
Place │ Time │ │ │Suspended│ Remarks │Reference
│ Nitrogen │Total│Chlorine│ Matter │ │
────────────┼────────────┼─────┼────────┼─────────┼──────────┼─────────
Marion, Ohio│Mid’t-noon, │ │ │ │Industrial│ 1
│ 5–21–06. │ 45│ 53│ 190│ │
│Noon-mid’t │ │ │ │Domestic │ 1
│ 5–21–06. │ 37│ 94│ 133│ │
Westerville,│Day │ │ │ │college │ 1
Ohio │ │ 10.2│ 76│ 118│ town │
│Night │ 2.6│ 74│ 41│ │ 1
Columbus, │1904–1905 │ │ │ │ │
Ohio │ │ │ │ │ │
│Mid’t to 2 │ │ │ │ │ 2
│ a.m. │ 4.6│ 50│ 131│ │
│2 a.m. to 4 │ │ │ │ │ 2
│ a.m. │ 3.0│ 52│ 95│ │
│4 a.m. to 6 │ │ │ │ │ 2
│ a.m. │ 2.3│ 51│ 83│ │
│6 a.m. to 8 │ │ │ │ │ 2
│ a.m. │ 2.7│ 48│ 83│ │
│8 a.m. to 10│ │ │ │ │ 2
│ a.m. │ 16.3│ 66│ 476│ │
│10 a.m. to │ │ │ │ │ 2
│ noon │ 11.4│ 100│ 324│ │
│Noon to 2 │ │ │ │ │ 2
│ p.m. │ 11.3│ 86│ 246│ │
│2 p.m. to 4 │ │ │ │ │ 2
│ p.m. │ 12.3│ 78│ 246│ │
│4 p.m. to 6 │ │ │ │ │ 2
│ p.m. │ 22.0│ 78│ 368│ │
│6 p.m. to 8 │ │ │ │ │ 2
│ p.m. │ 8.2│ 71│ 209│ │
│8 p.m. to 10│ │ │ │ │ 2
│ p.m. │ 7.8│ 80│ 120│ │
│10 p.m. to │ │ │ │ │ 2
│ mid’t │ 6.2│ 56│ 117│ │
│ │ │ │ │ │
Center Ave.,│Mid’t to 3 │ │ │ │ │ 3
Chicago. │ a.m. │ │ │ 123│ │
│4 a.m. to 7 │ │ │ │ │ 3
│ p.m. │ │ │ 316│ │
│8 a.m. to 11│ │ │ │ │ 3
│ p.m. │ │ │ 608│ │
│Noon to 3 │ │ │ │ │ 3
│ p.m. │ │ │ 785│ │
│4 p.m. to 7 │ │ │ │ │ 3
│ p.m. │ │ │ 717│ │
│8 p.m. to 11│ │ │ │ │ 3
│ p.m. │ │ │ 287│ │
│ │ │ │ │ │
Columbus, │Sunday │ │ │ │ │ 2
Ohio │ │ 6.7│ 55│ 858│ │
│Monday │ 9.1│ 66│ 1048│ │ 2
│Tuesday │ 9.4│ 69│ 1024│ │ 2
│Wednesday │ 9.6│ 68│ 1005│ │ 2
│Thursday │ 9.2│ 66│ 990│ │ 2
│Friday │ 9.2│ 67│ 1018│ │ 2
│Saturday │ 9.3│ 67│ 1016│ │ 2
│ │ │ │ │ │
Baltimore, │Aug. 1 to │ │ │ │ │ 4
1907–1908 │ Sept. 1 │ 16.0│ │ 246│ │
│Sept. 4 to │ │ │ │ │ 4
│ Oct. 3 │ 19.0│ │ 190│ │
│Oct. 6 to │ │ │ │ │ 4
│ Nov. 4 │ 20.0│ │ 188│ │
│Nov. 15 to │ │ │ │ │ 4
│ Nov. 29 │ 20.0│ │ 164│ │
│Dec. 3 to │ │ │ │ │ 4
│ Dec. 29 │ 20.0│ │ 123│ │
│Jan. 6 to │ │ │ │ │ 4
│ Jan. 21 │ 19.0│ │ 127│ │
│Feb. 2 to │ │ │ │ │ 4
│ Feb. 26 │ 20.0│ │ 149│ │
│Feb. 29 to │ │ │ │ │ 4
│ Mar. 24 │ 28.0│ │ 274│ │
│Mar. 27 to │ │ │ │ │ 4
│ April 29 │ 25.0│ │ 165│ │
│April 30 to │ │ │ │ │ 4
│ May 26 │ 19.0│ │ 104│ │
│June 8 to │ │ │ │ │ 4
│ July 11 │ 15.0│ │ 88│ │
│July 13 to │ │ │ │ │ 4
│ Aug. 8 │ 9.5│ │ 124│ │
────────────┴────────────┴─────┴────────┴─────────┴──────────┴─────────

References:

1. 1908 Report of the Ohio State Board of Health.

2. Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson,
1905.

3. Report on Industrial Wastes from the Stock Yards and Packingtown in
Chicago, by the Sanitary District of Chicago. 1921.

4. Report of the Baltimore Sewerage Commission, 1911.

=217. Sewage Disposal.=—Previous to the development of the
water-carriage method for removing human excreta and other liquid wastes
the solid matter was disposed of by burial and the liquid wastes were
allowed to seep into the ground or to run away over its surface.
Following the development of the water-carriage system, which
necessitated the development of sewers, the problem of ultimate disposal
was rendered more serious by the concentration of human excreta together
with a large volume of water. The unthinking citizen believes the
problem of sewage disposal is solved when the toilet is flushed or the
bath tub is drained. The problem may more truly be said to commence at
this point.

It would appear that the simplest method of disposal of sewage would be
to discharge it into the nearest water course. Unfortunately the nature
of sewage is such that it may be either highly offensive to the senses
or dangerous to health or both, when discharged in this manner. Only the
most fortunate communities are favored with a body of water of
sufficient size to receive sewage without creating a nuisance.

The problems of sewage disposal are to prevent nuisances causing offense
to sight and smell; to prevent the clogging of channels; to protect
pumping machinery; to protect public water supplies; to protect fish
life; to prevent the contamination of shell fish; to recover valuable
constituents of the sewage; to enrich and to irrigate the soil; to
safeguard bathing and boating; for other minor purposes; and in some
cases to comply with the law. Sewage may be treated to attain one or
more of these objects by methods of treatment varying as widely as the
objects to be attained.

=218. Methods of Sewage Treatment.=—In studying the subject of sewage
treatment it must be borne in mind that it is impossible to destroy any
of the elements present. They may be removed from the mixture only by
gasification, straining or sedimentation. Their chemical combinations
may be so changed, however, as to result in different substances than
those introduced to the treatment plant. It is with these chemical
changes that the student of sewage treatment is interested.

The methods of sewage treatment can be classified as mechanical,
chemical and biological. These classifications are not separated by
rigid lines but may overlap in certain treatment devices or methods.
Mechanical methods of treatment are exemplified by sedimentation, and
screening. Chemical precipitation and sterilization are examples of
chemical methods. The biological methods, the most important of all,
include dilution, septicization, filtration, sewage farming, activated
sludge, etc. If for any reason it is desired to treat sewage by more
than one of these methods the procedure should follow as nearly as
possible the order of the occurrence of the phenomena in the natural
biolysis of sewage. For example, in one treatment plant the sewage would
first pass through a grit chamber where the coarse sediment would be
removed, then through a screen where the floating matter and coarse
suspended matter would be removed, then to a sedimentation basin where
some finer suspended matter might settle out, then to a digestive tank
where the solid matter deposited would be worked upon by bacterial
action and partially liquefied. Simultaneous to the liquefaction of the
deposited solid matter the liquid effluent from the digestive tank might
proceed to an aërating device to expedite oxidation, then to an aërobic
filter, and finally to disposal by dilution.

CHAPTER XIV
DISPOSAL BY DILUTION

=219. Definition.=—Disposal of sewage by dilution is the discharge of
raw sewage or the effluent from a treatment plant into a body of water
of sufficient size to prevent offense to the senses of sight and smell,
and to avoid danger to the public health.

=220. Conditions Required for Success.=—Among the desired conditions for
successful disposal by dilution are: adequate currents to prevent
sedimentation and to carry the sewage away from all habitations before
putrefaction sets in, or sufficient diluting water high in dissolved
oxygen to prevent putrefaction; a fresh or non-septic sewage; absence of
floating or rapidly settling solids, grease or oil; and absence of back
eddies or quiet pools favorable to sedimentation in the stream into
which disposal is taking place. The conditions which should be prevented
are: offensive odors due to sludge banks, the rise of septic gases, and
unsightly floating or suspended matter. In some instances the pollution
of the receiving body of water is undesirable and the sewage must be
freed from pathogenic organisms and the danger of aftergrowths minimized
before disposal. Such conditions are typified at Baltimore, where the
sewage is discharged into Back Bay, an arm of Chesapeake Bay. One of the
important industries of the state of Maryland is the cultivation of
oysters. The pollution of the Bay was therefore so objectionable that
careful treatment of the Baltimore sewage has been a necessary
preliminary to final disposal by dilution. It is unwise to draw public
water supplies, without treatment, from a stream receiving a sewage
effluent, no matter how careful or thorough the treatment of the sewage.
The treatment of the sewage is a safeguard, and lightens the load on the
water purification plant, but under no considerations can it be depended
upon to protect the community consuming the diluted effluent.

The sewer outlet should be located well out in the current of the
stream, lake, or harbor. Deeply submerged outlets are usually better
than an outlet at the surface, as a better mixture of the sewage and
water is obtained. The discharge of sewage into a body of water of which
the surface level changes, alternately covering and exposing large areas
of the bottom is unwise, as the sludge which is deposited during
inundation will cause offensive odors when uncovered. Such conditions
must be carefully guarded against when selecting a point of disposal in
tidal estuaries because of the frequent fluctuations in level.

=221. Self-Purification of Running Streams.=—The self-purification of
running streams is due to dilution, sedimentation, and oxidation. The
action is physical, chemical, and biological. When putrescible organic
matter is discharged into water the offensive character of the organic
matter is minimized by dilution. If the dilution is sufficiently great,
it alone may be sufficient to prevent all nuisance. The oxidation of the
organic matter commences immediately on its discharge into the diluting
water due to the growth and activity of nitrifying and other oxidizing
organisms and to a slight degree to direct chemical reaction. So long as
there is sufficient oxygen present in the water septic conditions will
not exist and offensive odors will be absent. When the organic matter is
completely nitrified or oxidized there will be no further demand on the
oxygen content of the stream and the stream will be said to have
purified itself. At the same time that this oxidation is going on some
of the organic matter will be settling due to the action of
sedimentation. If oxidation is completed before the matter has settled
on the bottom the result will be an inoffensive silting up of the river.
If oxidation is not complete, however, the result will be offensive
putrefying sludge banks which may send their stinks up through the
superimposed layers of clean water to pollute the surrounding
atmosphere.

The most important condition for the successful self-purification of a
stream is an initial quantity of dissolved oxygen to oxidize all of the
organic matter contributed to it, or the addition of sufficient oxygen
subsequent to the contribution of sewage to complete the oxidation.
Oxygen may be added through the dilution received from tributaries,
through aëration over falls and rapids, or by quiescent absorption from
the atmosphere. The rapidity of self-purification is dependent on the
character of the organic matter, the presence of available oxygen, the
rate of reaëration, temperature, sedimentation, and the velocity of the
current. Sluggish streams are more likely to purify themselves in a
shorter distance and rapidly flowing turbulent streams are more likely
to purify themselves in a shorter time, other conditions being equal.
Although the absorption of oxygen by a stream whose surface is broken is
more rapid than through a smooth unbroken surface, the growth of algæ,
biological activity, the effect of sunlight, and sedimentation are more
potent factors and have a greater effect in sluggish streams than the
slightly more rapid absorption of oxygen in a turbulent stream. It is
frequently more advantageous to discharge sewage into a swiftly moving
stream, however, regardless of the conditions of self-purification, as
the undesirable conditions which may result occur far from the point of
disposal and may be offensive to no one.

The sewage from a population of about 3,000,000 persons residing in and
about Chicago is discharged into the Chicago Drainage Canal. It
ultimately reaches tide water through the Des Plaines, the Illinois, and
the Mississippi Rivers. The action occurring in these channels is one of
the best illustrations known of the self-purification of a stream. In
Table 75 are shown the results of analyses of samples taken at various
points below the mouth of the Chicago River where the diluting water
from Lake Michigan enters, to Grafton, Illinois, at the junction of the
Illinois and Mississippi Rivers about 40 miles above St. Louis. The
effect of the physical characteristics of the stream on its chemical
composition is well illustrated in this table. The rise in the chlorine
content between Lake Michigan and the entrance to the Drainage Canal is
a measure of the addition of sewage. Since the chlorine is an inorganic
substance which is not affected by biologic action, its loss in
concentration in the lower reaches of the rivers is due to dilution by
tributaries and sedimentation, e.g., between the end of the canal at
Lockport and the sampling point at Joliet, the entrance of the Des
Plaines River reduces the concentration of chlorine from 124.5 to 41.5
parts per million. The entrance of the Kankakee River at Dresden Heights
further reduces the chlorine to 24.5 p.p.m. The increase of albuminoid
and ammonia nitrogen accompanied by a decrease in nitrites and nitrates,
between the upper end of the canal at Bridgeport and its lower end at
Lockport indicates the reducing action proceeding therein. The oxidizing
action over the various dams and the effect of dilution with water
containing oxygen is shown between miles 34 and 38, at mile 79, and at
mile 294. The excellent effect of quiescent sedimentation and aëration
in Peoria Lakes is shown between miles 145, 161 and 165.

TABLE 75

ANALYSES OF CHICAGO, DES PLAINES AND ILLINOIS RIVERS

(Parts per million)

────────────┬────────┬──────────────────────────────────────────────
Sampling │Distance│January-June, 1900, from “Sewage Disposal,” by
Point │in Miles│ Kinnicutt, Winslow and Pratt
│ from │
│ Lake │
│Michigan│
────────────┼────────┼────────┬────────┬──────────┬────────┬────────
│ │Chlorine│ Ammonia│Albuminoid│Nitrates│Nitrates
│ │ │Nitrogen│ Nitrogen│ │
│ │ │ │ │ │
────────────┼────────┼────────┼────────┼──────────┼────────┼────────
Lake │ 0│ 3.0│ 0.03│ 60.13│ 0.002│ 0.008
Michigan │ │ │ │ │ │
│ │ │ │ │ │
Canal, │ 5│ 96.6│ 8.05│ 2.05│ .021│ .074
Bridgeport│ │ │ │ │ │
Canal, │ 34│ 124.5│ 10.90│ 2.07│ .013│ .066
Lockport │ │ │ │ │ │
Joliet │ 38│ 41.5│ 4.22│ 0.83│ .021│ .086
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Dresden │ 52│ │ │ │ │
Heights │ │ │ │ │ │
Dresden │ 52│ │ │ │ │
Heights │ │ │ │ │ │
Morris │ 62│ 24.5│ 2.46│ .60│ .075│ .424
│ │ │ │ │ │
Marseilles │ 79│ │ │ │ │
Marseilles │ 79│ │ │ │ │
Ottawa │ 85│ 15.3│ 1.55│ .41│ .197│ .966
La Salle │ 100│ 17.5│ 1.05│ .43│ .109│ .979
Henry │ 129│ 13.3│ .92│ .38│ .102│ .800
Chillicothe │ 145│ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Averyville │ 161│ 13.5│ .81│ .37│ .004│ 1.150
│ │ │ │ │ │
│ │ │ │ │ │
Wesley │ 165│ 12.0│ .57│ .41│ .083│ 1.03
│ │ │ │ │ │
Pekin │ 175│ 12.3│ .70│ .43│ .060│ .990
Havana │ 205│ 11.2│ .60│ .36│ .065│ .570
Beardstown │ 237│ 10.7│ .69│ .44│ .106│ .685
La Grange │ 249│ │ │ │ │
Kampsville │ 294│ 11.3│ .66│ .44│ .044│ .870
Kampsville │ 294│ │ │ │ │
Grafton │ 325│ 9.8│ .46│ .42│ .031│ 1.06
│ │ │ │ │ │
Grafton │ 325│ │ │ │ │
│ │ │ │ │ │
────────────┴────────┴────────┴────────┴──────────┴────────┴────────

────────────┬─────────────────────┬───────────
Sampling │ Dissolved Oxygen │ Remarks
Point │ │
│ │
│ │
│ │
────────────┼───────┬──────┬──────┼───────────
│ Jan. │ July │ Nov. │
│30–Feb.│ 8–15 │12–19,│
│2, 1912│ 1912 │ 1912 │
────────────┼───────┼──────┼──────┼───────────
Lake │ 14.1│ │ 10.8│Typical
Michigan │ │ │ │ chemical
│ │ │ │ analysis
Canal, │ │ │ 6.9│Kedzie
Bridgeport│ │ │ │ Avenue
Canal, │ 9.9│ │ 1.7│Above dam
Lockport │ │ │ │
Joliet │ │ 1.4│ 5.6│Aëration
│ │ │ │ over dam.
│ │ │ │ Dilution
│ │ │ │by Des
│ │ │ │ Plaines
│ │ │ │ River
Dresden │ │ 1.0│ 4.1│Des Plaines
Heights │ │ │ │ River
Dresden │ │ │ 10.4│Kankakee
Heights │ │ │ │ River
Morris │ 7.8│ │ 5.7│Illinois
│ │ │ │ River
Marseilles │ 5.7│ 0.6│ 6.8│Above dam
Marseilles │ 8.2│ 4.5│ 9.3│Below dam
Ottawa │ 10.0│ │ 8.1│
La Salle │ 5.4│ │ 7.8│
Henry │ │ │ 7.9│
Chillicothe │ 3.4│ 1.5│ 5.9│Above
│ │ │ │ Peoria
│ │ │ │ Lakes
Averyville │ 3.3│ 8.2│ 8.9│Below
│ │ │ │ Peoria
│ │ │ │ Lakes
Wesley │ │ │ 7.1│Below
│ │ │ │ Peoria
Pekin │ 4.9│ 3.2│ 8.9│
Havana │ 4.8│ │ 8.8│
Beardstown │ 6.5│ │ 9.1│
La Grange │ │ 4.1│ 9.4│Below dam
Kampsville │ │ 4.1│ 10.0│Above dam
Kampsville │ │ 4.6│ 10.0│Below dam
Grafton │ 6.6│ 4.7│ 10.4│Illinois
│ │ │ │ River
Grafton │ │ 7.3│ 12.0│Mississippi
│ │ │ │ River
────────────┴───────┴──────┴──────┴───────────

=222. Self-Purification of Lakes.=—Sewage may be disposed of into lakes
with as great success as into running streams if conditions exist which
are favorable to self-purification. Lakes and rivers purify themselves
from the same causes; oxidation, sedimentation, etc., but in the former
the currents are much less pronounced and may be entirely absent. In
shallow lakes (20 feet or less in depth) dependence must be placed on
horizontal currents and the stirring action of the wind to keep the
water in motion in order that the sewage and the diluting water may be
mixed. In deeper bodies of water, currents induced by the wind are
helpful but entire dependence need not be placed upon them. Vertical
currents, and the seasonal turnovers in the spring and fall completely
mix the waters of the lake above those layers of water whose temperature
never rises higher than 4° C.

In the early winter the cold air cools the surface waters of a lake. The
cooling increases the density of the surface water causing it to sink,
and allowing the warmer layers below to rise and become cooled. After
the temperature of the entire lake has reached 4° C. the vertical
currents induced by temperature cease, as continued cooling decreases
the density of the surface water maintaining the same layer at the
surface. In the spring as the temperature of the surface water rises to
4° C. and above it becomes heavier and drops through the colder water
below causing vertical currents. These phenomena are known as the fall
and spring turnovers. The former is more pronounced. These turnovers are
effective in assisting in the self-purification of lakes.

=223. Dilution in Salt Water.=—The oxygen content in salt water is about
20 per cent less than in fresh water at the same temperature. The
greater content of matter in solution in salt water reduces its capacity
to absorb many sewage solids. This, together with the chemical reaction
between the constituents of the salt water and those of the sewage serve
to precipitate some of the sewage solids and to form offensive sludge
banks. The evidence of the action which takes place in the absorption of
oxygen from the atmosphere by salt water and its effect on dissolved
sewage solids is conflicting, but in general fresh water is a better
diluting medium than salt water.

Black and Phelps have made valuable studies of the relative rates of
absorption of oxygen from the air by fresh and salt water. The results
of their experiments are published in a Report to the Board of Estimate
and Apportionment of N. Y. City, made March 23, 1911.[131] Concerning
these rates they conclude:

Therefore there is no reason to believe that the reaëration of
salt water follows any other laws than those we have determined
mathematically and experimentally for fresh water. In the absence
of fuller information on the effect of increased viscosity upon
the diffusion coefficient, it can only be stated that the rate of
reaëration of salt water is less than that of fresh water, in
proportion to the respective solubilities of oxygen in the two
waters, and still less, but to an unknown extent, by reason of the
greater viscosity and consequent small value of the diffusion
coefficient.

=224. Quantity of Diluting Water Needed.=—In a large majority of the
problems of disposal of sewage by dilution it is not necessary to add
sufficient diluting water to oxidize completely all organic matter
present. Ordinarily it is sufficient to prevent putrefactive conditions
until the flow of the stream, lake, or tidal current, has reached some
large body of diluting water or where putrefaction is no longer a
nuisance. It is never desirable to allow the oxygen content of a stream
to be exhausted as putrescible conditions will exist locally before
exhaustion is complete. The exact point to which oxygen can be reduced
in safety is in some dispute. Black and Phelps have assumed 70 per cent
of saturation as the allowable limit; Fuller has placed it at 30 per
cent; Kinnicutt, Winslow, and Pratt have placed it at 50 per cent. Since
the reaction between the oxygen and the organic matter is quantitative,
others have placed the limit in terms of parts per million of oxygen.
Wisner,[132] has recommended a minimum of 2.5 p.p.m. as the limit for
the sustenance of fish life, which is not far from Fuller’s limit for
hot-weather conditions.

Formulas of various types have been devised to express the rate of
absorption of oxygen with a given quantity of diluting water which is
mixed with a given quantity and quality of sewage. The quantity of
sewage is sometimes expressed in terms of the tributary population or in
other ways. Knowing the rate at which oxygen is exhausted and the
velocity of flow of the stream, the point at which the oxygen will be
reduced to the limit allowed is easily determined. The accuracy of none
of these formulas has been proven, and their use, without an
understanding of the effect of local conditions, may lead to error. They
may be used as a check on the bio-chemical oxygen demand determinations,
which should be conclusive.

The following formula, based on the work of Black and Phelps, is a guide
to the amount of sewage which can be added to a stream without causing a
nuisance. It is:

_C_ = (log(_O′_⁄_O_))⁄_kt_,

in which _C_ = per cent of sewage allowed in the water;

_O′_ = per cent of saturation or the p.p.m. of oxygen in the
mixture at the time of dilution;

_O_ = per cent of saturation or the p.p.m. of oxygen in the
stream after period of flow to point beyond which no
nuisance can be expected;

_t_ = time in hours required for the stream to flow to this
point;

_k_ = constant determined by test determinations of the
factors in the following expression:

_k_ = (log(_O′__{1}⁄_O__{1}))⁄_C__{1}_t__{1},

in which _O′__{1} = per cent of saturation or the p.p.m. of oxygen in
the diluting water before mixing with the sewage;

_O__{1} = per cent of saturation or the p.p.m. of oxygen in
an artificial mixture made in the laboratory,
after _t__{1} hours of incubation;

_C__{1} = per cent of sewage in the mixture;

_t__{1} = number of hours of incubation of the mixture of
sewage and diluting water under laboratory
conditions.

In the solution of these formulas it is desired to determine the
permissible amount of sewage to discharge into a given quantity of
diluting water. This value is expressed by C in the first equation. In
solving this equation:

_O′_ is determined by laboratory tests and should represent
the conditions to be expected during various seasons
of the year;

_O_ is determined by judgment. It may be 30 per cent or 50
per cent or more as previously explained;

_t_ is determined by float tests or other measurements of
the stream flow;

_k_ is determined by laboratory tests in which mixtures of
various strengths are incubated for various periods
of time. Different values of _k_ will be obtained for
different characteristics of the sewage; but for the
same sewage the value of _k_ should be unchanged for
different periods of incubation.

Rideal devised the formula:[133]

_XO_ = _C_(_M_ − _N_)_S_

in which _X_ = flow of the stream expressed in second-feet;

_O_ = grams of free oxygen in one cubic foot of water;

_S_ = rate of sewage discharge in second-feet;

_M_ = grams of oxygen required to consume the organic matter
in one cubic foot of diluted sewage as determined by
the permanganate test with 4 hours boiling;

_N_ = grams of oxygen available in the nitrites and nitrates
in one cubic foot of diluted sewage;

_C_ = ratio between the amount of oxygen in the stream and
that required to prevent putrefaction. Where _C_ is
equal to or greater than one, satisfactory conditions
have been attained.

In using this formula it is necessary to make analyses of trial mixtures
of sewage and water until the correct mixture has been found.

Hazen’s formula is:[134]

_D_ = _x_⁄_S_ = 4_m_⁄_O_,

in which _D_ = dilution ratio;

_x_ = volume of water;

_S_ = volume of sewage;

_m_ = result of the oxygen consumed test expressed in p.p.m.
after 5 minutes, boiling with potassium permanganate;

_O_ = amount of dissolved oxygen in the diluting water
expressed in p.p.m.

For comparison with Rideal’s formula the factor of 7 should be used
instead of 4 to allow for the increased time of boiling.

Since the amount of oxygen needed is dependent on the amount of organic
matter in the sewage rather than the total volume of the sewage, and
since the amount of organic matter is closely proportional to the
population, the amount of diluting water has sometimes been expressed in
terms of the population. Hering’s recommendation for the quantity of
diluting water necessary for Chicago sewage was 3.3 cubic feet of water
per second per thousand population. Experience has proven this to be too
small. Between a minimum limit of 2 second-feet and a maximum of 8
second-feet of diluting water per thousand population the success of
dilution is uncertain. Above this limit success is practically assured
and below this limit failure can be expected.

Even with these carefully devised formulas and empirical guides, the
factors of reaëration, dilution, sedimentation, temperature, etc., may
have so great an effect as to vitiate the conclusions. As shown in Table
75 dilution in winter is far more successful than in summer. The lower
temperatures so reduce the activity of the putrefying organisms that
consumption of oxygen is greatly retarded.

=225. Governmental Control.=—A comprehensive discussion of the legal
principles governing the pollution of inland waters is contained in “A
Review of the Laws Forbidding the Pollution of Inland Waters,” by E. B.
Goodell, published by the United States Geological Survey in 1905, as
Water Supply Paper No. 152.

The disposal of sewage by dilution is subject to statutory limitations
in many states. The enforcement of these laws is usually in the hands of
the state board of health, which is frequently given discretionary
powers to recommend and sometimes to enforce measures for the abatement
of an actual or potential nuisance. Such recommendations usually take
the form of a specification of certain forms of treatment preliminary to
disposal by dilution. No project for the disposal of sewage by dilution
should be consummated until the local, state, national, and in the case
of boundary waters, international laws have been complied with. The
attitude of the courts in different states has not been uniform. Little
guidance can be taken from the personal feeling of the persons
immediately interested. The opinion of the riparian owner 5 miles down
stream may differ materially from the popular will of the voters of a
city, and it is likely to receive a more favorable hearing from the
court. Statutes and legal precedents are the safest guides.

=226. Preliminary Treatment.=—If the sewage to be disposed of by
dilution contains unsightly floating matter, oil, or grease, no amount
of oxygen in the diluting water will prevent a nuisance to sight, or the
formation of putrefying sludge banks. Under such conditions it will be
necessary to introduce screens or sedimentation basins, or both, in
order to remove the floating and the settling solids. Biologic tanks,
filtration, or other methods of treatment may be necessary for the
removal of other undesirable constituents.

=227. Preliminary Investigations.=—Before adopting disposal of sewage by
dilution without preliminary treatment, or before considering the proper
form of treatment necessary to render disposal by dilution successful, a
study should be made of the character of the body of water into which
the sewage or effluent is to be discharged. This study should include:
measurements of the quantity of water available at all seasons of the
year; analyses of the diluting water to determine particularly the
available dissolved oxygen; observations of the velocity and direction
of currents, and the effect of winds thereon; a study of the effect on
public water supplies, bathing beaches, fish life, etc. Good judgment,
aided by the proper interpretation of such information should lead to
the most desirable location for the sewer outlet. If preliminary
treatment is found to be necessary tests should be made to determine the
necessary extent and thoroughness of the treatment.

CHAPTER XV
SCREENING AND SEDIMENTATION

=228. Purpose.=—The first step in the treatment of sewage is usually
that of coarse screening in order to remove the larger particles of
floating or suspended matter. Screens and sedimentation basins are used
to prevent the clogging of sewers, channels, and treatment plants; to
avoid clogging of and injuries to machinery; to overcome the
accumulation of putrefying sludge banks; to minimize the absorption of
oxygen in diluting water; and to intercept unsightly floating matter.

By the plain sedimentation of sewage is meant the removal of suspended
matter by quiescent subsidence unaffected by septic action or the
addition of chemicals or other precipitants. In order to prevent septic
action plain sedimentation tanks must be cleaned as frequently as once
or twice a week in warm weather but not quite so often in cold weather.

Fine screening may take the place of sedimentation where insufficient
space is available for sedimentation tanks, and it is desired to remove
only a small portion of the suspended matter. Recent American practice
has tended to restrict the field of fine screening to treatment
requiring less than 10 per cent removal of suspended matter, thus
eliminating screens from the field covered by plain sedimentation tanks.
The practice is well expressed by Potter, who states:[135]

Where a high degree of purification is sought, the use of fine
screens is of doubtful value. A modern settling tank will give
better results and at a less cost for a given degree of
purification. A settled liquid is also superior to a screened
liquid for subsequent biological treatment in filters.... Again
the storing of large quantities of screenings must necessarily be
more objectionable than the storing of the digested sludge of a
modern settling tank.

[Illustration:

FIG. 150.—Types of Moving Screens.

Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 893.
]

=229. Types of Screens.=—The definitions of some types of screens as
proposed by the American Public Health Association follow: A _bar
screen_ is composed of parallel bars or rods. A _mesh screen_ is
composed of a fabric, usually wire. A _grating_ consists of 2 sets of
parallel bars in the same plane in sets intersecting at right angles. A
_band screen_ consists of an endless perforated band or belt which
passes over upper and lower rollers. A _perforated plate screen_ is made
of an endless band of perforated plates similar to a band screen. A
_wing screen_ has radial vanes uniformly spaced which rotate on a
horizontal axis. A _disc screen_ consists of a circular perforated disc
with or without a central truncated cone of similar material mounted in
the center. The Reinsch Wurl screen is the best known type of disc
screen. A _cage screen_[136] consists of a rectangular box made up of
parallel bars with the upstream side of the box or cage omitted.
Allen[137] gives the following definitions: A _drum screen_ is a
cylinder or cone of perforated plates or wire mesh which rotates on a
horizontal axis. A _shovel vane screen_ is similar to a wing screen with
semicircular wings and a different method of removing the screenings.
Examples of a band screen, a wing screen, a shovel vane screen, a drum
screen and a disc screen are shown in Fig. 150. A bar screen is shown in
Fig. 151 and a cage screen is shown in Fig. 152.

[Illustration:

FIG. 151.—Sketch of a Bar Screen.
]

[Illustration:

FIG. 152.—Sketch of a Cage Screen.
]

Screens can be classed as fixed, movable, or moving. Fixed screens are
permanently set in position and must be cleaned by rakes or teeth that
are pulled between the bars. Movable screens are stationary when in
operation, but are lifted from the sewage for the purpose of cleaning.
Moving screens are in continuous motion when in operation and are
cleaned while in motion. Fixed bar screens may be set either vertical,
inclined, or horizontal.

Movable screens with a cage or box at the bottom are sometimes used. The
box should be of solid material to prevent the forcing of screenings
through it when the screen is being raised for cleaning. A mesh screen
should be used only under special circumstances because of the
difficulty in cleaning. Screens which must be raised from the sewage for
cleaning should be arranged in pairs in order that one may be working
when the other is being cleaned. Movable screens are undesirable for
small plants because the labor involved in raising and lowering is
greater than in cleaning with a rake and the screens are more likely to
be neglected. In a large plant rakes operated by hand are too small for
cleaning the screens. A fixed screen is sometimes used with moving teeth
fastened to endless chains. The teeth pass between the parallel bars and
comb out the screenings. If the screen chamber in a small plant is too
deep for accessibility a movable cage or box screen may be desirable.

Moving screens are generally of fine mesh or perforated plates. They are
kept moving in order to allow continuous cleaning. They are cleaned by
brushes or by jets of air, water, or steam.

=230. Sizes of Openings.=—The area or size of the opening of a screen is
dependent upon the character of the sewage to be treated and upon the
object to be attained.

Large screens, with openings between 1½ inches and 6 inches are used to
protect centrifugal pumps, tanks, automatic dosing devices, conduits,
and gate valves from large objects such as pieces of timber, dead
animals, etc., which are found in sewage. The quantity of material
removed is variable, and is usually small.

Medium-size screens with openings from ¼ inch to 1½ inches are used to
prepare sewage for passage through reciprocating pumps, complex dosing
apparatus, contact beds, and sand filters. The amount of material
removed varies from 0.5 to 10 cubic feet per million gallons of sewage
treated, dependent on the character of the sewage and the size of the
screen. Screenings before drying contain 75 to 90 per cent moisture and
weigh 40 to 50 pounds per cubic foot. At times the amount removed may
vary widely from the limits stated. Schaetzle and Davis[138] state:

Screenings differ greatly both in amount and character.... The
amount varies with the days of the week as well as during the
course of the day. It reaches its maximum about noon or shortly
before and commences to disappear about midnight, reaching a
minimum about 4 or 5 a.m. The material is almost wholly organic
and consists of scraps of vegetables or fruit, cloth, hair, wood,
paper and lumps of fecal matter. The amount varies so widely that
it is impossible to state just what to expect any definite size
screen to remove. The amount of water contained is small compared
with that in the sludge in sedimentation basins and amounts to
from 70 per cent to 80 per cent. On account of its organic origin
it is highly putrescible.

Medium-size screens are sometimes placed close together with the bars of
the one opposite the openings in the other, thus approaching a fine
screen.

Fine screens vary in size of opening from ¼ inch to 50 openings per
linear inch or 2,500 per square inch. They are used for removing solids
preparatory to disposal by dilution, to protect sprinkling filters,
complex dosing apparatus, sand filters, sewage farms, and to prevent the
formation of scum in subsequent tank treatment. In general, fine screens
will remove from 0.1 to 1 cubic yard of wet material per million gallons
of sewage treated. The wet screenings will contain about 75 per cent
moisture and will weigh about 60 pounds per cubic foot. The dry weight
of the screenings will therefore be about 10 to 400 pounds per million
gallons of sewage treated. The effect of the removal of this amount of
material is usually not detectable by methods of chemical analysis, the
amount of suspended matter before and after screening being found
unchanged.

In his conclusions on the discussion of the results to be expected from
fine screens, Allen states:[139]

With openings not more than 0.1 inch in size, fine screening
should remove at least 30 per cent of the suspended solids and 20
per cent of the suspended organic solids from ordinary domestic
sewage, or 0.1 cubic yard of screenings, containing 75 per cent
water per thousand population daily.

The effect of the use of different size openings under the same
conditions is shown in Fig. 153.[140] Some data covering the amount of
material removed by screening are given in Table 76. More extensive data
are given in Volume III of “American Sewerage Practice” by Metcalf and
Eddy.

TABLE 76

DATA ON SCREENS

(Trans. Am. Society Civil Engineers, Vol. 78, Page 942)

───────┬──────────┬──────────┬──────────────────────────
Type of│ Location │ Clear │ Screenings
Screen │ │ Opening, │
│ │in Inches │
│ │ │
│ │ │
│ │ │
───────┼──────────┼──────────┼────────────┬─────────────
│ │ │Per Million │ Per 1000
│ │ │ Gallons, │ Population
│ │ │_y_ = Cubic │ Daily,
│ │ │ Yard │ _y_ = Cubic
│ │ │ _t_ = Tons │ Yard
│ │ │ │ _t_ = Tons
───────┼──────────┼──────────┼────────────┼─────────────
Band │Hamburg │ 0.6 │ 0.34_y_ │ 0.018_y_
│Göttingen │ 0.4 │ 0.35_y_ │ 0.026_y_
│Sutton │0.375[141]│ 0.6_y_ │
│Chicago │ │ 2.4–3.1_t_ │
Wing │Frankfort │ 0.40 │ 0.7_y_ │ 0.040_y_
│Elberfeld │ 0.40 │ 1.15_y_ │ 0.053_y_
│Stralsund │ 0.20 │ │ 0.079_y_
│Wiesbaden │ 0.60 │ 1.1_y_ │ 0.033_y_
Shovel │Strassburg│ 0.10 │ 1.6_y_ │ 0.043_y_
vane │ │ │ │
│Gleiwitz │ 0.12 │ │ 0.192_y_
│Temesvar │ 0.12 │ 0.9–1.7_y_ │0.067–.133_y_
Drum │Bromberg │ 0.08 │ 4.75_t_ │
│Mainz │ Note 6 │ 0.52_y_ │
│Trier │ 0.10 │0.39–0.42_y_│ 0.13_y_
│Osnabruck │ 0.08 │ 3.2–4.0_y_ │ 0.08–.10_y_
Weand │Reading, │ 36[141] │ 1.0_y_ │
│ Pa. │ │ │
│Brockton │ 36[141] │ 1.4_t_ │
Reinsch│Dresden │ 0.08 │ 0.97_t_ │ 0.09_y_
Wurl │ │ │ │
───────┴──────────┴──────────┴────────────┴─────────────

───────┬────────┬───────────┬─────────┬────────────
Type of│Per Cent│Horse-Power│ Cost of │ Remarks
Screen │Moisture│Per Screen │Operation│
│ │ │ Per │
│ │ │ Million │
│ │ │Gallons, │
│ │ │ Dollars │
───────┼────────┼───────────┼─────────┼────────────
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
│ │ │ │
───────┼────────┼───────────┼─────────┼────────────
Band │ 87 │ 2.5 │ │Note 1
│ │ 2.0 │ │
│ │ │ │
│ 79 │ │ │Stock Yard
Wing │ │ 5.0 │ │Note 2
│ 75 │ │ │Note 3
│ │ 4.5 │ │
│ │hand power │ 1.64 │Note 4
Shovel │ 89.3 │ 3.35 │ │Note 5
vane │ │ │ │
│ │ │ 0.90 │
│ 60–70 │ │ small │
Drum │ 40–60 │ │ 2.45 │Experimental
│ 75 │ 5.2–6.8 │0.89–3.42│
│ 50–60 │ │ 2.41 │Experimental
│ │ 9.00 │ │Note 7
Weand │ 89.5 │ 2.0 │ 1.00± │
│ │ │ │
│ │ │ │
Reinsch│ 84 │ 2.5 │.325–1.76│
Wurl │ │ │ │
───────┴────────┴───────────┴─────────┴────────────

Notes:—1. After removal of ½ this volume of grit.

2. After removal of 16 per cent by the grit chamber.

3. Including 0.6 cubic yard grit per million gallons.

4. After passing 1.6 inch bar screen.

5. After removal of 0.132 cubic yard grit and coarse
screenings per 1000 population.

6. 0.12, 0.04–0.08.

7. Before removal of 0.4 cubic yard grit per million gallons.

[Illustration:

FIG. 153.—Screenings Collected on Different Sized Openings.

1921 Report on Industrial Wastes Disposal, Union Stock Yards District,
Chicago, Illinois, to the Sanitary District of Chicago.
]

=231. Design of Fixed and Movable Screens.=—The determination of the
size of the opening is the first step in the design of a sewage screen.
This is followed by the computation of the net area of openings in the
screen. The final steps are the determination of the overall dimensions
of the screen; the size of the bar, wire, or support; and the dimensions
of the screen chamber. The net area of openings is fixed by the
permissible velocity of flow through the screen and the quantity of
sewage to be treated. In determining the velocity of flow the general
principle should be followed that the velocity should not be reduced
sufficiently to allow sedimentation in the screen chamber. The velocity
of grit bearing sewage in passing through coarse screens should not be
reduced below 2 or 3 feet per second. If the sewage contains no grit, or
the screen is placed below a grit chamber the velocity through a medium
or fine screen should be from ½ to 1½ feet per minute. The velocity
through the screen in a direction normal to the plane of the screen can
be reduced without reducing the horizontal velocity of the sewage by
placing the screen in a sloping position.

The final steps are the design of the screen bar and the determination
of the dimensions of the screen and of the screen chamber. The size of
the bar in a bar screen, or as a support to a wire mesh, is dependent on
the unsupported length of the bar. The stresses in the bars are the
results of impact and bending, caused by cleaning, and of the load due
to the backing up of the sewage when the screen is clogged. Allowance
should be made for a head of 2 or 3 feet of sewage against the screen. A
generous allowance should be made in addition for the indeterminate
stresses due to cleaning. The screen should be supported only at the top
and bottom, as intermediate supports in a bar screen are undesirable
unless they are so arranged as not to interfere with the teeth of the
cleaning devices.

Fixed screens should be placed at an angle between 30° and 60° with the
horizontal, with the direction of slope such that the screenings are
caught on the upper portion of the screen. A small slope is desirable in
order to obtain a low velocity through the screen. The slope is limited
since the smaller the slope the longer the bars of the screen and the
greater the difficulty of hand cleaning. Small slopes will tend to make
the screens self cleaning. As the screen clogs, the increasing head of
sewage will push the accumulated screenings up the screen. The use of
flat screens in a vertical position is not desirable because of the
difficulty of cleaning and the accumulation of material at inaccessible
points. If a flat screen is placed in a horizontal position with the
flow of sewage downward difficulties are encountered in cleaning and
solid matter is forced through the screen as clogging increases. An
upward flow through a horizontal screen is undesirable as the material
is caught in a position inaccessible for cleaning. Movable screens are
more easily handled when placed in a vertical position.

In the construction of small screens, round bars are sometimes used
where the unsupported length of the bar is less than 3 or 4 feet. They
are not recommended, however, as the efficient area and the amount of
material removed by the screen are diminished. Bars which produce
openings with the larger end upstream are undesirable as particles
become wedged in the screen, and are either forced through or become
difficult to remove.[142] Rectangular bars are easily obtained and give
satisfactory service except where they are of insufficient strength
laterally. For greater lateral thickness a pear-shaped bar is sometimes
used, with the thicker side upstream. Fine mesh screens or perforated
plates are supported on grids or parallel bars of stronger material
designed to take up the heavy stresses on the screen.

The dimensions of the bar may be selected arbitrarily. The length and
width of the screen are fixed to give desirable dimensions to the screen
chamber and to give the necessary net opening in the screen. The width
of the screen chamber and the screen should be the same. The screen
chamber should be sufficiently long to prevent swirling and eddying
around the screen. If the dimensions thus fixed permit an undesirable,
velocity in the screen chamber they should be changed. A sufficient
length of screen should be allowed to project above the sewage for the
accumulation of screenings. The bars may be carried up and bent over at
the top as shown in Fig. 151 to simplify the removal of screenings.

Coarse screens are usually placed above all other portions of a
treatment plant. They may be followed by grit chambers or finer screens.
Coarse screens are occasionally placed as a protection above medium or
fine screens. In sewage containing grit the smaller screens are
sometimes placed below the grit chamber. It is desirable to provide some
means of diverting the sewage from a screen chamber to allow of repairs
to the screen and the cleaning of the chamber. Screen chambers are
sometimes designed in duplicate to allow for the cleaning of one while
the other is operating.

PLAIN SEDIMENTATION

=232. Theory of Sedimentation.=—Sedimentation takes place in sewage
because some particles of suspended matter have a greater specific
gravity than that of water. All particles do not settle at the same
rate. Since the weights of particles vary as the cubes of their
diameters, whereas the surface areas (upon which the action of the water
takes place) vary only as the squares of the diameters, the amount of
the skin friction on small particles is proportionally greater than that
on large particles, because of the relatively greater surface area
compared to their weight. As a result the smaller particles settle more
slowly. The velocity of sedimentation of large particles has been found
to vary about as the diameter and of small particles as the square root
of the diameter. The change takes place at a size of about 0.01 mm.

Sedimentation is accomplished by so retarding the velocity of flow of a
liquid that the settling particles will be given the opportunity to
settle out. The slowing down of the velocity is accomplished by passing
the sewage through a chamber of greater cross-sectional area than the
conduit from which it came. The time that the sewage is in this chamber
is called the period of retention. Although the shape of a basin, the
arrangement of the baffles and other details have a marked effect on the
results of sedimentation, the controlling factors are the period of
retention and the velocity of flow. Another factor affecting the
efficiency of the process is the quality of the sewage. Usually the
greater the amount of sediment in the sewage the greater the per cent of
suspended matter removed. A method for the determination of the proper
period of sedimentation has been developed by Hazen in Transactions of
the American Society of Civil Engineers, Volume 53, 1904, page 45. The
results of his studies are summarized in Fig. 154 which shows the per
cent of sediment remaining in a treated water after a certain period of
retention. This period of retention is expressed in terms of the
hydraulic coefficient[143] of the smallest size particle to be removed.
Table 77 shows the hydraulic coefficients of various particles. In Fig.
154 _a_ represents the period of retention and _t_ the time that it
would take a particle to fall to the bottom of the basin. The different
lines of the diagram represent the results to be expected by various
arrangements of settling basins. The meaning of these lines is given in
Table 78.

TABLE 77

HYDRAULIC VALUES OF SETTLING PARTICLES IN MILLIMETERS PER SECOND

───────────────────────────────────┬───────────────────────────────────
Diameter in mm. │ Hydraulic Value
───────────────────────────────────┼───────────────────────────────────
1.00 │ 100
0.80 │ 83
0.60 │ 63
0.50 │ 53
0.40 │ 42
0.30 │ 32
0.20 │ 21
0.15 │ 15
0.10 │ 8
0.08 │ 6
0.06 │ 3.8
0.05 │ 2.9
0.04 │ 2.1
0.03 │ 1.3
0.02 │ 0.62
0.015 │ 0.35
0.010 │ 0.154
0.008 │ 0.098
0.006 │ 0.055
0.005 │ 0.0385
0.004 │ 0.0247
0.003 │ 0.0138
0.002 │ 0.0062
0.0015 │ 0.0035
0.001 │ 0.00154
0.0001 │ 0.0000154
───────────────────────────────────┴───────────────────────────────────

An example will be given to illustrate the method of using the diagram
and tables to determine the size of a sedimentation basin to perform
certain required work.

Let it be required to determine the period of retention in a
continuously operated sedimentation basin with good baffling,
corresponding to two properly baffled sedimentation basins in
series. The basins are to remove 60 per cent of the finest
particles which are to have a size of .01 mm. The quantity to be
treated daily is 3,000,000 gallons.

1st. Entering Table 77, we find that the hydraulic value of the
finest particles is .154 mm. per second.

2d. Since we wish to remove 60 per cent of the finest particles,
40 per cent will remain. Since Fig. 154 shows the per cent
remaining after the time _a_⁄_t_ we enter Fig. 154 at 40 per cent
on the ordinates and run horizontally until we encounter Line 4
corresponding to good baffling in Table 78. We then run down
vertically from this intersection and find that the ratio of
_a_⁄_t_ is 1.0.

Then _a_ equals _t_, which means that the period of retention
should equal the time that it takes a particle 0.01 mm. in
diameter to drop from the top to the bottom of the basin. Since
this depends on the depth of the basin it is necessary to
determine the depth before the other dimensions of the basin can
be fixed.

Although this method is seldom used in practice for the final design of
a sedimentation basin, it is a guide to judgment and can be used to
supplement the data obtained from tests.

[Illustration:

FIG. 154.—Hazen’s Diagram, Showing the Relation between the Time of
Settling and the Period of Retention in Various Types of
Sedimentation Basins.

Trans. Am. Society Civil Engineers, Vol. 53, 1904, p. 45.
]

TABLE 78

COMPARISON OF DIFFERENT ARRANGEMENTS OF SETTLING BASINS

(From Hazen)
────────────────────────────────────────┬────────────┬─────────────────
Description of Basins │Line in Fig.│ Values of
│ 154 │ _a_⁄_t_.
────────────────────────────────────────┼────────────┼─────────────────
│ │ Per Cent of
│ │ Matter Removed
────────────────────────────────────────┼────────────┼─────┬─────┬─────
│ │ 50 │ 74 │87.5
────────────────────────────────────────┼────────────┼─────┼─────┼─────
Theoretical maximum. Cannot be reached.│ A │ 0.50│ 0.75│0.875
Surface skimming. Rockner Roth system. │ B │ 0.54│ 0.98│ 1.37
Intermittent basins, reckoned on time of│ C │ │ │
service only. │ │ 0.63│ 1.26│ 1.89
Continuous basin. Theoretical limit. │ D │ 0.69│ 1.38│ 2.08
Close approximation to the above. │ 16 │ 0.71│ 1.45│ 2.23
Very well baffled basin. │ 8 │ 0.73│ 1.62│ 2.37
Good baffling. │ 4 │ 0.76│ 1.66│ 2.75
Two basins, tandem. │ 2 │ 0.82│ 2.00│ 3.70
One long basin, well controlled. │ 1.5 │ 0.90│ 2.34│ 4.50
Intermittent basin in service half time.│ E │ 1.26│ 2.50│ 3.80
One basin, continuous. │ 1 │ 1.0│ 3.00│ 7.00
────────────────────────────────────────┴────────────┴─────┴─────┴─────

The design of sedimentation basins should be based on experimental
observations made upon the quantity of sediment removed at certain rates
of flow and periods of retention in different types of basins. Hazen’s
mathematical analysis is serviceable in making preliminary estimates and
in checking the results. The shape of the tank, period of retention and
rate of flow producing the most desirable results should be duplicated
with the expectation of obtaining similar results or results but
slightly modified from those obtained in the tests. This is the most
satisfactory method of determining the proper period of retention.

=233. Types of Sedimentation Basins.=—A sedimentation basin is a tank
for the removal of suspended matter either by quiescent settlement or by
continuous flow at such a velocity and time of retention as to allow
deposition of suspended matter.[144] The difference between
sedimentation tanks and other forms of tank treatment is that no
chemical or biological action is depended on for the successful
operation of the tank. Sedimentation tanks may be divided into two
classes, grit chambers and plain sedimentation basins.

A grit chamber is a chamber or enlarged channel in which the velocity of
flow is so controlled that only heavy solids, such as grit and sand, are
deposited while the lighter organic solids are carried forward in
suspension. If the velocity of flow is more than about one foot per
second, the tank is a grit chamber and below this velocity it is a plain
sedimentation basin.

There are six general types of plain sedimentation basins:

1st. Rectangular flat-bottom tanks operated on the continuous-flow
principle.

2nd. Rectangular flat-bottom tanks operated on the fill and draw
principle.

3rd. Rectangular or circular hopper-bottom tanks operated on the
continuous-flow principle, with horizontal flow.

4th. Rectangular or circular hopper-bottom tanks operated on the
fill and draw principle, with horizontal flow.

5th. Rectangular or circular hopper-bottom tanks operated on the
continuous-flow principle with vertical flow.

6th. Circular hopper-bottom tanks operated on the continuous-flow
principle with radial flow.

TABLE 79

CRITICAL VELOCITIES FOR THE TRANSPORTATION OF DEBRIS

Sedimentation will not Occur at Higher Velocities
───────────┬───────────────────────────────────┬──────────────────────
Diameter of│Critical Velocity, Feet per Second.│ Size of Screen or
Particle in│ │ Number of Meshes per
Millimeters│ │ Inch
───────────┼───────────────────────────────────┼──────────────────────
│ Specific Gravity │
───────────┼────────┬────────┬────────┬────────┼──────────────────────
│ 1.5 │ 2.0 │ 3.0 │ 5.0 │
───────────┼────────┼────────┼────────┼────────┼──────────────────────
0.010 │0.13 │0.20 │0.22 │0.28 │
0.050 │0.23 │0.34 │0.39 │0.50 │More than 200
0.100 │0.30 │0.42 │0.50 │0.65 │More than 150
0.500 │0.55 │0.73 │0.91 │1.15 │More than 28
1.0 │0.71 │0.92 │1.18 │1.50 │More than 14
1.25 │0.77 │1.00 │1.30 │1.60 │
2.0 │0.92 │1.20 │1.50 │1.90 │More than 10
5.0 │1.30 │1.70 │2.20 │2.60 │More than 4
10 │1.70 │2.20 │2.8 │3.4 │
│ │ │ │ │
Diameter in Millimeters for a Velocity of 1 Foot per Second
│ │ │ │ │
│2.5 │1.25 │0.65 │0.32 │
───────────┴────────┴────────┴────────┴────────┴──────────────────────

=234. Limiting Velocities.=—Sand, clay, bits of metal and other
particles of mineral matter will commence to deposit in appreciable
quantities when the velocity of flow falls below 3 feet per second. The
amount deposited will increase as the velocity decreases. In Table 79
are given the approximate horizontal velocities at which certain size
particles of mineral matter will deposit. At a velocity of about one
foot per second organic matter will commence to deposit. It will be
noticed by interpolation in Table 79,[145] that particles with the same
specific gravity as sand (2.6), larger than one mm. in diameter will
deposit at a velocity of about one foot per second or less, and that
smaller and lighter particles will not deposit at velocity of one foot
per second or greater. It will also be noticed that a velocity of one
foot per minute is sufficiently slow to permit the deposit of the
smallest and lightest particles. For this reason velocities of 1 or 2 or
even 3 feet per second have been adopted as the velocities in grit
chambers and velocities less than 1 foot per minute in plain
sedimentation basins.

=235. Quantity and Character of Grit.=—The amount of material deposited
in grit chambers varies approximately between 0.10 and 0.50 cubic yard
per million gallons. It is to be noted that grit chambers are used only
for combined and storm sewage and for certain industrial wastes. They
are unnecessary for ordinary domestic sewage. The material deposited in
grit chambers operating with a velocity greater than one foot per second
is non-putrescible, inorganic, and inoffensive. It can be used for
filling, for making paths and roadways, or as a filtering material for
sludge drying beds. An analysis of a typical grit chamber sludge is
shown in Table 80.

TABLE 80

ANALYSIS OF GRIT CHAMBER SLUDGE

───────────┬───────────┬───────────┬───────────────────────────────────
Velocity │ Specific │ Per Cent │Calculated to Dry Weight, Per Cent
Feet per │ Gravity │ Moisture │
Second │ │ │
───────────┼───────────┼───────────┼──────────┬──────────┬─────────────
│ │ │ Nitrogen │ Fixed │Miscellaneous
│ │ │ │ Matter │
───────────┼───────────┼───────────┼──────────┼──────────┼─────────────
1.0 │ 1.5 │ 45 │ 20 │ 78 │ 2
───────────┴───────────┴───────────┴──────────┴──────────┴─────────────

=236. Dimensions of Grit Chambers.=—The quantity of sewage to be treated
and the amount and character of the settling solids which it contains
should be determined by measurement and analysis, and the amount of
settling solids to be removed should be determined by a study of the
desired conditions of disposal, in order that a grit chamber that will
accomplish the desired results may be designed. The period of retention
and the velocity of flow are the controlling features in the successful
operation of any grit chamber. These should be determined by experiment
or as the result of experience. Where neither are available, Hazen’s
method can be followed or a decision made based on a study of other grit
chambers. In general, the period of retention in grit chambers is from
30 to 90 seconds, and the velocity of flow is about one foot per second.

After having determined the quantity of sewage to be treated, the
quantity of grit to be stored between cleanings, the period of
retention, the arrangement of the chambers, and the velocity of flow to
be used, the overall dimensions of the chambers are computed. The
capacity of the chamber is fixed as the sum of the quantity of sewage to
be treated during the period of retention and the required storage
capacity for grit accumulated between cleanings. The length of the
chamber is fixed as the product of the velocity of flow and the period
of retention. The cross-sectional area of the portion of the chamber
devoted to sedimentation is fixed as the quotient of the quantity of
flow of sewage per unit time and the velocity of flow. Only the relation
between the width and depth of the portion devoted to sedimentation and
the portion devoted to the storage of grit remain to be determined.
These should be so designed as to give the greatest economy of
construction commensurate with the required results. They will be
affected by the local conditions such as topography, available space,
difficulties of excavation, etc. Common depths in use lie between 8 and
12 feet, although wide variations can be found. A study of the
proportions of existing grit chambers will be of assistance in the
design of other basins.

=237. Existing Grit Chambers.=—The details of some typical grit chambers
are shown in Figs. 155 and 156. The grit chamber at the foot of 58th
Street, in Cleveland, Ohio, is shown in Fig. 155. The special feature of
this structure is the shape of the sedimentation basin, the bottom of
which is formed by sloping steel plates forming a 6–inch longitudinal
slot above the grit storage chamber. Flows between 8,000,000 and
16,000,000 gallons per day are controlled by the outlet weir so that the
velocity of flow remains at one foot per second. This is accomplished by
increasing the depth of flow in the same ratio as the increase in the
rate of flow. The bottoms of the two chambers differ, one having a
special hopper for grit and the other a flat bottom. This is due to the
method of cleaning the chambers, it being necessary in the one with a
flat bottom to shut off the flow when removing the grit while in the one
with the hopper bottom it is hoped to remove the grit by the use of sand
ejectors without stopping the sewage flow. The details of the chamber at
Hamilton, Ontario, are shown in Fig. 156. In studying these drawings the
following features should be noted: 1st, the smooth curves in the
channel to prevent eddies, undue deposition of organic matter, and
difficulties in cleaning; 2nd, the hopper in the upper end of the grit
storage chamber and the slope of the bottom of at least 1:20; and 3rd,
the simplicity of the inlet and outlet devices which may be either stop
planks or cast-iron sluice gates.

[Illustration:

FIG. 155.—Grit Chamber at Cleveland, Ohio.

Eng. Record, Vol. 73, 1916, p. 409.
]

[Illustration:

FIG. 156.—Grit Chamber at Hamilton, Ontario.

Eng. News, Vol. 73, 1915, p. 425.
]

The drawings shown are merely representative of some satisfactory types.
The number and variety of grit chambers in existence is great. In
designing grit chambers consideration must be given to the method of
cleaning. They are ordinarily cleaned by such methods as have been
described for the cleaning of catch-basins in Chapter XII. Continuous
bucket scrapers similar to excavating machines are sometimes used for
the cleaning of large grit chambers. The period between cleanings is
variable. The design should be such as not to require more frequent
cleanings than twice a month under the worst conditions. The
fluctuations in quality and quantity of grit will vary the period
between cleanings.

=238. Number of Grit Chambers.=—The period of retention in grit chambers
is so short and the velocity of flow so near the maximum and minimum
limitations that the wide fluctuations in the rate of discharge in storm
and combined sewers necessitates the construction of a number of
chambers which should be operated in parallel in order to maintain the
velocity between the proper limits. Unless arrangements are made
permitting the cleaning of grit chambers during operation, more than one
grit chamber should be installed in order that when one is being cleaned
the others may be in operation. The number of grit chambers must be
determined by the desired conditions of operation and the cost of
construction. The larger the number of basins the more nearly the flow
in any one basin can be maintained constant, but the more expensive the
construction. The increase in velocity of flow with increasing quantity
is dependent on the outlet arrangements. In a shallow chamber with
vertical sides and a standard sharp-crested rectangular weir at the
outlet the velocity will vary approximately as the cube root of the rate
of flow. Similarly if the outlet is a V notch the velocity will vary as
the fifth root of the rate of flow. In all cases the deeper the basin
the more nearly the velocity varies directly as the rate of flow. The
outlet weir can be arranged as at Cleveland, so that the velocity
remains constant for all rates of flow within certain limits. It is
seldom that more than three grit chambers are necessary to care for the
fluctuations in flow.

=239. Quantity and Characteristics of Sludge from Plain
Sedimentation.=—The sludge removed from plain sedimentation basins is
slimy, offensive, not easily dried, and is highly putrescible and
odoriferous. It contains about 90 per cent moisture and has a specific
gravity from 1.01 to 1.05. The amount removed varies between 2 and 5
cubic yards per million gallons of sewage. The percentage of suspended
matter removed varies between 20 and 60. The total amount removed and
the percentage removal depend on the character of the sewage, the type
of basin, and the period of detention.

=240. Dimensions of Sedimentation Basins.=—The dimensions of a
sedimentation basin are determined by a method similar to the one given
for the determination of the dimensions of a grit chamber in Art. 236.
The capacity of the basin is first fixed upon to give the required
period of sedimentation and sludge storage capacity. The length of the
basin is the product of the velocity and the period of retention. The
length, width, and depth of the basin are normally fixed by
considerations of economy and the limitations of the local conditions,
such as available area, topography, foundations, etc., and examples of
good practice. A study of basins in use shows the relation between
length and width to vary normally between 2:1 and 4:1. Widths greater
than 30 to 50 feet are undesirable because of the danger of cross
currents and back eddies which will reduce the efficiency of the
sedimentation. Depths used in practice vary too widely to act as guides
for any particular design. Theoretically the shallower the basin the
better the result. Tanks abroad have been built as shallow as 3 feet and
some in this country as deep as 16 feet. The economical dimensions can
be determined by trial or by calculus. They will serve as a guide in the
adoption of the final dimensions.

The method to be pursued in determining the economical dimensions of any
engineering structure are:

I. Express the total cost of the structure in terms of as few
variables as possible.

II. Express all of the variables in terms of any one and rewrite
the expression for the total cost in terms of this one variable.

III. Equate the first derivative of the expression with regard to
this variable to zero and solve for the variable. The result will
be the economical value of the variable. The values of the other
variables can be computed from the relations already expressed.

[Illustration:

FIG. 157.—Diagram for the Computation of Economical Basin Dimensions.
]

For example, let it be desired to determine the dimensions of two
continuous-flow sedimentation basins as shown in Fig. 157, in which the
period of retention in each is to be 2 hours, the velocity of flow is
not to exceed one foot per second, and the sludge accumulated will be 3
cubic yards per million gallons of sewage treated. The quantity of
sewage to be treated is 18,000,000 gallons per day. The shortest time
between cleanings will be 2 weeks.

The capacity of each basin must be 2/24 of 18,000,000 gallons, or
200,000 cubic feet in order to allow a period of retention of 2 hours.
To this volume should be added sufficient capacity to allow for the 2
weeks of sludge storage between cleanings. When a basin is being cleaned
the load must be put on the remaining basins. Then if _Q_ represents the
rate of accumulation of sludge per day, _n_ represents the number of
days between cleanings, _m_ represents the number of basins, and _S_ the
sludge capacity of one basin, then

_S_ = (_Q_(_n_ − 1))⁄_m_ + _Q_⁄(_m_ − 1)

The sludge storage capacity for the example given will be approximately
11,000 cubic feet.

In expressing the total cost of the basins let

_h_ = the depth in feet.
_l_ = the length in feet.
_b_ = the width in feet.

The cost of land, floor, etc., per square foot = _p_ dollars.
The cost of wall per foot length = _qh_^2 dollars.
The cost of pipes, valves and appurtenances = _P_ dollars.

Then the total cost _C_ = (3_l_ + 4_b_)_qh_^2 + 2_plb_ + _P_.

It is now necessary to express the three variables _b_, _l_, and
_h_, in terms of one of them. From the relation _Q_ = 2_blh_ it is
possible to rewrite the expression for the total cost as:

_C_ = (3_Q_⁄(2_bh_) + 4_b_)_qh_^2 + (_pQ_)⁄_h_ + _P_.

_C_ = (3_l_ + 2_Q_⁄(_lh_))_qh_^2 + (_pQ_)⁄_h_ + _P_.

Holding _h_ constant and differentiating with regard to _b_ in the
first expression and with regard to _l_ in the second expression,
equating to zero and solving we get:

_b_ = √((3_Q_)⁄(8_h_)) and _l_ = √((2_Q_)⁄(3_h_)).

The economical relation between _b_ and _l_ is therefore

_b_ = 0.75_l_

regardless of the value of _h_.

Substituting these values of _l_ and _b_ in the original
expression for the total cost, it becomes

_C_ = (3√((2_Q_)⁄(3_h_)) + 4√((3_Q_)⁄(8_h_)))_qh_^2 + (_pQ_)⁄_h_ +
_P_.

Differentiating with regard to _h_, equating to zero, and solving

_h_ = 0.45((_pQ_^½)⁄_q_)^⅔.

In the example given if _q_ = 0.2 and _p_ = 1.0 then

_h_ = 11.6 feet, _b_ = 120 feet and _l_ = 160 feet.

Since these are reasonable dimensions and in accord with good practice
they should be used, unless other conditions are unsuitable or the
velocity of flow is too great. A width of channel of 120 feet as
compared to a length of 160 feet is conducive to a poor distribution of
velocity across the basin. A ratio of width to length of about 1:4 is
desirable. In this case, by the use of three baffles parallel to the
length of the basin, thus dividing it into channels 40 feet wide and
11.6 feet deep, the ratio of width to length is changed to 1:4 and the
velocity will be increased only to 0.06 foot per second or 3.6 feet per
minute, which is a reasonable velocity. It could be reduced by
increasing the spacing of the baffles or the depth of the chamber.

Complicated baffling is undesirable. Two or three overflow baffles may
be used to permit quiescent sedimentation in the space thus formed, and
hanging baffles may be placed before the inlet and outlet to break up
surface currents, or to prevent the movement of scum. The hanging
baffles should not extend more than 12 to 18 inches below the water
surface. The inlet and outlet are sometimes arranged to permit the
reversal of flow, and the connecting channels between basins to allow
the operation of any number of basins in series or in parallel, although
such arrangements are more important in water purification. Sewage
should enter and leave at the top of the basin.

[Illustration:

FIG. 158.—Section through a Dortmund Tank.

Depth 20 to 30 feet.
]

Cleaning is facilitated by the location of a central gutter in the
bottom of the basin with the slope of the bottom of the basin towards
the gutter from 1:25 to 1:80 or steeper. A pipe, 2 inches or larger in
diameter, containing water under pressure with connections for hose
placed at frequent intervals is a useful adjunct in flushing the sludge
from the sedimentation basins. For equal capacity, deep vertical flow
tanks are more expensive and difficult to construct than the shallower
rectangular type. Deep tanks are advantageous, however, in that sludge
can sometimes be removed by gravity or by pumping without stopping the
operation of the tank. They will also operate successfully with shorter
periods of detention and higher velocities. The upward velocity should
not be greater than the velocity of sedimentation of the smallest
particle to be removed. The efficiency of sedimentation in them will be
increased by the sedimentation of the larger particles which drag some
of the smaller particles down with them. The Dortmund tank shown in Fig.
158 is an example of this type.

Ordinarily it is not necessary to roof sedimentation basins as the odors
created are not strong, and difficulties with ice are seldom serious.

CHEMICAL PRECIPITATION

=241. The Process.=—Chemical precipitation consists in adding to the
sewage such chemicals as will, by reaction with each other and the
constituents of the sewage, produce a flocculent precipitate and thus
hasten sedimentation. The advantages of this process over plain
sedimentation are a more rapid and thorough removal of suspended matter.
Its disadvantages include the accumulation of a large amount of sludge,
the necessity for skilled attendance, and the expense of chemicals. The
process is not in extensive use as the conditions under which the
advantages outweigh the disadvantages are unusual. Sewage containing
large quantities of substances which will react with a small amount of
an added chemical to produce the required precipitate are the most
favorable for this method of treatment.

Chemical precipitation accomplishes the same result as plain
sedimentation, although the effluent from the chemically precipitated
sewage may be of better quality than that from a plain sedimentation
basin.

=242. Chemicals.=—Lime is practically the only chemical used for the
precipitation of the solid matter in sewage. Commercial lime used for
precipitation consists of calcium oxide (CaO), with large quantities of
impurities. It should be stored in a dry place and protected from undue
exposure to the air to prevent the formation of calcium carbonate
(CaCO_{3}), the formation of which is commonly known as air slacking.
The active work in the formation of the precipitate is performed by the
lime (CaO) or calcium hydroxide (Ca(OH)_{2}). The lime should therefore
be purchased on the basis of available CaO, which may be as low as 10 to
15 per cent in some commercial products. The amount of lime necessary
depends on the quality of the sewage, the period of retention in the
sedimentation basin, the method of application, the required results,
and other less easily measured factors. Full scale tests for the amount
of lime needed to produce certain results are the most satisfactory. In
practice the amount of lime necessary when lime alone is used as a
precipitant has been found to be about 15 grains per gallon. This may be
markedly different, dependent on the quality of the sewage. For acid
sewages, lime alone is not suitable as a precipitant since it is
necessary to add sufficient lime to neutralize the sewage before the
calcium carbonate will be precipitated.

The use of copperas (FeSO_{4}) together with lime, leads to economy in
the use of chemicals as the flocculent precipitate of ferrous hydroxide
(Fe(OH)_{2}) is more voluminous than the precipitate of calcium
carbonate. This is commonly known as the lime and iron process. The
presence of iron in certain trade wastes may reduce the cost of chemical
precipitation, as the necessary amount of copperas is reduced. Where 15
grains of lime alone will be needed per gallon of sewage, the total
amount of chemicals used will be reduced to 8 to 10 grains per gallon
with the use of lime and iron. This combination is less expensive than
the use of lime alone, and is even cheaper where the iron is already
present in the sewage. Such a condition is well illustrated by the
sewage at Worcester, Mass., where the oldest and best known chemical
precipitation plant in the United States is located. The amount of lime
used at this plant has varied between 6 and 10 grains per gallon of
sewage, the normal amount being about 7 grains. No iron is added because
of the amount already in solution.

The results of a series of experiments on the chemical precipitation of
sewage by Allen Hazen, are given in the 1890 Report of the Massachusetts
State Board of Health, on p. 737 of the volume on the Purification of
Water and Sewage. Hazen concludes as the result of his experiments:
concerning lime,

There is a certain definite amount of lime ... which gives as good
or better results than either more or less. This amount is that
which exactly suffices to form normal carbonates with all the
carbonic acid of the sewage. This amount can be determined in a
few minutes by simple titration.

Concerning lime and iron (copperas) he states:

Ordinary house sewage is not sufficiently alkaline to precipitate
copperas, and a small amount of lime must be added to obtain good
results. The quantity of lime required depends both upon the
composition of the sewage and the amount of copperas used, and can
be calculated from titration of the sewage. Very imperfect results
are obtained from too little lime, and, when too much is used, the
excess is wasted, the result being the same as with a smaller
quantity.

In precipitation by ferric sulphate and crude alum, the addition
of lime was found unnecessary, as ordinary sewage contains enough
alkali to decompose these salts. Within reasonable limits the more
of these precipitants used the better is the result, but with very
large quantities the improvement does not compare with the
increased cost.

Using equal values of different precipitants, applied under the
most favorable conditions for each, upon the same sewage, the best
results were obtained from ferric sulphate. Nearly as good results
were obtained from copperas and lime used together, while lime and
alum each gave somewhat inferior effluents.... When lime is used
there is always so much lime left in solution that it is doubtful
if its use would ever be found satisfactory except in case of an
acid sewage.

It is quite impossible to obtain effluents by chemical
precipitation which will compare in organic purity with those
obtained by intermittent filtration through sand.

It is possible to remove from one-half to two-thirds of the
organic matter by precipitation ... and it seems probable that ...
a result may be obtained which will effectually prevent a public
nuisance.

=243. Preparation and Addition of Chemicals.=—Lime is not readily
soluble in water. Therefore, it is not best to add the lime as a powder
to the sewage, but to form a milk of lime, that is, a supersaturated
solution containing from 2,000 to 4,000 grains per gallon, although dry
slaked lime has sometimes been applied directly. The solution is
prepared in tanks in a quantity sufficient for some part of the day’s
run, commonly sufficient to last through one shift of 8 or 10 hours. The
lime is prepared by placing the amount necessary to fill one storage
tank into a slaking tank containing some cold water. Sufficient water is
added to keep the solution just at the boiling point, or steam may be
added to make it boil. After slaking, it is run into the milk-of-lime
solution tank and sufficient water added to bring to the proper
strength. The milk of lime is added in measured quantities, being
controlled by a variable head on a fixed orifice or weir, so that it may
be varied with the amount of sewage flowing through the plant. The
amount of lime to be added is determined by titration with
phenolphthalein, experience indicating the color to be obtained when the
proper amount of lime has been added.

The use of either copperas or alum has been so rare, for the
precipitation of sewage, that a description of the methods of handling
these chemicals as a sewage precipitant is not warranted. An excellent
description of the methods of handling these chemicals in water
purification will be found in “Water Purification” by Ellms.

TABLE 81

RESULTS OF CHEMICAL PRECIPITATION AT WORCESTER, MASSACHUSETTS[146]

──────────────────────────────────────┬──────────┬──────────┬──────────
│ 1900 │ 1910 │ 1920
──────────────────────────────────────┼──────────┼──────────┼──────────
Amount of sewage treated, million │ 4,781 │ 5,317 │ 8,893
gallons │ │ │
Amount of sewage chemically treated, │ 3,650 │ 3,574 │ 7,300
million gallons │ │ │
Gallons of wet sludge per million │ 4,450 │ 4,185 │
gallons of sewage treated │ │ │
Per cent of solids in sludge │ 4.42 │ 8.20 │4.64[147]
Tons of solids │ 7,294 │ 4,182 │6,431[147]
Pounds of lime added per million │ 999[148] │ 762[147] │ 534
gallons of sewage pumped │ │ │
Per cent of organic matter removed: │ │ │
By albuminoid ammonia: │ │ │
Total │52.7[149] │ 58.4 │ 51.9
Suspended │90.0[149] │ 88.7 │ 83.6
By oxygen consumed: │ │ │
Total │62.8[149] │ 61.1 │ 62.5
Suspended │86.6[149] │ 89.7 │ 86.2
──────────────────────────────────────┴──────────┴──────────┴──────────

=244. Results.=—The results of Hazen’s experiments indicate that a
greater amount of suspended matter can be removed in the same time by
chemical precipitation than by plain sedimentation. The percentage of
removal of suspended matter may be as high as 80 to 90 per cent with a
period of retention of 6 to 8 hours and the addition of a proper amount
of chemical. That the method is not always a success is shown by the
results of some tests at Canton, Ohio.[150] The report states:

... lime treatment removes about 50 per cent of the suspended
matter, and in the main about 50 per cent of the organic
matter.... These data are instructive as indicating that the
addition of lime to the Canton sewage in quantities as previously
stated does not materially improve the character of the resulting
effluent over and above that which could be produced by plain
sedimentation alone.

The plant at Worcester, Mass., is the largest in the United States and
information from it is of value. A summary of the results at Worcester
for 1900, 1910, and 1920 are shown in Table 81.

CHAPTER XVI
SEPTICIZATION

=245. The Process.=—Septic action is a biological process caused by the
activity of obligatory or facultative anaërobes as the result of which
certain organic compounds are reduced from higher to lower conditions of
oxidation, some of the solid organic substances are rendered soluble,
and a quantity of gas is given off. Among these gases are: methane,
hydrogen sulphide, and ammonia. The biologic process in the septic tank
represents the downward portion of the cycle of life and death, in which
complex organic compounds are reduced to a more simple condition
available as food for low forms of plant life. The disposal of sewage by
septic action, when introduced, promised the solution of all problems in
sewage treatment. Septic action is now better understood, and it is
known that some of the early claims were unfounded.

The principal advantage of septic action in sewage treatment is the
relatively small amount of sludge which must be cared for compared to
that produced by a plain sedimentation tank. The sludge from a septic
tank may be 25 to 30 per cent and in some cases 40 per cent less in
weight, and 75 to 80 per cent less in volume than the sludge from a
plain sedimentation tank. The most important results of septic action
and the greatest septic activity occur in the deposited organic matter
or sludge. The biologic changes due to septic action which occur in the
liquid portion of the tank contents are of little or no importance. The
installation of a septic tank, although it may fail to prevent the
nuisance calling for abatement, has a remarkable psychological effect in
stilling complaints. Among other advantages are the comparative
inexpensiveness of the tanks and the small amount of attention and
skilled attendance required. The tanks need cleaning once in 6 months to
a year. If properly designed no other attention is necessary.

The septic tank has fallen into some disrepute because of the better
results obtainable by other methods, the occasional discharge of
effluents worse than the influent, the occasional discharge of sludge in
the effluent caused by too violent septic boiling, and on account of
patent litigation. This last difficulty has been overcome as the Cameron
patents expired in 1916. Occasionally the odors given off by the septic
process are highly objectionable and are carried for a long distance.
These odors can be controlled to a large extent by housing the tanks.
Over-septicization must be guarded against as an over-septicized
effluent is more difficult of further treatment or of disposal than a
comparatively fresh, untreated sewage. An over-septicized or stale
sewage is indicated by the presence of large quantities of ammonias,
either free or albuminoid, frequently accompanied by hydrogen sulphide
and other foul-smelling gases. The oxygen demand in an over-septicized
sewage is greater than that in a fresh or more carefully treated sewage.

=246. The Septic Tank.=—A septic tank is a horizontal, continuous-flow,
one-story sedimentation tank through which sewage is allowed to flow
slowly to permit suspended matter to settle to the bottom where it is
retained until anaërobic decomposition is established, resulting in the
changing of some of the suspended organic matter into liquid and gaseous
substances, and a consequent reduction in the quantity of sludge to be
disposed of.[151] It is to be noted that a continuous flow is essential
to a septic tank. Small tanks containing stagnant household sewage are
called cesspools, although sometimes erroneously spoken of as septic
tanks.

Septic and sedimentation tanks differ in their method of operation only
in the period of storage and the frequency of cleaning. The period of
flow in a septic tank is longer and it is cleaned less frequently. The
results obtained by the two processes differ widely. A septic tank can
be converted into a sedimentation tank, or vice versa, by changing the
method of operation, no constructional features requiring alteration.
The purpose of the tank is to store the sludge for such a period of time
that partial liquefaction of the sludge may take place, and thus
minimize the difficulty of sludge disposal. For this reason the sludge
storage capacity of a septic tank is sometimes greater than would be
necessary for a plain sedimentation tank.

TABLE 82

EFFICIENCIES AND PERFORMANCE OF SEPTIC TANK AT COLUMBUS, OHIO

(Report of Sewage Purification, by G. A. Johnson, Nov. 10, 1905)
──────────────┬────┬─────┬────┬────┬────┬────┬────┬─────┬─────┬───┬────┬────
Month, │Aug.│Sept.│Oct.│Nov.│Dec.│Jan.│Feb.│March│April│May│June│Avg.
1904–1905 │ │ │ │ │ │ │ │ │ │ │ │
──────────────┼────┼─────┼────┼────┼────┼────┼────┼─────┼─────┼───┼────┼────
Temperature, │ │ │ │ │ │ │ │ │ │ │ │
degrees F. │ │ │ │ │ │ │ │ │ │ │ │
Influent │ 69 │ 70 │ 65 │ 60 │ 54 │ 51 │ 48 │ 50 │ 57 │61 │ 67 │
Effluent │ 69 │ 68 │ 64 │ 59 │ 52 │ 48 │ 45 │ 49 │ 57 │62 │ 68 │
Oxygen │ │ │ │ │ │ │ │ │ │ │ │
consumed, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │ 49 │ 50 │ 52 │ 47 │ 43 │ 51 │ 44 │ 47 │ 53 │33 │ 40 │ 47
Effluent │ 40 │ 36 │ 40 │ 39 │ 37 │ 35 │ 37 │ 39 │ 50 │34 │ 33 │ 38
Per cent │ 18 │ 28 │ 23 │ 15 │ 16 │ 31 │ 16 │ 17 │ 6 │–3 │ 18 │ 19
removal │ │ │ │ │ │ │ │ │ │ │ │
Organic │ │ │ │ │ │ │ │ │ │ │ │
nitrogen, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │6.5 │ 8.2 │9.3 │8.4 │8.8 │8.5 │6.7 │ 6.4 │ 7.9 │6.1│6.7 │7.8
Effluent │7.3 │ 5.5 │6.0 │7.4 │8.2 │7.0 │5.4 │ 5.5 │ 5.2 │ │ │
Per cent │–12 │ 32 │ 35 │ 12 │ 7 │ 18 │ 19 │ 14 │ 25 │30 │ 19 │ 19
removal │ │ │ │ │ │ │ │ │ │ │ │
Free ammonia, │ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Influent │9.7 │12.2 │12.4│16.3│14.7│10.8│8.3 │ 9.9 │12.3 │6.9│8.3 │11.7
Effluent │10.5│11.5 │12.4│17.2│14.3│11.1│8.9 │10.7 │14.9 │9.0│8.7 │12.1
Per cent │ –8 │ 6 │ 0 │ –6 │ 3 │ –3 │ –7 │ –8 │ –21 │–23│ –5 │ –3
removal │ │ │ │ │ │ │ │ │ │ │ │
│ │ │ │ │ │ │ │ │ │ │ │
Residue on │ │ │ │ │ │ │ │ │ │ │ │
Evaporation,│ │ │ │ │ │ │ │ │ │ │ │
parts per │ │ │ │ │ │ │ │ │ │ │ │
million: │ │ │ │ │ │ │ │ │ │ │ │
Total: │ │ │ │ │ │ │ │ │ │ │ │
Influent │990 │ 952 │993 │961 │989 │949 │890 │ 850 │1067 │912│945 │946
Effluent │935 │ 891 │893 │916 │925 │886 │843 │ 782 │ 895 │800│835 │873
Per cent │ 6 │ 6 │ 10 │ 5 │ 6 │ 6 │ 5 │ 8 │ 16 │12 │ 12 │ 8
removal │ │ │ │ │ │ │ │ │ │ │ │
Volatile: │ │ │ │ │ │ │ │ │ │ │ │
Influent │231 │ 184 │162 │175 │156 │167 │156 │ 168 │ 212 │122│162 │166
Effluent │206 │ 160 │129 │148 │137 │137 │134 │ 137 │ 147 │103│144 │139
Per cent │ 11 │ 13 │ 20 │ 15 │ 12 │ 18 │ 14 │ 18 │ 31 │16 │ 11 │ 16
removal │ │ │ │ │ │ │ │ │ │ │ │
Mineral: │ │ │ │ │ │ │ │ │ │ │ │
Influent │759 │ 768 │831 │786 │833 │782 │734 │ 682 │ 855 │700│783 │780
Effluent │729 │ 731 │764 │768 │788 │749 │709 │ 645 │ 748 │697│691 │734
Per cent │ 4 │ 5 │ 8 │ 2 │ 5 │ 4 │ 3 │ 5 │ 11 │ 1 │ 12 │ 6
removal │ │ │ │ │ │ │ │ │ │ │ │
Cubic yards │ │ │0.10│1.24│1.09│1.17│0.65│0.63 │0.57 │ │1.34│
wet sludge │ │ │ │ │ │ │ │ │ │ │ │
per million │ │ │ │ │ │ │ │ │ │ │ │
gallons: │ │ │ │ │ │ │ │ │ │ │ │
Per cent │ │ │ │ │ │ │ │ │ │ │ │
removal of │ │ │ │ │ │ │ │ │ │ │ │
suspended │ │ │ │ │ │ │ │ │ │ │ │
matter: │ │ │ │ │ │ │ │ │ │ │ │
Total │ 59 │ 54 │ 56 │ 51 │ 42 │ 48 │ 32 │ 47 │ 56 │67 │ 53 │ 50
Volatile │ 60 │ 41 │ 48 │ 52 │ 44 │ 55 │ 47 │ 47 │ 62 │80 │ 15 │ 48
Fixed │ 75 │ 65 │ 60 │ 51 │ 40 │ 38 │ 19 │ 48 │ 53 │64 │ 67 │ 51
Gas evolved, │ │ │ │ │ │ │ │ 29 │ 14 │41 │ 50 │
cubic feet │ │ │ │ │ │ │ │ │ │ │ │
per day: │ │ │ │ │ │ │ │ │ │ │ │
──────────────┴────┴─────┴────┴────┴────┴────┴────┴─────┴─────┴───┴────┴────

=247. Results of Septic Action.=—The results obtained from the septic
tanks at the Columbus Sewage Experiment Station are given in Table 82.
The effluent is higher than the influent in free ammonia, but the
reduction of other constituents, particularly suspended matter, is
marked.

Septic action is sensitive to temperature changes, and to certain
constituents of the incoming sewage. Cold weather or an acid influent
will inhibit septicization. In winter the liquefaction of sludge may
practically cease, whereas in summer liquefaction may exceed deposition.
The amount of gas generated is a measure of the relative amount of
septic action. The rapid generation of gas in warm weather disturbs the
settled sludge and may cause a deterioration of the quality of the
effluent because of the presence of decomposed sludge. The results in
Table 82 show the effect of cold weather on the process. In warm weather
the violent ebullition of gas sometimes causes the discharge of sludge
in the effluent, resulting in a liquid more difficult of disposal than
the incoming sewage. Since septic action is dependent on the presence of
certain forms of bacteria, where these are absent there will be no
septic action. Sewage generally contains the forms of bacteria necessary
for this action but it has occasionally been found necessary to seed new
tanks in order to start septic action.

The sludge from septic tanks is usually black, with a slight odor,
though in some cases this odor may be highly offensive. The sludge will
flow sluggishly. It can be pumped by centrifugal pumps and it will flow
through pipes and channels. It has a moisture content of about 90 per
cent and a specific gravity of about 1.03. It is dried with difficulty
on open-air drying beds, and it is worthless as a fertilizer. The
composition of some septic sludges are shown in Table 83.

=248. Design of Septic Tanks.=—The sedimentation chambers of a septic
tank are designed on the same principles as the sedimentation basins
described in Art. 240. The velocity of flow should not exceed one foot
per minute. The channels should be straight and free from obstructions
causing back eddies. The ratio of length to width of channel should be
between 2 : 1 to 4 : 1 with a width not exceeding 50 feet, and desirably
narrower. The depths used vary between 5 and 10 feet, exclusive of the
sludge storage capacity. Hanging baffles should be placed, one before
the inlet and the other in front of the outlet, so as to distribute the
incoming sewage over the tank, and to prevent scum from passing into the
outlet. The baffles should hang about 12 inches below the surface of the
sewage. Intermediate baffles are sometimes desirable to prevent the
movement of sludge or scum towards the outlet. The placing of baffles
must be considered carefully as injudicious baffling may lessen the
effectiveness of a tank by so concentrating the currents as to prevent
sedimentation or the accumulation of sludge. Baffles should be built of
concrete or brick, as wood or metal in contact with septic sewage
deteriorates rapidly. In designing the sludge storage chambers it may be
assumed that one-half of the organic matter and none of the mineral
matter will be liquefied or gasified. The net storage volume allowed is
about 2 to 3 cubic yards per million gallons of sewage treated.
Variations between 0.1 and 10.0 cubic yards have been recorded, however.
If grit is carried in the sewage to be treated, it should be removed by
the installation of a grit chamber before the sewage enters the septic
tank.

TABLE 83

ANALYSIS OF TANK SLUDGES

──────────┬────────┬────────┬────────────────────────────
Place │Specific│Per Cent│ Per Cent in Terms of Dry
│Gravity │Moisture│ Matter
│ │ │
│ │ │
│ │ │
──────────┼────────┼────────┼────────┬─────┬────────┬────
│ │ │Volatile│Fixed│Nitrogen│Fat
──────────┼────────┼────────┼────────┼─────┼────────┼────
Mansfield,│ 1.11│ 80.8│ │ │ │
O. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Chicago, │ 1.03│ 90│ 40│ 60│ 1.9│ 7.0
Ill. │ │ │ │ │ │
│ │ │ │ │ │
Columbus, │ 1.09│ 83.3│ 4.4│ 16.7│ 0.25│0.94
O. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Atlanta, │ 1.02│ 87.1│ 39.1│ 60.9│ 1.25│6.11
Ga. │ │ │ │ │ │
│ │ │ │ │ │
Baltimore,│ 1.02│ 91.9│ 66.2│ │ 2.45│4.02
Md. │ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ │ │ │ │
Baltimore,│ 1.02│ 92.4│ 62.7│ │ 2.75│
Md. │ │ │ │ │ │
Baltimore,│ │ 79.2│ 73.8│ │ 2.64│9.00
Md. │ │ │ │ │ │
Baltimore,│ │ 92.4│ 58.0│ 3.19│ │
Md. │ │ │ │ │ │
──────────┴────────┴────────┴────────┴─────┴────────┴────

──────────┬────────┬────────┬─────────┬────────────
Place │ Cubic │ Pounds │ Kind of │ Reference
│Yard per│ per │ Sludge │
│Million │Million │ │
│Gallons,│Gallons,│ │
│ Wet │ Dry │ │
──────────┼────────┼────────┼─────────┼────────────
│ │ │ │
──────────┼────────┼────────┼─────────┼────────────
Mansfield,│ │ │Septic │1908 Report,
O. │ │ │ │ State
│ │ │ │ Board of
│ │ │ │ Health
Chicago, │ 1.0│ 200│Septic │
Ill. │ │ │ │
│ 1.5│ 300│ │
Columbus, │ │ │Septic │G. A.
O. │ │ │ │ Johnson
│ │ │ │ 1905
│ │ │ │ Report
Atlanta, │ │ │Imhoff │Eng. Rec.,
Ga. │ │ │ │ V. 72,
│ │ │ │ 1915, p. 4
Baltimore,│ │ │Digestion│Eng.
Md. │ │ │ Tank │ News-Rec.,
│ │ │ │ V. 87,
│ │ │ │ 1921, p.
│ │ │ │ 98
Baltimore,│ │ │Imhoff │ do.
Md. │ │ │ │
Baltimore,│ │ │Raw │ do.
Md. │ │ │ Sludge │
Baltimore,│ │ │Settling │ do.
Md. │ │ │ Basin │
──────────┴────────┴────────┴─────────┴────────────

Two or more tanks should be constructed to allow for the shut down of
one for cleaning and to increase the elasticity of the plant. The number
of tanks to be used is dependent on the total quantity of sewage and the
fluctuations in rate of flow. An average period of retention of about 9
to 10 hours with a minimum period of 6 hours during maximum flow is a
fair average to be assumed for design. The period of retention should
not exceed about 24 hours, as the sewage may become over-septicized. The
sludge storage period should be from 6 to 12 months.

A cover is not necessary to the successful operation of a septic tank.
Covers are sometimes used with success, however, in reducing the
dissemination of odors from the tank. They are also useful in retaining
the heat of the sewage in cold weather and thus aid in promoting
bacterial activity. Types of covers vary from a building erected over
the tank to a flat slab set close to the surface of the sewage. In the
design of a cover, good ventilation should be provided to permit the
escape of the gases, and easy access should be provided for cleaning.
Tightly covered tanks or tanks with too little ventilation have resulted
in serious explosions, as at Saratoga Springs in 1906 and at
Florenceville, N. C., in 1915.[152]

The sludge may be removed through drains in the bottom of the tank as
described for sedimentation basins, or where such drains are not
feasible the sludge and sewage are pumped out. For this purpose a pump
may be installed permanently at the tank, or for small tanks portable
pumps are sometimes used. Septic tanks should be cleaned as infrequently
as possible without permitting the overflow of sludge into the effluent.
The less frequent the cleaning the less the amount of sludge removed
since digestion is continuous throughout the sludge. It is necessary to
clean when the tank becomes so filled with sludge, that the period of
retention is materially reduced, or sludge is being carried over into
the effluent.

The details of the septic tank at Champaign, Illinois, are shown in Fig.
159. This tank was designed by Prof. A. N. Talbot, and was put in
service on Nov. 1, 1897. It was among the first of such tanks to be
installed in the United States. The tank shown in Fig. 159 is an example
of present day practice in single-story septic tank design.

[Illustration:

FIG. 159.—Septic Tank at Champaign, Illinois.
]

[Illustration:

FIG. 160.—Design for a Residential Septic Tank for a Family of Ten.
Illinois State Board of Health.
]

Small septic tanks for rural homes of 5 to 15 persons, or on a slightly
larger scale for country schools and small institutions, are little more
than glorified cesspools. Nevertheless much attention has been given to
the construction of such tanks by the National Government and by state
boards of health.[153] The recommendations of some of these boards have
been compiled in Table 84. A typical method for the construction of such
tanks, as recommended by the Illinois State Board of Health, is shown in
Fig. 160. A subsurface filter, into which the effluent is discharged, is
an important adjunct where no adequate stream is available to receive
the discharge from the tank.

TABLE 84

CAPACITIES OF SEPTIC TANKS FOR SMALL INSTALLATIONS

──────────────┬─────────────┬─────────┬─────────┬──────────────────────
Rule │ Number, │Capacity,│Period of│ Remarks
Recommended by│ Persons │ Gallons │Retention│
State Board of│ │ per │ │
Health │ │ Person │ │
──────────────┼─────────────┼─────────┼─────────┼──────────────────────
Wisconsin │ │ 30 │24 hours │
Ohio │ 4 to 10 │ 50 │ │Not less than 560
│ │ │ │ gallons
Kentucky │ │ │24 to 48 │Not more than 5 feet
│ │ │ hours │ deep
Texas │ │ │24 hours │
Illinois │ │ 45 │24 hours │
U.S. Dept. │ │ 40 │24 hours │25 per cent additional
Agriculture.│ │ │ │
│ │ │ │capacity for sludge
North Carolina│Large Schools│ 15 │ │Not less than 500
│ │ │ │ gallons
North Carolina│ 20 pupils │ 25 │ │
North Carolina│Medium School│ 20 │ │
North Carolina│ Homes │25 to 30 │ │
──────────────┴─────────────┴─────────┴─────────┴──────────────────────

=249. Imhoff Tanks.=—In the discussion of septic tanks it has been
brought out that one of the objections to their use is the unloading of
sludge into the effluent which occasionally causes a greater amount of
suspended matter in the effluent than in the influent. The Imhoff tank
is a form of septic tank so arranged that this difficulty is overcome.
It combines the advantages of the septic and sedimentation tanks and
overcomes some of their disadvantages. An Imhoff tank is a device for
the treatment of sewage, consisting of a tank divided into 3
compartments. The upper compartment is called the sedimentation chamber.
In it the sedimentation of suspended solids causes them to drop through
a slot in the bottom of the chamber to the lower compartment called the
_digestion_ chamber. In this chamber the solid matter is humified by an
action similar to that in a plain septic tank. The generated gases
escape from the digestion chamber to the surface through the third
compartment called the _transition_ or _scum_ chamber. Sections of
Imhoff tanks are shown in Fig. 161. It is essential to the construction
of an Imhoff tank that the slot in the bottom of the sedimentation
chamber does not permit the return of gases through the sedimentation
chamber, and that there be no flow in the digestion chamber.

[Illustration:

FIG. 161.—Typical Sections through Imhoff Tanks.

Eng. News, Vol. 75, p. 15.
]

The Imhoff tank was invented by Dr. Karl Imhoff, director of the Emscher
Sewerage District in Germany. Its design is patented in the United
States, the control of the patent being in the hands of the Pacific
Flush Tank Co. of Chicago, which collects the royalties which are
payable when construction work begins. The fee for a tank serving 100
persons is $10, for 1,000 persons is $80 and for 100,000 persons is
$2550. The rate of the royalty reduces in proportion as the number of
persons served increases.[154] As designed by Imhoff and used in Germany
the tanks were of the radial flow type and quite deep. The depth, as
explained by Imhoff, is one of the chief requirements for the successful
operation of the tank. As adapted to American practice the tanks are
generally of the longitudinal flow type and are not made so deep. An
isometric view of a radial flow Imhoff tank is shown in Fig. 162. The
sewage enters at the center of the tank near the surface and flows
radially outward under the scum ring and over a weir placed near the
circumference of the tank. One type of longitudinal flow tank is shown
in isometric view in Fig. 163.

[Illustration:

FIG. 162.—Sketch of Radial Flow Imhoff Tank at Baltimore, Maryland.

Eng. Record, Vol. 70, p. 5.
]

[Illustration:

FIG. 163.—Isometric View of Longitudinal Flow Imhoff Tank at Cleburne,
Texas.

Eng. News, Vol. 76, p. 1029.
]

=250. Design of Imhoff Tanks.=—The velocity of flow, period of
retention, and the quantity of sewage to be treated determine the
dimensions of the _sedimentation chamber_ as in other forms of tanks.
The velocity of flow should not exceed one foot per minute, with a
period of retention of 2 to 3 hours. A greater velocity than one foot
per minute results in less efficient sedimentation. A longer period of
retention than the approximate limit set may result in a septic or stale
effluent, and a shorter period may result in loss of efficiency of
sedimentation. The bottom of the sedimentation chamber should slope not
less than 1½ vertical to 1 horizontal, in order that deposited material
will descend into the sludge digestion chamber. Provision should be made
for cleaning these sloping surfaces by placing a walk on the top of the
tank from which a squeegee can be handled to push down accumulated
deposits. It is desirable to make the material of the sides and bottom
of the sedimentation chamber as smooth as possible to assist in
preventing the retention of sludge in the sedimentation chamber. Wood,
glass, and concrete have been used. The latter is the more common and
has been found to be satisfactory. The length of the sedimentation
chamber is fixed by the velocity of flow and the period of retention.
Tanks are seldom built over 100 feet in length, however, because of the
resulting unevenness in the accumulation of sludge. Where longer flows
are desired two or more tanks may be operated in series. The width of
the chamber is fixed by considerations of economy and convenience. It
should not be made so great as to permit cross currents. In general a
narrow chamber is desirable. Satisfactory chambers have been constructed
at depths between 5 and 15 feet. The depth of the sedimentation chamber
and the depth of the digestion chamber each equal about one-half of the
total depth of the tank. This should be made as deep as possible up to a
limit of 30 to 35 feet, with due consideration of the difficulties of
excavation. C. F. Mebus states:[155]

In 9 of the largest representative United States installations,
the depth from the flow line to the slot varies from 10 feet 10
inches to 13 feet 6 inches.

Imhoff states, concerning the depth of tanks:

Deep tanks are to be preferred to shallow tanks because in them
the decomposition of the sludge is improved. This is so because in
the deeper tanks the temperature is maintained more uniformly and
because the stirring action of the rising gas bubbles is more
intense.

The stirring action of the gas bubbles is desirable as it brings the
fresh sludge more quickly under the influence of the active bacterial
agents. The greater pressure on the sludge in deep tanks also reduces
its moisture content.

Two or more sedimentation chambers are sometimes used over one sludge
digestion chamber in order to avoid the depths called for by the sloping
sides of a single sedimentation chamber. An objection to multiple-flow
chambers is the possibility of interchange of liquid from one chamber to
another through the common digestion chamber.

The inlet and outlet devices should be so constructed that the direction
of flow in the tank can be reversed in order that the accumulated sludge
may be more evenly distributed in the hoppers of the digestion chamber.
The sewage should leave the sedimentation chamber over a broad crested
weir in order to minimize fluctuations in the level of sewage in the
tank. The gases in the digesting sludge are sensitive to slight changes
in pressure. A lowering of the level of sewage will release compressed
gas and will too violently disturb the sludge in the digestion chamber.
Hanging baffles, submerged 12 to 16 inches and projecting 12 inches
above the surface of the sewage, should be placed in front of the inlet
and outlet, and in long tanks intermediate baffles should be placed to
prevent the movement of scum or its escape into the effluent. An Imhoff
tank which is operating properly should not have any scum on the surface
of the sewage in the sedimentation chamber.

The _slot_ or opening at the bottom of the sedimentation chamber should
not be less than 6 inches wide between the lips. Wider slots are
preferable, but too wide a slot will involve too much loss of volume in
the digestion chamber. One lip of the slot should project at least 3
inches horizontally under the other so as to prevent the return of gases
through the sedimentation chamber. A triangular beam may be used as
shown in Fig. 161 A. This method of construction is advantageous in
increasing the available capacity for sludge storage.

The _digestion chamber_ should be designed to store sludge from 6 to 12
months, the longer storage periods being used for smaller installations.
In warm climates a shorter period may be used with success. The amount
of sludge that will be accumulated is as uncertain as in other forms of
sewage treatment. A widely quoted empirical formula, presented in
“Sewage Sludge” by Allen, states:

_C_ = 10.5 _PD_ for combined sewage;
_C_ = 5.25 _PD_ for separate sewage,

in which _C_ = the effective capacity of the digestion chamber in
cubic feet;

_P_ = the population served, expressed in thousands;

_D_ = the number of days of storage of sludge.

The effective capacity of the chamber is measured as the entire volume
of the chamber approximately 18 inches below the lower lip of the slot.
The capacity as computed from the above formula is assumed as
satisfactory for a deep tank. Frank and Fries[156] recommend the
increase of the capacity for shallow tanks to compensate for the
decreased hydrostatic pressure. In any event the formula can be no more
than a guide to design. No formula can be of equal value to data
accumulated from tests on the sewage to be treated. The Illinois State
Board of Health requires 3 cubic yards of sludge digestion space per
million gallons of sewage treated. Frank and Fries recommend an
allowance of 0.007 cubic foot of storage per inhabitant per day for
combined sewage and one-half that amount for separate sewage. If this is
based on 80 per cent moisture content, the volume for other percentages
of moisture can be easily computed. An average figure used in the
Emscher District is one cubic foot capacity for each inhabitant for the
combined system, and three-fourths of this for the separate system.
Metcalf and Eddy[157] recommend the following method for the
determination of the sludge storage capacity: (1) From analyses of the
sewage or study of the sources ascertain the amount of suspended matter.
(2) Assume, or determine by test, the amount which will settle in the
period of detention selected, say 60 per cent in 3 hours. (3) Estimate
the amount which will be digested in the sludge chamber at about 25 per
cent, leaving 75 per cent to be stored. (4) Estimate the percentage
moisture in the sludge conservatively, say 85 per cent. The total volume
of sludge can then be computed. This method is more rational than the
use of empirical formulas, but because of the estimates which must be
made its results will probably be of no greater accuracy than those
obtained empirically.

The digestion chamber is made in the form of an inverted cone or pyramid
with side slopes at most about 2 horizontal to 1 vertical and preferably
much steeper without necessitating too great a depth of tank. The
purpose of the steep slope is to concentrate the sludge at the bottom of
the hopper thus formed. Concrete is ordinarily used as the material of
construction as a smooth surface can be obtained by proper workmanship.
Where flat slopes have been used, a water pipe perforated at intervals
of 6 to 12 inches may be placed at the top of the slopes, and water
admitted for a short time to move the sludge when the tank is being
cleaned.

A cast-iron pipe, 6 to 8 inches in diameter, is supported in an
approximately vertical position with its open lower end supported about
12 inches above the lowest point in the digestion chamber. This is used
for the removal of sludge. A straight pipe from the bottom of the tank
to a free opening in the atmosphere is desirable in order to allow the
cleaning of the pipe or the loosening of sludge at the start, and to
prevent the accumulation of gas pockets. The sludge is led off through
an approximately horizontal branch so located that from 4 to 6 feet of
head are available for the discharge of the sludge. A valve is placed on
the horizontal section of the pipe. A sludge pipe is shown in Fig. 162
and 163. Under such conditions, when the sludge valve is opened the
sludge should flow freely. The hydraulic slope to insure proper sludge
flow should not be less than 12 to 16 per cent. Where it is not possible
to remove the sludge by gravity an air lift is the best method of
raising it.

The volume of the _transition_ or _scum_ chamber should equal about
one-half that of the digestion chamber. The surface area of the scum
chamber exposed to the atmosphere should be 25 to 30 per cent of the
horizontal projection of the top of the digestion chamber. Some tanks
have operated successfully with only 10 per cent, but troubles from
foaming can usually be anticipated unless ample area for the escape of
gases has been provided.

All portions of the surface of the tank should be made accessible in
order that scum and floating objects can be broken up or removed. The
gas vents should be made large enough so that access can be gained to
the sludge chamber through them when the tank is empty.

Precautions should be taken against the wrecking of the tank by high
ground water when the tank is emptied. With an empty tank and high
ground water there is a tendency for the tank to float. The flotation of
the tank may be prevented by building the tank of massive concrete with
a heavy concrete roof, by underdraining the foundation, or by the
installation of valves which will open inwards when the ground water is
higher than the sewage in the tank. Dependence should not be placed on
the attendant to keep the tank full during periods of high ground water.

Roofs are not essential to the successful operation of Imhoff tanks.
They are sometimes used, however, as for septic tanks, to assist in
controlling the dissemination of odors, to minimize the tendency of the
sewage to freeze, and to aid in bacterial activity. In the construction
of a roof, ventilation must be provided as well as ready access to the
tank for inspection, cleaning, and repairs.

=251. Imhoff Tank Results.=—The Imhoff tank has the advantage over the
septic tank that it will not deliver sludge in the effluent, except
under unusual conditions. The Imhoff tank serves to digest sludge better
than a septic tank and it will deliver a fresher effluent than a plain
sedimentation tank. Imhoff sludge is more easily dried and disposed of
than the sludge from either a septic or a sedimentation tank. This is
because it has been more thoroughly humified and contains only about 80
per cent of moisture. As it comes from the tank it is almost black,
flows freely and is filled with small bubbles of gas which expand on the
release of pressure from the bottom of the tank, thus giving the sludge
a porous, sponge-like consistency which aids in drying. When dry it has
an inoffensive odor like garden soil, and it can be used for filling
waste land, without further putrefaction. It has not been used
successfully as a fertilizer.

Offensive odors are occasionally given off by Imhoff tanks, even when
properly operated. They also have a tendency to “boil” or foam. The
boiling may be quite violent, forcing scum over the top of the
transition chamber and sludge through the slot in the sedimentation
chamber, thus injuring the quality of the effluent. The scum on the
surface of the transition chamber may become so thick or so solidly
frozen as to prevent the escape of gas with the result that sludge may
be driven into the sedimentation chamber.

Some chemical analyses of Imhoff tank influents and effluents are given
in Table 86 and the analyses of some sludges from Imhoff tanks are given
in Table 83. It is to be noted that the nitrites and nitrates are still
present in the effluent, whereas they are seldom present in the effluent
from septic tanks. The per cent of moisture in the Imhoff sludge is less
than that in the septic tank sludge, and its specific gravity is higher.
It is heavier and more compact because of the longer time and the
greater pressure it has been subjected to in the digestion chamber of
the Imhoff tank.

=252. Status of Imhoff Tanks.=—The introduction of the Imhoff tank into
the United States, like the introduction of the Burkli-Ziegler Run-Off
Formula, and Kutter’s Formula, is to be credited to Dr. Rudolph Hering.
He advised Dr. Imhoff to come to the United States to introduce his tank
and gave him material aid through recommendations and introductions to
engineers. Shortly after its introduction, in 1907, the tank became very
popular and installations were made in many cities. This popularity was
caused by a growing dissatisfaction with the septic tank, the litigation
then progressing over septic patents, the production of inoffensive
sludge, and the promising results which had been obtained in Germany. As
a result of the extended experience obtained in the use of Imhoff tanks
American engineers have learned that, like all other sewage treatment
devices introduced up to the present time, the Imhoff tank requires
experienced attention for its successful operation. These tanks are now
being installed in the place of septic tanks, and they are frequently
used in conjunction with sprinkling filters.

=253. Operation of Imhoff Tanks.=—The important feature in the
successful operation of an Imhoff tank is the proper control of the
sludge and transition chambers. During the ripening process, which may
occupy 2 weeks to 3 months after the start of the tank, offensive odors
may be given off, the tank may foam violently, and scum may boil over
into the sedimentation chamber. This is usually due to an acid condition
in the digestion chamber which may possibly be overcome by the addition
of lime. A very fresh influent will have a similar effect. Too violent
boiling is not likely to occur where the area for the escape of gas has
been made large and the gas is not confined. Any accumulation of scum
should be broken up and pushed down into the digestion chamber, or
removed from the tank. The stream from a fire hose is useful in breaking
up scum. The side walls of the sedimentation chamber should be squeegeed
as frequently as is necessary to keep them free from sludge, which may
be as often as once or twice a week. Material floating on the surface of
the sedimentation chamber should be removed from the tank or sunk into
the digestion chamber through the gas vents in the transition chamber.

No sludge should be removed, except for the taking of samples, until the
tank is well ripened. The ripening of the sludge can be determined by
examining a sample and observing its color and odor. An odorless, black,
granular, well humified sludge is indicative of a ripened tank. After
the tank has ripened, sludge should be removed in small quantities at 2
to 3–week intervals, except in cold or rainy weather. The sludge should
be drawn off slowly to insure the removal of the oldest sludge at the
bottom of the digestion chamber. After the drawing off of the sludge has
ceased the pipe should be flushed with fresh water to prevent its
clogging with dried sludge in the interim until the next removal. Under
no circumstances should all the sludge in the tank be removed at any
time. The removal of some sludge during foaming after ripening may
reduce or stop the foaming. The ripening of a tank can be hastened by
adding some sludge from a tank already ripened.

Sludge should not be allowed to accumulate within 18 inches of the slot
at the bottom of the digestion chamber. The elevation of the surface of
the sludge can be located by lowering into the tank, a stoppered,
wide-mouthed bottle on the end of a stick. The stopper is pulled out by
a string when the bottle is at some known elevation. The bottle is then
carefully raised and observed for the presence of sludge. The process is
repeated with the bottle at different elevations until the surface of
the sludge has been discovered. Another method is to place the suction
pipe of a small hand pump at known points, successively increasing in
depth, and to pump in each position until one position is found at which
sludge appears in the pump. When the sludge in one portion of the
digestion chamber has risen higher than in another portion, the
direction of flow in the sedimentation chamber should be reversed if
possible. In the ordinary routine of operation it is never necessary to
shut down an Imhoff tank. Sludge is removed while the tank is operating.
The shut down of a tank will be caused by accidents and breaks to the
structure or control devices.

=254. Other Tanks.=—The Travis Hydrolytic Tank represents a step in the
development from the septic tank to the Imhoff tank. The Doten tank and
the Alvord tank are recent developments, and are somewhat similar in
operation to the Imhoff tank.

The Travis Hydrolytic Tank when first designed differed from the later
design of the Imhoff tank in the slot between the sedimentation chamber
and the digestion chamber which was not trapped against the escape of
gas from the latter to the former, and in operation a small quantity of
fresh sewage was allowed to flow through the digestion chamber. The tank
is called a hydrolytic tank because some solids are liquefied in it. The
tank is mainly of historic interest as designs similar to it are rarely
made to-day. Better results are obtained from the use of the Imhoff
tank. Recent developments have altered the original design of the Travis
tank so that it is hardly recognizable. The Travis tank at Luton, Eng.,
is shown in Fig. 164. The detailed description given in the _Engineering
News_ in connection with this illustration shows that the governing
object of the design is to separate as quickly as possible the sludge
deposited by the sewage without septic action being set up. To aid in
the collection and settlement of flocculent matter vertical wooden grids
or colloiders are used. The suspended matter strikes these and forms a
slimy deposit on them that in a short time slips off in pieces large
enough to settle readily.

[Illustration:

FIG. 164.—Plan and Section of Hydrolytic Tank at Luton, England.

Eng. News, Vol. 76, 1916, p. 194.
]

[Illustration:

FIG. 165.—Doten Tank for Army Cantonment Sewage Disposal.

Eng. News-Record, Vol. 79, 1917, p. 931.
]

The Doten tank[158] is a single-storied, hopper-bottomed septic tank,
views of which are shown in Fig. 165. It was devised by L. S. Doten for
army cantonments during the War. Its chief purpose was to avoid the
foaming and frothing so common to Imhoff tanks when overdosed with fresh
sewage. The first Alvord tank was constructed in Madison, Wis., in
1913.[159] As now constructed the tank consists of three deep,
single-story compartments with hopper bottoms. These compartments are
arranged side by side in any one unit. Sewage enters at the surface of
one of the compartments and is retained here during one-half of the
total period of retention. It leaves the first compartment over a weir
and passes in a channel over the top of the intermediate compartment to
the third or effluent compartment, where it is held for the remainder of
the period of detention. Accumulated scum and sludge are drawn off into
the intermediate compartment at the will of the operator, this
compartment being used for sludge digestion only. Such tanks as the
Doten and the Alvord have been used for plants receiving very fresh
sewages such as is discharged from military cantonments, in order to
assist in the prevention of the foaming to be expected from an Imhoff
tank receiving such a fresh influent. The tanks are suitable for small
installations, or where excavation to the depth required for an Imhoff
tank is not practicable.

CHAPTER XVII
FILTRATION AND IRRIGATION

=255. Theory.=—The cycle through which the elements forming organic
matter pass from life to death and back to life again has been described
in Chapter XIII. It has been shown in Chapter XVI that septic action
occupies that portion of the cycle in which the combinations of these
elements are broken down or reduced to simpler forms and the lower
stages of the cycle are reached. The action in the filtration of sewage
builds up the compounds again in a more stable form and almost complete
oxidation is attained, dependent on the thoroughness of the filtration.
In the filtration of sewage only the coarsest particles of suspended
matter are removed by mechanical straining. The success of the
filtration is dependent on biologic action. The desirable form of life
in a filter is the so-called nitrifying bacteria which live in the
interstices of the filter bed and feed upon the organic matter in the
sewage. Anything which injures the growth of these bacteria injures the
action of the filter. In a properly constructed and operated filter, all
matter which enters in the influent, leaves with the effluent, but in a
different molecular form. A slight amount may be lost by evaporation and
gasification but this is more than made up by the nitrogen and oxygen
absorbed from the atmosphere. The nitrifying action in sewage filtration
is shown by the analysis of sewage passing through a trickling filter,
as given in Tables 86 and 87. It is shown by the reduction of the
content of organic nitrogen, free ammonia, oxygen consumed, and the
increase in nitrites, nitrates, and dissolved oxygen. The reduction of
suspended matter is interrupted periodically when the filter “unloads.”
The suspended matter in the effluent is then greater than in the
influent.

The nitrifying organisms have been isolated and divided into two
groups—_nitrosomonas_, the nitrite formers, and _nitrobacter_, the
nitrate formers. Experiments indicate that the growth of the nitrobacter
organisms is dependent on the presence of the nitrosomonas organisms,
which are in turn dependent on the presence of the putrefactive
compounds resulting from the action of putrefying bacteria. The
existence of these organisms is an example of symbiotic action in
bacterial growth. The organisms have been found to grow best on rough
porous material on which their zoögleal jelly can be easily deposited
and affixed. Sewage filters were constructed to provide these ideal
conditions before the action of a filter was thoroughly understood.

The action in irrigation is similar to that in filtration. Although more
strictly a method of final disposal rather than preliminary treatment,
the similarity of the actions which take place, and the grading of sand
filtration into broad irrigation with no distinct line of difference has
resulted in the inclusion of the discussion of irrigation in the same
chapter with filtration.

=256. The Contact Bed.=—A contact bed is a water-tight basin filled with
coarse material, such as broken stone, with which sewage and air are
alternately placed in contact in such a manner that oxidation of the
sewage is effected. A contact bed has some of the features of a
sedimentation tank and an oxidizing filter. As such it marks a
transitory step from anaërobic to aërobic treatment of sewage. A plan
and a section of a contact bed are shown in Fig. 166.

Because of its dependence on biologic action a contact bed must be
ripened before a good effluent can be obtained. The ripening or maturing
occurs progressively during the first few weeks of operation, the
earlier stages being more rapidly developed. The time required to reach
such a stage of maturity that a good effluent will be developed will
vary between one and six or eight weeks, dependent on the weather and
the character of the influent. During the period of maturing the load on
the bed should be made light.

The use of contact beds has been extensive where a more stable effluent
than could be obtained from tank treatment has been desired, yet the
best quality of effluent was not required. The sewage to undergo
treatment in a contact bed should be given a preliminary treatment to
remove coarse suspended matter. The efficiency of the contact treatment
can be increased by passing the sewage through two or three contact beds
in series. In double contact treatment the primary beds are filled with
coarser material and operate at a more rapid rate than the secondary
beds. Double contact gives better results than single contact, but
triple contact treatment, though showing excellent results, is hardly
worth the extra cost. An advantage which contact treatment has over all
other methods of sewage filtration is that the bed can be so operated
that the sewage is never exposed to view. As a result the odors from
well-operated contact beds are slight or are entirely absent and there
should be no trouble from flying insects. Such a method of treatment is
favorable to plants located in populous districts and to the fancies of
a landscape architect. Another advantage of the contact bed is the small
amount of head required for its operation, which may be as low as 4 to 5
feet. This low head consumption by a sewage filter is equaled only by
the intermittent sand filter.

[Illustration:

FIG. 166.—Plan and Section of Treatment Plant at Marion, Ohio, Showing
Septic Tank, Contact Bed, and Sand Filter.

1908 Report Ohio State Board of Health.
]

The quality of the effluent from some contact beds is shown in Table 85.
It is to be noted that nitrification has been carried to a fair degree
of completion, and that the reduction of oxygen consumed has been
marked. In comparison with the effluent from filters, contact effluent
contains a smaller amount of nitrogen as nitrites and nitrates, and
suspended solids. Contact effluent is usually clear and odorless, but it
is not stable without dilution. The absence of nitrites and nitrates is
sometimes advantageous as the effluent will not support vegetable
growths dependent on this form of nitrogen. The absence of suspended
solids obviates the use of secondary sedimentation basins which are
needed with trickling filters. The head of 5 to 8 feet required for
contact treatment is low in comparison to the 10 to 15 feet required for
trickling filters, but is slightly higher than the head required for
intermittent sand filtration. The cost of contact treatment is higher
than the cost of trickling filters but is lower than the cost of
intermittent sand filtration, as shown in Table 90.

TABLE 85

QUALITY OF EFFLUENTS FROM CONTACT BEDS

Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905.

──────┬──────┬─────────┬───────┬────────┬─────────────────────────────────
Filter│Depth,│ Size of │ Rate, │ Oxygen │ Nitrogen as
│ Feet │Material │Million│Consumed│
│ │in Inches│Gallons│ │
│ │ │ per │ │
│ │ │ Acre │ │
│ │ │per Day│ │
──────┼──────┼─────────┼───────┼────────┼───────┬───────┬────────┬────────
│ │ │ │ │Organic│ Free │Nitrites│Nitrates
│ │ │ │ │ │Ammonia│ │
──────┼──────┼─────────┼───────┼────────┼───────┴───────┴────────┴────────
│ │ │ │ │ Parts per Million
│ │ │ │ │ │ │ │
A │ 5│0.25–1.00│ 0.953│ 23│ 3.5│ 8.7│ 0.20│ 1.6
B │ 5│0.25–2.00│ 1.514│ 21│ 4.0│ 8.4│ 0.15│ 1.4
C │ 5│0.25–1.50│ 1.222│ 24│ 3.5│ 10.8│ 0.11│ 0.6
D │ 5│0.50–1.50│ 1.405│ 22│ 3.3│ 9.5│ 0.13│ 0.9
│ │ │ │ │ │ │ │
│ │ │Per Cent Removal of Constituents of Applied Sewage
│ │ │ │ │ │ │ │
A │ 5│0.25–1.00│ 0.953│ 48│ 49│ 10│ │
B │ 5│0.25–2.00│ 1.514│ 52│ 40│ 11│ │
C │ 5│0.25–1.50│ 1.222│ 47│ 31│ 12│ │
D │ 5│0.50–1.50│ 1.405│ 46│ 37│ 19│ │
──────┴──────┴─────────┴───────┴────────┴───────┴───────┴────────┴────────

──────┬──────┬────────────────────┬─────────
Filter│Depth,│ Suspended Matter │Dissolved
│ Feet │ │ Oxygen
│ │ │
│ │ │
│ │ │
│ │ │
──────┼──────┼─────┬────────┬─────┼─────────
│ │Total│Volatile│Fixed│
│ │ │ │ │
──────┼──────┴─────┴────────┴─────┴─────────
│ Parts per Million
│ │ │ │ │
A │ 5│ 832│ 94│ 737│ 0.3
B │ 5│ 831│ 85│ 746│ 0.1
C │ 5│ 826│ 92│ 734│ 0.8
D │ 5│ 810│ 91│ 717│ 0.9
│ │ │ │ │
│ Per Cent Removal of Constituents of Applied Sewage
│ │ │ │ │
A │ 5│ 73│ 70│ 76│
B │ 5│ 80│ 77│ 83│
C │ 5│ 70│ 70│ 70│
D │ 5│ 67│ 61│ 72│
──────┴──────┴─────┴────────┴─────┴─────────

The depth of the contact bed is generally made from 4 to 6 feet. The
deeper beds are less expensive per unit of volume, to construct, as the
cost of the underdrains and the distribution system is reduced in
relation to the capacity of the filter. The increased depth reduces the
aëration, and the periods of filling and emptying are so increased as to
limit the depths to the figures stated. The other dimensions of the bed
are controlled by economy and local conditions, as the success of the
contact treatment is not affected by the shape of the bed. Contact units
are seldom constructed larger than one-half an acre in area, as larger
beds require too much time for filling and emptying. A large number of
small units is also undesirable because of the increased difficulty of
control. In general it is well to build as large units as are compatible
with efficient operation, elasticity of plant, and which can be filled
within the time allowed at the average rate of sewage flow, or from
dosing tanks in which the storage period is not so long as to produce
septic conditions.

The interstices in a contact bed will gradually fill up, due to the
deposition of solid matter on the contact material, the disintegration
of the material, and the presence of organic growths. The period of rest
allowed every five or six weeks tends to restore partially some of this
lost capacity through the drying of the organic growths. It is
occasionally necessary to remove the material from the bed and wash it
in order to restore the original capacity. It may be necessary to do
this three or four times a year, in an overloaded plant, or as
infrequently as once in five or six years in a more lightly loaded bed.
The period is also dependent on the character of the contact material
and the quality of the influent. This loss of capacity may reduce the
voids from an original amount of 40 to 50 per cent of voids to 10 to 15
per cent. If the bed is not overloaded the loss of capacity will not
increase beyond these figures.

The rate of filtration depends on the strength of the sewage, the
character of the contact material, and the required effluent. It should
be determined for any particular plant as the result of a series of
tests. For the purposes of estimation and comparison the approximate
rate of filtration should be taken at about 94 gallons per cubic yard of
filtering material per day on the basis of three complete fillings and
emptyings of the tank. This is equivalent to 150,000 gallons per acre
foot of depth per day, or for a bed 5 feet deep to a rate of 750,000
gallons per acre per day. The net rate for double or triple filtration
is less than these figures, but on each filter the rates are higher.

The material of the contact bed should be hard, rough, and angular. It
should be as fine as possible without causing clogging of the bed.
Materials in successful use are: crushed trap rock or other hard stone,
broken bricks, slag, coal, etc. Soft crumbling materials such as coke
are not suitable as the weight of the superimposed material and the
movement of the sewage crushes and breaks it into fine particles which
accumulate in the lower portion of the filter and clog it. Roughness,
porosity, and small size are desirable, as the greater the surface area
the more rapid the deposition of material. After a short time, however,
the advantages of roughness and porosity are lost, as the sediment soon
covers all unevenness alike. The minimum size of the material is limited
by the tendency towards clogging. The sizes in successful use vary
between ¼ and ¾ of an inch, ½ inch being a common size. The same size of
material is used throughout the depth of the bed except that the upper 6
inches may be composed of small white pebbles or other clean material,
which does not come in contact with the sewage and which will give an
attractive appearance to the plant. In double or triple contact beds 3
or 4–inch material is sometimes used for the primary beds, and ¼-inch
material in the final bed.

Sewage may be applied at any point on or below the surface. The sewage
is withdrawn from the bottom of the bed. It is undesirable to have too
few inlet or outlet openings as the velocity of flow about the openings
will be so great as to disturb the deposit on the contact material. The
distribution system and the underdrains for the bed at Marion, Ohio, are
shown in Fig. 166.

The cycle of operation of a contact bed is divided into four periods. A
representative cycle might be: time of filling, one hour; standing full,
2 hours; emptying, one hour; standing empty, 4 hours. The length of
these periods is the result of long experience based on many tests and
are an average of the conclusions reached. Wide variations from them may
be found in different plants, and tests may show successful results with
different periods. The combination of these four periods is known as the
contact cycle.

The period of filling should be made as short as possible without
disturbing the material of the bed nor washing off the accumulated
deposits. The sewage should not rise more rapidly than one vertical foot
per minute. During the contact or standing full period sedimentation and
adsorption of the colloids are occurring on the area of surface exposed
to the sewage. This period should be of such length that septic action
does not become pronounced, and long enough to permit of thorough
sedimentation. The period of emptying should be made as short as
possible without disturbing the bed, on the same basis that the period
of filling is determined. During the period of standing empty, air is in
contact with the sediment deposited in thin layers on the contact
material, and the oxidizing activities of the filter are taking place.
The filter is given a rest period of one or two days every five or six
weeks, in order that it may increase its capacity and its biologic
activity.

The control of a contact bed may be either by hand or automatic, the
latter being the more common. Hand control requires the constant
attention of an operator and results in irregularity of operation,
whereas automatic control will require inspection not more than once a
day and insures regularity of operation. A number of automatic devices
have been invented which give more or less satisfaction. The air-locked
automatic siphons, without moving parts, have proven satisfactory and
are practically “fool-proof.” The operation of these devices is
explained in Chapter XXI.

=257. The Trickling Filter.=—A trickling or sprinkling filter is a bed
of coarse, rough, hard material over which sewage is sprayed or
otherwise distributed and allowed to trickle slowly through the filter
in contact with the atmosphere. A general view of a trickling filter in
operation at Baltimore is shown in Fig. 167. The action of the trickling
filter is due to oxidation by organisms attached to the material of the
filter. The solid organic matter of the sewage deposited on the surface
of the material, is worked over and oxidized by the aërobic bacteria,
and is discharged in the effluent in a more highly nitrified condition.
At times the discharge of suspended matter becomes so great that the
filter is said to be unloading. The action differs from that in a
contact bed in that there is no period of septic or anaërobic action and
the filter never stands full of sewage.

The effluent from a trickling filter is dark, odorless, and is
ordinarily non-putrescible. Analyses of typical effluents are given in
Tables 86 and 87. The unloading of the filter may occur at any time, but
is most likely to occur in the spring or in a warm period following a
period of low temperatures. It causes higher suspended matter in the
effluent than in the influent and may render the effluent putrescible.
The action is marked by the discharge of solid matter which has sloughed
off of the filter material and which increases the turbidity of the
effluent. Where the diluting water is insufficient to care for the
solids so carried in the effluent, they can be removed by a 2–hour
period of sedimentation. The effluent may become septic during this
time, however. The nitrogen in the effluent is almost entirely in the
form of nitrates, and the percentage of saturation with dissolved oxygen
is high. The effluent is more highly nitrified than that from a contact
bed, and its relative stability is also higher, thus demanding a smaller
volume of diluting water.

[Illustration:

FIG. 167.—Sprinkling Filter in Operation in Winter at Baltimore.
]

The principal advantage of a trickling filter over other methods of
treatment is its high rate which is from two to four times faster than a
contact bed, and about seventy times faster than an intermittent sand
filter. The greatest disadvantage is the head of 12 to 15 feet or more
necessary for its operation. Sedimentation of the effluent is usually
necessary to remove the settleable solids. During the period of
secondary sedimentation the quality of the filter effluent may
deteriorate in relative stability. In winter the formation of ice on the
filter results in an effluent of inferior quality, but as the diluting
water can care for such an effluent at this time the condition is not
detrimental to the use of the trickling filter. In summer the filters
sometimes give off offensive odors that can be noticed at a distance of
half a mile, and flying insects may breed in the filter in sufficient
quantities to become a nuisance if preventive steps are not taken. The
dissemination of odors is especially marked when treating a stale or
septic sewage. The treatment of a fresh sewage seldom results in the
creation of offensive odors.

TABLE 86

ANALYSIS OF CRUDE SEWAGE, IMHOFF TANK, AND SPRINKLING FILTER EFFLUENTS
AT ATLANTA, GEORGIA

(Engineering Record, Vol. 72, p. 4)

─────────┬───────────┬──────────────────────────────────────────
│Temperature│ Parts per Million
│Fahrenheit │
│ │
│ │
─────────┼───────────┼─────────────────────────────────┬────────
│ │ Nitrogen as │ Oxygen
│ │ │Consumed
─────────┼───────────┼───────┬───────┬────────┬────────┼────────
│ │Organic│ Free │Nitrites│Nitrates│
│ │ │Ammonia│ │ │
─────────┴───────────┴───────┴───────┴────────┴────────┴────────

_Crude Sewage_

─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 77│ 15.6│ 21.8│ 0.1│ 3.0│ 100.0
Minimum │ 61│ 10.4│ 16.5│ 0.1│ 1.4│ 78.3
Average │ 70│ 12.8│ 18.8│ 0.1│ 2.2│ 90.6
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 74│ 16.0│ 33.4│ │ 2.3│
Minimum │ 60│ 9.5│ 18.1│ │ 1.6│
Average │ 66│ 13.4│ 27.1│ │ 2.0│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────

_Imhoff Effluent_

─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 78│ 13.2│ 21.9│ 0.2│ 3.1│ 68.0
Minimum │ 58│ 6.5│ 16.8│ 0.1│ 1.1│ 53.1
Average │ 68│ 9.0│ 20.0│ 0.2│ 2.1│ 60.1
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 77│ 10.3│ 30.3│ │ 2.0│
Minimum │ 59│ 4.1│ 18.0│ │ 1.5│
Average │ 65│ 7.7│ 25.9│ │ 1.8│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────

_Sprinkling Filter Effluent_

─────────┬───────────┬───────┬───────┬────────┬────────┬────────
1913 │ │ │ │ │ │
Maximum │ 79│ 5.6│ 14.2│ 0.8│ 11.3│ 32.1
Minimum │ 55│ 2.6│ 6.2│ 0.5│ 5.8│ 23.6
Average │ 66│ 3.8│ 9.9│ 0.7│ 8.2│ 28.2
1914 (7 │ │ │ │ │ │
months)│ │ │ │ │ │
Maximum │ 77│ 8.5│ 20.7│ │ 11.2│
Minimum │ 55│ 4.4│ 8.8│ │ 3.6│
Average │ 63│ 5.7│ 15.2│ │ 7.2│
─────────┴───────────┴───────┴───────┴────────┴────────┴────────

─────────┬────────────────────┬──────────┬─────────
│ Parts per Million │ Per Cent │Relative
│ │Saturation│Stability
│ │Dissolved │
│ │ Oxygen │
─────────┼────────────────────┼──────────┼─────────
│ Suspended Matter │ │
│ │ │
─────────┼─────┬────────┬─────┼──────────┼─────────
│Total│Volatile│Fixed│ │
│ │ │ │ │
─────────┴─────┴────────┴─────┴──────────┴─────────

_Crude Sewage_

─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 371│ 154│ 163│ 47│
Minimum │ 222│ 98│ 112│ 11│
Average │ 285│ 126│ 138│ 28│
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 431│ │ │ 48│
Minimum │ 279│ │ │ 12│
Average │ 351│ │ │ 30│
─────────┴─────┴────────┴─────┴──────────┴─────────

_Imhoff Effluent_

─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 90│ 50│ 41│ │
Minimum │ 35│ 42│ 21│ │
Average │ 68│ 46│ 33│ │
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 73│ │ │ 48│
Minimum │ 49│ │ │ 34│
Average │ 65│ │ │ 43│
─────────┴─────┴────────┴─────┴──────────┴─────────

_Sprinkling Filter Effluent_

─────────┬─────┬────────┬─────┬──────────┬─────────
1913 │ │ │ │ │
Maximum │ 60│ 31│ 28│ 76│ 99
Minimum │ 33│ 26│ 28│ 52│ 88
Average │ 49│ 28│ 28│ 64│ 89
1914 (7 │ │ │ │ │
months)│ │ │ │ │
Maximum │ 106│ │ │ 79│ 99
Minimum │ 40│ │ │ 55│ 89
Average │ 62│ │ │ 65│ 95
─────────┴─────┴────────┴─────┴──────────┴─────────

TABLE 87

EFFICIENCY OF SPRINKLING FILTER CHICAGO, ILLINOIS

Depth of Filter 9 feet. Size of stone 2 in. to 3 in.

────────┬───────────────────────────┬───────────────────────────
Month │ Organic Nitrogen │ Free Ammonia
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ 5.1│ 2.8│ 45│ 12.0│ 4.6│ 62
November│ 5.9│ 2.5│ 58│ 12.0│ 5.9│ 51
December│ 4.6│ 3.0│ 35│ 12.0│ 6.9│ 42
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ 6.3│ 4.8│ 24│ 11.0│ 7.0│ 36
February│ 9.0│ 4.8│ 47│ 10.0│ 7.2│ 28
March │ 8.3│ 3.5│ 58│ 9.9│ 6.4│ 35
April │ 6.4│ 4.0│ 37│ 8.3│ 3.6│ 69
May │ 7.6│ 5.4│ 29│ 9.2│ 2.4│ 74
June │ 5.9│ 3.2│ 46│ 11.0│ 0.6│ 95
July │ 6.2│ 4.2│ 32│ 11.0│ 1.3│ 88
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────

────────┬───────────────────────────┬───────────────────────────
Month │ Oxygen Consumed │ Nitrites
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ 30│ 15│ 50│ │ .90│
November│ 35│ 15│ 57│ │ .76│
December│ 39│ 20│ 49│ .07│ .45│ 6.4
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ 42│ 20│ 52│ .08│ .15│ 1.9
February│ 46│ 20│ 56│ .09│ .15│ 1.7
March │ 47│ 21│ 56│ .09│ .15│ 1.7
April │ 38│ 21│ 45│ .16│ .21│ 1.3
May │ 33│ 31│ 6│ .08│ .38│ 4.8
June │ 28│ 16│ 43│ .00│ .30│ ∞
July │ 34│ 26│ 24│ .00│ .36│ ∞
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────

────────┬───────────────────────────┬───────────────────────────
Month │ Nitrates │ Dissolved Oxygen
│ │
────────┼───────────────────────────┼───────────────────────────
│ │
────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ Million │ Million │Removed│ Million │ Million │Removed
────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │
October │ │ 7.8│ │ 0.0│ 8.5│ ∞
November│ │ 5.9│ │ 0.0│ 8.1│ ∞
December│ .15│ 2.6│ 17│ 2.0│ 8.4│ 4.2
│ │ │ │ │ │
1911 │ │ │ │ │ │
January │ .27│ 2.2│ 8.2│ 3.0│ 7.8│ 2.9
February│ .50│ 2.6│ 5.2│ 2.6│ 8.0│ 3.1
March │ .34│ 3.2│ 9.4│ 2.2│ 6.6│ 3.0
April │ .53│ 4.5│ 8.5│ 2.1│ 7.1│ 3.4
May │ .15│ 7.5│ 4.3│ 0.1│ 7.7│ 77
June │ .16│ 8.3│ 5.2│ 0.0│ 7.6│ ∞
July │ .09│ 7.7│ 8.0│ 0.0│ 6.5│ ∞
────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────

────────┬───────────┬───────────────────────────────────────────────────────
Month │ Per Cent │ Suspended Matter
│Putrescible│
────────┼───────────┼───────────────────────────┬───────────────────────────
│ │ Total │ Volatile
────────┼───────────┼─────────┬─────────┬───────┼─────────┬─────────┬───────
│ │Influent,│Effluent,│ Per │Influent,│Effluent,│ Per
│ │Parts per│Parts per│ Cent │Parts per│Parts per│ Cent
│ │ Million │ Million │Removed│ Million │ Million │Removed
────────┼───────────┼─────────┼─────────┼───────┼─────────┼─────────┼───────
1910 │ │ │ │ │ │ │
October │ 0│ 75│ 40│ 47│ 54│ 25│ 54
November│ 5│ 61│ 16│ 74│ 52│ 15│ 71
December│ 35│ 85│ 40│ 53│ 60│ 26│ 57
│ │ │ │ │ │ │
1911 │ │ │ │ │ │ │
January │ 38│ 112│ 43│ 63│ 68│ 29│ 57
February│ 29│ 100│ 49│ 51│ 64│ 32│ 50
March │ 28│ 106│ 37│ 65│ 63│ 22│ 65
April │ 9│ 113│ 68│ 40│ 59│ 35│ 41
May │ 6│ 88│ 150│ _1.7_│ 54│ 70│ _1.3_
June │ 1│ 92│ 77│ 18│ 56│ 36│ 36
July │ 4│ 155│ 130│ 16│ 74│ 61│ 18
────────┴───────────┴─────────┴─────────┴───────┴─────────┴─────────┴───────

────────┬───────────────────────────
Month │ Suspended Matter
│
────────┼───────────────────────────
│ Fixed
────────┼─────────┬─────────┬───────
│Influent,│Effluent,│ Per
│Parts per│Parts per│ Cent
│ Million │ Million │Removed
────────┼─────────┼─────────┼───────
1910 │ │ │
October │ 21│ 15│ 29
November│ 9│ 1│ 89
December│ 25│ 14│ 44
│ │ │
1911 │ │ │
January │ 44│ 13│ 70
February│ 37│ 17│ 53
March │ 43│ 15│ 65
April │ 54│ 33│ 39
May │ 34│ 80│ _2.4_
June │ 36│ 41│ _1.1_
July │ 81│ 69│ 15
────────┴─────────┴─────────┴───────

NOTE.—Italic figures represent increases.

Raw sewage cannot be treated successfully on a trickling filter. Coarse
solid particles should be screened and settled out, in order that the
distributing devices or the filter may not become clogged. The effluent
from an Imhoff tank has proven to be a satisfactory influent for a
trickling filter. A septic tank effluent may be so stale as to be
detrimental to the biologic action in the filter.

In the operation of a trickling filter the sewage is sprayed or
otherwise distributed as evenly as possible in a fine spray or stream,
over the top of the filtering material. The sewage then trickles slowly
through the filter to the underdrains through which it passes to the
final outlet. The distribution of the sewage on the bed is intermittent
in order to allow air to enter the filter with the sewage. The cycle of
operation should be completed in 5 to 15 minutes, with approximately
equal periods of rest and distribution. Cycles of too great length will
expose the filter to drying or freezing and will give poorer
distribution throughout the filter. Cycles which are too short will
operate successfully only with but slight variation in the rate of
sewage flow. In some plants it has been found advantageous to allow the
filters to rest for one day in 3 to 6 weeks or longer, dependent on the
quality of the effluent.

The rate of filtration may be as high as 2,000,000 gallons per acre per
day, which is equivalent to 200 gallons per cubic yard of material per
day in a bed 6 feet deep. This is more than double the rate permissible
in a contact bed. The exact rate to be used for any particular plant
should be determined by tests. It is dependent on the quality of the
sewage to be treated, on the depth of the bed, the size of the filling
material, the weather, and other minor factors.

The filtering material is similar to that used in a contact bed. It
should consist of hard, rough, angular material, about 1 to 2 inches in
size. Larger sizes will permit more rapid rates of filtration, but will
not produce so good an effluent. Smaller sizes will clog too rapidly.

The depth of the filter is limited by the possibility of ventilation and
the strength of the filtering material to withstand crushing. The deeper
the bed the less the expense of the distribution and collecting system
for the same volume of material, and the more rapid the permissible rate
of filtration. The depths in use vary between 6 and 10 feet, with 6 to 8
feet as a satisfactory mean. From a biologic standpoint the action of
the filter seems to be proportional to the volume of the filtering
material and therefore proportional to the depth of the bed, being
limited to a minimum depth of about 5 feet, below which sewage may pass
through the filter without treatment. The shape and other dimensions of
the filter depend on the local conditions and the economy of
construction. The filters need not be broken up into units by
water-tight dividing walls. One filter can be constructed sufficient for
all needs and various portions of it can be isolated as units by the
manipulation of valves in the distribution system. Ventilation is
provided by the air entrained with the sewage as it falls upon the
surface. If the sides of the filter are built of open stone crib work
the ventilation will be greatly improved, but it will not be possible to
flood the filters to keep down flies, and in cold climates these
openings must be covered in winter to prevent freezing. Filters have
been constructed without side walls, the filtering material being
allowed to assume its natural angle of repose. This has usually been
found to be more expensive than the construction of side retaining
walls, due to the unused filling material and the extra underdrains
required.

The distribution of sewage is ordinarily effected by a system of pipes
and spray nozzles as shown in Fig. 168 and 169. Other methods of
distribution have been used. At Springfield, Mo.,[160] a moving trough
from which the sewage flows continuously is drawn back and forth across
the bed by means of a cable. In England circular beds have been
constructed and the sewage distributed on them through revolving
perforated pipes. At the Great Lakes Naval Training Station[161] the
distributing pipes in the plant, now abandoned, were supported above the
surface of the filter. The sewage fell from holes in the lower side of
these pipes on to brass splash plates 14 inches above the filter. It was
deflected horizontally from these plates over the filter surface. Pipes
and spray nozzles have been adopted almost universally in the United
States. Splash plates, traveling distributors, and other forms of
distribution have been used only in exceptional cases. In a distributing
system consisting of pipes and nozzles, a network of pipes is laid out
somewhat as shown in Fig. 168, in such a manner that the head loss to
all points is approximately equal. The number of valves required should
be reduced to a minimum. The pipes may be laid out with the main feeders
leading from a central point and branches at right angles to them,
somewhat on the order of a spider’s web, or they may be laid out on a
rectangular or gridiron system. The radial system is advantageous
because of the central location of the control house, but it does not
always lend itself favorably to the local conditions, and the piping and
nozzle location are not so simple. The gridiron system lends itself
favorably to the equalization of head losses. The pipes used should be
larger than would be demanded by considerations of economy alone, both
for the purpose of reduction of head loss and ease in cleaning. No pipe
less than 6 inches in diameter should be used, and the average velocity
of flow should not exceed one foot per second. Cast-iron, concrete, or
vitrified clay pipe may be used, but cast iron is the material commonly
used. The system should be arranged for easy flushing and cleaning and
the pipes so sloped that the entire system can be drained in case of a
shut down in cold weather.

[Illustration:

FIG. 168.—Section through Sprinkling Filter at Fitchburg, Mass.,
Showing Distribution System.

Eng. Record, Vol. 67, p. 634.
]

The pipes are placed far enough below the surface of the filling
material so that the top of the spraying nozzle is 6 to 12 inches above
the surface of the filter. If the pipes are placed near the surface they
are accessible for repairs, but are exposed to temperature changes. If
the pipes are large their presence near the surface of the filter may
seriously affect the distribution of the sewage through the filter. If
the distributing pipes are placed near the bottom of the filter they are
inaccessible for repairs and the nozzles must be connected to them by
means of long riser pipes. The distributing pipes should be supported by
columns extending to the foundation of the filter bed, there being a
column at every pipe joint with such intermediate supports as may be
required. In some plants the pipes have been supported by the filtering
material. Although slightly less expensive in first cost the practice of
so supporting the pipes is poor, as settling of the material may break
the pipe or cause leaks, and if the bed becomes clogged, removal of the
material is made more difficult. Valves should be placed in the
distributing system in such a manner that different sets of nozzles can
be cut out at will, thus resting those portions of the filter and
permitting repairs without shutting down the entire filter.

The spacing of the nozzles is fixed by the type and size of the nozzle,
the available head, and the rate of filtration. Various types of
sprinkler nozzles are shown in Fig. 169 and the discharge rates, head
losses, and distances to which sewage is thrown for the Taylor nozzles,
are shown in Fig. 170. Nozzles are available which will throw circular,
square, or semicircular sprays. In the use of circular sprays there is
necessarily some portion of the filter which is underdosed if the
nozzles are placed at the corners of squares with the sprays tangent,
and there is an overdosing of other portions if the sprays are allowed
to overlap so that no portion of the filter is left without a dose.
Rectangular sprays will apparently overcome these difficulties, but
studies have shown that circular sprays with some overlapping, and the
nozzles placed at the apexes of equilateral triangles as shown in Fig.
172 will give as satisfactory distribution as other forms.

[Illustration:

FIG. 169.—Sprinkling Filter Nozzles.

Bulletin No. 3, Engineering Experiment Station, Purdue University.
]

[Illustration:

FIG. 170.—Diagram Showing the Discharge and Spacing of Taylor Nozzles.
]

The nozzles should be selected to give the best distribution, to consume
all of the head available, and to give the proper cycle of operation.
The entire head available should be consumed in order that the fewest
number of nozzles may be used. An excellent study of the characteristics
of various types of nozzles has been published in Bulletin No. 3 of the
Engineering Experiment Station at Purdue University, 1920. As a result
of the tests on the nozzles shown in Fig. 169, it was determined for all
nozzles, except No. 8, that

_Q_ = _Ca_√(2_gh_);

in which _Q_ = the rate of discharge in cubic feet per second;

_C_ = a coefficient shown in Table 88;

_a_ = the net cross-sectional opening of the nozzle in square
feet;

_h_ = the pressure on the nozzle in feet of water.

TABLE 88

COEFFICIENTS OF DISCHARGE FOR SPRINKLER NOZZLES SHOWN IN FIG. 169

──────────────────────┬──────┬──────┬──────┬──────┬──────┬──────┬──────
Nozzle Number │ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7
──────────────────────┼──────┼──────┼──────┼──────┼──────┼──────┼──────
Coefficient │ .648 │ .756 │ .696 │ .666 │ .675 │ .598 │ .569
──────────────────────┴──────┴──────┴──────┴──────┴──────┴──────┴──────

It is evident that if the head on the nozzles is constant and the nozzle
throws a circular spray, the intensity of dosing at the circumference
will be greater than nearer the center. This difficulty is overcome by
so designing the dosing tank from which the sewage is fed that the head
on the nozzle and the quantity thrown will vary in such a manner that
the distribution over the bed is equalized. Intermittent action is
obtained by an automatic siphon which commences to discharge when the
tank is full and empties the tank in the period allowed for dosing.
Under such conditions the tank should discharge for a longer time at the
higher heads than at the lower heads as there is more territory to be
covered at the higher heads. The design of the tank to do this with
exactness is difficult, and the construction of the necessary curved
surfaces is expensive. Where a dosing tank is used for such conditions
it has been found satisfactory to construct the tank with plane sides
sloping at approximately 45 degrees from the vertical (or horizontal). A
tank with curved surfaces is shown in Fig. 171. The dosing siphon is
usually placed in the tank as shown in the figure. The head and quantity
of discharge through the nozzles can be varied also by maintaining a
constant depth in a dosing tank by means of a float feed valve, and
varying the head and quantity discharged to the nozzles by a butterfly
valve in the main feed line, or by the use of a Taylor undulating valve
designed for this purpose. The butterfly valve is opened and closed by a
cam so designed and driven at such a rate that the required distribution
is obtained. The Taylor undulating valve is opened and closed at a
constant rate, the shape of the valve giving the required variations in
head and discharge. Other methods of control have been attempted but
have not been used extensively.

[Illustration:

FIG. 171.—Section of 12–inch Siphon and Dosing Tank, for King’s Park,
Long Island.
]

An example of the design of the nozzle layout and dosing tank for a
sprinkling filter follows:

Let it be required to determine the nozzle layout for one acre of
sprinkling filters with 5 feet available head on the nozzles.

The selection of the type of nozzle and the size of opening is a
matter of judgment and experience. Nozzles with large openings are
less liable to clog and fewer nozzles are needed than where small
nozzles are used, but the distribution of sewage is not so even as
with the use of small nozzles. In this example Taylor circular
spray nozzles will be selected. Fig. 170 shows that a Taylor
circular spray nozzle will discharge 22.3 g.p.m. under a head of 5
feet, and that the economical nozzle spacing will be 15.3 feet.
The least number of nozzles at this spacing required for a bed of
one acre in area is found as follows: In Fig. 172, let _n_ equal
the number of nozzles in a horizontal row, counting half-spray
nozzles as ½, and let _m_ equal the number of rows counting rows
of half-spray nozzles as half rows.[162] Then the number of
nozzles, _N_, equals _mn_, and 15.3_m_ × 13.2_n_ equals 43,560 or
_mn_ equals 215.

[Illustration:

FIG. 172.—Typical Sprinkler Nozzle Layout.
]

The next step should be the design of the dosing tank and siphon. It is
possible to design a tank which will give equal distribution over equal
areas of filter surface. It has been found, however, that the expense of
this refinement is unwarranted as there are a number of outside factors
which tend to overcome the theoretical design. The effect of wind,
unequal spacing, and irregularities in the elevation of the nozzles have
a tendency to offset refinements in the design of a dosing tank. It is
therefore the general practice to slope the sides of the tank at an
angle of about 45 degrees as previously stated. The dosing tank is
generally designed to have a capacity which will give a complete cycle
of operation once in 15 minutes. In the ordinary design the factors
given are the rate of inflow and the given time of filling. In the
following example the time of filling will be taken as 10 minutes, the
time of emptying as 5 minutes, and the rate of flow as 1,000,000 gallons
per day. The capacity of the tank will therefore be (1,000,000)⁄24 x 6 =
7,000 gallons. The diameter of the siphon to be selected can be computed
as follows:

Let _Q_ = the capacity of the tank in cubic feet;
_q__{1} = the rate of discharge of the siphon in cubic feet per
second;
_q__{2} = the rate of inflow to the tank in cubic feet per second;
_q_ = the rate of emptying the tank in cubic feet per second =
(_q__{1} − _q__{2});
_A_ = the cross-sectional area of the free surface of the water
in the tank at any instant, in square feet;
_a_ = the cross-sectional area of the siphon in square feet;
_b_ = the small dimension of the base of the tank in feet;
_h_ = the head of water, in feet, on the discharge siphon;
_h__{1} = the initial head of water, in feet, on the siphon;
_h__{2} = the final head of water in feet, on the siphon;
_t_ = the time, in seconds, required to empty the tank,

then _dQ_ = -_Adh_ = _q__{1}_dt_ − _q__{2}_dt_,

and _dt_ = (_dQ_)⁄_q_ = − _Adh_⁄(_q__{1} − _q__{2}),

but _q__{1} = 0.4 _A_ √((2_gh_)),[163]

therefore _t_ = ∫_{_h__{2}}^{_h__{1}} -_Adh_⁄(0.4_a_√(2_gh_) −
_q__{2}),

but _A_ = 4_h_^2 + 4_bh_ + _b_^2,

therefore _t_ = ∫_{_h__{1}}^{_h__{2}} ((_b_^2 + 4_bh_ +
4_h_^2)_dh_)⁄0.4_a_√(2_gh_) − _q__{2}.

The integration of this expression is tedious. Its solution for siphons
between 6 inches and 12 inches operating under heads commencing from 3
feet to 6 feet, with a time of emptying of 5 minutes and time of filling
of 10 minutes is given in Fig. 173. In the example given the rate of
inflow is 1.55 sec. feet and the head is 5 feet. Then from Fig. 173 the
size of the siphon to be used is 12 inches. Where a siphon of the size
required to empty the tank in the time fixed is not available,
combinations of available sizes can sometimes be used.

[Illustration:

FIG. 173.—Diagram for the Determination of the Capacities of Dosing
Tanks for Trickling Filters.

Time of emptying, 5 minutes. Time of filling, 10 minutes. Shape of
tank is a right pyramid or a truncated right pyramid with all four
sides making an angle of 45 degrees with the vertical. All
horizontal cross-sections are squares.
]

For example, if the given head is 6 feet, and the rate of inflow
is 1.4 sec. feet, it is evident from Fig. 173 that a 6,300–gallon
dosing tank and two 8–inch siphons will give the required cycle.

The method used for the design of the setting of Taylor nozzles by the
Pacific Flush Tank Co., is less rational but more simple and probably as
satisfactory. In this method the steps are as follows:

(1) Divide the maximum daily rate of sewage flow by 1,000 to get
the maximum minute inflow.

(2) The number of nozzles required is determined by dividing the
preceding figure by 6. Generally a Taylor nozzle with an orifice
of ⅞ of an inch will discharge about 20 g.p.m. at the high head
and about 8 g.p.m. at the low head, and as the nozzles must have a
capacity which will take care of the inflow at the low head, the
divisor 6 is used as a factor of safety instead of using 8 as the
divisor.

(3) The type of nozzle to be used is selected from experience or
as a matter of judgment. Circular-spray nozzles are more generally
used.

(4) The spacings are determined from Fig. 170.

(5) The dosing tank of the shape described is then designed. The
capacity is such as to give a complete cycle once every 15
minutes. The method of this design is similar to that followed
previously.

(6) The dosing siphons are designed so that they will have a
capacity at the minimum head of from 40 to 50 per cent in excess
of the maximum minute inflow, and the draining depth of the siphon
will be limited to a maximum of 5 to 5½ feet. The siphons are all
made adjustable with a variation of 6 inches or more on either
side of the normal discharge line so that the spraying area and
cycle can be varied to secure the best results.

The underdrainage of a trickling filter should consist of some form of
false bottom such as the types shown in Fig. 174. Where possible the
underdrains should be open at both ends for the purpose of ventilation
and flushing. It is desirable that the drains be so arranged that a
light can be seen through them in order that clogging can be easily
located. The drains should be placed on a slope of approximately 2 in
100 towards a main collector. The length of the drains is limited by
their capacity to carry the average dose from the area drained by them.
The main collecting conduits must be designed in accordance with the
hydraulic principles given in Chapter IV. No valves, or other
controlling apparatus, are placed on the underdrains or outlets from the
filter.

Covers have been provided in winter for some trickling filters in cold
climates. The Taylor sprinkling nozzle has been found to work
successfully in extremely cold weather, and it is generally accepted
that the covering of filters is unnecessary, if the filter is not to be
shut down for any length of time in cold weather.

The operation of devices for automatically controlling the operation of
a trickling filter is explained in Chapter XXI.

[Illustration:

FIG. 174.—Types of False Bottoms for Trickling Filters.

Eng. News, Vol. 74, p. 5.
]

=258. Intermittent Sand Filter.=—An intermittent sand filter is a
specially prepared bed of sand, or other fine grained material, on the
surface of which sewage is applied intermittently, and from which the
sewage is removed by a system of underdrains. It differs from broad
irrigation in the character of the material, the care and preparation of
the bed, and the thoroughness of the underdrainage. A distinctive
feature of the intermittent sand filter is the quality of the effluent
delivered by it. In a properly designed and operated plant the effluent
is clear, colorless, odorless, and sparkling. It is completely
nitrified, is stable and contains a high percentage of dissolved oxygen.
It contains no settleable solids except at widely separated periods when
a small quantity may appear in the effluent. The percentage removal of
bacteria may be from 98 to 99 per cent. Some analyses of sand filter
effluents are given in Table 89. The dissolved solids, the remaining
bacteria, and the antecedents of the effluent are the only differences
between it and potable water. An effluent from an intermittent sand
filter is the most highly purified effluent delivered by any form of
sewage treatment. The effluent can be disposed of without dilution, on
account of its high stability. The treatment of sewage to so high a
degree is seldom required, so that the use of intermittent filters is
not common. Other drawbacks to their use are the relatively large area
of land necessary and the difficulty of obtaining good filter sand in
all localities.

TABLE 89

QUALITY OF EFFLUENTS FROM SAND FILTERS

(Report on Sewage Purification at Columbus, Ohio, by G. A. Johnson, 1905)
────────────┬───────────────────────────────────────────────────────┬───────
Source of │ Parts per Million │Rate of
Sample │ │Filtra-
│ │ tion
│ │Gallons
│ │ per
│ │ Acre,
│ │per Day
────────────┼────────────────────────────────────┬────────┬─────────┼───────
│ Nitrogen as │ Oxygen │ Oxygen │
│ │Consumed│Dissolved│
────────────┼───────┬──────────┬────────┬────────┼────────┼─────────┼───────
│ Free │Albuminoid│Nitrites│Nitrates│ │ │
│Ammonia│ Ammonia │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 11.0 │ 8.6 │ │ │ 59. │ │
influent │ │ │ │ │ │ │
from grit │ │ │ │ │ │ │
chamber │ │ │ │ │ │ │
Filter │ 1.12 │ 0.88 │ 0.08 │ 11.5 │ 6.9 │ 6.3 │ 0.081
effluent │ │ │ │ │ │ │
Filter │ 0.81 │ 0.88 │ 0.10 │ 12.6 │ 6.5 │ 6.2 │ 0.118
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 9.7 │ 5.4 │ │ │ 33. │ │
influent │ │ │ │ │ │ │
from plain│ │ │ │ │ │ │
settling │ │ │ │ │ │ │
tank │ │ │ │ │ │ │
Filter │ 0.62 │ 0.77 │ 0.11 │ 14.9 │ 6.0 │ 8.2 │ 0.139
effluent │ │ │ │ │ │ │
Filter │ 0.99 │ 1.10 │ 0.10 │ 12.6 │ 7.8 │ 6.5 │ 0.274
effluent │ │ │ │ │ │ │
Filter │ 2.61 │ 1.39 │ 0.09 │ 9.0 │ 9.7 │ 3.9 │ 0.357
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 10.7 │ 5.6 │ │ │ 38. │ │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
septic │ │ │ │ │ │ │
tank │ │ │ │ │ │ │
Filter │ 1.63 │ 1.16 │ 0.09 │ 11.2 │ 8.0 │ 5.8 │ 0.357
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 13.4 │ 4.7 │ │ │ 40. │ │
influent │ │ │ │ │ │ │
from coke │ │ │ │ │ │ │
strainer │ │ │ │ │ │ │
Filter │ 2.24 │ 1.35 │ 1.03 │ 14.6 │ 10.1 │ 6.9 │ 0.372
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 8.6 │ 3.6 │ 0.19 │ 1.6 │ 24. │ 0.3 │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
contact │ │ │ │ │ │ │
bed │ │ │ │ │ │ │
Filter │ 2.62 │ 1.35 │ 0.31 │ 8.1 │ 8.3 │ 5.8 │ 0.516
effluent │ │ │ │ │ │ │
Filter │ 2.44 │ 2.41 │ 0.16 │ 9.4 │ 12.5 │ 5.0 │ 0.525
effluent │ │ │ │ │ │ │
Filter │ 3.40 │ 1.15 │ 0.20 │ 10.9 │ 9.7 │ 5.2 │ 0.525
effluent │ │ │ │ │ │ │
────────────┼───────┼──────────┼────────┼────────┼────────┼─────────┼───────
Filter │ 9.0 │ 4.8 │ 0.42 │ 1.3 │ 27. │ 3.4 │
influent │ │ │ │ │ │ │
from │ │ │ │ │ │ │
sprinkling│ │ │ │ │ │ │
filter │ │ │ │ │ │ │
after │ │ │ │ │ │ │
sedimen- │ │ │ │ │ │ │
tation │ │ │ │ │ │ │
Filter │ 2.95 │ 1.25 │ 0.19 │ 7.0 │ 8.8 │ 3.8 │ 0.675
effluent │ │ │ │ │ │ │
Filter │ 4.77 │ 2.63 │ 0.51 │ 4.6 │ 11.8 │ 2.5 │ 0.749
effluent │ │ │ │ │ │ │
Filter │ 3.47 │ 1.61 │ 0.31 │ 7.2 │ 11.9 │ 3.7 │ 1.129
effluent │ │ │ │ │ │ │
────────────┴───────┴──────────┴────────┴────────┴────────┴─────────┴───────

The action in an intermittent sand filter is more complete than in other
forms of filters because a greater surface is exposed to the passage of
sewage by the fine sand particles, and the sewage is in contact with the
filtering material a longer time due to the lower rate of filtration and
the slow velocity of flow through the filter. It is essential that the
sewage be applied to the bed intermittently in order that air shall be
entrained in the filter. The period between doses should not be so long
that the filter becomes dry.

In the operation of an intermittent sand filter one dose per day is
considered an ordinary rate of application, although some plants operate
with as many as four doses per day per filter, and others on one dose at
long and irregular intervals. It is not always necessary to rest the
filter for any length of time unless signs of overloading and clogging
are shown. The intermittent dosing action may be obtained by the action
of an automatic siphon as is described in Chapter XXI. The sewage is
distributed on the beds through a number of openings in the sides of
distributing troughs resting on the surface of the filter. The sewage is
withdrawn from the bottom of the filter through a system of underdrains,
into which it enters after its passage through the bed. There are no
control devices on the outlet, as the rate of filtration is controlled
by the action of the dosing apparatus and the rate at which sewage is
delivered to it. The action of the dosing apparatus should respond
quickly to variations in sewage flow. As the doses are applied to a sand
filter, a mat of organic matter or bacterial zoöglea is formed on the
surface of the bed. The mat is held together by hair, paper, and the
tenacity of the materials. It may attain a thickness of ¼ to ½ an inch
before it is necessary to remove it. So long as the filter is draining
with sufficient rapidity this mat need not be removed, but if the bed
shows signs of clogging, the only cleaning that may be necessary will be
the rolling up of this dried mat. It is believed that the greater
portion of the action in the filter occurs in the upper 5 to 8 inches of
the bed, but occasionally the beds become so clogged that it is
necessary to remove ¾ of an inch to 2 inches of sand in addition to the
surface mat, or to loosen up the surface by shallow plowing or
harrowing. The necessity for such treatment may indicate that the filter
is being overloaded as a result of which the rate of filtration should
be decreased or the preliminary treatment should be improved. The
plowing of clogging material into the bed should be avoided as under
these conditions the final condition of the bed will be worse than its
condition when trouble was first observed.

In winter the surface of the bed should be plowed up into ridges and
valleys. The freezing sewage forms a roof of ice which rests on the
ridges and the subsequent applications of sewage find their way into the
filter through the valleys under the ice. In a properly operated bed the
filtering material will last indefinitely without change. If a filter is
operated at too high a rate, however, although the quality of the
effluent may be satisfactory, it will be necessary at some time to
remove the sand and restore the filter.

The rate of filtration depends on the character of the influent, the
desired quality of the effluent, and the depth and character of the
filtering material. Filters can be found operating at rates of 50,000
gallons per acre per day and others at eight times this rate. For sewage
which has had some preliminary treatment, the rate should not exceed
100,000 gallons per acre per day, whereas the rate for raw sewage should
be less than this. For rough estimates made without tests of the sewage
in question, the rate should not be taken at more than 1,000 persons per
acre. If the preliminary treatment of the sewage has been thorough and
the material of the sand filter is coarser than ordinary the rate of
filtration can be high. For less careful preliminary treatment and fine
filtering material the rates must be reduced. The sewage must undergo
sufficient preliminary treatment to remove large particles of solid
matter which would otherwise clog the dosing apparatus and the filter.
This treatment should include grit removal, screening, and some form of
tank treatment. Some plants have operated successfully with a stale
sewage and no preliminary treatment, as at Brockton, Mass. Septic tank
effluent can be treated successfully on an intermittent sand filter, but
not so satisfactorily as the effluent from a tank delivering a fresh
sewage.

The material of the filter should consist of clean, sharp, quartz or
silica sand with an effective size[164] of 0.2 to 0.4 mm., preferably
about 0.25 to 0.35 mm., and a uniformity coefficient[165] of 2 to 4.
Within the limits mentioned no careful attention need be given to the
size of the material. Natural sand found in place has been underdrained
and used successfully for sewage treatment. The size of the sand is
fixed by the rate of filtration rather than the bacteriological action
of the filter. A coarse sand will permit the sewage to pass through the
bed too rapidly, and a fine sand will hold it too long or will become
clogged. The same size of material should be used throughout the bed,
except that a layer of gravel from 6 to 12 inches thick, graded from
very small sizes to stones just passing a 2–inch ring should be placed
at the bottom to facilitate the drainage of the bed.

The thickness of the sand layer should not be less than 30 inches to
insure complete treatment of the sewage. In shallower beds the sewage
might trickle through without adequate treatment. Beds are ordinarily
made from 30 to 36 inches deep, but when deeper layers of sand are found
in place there is no set limit to the depth which may be used. The shape
and overall dimensions of the bed should conform to the topography of
the site and the rate of filtration adopted. A plan and cross-section of
an intermittent sand filter showing the distribution and under drainage
systems are given in Fig. 166 and 175.

The distribution system consists of a system of troughs on the surface
of the filter, laid out in a branching form, as shown in the figure. The
openings in the troughs should be so located that the maximum distance
from any point on the bed to the nearest opening should not exceed 20 to
30 feet. If the filters are small enough, troughs need not be used, the
sewage being distributed from one corner, or from mid-points on the
sides. Where troughs are used they should be supported from the bottom
of the filter in order to prevent uneven settling due to the washing of
the sand. The openings in the troughs are made adjustable by swinging
gates as shown in Fig. 176, or by other means so that after the filter
is in operation the intensity of the dose on any portion of the filter
can be changed. The troughs may be placed with their bottoms level with
the surface of the sand and with sides of sufficient height to give the
required gradient to the water surface, or they may be built up above
the surface of the filter and given the required slope so that the
surface of the flowing water is parallel to the bottom of the trough. In
either case a splash plate should be placed at each opening, so that not
less than 2 feet of the surface of the sand is protected in all
directions from the opening. A stone or concrete slab 2 to 4 inches
thick makes a satisfactory splash plate. Either wood or concrete may be
used for the construction of the troughs. The former is less durable,
but also less expensive in first cost. The capacity of the troughs may
be computed by Kutter’s formula with the quantity to be carried equal to
the maximum rate of discharge of the feeding siphon, with a reduction in
size below each branch or outlet proportional to the amount which will
be discharged above this point.

[Illustration:

FIG. 175.—Plan and Section of an Intermittent Sand Filter Showing
Central Location of Control House.
]

The operation of automatic devices for dosing the bed is explained in
Chapter XXI. The dosing tank should have a capacity sufficient to cover
the bed to a depth of about 1 to 3 inches at one dose, and the siphon
should discharge at a rate of about one second-foot for each 5,000
square feet of filter area. A dose should disappear within 20 minutes to
half an hour after it is applied to the filter. With the rate stated and
four applications per day to a depth of 1 inch at each dose, the rate
per acre per day will be 109,000 gallons.

[Illustration:

FIG. 176.—Distributing Trough with Adjustable Openings.
]

The filtration of sewage through sand in a manner similar to the _rapid
sand filtration_ of water is being attempted at the Great Lakes Naval
Training Station. No results of this treatment have been published and
the practical success of the method has not been assured.

=259. Cost of Filtration.=—Only comparative figures can be given in
stating the costs of filtration, as most data available are based on
pre-war conditions, and are therefore unreliable for present conditions.
The variations from the figures given may be very large but in general
the relative costs have not changed. The figures given in Table 90 are
suggestive of the relative costs of the different forms of filtration.

TABLE 90

RELATIVE COSTS OF DIFFERENT METHODS OF SEWAGE TREATMENT

Costs in Dollars per Million Gallons per Day
─────────────────────────┬───────────────┬──────────────┬──────────────
Form of Treatment │First Cost[166]│Operation and │ Total
│ │ Maintenance │
─────────────────────────┼───────────────┼──────────────┼──────────────
Coarse screens │ │ │ 0.20
Fine screens │ │ │ 3.00
Plain sedimentation │ 7.00│ 3.00│ 10.00
Chemical precipitation │ │ │ 22.00[167]
Septic tank │ 7.00│ 1.00│ 8.00
Imhoff tank │ 10.00│ 1.00│ 11.00
Contact bed │ 8.00│ 2.00│ 10.00
Trickling filter │ 4.00│ 2.00│ 6.00
Intermittent sand filter │ 15.00│ 10.00│ 25.00
Activated sludge │ 6.50│ 8.50│ 15.00[168]
─────────────────────────┴───────────────┴──────────────┴──────────────

IRRIGATION

=260. The Process.=—Broad irrigation is the discharge of sewage upon the
surface of the ground, from which a part of the sewage evaporates and
through which the remainder percolates, ultimately to escape in surface
drainage channels. Sewage farming is broad irrigation practiced with the
object of raising crops. Broad irrigation can be accomplished
successfully without the growing of crops, but it is seldom attempted as
some return and sometimes even a profit can be obtained from the crops
raised. Broad irrigation and sewage farming differ from intermittent
sand filtration in the intensity of the application of the sewage, the
method of preparing the area on which the sewage is to be treated, and
the care in operation. In broad irrigation and intermittent sand
filtration the paramount consideration is successful disposal of the
sewage. In sewage farming the paramount consideration is the growing of
crops. The growing of crops may be combined with irrigation and
filtration, however, but the crop should be sacrificed to the successful
disposal of the sewage.

The change which occurs in the characteristics of the sewage due to its
filtration through the ground is the same as occurs in aërobic
filtration. The effect on the crops is mainly that of an irrigant, as
the manurial value of the sewage is small.

=261. Status.=—The disposal of sewage by broad irrigation was practiced
in England previous to the development of any of the more intensive
biologic methods of treatment. It was considered the only safe and
sanitary method for the disposal of sewage, and as a result, areas
irrigated by sewage were common throughout England. Crops were grown on
these areas as a minor consideration, and sewage farming gained some of
its popularity from the apparent success of these disposal areas. The
success of sewage farms is due more to generous irrigation in dry years
than to fertilization by sewage.

The sewage farms of Paris and Berlin are frequently cited as examples of
the successful and remunerative disposal of sewage by farming in
connection with broad irrigation. Kinnicutt, Winslow, and Pratt[169]
state:

The Berlin Sewage farms offer examples of broad irrigation under
better conditions ... of 21,008 acres receiving sewage, 16,657
acres were farmed by the city, 3,956 acres were leased to farmers,
and only 395 acres were unproductive. The contributing population
at this time was 2,064,000 and the average amount of sewage
treated was 77,000,000 gallons, giving a daily rate of treatment
of about 3,700 gallons per acre of prepared land. The soil is
sandy and of excellent quality. A quarter of the area operated by
the authorities is devoted to pasturage, and about a third to the
cultivation of cereals, of which winter rye and oats are the most
important. Potatoes and beets are grown in considerable amounts
and a wide variety of other crops in smaller proportions.... Even
fish ponds are made to yield a part of the revenue, and the drains
on some of the farms have been successfully stocked with breed
trout.

The cost of the Berlin farms to March 31, 1910, was $17,470,000,
somewhat more than half being the purchase price of the land. The
expenses for this year amounted to $1,300,385 for maintenance, and
$741,818 for interest charges. The receipts were $1,240,773 and
there was an estimated increase of $122,593 in value of live stock
and other property.

The conditions at Berlin are quoted at length to indicate the success
which can accompany broad irrigation, and as an example of what is being
done abroad, where the rainfall is light and the soil is suitable.

In the United States success in sewage farming has not been marked. This
may be due partially to the relative weakness of American sewages, to
the cost of labor, to lack of satisfactory irrigation areas, and to
inattention to details. An attempt was made to grow crops on the sand
filters at Brockton, Mass., but it was finally abandoned as the
interests of the crops and the successful treatment of the sewage could
not both be satisfied. At Pullman, Illinois,[170] in 1880, there was
commenced probably the most extensive attempt at sewage farming in
eastern United States. The farm was a failure from the start, because of
the clay soil, and it was subsequently abandoned. Sewage farming, mainly
as a subsidiary consideration to the filtration of sewage, is practiced
in a few cities in the eastern portion of the United States to-day.
Among the cities mentioned by Metcalf and Eddy[171] are Danbury, Conn.,
and Fostoria, Ohio. In the western portion of the United States where
water is scarce and the ground is porous, sewage has been used as an
irrigant with some success. Such use of sewage cannot be considered as a
method of treatment since the prime consideration is the growing of
crops. In this process all sewage not used as an irrigant is discharged
without treatment into water courses. According to Metcalf and Eddy
there were 35 cities in California in 1914 that were operating sewage
farms. Among these are Pasadena, Fresno, and Pomona. Other farms,
notably the pioneer farm at Cheyenne, Wyo., have been abandoned because
of the local nuisance created and the lack of financial success.

=262. Preparation and Operation.=—A porous sandy soil on a good slope
and with good underdrainage is most suitable for broad irrigation.
Impervious clay or gumbo soils are unsuitable and should not be used.
They become clogged at the surface, forming pools of putrefying sewage,
or in hot weather form cracks which may permit untreated sewage to
escape into the underdrains.

The sewage may be distributed to the irrigated area in any one of five
ways which are known as: flooding, surface irrigation, ridge and furrow
irrigation, filtration, and subsurface irrigation. In flooding, sewage
is applied to a level area surrounded by low dikes. The depth of the
dose may be from 1 inch to 2 feet. In surface irrigation the sewage is
allowed to overflow from a ditch over the surface of the ground into
which it sinks or over which it flows into another ditch placed lower
down. This ditch conducts it to a point of disposal or to another area
requiring irrigation. Ridge and furrow irrigation consists in plowing a
field into ridges and furrows and filling the furrows with sewage while
crops are grown on or between the ridges. In filtration the sewage is
distributed in any desired fashion on the surface and is collected by a
system of underdrains after it has filtered through the soil. In
subsurface irrigation the sewage is applied to the land through a system
of open-joint pipes laid immediately below the surface, similarly to a
system of underdrains. Combinations of and modifications to these
methods are sometimes made. Underdrains may be used in connection with
any of these forms of distribution.

The preparation of the ground consists in: the construction of ditches
or dikes to permit of any of the above described methods of application,
grading of the surface to prevent pooling, the laying of underdrains,
and the grubbing and clearing of the land. The main carriers may be
excavated in open earth or earth lined with an impervious material. The
distribution of the sewage from the main carriers to groups of laterals
may be controlled by hand-operated stop planks. If the soil has a
tendency to become waterlogged it may be relieved by installing
underdrains at depths of 3 to 6 feet, and 40 to 100 feet apart. The tile
underdrains may discharge into open ditches excavated for the purpose
which serve also to drain the land. Drains should be used where the
ground water is within 4 feet of the surface, and the open ditches
should be cut below the drains to keep the ground water out of them.
Four or 6–inch open-joint farm tile may be used for underdrains. The
porosity of the soil will be increased by cultivation. Where particular
care is taken in the cultivation of the soil so that sewage can be
applied at a high rate, broad irrigation merges into the more intensive
intermittent filtration through sand.

Before being turned on to the land, sewage should be screened and
heavy-settling particles should be removed. The rate of application may
be increased as the intensity of the preliminary treatment is increased.
The rate at which sewage may be applied is dependent also on the
character of the soil, and may vary between 4,000 and 30,000 gallons per
acre per day, although higher rates have been used with the effluent
from treatment plants and on favorable soil. The sewage should be
applied intermittently in doses, the time between doses varying between
one day and two or three weeks or more, dependent on the weather and the
condition of the soil. The methods of dosing vary as widely as the
rates. The dose may be applied continuously for one or two weeks with
correspondingly long rests, or it may be applied with frequent
intermittency alternated with short rests, interspersed with long rest
periods at longer intervals of time. When applying the sewage to the
land the rate of application of the dose is about 10,000 to 150,000
gallons per acre per day. The area under irrigation at any one time may
be as much as 10 to 15 acres. The rate of the application of the sewage
is also dependent on the weather and may vary widely between seasons. It
is obvious that a rain-soaked pasture cannot receive a large dose of
sewage without danger of undue flooding. One of the principal
difficulties with the treatment or disposal of sewage by broad
irrigation is that the greatest load of sewage must be cared for in wet
seasons when the ground is least able to absorb the additional moisture.

=263. Sanitary Aspects.=—A well-operated sewage farm should cause no
offense to the eye or nose, and is not a danger to the public health. In
Berlin, a portion of the sewage farms are laid out as city parks. The
liquid in the drainage ditches or underdrains may be clear, odorless,
and colorless, high in nitrates and non-putrescible. Where the farm has
been improperly managed or overdosed the condition may be serious from
both esthetic and health considerations. Sewage may be spread out to
pollute the atmosphere and to supply breeding places for flying insects
which will spread the filth for long distances surrounding the farm. The
character of the crop is also a sanitary consideration.

=264. The Crop.=—From a sanitary viewpoint no crops which come in
contact with the sewage should be cultivated on a sewage farm. Such
products as lettuce, strawberries, asparagus, potatoes, radishes, etc.,
should not be grown. Grains, fruits, and nuts are grown successfully and
as they do not come in contact with the sewage there is no sanitary
objection to their cultivation in this manner. Italian rye grass and
other forms of hay are grown with the best success as they will stand a
large amount of water without injury. The raising of stock is also
advisable for sewage farms where hay and grain are cultivated. The stock
should be fed with the fodder raised on the irrigated lands and should
not be allowed to graze on the crops during the time that they are being
irrigated. This is due as much to the danger of injury to the
distributing ditches and the formation of bogs by the trampling of the
cattle, as to the danger to the health of the cattle.

CHAPTER XVIII
ACTIVATED SLUDGE

=265. The Process.=—In the treatment of sewage by the activated sludge
process the sewage enters an _aëration tank_ after it has been screened
and grit has been removed. As it enters the aëration tank it is mixed
with about 30 per cent of its volume of activated sludge. The sewage
passes through the aëration tank in about two to four hours during which
time air is blown through it in finely divided bubbles. The effluent
from the aëration tank passes to a _sedimentation tank_ where it remains
for one-half an hour to an hour to allow the sedimentation of the
activated sludge. The supernatant liquid from the sedimentation tank is
passed to the point of final disposal. A portion of the sludge removed
from the tank is returned to the influent of the aëration tank. The
remainder may be sent to any or all of the following: the _sludge drying
process_, the reaëration tanks, or to some point for final disposal.
Sections of the activated sludge plant at Houston, Texas, are shown in
Fig. 177.

The biological changes in the process occur in the aëration tank. These
changes are dependent on the aërobic organisms which are intensively
cultivated in the activated sludge. When placed in intimate contact with
fresh sewage, brought about by the agitation caused by the rising air,
and in the presence of an abundance of oxygen, the organic matter is
partially oxidized. The putrefactive stage of the organic cycle is
avoided. Colloids and bacteria are partially removed probably by the
agitation effected in the presence of activated sludge but the exact
action which takes place is not well understood.

[Illustration:

FIG. 177.—Activated Sludge Plant at Houston, Texas.

Eng. News, Vol. 77, p. 236.
]

=266. Composition.=—Activated sludge is the material obtained by
agitating ordinary sewage with air until the sludge has assumed a
flocculent appearance, will settle quickly, and contain aërobic and
facultative bacteria in such numbers that similar characteristics can be
readily imparted to ordinary sewage sludge when agitated with air in the
presence of activated sludge. Copeland described activated sludge as
follows:[172]

The sludge embodied in sewage and consisting of suspended organic
solids, including those of a colloidal nature, when agitated with
air for a sufficient period assumes a flocculent appearance very
similar to small pieces of sponge. Aërobic and facultative
bacteria gather in these flocculi in immense numbers—from 12 to 14
million per c.c.—some having been strained from the sewage and
others developed by natural growth. Among the latter are species
that have the power to decompose organic matter, especially of an
albuminoid or nitrogenous nature, setting the nitrogen free; and
others absorbing the nitrogen convert it into nitrites and
nitrates. These biological processes require time, air, and
favorable environment such as suitable temperature, food supply
and sufficient agitation to distribute them throughout all parts
of the sewage.

Ardern states that the sludge differs entirely from the usual tank
sludge. It is inoffensive and flocculent in character. The percentage of
moisture is from 95 to 99 per cent. American experience has generally
been that the sludge does not readily separate from its moisture by
treatment on fine-grain filters, but the results in England and at
Milwaukee, Wisconsin, are in conflict with this general experience. Upon
standing 24 hours or more partially dried activated sludge may start to
decompose accompanied by the production of offensive odors.

Duckworth states:

The activated sludge at Salford contained three times as much
nitrogen, twice as much phosphoric acid and one-half as much fatty
matter as ordinary sludge.

TABLE 91

COMPOSITION OF SEWAGE, IMHOFF SLUDGE, AND ACTIVATED SLUDGE AND EFFLUENT AT
MILWAUKEE

(W. R. Copeland, Eng. News, Vol. 76, p. 665)
──────┬──────────┬────────────────────────────────────────────────────────────
Period│Source of │ Parts per Million
of │ Sample │
Test │ │
──────┼──────────┼──────┬──────────────────────────────────────┬──────────────
│ │ Sus- │ Nitrogen as │Nitrogen
│ │pended│ │ Reported as
│ │Matter│ │ Ammonia on a
│ │ │ │ Basis of
│ │ │ │ Sludge Dried
│ │ │ │ to 10 Per
│ │ │ │ Cent
│ │ │ │ Moisture.
│ │ │ │ Three
│ │ │ │ samples of
│ │ │ │ Sludge
──────┼──────────┼──────┼───────┬───────┬────────┬──────┬──────┼──────────────
│ │ │ Free │ Albu- │Organic │ Ni- │ Ni- │
│ │ │Ammonia│minoid │Nitrogen│trites│trates│
│ │ │ │Ammonia│ │ │ │
──────┼──────────┼──────┼───────┼───────┼────────┼──────┼──────┼────┬────┬────
Aug., │Sewage │ 253│ 14.6│ 7.88│ 29│ 0.15│ 0.13│ │ │
1915│ │ │ │ │ │ │ │ │ │
│Imhoff │ 105│ 16.2│ 6.10│ 27│ 0.19│ 0.13│2.87│3.82│
│ effluent│ │ │ │ │ │ │ │ │
│Activated │ 14│ 3.8│ 3.19│ 6│ 0.29│ 6.00│5.71│4.97│7.04
│ sludge │ │ │ │ │ │ │ │ │
│ effluent│ │ │ │ │ │ │ │ │
──────┼──────────┼──────┼───────┼───────┼────────┼──────┼──────┼────┼────┼────
Sept.,│Sewage │ 300│ 13.5│ 8.81│ 29│ 0.25│ 0.14│ │ │
1915│ │ │ │ │ │ │ │ │ │
│Imhoff │ 116│ 15.4│ 7.10│ 27│ 0.12│ 0.09│3.88│ │
│ effluent│ │ │ │ │ │ │ │ │
│Activated │ 8│ 5.7│ 2.22│ 9│ 0.24│ 5.01│8.69│9.00│
│ sludge │ │ │ │ │ │ │ │ │
│ effluent│ │ │ │ │ │ │ │ │
──────┴──────────┴──────┴───────┴───────┴────────┴──────┴──────┴────┴────┴────

These results have been roughly checked by American experimenters as
shown in Table 91.[173] In the recovery of nitrogen from sewage the
activated sludge process is the most promising for satisfactory results.
In all other processes of sewage treatment the sludge is digested to
some extent and nitrogen lost in the gases or in the soluble matter
which passes off with the effluent. In the activated sludge process a
negligible amount of gasification and liquefaction take place and only a
small amount of nitrogen passes off with the effluent as compared with
the loss from the Imhoff process as shown in Table 91. The percentage of
nitrogen in dried activated sludge is shown in Table 92.

TABLE 92

NITROGEN CONTENT OF DRY ACTIVATED SLUDGE AND SLUDGE FROM OTHER
PROCESSES

(G. W. Fuller, Eng. News, Vol. 76, p. 667)
────────────────────────────────────────┬──────────────────────────────
Source │ Per Cent Nitrogen
────────────────────────────────────────┼──────────────────────────────
Milwaukee (Copeland) │ 4.40
Manchester, England (Ardern) │ 4.60
Salford, England (Melling) │ 3.75
Urbana, Illinois (Bartow) │ 3.5 to 6.4
Armour and Co. (Noble) │ 4.6
Approximate range of all other processes│ 1.0 to 3.0
────────────────────────────────────────┴──────────────────────────────
These figures are expressed in terms of nitrogen and not of ammonia.
Nitrogen is only 82 per cent of the ammonia content.

Nitrifying bacteria and other species which have the power of destroying
organic matter have been isolated from the sludge. An analysis of the
dried sludge at Urbana[174] showed the following results after the
weight had been reduced 95.5 per cent by drying: 6.3 per cent nitrogen,
4.00 per cent fat, 1.44 per cent phosphorus, and 75 per cent volatile
matter or loss on ignition. Analyses of other domestic sewages have not
shown such high contents of these desirable constituents.

The dewatering of activated sludge is a problem which offers serious
obstacles to the successful operation of the process. It is its greatest
disadvantage. Five to ten times the volume of sludge may be produced by
the activated sludge process as by an Imhoff tank, and the activated
sludge contains a greater percentage of water. According to Copeland:

The best information now available points to a combination of
settling and decantation as a preliminary dewatering process. By
this means the water will be cut down from about 99 per cent to 96
per cent. On passing the concentrated residue through a pressure
filter the moisture can be cut down to 75 per cent. The press cake
can be dewatered in a heat drier to 10 per cent moisture or
less.[175]

The quantity of sludge produced at Milwaukee[176] is about 15 cubic
yards per million gallons of sewage, the sludge having about 98 per cent
moisture. On the basis of 10 per cent moisture it produces ½ ton of dry
sludge per million gallons of sewage treated. At Cleveland,[177] 20
cubic yards per million gallons at 97.5 per cent moisture are produced.
Methods of drying sludge are discussed in Chapter XX.

Chemical analyses and biological tests indicate that the fertilizing
value of the sludge is appreciable. Professor C. B. Lipman states, as
the result of a series of tests in which a sludge and a soil were
incubated for one month, as follows:[178]

The amounts of nitrates produced in one month’s incubation from
the soil’s own nitrogen and from the nitrogen from the sludge
mixed with the soil in the ratio of one part of sludge to 100 of
soil is, in milligrams of nitrate, as follows: Anaheim soil
without sludge 6.0, with sludge 10.0; Davis soil without sludge
4.2, with sludge 14.0; Oakley soil without sludge 2.2, with sludge
4.0.

The effect of the sludge on plant growth is shown in Table 93.[179] The
results represent the growth obtained after fifteen weeks from the
planting of 30 wheat seeds in each pot.

=267. Advantages and Disadvantages.=—Some of the advantages of the
process are: a clear, sparkling, and non-putrescible effluent is
obtained; the degree of nitrification is controllable within certain
limits; the character of the effluent can be varied to accord with the
quantity and character of the diluting water available; more than 90 per
cent of the bacteria can be removed; the cost of installation is
relatively low; and the sludge has some commercial value.

TABLE 93

FERTILIZING VALUE OF ACTIVATED SLUDGE

(E. Bartow, Journal Am. Water Works Ass’n, Vol. 3, p. 327)
───────────────────────────────────────┬───────────────────────────────
Cultivating Medium │Grams Contained in Experimental
│ Pot
───────────────────────────────────────┼───────┬───────┬───────┬───────
│ 1 │ 2 │ 3 │ 4
───────────────────────────────────────┼───────┼───────┼───────┼───────
White sand │ 19,820│ 19,820│ 19,820│ 19,820
Dolomite │ 60│ 60│ 60│ 60
Bone meal │ 6│ 6│ 6│ 6
Potassium sulphate │ 3│ 3│ 3│ 3
Activated sludge │ 0│ 0│ 20│ 0
Activated sludge extracted with Ligroin│ 0│ 0│ 0│ 20
Dried blood │ 0│ 8.61│ 0│ 0
───────────────────────────────────────┼───────┼───────┼───────┼───────
Number of heads of wheat │ 14│ 15│ 22│ 23
Number of seeds │ 85│ 189│ 491│ 518
Weight of seeds, grams │ 2.38│ 5.29│ 13.748│ 14.504
Bushels per acre, calculated │ 6.20│ 13.6│ 35.9│ 38.7
Average length of stalk, inches │ 19.40│ 23.0│ 35.4│ 37.1
Weight of straw, grams │ 2.25│ 8.25│ 26.75│ 26.21
Tons per acre, calculated │ 0.18│ 0.68│ 2.23│ 2.18
───────────────────────────────────────┴───────┴───────┴───────┴───────

Among the disadvantages of the process can be included, uncertainty due
to the lack of information concerning the results to be expected under
all conditions, high cost of operation under certain conditions, the
necessity for constant and skilled attendance, and the difficulty of
dewatering the sludge.

=268. Historical.=—The most notable work in the aëration of sewage
within recent years was that performed by Black and Phelps for the
Metropolitan Sewerage Commission of New York, in 1910,[180] and by Clark
and Gage at the Lawrence, Massachusetts, Sewage Experiment Station in
1912 and 1913.[181] The results of these investigations showed that the
treatment of sewage by forced aëration might give a satisfactory
effluent, but that the time and expense in connection thereto rendered
the method impractical.

It remained for Messrs. Ardern and Lockett of Manchester, England, to
introduce the process of the aëration of sewage in the presence of
activated sludge, as a result of their connection with Dr. Fowler, who
attributes his inspiration to his visit to the Lawrence Experiment
Station and observing the work of Clark and Gage. Ardern and Lockett
commenced their experiments in 1913. Their results were published in the
_Journal of the Society of Chemical Industry_, May 30, 1914, Vol. 33, p.
523. Shortly thereafter experiments were started at the University of
Illinois by Dr. Edw. Bartow and Mr. F. W. Mohlmann of the Illinois State
Water Survey. At about the same time an experimental plant was started
at Milwaukee, by T. C. Hatton, Chief Engineer of the Milwaukee Sewerage
Commission. The United States Public Health Service became actively
interested in December, 1914, and on February 20, 1915, announced its
intention to co-operate with the Baltimore Sewerage Commission in the
conduct of experiments. In May, 1915, patent number 1,139,024 was
granted to Leslie C. Frank, Sanitary Engineer of the U. S. Public Health
Service, covering certain features of the process. Mr. Frank generously
donated this patent to the public for the use of municipalities.

The first full sized plant for the treatment of sewage by this method
was erected in Milwaukee in December, 1915. This plant had a capacity of
1,600,000 gallons per day. It was used for experimental purposes and is
not now in use. The Champaign, Illinois, septic tank, among the first of
its kind in the country, was converted into an activated sludge tank on
April 13, 1916. The changes, developments, and the results obtained from
these and other plants have been reported in the technical press from
time to time.

=269. Aëration Tank.=—The sewage on leaving the screen and grit chamber
enters the aëration tank, which is usually operated on the
continuous-flow principle, although in the early days of experimentation
the fill and draw method was practiced. This tank should be rectangular
with a depth of about 15 feet and a width of channel not to exceed 6 to
8 feet. Such proportions allow better air and current distribution than
larger tanks. The bottom should be level to insure an even distribution
of air. The velocity of flow of sewage through the tank is usually in
the neighborhood of 5 feet per minute, dependent on the length of the
tank and the period of retention. The period of retention is in turn
dependent on the desired quality of the effluent. The process is
flexible and the quality of the effluent can be changed by changing the
period of retention or by changing the rate of application of the air,
or both. The period of retention in the aëration tank is usually about 4
hours.

The bottom of the aëration tank is usually made of concrete arranged in
ridges and valleys, or small shallow hoppers, at the bottom of which the
air-diffusing devices are located, as shown in Fig. 177. The inlet and
outlet devices are similar to those in a plain sedimentation tank.

=270. Sedimentation Tank.=—It is evident that as no sedimentation is
permitted in the aëration tank, the settleable particles will be
discharged in the effluent unless some provision is made for their
detention. The effluent from the aëration tank is therefore run through
a plain sedimentation tank, usually with a hopper bottom, which has been
arranged to permit frequent and easy cleaning. An air lift or a
centrifugal sludge pump is satisfactory for this purpose. Another type
of sedimentation tank which has been used has a smooth bottom with a
slight slope towards the center. A revolving scraper collects the sludge
continuously, scraping it towards the center of the tank. Although this
arrangement gives better results than the hopper-bottom tank, its
expense has usually prevented its installation.[182]

The period of sedimentation in different plants varies from 30 minutes
to one hour, although the longer periods usually give the better
results. Approximately 65 per cent of the sludge will settle in the
first 10 minutes, 80 per cent in the first 30 minutes, and about 5 per
cent more in the next half hour.

The effluent from the sedimentation tank is ready for final disposal or
if desired, for further treatment by some other method. The sludge, or a
portion of it, is pumped back into the influent of the aëration tank,
provided the sludge is in a satisfactory state of nitrification.
Otherwise it should be pumped to the reaëration tanks. The remainder of
the sludge which is not to be used in the process is ready for drying
and final disposal.

=271. Reaëration Tank.=—The purpose of the reaëration or sludge aëration
tank is to reactivate the sludge which has gone through the aëration
tank. During the process of the aëration of the sewage in the aëration
tank the activated sludge may lose some of its qualities because of the
deficiency of oxygen to maintain aërobic conditions. By blowing air
through the sludge in the reaëration tank these properties are returned
and the sludge made available to be pumped back into the aëration tank.
The reactivation of the sludge obviates the necessity for supplying
sufficient air to the entire mass of the sewage to maintain aërobic
conditions, and results in an economy in the use of air. The use of
mechanical agitators has also been attempted both in the reaëration and
the aëration tanks with the expectation of saving in the use of air, but
with indifferent success.

It is difficult to say, without experimentation, what the size of the
reaëration tank should be, as the necessary amount or reactivation is
uncertain. In the experimental plant at Milwaukee, there were eight
units of aëration tanks, one sedimentation tank, and two reaëration
tanks, all of the same capacity and general design. This represents a
ration of about one reaëration tank to four aëration tanks.

=272. Air Distribution.=—Air is applied to the sewage at the bottom of
the aëration tank at a pressure in the neighborhood of 5.5 to 6.0 pounds
per square inch, dependent on the depth of the sewage, the loss of head
through the distributing pipes, and the rate of application. In
different experimental plants the pressure has varied from 3 to 30
pounds per square inch. Such pressures are on the line which divides the
use of direct blowers for low pressures from turbo and reciprocating
pressure machines for pressures above 10 pounds per square inch.
Positive-pressure blowers or direct blowers operate on the principle of
a centrifugal pump and because of the lighter specific gravity of air
they rotate at a very high speed. The Nash Hytor Turbo Blower consists
of a rotor with a large number of long teeth slightly bent in the
direction of rotation. The rotor, which has a circular circumference,
revolves in an elliptical casing. At the commencement of operation the
rotor and casing are partially filled with water. The revolution of the
rotor throws the water to the outside of the elliptical casing thus
forming a partial vacuum between any two teeth as the water is thrown
from near the center of the short diameter of the casing to the
extremity of the long diameter of the casing. Air is allowed to enter
through the inlet port to relieve the vacuum. As the teeth pass from the
long diameter to the short diameter of the ellipse, the water again
approaches the center of the rotor compressing the air trapped between
the teeth and forcing it out under pressure into the exhaust pipe. Among
the advantages of this compressor are the washing of the air, cooling,
and ease in operation. Reciprocating air compressors operate similarly
to direct-acting steam pumps or crank-and-fly-wheel pumps but at much
higher speeds, and they require more floor space than either of the
other types. Fig. 178 shows the field of serviceability of various types
of air compression machinery.

[Illustration:

FIG. 178.—Economic Range of Air Compressors.

From Eng. News, Vol. 74, p. 906.
]

For pressures up to about 10 pounds per square inch the positive blower
seems most desirable. It has a low first cost and a relatively high
efficiency of about 75 to 80 per cent of the power input. No oil or dirt
is added to the air to clog the distributing plates, as in the
reciprocating machine. A disadvantage is the difficulty of varying the
pressure or quantity of the output of the machine. As the required
pressure and volume of air increases the turbo blower becomes more and
more desirable within the limits of pressure which are ordinarily used
in this process. For small installations the best form of power is
probably the electric drive, but when the capacity becomes such as to
make turbo blowers advisable they should be driven by directly connected
steam turbines.

The quantity of air required varies between 0.5 to 6.0 cubic feet per
gallon of sewage, with from 3 to 6 hours of aëration. The quantity of
air depends on the degree of treatment required, the strength of the
sewage, the depth of the tank, and the period of aëration. The deeper
the tank the less the amount of air needed because of the greater travel
of the bubble in passing through the sewage, but the higher the pressure
at which the air must be delivered. Shallow tanks usually require a
longer period of retention. The depth of the tank then has very little
to do with economy in the use of air. Hatton states:[183]

The purification of sewage obtained varies decidedly with the
volume of air applied. Small volumes applied for 5 or 6 hours do
as well as larger volumes applied for 3 or 4 hours, but the time
of aëration required to obtain a like effluent does not vary
directly with the volume of air applied per unit of time. For
instance air applied at a rate of 2 cubic feet per minute purifies
the sewage in less time than one cubic foot of air per minute, but
will not accomplish an equal degree of purification in half the
time.

It has been found that although a low temperature has a deleterious
effect on the process, by the use of an additional quantity of air good
results can be maintained. The effect of changing the quantity of air
and the period of aëration are shown in Table 94 taken from Hatton.

The velocity of the air in the pipes should be about 1,000 feet per
minute. There should be relatively few sharp turns in the line, and the
distributing mains should be arranged without dead ends. It is desirable
to use as little piping as possible and at the same time to make the
travel of the sewage long in order to maintain a non-settling velocity
and intimate contact with the air. The piping should be accessible and
well provided with valves. It should be non-corrodible, particularly on
the inside, as flakes of rust will quickly clog the air diffusers. It
should drain to one point in order that it can be emptied when flooded,
as occasionally happens.

TABLE 94

EFFECT OF VARIOUS RATES AND PERIODS OF APPLICATION OF AIR ON THE RESULTS
OBTAINED FROM THE TREATMENT OF SEWAGE BY THE ACTIVATED SLUDGE PROCESS

(Milwaukee Results)

─────────┬──────┬──────┬──────────┬────────
Time of │Cubic │Cubic │Appearance│Per Cent
Aëration,│ Feet │ Feet │of Settled│Removal
Hours │ Free │ Air │ Liquid │Bacteria
│ Air │ per │ │
│ Per │Gallon│ │
│Minute│ of │ │
│ │Sewage│ │
─────────┼──────┼──────┼──────────┼────────
│ │ │ │
│ │ │ │
─────────┼──────┼──────┼──────────┼────────
│ │ │ │
│ │ │ │
─────────┼──────┼──────┼──────────┼────────
0│ 0│ 0.0│ Turbid │ 0
1│ 160│ 0.67│ Clear │ 52
2│ 160│ 1.32│ Clear │ 81
3│ 160│ 1.98│ Clear │ 92
4│ 160│ 2.64│ Clear │ 94
5│ 160│ 3.31│ Clear │ 98
2.5│ 90│ 1.07│ │ 92
3│ 90│ 1.28│ │ 96
4│ 90│ 1.71│ │ 98
4│ 80│ 1.82│ │ 97.7
4│ 70│ 1.60│ │ 99.6
4│ 46│ 1.67│ │ 88.3
4│ 105│ 1.75│ │ 92.7
3│ 140│ 1.75│ │ 91.2
2.5│ 168│ 1.74│ │ 96.7
│ │ 1.80│ │ 98.1
│ │ 1.53│ │ 99
│ │ 1.12│ │ 91
─────────┴──────┴──────┴──────────┴────────

─────────┬─────────────────────────────────────────────────────┬──────────
Time of │ Parts per Million │Stability,
Aëration,│ │ Hours
Hours │ │
│ │
│ │
│ │
│ │
─────────┼─────────────────────────────────┬─────────┬─────────┼──────────
│ Nitrogen as │Dissolved│Suspended│
│ │ Oxygen │ Matter │
─────────┼───────┬────────┬────────┬───────┼─────────┼─────────┼──────────
│ Free │Nitrites│Nitrates│Organic│ │ │
│Ammonia│ │ │ │ │ │
─────────┼───────┼────────┼────────┼───────┼─────────┼─────────┼──────────
0│ 22│ 0.08│ 0.08│ │ 0.00│ │ 000
1│ 17│ 0.00│ 0.04│ │ 0.30│ │ 2
2│ 15│ 0.95│ 0.70│ │ 1.90│ │ 33
3│ 11│ 1.75│ 2.80│ │ 4.30│ │ 120
4│ 7│ 2.20│ 5.60│ │ 5.90│ │ 120
5│ 5│ 2.50│ 8.20│ │ 6.70│ │ 120
2.5│ 11│ 0.05│ 2.00│ │ │ │ 69
3│ 9.9│ 0.12│ 2.9│ │ │ │ 95
4│ 1.8│ 0.14│ 5.2│ │ │ │ 120
4│ 1.95│ 0.08│ 8.5│ │ │ │ 120
4│ 5.79│ 0.14│ 9.0│ │ │ │ 120
4│ 7.90│ 0.02│ 2.0│ │ │ │ 61
4│ 4.86│ 0.36│ 4.9│ │ │ │ 120
3│ 9.39│ 0.60│ 3.0│ │ │ │ 120
2.5│ 11.2│ 0.36│ 1.1│ │ │ │ 84
│ │ │ 8.5│ 4│ │ 11│ 120
│ 5.79│ │ 9.0│ 8│ │ 9│ 120
│ 10.1│ │ 2.3│ 14│ │ 42│ 73
─────────┴───────┴────────┴────────┴───────┴─────────┴─────────┴──────────

TABLE 95

COMPARATIVE RESULTS FROM THE AËRATION OF SEWAGE IN THE PRESENCE OF
ACTIVATED SLUDGE WITH THE USE OF DIFFERENT DISTRIBUTING MEDIA

(T. C. Hatton, Eng. Record, Vol. 73, p. 255)
─────────────┬──────────┬────────┬────────┬────────┬─────────┬─────────
Diffusers │Months in │ Pounds │ Air, │Per Cent│Nitrates,│Stability
│ 1915 │ per │ Cubic │Bacteria│Parts per│Effluent
│ │ Square │Feet per│Removed │ Million │in Hours
│ │ Inch │ Gallon │ │ │
─────────────┼──────────┼────────┼────────┼────────┼─────────┼─────────
Filtros plate│June 1 to│ 4.3│ 2.06│ 91│ 3.4│ 78
│ Aug. 15 │ │ │ │ │
Air jet │June 1 to│ 3.5│ 1.94│ 91│ 2.2│ 52
│ Aug. 15 │ │ │ │ │
Filtros plate│Nov. 18 to│ 4.6│ 1.71│ 90│ 0.3│ 113
│ Dec. 7 │ │ │ │ │
Monel metal │Nov. 18 to│ 3.0│ 1.71│ 80│ 0.2│ 63
│ Dec. 7 │ │ │ │ │
─────────────┴──────────┴────────┴────────┴────────┴─────────┴─────────

It is desirable to diffuse the air in small bubbles as by this means the
greatest efficiency seems to be obtained from the amount of air added. A
diameter 1/16 to ⅛ of an inch is approximately the maximum limit for the
size of an effective bubble. Monel metal cloth, porous wood blocks, open
jets, paddles, and other forms of diffusers have been tried, but none
have given the satisfaction of the filtros plate. The relative value of
different types of diffusers is shown in Table 95 taken from
Hatton.[184] The Filtros plates are a proprietary article manufactured
by the General Filtration Company of Rochester, N. Y. They are made of a
quartz sand firmly cemented together and can be obtained with
practically any degree of porosity, size of pore opening or dimension of
plate, but they are made in a standard size 12 inches square by 1½
inches thick. The frictional loss through the plate is not very great
for the amount of air ordinarily used. The plates are classified in
accordance with the volume of air which will pass through them, when
dry, per minute when under a pressure of 2 inches of water. These
classes run from ½ to 12 cubic feet of air per minute. The type usually
specified passes about 2 cubic feet of air per minute. The loss of head
through these plates as tested at Milwaukee showed an initial loss of ¾
of a pound and an additional loss of about ¼ of a pound for every cubic
foot of air per minute per square foot of surface. It is necessary to
screen and wash the air before blowing it through the filtros plate as
ordinary air is so filled with dirt as to clog the pores of the diffuser
quite rapidly.

The area of filtros plates required in the bottom of the tank is usually
expressed in terms of the free surface of the tank or as a ratio
thereto. In the Urbana tests the best ratio was found to be less than 1
: 3 and more than 1 : 9. In Milwaukee[185] the ratio adopted is in the
neighborhood of 1 : 4 or 1 : 5. At Fort Worth the ratio will be about 1
: 7 and at Chicago it will be 1 : 8. The exact ratio should be
determined by experiment and will depend on the construction of the tank
and the character of the raw sewage and the desired effluent. It is
essential that the filtros plates be placed level and at the same
elevation as otherwise the distribution of air will be uneven.

=273. Obtaining Activated Sludge.=—After a plant is once started
activated sludge is generated during the process of treatment and with
careful management a stock of activated sludge can be kept on hand. When
a plant is new, or if shut down for such a length of time that the
sludge loses its activation, it is necessary to activate some new
sludge. This is done by blowing air continuously through sewage either
on the fill and draw method with periodic decantations of the
supernatant liquid, or by the continuous-flow process, but more
preferably by the latter. Where activated sludge is to be obtained from
fresh sewage alone the time required is in the neighborhood of 10 to 14
days, and purification begins at the start. An estimate of the quantity
which will be obtained can not be made with accuracy. After the initial
quantity of sludge has been obtained activated sludge can be maintained
during the process of aëration of the raw sewage, or by means of the
reaëration tanks previously described.

The volume of activated sludge present in the aëration tank should be
about 25 per cent of the volume of the tank. The volume of the sludge is
measured in a somewhat arbitrary manner as the amount by volume which
will settle in 30 minutes in an ordinary test tube. It is found that
this is almost 90 per cent of the solids settling in 4 to 6 hours.

=274. Cost.=—The available information on the cost of the activated
sludge process is meager and unreliable. The factors entering into the
cost are: the price of fuel, the size of the plant, the period of
sedimentation, the amount of air per gallon of sewage, the air pressure,
and the percentage of sludge to be aërated in the mixture. In
Milwaukee[186] the cost of construction is estimated at $44,000 per
million gallons, and $4.75 per million gallons for operation. At
Houston, Texas, the cost is estimated at $24,000 per million gallons,
exclusive of the sludge drying plant, which may cost $40,000 per million
gallons. At Milwaukee, the cost of pressing the sludge is $4.82 per dry
ton and of drying is $3.93 per dry ton. The sludge may be sold at the
normal rate of $2.50 per unit of nitrogen. Based on the normal value the
evident profit will be $3.75 per ton. The net cost of disposing of
Milwaukee sewage is estimated at $9.64 per million gallons of which
$4.89 is chargeable to overhead and $4.75 to repairs, operation and
renewal. In a comparison of the costs of activated sludge and Imhoff
tanks with sprinkling filters,[187] the information given by Eddy has
been summarized in Table 96. In comparing the relative areas required
for different methods of sewage treatment, activated sludge should be
allowed about 15 million gallons per acre per day on the basis of
aëration tanks 15 feet deep. This figure represents approximately the
gross area of the plants at Milwaukee and at Cleveland.

TABLE 96

COMPARATIVE COSTS OF ACTIVATED SLUDGE, AND OF IMHOFF TANKS FOLLOWED BY
SPRINKLING FILTERS

(H. P. Eddy, Eng. Record, Vol. 74, p. 557)
─────────────┬─────────────┬─────────────┬─────────────────────────────
Process │ First Cost │Operation per│Total Annual Cost at 4 Per
│ per Million │ Million │ Cent with Sinking Fund at
│ Gallons, │ Gallons, │ 2.5 Per Cent per
│ Dollars │ Dollars │
─────────────┼─────────────┼─────────────┼──────────────┬──────────────
│ │ │ Million │ Capita,
│ │ │ Gallons, │ Dollars
│ │ │ Dollars │
─────────────┼─────────────┼─────────────┼──────────────┼──────────────
Activated │ 57,100│ 20.00│ 29.85│ 1.09
sludge │ │ │ │
Imhoff tank │ 78,500│ 8.50│ 21.84│ 0.80
and │ │ │ │
sprinkling │ │ │ │
filter │ │ │ │
─────────────┴─────────────┴─────────────┴──────────────┴──────────────

REFERENCES AND BIBLIOGRAPHY ON ACTIVATED SLUDGE

The following abbreviations will be used: A.S. for Activated Sludge,
E.C. for Engineering and Contracting, E.N. for Engineering News, E.R.
for Engineering Record, E.N.R. for Engineering News-Record, p. for page,
and V. for volume.

No.

1. Cooperation Sought in Conducting A.S. Experiments at Baltimore, by
Franks and Hendrick. E.R. V. 71, 1915, pp. 521, 724, and 784. V.
72, 1915, pp. 23, and 640.

2. Sewage Treatment Experiments with Aëration and A.S., by Bartow and
Mohlman. E.N. V. 73, 1915, p. 647, and E.R. V. 71, 1915, p. 421.

3. A.S. Experiments at Milwaukee, Wisconsin, by Hatton. E.N. V. 74,
1915, p. 134.

4. A.S. in America, An Editorial Survey, by Baker. E.N. V. 74, 1915,
p. 164.

5. Choosing Air Compressors for A.S., by Nordell, E.N. V. 74, 1915, p.
904.

6. A Year of A.S. at Milwaukee, by Fuller. E.N. V. 74, 1915, p. 1146.

7. A.S. Experiments at Urbana. E.N. V. 74, 1915, p. 1097.

8. Experiments on the A.S. Process, by Bartow and Mohlman. E.C. V. 44,
1915, p. 433.

9. Milwaukee’s A.S. Plant, the Pioneer Large Scale Installation, by
Hatton. E.R. V. 72, 1915, p. 481 and E.C. V. 44, 1915, p. 322.

10. A.S. Experiments at Milwaukee, by Hatton. Journal American
Waterworks Association and Proceedings Illinois Society of
Engineers, 1916. Also E.R. V. 73, 1916, p. 255. E.C. V. 45, 1916,
p. 104, and E.N. V. 75, 1916, pp. 262 and 306.

11. A.S. Defined. E.N. V. 75, 1916, p. 503, and E.N.R. V. 80, 1918, p.
205.

12. Status of A.S. Sewage Treatment, by Hammond. E.N. V. 75, 1916, p.
798.

13. Trial A.S. Unit at Cleveland, by Pratt. E.N. V. 75, 1916, p. 671.

14. Air Diffuser Experience with A.S. E.N. V. 76, 1916, p. 106.

15. Nitrogen from Sewage Sludge, Plain and Activated, by Copeland,
Journal American Chemical Society, Sept. 28, 1916. E.N. V. 76,
1916, p. 665. E.R. V. 74, 1916, p. 444.

16. Tests Show A.S. Process Adapted to Treatment of Stock Yards Wastes.
E.R. V. 74, 1916, p. 137.

17. Aëration Suggestions for Disposal of Sludge, by Hammond. Journal
American Chemical Society, Sept. 25, 1916. E.R. V. 74, 1916, p.
448.

18. Cost Comparison of Sewage Treatment. Imhoff Tank and Sprinkling
Filters vs. A.S., by Eddy. E.R. V. 74, 1916, p. 557.

19. Large A.S. Plant at Milwaukee. E.N. V. 76, 1916, p. 686.

20. A.S. Novelties at Hermosa Beach, Cal. E.N. V. 76, 1916, p. 890.

21. A.S. Experiments at University of Illinois, by Bartow, Mohlman, and
Schnellbach. E.N. V. 76, 1916, p. 972.

22. A.S. Results at Cleveland Reviewed, by Pratt and Gascoigne. E.N. V.
76, 1916, pp. 1061 and 1124.

23. Sewage Treatment by Aëration and Activation, by Hammond.
Proceedings American Society Municipal Improvements, 1916.

24. A.S., by Bartow and Mohlman, Proceedings Illinois Society of
Engineers, 1916.

25. The Latest Method of Sewage Treatment, by Bartow. Journal American
Waterworks Association, V. 3, March, 1916, p. 327.

26. Winter Experiences with A.S., by Copeland. Journal American Society
of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916, p. 386.

27. A.S. Process Firmly Established, by Hatton. E.R. V. 75, 1917, p.
16.

28. Operate Continuous Flow A.S. Plant, by Bartow, Mohlman, and
Schnellbach. E.R. V. 75, 1917, p. 380.

29. Chicago Stock Yards Sewage and A.S., by Lederer. Journal American
Society of Chemical Engineers, April 21, 1916. E.C. V. 45, 1916,
p. 388.

30. The Patent Situation Concerning A.S. E.C. V. 45, 1916, p. 208.

31. “Sewage Disposal” by Kinnicutt, Winslow, and Pratt, published by
John Wiley & Sons. 2d Edition, Chapter 12.

32. A.S. Tests Made by California Cities. E.N.R. V. 79, 1917, p. 1009.

33. Conclusions on the A.S. Process at Milwaukee. Journal American
Public Health Association, 1917. E.N.R. V. 79, 1917, p. 840.

34. Dewatering A.S. at Urbana, by Bartow. Journal American Institute of
Chemical Engineers, 1917. E.N.R. V. 79, 1917, p. 269.

35. Milwaukee Air Diffusion Studies in A.S. E.N.R. V. 78, 1917, p. 628.

36. A.S. Bibliography (up to May 1, 1917) by J. E. Porter.

37. Air Diffusion in A.S. E.N.R. V. 78, 1917, p. 255.

38. A.S. Plant at Houston, Texas. E.N. V. 77, 1917, p. 236, E.N.R. 83,
1919, p. 1003, and V. 84, 1920, p. 75.

39. A.S. Power Costs, by Requardt. E.N. V. 77, 1917, p. 18.

40. A.S. at San Marcos, Texas, by Elrod. E.N. V. 77, 1917, p. 249.

41. Filtros Plates Made the Best Showing in Air Diffuser Tests. E.N.R.
V. 79, 1917, p. 269.

42. Results of Experiments on A.S., by Ardern and Lockett. Journal
Society for Chemical Research, V. 33, May 30, 1914, p. 523.

43. Final Plans at Milwaukee. E.N.R. V. 84, 1920, p. 990.

44. A.S. Bibliography, published by General Filtration Co., Rochester,
N. Y., 1921.

45. A.S. at Manchester, Eng. by Ardern. Journal Society Chemical
Industry, 1921. E.C. V. 55, 1921, p. 310.

46. The Des Plaines River A.S. Plant, by Pearse. E.N.R. V. 88, 1920, p.
1134.

47. Sewage Treatment by the Dorr System, by Eagles. Proceedings, Boston
Society of Engineers, 1920. Public Works V. 50, 1920, p. 53.

CHAPTER XIX
ACID PRECIPITATION, LIME AND ELECTRICITY, AND DISINFECTION

=275. The Miles Acid Process.=—The Miles Acid Process for the treatment
of sewage was devised and patented by G. W. Miles. It was tried
experimentally at the Calf Pasture sewage pumping station, Boston,
Mass., 1911 to 1914. In 1916 it was tried experimentally at the
Massachusetts Institute of Technology, and it has been tested
subsequently at other places, notably at New Haven, Conn., in 1917 and
1918. It is one of the most recent developments in sewage treatment and
no extensive experience has been had with it. The process consists in
the acidification of sewage with sulphuric or sulphurous acid, as the
result of which the suspended matter and grease are precipitated and
bacteria are removed. The equipment required for the process consists of
devices for the production of sulphur dioxide (SO_{2}), and for feeding
niter cake or other forms of acid; subsiding basins; sludge-handling
apparatus; sludge driers; grease extractors; grease stills; and tankage
driers and grinders.

The first step is the acidification of the sewage. The period of contact
with the acid is about 4 hours. Sulphurous acid seems to give better
results than sulphuric because of the ease in which it can be
manufactured on the spot. It seems also to be more virulent in attacking
bacteria than an equal strength of sulphuric acid. In experimental
plants the acidulation has been accomplished in different ways such as:
by the addition of compressed sulphur dioxide from tanks; by the
addition of sulphur dioxide made from burning sulphur; or by the
roasting of iron pyrite (FeS_{2}). The acidulation precipitates most of
the grease as well as the suspended matter and results in a sludge which
gives some promise of commercial value. In referring to the process R.
S. Weston states:[188]

(1) It disinfects the sewage by reducing the numbers of bacteria
from millions to hundreds per c.c.

(2) If the drying of the sludge and the extraction of the grease
can be accomplished economically, it is possible that a large
part, if not all, of the cost of the acid treatment may be met by
the sale of the grease and fertilizer recovered from the sewage.

(3) The use of so strong a deodorizer and disinfectant as sulphur
dioxide would prevent the usual nuisances of treatment works.

(4) The addition of sulphur dioxide to the sewage also avoids any
fly nuisance, which is a handicap to the operation of Imhoff tanks
and trickling filters.

The amount of acid used varies with the quality of the sewage and the
desired character of the effluent. At Bradford, England,[189] 5,500
pounds of sulphuric acid are used per million gallons, producing about
2,340 pounds of grease or 0.43 pound of grease per pound of sulphuric
acid. At Boston only 0.215 pound of grease were produced per pound of
sulphuric acid. The difference is probably due to the great difference
in the amount of grease in the raw sewage. In the East Street sewer at
New Haven, Conn.,[190] only 700 pounds of acid are used per million
gallons of sewage as the alkalinity is only 50 p.p.m. This amount of
acid secures an acidity of 50 p.p.m. whereas in the Boulevard sewer
1,130 pounds of acid had to be added to produce the same result. The
results obtained by the experiments conducted by the Massachusetts
State Board of Health in 1917 are shown in Table 97. The character of
the sludge from the same tests is shown in Table 98. After
acidification[191] the sewage contains bisulphites and some free
sulphurous acid, with some lime and magnesium soaps which are attacked
by the acid liberating the free fatty acids. Part of the bisulphites
and sulphurous acid are oxidized to bisulphates and sulphuric acid. It
was found as a result of the New Haven[191] experiments that the
presence of sulphur dioxide in the effluent caused an abnormal oxygen
demand from the diluting water and that this difficulty could be
partly overcome by the aëration of the effluent after acidulation and
sedimentation, without prohibitory expense. The effluent and sludge
are both stable for appreciable periods of time and are suitable for
disposal by dilution. The character of the sludge as determined by the
New Haven tests[192] is shown in Table 99.

TABLE 97

AVERAGE ANALYSIS OF SEWAGE ENTERING BOSTON HARBOR, BEFORE AND AFTER
TREATMENT, JULY 17 TO SEPTEMBER 27, 1917

(Eng. News-Record, Vol. 80, p. 319)
─────────┬───────────────────────────────────────────────┬───────────────
Sample │ Parts per Million │ Bacteria,
│ │ Millions
─────────┼─────────────────┬───────────┬────────┬────────┼─────┬─────────
│ Ammonia │ Kjeldahl │Chlorine│ Oxygen │ │
│ │ Nitrogen │ │Consumed│ │
─────────┼─────┬───────────┼─────┬─────┼────────┼────────┼─────┼─────────
│Free │Albuminoid │ │ │ │ │ │
─────────┼─────┼─────┬─────┼─────┼─────┼────────┼────────┼─────┼─────────
│Total│Total│Diss.│Total│Diss.│ │ │ 20° │ 37°
─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────

_Paddock’s Island_

─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬─────────
Raw │ 14.0│ 3.3│ 1.8│ 6.8│ 3.6│ 134│ 23.1│1.86 │ 4.15
sewage │ │ │ │ │ │ │ │ │
Settled │ 12.2│ 1.6│ 1.1│ 3.5│ 2.2│ │ 15.4│ │
Sewage │ │ │ │ │ │ │ │ │
Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 91
and │ │ │ │ │ │ │ │ 94 │
settled│ │ │ │ │ │ │ │ │
sewage │ │ │ │ │ │ │ │ │
─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────

_Deer Island_

─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬─────────
Raw │ 23.3│ 8.2│ 4.8│ 16.8│ 8.9│ 3100│ 87.3│2.63 │ 1.50
sewage │ │ │ │ │ │ │ │ │
Settled │ 21.1│ 5.6│ 3.9│ 10.7│ 7.3│ │ 62.2│ │
sewage │ │ │ │ │ │ │ │ │
Acidified│ 20.9│ 5.2│ 3.9│ 10.0│ 7.5│ │ │units│units 85
and │ │ │ │ │ │ │ │ 147 │
settled│ │ │ │ │ │ │ │ │
sewage │ │ │ │ │ │ │ │ │
─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────

_Calf Pasture_

─────────┬─────┬─────┬─────┬─────┬─────┬────────┬────────┬─────┬─────────
Raw │ 18.0│ 4.5│ 2.0│ 9.7│ 4.1│ 3254│ 41.2│1.89 │ 0.98
sewage │ │ │ │ │ │ │ │ │
Settled │ 19.1│ 2.3│ 1.4│ 4.9│ 3.3│ │ 25.8│ │
sewage │ │ │ │ │ │ │ │ │
Acidified│ 17.8│ 2.4│ 1.6│ 4.9│ 3.3│ │ │units│units 149
and │ │ │ │ │ │ │ │ 277 │
settled│ │ │ │ │ │ │ │ │
sewage │ │ │ │ │ │ │ │ │
─────────┴─────┴─────┴─────┴─────┴─────┴────────┴────────┴─────┴─────────

The success of the Miles Acid Process in comparison with other processes
is dependent on the commercial value of the sludge produced. The New
Haven experiments indicate that 16 to 21 per cent of the grease in the
sludge is unsaponifiable and seriously impairs the value of the process.

TABLE 98

AVERAGE AMOUNT OF SLUDGE AND FATS OBTAINED FROM SEWAGE ENTERING BOSTON
HARBOR AFTER EIGHTEEN HOURS SEDIMENTATION WITH AND WITHOUT
ACIDIFICATION

(Eng. News-Record, Vol. 80, p. 319)
────────────────────┬────────────────┬────────────────┬────────────────
│Paddock’s Island│ Deer Island │ Calf Pasture
────────────────────┼────────────────┼────────────────┼────────────────
│ Sedimentation │ Sedimentation │ Sedimentation
────────────────────┼─────┬──────────┼─────┬──────────┼─────┬──────────
│Plain│Acidulated│Plain│Acidulated│Plain│Acidulated
────────────────────┼─────┼──────────┼─────┼──────────┼─────┼──────────
Pounds of SO_{2} │ │ 818│ │ 1513│ │ 1189
used per million │ │ │ │ │ │
gallons of sewage │ │ │ │ │ │
treated │ │ │ │ │ │
Dry sludge per │ 782│ 959│ 1709│ 1939│ 1208│ 1427
million gallons │ │ │ │ │ │
Per cent Nitrogen in│ 3.10│ 3.38│ 3.57│ 3.45│ 3.18│ 2.83
sludge │ │ │ │ │ │
Per cent fats in │27.30│ 27.30│24.60│ 19.40│24.30│ 26.30
sludge │ │ │ │ │ │
────────────────────┴─────┴──────────┴─────┴──────────┴─────┴──────────

TABLE 99

CHARACTER OF MILES ACID SLUDGE AT NEW HAVEN

(Eng. News-Record, Vol. 81, p. 1034)
─────────────────────────┬───────────────────────────────────┬─────────
│ East Street Sewer │Boulevard
│ │ Sewer
─────────────────────────┼────────┬────────┬────────┬────────┼─────────
Length of run in days │ 25│ 24│ 44│ 70│ 29
Total sewage treated, │ 260│ 239.4│ 407.8│ 602.2│ 145.5
thousand gallons │ │ │ │ │
Gallons wet sludge per │ 3750│ 4025│ 3200│ 2600│ 5375
million gallons sewage │ │ │ │ │
Specific gravity │ 1.067│ 1.048│ 1.054│ 1.061│
Per cent moisture │ 86.6│ 88│ 86.3│ 85.7│ 92.5
Pounds of dry sludge per │ 503│ 483│ 439│ 368│ 403
million gallons sewage │ │ │ │ │
Ether extract, per cent │ 23.7│ 24.0│ 29│ 32.6│ 30.9
dry sludge │ │ │ │ │
Ether extract, pounds per│ 119│ 116│ 127│ 120│ 124
million gallons │ │ │ │ │
Volatile matter, per cent│ 47.2│ 51.2│ 57.3│ 63.8│ 78.5
dry sludge │ │ │ │ │
Nitrogen, per cent dry │ 1.6│ 1.6│ 2.4│ 2.0│ 3.0
sludge │ │ │ │ │
─────────────────────────┴────────┴────────┴────────┴────────┴─────────

The conclusions reached as a result of the New Haven experiments
are:[193]

Our experience with New Haven sewage lends no color to the hope
that a net financial profit can be obtained by the use of the
Miles Acid Process, except with sewage of exceptionally high
grease content and low alkalinity. They do, however, suggest that
for communities where clarification and disinfection are
desirable—where screening would be insufficient and nitrification
unnecessary—the process of acid treatment comes fairly into
competition with the other processes of tank treatment, and that
it is particularly suited to dealing with sewages that contain
industrial wastes, and to use in localities where local nuisances
must be avoided at all costs and where sludge disposal could be
provided for only with difficulty.

The conclusions reached as a result of the Chicago experiments are:[194]

The results on hand indicate that treatment of this sewage with
acid results in a somewhat greater retention of fat. An apparent
reduction in the oxygen demand over that resulting from plain
sedimentation, while remarkable, is probably not real, being
simply due to a retardation of decomposition by the sterilization
of the bacteria present, the organic matter being left in
solution.... However, there appears the added cost of acid
treatment and the cost of recovery of the grease, as well as the
uncertainty of the price to be received for the grease recovered.

The cost of the treatment is estimated by Dorr to be $18 per million
gallons, and the value of the sludge obtained from the Boston sewage as
$24 per million gallons, giving a net margin of profit of $6 per million
gallons. At New Haven, the total return is estimated at $7.09 per
million gallons. Based on the production of sulphur dioxide by burning
sulphur (assumed to cost $36 per long ton) and on drying from 85 per
cent to 10 per cent moisture with coal assumed to cost $7.50 per ton, it
appears that the acid treatment of sewage should be materially cheaper
than either the Imhoff treatment or fine screening under the local
conditions. A comparison of the cost of the treatment of the East Street
and the Boulevard sewage at New Haven and the Calf Pasture sewage in
Boston is given in Table 100. The cost of construction was estimated by
Dorr and Weston in 1919 as greater than $15,000 per million gallons of
sewage per day capacity.

TABLE 100

ESTIMATED COST OF SEWAGE TREATMENT AT NEW HAVEN AND BOSTON BY THREE
DIFFERENT PROCESSES

Cost in Dollars per Million Gallons Treated

(Engineering and Contracting, Vol. 51, p. 510)
────────────────┬────────────────────┬────────────────────┬────────────
│ Miles Acid Process │ Imhoff Tank and │Fine Screens
│ │ Chlorination │ and
│ │ │Chlorination
────────────────┼──────┬─────┬───────┼──────┬─────┬───────┼──────┬─────
│ East │Boul-│ Calf │ East │Boul-│ Calf │ East │Boul-
│Street│evard│Pasture│Street│evard│Pasture│Street│evard
────────────────┼──────┼─────┼───────┼──────┼─────┼───────┼──────┼─────
Tanks and │ 2.47│ 2.47│ 2.47│ 5.28│ 4.44│ │ 4.60│ 4.60
Buildings Int.│ │ │ │ │ │ │ │
and Dep. │ │ │ │ │ │ │ │
Acid treatment │ 6.93│10.74│ 18.65│ │ │ │ │
Drying sludge │ 2.09│ 2.04│ 10.34│ │ │ │ │
Degreasing │ 1.78│ 1.91│ 9.12│ │ │ │ │
sludge │ │ │ │ │ │ │ │
Superintendence │ 1.06│ 2.65│ 1.06│ 0.46│ 1.15│ │ 0.47│ 1.15
Labor on tanks │ 1.00│ 1.00│ 1.00│ 1.20│ 1.50│ │ 1.42│ 2.05
and screens │ │ │ │ │ │ │ │
Disposal of │ │ │ │ 1.00│ 1.00│ │ 0.50│ 0.50
sludge or │ │ │ │ │ │ │ │
screenings │ │ │ │ │ │ │ │
Chlorination │ │ │ │ 4.05│ 4.05│ │ 4.05│ 4.05
Gross cost │ 15.50│20.98│ 42.75│ 11.99│12.14│ │ 11.03│12.35
Revenue │ 6.57│10.66│ 47.59│ │ │ │ │
Net cost │ 8.93│10.32│ 4.84│ 11.99│12.14│ │ 11.03│12.35
────────────────┴──────┴─────┴───────┴──────┴─────┴───────┴──────┴─────

ELECTROLYTIC TREATMENT

=276. The Process.=—This process has been generally unsuccessful in the
treatment of sewage and has grown into disrepute. In the words of the
editor of the _Engineering News-Record_:[195]

Thirty years of experiments and demonstrations with only a few
small working plants built and most of them abandoned—such in
epitome is the record of the electrolytic process of sewage
treatment.

It is probably true that the process has never received a thorough and
exhaustive test on a large scale, but the small-scale tests have not
been promising of good results. Among the most extensive tests have been
those at Elmhurst, Long Island,[196] Decatur, Ill.,[197] and Easton,
Pa.[198]

Whatever degree of popularity the method has possessed has been due
possibly to the mystery and romance of “electricity” and to the
personality of its promoters. The process should, nevertheless, be
understood by the engineer in order that it may be explained
satisfactorily to the layman interested in its adoption.

In this process, sometimes called the direct-oxidation process, all grit
is removed and the sewage is passed through fine screens before entering
the electrolytic tank. In the electrolytic tank the sewage passes in
thin sheets between electrodes and an electric current is discharged
through it. A recent development has been the addition of lime to the
sewage at some point in its passage through the electrolytic tank. From
the electrolytic tank the sewage flows to a sedimentation tank, where
sludge is accumulated, and from which the liquid effluent is finally
disposed of.

It is claimed that the action of the electricity electrolyzes the
sewage, releasing chlorine, which acts as a powerful disinfectant. The
constituents of the sewage are oxidized so that the dissolved oxygen,
nitrates, and relative stability are increased and the sludge is
rendered non-putrescible. It is said that the addition of lime increases
the efficiency of sedimentation and enhances the effect of the electric
current. The results obtained by tests at Easton, Pa., are shown in
Table 101. It will be observed from this table that the combination of
lime and electricity does not have a more beneficial effect than either
one of them alone. The amount of sludge produced by the combination is
about the same as by chemical precipitation alone, but the character of
the sludge produced with electricity is less putrescible. The cost of
the treatment as estimated at Elmhurst is shown in Table 102.

As a result of the tests at Decatur, comparing lime alone with lime and
electricity together, Dr. Ed. Bartow stated:

The purification by treatment with lime alone was greater than
that obtained in several of the individual samples treated with
lime and electricity.

TABLE 101

COMPARATIVE RESULTS OBTAINED FROM THE TREATMENT OF SEWAGE BY LIME
ALONE, ELECTRICITY ALONE, AND LIME AND ELECTRICITY COMBINED

(Creighton and Franklin, Journal of the Franklin Institute, August,
1919)
───────────────────────┬───────────────┬───────────────┬───────────────
│ Lime and │ Lime Alone │ Electricity
│ Electricity │ │ Alone
───────────────────────┼───────┬───────┼───────┬───────┼───────┬───────
│Change,│Change,│Change,│Change,│Change,│Change,
│ Parts │ Per │ Parts │ Per │ Parts │ Per
│ per │ Cent │ per │ Cent │ per │ Cent
│Million│ │Million│ │Million│
───────────────────────┼───────┼───────┼───────┼───────┼───────┼───────
Chlorine │ +1.2│ +1.9│ +12.3│ +18.2│ +1.6│ +2.2
Nitrites │ +0.014│ +58.3│ -.005│ –10.0│ –0.01│ –20.0
Nitrates │ +0.13│ +23.6│ +.005│ +0.8│ –0.15│ –20.0
Ammonia │ –3.3│ –18.3│ +0.2│ +1.3│ +0.9│ +6.6
Albuminoid ammonia │ –3.6│ –12.1│ –0.4│ –1.7│ –0.5│ –2.3
Oxygen demand │ –13.0│ –20.5│ –7.7│ –8.9│ –6.5│ –10.0
Dissolved oxygen │ +1.78│ +40.9│ –0.93│ –19.1│ +1.61│ +40.1
Total bacteria at 37° │ –343│ –92.7│ –373│ –82.4│ –165│ –37.8
(Thousands) │ │ │ │ │ │
Total bacteria at 20° │ –688│ –92.7│ –1074│ –90.1│ –635│ –70.0
(Thousands) │ │ │ │ │ │
B. Coli (Thousands) │ –77.9│ –99.85│ –96.3│ –92.3│ –45│ –81.8
Oxygen absorbed in 5 │ –3.40│ –81.6│ –1.03│ –21.│ +1.24│ +31
days │ │ │ │ │ │
───────────────────────┴───────┴───────┴───────┴───────┴───────┴───────

DISINFECTION

=277. Disinfection of Sewage.=—Sewage is disinfected in order to protect
public water supplies, shell fish, and bathing beaches; to prevent the
spread of disease; to keep down odors, and to delay putrefaction.
Disinfection is the treatment of sewage by which the number of bacteria
is greatly reduced. Sterilization is the destruction of all bacterial
life, including spores. Ordinarily even the most destructive agents do
not accomplish complete sterilization. Chlorine and its compounds are
practically the only substances used for the disinfection of sewage. The
lime used in chemical precipitation, the acid used in the Miles Acid
Process, the aëration in the activated sludge process, all serve to
disinfect sewage, but are not used primarily for that purpose. Copper
sulphate has been used as an algaecide but never on a large scale as a
bactericide.[199] Heat has been suggested, but its high cost has
prevented its practical application to the disinfection of sewage.

TABLE 102

COST OF ELECTROLYTIC TREATMENT, ELMHURST, LONG ISLAND, AND EASTON,
PENNSYLVANIA

────────────────────────────────────────┬───────────────────┬─────────
│ │ Three
Item │One Million Gallon │ Million
│ │ Gallon
────────────────────────────────────────┼─────────┬─────────┼─────────
│ unit at │ unit at │ unit at
│ Easton, │Elmhurst,│Elmhurst,
│ Dollars │ Dollars │ Dollars
────────────────────────────────────────┼─────────┼─────────┼─────────
Hydrated lime: │ │ │
Elmhurst, 1300 pounds at $7.90 ton. │ 12.56│ 5.14│ 15.42
Easton, 3720 pounds at $6.75 ton. │ │ │
Electric power electrolysis: │ │ │
Elmhurst, 85 kw-h. at 4 cents │ 4.19│ 3.40│ 9.60
Easton, 6.25 kw-h. at 8.05 cents │ │ │
Electric power, light and agitation: │ │ │
Elmhurst, 60 kw-h. at 4 cents │ 0.50│ 2.40│ 7.20
Easton, 6.25 kw-h at 8.05 cents │ │ │
Heating │ 1.25│ │
Labor and supervision │ 15.00│ 12.50│ 15.00
Maintenance, repairs and supplies │ 1.50│ 1.00│ 3.00
Sludge pressing and removal │ │ 5.11│ 15.33
────────────────────────────────────────┼─────────┼─────────┼─────────
Total │ 35.00│ 29.55│ 65.55
Cost per million gallons │ 35.00│ 29.55│ 21.85
────────────────────────────────────────┴─────────┴─────────┴─────────

The action which takes place on the addition to sewage of chlorine or
its compounds is not well understood. The idea that the bacteria are
burned up with “nascent” or freshly born oxygen, has been exploded.[200]
Likewise the idea that the toxic properties of chlorine have no effect
has not been borne out by experiments. It has been demonstrated,
particularly by tests on strong tannery wastes, that the action of
chlorine gas is more effective than the application of the same amount
of chlorine in the form of hypochlorite. All that we are certain of at
present is that the greater the amount of chlorine added under the same
conditions, the greater the bactericidal effect.

Chlorine is applied either in the form of a bleaching powder or a gas.
In ordinary commercial bleach (calcium hypochlorite) the available
chlorine is about 35 to 40 per cent by weight. In order to add one part
per million of available chlorine to sewage it is necessary to add about
25 pounds of bleaching powder or 8½ pounds of liquid chlorine per
million gallons of sewage. This can be computed as follows:

The molecular weight of calcium hypochlorite is 127.0. This reacts
to produce two atoms of available chlorine with a molecular weight
of 70.9. If the bleaching powder were pure the available chlorine
would therefore represent 70.9 ÷ 127, or 56 per cent of its
weight. Then to obtain one pound of chlorine it would be necessary
to have 1.79 pounds of pure bleaching powder. Since 1,000,000
gallons of water weigh approximately 8,300,000 pounds, in order to
apply one part per million of chlorine to 1,000,000 gallons of
sewage it is necessary to apply 1.79 × 8.3 or 14.9 pounds of pure
bleaching powder. Commercial bleaching powder is only about 60 per
cent calcium hypochlorite. It is therefore necessary to add 14.9 ÷
0.60 or about 25 pounds of commercial bleach.

Since liquid chlorine is very nearly pure, approximately 8½ pounds
of it applied to 1,000,000 gallons of sewage are equivalent to a
dose of one part per million.

Commercial bleaching powder is a dry white powder which absorbs moisture
slowly, and which loses its strength rapidly when exposed to the air. It
is packed in air-tight sheet iron containers, which should be opened
under water, or emptied into water immediately on being opened. The
strength of the solution should be from ½ to 1 per cent. The rate of the
application of the solution to the sewage may be controlled by automatic
feed devices, or by hand-controlled devices.

Commercial liquid chlorine is sold in heavy cast steel containers, which
hold 100 to 140 pounds of liquid chlorine under a pressure of 54 pounds
per square inch at zero degrees C. or 121 pounds per square inch at 20
degrees.

The amount of chlorine used is dependent on the character of the sewage
to be treated, the stage of decomposition of the organic matter, the
desired degree of disinfection, the period of contact, and the
temperature. The amount of chlorine is expressed in parts per million of
available chlorine, regardless of the form in which the chlorine is
applied. In general about 15 to 20 parts per million of available
chlorine with 30 minutes’ contact at a temperature of about 15° C. will
effect an apparent removal of 99 per cent of the bacteria from the raw
sewage. The effect is only apparent because many of the bacteria encased
in the solid matter of the sewage escape the effect of the chlorine, or
detection in the bacterial analysis. Stronger and older sewages, higher
temperatures, and shorter periods of contact will demand more chlorine
to produce the same results. A septic effluent will require more
chlorine than a raw sewage because of the greater oxygen demand by the
septic sewage. The results of experiments on disinfection made at
different testing stations have shown such wide variations in the amount
of chlorine necessary, as to demonstrate the necessity for independent
studies of any particular sewage which is to be chlorinated. For
instance, at Milwaukee approximately 13 p.p.m. of available chlorine
applied to an Imhoff tank effluent effected a 99 per cent removal of
bacteria, whereas the same result was obtained at Lawrence, Mass., on
crude sewage with only 6.6 p.p.m. and at Marion, Ohio, only 9 per cent
removal of bacteria was obtained by the addition of 4,815 p.p.m. to
crude sewage. The Ohio and Massachusetts reports show irrational
variations among themselves. For instance, 6.2 p.p.m. applied to a
septic effluent effected 88 per cent removal whereas in another case 7.6
p.p.m. effected only 36 per cent removal. At Lawrence in one case it
took 8.6 p.p.m. to remove 99 per cent from a sand filter effluent, but
only 6.3 p.p.m. to effect the same result in the effluent from a septic
tank. The most consistent results are those found at Milwaukee which
show a steadily increasing percentage removal with increasing amounts of
chlorine.

Some time after sewage has received its dose of chlorine the number of
bacteria may be greater than in the raw sewage. Such bacteria are called
aftergrowths. Certain forms of bacteria, particularly the pathogenic or
body temperature types, are most susceptible to disinfecting agents.
These are killed off and leave the sewage in a condition more favorable
to the growth of more resistant forms of bacteria. As the latter are
non-pathogenic and are generally aërobic their presence is usually more
beneficial than detrimental, as they hasten the action of
self-purification.

REFERENCES

The following abbreviations will be used: E.C. for Engineering and
Contracting, E.N. for Engineering News, E.R. for Engineering Record,
E.N.R. for Engineering News-Record, M.J. for Municipal Journal, p. for
page, and V. for volume.

No.

1. Grease and Fertilizer Base for Boston Sewage, by Weston, E.N. V.
75, 1916, p. 913 and Journal American Public Health Association,
April, 1916.

2. Getting Grease and Fertilizer from City Sewage, by Allen. E.N. V.
75, 1916, p. 1005.

3. New Haven Tests Five Processes of Sewage Treatment. E.N.R. V. 79,
1917, p. 829.

4. Recovery of Grease and Fertilizer from Sewage Comes to the Front.
E.N.R. V. 80, 1916, p. 319.

5. Miles Acid Process may Require Aëration of Effluent, by Mohlman.
E.N.R. V. 81, 1918, p. 235.

6. Promising Results with Miles Acid Process in New Haven Tests.
E.N.R. V. 81, 1918, p. 1034.

7. Baltimore Experiments on Grease from Sewage. E.N. V. 75, 1916, p.
1155.

8. Report on Industrial Wastes from the Stock Yards and Packingtown in
Chicago to the Trustees of the Sanitary District of Chicago,
1914, pp. 187–195.

9. The Separation of Grease from Sewage, by Daniels and Rosenfeld.
Cornell Civil Engineer. V. 24, p. 13.

10. The Separation of Grease from Sewage Sludge with Special Reference
to Plants and Methods Employed at Bradford and Oldham, England,
by Allen. E.C. V. 40, 1913, p. 611.

11. Acid Treatment of Sewage, by Dorr and Weston. Journal Boston
Society of Civil Engineers, April, 1919. E.C. V. 51, 1919, p.
510. M.J. V. 46, 1919, p. 365.

12. The Miles Acid Process for Sewage Disposal. Metallurgical and
Chemical Engineering, V. 18, p. 591.

13. Miles Acid Treatment of Sewage, by Winslow and Mohlman. Journal
American Society Municipal Improvements, Oct., 1918. M.J. V. 45,
1918, pp. 280, 297, and 321.

14. New Electrolytic Sewage Treatment. M.J. V. 37, 1914, p. 556.

15. Electrolytic Sewage Treatment. M.J. V. 47, 1919, p. 131.

16. Electrolytic Treatment of Sewage at Durant, Oklahoma, by Benham.
E.N. V. 76, 1916, p. 547. Municipal Engineering, V. 49, 1916, p.
141.

17. Electrolytic Treatment of Sewage at Elmhurst, Long Island, by
Travis. Report to the President of the Borough of Queens, Aug.
31, 1914. E.R. V. 70, 1914, pp. 292, 315, and 429. M.J. V. 39, p.
551. Municipal Engineering, V. 47, p. 281.

18. Tests of the Electrolysis of Sewage at Toronto, by Nevitt. E.N. V.
71, 1914, p. 1076.

19. Electrolytic Treatment of Sewage Little Better than Lime Alone, by
Bartow. E.R. V. 74, 1916, p. 596.

20. Electrolytic Sewage Treatment Not Yet an Established Process.
E.N.R. V. 83, 1919, p. 541.

21. Tests of Electrolytic Sewage Treatment Process at Easton, Pa.
Journal of the Franklin Institute, Aug., 1919. E.N.R. V. 83,
1919, p. 569.

22. The Disinfection of Sewage. U. S. Geological Survey, Water Supply
Paper, No. 229.

23. Sewage Disinfection in Actual Practice, by Orchard. E.R. V. 70,
1914, p. 164.

24. Water and Sewage Purification in Ohio. Report of the Ohio State
Board of Health, 1908, pp. 738–762.

25. Water Purification, by Ellms. Published in 1917 by McGraw-Hill Book
Co.

26. Electrolytic Sewage Treatment, A Half Century of Invention and
Promotion. E.N.R. V. 86, 1921, p. 25.

CHAPTER XX
SLUDGE

=278. Methods of Disposal.=—Sludge is the deposited suspended matter
which accumulates as the result of the sedimentation of sewage. The
methods for the disposal of sludge as discussed herein will include the
disposal of scum. Scum is a floating mass of sewage solids buoyed up in
part by entrained gas or grease, forming a greasy mat which remains on
the surface of the sewage.[201] The sludges formed by different methods
of sewage treatment are described in the chapter devoted to the
particular method. The disposal of sludge is a problem common to all
methods of sewage treatment involving the use of sedimentation tanks.

Sludge is disposed of by: dilution, burial, lagooning, burning, filling
land, and as a fertilizer or fertilizer base. Certain methods of
disposal, such as burning or as a fertilizer, demand that the sludge be
dried preparatory to disposal. Sludge is dried on drying beds, in a
centrifuge, in a press, in a hot-air dryer, or by acid precipitation.

=279. Lagooning.=—This is a method of sludge disposal in which fresh
sludge is run on to previously prepared beds to a depth of 12 to 18
inches or more, and allowed to stand without further attention. The
preparation of the lagoons requires leveling the ground, building of
embankments, and, if the ground is not porous, the placing of
underdrains laid in sand or gravel. At Reading, Pa.,[202] approximately
one acre was required for 1,700 cubic yards of wet sludge. The results
of lagooning at Philadelphia are given in Table 103.[202]

TABLE 103

RESULTS OF DRYING SLUDGE IN LAGOONS AT PHILADELPHIA

(“Sewage Sludge” by Allen)
─────────────────────┬─────────┬─────────┬─────────┬─────────┬─────────
Treatment │ Days │ Depth, │Per Cent,│Rainfall,│ Cubic
│ │ Inches │Moisture │ Inches │Yards per
│ │ │ │ │ Acre
─────────────────────┼─────────┼─────────┼─────────┼─────────┼─────────
Screened │ 0│ 12.20│ 82.8│ 0│ 1600
Screened │ 26│ 7.67│ 57.0│ 0│ 1000
Screened │ 49│ 3.50│ 51.6│ 0.43│ 470
Screened │ 0│ 13.50│ 90.1│ 0│ 1800
Screened │ 62│ 7.00│ 61.0│ 3.14│ 950
Crude │ 0│ 12.00│ 88.7│ 0│ 1600
Crude │ 59│ 4.70│ 62.8│ 2.59│ 640
─────────────────────┴─────────┴─────────┴─────────┴─────────┴─────────

During the period of standing in the lagoon the moisture drains out and
evaporates and the organic matter putrefies, giving off gases and foul
odors. In the course of three to six months, biological action ceases
and the sludge has become humified and reduced to about 75 per cent
moisture. In the utilization of this method of disposal the lagoons must
be removed from settled districts and should occupy land of little value
for other purposes. The odors created at the lagoons may be intense and
offensive. The land so used is rendered unfit for other purposes for
many years.

The digestion of sludge in special tanks is a form of lagooning in which
an attempt is made to maintain septic action as a result of which a
portion of the sludge is gasified or liquefied, leaving less to be cared
for by some of the other methods of treatment or disposal. The results
obtained by digestion tanks have not been entirely satisfactory. A
partial drying and consolidation of the sludge may be effected, however,
by the process of decantation, in which the supernatant liquid is run
off, followed by further sedimentation, rendering the final product more
compact.

=280. Dilution.=—In the disposal of sludge by dilution, as in the
disposal of sewage by dilution, there must be sufficient oxygen
available in the diluting water to prevent putrefaction, and a swift
current to prevent sedimentation. Such conditions exist in localities
along the sea coast, and in communities situated near rivers, when the
rivers are in flood. In some seacoast towns, for example at London and
Glasgow, the sludge is taken out to sea in boats, and dumped. Since it
is not necessary to discharge sludge continuously, it can be stored to
advantage in the digestion chamber of a tank, until the conditions in
the body of diluting water are suitable to receive it.

The amount of diluting water to receive sewage sludge has not been
sufficiently well determined to draw reliable general conclusions. A
dilution of 1,500 to 2,000 volumes may be considered sufficiently safe
to avoid a nuisance provided there is a sufficient velocity to prevent
sedimentation. Johnson’s Report on Sewage Purification at Columbus, Ohio
(1905), states that a dilution of 1 to 800 is sufficient to avoid a
nuisance. The character of the sludge has a marked effect on the proper
ratio of dilution, the sludge from septic and sedimentation tanks
requiring a greater dilution than that from Imhoff tanks.

=281. Burial.=—Sludge can be disposed of by burial in trenches about 24
inches deep with at least 12 inches of earth cover, without causing a
nuisance. The ground used for this purpose should be well drained. This
method of disposal is generally used as a makeshift and has not been
practiced extensively because of the large amount of land required.
Insufficient information is available to generalize on the amount of
land required or the time before the land can be used for further sludge
burial, or for other purposes. Indications are that the sludge may
remain moist and malodorous for years and that the land may be rendered
permanently unfit for further sludge burial. Under some conditions the
land may be used again for the same or other purposes. For example,
Kinnicutt, Winslow and Pratt[203] state that 500 tons of wet sludge can
be applied per acre and:

The same land, it is claimed, can be used again after a period of
a year and a half to two years, if in two months or so after
covering the sludge with earth, the ground is broken up, planted,
and, when the crop is removed, again plowed and allowed to remain
fallow for about a year.

=282. Drying.=—Before sludge can be disposed of to fill land, by
burning, or for use as a fertilizer filler it must be dried to a
suitable degree of moisture. The removal of moisture from the sludge
decreases its volume and changes its characteristics so that sludge
containing 75 per cent moisture has lost all the characteristics of a
liquid. It can be moved with a shovel or fork, and can be transported in
non-watertight containers. A reduction in moisture from 95 to 90 per
cent will cut the volume in half.

The change in volume on the removal of moisture can be represented as:

_V__{1} = _V_(100 − _P_)⁄(100 − _P__{1}),

in which _P_ = the original percentage of moisture;

_P__{1} = the final percentage of moisture;

_V_ = the original volume;

_V__{1} = the final volume.

The drying of sludge on coarse sand filter beds is more particularly
suited to sludge from Imhoff tanks. This sludge does not decompose
during drying, and is sufficiently light and porous in texture to permit
of thorough draining. The sludge from plain sedimentation or chemical
precipitation tanks is high in moisture, putrescible, and when placed on
a filter bed it settles into a heavy, compact, impervious mass which
dries slowly. In order to avoid this condition the sludge is run on to
the beds as quickly as possible, to a depth of not more than 6 to 10
inches. Lime is sometimes added to the sludge at this time as it aids
drying by assisting in the maintenance of the porosity of the sludge,
and it is advantageous in keeping down odors and insects.

Sludge filter beds are made up of 12 to 24 inches of coarse sand,
well-screened cinders, or other gritty material, underlaid by 6 inches
of coarse gravel and 6 or 8–inch open-joint tile underdrains, laid 4 to
10 feet apart on centers, dependent on the porosity of the subsoil. The
side walls of the filters are made of planks or of low earth
embankments. The sludge filters at Hamilton, Ontario, are shown in Fig.
179.

[Illustration:

FIG. 179.—Sludge drying Beds at Hamilton, Ontario.

Eng. News, Vol. 73, p. 426.
]

The size of the bed is dependent mainly upon the characteristics of the
sludge. For Imhoff tank sludge which comes from the tank with about 85
per cent moisture, the practice is to allow about 350[204] square feet
of filter surface per 1,000 population contributing sludge. For other
types of sludge the area varies from 900 to 9,000 square foot per 1,000
population contributing sludge, and only experiments with the sludge in
hand can determine the proper allowance. Imhoff recommends 1,080 square
feet per 1,000 population for septic tank sludge, and 6,480 square feet
for sludge from plain sedimentation tanks.[205] Kinnicutt, Winslow, and
Pratt in their book on Sewage Disposal state:

With an average depth of 10 inches per dose of sludge of 87 per
cent water content, one square foot of covered (glass) bed should
dry to a spadable condition one cubic yard of sludge per year.

The sludge is run on the bed in small quantities at periods from two
weeks to a month apart. In favorable weather Imhoff sludge will dry in
two weeks or less to approximately 50 to 60 per cent moisture. It is
then suitable for use as a filling material on waste land, for burning,
or for further drying by heat. Glass roofs, similar to those used on
green-houses, have been used to speed the drying process by preventing
the moistening of partly dried sludge during rainy weather. In some
instances sludge has dried to 10 per cent moisture on such beds. Imhoff
sludge can be removed from the drying beds with a manure or hay fork. It
has an odor similar to well-fertilized garden soil. It is stable, dark
brownish-gray in color, is of light coarse material, and is granular in
texture.

Sludge presses are suitable for removing moisture from the bulky wet
sludge obtained from plain sedimentation, chemical precipitation, and
the activated sludge process. The details of a typical sludge press are
shown in Fig. 180. The press shown is made up of a number of corrugated
metal plates about 30 inches in diameter with a hole in the center about
8 inches in diameter. The corrugations run vertically except for a
distance about 3 inches wide around the outer rim, which is smooth. To
this smooth portion is fastened, on each side of the plate, an annular
ring about an inch thick and 2 to 3 inches wide, of the same outside
diameter as the plate. A circular piece of burlap, canvas, or other
heavy cloth is fastened to this ring, covering the plate completely. A
hole is cut in the center of the cloth slightly smaller in diameter than
the center hole in the plate, and the edges of the cloth on opposite
sides of the plate are sewed together. The plates are then pressed
tightly together by means of the screw motion at the left end of the
machine, thus making a water-tight joint at the outer rim. Sludge is
then forced under pressure into the space between the plates, passing
through the machine by means of the central hole. The pressure on the
sludge may be from 50 to 100 pounds per square inch. This pressure
forces the water out of the sludge through the porous cloth from which
it escapes to the bottom of the press along the corrugations of the
separating plate. After a period of 10 to 30 minutes the pressure is
released, the cells are opened, and the moist sludge cake is removed.
The liquid pressed from the sludge is highly putrescible and should be
returned to the influent of the treatment plant. The pressing of wet
greasy sludges is facilitated by the addition of from 8 to 10 pounds of
lime per cubic yard of sludge. The cake thus formed is more cohesive and
easy to handle. The output of the press depends so much on the character
of the sludge that a definite guarantee of capacity is seldom given by
the manufacturer.

[Illustration:

FIG. 180.—Filter Press.
]

The simplest form of centrifugal sludge dryer is a machine which
consists of a perforated metal bowl lined with porous cloth in which the
sludge is placed. Surrounding this bowl is a second water-tight metal
bowl so arranged as to intercept the water thrown from the sides of the
inner bowl as it revolves. The peripheral velocity of the inner bowl is
about 6,000 feet per minute, which makes the effective weight of each
particle about 250 times its normal weight when at rest. Very few data
are available on the operation of such machines, and their use has not
been extensive because of the difficulty of starting and stopping the
machine at each filling, and the difficulty of removing the partially
dried sludge from the inner basket. The Besco-ter-Meer centrifuge,
manufactured by the Barth Engineering and Sanitation Co., can be
operated continuously and the difficulties of removing the dried sludge
from the machine have been overcome. According to the manufacturers the
centrifuge has been operated very successfully in Germany on plain
septic tank sludge. A removal of 70 per cent of suspended solids in the
raw sludge and a production of 3,600 pounds of sludge per hour,
containing 60 to 70 per cent of moisture, can be obtained at less than
900 r.p.m. with a consumption of 15 horse-power. Extensive tests of the
machine were made at Milwaukee from October, 1920, to September, 1921,
on activated sludge, but results of these tests are not as yet
available. Indications are that the centrifuge has acted as a
classifier. The coarser particles of sludge have been removed but the
finer particles have been continuously returned with the liquid to the
sedimentation tank, ultimately filling this tank with fine particles of
sludge. An illustration of the unit tested at Milwaukee is shown on this
page.

[Illustration:

Besco-ter-Meer Sludge Drying Centrifuge at Milwaukee, Wisconsin
Courtesy, Barth Engineering and Sanitation Co.
]

Experiments on the drying of sludge by acid flotation have not
progressed sufficiently to allow the installation of a working unit. The
method, which has been applied principally to activated sludge, consists
in adding a small amount of sulphuric acid to the sludge as it leaves
the storage tank. The sludge is coagulated by this action, the
coagulated material rising to the surface as a scum containing about 86
per cent moisture. The consistency is such that it can be removed with a
shovel. The liquid can be withdrawn continuously from below the scum.

[Illustration:

FIG. 181.—Direct-Indirect Sludge Dryer.

Courtesy, the Buckeye Dryer Co.
]

The moisture content of sludge to be used in the manufacture of
fertilizer must be reduced to 10 per cent or less. None of the methods
of drying described so far can be relied upon for such a product and it
becomes necessary to use direct or indirect heat dryers. There are
various types of dryers on the market. The details of a Buckeye dryer
are shown in Fig. 181. In the operation of this machine moist sludge is
fed in at the left end at the point marked “feed.” The hot gases pass
from the fire box up and around the cylinder which revolves at about
eight r.p.m. The gases are drawn into the inner cylinder through the
openings marked A which revolve with the two cylinders. The gases escape
from the inner cylinder through the openings to the right and flow
towards the left in the outer cylinder. They come in contact with the
sludge at this point. The gases then pass off through the fan at the
left. The sludge is lifted by the small longitudinal baffles fastened to
the outer cylinder, as the drying cylinders revolve. The right end of
the cylinder is placed lower than the left so that the drying sludge is
lifted and dropped through the cylinder at the same time that it moves
slowly toward the right hand end of the cylinder. These dryers require
about one pound of fuel for 10 pounds of water evaporated. The odors
from the dryer can be suppressed by passing the gases through a dust
chamber and washer.

A summary of the results from methods of sludge drying at Milwaukee[206]
follows:

Excess sludge produced, 12,100 gallons, having 97.5 per cent
moisture, per million gallons of sewage treated.

Sludge cake produced (by presses), 10,083 pounds having 80.3 per
cent moisture, per million gallons of sewage treated.

Dried sludge (from heat driers) produced, 2,521 pounds having 10
per cent moisture, per million gallons of sewage treated.

Press will produce 3 pounds of cake per square foot of filter
cloth in four and a half hours, or five operations per twenty-four
hours.

Dryers will reduce 6,700 pounds of sludge cake at 80 per cent
moisture to 10 per cent moisture, and will evaporate 8 pounds of
water per pound of combustible.

Thickening devices known as Dorr thickeners, patented and manufactured
by the Dorr Co. and originally intended for metallurgical purposes, have
been adapted to the thickening of sewage sludge. These thickeners are
circular sedimentation tanks, from 8 to 12 feet deep, more or less, and
are made in any diameter up to 200 feet or more. An arm, pivoted in the
center and extending to the circumference, is provided at the bottom
with a number of baffles or squeegees set at an angle with the arm. The
arm revolves at from one to fifteen revolutions per hour, and the
squeegees, in contact with the bottom of the tank, scrape the deposited
sludge towards a central sump, from which it is removed by a pump or by
gravity, without interrupting the operation of the thickener. The sludge
so thickened may be reduced to 95 or 96 per cent moisture. These devices
are ordinarily used only in the activated sludge process in which they
have been a pronounced success.

CHAPTER XXI
AUTOMATIC DOSING DEVICES

=283. Types.=—Automatic dosing devices are used to apply sewage to
contact beds, trickling filters, and intermittent sand filters. These
devices can be separated into two classes; those with moving parts and
those without moving parts. The latter are better known as air-locked
dosing devices. Simple devices without moving parts are less liable to
disorders and are nearer “fool-proof” than any device depending on
moving parts for its operation.

No one type of moving part device has been used extensively in different
sewage treatment plants. Designing engineers have exercised their
ingenuity at different plants, resulting in the production of different
types.[207] Among the best known forms is the apparatus designed by J.
W. Alvord for the intermittent sand filters at Lake Forest,
Illinois.[208] In its operation....

A float in the dosing chamber lifts an iron ball in one of a
series of wooden columns, and at a certain height the ball rolls
through a trough from one column to the next, in its passage
striking a catch, which opens an air valve attached to one of ten
bell-siphons in the dosing chamber. Each of the siphons discharges
on one of the ten sand beds, which are thus dosed in rotation.

Since air-locked dosing devices are in more general use their operation
will be explained in greater detail.

=284. Operation.=—The simplest form of these devices is the automatic
siphon used for flush-tanks, the operation of which is described in Art.
61.

In the operation of sand filters, sprinkling filters, or other forms of
treatment where there are two or more units to be dosed it is desirable
that the dosing of the beds be done alternately. A simple arrangement
for two siphons operating alternately is shown in Fig. 182. They operate
as follows: with the dosing tank empty at the start water will stand at
_bb′_ in siphon No. 2 and at _aa′_ in siphon No. 1. As the water enters
through the inlet on the left the tank fills. When the water rises
sufficiently, air is trapped in the bells, and as the water continues to
rise in the tank, surfaces _a_ and _b_ are depressed an equal amount.
When _b_ has been depressed to _d_, _a_ has been depressed to _c_. Air
is released from siphon No. 2 through the short leg, and siphon No. 2
goes into operation. Surface _c_ rises in siphon No. 1 as the tank
empties and when the action of Siphon No. 2 is broken by the admission
of air when the bottom of the bell is uncovered the water in siphon No.
1 has assumed the position of _bb′_ and that in No. 2 is at _aa′_. The
conditions of the two siphons are now reversed from that at the
beginning of the operation and as the tank refills siphon No. 1 will go
into operation. It is to be noted that these siphons are made to
alternate by weakening the seal of the next one to discharge and by
strengthening the seal of the one which has just discharged.

[Illustration:

FIG. 182.—Diagram Showing the Operation of Two Alternating Siphons.
]

[Illustration:

FIG. 183.—Diagram Showing the Operation of Three Alternating Siphons.
]

=285. Three Alternating Siphons.=—This principle can be extended to the
operation of three alternating siphons as shown in Fig. No. 183. These
operate as follows: with the dosing tank empty at the start and water at
_aa′_ in siphons 1 and 2, and at _bb′_ in siphon No. 3, the dosing tank
will be allowed to fill. As the water rises in the tank air is trapped
in all the bells and surfaces _a_ and _b_ are depressed. When surface
_b_ has been depressed to _d_, _a_ has been depressed to _c_. Air is
released from siphon No. 3 and this siphon goes into action. Surface _c_
rises in siphons 1 and 2 to the position _b_, as the dosing tank is
emptied. At the same time a small amount of water is passed from siphon
No. 3 to the short leg of siphon No. 1, through the small pipes shown,
thus filling this leg so that when siphon No. 3 ceases to operate the
water in siphons 1 and 3 stands at _aa′_ and that in No. 2 stands at
_bb′_. Siphon No. 2, having the weaker seal, will be the next to
operate. During its operation it will fill siphon No. 3, leaving No. 1
weak. When No. 1 operates it will refill No. 2, leaving No. 3 weak, thus
completing a cycle for the three siphons. This principle has not been
applied to the operation of more than three alternating siphons and is
seldom used on recent installations.

[Illustration:

FIG. 184.—Miller Plural Alternating Siphons.

Courtesy, Pacific Flush Tank Co.
]

=286. Four or More Alternating Siphons.=—An arrangement for the
alternation of four or more siphons is illustrated in Fig. 184. At the
commencement of the cycle it will be assumed that all starting wells are
filled with water except well No. 1, and that all main and all blow-off
traps are filled with water. The following description of the operation
of the siphons is taken from the catalog of the Pacific Flush Tank
Company:

The liquid in the tank gradually rises and finally overflows into
the starting well No. 1 and the starting bell being filled with
air, pressure is developed which is transmitted, as shown by the
arrows, to the blow-off trap connected with siphon No. 2. When the
discharge line is reached, sufficient head is obtained on the
starting bell to force the seal in blow-off trap No. 2, thus
releasing the air confined in siphon No. 2 and bringing it into
full operation.

During the time that siphon No. 2 is operating, siphonic action is
developed in the draining siphon connected with starting well No.
2 and as soon as the level in the tank is below the top of the
well it is drained down to a point below the bottom of starting
well No. 2. It can now be seen that after the first discharge
starting well No. 2 is empty, whereas the other three are full....
Therefore when the tank is filled the second time, pressure is
developed in starting bell No. 2, which forces the seal of
blow-off trap No. 3, thus starting siphon No. 3....

This alternation can be continued for any number of siphons. Other
arrangements have been devised for the automatic control of alternating
siphons, but these principles of the air-locked devices are fundamental.

=287. Timed Siphons.=—In the operation of a number of contact beds not
only must the dosing of the tanks be alternated, but some method is
needed by which the beds shall be automatically emptied after the proper
period of standing full. To fulfill this need the principle of the timed
siphon must be employed in conjunction with the alternating siphons.
Fig. 185 illustrates the operation of the Miller timed siphon. Its
operation is as follows: water is admitted to the contact bed and
transmitted to the main siphon chamber through the “opening into bed.”
Water flows from the main siphon chamber into the timing chamber at a
rate determined by the timing valve. The contact bed is held full during
this period. As the timing chamber fills with water air is caught in the
starting bell and the pressure is increased until the seal in the main
blow-off trap is blown and the main siphon is put into operation. As the
water level in the main siphon chamber descends, water flows from the
timing chamber into the main siphon through the draining siphon and the
timing chamber is emptied, ready to commence another cycle.

=288. Multiple Alternating and Timed Siphons.=[209]—The alternating and
timing of a number of beds is more complicated. The arrangement
necessary for this is shown in Fig. 186. It will be assumed at the start
that all beds are empty and that all feeds are air locked as shown in
Section _AB_ except that to bed No. 4 into which sewage is running. As
bed No. 4 fills, sewage is transmitted through the opening in the wall
into the timed siphon chamber No. 4. When the level of the water in the
bed and therefore in this chamber has reached the top of the withdraw
siphon leading to the compression dome chamber No. 4, this latter
chamber is quickly filled. The air pressure in starting bell No. 4_a_ is
transmitted to blow-off trap No. 1_a_. The seal of this trap is blown,
releasing the air lock in feed No. 1 and the flow into bed No. 1 is
commenced. At the same time the air pressure in compression dome No. 4
is transmitted to feed No. 4, air locking this feed and stopping the
flow into bed No. 4. The alternation of the feed into the different beds
is continued in this manner.

[Illustration:

FIG. 185.—Miller Timed Siphon.

Courtesy, Pacific Flush Tank Co.
]

Bed No. 4 is now standing full and No. 1 is filling. When compression
dome chamber No. 4 was filled, water started flowing through timing
siphon valve No. 4 into timing chamber No. 4 at a rate determined by the
amount of the opening of the timing valve. As this chamber fills
compression is transmitted to blow-off trap 4_b_ and when sufficiently
great this trap is blown and timed siphon No. 4 is put into operation.
Bed No. 4 is emptied by it, and compression dome chamber No. 4 is
emptied through the withdraw siphon at the same time. This completes a
cycle for the filling and emptying of one bed and the method of passing
the dose on to another bed has been explained. The principle can be
extended to the operation of any number of beds.

[Illustration:

FIG. 186.—Plural Timed and Alternating Siphons for Contact Bed
Control.

Courtesy, Pacific Flush Tank Co.
]

INDEX

A. B. C. process of sewage treatment, 4

Abandonment of contract, 225

Access to work, 228, 229

Accident, contractor’s responsibility, 221, 224

Acetylene, explosive, 347

Acid precipitation. _See_ Miles Acid Process.
of sludge, 503

Acids as disinfectants, 489, 490

Activated sludge. Chapter XVIII, 465–479
advantages and disadvantages, 469, 470
aëration tank, 471, 472
air diffusion, 475, 477
air distribution, 473–478
air quantity, 475, 476
area of filtros plates, 478
colloid removal, 358
composition, 465–469
cost, 478, 479
definition, 466
dewatering, 468, 469, 497–505
fertilizing value, 469, 470
historical, 470, 471
how obtained, 478
nitrogen content, 468
patent, 471
process, 465
quantity, 469
reaëration tank, 473
results, 467, 468, 476
sedimentation tank, 472

Advertisement, 214

Aëration, effect on oxygen dissolved, 373–375
of sewage, 371, 376, 465–479

Aërobes, 363

Aërobic decomposition, 366, 367

Aftergrowths, 492

Aggregates, specifications, 172–174

Air, see also ventilation, activated sludge, compressed air, etc.
ejectors, 150
lock dosing apparatus. Chap. XXI, 506–512
machinery for activated sludge, 473, 474

Algæ, 363

Alkalinity, 358

Alleys, sewers in, 80

Alum, 407, 408

Alvord tank, 427, 429

Ammonia, 366, 367, 374, 375, 410
explosives, 297

Analyses, bacteriological, 364
chemical, 354, 355
mechanical of sand, 182
physical, 352–354
sewage, 352–364

Anaërobes, 363, 365–367

Anaërobic, action, 410
bacteria, 363
conditions, 367
decomposition, 365–367

Ann Arbor, Michigan, Population, 14

Annual expense, method of financing, 157, 158

Ansonia air ejector, 150, 151

Antibiosis, definition, 363

Appurtenances to sewers. Chap. VI, 99–115

Arch, analyses, 204–208
elastic method, 206–208
vouissoir analysis, 204–206
brick construction, 312, 313
centers for brick sewers, 313
concrete construction, 318–321

Ardern and Lockett, development of activated sludge, 467, 468, 471

Area of cities, 31

Asphyxiation in sewer gas, 336

Assessments, special, 15, 16

Augers, earth, 21

Automatic, regulators, 117–121
siphons, flush-tanks, 110
double alternating, 507
multiple alternating, 508–512
timed, 510
timed and multiple alternating, 510–512
triple alternating, 508

Bacillus, definition and morphology, 362, 363

Backfilling, 328–331

Backfill, puddling, 330
weight of, 199, 201

Backwater curve, 73

Bacteria, definition and morphology, 362, 363
good and bad, 363, 364
nature of, 362, 363
nitrifying, 431, 432
sanitary significance of, 364
in sewage, 362, 363
total count, 364

Bacterial analyses, results in sewage, 364

Baffles, scum, 404, 413, 414, 421
in sedimentation tanks, 404
in septic tanks, 413, 414
in Imhoff tanks, 421

Balls, for cleaning sewers, 338

Band screen, 384

Barring, definition, 263

Bars for screens, 390

Basins, sedimentation, baffling, 404
bottoms, 404
cleaning arrangements, 404
depth, 401
economical dimensions, 401–403
inlets and outlets, 404
scum boards, 404
types, 395

Basket handle sewer section, 67, 69

Bathing beaches, pollution, 381

Bazin’s formula, 54

Bearings, for centrifugal pumps, 131, 137, 138
thrust, 138

Bellmouth, 121, 122

Bends in pipe, loss of head in, 116

Berlin, sewage farm, 460, 461
sewers, date of, 3

Bids, proposal, 217–219

Bidder’s duties, 215–217

Bio-chemical oxygen demand, 359–361

Biolysis of sewage, 366, 367

Black and Phelps dilution formulas, 377–379

Blasting and explosives, 294–304
caps, 297, 299, 300
detonators, 294, 297–300
firing, 302–304
fuses and detonators, 297–300
fuses, delayed action, 291, 300
fuses, electric, 299, 300
splicing, 303
gelatine, 296
loading holes, 303
powder, 295
precautions, 300–302
priming and loading, 303
rock, 269
size of charge, 304, 305
tunneling, 290, 291

Bleach, characteristics of for disinfection, 491

Block sewer, construction, 311–314
hollow tile as underdrains, 126

Blocks, vitrified clay, 189, 190

Boilers, steam, 147–150

Boilers, efficiencies, 149
horse-power, 149

Bond, contractor’s, 213, 214, 232
issues, 14

Bonds, definition and types, 14–16

Boring underground, 20

Bottom, activated sludge aëration tank, 472
Imhoff tanks, 423
sedimentation tanks, 404
trickling filter, 451, 452

Box sheeting, 272

Branch sewer, defined, 7

Breast boards, 288

Brick, arch construction, 312, 313
and block sewer construction, 311–315
invert construction, 311, 312
sewer construction, 311–315
arch centers, 313
invert, 311–312
organization, 314, 315
progress, 314
row lock bond, 312
specifications, 188, 189
sewers, life of, 351

Bricks for sewers, 316

British Royal Commission on Sewage Disposal, 4

Broad irrigation. _See_ under Irrigation.

Bucket excavators, 246, 255, 256

Building material, weight of, 201

Burkli-Ziegler formula, 47, 425

Butyrine, 366

Cableway excavators, 246, 250–252

Cage screen, 384, 385

Caisson excavation, 285, 286

Calcium carbide, explosive, 347

Calumet pumping station, 128, 142

Cameron septic patent, 411

Capacity of sewers, diagrams, 57–60

Capital, private invested in sewers, 17

Capitalization, method of financing, 157–160

Caps, blasting. _See_ blasting.

Carbohydrate, 366, 367

Carbon, analysis for, 356
dioxide, 366, 367

Carson Trench machine, 250, 251

Cast-iron pipe, 122, 164, 190, 191
joints, 164
quality, 101, 102, 190

Castings, iron, 101, 102

Catch-basins, 99, 107–108, 217
cleaning, 343, 344
inspection, 337

Catenary sewer section, 69

Cellars, depth of, 88

Cellulose, 367

Cement. _See also_ Concrete,
pipe, specifications, manufacture and sizes, 171–179
vs. concrete, 164

Centrifugal pumps. _See_ pumps, centrifugal.

Centrifuge for sludge drying, 501, 502

Cesspool, 411, 416, 417

Champaign, Illinois, septic tank, 415, 416

Changes in plan, 222, 223

Channeling, definition, 263

Character of surface, 44

Chemical analyses, 354–362

Chemical precipitation, 371, 405–409
chemicals used, 405–407
preparation of chemicals, 407, 408
results, 408, 409
at Worcester, 408

Chezy formula, 52, 53

Chicago. _See also_ Sanitary District of Chicago.
drainage canal, 374, 375
dilution requirement for sewage, 380
early sewers, 3
method of sewage disposal, 374
population and density, 29, 30
trench excavation in, 248

Chlorine. _See also_ Disinfection.
disinfectant, 489–493
in sewage, 358, 374, 375

Chlorine liquid, application, 491, 492

Cholera, transmittable disease, 364

Chromatin, 365

Chutes for concrete, 187

Circular sewer section, hydraulic elements, 65, 66, 69
types, 70, 71

City, growth of area, 31
growth of population, 24–28
legal powers, 219

Clay, life of pipe, 349–351
manufacture of pipe, 165–167
specifications for pipe, 168–170
unglazed for pipe, 165
vitrified blocks, 167, 189, 190
vitrified pipe, 165–171

Cleaning, grit chambers, 398, 400
sedimentation basins, 404
sewers, cost, 341
in N. Y. City, 332
methods, 337–343
tools, 338–340
up after completion of work, 228

Coccus, 362

Coefficient of uniformity of sand, 456

Coffin sewer regulator, 117, 118

Colloid, nature of, 358
treatment for, 358

Color of sewage, 352, 353

Combined sewer system, 78, 79

Commercial districts, characteristics of and sewage from, 32, 34, 35

Compensators for pumps, 142

Compressed air. _See also_ ventilation, tunneling, drilling, etc.
activated sludge, 473–475
for drilling, 264–268
in tunnels, 292–294
transporting concrete, 320, 321

Concentration, time of flood flow, 41–43, 96, 97

Concrete, aggregates, 172–174
mixing and placing, 184–188
pipe, details, 175–179
manufacture, 171–179
reinforcement, 177, 178, 209, 210
pipe, steam process, 176
sizes, 175
pressure against forms, 232, 323

Concrete, proportioning, 179–183
qualities, 179, 180
reinforcement, placing, 178, 326, 327
reinforcing steel, quality, 191
sewer construction, 314–328
arch, 318–321
form length, 319
labor costs, 327, 328
in open cut, 314–320
in tunnel, 320, 321
invert, 315–320
organization for, 328
working joints, 319
sewer costs, 327–329
strength, 181
waterproofing, 184

Conduits, special sections, 67, 70, 71

Connections to sewers, ordinances, 344, 345
record of 92, 238

Construction of sewers, Chap. XI, 233–331

Construction, elements of, 233
organizations, 315, 328

Contact bed, 432–437, 506
advantages and disadvantages, 432–434
automatic control, 437, 506
cleaning, 435
clogging, 435
construction, 434–436
control, 437, 506
cycle, 436, 437
depth, 434
description, 432, 433
design, 434–436
dimensions, 434, 435
loss of capacity, 435
material, 435, 436
multiple, 433, 435
operating conditions, 432–437
rate, 435
results, 433, 434
ripening, 432

Continuous bucket excavators, 246–250

Contour interval on maps, 79, 80

Contracts, Chap. X, 211–232
abandonment of, 225
assignment, 228
completion of, 222, 228
bond, 213, 222
content, 213, 230, 231
cost-plus, 212, 213
disputes, 220
divisions of, 213
drawings, 213
engineer as an arbitrator, 220
the instrument, 230, 231
interpretation of, 220, 234, 235
lump sum, 212
nature of, 211, 212
sample, 230, 231
time allowed, 222
types, 212, 213
unit-price, 213

Contractor, absence of, 222
bond, 232
claims against, 228
duties, 221
liability, 224
relations with other contractors, 228, 229

Contractor’s powder, 294

Control devices, automatic, for sewers, 117–121
for filters, 500–512
inspection of, 336, 337

Copper sulphate, disinfectant, 490

Copperas, precipitant, 406–408

Cordeau Bickford, 298, 303

Corrugated iron pipe, 165

Cost. _See_ under item wanted.

Cost, annual. Method of financing, 157–160
capitalized. Method of financing, 157–160
classification of, 235–238
comparisons of. Methods for
making, 157–160
collection of data, 10–14, 235–238
estimate. Method of making, 10–14
overhead, 237, 238

Couplings, flexible for shafts, 138

Covers, Imhoff tanks, 424
septic tanks, 415
trickling filters, 451

Crops on sewage farms, 463, 464

Cunette, 67, 70

Cut, depth of excavation, 88, 92

Cycle, contact bed, 436
life and death, 367, 431
nitrogen, 367, 368
trickling filter, 441

Cylinders, stresses in, 194, 202–204

Cytoplasm, 365

Damages, liquidated, 222
material, 221, 224

Darcy’s formula, 52

Day labor, 211

Decomposition of sewage, 365–367

Definitions. _See_ word defined.

Deflagration, definition, 294

Delays in contract work, 228

Delayed action fuses, 291, 300

Densities. _See_ population.

Depreciation, of sewers, 348–351
rate of, financial, 158

Depth of sewers, 88

Design conditions, 88–92
economical, mathematics of, 401–403
preparations for, 17–23

Detention period, grit chamber, 397
Imhoff tank, 419
plain sedimentation, 392–395, 401
septic tank, 415

Detonation, definition, 294

Detonator. _See_ blasting cap.

Diameter of sewers, 57–60, 72, 88–92

Diaphragm pump, 257, 258

Diesel engine, 152, 154

Digestion chamber, Imhoff tank, 422, 423

Digestion of sludge in separate tank, 427–430, 497

Dilution, amount needed, 377–380
conditions for success, 372, 373

Dilution, definition, 372
formulas for quantity, 378–380
governmental control, 380, 381
preliminary studies, 381, 382
in salt water, 376, 377
in streams, 372–376
of sewage, 370 and Chap. XIV, 372–382

Diseases, water-borne, 364

Disinfection, 489–493
action of, 489–491
bleaching powder, 491
chlorine, liquid, 491
amount of, 492
disinfectants, 489, 490
purpose, 489
selective action of disinfectants, 492, 493

Disk screen, 384

Disposal of sewage, _See_ sewage treatment.

Disputes, engineer to settle, 220

Dissolved oxygen. _See_ Oxygen dissolved.

Distribution of sewage,
contact beds, 436
irrigation, 461, 462
nozzles, 442–449
sand filter, 450–458
traveling distributor, 442
trickling filters, 441–451

Districts, character of, 29, 30, 32–37
classification of, 34, 35

Domestic sewage, defined, 6, 7, 352

Dorr Thickeners, 472, 504

Dortmund tank, 404

Dosing devices, 506–512
alternating and timed siphons, 500–512
Alvord device at Lake Forest, 506
four or more alternating siphons, 509
operation of automatic siphon, 110
three alternating siphons, 508
timed siphons, 510
two alternating siphons, 507
types, 506

Dosing tank design, for trickling
filter, 446–450

Doten tank, 429, 430

Drag line excavators, 255, 256

Drainage areas, 81, 84, 94

Drills, electric, 267
jack hammer, 264, 265
punch, 20
size of cylinder for, 266
tripod, 264, 265

Drilling, methods, 20–23, 264–270
depth, diameter and spacing of
holes, 268–270
power for, 267, 268
rate of, in rock, 267
steam and air, 267, 268

Drop manhole, 100, 101

Drop-down curve, 73, 77

Drum screen, 384

Dry weather flow, 24, 38

Drying sludge. _See_ sludge drying.

Dualin, 296

Duty of contractor. _See_ Contractor, duties

Duty of engineer. _See_ Engineer, duties.

Duty of inspector. _See_ Inspector, duties.

Duty of a pump, defined, 135

Dynamite, 296–298, 300–302, 304, 305
cartridge, 268, 296, 302
thawing, 301, 302

Dysentery, 365

Earth pressures, theories, 274, 275

Economical dimensions, mathematics of, 401–403

Effective size of sand, defined, 456

Efficiency of a pump, defined, 135

Effluents, character of
activated sludge, 467, 468
chemical precipitation, 408
contact bed, 434
Imhoff tank, 414, 424, 425, 432
lime and electricity, 489
Miles acid process, 484, 485
sand filter, 453

Effluents, sedimentation tank, 401
septic tank, 412–414

Egg-shaped section, 67, 68, 70

Ejectors, air, 150, 151

Elastic arch analysis, 206–208

Electric motors, 150–152

Electrolytic treatment, 487–489

Elevations, method of recording, 92

Emergencies, duties of engineer, 235

Emerson pump, 261

Engines, internal combustion, 152–154
steam, types, 142–144.

Engineer, absence of, 221
defined, 220
disputes settled by, 220, 234
duties of, 9, 10, 220, 233, 234, 238
individuality and personality, 9, 234
qualifications, 9
sanitary, definition, 2

Engineering News pile formula, 125, 126

Entering sewers, precautions, 335, 336

Enzymes, 365

Equipment for construction, 237

Equivalent sections, defined, 72
solution of problems in, 67–72

Estimates, cost and work done, 10–14
when made, 226
data for, 235

Excavation, depth of open cut, 284
drainage, 252, 262
hand, 242–245, 249
economy, 245
laborer’s ability, 243
lay out of tasks, 243

Excavation, hand, opening trench, 243
vs. machine, 245, 249
tools, 242
machine, 244–246
economy, 245
limitations, 246
vs. hand, 245, 249
specifications, 240, 241

Excavating machines, bucket, 246, 255
cableway and trestle, 246, 250–252
Carson machine, 250, 251
continuous belt, 246
bucket, 246, 247
drag line, 255
Potter machine, 251
steam shovel, 252–254
tower cableway, 252
wheel excavators, 246–250

Excavation, machine, organization, 249
pumping and drainage, 256, 257
quicksand, 256
rock, 263, 264
payment for, 230
specifications, 240, 241
trench bottom, 241, 304, 311

Explosions in sewers, 108, 336, 346–348
causes of, 346
historical, 346
prevention, 108, 348

Explosives. _See also_ Blasting.

Explosives, and blasting, 294–304
ammonia compounds, 297
blasting gelatine, 296
contractor’s powder, 294
deflagrating, 294
detonating, 294
detonators, 294, 297–300
“Don’ts,” 300, 301
dynamite, 296–298, 300–302, 304, 305
fuses and detonators, 297–300
gelatine dynamite, 296
gunpowder, 295
handling, 300–302
nitro-glycerine, 295
nitro-substitution compounds, 295
permissible, 297
quantity, 304, 305
requirements, 294
strength of, 297, 298
T.N.T., 295
types, 294–297

Exponential formulas for flow of water, 54, 55

Extra work, compensation, 227

Facultative bacteria, 363

Fanning’s run-off formula, 49

Farms, septic tanks for, 416, 417

Farming with sewage. _See_ irrigation.

Fats in sewage, 357–359, 366, 367
from Miles acid process, 485–487

Feathers, for splitting rock, 264

Ferrous sulphate, precipitant, 406–408

Fertilizer from sludge, 470, 495, 497

Fertilizing value of, activated sludge, 470
sewage, 459, 460

Filter press for sludge, 500, 501

Filters. _See_ under name of filter.

Filtration, of sewage, 370, 371, 431–459
action in, theory of, 431
cost, 458, 459

Filtros plates, 477, 478

Finances, mathematics of, 157–160

Financing, methods of, 14–17

Flamant’s formula, 54, 56

Flies on trickling filters, 438

Flight sewer, 101, 102

Flood, crest velocities, 42, 43
flow computations, 94–98
McMath formula, 94, 96, 97
Rational method, 95–98

Flow, laws of, 52
velocity of, 52, 90, 91

Fluctuations, in rate of sewage flow, 33–38
in quality of sewage, 368–370

Flush-tanks, automatic, 109–113
capacity, 111
details, 110, 112
inspection of, 336, 337
payment for, 217
siphon sizes, 111

Flushing, 109–113, 341–343
amount of water needed, 112
methods, 341–343
manhole, 109
sewer, defined, 8

Foaming of Imhoff tanks, 425, 426

Foot valves, 141

Force main, defined, 8

Forms, design of, 322, 323
length of, 319
materials, 321, 322
oiling, 174, 186, 322
specifications, 322
steel, 325, 326
steel-lined, 325
support for, 316, 318
time in place, 319
wooden, 323, 324

Formulas, hydraulic, methods for solution, 55–61
for flow of water, 52–55
for rainfall. _See_ Rainfall,
for run-off. _See_ Run-off.

Foundations, 99, 124–126

Franchises for sewers, 17

Free ammonia, 366, 367, 374, 375, 410

Freezing, catch-basins, 108
concrete, 186, 187
dynamite, 301, 302

Fresh sewage, characteristics, 352–354

Friction losses. _See_ Head losses.
flow in pipe, 51, 52

Fuel, consumption by prime movers, 153
costs, 153
heat value, 150

Fungus growth in sewers, 333

Fuses. _See_ blasting fuses.

Ganguillet and Kutter’s formula, 52–65

Gas, chamber in Imhoff tank. _See_ Scum chamber.
engines, 152–154
illuminating, explosive, 347
sewer, 335, 336

Gasoline, explosive, 108, 109, 335, 346, 347
engines, 152–154
and oil separator, 109
odors, significance, 335, 353

Gearing, reduction for turbines, 140, 146

Gelatine dynamite, 296

Glycerol, 366

Gothic section, 67

Governmental control, stream pollution, 380, 381

Grade, of sewers. _See also_ Slope.
how given, 281–284
selection of, 90
stakes, 221, 281–283

Gravel, specifications, 172

Grease, in sewers, 99, 108, 333, 345
cutter, 340
ordinance concerning, 345
traps, 99, 108

Gregory’s imperviousness formulas, 44, 46

Grit, clogs sewers, 333
chambers, 127, 397–401
description, 395, 398
design, 397, 398
dimensions, 397, 398
existing, 398–400
outlet arrangements, 400
results, 397
retention period, 397
sludge analyses, 397
units, number of, 400, 401
velocity of flow in, 396–398
quantity and character of, 397

Grooves in concrete, working joints, 319

Ground water in sewers, 38, 39, 85, 87, 256, 352

Gun cotton, 296

Gunpowder, 295

Hazen, theory of sedimentation, 392–395
dilution formula, 380

Hazen and William’s formula, 55, 57

Head loss, in bends, 116
entrance, 115
friction in straight pipe, 51, 52, 115

Hercules powder, 296

Hering, Rudolph, dilution recommendations, 380

Hering, Rudolph, introduction of Imhoff tank and hydraulic formulas,
425

Historical résumé of sewerage and sewage treatment, 2–5

Hitch, tunnel frame, 286, 287

Holes, drill. _See_ Drill holes.

Holidays, work on, 221

Hook for lifting pipe, 304, 306

Horse-power, boiler, 149, 150
of pumps, 144–146

Horseshoe sewer section, 71

House, connections, record of, 92, 234
drains, 7, 88, 90
sewer, defined, 7

Hydraulic, elements, 65, 69
formulas, 52–55
jump, 73–74
principles, 51, 52, 72, 73
value of settling particles, 393

Hydraulics of, sewers, Chap. IV, 51–77
circular pipes partly full, 65, 66
equivalent sections, 72
non-uniform flow, 72–77
sections other than circular, 67–72
use of diagrams, 61–65

Hydrocarbon, 367

Hydrogen sulphide, 353, 366, 410

Hydrolytic tank, 427, 428

“Hypo” as a disinfectant, 491

Hytor Turbo blower, 473, 474

Illinois River, self-purification, 374–376

Imhoff tank, and chlorination, costs, 487
cover, 424
description, 417–419
design, 419–424
digestion chamber, 422
inlet and outlet, 421
operation, 426–427
patent, 418
results, 414, 424, 425, 439, 467
sedimentation chamber, 419–422
scum chamber, 424
slot, 422
sludge, 414, 467
sludge pipe, 423, 424
status, 425, 426
and trickling filter, cost, 479

Impeller, for centrifugal pump, 131, 136

Imperviousness, relative, 40, 42, 44–46, 95–97

Industrial, districts, 32–37
wastes, defined, 7, 352
tannery, 491

Information and instructions for bidders, 213, 215–217

Inlets, street, 93, 94, 99, 104–107

Inspection, contract stipulations, 221–224
during construction, 233, 234
for maintenance, 104, 333–337, 348, 349

Inspector, absence of, 221, 222
duties, 233–234
qualifications, 234

Institutional sewage treatment plants, 416, 417

Intercepting sewer, defined, 7

Intermittentsand filter. _See_ Sand filter.

Internal combustion engines, 152–154

Inverted siphon, 113–116

Iron, ferrous sulphate, precipitant, 406–408
cast. _See_ cast iron.

Irrigation. _See also_ Farming and Sewage farming.
area required, 463
Berlin sewage farm, 460, 461
crops, 463, 464
description, 459
fertilizing value of sewage, 460, 470, 495, 498
vs. farming, 459
operation, 461–463
preliminary treatment, 462, 463
preparation for, 461–463
process, 459, 460
sanitary aspects 463
status, 460, 461
theory, 432
in the United States, 461

Jack hammer drill, 264, 265

Jetting method, 21–23

Jet pump, 259, 341, 343

Joints, bituminous, 309–311
in cast-iron pipe, 164
cement, 307, 308
inspection of, 234
lead, 164
mortar, 307
open, 307
poured, 309–311
cement, 309, 311
riveted steel, 195, 196
sulphur and sand, 309
types, for pipe, 307
working, in concrete, 319

Junctions, 99

Kuichling, run-off rules, 46, 47, 49
storm intensity formulas, 50

Kutter’s formula, 52–65

Labor, day vs. contract, 211
costs on concrete sewer, 328, 329

Labyrinth packing rings, 136, 137

Lagging, tunnel frames, 287
for forms, 322

Lagooning sludge, 495–497

Laitance, 186, 188

Lakes, self-purification of, 376

Lampé’s formula, 54

Lampholes, 99, 104

Lateral sewer, defined, 7

Lawrence Experiment Station, 4

Leaping weir, 118–121, 337

Legal requirements, construction, 224
dilution, 380, 381
in design, 9

Liernur system, 5

Life, organic in sewage, 363, 364
of sewers, 348–351

Lime as a precipitant, 405–408
with electricity, 488, 489
with iron, 406, 407

Line and grade, 281–284
how given, 281–283

Liquefaction of sludge, 411–413, 496, 497

Liquid chlorine. _See also_ Chlorine, 491

Liquidated damages, 222

Loads on, pipe, 198–202
Marston’s method, 198–202
trench, 199–202

Lock bar pipe, 197

Lock-joint pipe, 177

Long loads, 201

Machine excavation. _See_ Excavation.

Macroscopic organisms, 363, 368

Main sewer, defined, 7

Maintenance of sewers, Chap. XII, 332–351
catch-basin cleaning, 343, 344
cleaning sewers, 337–343
complaints, 333
cost, 341
entering sewers, 335, 336
flushing, 109–113, 341–343
hand cleaning, 341
inspection, 333–337
organization, 332
protection of sewers, 344, 345
repairs, 337
tools, 338–341
troubles, 333
work involved, 332

Man, shoveling ability, 243

Manholes, 81, 99–104
bottom, 100
cover, 102–103
drop, 101
flushing, 109, 342
location and numbering, 81
payment methods, 217, 218
steps, 100, 103, 104

Manning’s formula, 55

Map, preliminary, 17, 79, 80, 82, 83

Marsh gas, 347, 366, 367, 410, 415

Marston’s methods for external loads on buried pipe, 198–202

Materials, for sewers, Chap. VIII, 164–193
measurement of, 236, 237
record of, 237
unit weights, 201, 202

McMath’s formula, 47, 48, 94, 95

Meem’s theory of earth pressure, 274, 275

Mercaptan, 367

Metabolism, 365

Methane, 347, 366, 367, 410, 415

Methylene blue, 360

Microscopic organisms, 363, 364, 368

Miles acid process, costs, 487
amount of acid, 483
analyses of sludge, 485
description, 482
results, 483–487
sludge, 485

Mineral matter in sewage, 357

Mirror, inspecting device, 334

Money retained by city, 227

Mosquitoes in catch-basins, 108

Motors, electric, 150–152

Municipal, bond, 14, 15
corporations, 15

_n_, value of in Kutter’s formula, 53

New York City, density of population, 29, 31
siphons under subway, 114
grease and gasoline trap, 108, 109
aëration of sewage, 377, 470
cleaning sewers, 332
depreciation of sewers, 348–351

Needle beam, 286, 287

Night, soil, 5
work, 221

Nitrates, 355, 356

Nitrites, 355, 356

Nitrifying organisms, 431, 432

Nitrobacter, 431, 432

Nitro explosives, 295, 296

Nitrogen, cycle, 367, 368
organic, 355, 356

Nitro-glycerine, 295

Nitrosomonas, 431, 432

Nomograph, 55, 56

Non-uniform flow, 72–77

Nozzles. _See also_ Trickling filters.
coefficients of discharge, 446
types, 445

Numbering, drainage areas, 81, 94
manholes, 81

Nye steam pump, 260, 263

Obstructions to construction, 235

Odor of sewage, 353

Oil in sewage, 108, 344–348

Oiling forms, 174, 186, 322

Olein, 366

Ordinances, for protection of sewers, 344, 345

Organisms in sewage, 363, 364, 368

Organic matter, composition, 366

Organizations for construction, 315, 317, 328

Orders, to whom given, 222

Outfall sewer, defined, 8

Outlets, 99, 122–124, 373

Overflow weir, 118–121
inspection of, 337

Overhead, costs, division of, 10, 237, 238
-track excavators, 246, 250, 251

Oxidation in streams, 373–376

Oxygen, absorption of, 374–377
consumed, 355, 356
demand, 359–361
computation of, 360
bio-chemical, 359–361

Oxygen dissolved
exhaustion of, 366
in dilution, 381
solubility, 362
supersaturation, 361
concentration for successful dilution, 377–380
formulas for concentration, 378–380
significance of in sewage, 359–362

Oysters, contamination of, 372, 489

Packing rings, labyrinth type, 136, 137

Palmatin, 366

Parasites, 365

Paris sewage farm, 460

Patents. Protection of City by contractor, 224, 225

Pathogenic bacteria, 364

Pavement, replacing, 329

Payment, final on contract, 228

Payments, methods of making, 217, 218

Periscope inspecting device, 334, 335

Permissible explosives, 297

Phenolphthalein indicator, 408

Photographic records, 238

Piles for foundations, 123–126

Pills for cleaning sewers, 338

Pipe, bedding, 230, 304, 328
cast-iron. _See_ under cast-iron pipe.
design of ring, Chap. IX, 194–210
external loads on, 198–202
joints. _See_ Joints.
sewer construction, 304–311
laying, line and grade, 282–284
organization, 311
method of laying, 304, 306, 307
steel, design, 195–197
stresses in, external forces, 194, 202–204
stresses due to internal pressure, 194
stresses in buried pipe, 198–204
stresses in circular ring, 202–204
wood design, 197, 198

Plankton, defined, 363
in sewage, 368

Plans, changes in contract, 222, 223

Plug and feathers for splitting rock, 264

Pneumatic, collection system, 5
concreting, 320, 321

Poling boards, in open cut, 271, 272
in tunnel, 287

Pollution, legal features, 380, 381

Population, density, 28–31
predictions, 24–27
served by sewers in the U. S., 3
sources of information, 27, 28
and quantity of sewage, 31, 32

Potter trench machine, 251

Powder. _See_ Blasting.

Power pump, 132, 133

Precautions in entering sewers, 335, 336

Precipitants, chemical, 405–407

Preliminary, map, 17, 79, 80, 82, 83
work, 9, 17–23

Present worth, 158, 160

Pressing sludge, 500, 501

Priming explosives, 302–304

Private, capital, 17
sewers, 17

Privy, 5

Profile, for brick sewers, 312
sewer, 92
surface, 88

Progress, rate of, 222
reports, 238

Promotion (inception of sewers), 9

Proportioning concrete. _See_ Concrete proportioning.

Proposal (contract), 213, 217–219

Protection of sewers (ordinances), 344, 345

Protein, 366

Puddling, backfill, 330

Pulsometer pump, 260, 261

Pumping, in excavations, 256–263
selection of machinery, 154–156
equipment, cost comparison, 162
station, 128, 142
costs, 156–163
equipment, 127, 128

Pumps, air ejector, 150, 151
capacity, 129, 160–163
capacity of units, 160–163
centrifugal, details, 130, 131, 136–138
automatic control, 141, 142
characteristics, 138–140
efficiency, 140
for excavation, 262
motors for driving, 150–152
performance, 138–140
protection of, by screens, 386
selection of, 154–156
setting, 140–142
turbine, 130–132, 154
types, 130, 131

Pumps, centrifugal, volute, 130–132, 154
character of load, 129
costs, 156, 157
description of types, 130–134
for construction work, 256–263
diaphragm, 257, 258
direct-acting, 133
duty of, 135, 136
efficiencies, 135, 136
ejector, 134, 150, 151, 259, 341, 343
jet, 259
need for, 127
number of units, 160–163
packing of, 133, 134
piston, 133
speed, 133, 134
plunger, 133
power, 132, 133
reciprocating, 130, 132–135, 154–156
for excavation, 262
reliability, 127
sizes, 135
steam, 134, 135, 142–146
consumption, 144, 145
vacuum, 259, 262
improvised for trench work, 257
turbine, 130–132, 154
volute, 130–132, 154

Putrescibility, 359, 360

Quantity, of sewage, 24–50, 84–87
variations, 33–38
storm water, 40–50, 94–98

Quicksand, definition, 256
excavation in, 256
safeguards, 235

Quiescent water, self-purification, 374

Racks. _See_ Screens.

Rainfall, 17, 40, 41, 50, 96, 97
data, 17
rate, 96, 97

Rangers, 270–274, 276–279

Rankine’s theory of earth pressure, 275

Rapid sand filtration of sewage, 458

Rational method of run-off determination, 40, 95–98

Reaëration tank in activated sludge, 473

Receiving well, capacity, 129, 130

Reciprocating pumps. _See_ Pumps, reciprocating.

Records, character of, on construction, 238–240

Rectangular sewer section, 67–69

Regulators, 99, 117–121, 337
inspection of, 337

Reinforced concrete sewer design, 209, 210

Reinforcing steel, specifications, 191
placing, 326, 327

Reinsch Wurl screen, 384

Relative stability numbers, 359

Relief sewer, defined, 7

Repairs to sewers, 337

Report, engineer’s preliminary, 10

Reservoir, collecting capacity, 129, 130

Residences, septic tanks for, 416, 417

Residential districts, characteristics, 32–37

Residue on evaporation, 356, 357

Rideal’s dilution formula, 379

Ring, design. Chap. IX, 194–210
stresses in circular, 202–204

River pollution, legal features, 380, 381

Rivers, self-purification of, 373–376

Riveted joints, properties, 196

Rock, blasting, 268, 290, 291
definition, 263
drill, data on, 266, 267
drilling. _See_ also Drilling.
by hand, 264
by power, 264–268
rates, 267
excavation. _See also_ Excavation.
payment for, 230
measurement of, in place, 235
tunnels, 290, 291

Rods, sewer, 338

Roman ordinance relative to sewers, 2

Roofs. _See_ Covers.

Root cutters, 340

Roots, 333, 340

Row lock bond for bricks, 312

Running water, self-purification, 373–376

Run-off, computations, 17, 40, 46–50, 94–98

Safeguards during construction, 221, 241

Salt water, dilution in, 376, 377

Sand, effective size, 456
uniformity coefficient, 456
filters, 452–459
action in, 431, 432, 452–454
control, 458, 506–510
description, 452
dimensions, 456
distribution systems, 433, 456–458
dosing, 454–456
dosing devices, 506–510
materials, 456
operation, 454, 455
preliminary treatment, 455
rate, 455
results, 452, 453
size of sand for, 456
thickness, 456
in winter, 455

Sanitary District of Chicago,
dilution factor, 380
specifications, for manhole covers, 101, 102
tunnel cover, 284
tunnel ventilation, 291

Sanitary engineering, 1, 2

Sanitary sewage, defined, 7, 352

Saph and Schoder’s formula, 54

Saprophytes, 365

Screed, 316

Screens, 383–391
chlorination and fine screens, costs, 487
coarse, 386, 391
data on fine, 388, 389
design of, 389–391
fine, 381, 382, 387–389
fixed, 385, 390
medium, 386
movable, 385, 386, 389–391
moving, 384–386
openings, 386–389
protection to pumps, 127, 141
purpose, 383
results, 386–389
sewage treatment by, 371, 381
size and performance, 386–389
sizes, 386–391
types, 384–386

Screening, vs. sedimentation, 383
purpose, object, 383

Screenings, character of, 386–389

Scum, boards for, septic tanks, 413, 414
Imhoff tanks, 421
chamber in an Imhoff tank, 424
definition, 495

Sediment, velocity of transportation, 396, 397

Sedimentation, 383–405
definition, 383
Hazen’s analysis, 392–395
hydraulic values, 393
a method of treatment, 370
object, 383
Peoria Lakes, 376
protection of siphons, 113, 114
results from plain sedimentation, 401
theory of, 391–395
transportation of debris, 396
velocity of, 392, 393
vs. screening, 383
velocities, limiting, 396, 397

Sedimentation, basins, arrangement, 394
baffling, 404
cleaning, 404
dimensions, 401–403
inlet and outlet, 404
operation, 411
types, 395
chamber, Imhoff tank, 419–422

Self-purification of lakes, 376

Self-purification of streams, 373–376

Separate sewer systems, 78–80

Septic action, 353, 365–368, 371, 410, 411, 496, 497
results, 412, 413
vs. sedimentation, 411

Septic tank, 411
baffling, 413, 414
capacities of small tanks, 417
for country homes, 416, 417
covers for, 415
definition, 411
design, 413–417
explosions in, 415
results, 412, 413
seeding, 413
sludge storage, 414
small, 416, 417
units, 415

Septic sludge analysis, 414

Septicization. Chap. XVI, 410–430
a method of treatment, 371
the process, 410, 411
results, 412, 413

Settling solids, 357

Sewage and water supply, 32
aëration, 371, 376, 465–479
alkalinity of, 358
analyses, chemical, 355, 369, 467
interpretation of, 356–362
physical, 352–354
average, 352–355
bacteria, 362–365
biolysis of, 366, 367
changes in, rate of discharge of, 33–38
characteristics, 368–370
characteristics of, 352–354
chemical constituents, 354–356
classification of, 6, 7, 352
collection, 5
color, 352, 353
components and properties, 352–356
decomposition of, 365–367
definition, 6, 7, 352
disposal. _See also_ Sewage treatment.
methods, 6, 370, 371
purposes, 370, 371
domestic, 7, 352
farming. _See_ Irrigation.
fertilizing value, 459, 460
flow fluctuations, 33–38
ratio of maximum to average, 36, 37, 85
fresh, 352–354
gas, 335, 336, 353
industrial, defined, 7, 352
life in, 363–365, 368
odor, 353
physical, analyses, 352–354
characteristics, 352–354
quality variations, 368–370
quantity. Chap. III, 24–50, and 84, 87
and population, 31, 32
of sanitary, 24–40
variations, 33–38
sanitary, defined, 7, 352
septic, 353, 365–368, 371, 410, 411, 496, 497
stability, 359, 360
stale, 353
storm, defined, 7, 352
strong, 355
temperature, 353
turbidity, 353
treatment processes, 370, 371
A. B. C., 4
activated sludge, Chap. XVIII, 465–479
biological, 371
chemical, 371
contact bed, 432–437, 506
costs, 459
dilution. Chap. XIV, 372–382
disinfection, 489–493
electrolytic, 487–489
filtration, 431–459
increase of, 3
irrigation, 431, 459–464
mechanical, 471
Miles acid process, 482–487
purpose of, 6, 370
résumé, 6, 370, 371
sand filter, 452–458
screening, 383–391
sedimentation, 391–409, 411
septicization. Chap. XVI, 410–430
trickling filters, 437–452
weak, 355
and water supplies, 31, 32

Sewerage, definition, 7
demand for, 2
design, 78–98
growth of, 2–4
historical, 2–4

Sewers, ancient, 2, 3
capacity, diagrams, 56–60
cost, 10–14
definitions of various types, 7, 8
depth of, 88
diameter, 58–60, 88–92
flat grades, 73, 109
flight, 101, 102
inspection of, 333–337
life of, 348–351
location of, 80, 81, 94
materials. Chap. VIII, 164–193
medieval, 3
pipe, properties of concrete, 175
design. Chap. IX, 194–210
vitrified clay, properties, 169–171
profile, 89, 92
section of different types, 67–72
separate system, 78, 79, 82, 86, 87
slope, 88–92
storm-water system, 78, 79, 83, 93, 94
stresses in, 194, 198–204

Shafts, for tunnels, 284–287

Sheeting, 270–280
alignment, 240, 241
backfilling, 330
box, 272
design, 275–280
driving, 273
length, 273
lumber, 277
moving, 248
poling boards, 271, 272, 287
pulling, 274
skeleton, 270, 271
stay bracing, 270
steel, 252, 280, 281
thickness, 276–278
types, 270
vertical, 270, 272–274
Wakefield piling, 273

Shellfish contamination, 372, 489

Shields, tunnel, 288–290

Short loads on trenches, 202

Shovels, for hand excavation, 242
steam. _See_ Steam shovels.

Shovel vane screen, 384

Shoveling by hand, height raised, 244
performance by one man, 243

Symbiosis, definition, 363
example, 432

Sinking fund, 158

Siphons, automatic. Chap. XXI, 506–512. _See also under_ Dosing
devices.
in flush-tanks, 109–110
inspection, 337
operation, 109–110, 506–512
for trickling filter, 448–451
true and inverted, 113–117

Skeleton sheeting, 270, 271

Slope, of sewers, 88–92
of tank bottoms, Imhoff, 419, 423
sedimentation tank, 404

Skewback, 204

Sludge. Chap. XX, 495–505
activated. Chap. XVIII, 465–479. _See also under_ Activated sludge.
analyses, 414, 467, 468, 485, 496
characteristics, 495
definition, 495
digestion tanks, 427–430, 497
disposal methods, 495
drying, 497–505
acid flotation, 503
beds, 498, 500
centrifuge, 501–502
heat, 502, 503
press, 500–501
thickeners, 504, 505
fertilizing value, 470, 495, 497

Sludge, filters, 498–500
lagooning, 495, 496
measurement, 427
press, 500, 501
sedimentation, 401
septic analysis, 434
treatment methods, 495

Soaps, 357

Soil, bearing value, 125
stack, definition, 7

Solids in sewage, 356–368

Special assessment, 15, 16

Specifications. Chap. X, 211–232
general, 219–229
special, 230
technical, 229, 230

Spiling. _See_ Piles.

Spirillum, 362

Spores, 363

Springing line, 204

Sprinkling filter. _See_ Trickling filter

Square sewer section, 68, 69

Stability, relative, 359–361

Stagnant water, 374

Stakes, contractor to provide, 221
where driven, 281, 282

Stationing, 92

Stay bracing, 270

Steam boilers, 147–150

Steam, consumption by, pumps, 144, 145
turbines, 144, 147
engines, 144, 145
pumping engines, 142–146
pumps. _See_ Pumps, steam.
shovels, 246, 252–254
turbines, 146, 147

Stearin, 366

Steel, forms. _See_ Forms, steel.
pipe, 164, 191, 192
design, 195–197
specifications, 191
reinforcement for concrete, 191, 326–327
sheet piling, 252, 280, 281

Stench, historic in London, 4

Sterilization. _See_ Disinfection.

Storm, sewage, definition, 7, 352
Storm, sewer system design, 93–98
water, quantity, 40–50

Storms, extent and intensity, 50

Stream pollution, regulation, 380, 381

Streams, self-purification, 373–376

Street, inlet. _See_ Inlets.
wash, definition, 352

Stresses, in buried pipe, 198–204
in circular ring, 194, 202–204

Sub-main, defined, 7

Subsurface surveys, 18–20

Suction for centrifugal pump, 141

Sulphur and sand joint compound, 309

Sunday work, 221

Surface, elevation, 92
of ground, character, 44–46
profile, 88
water, 7, 352

Surveys, underground, 18–20

Suspended matter, 357

Talbot’s run-off formula, 49

Tamping, backfilling, 328–331

Tannery wastes, disinfection, 491

Taxation, general, 16, 17

Taylor nozzles, 444, 445

Temperature of sewage, 353

Templates, brick sewers, 312

Thawing dynamite, 301, 302

Tide gate, 122

Timbering tunnels, 286–288

Timber, strength of, 277

Time of concentration, 41–43, 95–97

Tools, for cleaning sewers, 337–341
excavating, 242, 246

Tower cableways, 252

Trade wastes. _See_ Industrial wastes.

Traps, in catch-basins, 107
grease, gasoline, and oil, 108, 109
in street inlets, 104, 105

Travis tank, 427, 428

Tree roots, 333, 340

Tremie, 187, 188

Trench, backfilling, 328–331
blasting in, 244, 269
bottom, shape of, 241, 304, 311
breaking surface, 243, 244
drainage, 256–263
excavating, by hand, 242–245
machine, 244–256
guarding and lighting, 221
layout of tasks, 243
length of open, 241, 248
line and grade, 281–284
location, 243, 281
opening, 243, 244
pumps, 256–263
sheeting, 270–280
width, 240, 241, 246

Trestle excavators, 250, 251

Trickling filter, 437–452
advantages, 438, 439
covers for, 451
depth, 441, 442
description, 437, 438
dimensions, 442
distribution of sewage, 442–451
dosing siphon, 446–451
dosing tank, 446–451
head lost, 438
insects, 438
material, 441
nozzles, 442–451
layout, 447–451
odors, 438, 439
operation, 441
rate, 441
results, 439, 440
siphon size, 449–451
underdrainage, 451, 452
unloading, 431, 437

Tripod drill, 265

Triton, 295

Troubles with sewers, causes, 333

Trumpet arch, 121

Trunk sewer, defined, 7

Tunnels, 283–294
backfilling, 331
breast boards, 288
brick invert, 313
compressed air in, 292–294
concrete construction, 320, 321
depth of cover, 284
line and grade in, 283
machines, 290
rock, 290–292
shafts, 284–286
shield, 288–290
timbering, 284–288
ventilation, 291, 292

Turbidity of sewage, 353

Turbine, for cleaning sewers, 340
pumps, 130, 132
steam, 146, 147

Typhoid fever, 364

U-shaped sewer section, 67, 69, 71

Underdrains for, sewers, 126
trickling filters, 451, 452

Underground surveys, 18–20

Unexpected situations, 235

Uniformity coefficient of sand, 456

Unloading of filters, 431, 437

Urea, 367

Valuation of sewers, 332, 348–351

Velocities, depositing, 395–397
distribution of, 51
flow in sewers, 90
over surface of ground, 42
limiting for sedimentation, 396, 397
limiting in sewers, 396, 397
principles of flow in sewers, 51
transporting, 396

Ventilation, air pressures, 291
compressed air, 292–294
pipes, 291

Ventilation, of sewers, 102, 103, 335
tunnel, 291

Vertical sheeting, 270–274

Vitrified clay. _See_ Clay vitrified.

Volatile matter in sewage, 357

Volute pumps, 130, 132, 154

Vouissoir arch analysis, 204

Wakefield piling, 273

Wales, 288

Waste pipe, defined, 7

Wastes. _See_ Industrial wastes.

Water consumption, 31–33
flow of, 51–77
rate of steam engines, 144, 145
supply and sewage flow, 31–33

Watershed. _See_ Drainage area.

Weight, of backfill, 199
of building material, 201
of moving loads, 200, 202

Well, hole, 101
points, 262, 263

Wheel excavator, 246–250

Wing screen, 384

Wood, forms. _See_ Forms.
pipe, materials, 164, 165, 190, 192, 193
design, 197, 198
working strength of, 277

Work, extra, 227
preliminary to design, 9
Sunday, night, and holiday, 221

Workmen, competent, 227
dishonesty, 233, 234

-----

Footnote 1:

Frontinus and the Water Supply of Rome, p. 81, by Clemens Herschel.

Footnote 2:

Estimated by G. W. Fuller, Trans. Am. Society of Civil Engineers, Vol.
44, 1905, p. 148. The total population connected with sewerage systems
was assumed to be the total population in the United States in cities
over 4000 in population.

Footnote 3:

Estimated by Metcalf and Eddy, American Sewerage Practice, Vol. III,
p. 240.

Footnote 4:

Computed from report of the United States Census, 1920, on the same
basis as Fuller’s estimate for 1905.

Footnote 5:

Cosgrove, History of Sanitation.

Footnote 6:

Sedgwick: Sanitary Science and Public Health.

Footnote 7:

No detrimental effect on the public health was noted as a result of
this condition however. It has never been conclusively proven that
such nuisances are detrimental to the public health.

Footnote 8:

Moore and Silcock, Sanitary Engineering, p. 67, 1909.

Footnote 9:

Similar to the definition proposed by the Am. Public Health Assn.

Footnote 10:

Definition recommended by Am. Public Health Assn.

Footnote 11:

Ibid.

Footnote 12:

Ibid.

Footnote 13:

Eng. News, Vol. 76, 1916, p. 781. See also Eng. News-Record, Vol. 85,
1920, pp. 22, 1175.

Footnote 14:

For a more extensive treatment of the subject see Principles and
Methods of Municipal Administration by W. B. Munro, 1916.

Footnote 15:

Eng. Record, Vol. 74, 1916, p. 263.

Footnote 16:

Professional paper No. 46, United States Geological Survey, 1906, p.
97.

Footnote 17:

United States Geological Survey, Water Supply paper No. 257, 1911.

Footnote 18:

From Eng. Cont., Vol. 41, 1914, p. 698.

Footnote 19:

Max. represents only the average maximum, not the greatest maximum.

Footnote 20:

Eng. News-Record, Vol. 80, page 1233, 1918.

Footnote 21:

Infiltration of Ground Water into Sewers. Transactions of the American
Society of Civil Engineers, Vol. 76, 1913, p. 1909.

Footnote 22:

A comprehensive discussion of rainfall formulas will be found in Vol.
54 of the Transactions Am. Society of Civil Engineers, 1905.

Footnote 23:

Formula devised by H. E. Babbitt from Allen’s 25–year curve.

Footnote 24:

See Note under Table 14.

Footnote 25:

Sewerage by A. P. Folwell.

Footnote 26:

From an article by E. Kuichling in Transactions American Society of
Civil Engineers, Vol. 65, 1909, p. 399.

Footnote 27:

Trans. Am. Society Civil Engineers, Vol. 58, 1907, p. 483.

Footnote 28:

Trans. American Society of Civil Engineers, Vol. 58, 1907, p. 498.

Footnote 29:

Ibid.

Footnote 30:

The principles governing the run-off from large areas are explained in
Elements of Hydrology, by A. F. Meyer, 1917.

Footnote 31:

Transactions of the American Society of Civil Engineers, Vol. 51,
1903, p. 11.

Footnote 32:

Municipal and County Engineering, Vol. 58. 1920, p. 164.

Footnote 33:

Industrial waste Treated as ground water.

Footnote 34:

For diagrams for the Solution of the Rational Method, see Eng.
News-Record, Vol. 83, 1919, p. 868 and Vol. 85, 1920, p. 151.

Footnote 35:

Municipal and County Engineering, October, 1909.

Footnote 36:

“Cleaning and Flushing Sewers.” Journal of the Association of
Engineering Societies, Vol. 33, 1904, p. 212.

Footnote 37:

Notes on the Design and Principles of Sewage Siphons, Eng.
News-Record, Vol. 85, 1920, p. 1041.

Footnote 38:

From A. E. Phillips, Trans. Am. Society of Municipal Improvements,
1898, p. 70.

Footnote 39:

Trans. Am. Society of Civil Engineers, Vol. 15, 1886.

Footnote 40:

True Siphon at East Providence, Eng. News-Record, Vol. 85, 1920, p.
862.

Footnote 41:

“The Effect of Mouthpieces on The Flow of Water Through a Submerged
Short Pipe,” by F. B. Seely. Bulletin No. 96, 1917, of the Eng’g.
Experiment Station of the University of Illinois.

Footnote 42:

Trans. Am. Society of Civil Engineers, Vol. 49, 1902.

Footnote 43:

Described by W. L. Stevenson before the Boston Society of Civil
Engineers in 1916.

Footnote 44:

Multiple Outlet for Calumet Intercepting Sewer, by S. T. Smetters,
Eng. News-Record, Vol. 83, 1919, p. 728.

Footnote 45:

“Direct Acting Steam Pumps,” by F. R. Nickel, 1915.

Footnote 46:

From Heat Engines, by Allen and Bursley.

Footnote 47:

“The Economy Resulting from the Use of Variable Speed Induction Motors
for Driving Centrifugal Pumps” by M. L. Enger and W. J. Putnam.
Journal Am. Water Works Ass’n., 1920, Vol. 7, p. 536.

Footnote 48:

C. A. Hague in Trans. Am. Society of Civil Engineers, Vol. 74, 1911,
p. 20.

Footnote 49:

Includes screen chamber, collecting reservoir, and building.

Footnote 50:

Computed on the assumption that the pumps may be operated at 50 per
cent overload for short periods, the rated capacity being equal to the
loads given in Table 33.

Footnote 51:

For description of type see note under Table 35.

Footnote 52:

Proceedings Illinois Society of Engineers, 1916, page 81.

Footnote 53:

Municipal Engineers’ Journal for April, 1918.

Footnote 54:

Workability involves ease in placing and smoothness of working.

Footnote 55:

Johnson’s Materials of Construction, 5th Edition, 1918, p. 432.

Footnote 56:

Trans. Am. Society of Civil Engineers, Vol. 59, 1907, p. 146.

Footnote 57:

L. N. Edwards, Trans. Am. Society Testing Materials, 1918, and R. B.
Young, Eng. News-Record, Vol. 82, 1919, p. 33.

Footnote 58:

Bulletin No. 1, Structural Materials Research Laboratory, Lewis
Institute, Chicago, Illinois.

Footnote 59:

Proportioning Concrete by Voids in the Mortar, A. N. Talbot, read
before Am. Society Testing Materials, June 22, 1921. Abstract in Eng.
News-Record, Vol. 87, 1921, p. 147.

Footnote 60:

Trans. Am. Society of Civil Engineers, Vol. 81, 1917, p. 1122.

Footnote 61:

See also Tentative Specifications for Concrete and Reinforced Concrete
submitted by the Joint Committee to its Constituent Organizations,
June 4, 1921.

Footnote 62:

Journal Illinois Society of Engineers for 1916, p. 75.

Footnote 63:

See A. S. T. M. Standards for 1918, p. 148.

Footnote 64:

Trans. Am. Society Civil Engrs., Vol. 82, 1918, p. 459.

Footnote 65:

See Trans. Am. Society Civil Eng., Vol. 82, 1918, p. 482.

Footnote 66:

See Trans. Am. Society Civil Engr., Vol. 41, 1899, p. 76, and Vol. 82,
1918, p. 433, Eng. News, Vol. 74, 1915, p. 400, and Vol. 75, 1916, p.
911.

Footnote 67:

Trans. Am. Soc. Civil Engrs., Vol. 82, 1918, p. 433.

Footnote 68:

Bulletin No. 31 of the Engineering Experiment Station of the Iowa
State College of Agriculture.

Footnote 69:

From bulletin No. 31, Engineering Experiment Station, Iowa State
College of Agriculture.

Footnote 70:

From Bulletin No. 31, Engineering Experiment Station, Iowa State
College of Agriculture.

Footnote 71:

From Bulletin No. 31, Engineering Experiment Station, Iowa State
College of Agriculture.

Footnote 72:

From Vouissoir Arches by Cain.

Footnote 73:

Baker’s Masonry, 10th Edition, p. 676.

Footnote 74:

Business Law for Engineers, C. Frank Allen, McGraw-Hill, 1917;
Engineering Contracts and Specifications, J. B. Johnson, McGraw-Hill,
1904; Contracts in Engineering, J. I. Tucker, McGraw-Hill, 1910; The
Law Affecting Engineers, W. V. Ball, Archibald Constable, 1909; Law
and Business of Engineering and Contracting, C. E. Fowler,
McGraw-Hill, 1909; The Economics of Contracting, D. J. Hauer, E. H.
Baumgartner, 1915; The Elements of Specification Writing, R. S. Kirby,
John Wiley & Son, 1913; Contracts, Specifications and Engineering
Relations, D. W. Mead, McGraw-Hill, 1916; Engineering and
Architectural Jurisprudence, J. C. Wait, John Wiley, 1912.

Footnote 75:

See article by E. W. Bush in Eng. News-Record, Vol. 85, 1920, p. 122.

Footnote 76:

An unbalanced proposal is one in which the bids on some of the items
are obviously low and on other items are obviously or suspiciously
high. The purpose of submitting unbalanced bids is to keep secret the
true or supposed cost of the work to the contractor or to obtain more
money by bidding high on those items which are believed to have been
underestimated by the Engineer. A low bid is made on other items in
order to keep down the total amount of the bid.

Footnote 77:

Taken mainly from specifications of the Sanitary District of Chicago
and the Baltimore Sewerage Commission, with miscellaneous selections
from other sources.

Footnote 78:

Restrictions are placed on work done outside of ordinary working hours
in order that the Contractor may not perform work in the absence of an
engineer or inspector.

Footnote 79:

Cost Keeping and Management, by Gillette and Dana. Practical Cost
Keeping for Contractors, by F. R. Walker. Cost Keeping in Sewer Work,
by K. O. Guthrie in Eng. Contracting, Vol. 28, p. 238, 1905. Sewer
Construction Records at Scarsdale, Eng. News-Record, Vol. 83, p. 111,
1919.

Footnote 80:

See Planning and Progress on a Big Construction Job, by Chas. Penrose,
Eng. News-Record, Vol. 84, 1920, pp. 554 and 627.

Footnote 81:

See also “Ownership and Operation of Trench Excavators by the Water
Department of Baltimore,” by V. B. Seims, presented before Am. Water
Works Association, June 9, 1921.

Footnote 82:

Eng. and Contracting, Vol. 48, 1917, p. 492.

Footnote 83:

Earth Excavation by A. B. McDaniel.

Footnote 84:

Courtesy, Sanitary District of Chicago.

Footnote 85:

See article by J. R. Gow, Journal New England Waterworks Ass’n, Sept.,
1920, also Public Works, Vol. 50, p. 98.

Footnote 86:

Diameter of diaphragm.

Footnote 87:

Gallons per minute.

Footnote 88:

Eng. News, Vol. 75, 1916 p. 1050.

Footnote 89:

Mun. Engineering, Vol. 53, p. 6.

Footnote 90:

For types of drill bits see article by T. H. Proske, Mining and
Scientific Press, March 5, 1910.

Footnote 91:

These intermediate holes are seldom more than 3 feet apart.

Footnote 92:

Earth Pressures, Old Theories and New Test Results, Eng. News-Record,
Vol. 85, 1920, p. 632.

Footnote 93:

Trans. Am. Society Civil Eng’rs, Vol. 60, 1908.

Footnote 94:

Adopted by the Am. Ry. and Maintenance of Way Ass’n in 1907.

Footnote 95:

Tunneling Machines Successful on Detroit Sewers, Eng. News-Record,
Vol. 84, 1920, p. 329.

Footnote 96:

Rules on Compressed-Air Work of N. Y. State Industrial Commission,
Eng. News-Record, Vol. 85, 1920, p. 1225.

Footnote 97:

Taken mainly from the Engineer Field Manual of the U. S. Army; Safety
Factors in the Use of Explosives by W. O. Snelling, Technical Paper
No. 18, U. S. Bureau of Mines; and an article in Eng’g and
Contracting, Vol. 52, 1919, p. 585.

Footnote 98:

See paper by C. T. Hall before Am. Inst. Chemical Engineers.

Footnote 99:

Per cubic yard of material displaced.

Footnote 100:

Eng. News, Vol. 75, 1916, p. 592.

Footnote 101:

Pressure of Concrete on Forms Measured in Tests, by E. B. Smith,
before Am. Concrete Institute, Feb. 15, 1920. Abstracted in Eng.
News-Record, Vol. 84, 1920, p. 665.

Footnote 102:

See, also, Concrete Form Design, by E. F. Rockwood, Eng. and
Contracting, Vol. 55, 1921, p. 528.

Footnote 103:

Includes 6 cents per foot for excavation. Labor for this was 58 per
cent of the total labor cost.

Footnote 104:

Cement at $1.25 per barrel.

Footnote 105:

Mun. Journal, Vol. 36, 1914, p. 736.

Footnote 106:

Mun. Journal, Vol. 39, 1915, p. 911.

Footnote 107:

Formerly the Municipal Journal.

Footnote 108:

See Eng. Record, Vol. 75, 1917, p. 463.

Footnote 109:

Eng. Record, Vol. 73, 1916, p. 141, and Eng. News-Record, Vol. 79,
1917, p. 1019.

Footnote 110:

Eng. Record, Vol. 72, 1915, p. 690.

Footnote 111:

Eng. Record, Vol. 71, 1915, p. 256.

Footnote 112:

Eng. and Contr., Vol. 41, 1914, p. 250.

Footnote 113:

H. J. Kellogg in Journal Connecticut Society of Civil Engineers, 1914,
and Technical Paper 117, U. S. Bureau of Mines.

Footnote 114:

Eng. News, Vol. 70, 1913, p. 1157.

Footnote 115:

Technical Paper No. 117, U. S. Bureau of Mines.

Footnote 116:

Eng. News, Vol. 71, 1914, p. 84.

Footnote 117:

Eng. News, Vol. 71, 1914, p. 82.

Footnote 118:

Similar to definition proposed by the Am. Public Health Ass’n.

Footnote 119:

Economic Values in Sewage and Sewage Sludge, by Raymond Wells,
Proceedings Am. Society Municipal Improvements, Nov. 12, 1919. Eng.
News-Record, Vol. 83, 1919, p. 948.

Footnote 120:

Sample boiled for five minutes.

Footnote 121:

Sample immersed in boiling water for 30 minutes.

Footnote 122:

Four months.

Footnote 123:

One week in March, 1914.

Footnote 124:

R represents any chemical element such as K, Na, etc.

Footnote 125:

Standard Methods of Water Analysis, American Public Health
Association, 1920.

Footnote 126:

Routine tests are ordinarily incubated for this period only, and if
not decolorized in this time are recorded as stable.

Footnote 127:

Determination of the Biochemical Oxygen Demand of Sewage and
Industrial Wastes, by E. J. Theriault, Report of the U. S. Public
Health Service, Vol. 35, May 7, 1920, No. 19, p. 1087.

Footnote 128:

Standard Methods of Water Analysis, American Public Health
Association, 1920.

Footnote 129:

Jordan, General Bacteriology, 1909, p. 91.

Footnote 130:

Ibid.

Footnote 131:

Reprinted in Vol. III of Contributions from the Sanitary Research
Laboratory of Massachusetts Institute of Technology.

Footnote 132:

Formerly Chief Engineer of the Sanitary District of Chicago.

Footnote 133:

From “Sewage,” by Samuel Rideal, 1900, p. 16.

Footnote 134:

See Am. Civil Engineers’ Pocket Book, Second Edition, p. 982.

Footnote 135:

Trans. Am. Society Civil Engineers, Vol. 58, 1907, p. 988.

Footnote 136:

Not defined by the American Public Health Association.

Footnote 137:

Trans. Am. Society Civil Engineers, Vol. 78, 1915, p. 892.

Footnote 138:

Removal of Suspended Matter by Sewage Screens, Cornell Civil Engineer,
1914. Abstracted in Engineering and Contracting, Vol. 41, 1914, p.
451.

Footnote 139:

“The Clarification of Sewage by Fine Screens,” Trans. Am. Society
Civil Engineers, Vol. 78, 1915, p. 1000.

Footnote 140:

Langdon Pearse, Trans. Am. Society Civil Engineers, Vol. 78, 1915, p.
1000.

Footnote 141:

Meshes per inch.

Footnote 142:

See article by Henry Ryon in Cornell Civil Engineer, 1910.

Footnote 143:

The hydraulic coefficient is defined as the rate of settling in mm.
per second.

Footnote 144:

Definition suggested by the American Public Health Association.

Footnote 145:

Computed from formula by Gilbert in “Transportation of Debris by
Running Water,” U. S. Geological Survey, Professional Paper No. 86,
1914. Diameter in mm. = (1.28 (velocity)^{2.7})⁄(Sp. gv. − 1).

Footnote 146:

Computed from Annual Report of the Superintendent of Sewers, Nov. 30,
1919, and 1920.

Footnote 147:

These figures are for 1919.

Footnote 148:

These figures are for 1905.

Footnote 149:

These figures are for 1902.

Footnote 150:

Report of the Ohio State Board of Health, 1908, page 425.

Footnote 151:

Definition proposed by the Am. Public Health Assn.

Footnote 152:

See Eng. News. Vol. 73, 1915, p. 410.

Footnote 153:

Sewage Treatment from Single Houses and Small Communities, by L. C.
Frank. U. S. Public Health Service, Bulletin 101, 1920.

Footnote 154:

Eng. News-Record, Vol. 78, 1917, p. 566.

Footnote 155:

Municipal Engineering, Vol. 54, p. 149.

Footnote 156:

Eng. Record, Vol. 68, 1913, p. 452.

Footnote 157:

Am. Sewerage Practice, Vol. III, p. 437.

Footnote 158:

Trans. Am. Society Civil Engineers, Vol. 83, 1920, p. 337.

Footnote 159:

Eng. News-Record, Vol. 83, 1919, p. 510.

Footnote 160:

See Eng. News, Vol. 70, 1913, p. 1112; Eng. Record, Vol. 68, 1913, p.
440, and Eng. News, Vol. 75, 1916, p. 1028.

Footnote 161:

See Eng. Record, Vol. 67, 1913, p. 232.

Footnote 162:

The use of half-spray nozzles is not always advocated as it is
considered that their use does not markedly improve the distribution.
Where half nozzles are used, a margin of 18 inches to 2 feet should be
allowed between the edge of the filter and the nozzle, to prevent the
blowing of raw sewage from the filter.

Footnote 163:

From paper by E. G. Bradbury in Proceedings of the Ohio Eng. Society,
1910, p. 79.

Footnote 164:

The effective size of sand is the diameter in millimeters of the
largest grain in that 10 per cent, by weight, of the material which
contains the smallest grains.

Footnote 165:

The uniformity coefficient is the ratio of the diameter of the largest
particle of the smallest 60 per cent, by weight, to the effective
size.

Footnote 166:

Interest at 6 per cent.

Footnote 167:

Worcester figures.

Footnote 168:

This method may show a profit from the sale of sludge.

Footnote 169:

Sewage Disposal, 1919, p. 223.

Footnote 170:

See Eng. News, Vol. 9, 1883, p. 203, and Vol. 29, 1893, p. 27.

Footnote 171:

American Sewerage Practice, Vol. III.

Footnote 172:

Reference 11, at end of this chapter.

Footnote 173:

Reference 15.

Footnote 174:

Reference 2.

Footnote 175:

For mechanical methods of drying sludge, see Reference 22, p. 1127,
and No. 33, p. 843.

Footnote 176:

Reference 10.

Footnote 177:

Reference 13.

Footnote 178:

University of California, Bulletin 251, 1915.

Footnote 179:

Reference 25.

Footnote 180:

See Report by Black & Phelps of Metropolitan Sewerage Commission,
1911, reprinted as Vol. VII of Contributions from the Sanitary
Research Laboratory of the Massachusetts Institute of Technology.

Footnote 181:

See Reports, Mass. State Board of Health.

Footnote 182:

Reference 47.

Footnote 183:

Reference 10.

Footnote 184:

Reference 10.

Footnote 185:

Reference 10.

Footnote 186:

Hatton, reference 33.

Footnote 187:

Reference 18.

Footnote 188:

Reference 1, at end of this chapter.

Footnote 189:

Reference 2.

Footnote 190:

Reference 6.

Footnote 191:

Reference 5.

Footnote 192:

Reference 6.

Footnote 193:

Reference 6.

Footnote 194:

Reference 8.

Footnote 195:

Reference 20.

Footnote 196:

Reference 17.

Footnote 197:

Reference 19.

Footnote 198:

Reference 21.

Footnote 199:

Reference 24.

Footnote 200:

Inorganic Chemistry, by Alexander Smith.

Footnote 201:

American Public Health Association definition.

Footnote 202:

Sewage Sludge by Allen.

Footnote 203:

Sewage Disposal by Kinnicutt, Winslow and Pratt.

Footnote 204:

Sewage Disposal by Fuller.

Footnote 205:

Sewage Sludge by Allen.

Footnote 206:

From Eng. News-Record, Vol. 84, 1920, p. 995.

Footnote 207:

A Simple Mechanical Control for Dosing Sewage Beds, by P. Thompson,
Eng. News-Record, Vol. 84, 1920, p. 1018.

Footnote 208:

Sewage Disposal by Kinnicutt, Winslow and Pratt.

Footnote 209:

Design of Siphon by G. H. Bayles, Eng. News-Record, Vol. 84, 1920, p.
974.

------------------------------------------------------------------------

TRANSCRIBER’S NOTES

1. Silently corrected typographical errors and variations in spelling.
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