The filtration of public water-supplies: Third edition, revised and enlarged.

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Title: The filtration of public water-supplies: Third edition, revised and enlarged.
Author: Hazen, Allen
Language: English
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*** Start of this LibraryBlog Digital Book "The filtration of public water-supplies: Third edition, revised and enlarged." ***
WATER-SUPPLIES ***

Transcriber’s Notes:

The spelling, hyphenation, punctuation and accentuation are as the
original, except for apparent typographical errors which have been
corrected.

Italic text is denoted _thus_.
Bold text is denoted =thus=.

See further notes at the end of the book.

[Illustration: GENERAL VIEW OF FILTERS AT HAMBURG.

[_Frontispiece._]
]

THE FILTRATION

PUBLIC WATER-SUPPLIES.

BY
ALLEN HAZEN,

MEMBER OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS, THE BOSTON SOCIETY
OF CIVIL ENGINEERS, THE AMERICAN WATER-WORKS ASSOCIATION, THE NEW
ENGLAND WATER-WORKS ASSOCIATION, THE AMERICAN CHEMICAL SOCIETY, THE
AMERICAN PUBLIC HEALTH ASSOCIATION, ETC.

_THIRD EDITION, REVISED AND ENLARGED._
SECOND THOUSAND.

NEW YORK:
JOHN WILEY & SONS.
LONDON: CHAPMAN & HALL, LIMITED.
1905.

ROBERT DRUMMOND, ELECTROTYPER AND PRINTER, NEW YORK.

PREFACE TO FIRST EDITION.

The subject of water-filtration is commencing to receive a great deal
of attention in the United States. The more densely populated European
countries were forced to adopt filtration many years ago, to prevent
the evils arising from the unavoidable contaminations of the rivers
and lakes which were the only available sources for their public
water-supplies; and it has been found to answer its purpose so well
that at the present time cities in Europe nearly if not quite equal in
population to all the cities of the United States are supplied with
filtered water.

Many years ago, when the whole subject of water-supply was still
comparatively new in this country, filtration was considered as a means
for rendering the waters of our rivers suitable for the purpose of
domestic water-supply. St. Louis investigated this subject in 1866,
and the engineer of the St. Louis Water Board, the late Mr. J. P.
Kirkwood, made an investigation and report upon European methods of
filtration which was published in 1869, and was such a model of full
and accurate statement combined with clearly-drawn conclusions that, up
to the present time, it has remained the only treatise upon the subject
in English, notwithstanding the great advances which have been made,
particularly in the last ten years, with the aid of knowledge of the
bacteria and the germs of certain diseases in water.

Unfortunately the interest in the subject was not maintained in
America, but was allowed to lag for many years; it was cheaper to use
the water in its raw state than it was to purify it; the people became
indifferent to the danger of such use, and the disastrous epidemics
of cholera and typhoid fever, as well as of minor diseases, which so
often resulted from the use of polluted water, were attributed to other
causes. With increasing study and diffusion of knowledge the relations
of water and disease are becoming better known, and the present state
of things will not be allowed to continue; indeed at present there is
inquiry at every hand as to the methods of improving waters.

The one unfortunate feature is the question of cost. Not that the cost
of filtration is excessive or beyond the means of American communities;
in point of fact, exactly the reverse is the case; but we have been so
long accustomed to obtain drinking-water without expense other than
pumping that any cost tending to improved quality seems excessive, thus
affording a chance for the installation of inferior filters, which by
failing to produce the promised results tend to bring the whole process
into disrepute, since few people can distinguish between an adequate
filtration and a poor substitute for it. It is undoubtedly true that
improvements are made, and will continue to be made, in processes of
filtration; so it will often be possible to reduce the expense of the
process without decreasing the efficiency, but great care must be
exercised in such cases to maintain the conditions really essential to
success.

In the present volume I have endeavored to explain briefly the nature
of filtration and the conditions which, in half a century of European
practice, have been found essential for successful practice, with a
view of stimulating interest in the subject, and of preventing the
unfortunate and disappointing results which so easily result from the
construction of inferior filters. The economies which may possibly
result by the use of an inferior filtration are comparatively small,
and it is believed that in those American cities where filtration is
necessary or desirable it will be found best in every case to furnish
filters of the best construction, fully able to do what is required of
them with ease and certainty.

PREFACE TO THIRD EDITION.

There have been several distinct epochs in the development of water
purification in the United States. The first may be said to date
from Kirkwood’s report on the “Filtration of River Waters,” and the
second from the inauguration of the Lawrence Experiment Station by
the Massachusetts State Board of Health, and the construction of
the Lawrence city filter, with the demonstration of the wonderful
biological action of filters upon highly polluted waters.

The third epoch is marked by the experiments at Louisville, Pittsburg
and Cincinnati, which have greatly increased our knowledge of the
treatment of waters containing enormous quantities of suspended matter,
and have reduced to something like order the previously existing
confused mass of data regarding coagulation and rapid filtration.

The first edition of this book represented the earlier epochs
before the opening of the third. In the five years since it was
written, progress in the art of water purification has been rapid
and substantial. No apology is needed for the very complete revision
required to treat these newly investigated subjects as fully as were
other matters in the earlier editions.

In the present edition the first seven chapters remain with but few
additions. Experience has strengthened the propositions contained
in them. New data might have been added, but in few cases would the
conclusions have been altered. The remaining chapters of the book have
been entirely rewritten and enlarged to represent the added information
now available, so that the present edition is nearly twice as large as
the earlier ones. In the appendices, also, much matter has been added
relating to works in operation, particularly to those in America.

NEW YORK January, 1900.

CONTENTS.

PAGE

Chapter I. INTRODUCTION. 1
II. CONTINUOUS FILTERS AND THEIR CONSTRUCTION 5
Sedimentation-basins 8
Size of Filter-beds 10
Form of Filter-beds 11
Covers for Filters 12
III. FILTERING-MATERIALS 20
Sand 20
Sands Used in European Filters 24
Effect of Size of Grain Upon Efficiency of Filtration 30
Effect of Grain Size Upon Frequency of Scraping 32
Selection of Sand 33
Thickness of the Sand Layer 34
Underdraining 35
Gravel Layers 35
Underdrains 39
Depth of Water on Filters 45
IV. RATE OF FILTRATION AND LOSS OF HEAD 47
Effect of Rate Upon Cost of Filtration 48
Effect of Rate Upon Efficiency of Filtration 50
The Loss of Head 52
Regulation of the Rate and Loss of Head in the
Older Filters 52
Apparatus For Regulating the Rate and Loss of Head 55
Apparatus For Regulating the Rate Directly 57
Apparatus For Regulating the Height of Water Upon
Filters 59
Limit to the Loss of Head 60
V. CLEANING FILTERS 68
Frequency of Scraping 72
Quantity of Sand to Be Removed 74
Wasting the Effluents After Scraping 74
Sand-washing 76
VI. THEORY AND EFFICIENCY OF FILTRATION 83
Bacterial Examination of Waters 93
VII. INTERMITTENT FILTRATION 97
The Lawrence Filter 100
Chemnitz Water-Works 107
Application of Intermittent Filtration 111
VIII. TURBIDITY AND COLOR, AND THE EFFECT OF MUD UPON
SAND FILTERS 113
The Measurement of Color 114
Amount of Color in American Waters 115
Removal of Color 117
Measurement of Turbidity 117
Relation of Platinum-wire Turbidities to Suspended
Matters 122
Source of Turbidity 123
The Amounts of Suspended Matters in Water 129
Preliminary Processes to remove Mud 133
Effect of Mud upon Sand Filters 137
Effect of Turbidity Upon the Length of Period 137
Power of Sand Filters to Produce Clear Effluents
from Muddy Water 139
Effect of Mud Upon Bacterial Efficiency of Filters 141
Limits to the Use of Subsidence for the Preliminary
Treatment of Muddy Waters 142
IX. COAGULATION OF WATERS 144
Substances used for Coagulation 145
Coagulants Which Have Been Used 150
Amount of Coagulant required to remove Turbidity 150
Amount of Coagulant required to remove Color 153
Successive Applications of Coagulant 154
The Amount of Coagulant which Various Waters will
receive 155
X. MECHANICAL FILTERS 159
Providence Experiments 159
Louisville_Experiments 161
Lorain Tests 161
Pittsburg Experiments 162
Wasting Effluent After Washing Filters 163
Influence of Amount of Sulphate of Alumina on
Bacterial Efficiency of Mechanical Filters 165
Influence of Degree of Turbidity upon Bacterial
Efficiency of Mechanical Filters 167
Average Results Obtained with Various Quantities of
Sulphate of Alumina 171
Types of Mechanical Filters 172
Efficiency of Mechanical Filters 179
Pressure Filters 180
XI. OTHER METHODS OF FILTRATION 181
Worms Tile System 181
The Use of Asbestos 181
Filters Using High Rates of Filtration Without
Coagulants 182
Household Filters 183
XII. REMOVAL OF IRON FROM GROUND-WATERS 186
Amount of Iron Required to Render Water Objectionable 186
Cause of Iron in Ground-waters 187
Treatment of Iron-containing Waters 189
Iron-removal Plants in Operation 192
XIII. TREATMENT OF WATERS 197
Cost of Filtration 200
What Waters Require Filtration 207
XIV. WATER-SUPPLY AND DISEASE—CONCLUSIONS 210
Appendix I. RULES OF THE GERMAN GOVERNMENT IN REGARD TO THE
FILTRATION OF SURFACE-WATERS USED FOR PUBLIC
WATER-SUPPLIES 221
II. EXTRACTS FROM “BERICHT DES MEDICINAL-INSPECTORATS
DES HAMBURGISCHEN STAATES FÜR DAS JAHR 1892” 226
III. METHODS OF SAND-ANALYSIS 233
IV. FILTER STATISTICS 241
Statistics of Operation of Sand Filters 241
Partial List of Cities Using Sand Filters 244
List of Cities and Towns Using Mechanical Filters 247
Notes Regarding Sand Filters in the United States 251
Capacity of Filters 254
V. LONDON’S WATER-SUPPLY 255
VI. THE BERLIN WATER-WORKS 261
VII. ALTONA WATER-WORKS 265
VIII. HAMBURG WATER-WORKS 269
IX. NOTES ON SOME OTHER EUROPEAN WATER-SUPPLIES 272
The Use of Unfiltered Surface-waters. 275
The Use of Ground-water. 276
X. LITERATURE OF FILTRATION 277
XI. THE ALBANY WATER-FILTRATION PLANT 288
Description of Plant. 289
Capacity of Plant and Means of Regulation. 308
Results of Operation. 314
Cost of Construction. 314
INDEX 317

UNITS EMPLOYED.

The units used in this work are uniformly those in common use in
America, with the single exception of data in regard to sand-grain
sizes, which are given in millimeters. The American units were not
selected because the author prefers them or considers them particularly
well suited to filtration, but because he feared that the use of the
more convenient metric units in which the very comprehensive records
of Continental filter plants are kept would add to the difficulty of
a clear comprehension of the subject by those not familiar with those
units, and so in a measure defeat the object of the book.

TABLE OF EQUIVALENTS.

Unit. Metric Equivalent. Reciprocal.
Foot 0.3048 meter 3.2808
Mile 1609.34 meters 0.0006214
Acre 4047 square meters 0.0002471
Gallon[1] 3.785 liters 0.26417
1 million gallons 3785 cubic meters 0.00026417
Cubic yard 0.7645 cubic meters 1.308
1 million gallons per } { meter in depth }
acre daily } 0.9354 { of water daily } 1.070

ACKNOWLEDGMENT.

I wish to acknowledge my deep obligation to the large number of
European engineers, directors, and superintendents of water-works, and
to the health officers, chemists, bacteriologists, and other officials
who have kindly aided me in studying the filtration-works in their
respective cities, and who have repeatedly furnished me with valuable
information, statistics, plans, and reports.

To mention all of them would be impossible, but I wish particularly to
mention Major-General Scott, Water-examiner of London; Mr. Mansergh,
Member of the Royal Commission on the Water-supply of the Metropolis;
Mr. Bryan, Engineer of the East London Water Company; and Mr. Wilson,
Manager of the Middlesborough Water-works, who have favored me with
much valuable information.

In Holland and Belgium I am under special obligations to Messrs. Van
Hasselt and Kemna, Directors of the water companies at Amsterdam and
Antwerp respectively; to Director Stang of the Hague Water-works; to
Dr. Van’t Hoff, Superintendent of the Rotterdam filters; and to my
friend H. P. N. Halbertsma, who, as consulting engineer, has built many
of the Dutch water-works.

In Germany I must mention Profs. Frühling, at Dresden, and Flügge, at
Breslau; Andreas Meyer, City Engineer of Hamburg; and the Directors of
water-works, Beer at Berlin, Dieckmann at Magdeburg, Nau at Chemnitz,
and Jockmann at Liegnitz, as well as the Superintendent Engineers
Schroeder at Hamburg, Debusmann at Breslau, and Anklamm and Piefke at
Berlin, the latter the distinguished head of the Stralau works, the
first and most widely known upon the Continent of Europe.

I have to acknowledge my obligation to City Engineer Sechner at
Budapest, and to the Assistant Engineer in charge of water-works,
Kajlinger; to City Engineer Peters and City Chemist Bertschinger at
Zürich; and to Assistant Engineer Regnard of the Compagnie Générale des
Eaux at Paris.

On this side of the Atlantic also I am indebted to Hiram F. Mills,
C.E., under whose direction I had the privilege of conducting
for nearly five years the Lawrence experiments on filtration; to
Profs. Sedgwick and Drown for the numerous suggestions and friendly
criticisms, and to the latter for kindly reading the proof of this
volume; to Mr. G. W. Fuller for full information in regard to the
more recent Lawrence results; to Mr. H. W. Clark for the laborious
examination of the large number of samples of sands used in actual
filters and mentioned in this volume; and to Mr. Desmond FitzGerald
for unpublished information in regard to the results of his valuable
experiments on filtration at the Chestnut Hill Reservoir, Boston.

ALLEN HAZEN.

BOSTON, April, 1895.

FILTRATION OF PUBLIC WATER-SUPPLIES.

CHAPTER I.

INTRODUCTION.

The rapid and enormous development and extension of water-works in
every civilized country during the past forty years is a matter which
deserves our most careful consideration, as there is hardly a subject
which more directly affects the health and happiness of almost every
single inhabitant of all cities and large towns.

Considering the modern methods of communication, and the free exchange
of ideas between nations, it is really marvellous how each country has
met its problems of water-supply from its own resources, and often
without much regard to the methods which had been found most useful
elsewhere. England has secured a whole series of magnificent supplies
by impounding the waters of small streams in reservoirs holding enough
water to last through dry periods, while on Continental Europe such
supplies are hardly known. Germany has spent millions upon millions in
purifying turbid and polluted river-waters, while France and Austria
have striven for mountain-spring waters and have built hundreds of
miles of costly aqueducts to secure them. In the United States an
abundant supply of some liquid has too often been the objective point,
and the efforts have been most successful, the American works being
entirely unrivalled in the volumes of their supplies. I do not wish
to imply that quality has been entirely neglected in our country, for
many cities and towns have seriously and successfully studied their
problems, with the result that there are hundreds of water-supplies
in the United States which will compare favorably upon any basis with
supplies in any part of the world; but on the other hand it is equally
true that there are hundreds of other cities, including some among
the largest in the country, which supply their citizens with turbid
and unhealthy waters which cannot be regarded as anything else than a
national disgrace and a menace to our prosperity.

One can travel through England, Belgium, Holland, Germany, and large
portions of other European countries and drink the water at every city
visited without anxiety as to its effect upon his health. It has not
always been so. Formerly European capitals drank water no better than
that so often dispensed now in America. As recently as 1892 Germany’s
great commercial centre, Hamburg, having a water-supply essentially
like those of Philadelphia, Pittsburg, Cincinnati, St. Louis, New
Orleans, and a hundred other American cities, paid a penalty in one
month of eight thousand lives for its carelessness. The lesson was a
dear one, but it was not wasted. Hamburg now has a new and wholesome
supply, and other German cities the qualities of whose waters were open
to question have been forced to take active measures to better their
conditions. We also can learn something from their experience.

There are three principal methods of securing a good water-supply for a
large city. The first consists of damming a stream from an uninhabited
or but sparsely inhabited watershed, thus forming an impounding
reservoir. This method is extensively used in England and in the United
States. In the latter most of the really good and large supplies are so
obtained. It is only applicable to places having suitable watersheds
within a reasonable distance, and there are large regions where, owing
to geological and other conditions, it cannot be applied. It is most
useful in hilly and poor farming countries, as in parts of England and
Wales, in the Atlantic States, and in California. It cannot be used to
any considerable extent in level and fertile countries which are sure
to be or to become densely populated, as is the case with large parts
of France and Germany and in the Middle States.

The second method is to secure ground-water, that is, spring or well
water, which by its passage through the ground has become thoroughly
purified from any impurities which it may have contained. This was the
earliest and is the most widely used method of securing good water.
It is specially adapted to small supplies. Under favorable geological
conditions very large supplies have been obtained in this manner. In
Europe Paris, Vienna, Budapest, Munich, Cologne, Leipzig, Dresden, a
part of London, and very many smaller places are so supplied. This
method is also extensively used in the United States for small and
medium-sized places, and deserves to be most carefully studied, and
used whenever possible, but is unfortunately limited by geological
conditions and cannot be used except in a fraction of the cases where
supplies are required. No ground-water supplies yet developed in the
United States are comparable in size to those used in Europe.

The third process of securing a good water-supply is by means of
filtration of surface waters which would otherwise be unsuitable for
domestic purposes. The methods of filtration, which it is the purpose
of this volume to explain, are beyond the experimental stage; they
are now applied to the purification of the water-supplies of European
cities with an aggregate population of at least 20,000,000 people. In
the United States the use of filters is much less common, and most of
the filters in use are of comparatively recent installation.

Great interest has been shown in the subject during the last few
years, and the peculiar character of some American waters, which differ
widely in their properties from those of many European streams, has
received careful and exhaustive consideration. In Europe filtration has
been practised with continually improving methods since 1829, and the
process has steadily received wider and wider application. It has been
most searchingly investigated in its hygienic relations, and has been
repeatedly found to be a most valuable aid in reducing mortality. The
conditions under which satisfactory results can be obtained are now
tolerably well known, so that filters can be built in the United States
with the utmost confidence that the result will not be disappointing.

The cost of filtration, although considerable, is not so great as to
put it beyond the reach of American cities. It may be roughly estimated
that the cost of filtration, with all necessary interest and sinking
funds, will add 10 per cent to the average cost of water as at present
supplied.

It may be confidently expected that when the facts are better
understood and realized by the American public, we shall abandon the
present filthy and unhealthy habit of drinking polluted river and
lake waters, and shall put the quality as well as the quantity of our
supplies upon a level not exceeded by those of any country.

CHAPTER II.

CONTINUOUS FILTERS AND THEIR CONSTRUCTION.

Filtration of water consists in passing it through some substance
which retains or removes some of its impurities. In its simplest form
filtration is a straining process, and the results obtained depend upon
the fineness of the strainer, and this in turn is regulated by the
character of the water and the uses to which it is to be put. Thus in
the manufacture of paper an enormous volume of water is required free
from particles which, if they should become imbedded in the paper,
would injure its appearance or texture. Obviously for this purpose the
removal of the smaller particles separately invisible to the unaided
eye, and thus not affecting the appearance of the paper, and the
removal of which would require the use of a finer filter at increased
expense, would be a simple waste of money. When, however, a water is
to be used for a domestic water supply and transparency is an object,
the still finer particles which would not show themselves in paper, but
which are still able, in bulk, to render a water turbid, should be as
far as possible removed, thus necessitating a finer filter; and, when
there is reason to think that the water contains the germs of disease,
the filter must be fine enough to remove with certainty those organisms
so extraordinarily small that millions of them may exist in a glass of
water without imparting a visible turbidity.

It is now something over half a century since the first successful
attempts were made to filter public water-supplies, and there are
now hundreds of cities supplied with clear, healthy, filtered water.
(Appendix IV.) While the details of the filters used in different
places present considerable variations, the general form is, in
Europe at least, everywhere the same. The most important parts of a
filter are shown by the accompanying sketch, in which the dimensions
are much exaggerated. The raw water is taken from the river into a
settling-basin, where the heaviest mud is allowed to settle. In the
case of lake and pond waters the settling-tank is dispensed with, but
it is essential for turbid river-water, as otherwise the mud clogs
the filter too rapidly. The partially clarified water then passes to
the filter, which consists of a horizontal layer of rather fine sand
supported by gravel and underdrained, the whole being enclosed in a
suitable basin or tank. The water in passing through the sand leaves
behind upon the sand grains the extremely small particles which were
too fine to settle out in the settling-basin, and is quite clear as it
goes from the gravel to the drains and the pumps, which forward it to
the reservoir or city.

[Illustration: FIG. 1.—SKETCH SHOWING GENERAL ARRANGEMENT OF FILTER
PLANTS.]

The passages between the grains of sand through which the water must
pass are extremely small. If the sand grains were spherical and 1/50 of
an inch in diameter, the openings would only allow the passage of other
spheres 1/320 of an inch in diameter, and with actual irregular sands
much finer particles are held back. As a result the coarser matters
in the water are retained on the surface of the sand, where they
quickly form a layer of sediment, which itself becomes a filter much
finer than the sand alone, and which is capable of holding back under
suitable conditions even the bacteria of the passing water. The water
which passes before this takes place may be less perfectly filtered,
but even then, the filter may be so operated that nearly all of the
bacteria will be deposited in the sand and not allowed to pass through
into the effluent.

As the sediment layer increases in thickness with continued filtration,
increased pressure is required to drive the desired volume of water
through its pores, which are ever becoming smaller and reduced in
number. When the required quantity of water will no longer pass with
the maximum pressure allowed, it is necessary to remove, by scraping,
the sediment layer, which should not be more than an inch deep. This
layer contains most of the sediment, and the remaining sand will then
act almost as new sand would do. The sand removed may be washed for use
again, and eventually replaced when the sand layer becomes too thin
by repeated scrapings. These operations require that the filter shall
be temporarily out of use, and as water must in general be supplied
without intermission, a number of filters are built together, so that
any of them can be shut out without interfering with the action of the
others.

The arrangement of filters in relation to the pumps varies with local
conditions. With gravity supplies the filters are usually located below
the storage reservoir, and, properly placed, involve only a few feet
loss of head.

In the case of tidal rivers, as at Antwerp and Rotterdam, the quality
of the raw water varies with the tide, and there is a great advantage
in having the settling-basins low enough so that a whole day’s supply
can be rapidly let in when the water is at its best, without pumping.
At Antwerp the filters are higher, and the water is pumped from the
settling basins to them, and again from the reservoir receiving the
effluents from the filters to the city. In several of the London
works (East London, Grand Junction, Southwark and Vauxhall, etc.) the
settling-basins are lower than the river, and the filters are still
lower, so that a single pumping suffices, that coming between the
filter and the city, or elevated distributing reservoir.

In many other English filters and in most German works the
settling-basins and filters are placed together a little higher than
the river, thus avoiding at once trouble from floods and cost for
excavation. The water requires to be pumped twice, once before and once
after filtration. At Altona the settling-basins and filters are placed
upon a hill, to which the raw Elbe water is pumped, and from which it
is supplied to the city after filtration by gravity without further
pumping. The location of the works in this case is said to have been
determined by the location of a bed of sand suitable for filtration on
the spot where the filters were built.

When two pumpings are required they are frequently done, especially in
the smaller places, in the same pumping-station, with but one set of
boilers and engines, the two pumps being connected to the same engine.
The cost is said to be only slightly greater than that of a single
lift of the same total height. In very large works, as at Berlin and
Hamburg and some of the London companies, two separate sets of pumping
machinery involve less extra cost relatively than would be the case
with smaller works.

SEDIMENTATION-BASINS.

Kirkwood[2] found in 1866 that sedimentation-basins were essential
to the successful treatment of turbid river-waters, and subsequent
experience has not in any way shaken his conclusion. The German works
visited by him, Berlin (Stralau) and Altona, were both built by English
engineers, and their settling-basins did not differ materially from
those of corresponding works in England. Since that time, however,
there has been a well-marked tendency on the part of the German
engineers to use smaller, while the English engineers have used much
larger sedimentation-basins, so that the practices of the two countries
are now widely separated, the difference no doubt being in part at
least due to local causes.

Kirkwood found sedimentation-basins at Altona with a capacity of
2-1/4 times the daily supply. In 1894 the same basins were in use,
although the filtering area had been increased from 0.82 acre to 2.20
acres, and still more filters were in course of construction, and
the average daily quantity of water had increased from 600,000 to
4,150,000 gallons in 1891-2, or more than three times the capacity of
the sedimentation-basins. In 1890 the depth of mud deposited in these
basins was reported to be two feet deep in three months. At Stralau in
Berlin, also, in the same time the filtering area was nearly doubled
without increasing the size of the sedimentation-basins, but the Spree
at this point has such a slow current that it forms itself a natural
sedimentation-basin. At Magdeburg on the Elbe works were built in 1876
with a filtering area of 1.92 acres, and a sedimentation-basin capacity
of 11,300,000 gallons, but in 1894 half of the latter had been built
over into filters, which with two other filters gave a total filtering
surface of 3.90 acres, with a sedimentation-basin capacity of only
5,650,000 gallons. The daily quantity of water pumped for 1891-2 was
5,000,000 gallons, so that the present sedimentation-basin capacity is
about equal to one day’s supply, or relatively less than a third of the
original provision. The idea followed is that most of the particles
which will settle at all will do so within twenty-four hours, and that
a greater storage capacity may allow the growth of algæ, and that the
water may deteriorate rather than improve in larger tanks.

[Illustration: PAVED EMBANKMENT BETWEEN TWO FILTERS, EAST LONDON.]

[Illustration: FILTERS AND CHANNELS FOR RAW WATER, ANTWERP.]

[_To face page 10._]

At London, on the other hand, the authorities consider a large storage
capacity for unfiltered water as one of the most important conditions
of successful filtration, the object however, being perhaps as much to
secure storage as to allow sedimentation. In 1893 thirty-nine places
were reported upon the Thames and the Lea which were giving their
sewage systematic treatment before discharging it into the streams
from which London’s water is drawn. These sewage treatments are, with
hardly an exception, dry-weather treatments, and as soon as there is
a considerable storm crude sewage is discharged into the rivers at
every point. The rivers are both short, and are quickly flooded, and
afterwards are soon back in their usual condition. At these times of
flood, the raw water is both very turbid and more polluted by sewage
than at other times, and it is the aim of the authorities to have the
water companies provide reservoir capacity enough to carry them through
times of flood without drawing any water whatever from the rivers. This
obviously involves much more extensive reservoirs than those used in
Germany, and the companies actually have large basins and are still
adding to them. The storage capacities of the various companies vary
from 3 to 18 times the respective average daily supplies, and together
equal 9 times the total supply.

In case the raw water is taken from a lake or a river at a point where
there is but little current, as in a natural or artificial pond,
sedimentation-basins are unnecessary. This is the case at Zürich (lake
water), at Berlin when the rivers Havel and Spree spread into lakes, at
Tegel and Müggel, and at numerous other works.

SIZE OF FILTER-BEDS.

The total area of filters required in any case is calculated from the
quantity of water required, the rate of filtration, and an allowance
for filters out of use while being cleaned. To prevent interruptions
of the supply at times of cleaning, the filtering area is divided into
beds which are operated separately, the number and size of the beds
depending upon local conditions. The cost per acre is decreased with
large beds on account of there being less wall or embankment required,
while, on the other hand, the convenience of operation may suffer,
especially in small works. It is also frequently urged that with large
filters it is difficult or impossible to get an even rate of filtration
over the entire area owing to the frictional resistance of the
underdrains for the more distant parts of the filter. A discussion of
this point is given in Chapter III, page 41. At Hamburg, where the size
of the single beds, 1.88 acres each, is larger than at any other place,
it is shown that there is no serious cause for anxiety; and even if
there were, the objectionable resistance could be still farther reduced
by a few changes in the under-drains. The sizes of filter-beds used at
a large number of places are given in Appendix IV.

At a number of places having severe winters, filters are vaulted over
as a protection from cold, and in the most important of these, Berlin,
Warsaw, and St. Petersburg, the areas of the single beds are nearly
the same, namely, from 0.52 to 0.59 acre. The works with open filters
at London (seven companies), Amsterdam, and Breslau have filter-beds
from 0.82 to 1.50 acres each. Liverpool and Hamburg alone use filters
with somewhat larger areas. Large numbers of works with both covered
and open filters have much smaller beds than these sizes, but generally
this is to avoid too small a number of divisions in a small total area,
although such works have sometimes been extended with the growth of the
cities until they now have a considerable number of very small basins.

FORM OF FILTER-BEDS.

The form and construction of the filter-beds depend upon local
conditions, the foundations, and building materials available, the
principles governing these points being in general the same as for the
construction of ordinary reservoirs. The bottoms require to be made
water-tight, either by a thin layer of concrete or by a pavement upon
a puddle layer. For the sides either masonry walls or embankments are
used, the former saving space, but being in general more expensive in
construction. Embankments must, of course, be substantially paved near
the water-line to withstand the action of ice, and must not be injured
by rapid fluctuations in the water-levels in the filters.

Failure to make the bottoms water-tight has perhaps caused more
annoyance than any other single point. With a leaky bottom there
is either a loss of water when the water in the filters is higher
than the ground-water, or under reverse conditions, the ground-water
comes in and mixes with the filtered water, and the latter is rarely
improved and may be seriously damaged by the admixture. And with very
bad conditions water may pass from one filter to another, with the
differences in pressure always existing in neighboring filters, with
most unsatisfactory results.

COVERS FOR FILTERS.

The filters in England and Holland are built open, without protection
from the weather. In Germany the filters first built were also open,
but in the colder climates more or less difficulty was experienced
in keeping the filters in operation in cold weather. An addition to
the Berlin filters, built in 1874, was covered with masonry vaulting,
over which several feet of earth were placed, affording a complete
protection against frost. The filters at Magdeburg built two years
later were covered in the same way, and since that time covered filters
have been built at perhaps a dozen different places.

[Illustration: INTERIOR VIEW OF COVERED FILTER, ASHLAND, WIS.

When in use the water rises nearly to the springing line of the arches.]

[Illustration: COVERED FILTER IN COURSE OF CONSTRUCTION, SHOWING WOODEN
CENTERS FOR MASONRY VAULTING, SOMERSWORTH, N. H.]

[_To face page 12._]

It was found at Berlin that, owing to the difficulty of properly
cleaning the open filters in winter, it was impossible to keep the
usual proportion of the area in effective service, and as a result
portions of the filters were greatly overtaxed during prolonged
periods of cold weather. This resulted in greatly decreased bacterial
efficiency, the bacteria in March, 1889, reaching 3000 to 4000 per
cc. (with 100,000 in the raw water), although ordinarily the effluent
contained less than 100. An epidemic of typhoid fever followed, and was
confined to that part of the city supplied from the Stralau works,
the wards supplied from the covered Tegel filters remaining free from
fever. Open filters have since been abandoned in Berlin.

At Altona also, where the water is taken from an excessively polluted
source, decreased bacterial efficiency has repeatedly resulted in
winter, and the occasional epidemics of typhoid fever in that city,
which have invariably come in winter, appear to have been directly due
to the effect of cold upon the open filters. The city has just extended
the open filters, and hopes with an increased reserve area to avoid
the difficulty in future without resource to covered filters. (See
Appendices II and VII.)

Brunswick, Lübeck, and Frankfort on Oder with cold winters have open
filters, but draw their water-supplies from less polluted sources, and
have thus far escaped the fate of Berlin and Altona. The new filters
at Hamburg also are open. At Zürich, where open and covered filters
were long used side by side, the covered filters were much more
satisfactory, and the old open filters have recently been vaulted over.

Königsberg originally built open filters, but was afterward obliged to
cover them, on account of the severe winters; and at Breslau, where
open filters have long been used, the recent additions are vaulted over.

The fact that inferior efficiency of filtration results with open
filters during prolonged and severe winter weather is generally
admitted, although there is some doubt as to the exact way in which
the disturbance is caused. In some works I am informed that in cutting
the ice around the edges of the filter and repeatedly piling the
loose pieces upon the floating cake, the latter eventually becomes so
thickened at the sides that the projecting lower corners actually touch
the sand, with the fluctuating levels which often prevail in these
works, and that in this way the sediment layer upon the top of the sand
is broken and the water rapidly passes without adequate purification at
the points of disturbance.

This theory is, however, inadequate to account for many cases where
such an accumulation of ice is not allowed. In these cases the poor
work is not obtained until after the filters have been scraped. The
sand apparently freezes slightly while the water is off, and when water
is brought back and filtration resumed, normal results are for some
reason not again obtained for a time.

In addition to the poorer work from open filters in cold weather, the
cost of removing the ice adds materially to the operating expenses, and
in very cold climates would in itself make covers advisable.

I have arranged the European filter plants, in regard to which I have
sufficient information, in the table on page 15, in the order of the
normal mean January temperatures of the respective places. This may not
be an ideal criterion of the necessity of covering filters, but it is
at least approximate, and in the absence of more detailed comparisons
it will serve to give a good general idea of the case. I have not
found a single case where covered filters are used where the January
temperature is 32° F. or above. In some of these places some trouble is
experienced in unusually cold weather, but I have not heard of any very
serious difficulty or of any talk of covering filters at these places
except at Rotterdam, where a project for covering was being discussed.

Those places having January temperatures below 30° experience a great
deal of difficulty with open filters; so much so, that covered filters
may be regarded as necessary for them, although it is possible to keep
open filters running with decreased efficiency and increased expense by
freely removing the ice, with January temperatures some degrees lower.

Where the mean January temperature is 30° to 32° F. there is room for
doubt as to the necessity of covering filters, but, judging from the
experience of Berlin and Altona, the covered filters are much safer at
this temperature.

TABLE OF PLACES HAVING OPEN AND COVERED FILTERS.

ARRANGED ACCORDING TO THE MEAN JANUARY TEMPERATURES.

In case the raw water was drawn from a lake at a depth where its
minimum temperature was above 32°, which is the temperature which must
ordinarily be expected in surface-waters in winter, open filters might
be successfully used in slightly colder places.

The covers are usually of brick or concrete vaulting supported by
pillars at distances of 11 to 15 feet in each direction, the whole
being covered by 2 or 3 feet of earth; and the top can be laid out as
a garden if desired. Small holes for the admission of air and light
are usually left at intervals. The thickness of the masonry and the
sizes of the pillars used in some of the earlier German vaultings are
unnecessarily great, and some of the newer works are much lighter. For
American use, vaulting like that used for the Newton, Mass., covered
reservoir[3] should be amply strong.

Roofs have been used at Königsberg, Posen, and Budapest instead of
the masonry vaulting. They are cheaper, but do not afford as good
protection against frost, and even with great care some ice will form
under them.

Provision must be made for entering the filters freely to introduce and
remove sand. This is usually accomplished by raising one section of
vaulting and building a permanent incline under it from the sand line
to a door above the high-water line in the filter.

The cost of building covered filters is said to average fully one half
more than open filters.

Among the incidental advantages of covered filters is that with the
comparative darkness there is no tendency to algæ growths on the
filters in summer, and the frequency of scraping is therefore somewhat
reduced. At Zürich, in 1892, where both covered and open filters were
in use side by side, the periods between scrapings averaged a third
longer in the covered than in the open filters.

It has been supposed that covered filters kept the water cool in summer
and warm in winter, but owing to the large volume of water passing, the
change in temperature in any case is very slight; Frühling found that
even in extreme cases a change of over 3° F. in either direction is
rarely observed.

[Illustration: REMOVING ICE FROM A FILTER, EAST LONDON.

This represents the greatest accumulation of ice in the history of the
works.

[_To face page 16._]]

At Berlin, where open and covered filters were used side by side at
Stralau for twenty years, it was found that, bacterially, the open
filters were, except in severe winter weather, more efficient. It was
long supposed that this was caused by the sterilizing action of the
sunlight upon the water in the open filters. This result, however, was
not confirmed elsewhere, and it was finally discovered, in 1893, that
the higher numbers were due to the existence of passages in corners
on the columns of the vaulted roof and around the ventilators for the
underdrains, through which, practically, unfiltered water found its
way into the effluent. This at once removes the evidence in favor of
the superior bacterial efficiency of open filters and suggests the
necessity of preventing such passages. The construction of a ledge all
around the walls and pillars four inches wide and a little above the
gravel, as shown in the sketch, might be useful in this way, and the
slight lateral movement of the water in the sand above would be of no
consequence. The sand would evidently make a closer joint with the
horizontal ledge than with the vertical wall.

[Illustration: FIG. 2.]

In regard to the probable requirement or advisability of covers for
filters in the United States, I judge, from the European experience,
that places having January temperatures below the freezing-point will
have considerable trouble from open filters, and would best have
covered filters. Places having higher winter temperatures will be
able to get along with the ice which may form on open filters, and
the construction of covers would hardly be advisable except under
exceptional local conditions, as, for instance, with a water with an
unusual tendency to algæ growths.

I have drawn a line across a map of the United States on this basis
(shown by the accompanying plate) and it would appear that places far
north of the line would require covered filters, and that those south
of it would not, while for the places in the immediate vicinity of the
line (comparable to Hamburg and Altona) there is room for discussion.

In the United States covered filters have been constructed at St.
Johnsbury, Vt., Somersworth, N. H., Albany, N. Y., Ashland, Wis., and
Grand Forks, N. Dak., all of these places being considerably north of
the above-mentioned line.

The filter at Lawrence, Mass., with a mean January temperature of
about 25°, is not covered, but serious difficulty and expense have
been experienced at times from the ice, so much so that it has been
repeatedly recommended to cover it. Open filters have also been in use
for many years at Hudson and Poughkeepsie, N. Y., with mean January
temperatures about 24°; and although considerable difficulty has been
experienced from ice at times, these filters, particularly the ones
at Poughkeepsie, have been kept in very serviceable condition at all
times, notwithstanding the ice.

At Mount Vernon, N. Y., with a mean January temperature of about
31°, and with a reservoir water, no serious difficulty has been
experienced with ice; and at Far Rockaway, L. I., with a slightly
higher temperature and well-water, no difficulty whatever has been
experienced with open filters. Filters at Ilion, N. Y., with a mean
January temperature of about 23°, are not covered, and are fed from a
reservoir. No serious difficulty has been experienced with ice, which
is probably due to the fact that the water applied to them is taken
from near the bottom of the reservoir, and ordinarily has a temperature
somewhat above the freezing-point throughout the winter.

[Illustration: Map showing

Normal Mean January Temperatures

IN THE UNITED STATES

and the Area in which Filters should be covered]

The cost of removing ice from filters depends, among other things,
upon the amount of reserve filter area. When this reserve is small
the filters must be kept constantly at work nearly up to their rated
capacity; the ice must be removed promptly whenever the filters
require cleaning, and under some conditions the expense of doing this
may be considerable. If, on the other hand, there is a considerable
reserve area, so that when a filter becomes clogged in severe weather,
the work can be turned upon other filters and the clogged filter
allowed to remain until more moderate weather, or until a thaw, the
expense of ice removal may be kept at a materially lower figure.

In case open filters are built near or north of this line, I would
suggest that plenty of space between and around the filters for piling
up ice in case of necessity may be found advantageous, and that a
greater reserve of filtering area for use in emergencies should be
provided than would be considered necessary with vaulted filters or
with open filters in a warmer climate.

CHAPTER III.

FILTERING MATERIALS.

SAND.

The sand used for filtration may be obtained from the sea-shore, from
river-beds or from sand-banks. It consists mainly of sharp quartz
grains, but may also contain hard silicates. As it occurs in nature it
is frequently mixed with clayey or other fine particles, which must be
removed from it by washing before it is used. Some of the New England
sands, however, as that used for the Lawrence City filter, are so clean
that washing would be superfluous.

The grain size of the sand best adapted to filtration has been
variously stated at from 1/8 to 1 mm., or from 0.013 to 0.040 inch.
The variations in the figures, however, are due more to the way that
the same sand appears to different observers than to actual variations
in the size of sands used, which are but a small fraction of those
indicated by these figures.

As a result of experiments made at the Lawrence Experiment Station[4]
we have a standard by which we can definitely compare various sands.
The size of a sand-grain is uniformly taken as the diameter of a sphere
of equal volume, regardless of its shape. As a result of numerous
measurements of grains of Lawrence sands, it is found that when the
diameter, as given above, is 1, the three axes of the grain, selecting
the longest possible and taking the other two at right angles to it,
are, on an average, 1.38, 1.05, and 0.69, respectively and the mean
diameter is equal to the cube root of their product.

It was also found that in mixed materials containing particles of
various sizes the water is forced to go around the larger particles and
through the finer portions which occupy the intervening spaces, so that
it is the finest portion which mainly determines the character of the
sand for filtration. As a provisional basis which best accounts for the
known facts, the size of grain such that 10 per cent by weight of the
particles are smaller and 90 per cent larger than itself, is considered
to be the _effective size_. The size so calculated is uniformly
referred to in speaking of the size of grain in this work.

[Illustration: FIG. 3.—APPARATUS USED FOR MEASURING THE FRICTION OF
WATER IN SANDS.]

Another important point in regard to a material is its degree of
uniformity—whether the particles are mainly of the same size or whether
there is a great range in their diameters. This is shown by the
_uniformity coefficient_, a term used to designate the ratio of the
size of the grain which has 60 per cent of the sample finer than itself
to the size which has 10 per cent finer than itself.

The frictional resistance of sand to water when closely packed, with
the pores completely filled with water and in the entire absence of
clogging, was found to be expressed by the formula

_v_ = _cd_^2(_h_/_l_)(_t_ Fah. + 10°)/60,

where _v_ is the velocity of the water in meters daily in a solid column
of the same area as that of the sand, or approximately in
million gallons per acre daily;
_c_ is an approximately constant factor;
_d_ is the effective size of sand grain in millimeters;
_h_ is the loss of head (Fig. 3);
_l_ is the thickness of sand through which the water passes;
_t_ is the temperature (Fahr.).

TABLE SHOWING RATE AT WHICH WATER WILL PASS THROUGH EVEN-GRAINED AND
CLEAN SANDS OF THE STATED GRAIN SIZES AND WITH VARIOUS HEADS AT A
TEMPERATURE OF 50°.

-------+--------------------------------------------------------------
| Effective Size in Millimeters 10 per cent finer than:
_h_/_l_+------+------+-------+--------+--------+--------+-------+-----
| 0.10 | 0.20 | 0.30 | 0.35 | 0.40 | 0.50 | 1.00 | 3.00
-------+------+------+-------+--------+--------+--------+-------+-----
| | | Million Gallons per Acre daily. | |
.001 | .01 | .04 | .10 | .13 | .17 | .27 | 1.07 | 9.63
.005 | .05 | .21 | .48 | .65 | .85 | 1.34 | 5.35 |48.15
.010 | .11 | .43 | .96 | 1.31 | 1.71 | 2.67 | 10.70 |96.30
.050 | .54 | 2.14 | 4.82 | 6.55 | 8.55 | 13.40 | 53.50 |
.100 | 1.07 | 4.28 | 9.63 | 13.10 | 17.10 | 26.70 |107.00 |
1.000 |10.70 |42.80 | 96.30 | 131.00 | 171.00 | 267.00 | |
-------+------+------+-------+--------+--------+--------+-------+-----

The above table is computed with the value _c_ taken as 1000, this
being approximately the values deduced from the earliest experiments.
More recent and extended data have shown that the value of _c_ is not
entirely constant, but depends upon the uniformity coefficient, upon
the shape of the sand grains, upon their chemical composition, and upon
the cleanliness and closeness of packing of the sand. The value may be
as high as 1200 for very uniform, and perfectly clean sand, and maybe
as low as 400 for very closely packed sands containing a good deal
of alumina or iron, and especially if they are not quite clean. The
friction is usually less in new sand than in sand which has been in use
for some years. In making computations of the frictional resistance
of filters, the average value of _c_ may be taken at from 700 to 1000
for new sand, and from 500 to 700 for sand which has been in use for a
number of years.

The value of _c_ decreases as the uniformity coefficient increases.
With ordinary filter sands with uniformity coefficients of 3 or less
the differences are not great. With mixed sands having much higher
uniformity coefficients, lower and less constant values of _c_ are
obtained, and the arrangement of the particles becomes a controlling
factor in the increase in friction.

The friction of the surface layer of a filter is often greater than
that of all the sand below the surface. It must be separately computed
and added to the resistances computed by the formula, as it depends
largely upon other conditions than those controlling the resistance of
the sand.

While the value of _c_ is thus not entirely constant, it can be
estimated with approximate accuracy for various conditions, from a
knowledge of the composition, condition, and cleanliness of the sand,
and closeness of packing.

The following table shows the quantity of water passing sands at
different temperatures. This table was computed with temperature
factors as given above, which were based upon experiments upon the
flow of water through sands, checked by the coefficients obtained from
experiments with long capillary tubes entirely submerged in water of
the required temperature.

RELATIVE QUANTITIES OF WATER PASSING AT DIFFERENT TEMPERATURES.

32° 0.70
35° 0.75
38° 0.80
41° 0.85
44° 0.90
47° 0.95
50° 1.00
53° 1.05
56° 1.10
59° 1.15
62° 1.20
65° 1.25
68° 1.30
71° 1.35
74° 1.40
77° 1.45

The effect of temperature upon the passage of water through sands and
soils has been further discussed by Prof. L. G. Carpenter, _Engineering
News_, Vol. XXXIX, p. 422. This article reviews briefly the literature
of the subject, and refers at length to the formula of Poiseuille,
published in the _Memoires des Savants Etrangers_, Vol. XI, p. 433
(1846). This formula, in which the quantity of water passing at 0.0°
Cent., is taken as unity, is as follows:

Temperature factor = 1 + 0.033679_t_ + 0.000221_t_^2.

The results obtained by this formula agree very closely with those
given in the above table throughout the temperature range for
which computations are most frequently required. At the higher and
lower temperatures the divergencies are greater, as is shown in a
communication in the _Engineering News_, Vol. XL, p. 26.

The quantity of water passing at a temperature of 50° Fahr. is in many
respects more convenient as a standard than the quantity passing at the
freezing-point. Near the freezing-point, owing to molecular changes in
the water, the changes in its action are rapid, and the results are
less certain, and also 50° Fahr. is a much more convenient temperature
for precise experiments than is the freezing point.

SANDS USED IN EUROPEAN FILTERS.

To secure definite information in regard to the qualities of the sands
actually used in filtration, a large number of European works were
visited in 1894, and samples of sand were collected for analysis. These
samples were examined at the Lawrence Experiment Station by Mr. H. W.
Clark, the author’s method of analysis described in Appendix III being
used. In the following table, for the sake of compactness, only the
leading points of the analyses, namely, effective size, uniformity
coefficient, and albuminoid ammonia, are given. On page 28 full
analyses of some samples from a few of the leading works are given.

ANALYSES OF SANDS USED IN WATER FILTRATION.

---------------------+---------+--------+--------+--------------------
|Effective| | Albu- |
|Size; 10%| Uni- | minoid |
| Finer |formity | Ammo- |
Source. | than | Coeffi-| nia. | Remarks.
|(Milli- | cient. |Parts in|
| meters).| |100,000.|
---------------------+---------+--------+--------+--------------------
London, E. London Co.| 0.44 | 1.8 | 0.45 |New sand, never
| | | | used or washed.
London, E. London Co.| 0.39 | 2.1 | 26.20 |Dirty sand, very
| | | | old.
London, E. London Co.| 0.37 | 2.0 | 8.60 |Same, washed by
| | | | hand.
London, Grand Junc. | 0.26 | 1.9 | 1.90 |Sand from rough
| | filter.
London, Grand Junc. | 0.40 | 3.5 | 10.00 |Old sand in final
| | | | filter.
London, Grand Junc. | 0.41 | 3.7 | 2.70 |Freshly washed old
| | | | sand.
London, Southw’k & V.| 0.38 | 3.5 | 5.00 |Freshly washed old
| | | | sand.
London, Southw’k & V.| 0.30 | 1.8 | 2.80 |Freshly washed new
| | | | sand.
London, Lambeth | 0.36 | 2.3 | 2.60 |Freshly washed old
| | | | sand.
London, Lambeth | 0.36 | 2.4 | 0.35 |New unused sand,
| | | washed.
London, Lambeth | 0.25 | 1.7 | 0.70 |New extremely fine
| | | | sand.
London, Chelsea | 0.36 | 2.4 | 2.10 |Freshly washed old
| | | | sand.
Middlesborough | 0.42 | 1.6 | 17.60 |Dirty sand, ordinary
| | | | scraping.
Middlesborough | 0.43 | 1.6 | 7.30 |Same, after washing.
Birmingham | 0.29 | 1.9 | 33.20 |Dirty sand.
Birmingham | 0.29 | 1.9 | 7.20 |Sand below surface
| | | | of filter.
Reading | 0.30 | 2.5 | 4.00 |Dirty sand.
Reading | 0.22 | 2.0 | 1.50 |Same, after washing.
Antwerp | 0.38 | 1.6 | 7.80 |Dirty sand.
Antwerp | 0.39 | 1.6 | 3.40 |Same, after washing.
Hamburg | 0.28 | 2.5 | 8.50 |Dirty sand.
Hamburg | 0.31 | 2.3 | 0.80 |Same, after washing.
Hamburg | 0.34 | 2.2 | 7.90 |Dirty sand, another
| | | | sample.
Hamburg | 0.30 | 2.0 | 0.90 |Same, after washing
| | | | drums.
Hamburg | 0.34 | 2.3 | 1.50 |Same, after washing
| | | | ejectors.
Altona | 0.32 | 2.0 | 9.00 |Dirty sand, old
| | | | filters.
Altona | 0.37 | 2.0 | 1.50 |Same, after washing.
Altona | 0.33 | 2.8 | 0.50 |Washed sand for new
| | | | filters.
Berlin, Stralau | 0.33 | 1.9 | 12.20 |Dirty sand-pile.
Berlin, Stralau | 0.35 | 1.7 | 4.50 |Filter No. 6,
| | | | 3″ below surface.
Berlin, Stralau | 0.34 | 1.7 | 6.30 |Filter No. 7,
| | | | 3″ below surface.
Berlin, Stralau | 0.35 | 1.7 | 4.00 |Filter No. 10,
| | | | 3″ below surface.
Berlin, Tegel | 0.38 | 1.6 | 11.00 |Dirty sand, old
| | | | filters.
Berlin, Tegel | 0.38 | 1.5 | 2.80 |Same, after washing,
| | | | old filters.
Berlin, Tegel | 0.35 | 1.6 | 3.20 |Same, after washing,
| | | | new filters.
Berlin, Müggel | 0.35 | 1.8 | 0.80 |Sand from filters
| | | | below surface.
Berlin, Müggel | 0.33 | 2.0 | 6.30 |Dirty sand, ordinary
| | | | scraping.
Berlin, Müggel | 0.34 | 2.0 | 15.30 |Dirty sand, another
| | | | sample.
Charlottenburg | 0.40 | 2.3 | 7.20 |Dirty sand.
Chemnitz | 0.35 | 2.6 | 0.20 |New sand not yet
| | | | used.
Magdeburg | 0.39 | 2.0 | 9.50 |Dirty sand.
Magdeburg | 0.40 | 2.0 | 2.80 |Same, after washing.
Breslau | 0.39 | 1.8 | 1.40 |Normal new sand.
Budapest | 0.20 | 2.0 | 0.80 |New washed Danube
| | | | sand.
Zürich | 0.28 | 3.2 | 6.20 |Dirty sand.
Zürich | 0.30 | 3.1 | 1.50 |Same, after washing.
Hague | 0.19 | 1.6 | 0.70 |Dune-sand used for
| | | | filtration.
Schiedam | 0.18 | 1.6 | 5.60 |Dune-sand used for
| | | | filtration; dirty.
Schiedam | 0.31 | 1.5 | 13.50 |River-sand; dirty.
Amsterdam | 0.17 | 1.6 | 2.40 |Dune-sand.
Rotterdam | 0.34 | 1.5 | 2.30 |River-sand; new.
Liverpool, Rivington | 0.43 | 2.0 | 0.76 |Sand from bottom of
| | | | filter.
Liverpool, Rivington | 0.32 | 2.5 | 1.00 |New sand unwashed
| | | | and unscreened.
Liverpool, Rivington | 0.43 | 2.7 | 4.10 |Washed sand which
| | | | has been in use
| | | | 30 to 40 years.
Liverpool, Oswestry | 0.30 | 2.6 | 9.40 |Dirty sand.
Liverpool, Oswestry | 0.31 | 4.7 | 2.20 |Same, after washing.
---------------------+---------+--------+--------+--------------------

NOTE.—It is obvious that in case the sands used at any place are not
always of the same character, as is shown to be the case by different
samples from some of the works, the examination of such a limited
number of samples as the above from each place is entirely inadequate
to establish accurately the sizes of sand used at that particular
place, or to allow close comparisons between the different works, and
for this reason no such comparisons will be made. The object of these
investigations was to determine the sizes of the sands commonly used
in Europe, and, considering the number and character of the different
works represented, it is believed that the results are ample for this
purpose.

The English and most of the German sands are washed, even when entirely
new, before being used, to remove fine particles. At Breslau, however,
sand dredged from the river Oder is used in its natural state, and
new sand is used for replacing that removed by scraping. At Budapest,
Danube sand is used in the same way, but with a very crude washing, and
it is said that only new unwashed sand is used at Warsaw.

In Holland, so far as I learned, no sand is washed, but new sand is
always used for refilling. At most of the works visited dune-sand
with an effective size of only 0.17 to 0.19 mm. is used, and this is
the finest sand which I have ever found used for water filtration on
a large scale. It should be said, however, that the waters filtered
through these fine sands are fairly clear before filtration, and are
not comparable to the turbid river-waters often filtered elsewhere,
and their tendency to choke the filters is consequently much less. At
Rotterdam and Schiedam, where the raw water is drawn from the Maas, as
the principal stream of the Rhine is called in Holland, river-sand of
much larger grain size is employed. It is obtained by dredging in the
river and is never washed, new sand always being employed for refilling.

The average results of the complete analyses of sands from ten leading
works are shown in the table on page 28. These figures are the average
of all the analyses for the respective places, except that one sample
from the Lambeth Co., which was not a representative one, was omitted.

The London companies were selected for this comparison both on
account of their long and favorable records in filtering the polluted
waters of the Thames and Lea, and because they are subject to close
inspection; and there is ample evidence that the filtration obtained is
good—evidence which is often lacking in the smaller and less closely
watched works. For the German works Altona was selected because of
its escape from cholera in 1892, due to the efficient action of its
filters, and Stralau because of its long and favorable record when
filtering the much-polluted Spree water. These two works also have
perhaps contributed more to the modern theories of filtration than all
the other works in existence. The remaining works are included because
they are comparatively new, and have been constructed with the greatest
care and attention to details throughout, and the results obtained are
most carefully recorded.

Some of the most interesting of these results are shown graphically on
page 29. The method of plotting is that described in Appendix III.

TABLE SHOWING THE AVERAGE PER CENT OF THE GRAINS FINER THAN VARIOUS
SIZES IN SANDS FROM LEADING WORKS.

--------------+----------------------------------------------------
| Per Cent by Weight Finer than
+------+------+------+-----+-----+-----+-----+-------
|0.106 |0.186 |0.316 |0.46 |0.93 |2.04 |3.89 |5.89
| mm. | mm. | mm. | mm. | mm. | mm. | mm. | mm.
--------------+------+------+------+-----+-----+-----+-----+-------
East London | 0.2 | 0.5 | 3.6 |22.2 |69.7 |89.8 |95.0 |99.0
Grand Junction| 0 | 0.2 | 3.1 |17.4 |47.1 |68.2 |84.7 |93.6
Southwark and | | | | | | | |
Vauxhall | | 0.7 | 8.0 |34.1 |69.7 |83.5 |90.0 |94.0
Lambeth | 0 | 0.5 | 5.5 |26.6 |63.0 |79.2 |88.0 |94.3
Chelsea | 0 | 0.1 | 5.0 |28.6 |63.0 |76.7 |86.0 |93.6
Hamburg | 0.2 | 1.5 |10.9 |33.2 |74.4 |95.7 |99.5 |
Altona | 0.1 | 1.1 | 7.8 |28.7 |72.1 |92.1 |95.8 |
Stralau | | 0.3 | 7.0 |37.3 |86.9 |95.4 |97.6 |
Tegel | | 0.2 | 4.5 |35.4 |94.3 |98.5 |99.1 |
Müggel | 0.1 | 0.5 | 7.9 |33.6 |79.7 |94.3 |98.5 |
+------+------+------+-----+-----+-----+-----+-------
Average of all| 0.06 | 0.56 | 6.33 |29.71|71.99|87.34|93.42|(97.45)
--------------+------+------+------+-----+-----+-----+-----+-------

AVERAGE EFFECTIVE SIZE, UNIFORMITY COEFFICIENT, AND ALBUMINOID AMMONIA
IN SANDS FROM TEN LEADING WORKS.

I. LONDON FILTERS.
----------------------+--------------+------------+-------------------
| Effective | Uniformity |Albuminoid Ammonia.
| Size; 10% |Coefficient.+------------+------
| Finer than | | Dirty Sand.|Washed
|(Millimeters).| | | Sand.
----------------------+--------------+------------+------------+------
East London | 0.40 | 2.0 | 26.00 | 8.60
Grand Junction | 0.40 | 3.6 | 10.00 | 2.70
Southwark and Vauxhall| 0.34 | 2.5 | | 3.90
Lambeth | 0.36 | 2.4 | | 2.60
Chelsea | 0.36 | 2.4 | | 2.10
+--------------+------------+------------+------
Average | 0.37 | 2.6 | 18.00 | 3.98
----------------------+--------------+------------+------------+------

II. GERMAN WORKS.
----------------------+--------------+------------+------------+------
Stralau | 0.34 | 1.7 | 12.20 | 4.00
Tegel | 0.37 | 1.6 | 11.00 | 3.00
Müggel | 0.34 | 2.0 | 10.80 | 0.80
Altona | 0.34 | 2.3 | 9.00 | 1.50
Hamburg | 0.31 | 2.3 | 8.20 | 1.07
+--------------+------------+------------+------
Average | 0.34 | 2.0 | 10.25 | 2.07
----------------------+--------------+------------+------------+------

[Illustration: PLACING SAND IN A FILTER, HAMBURG.

[_To face page 28._]]

The averages show the effective size of the English sands to be
slightly greater than that of the German sands—0.37 instead of 0.34
mm.—but the difference is very small. The entire range for the ten
works is only from 0.31 to 0.40 mm., and these may be taken as the
ordinary limits of effective size of the sands employed in the best
European works. The average for the other sixteen works given above,
including dune-sands, is 0.31 mm., or, omitting the dune-sands, 0.34 mm.

[Illustration: FIG. 3_a_.—SAND ANALYSIS SHEET, WITH ANALYSES OF SEVERAL
EUROPEAN FILTER SANDS.]

It is important that filter sands should be free from lime. When water
is filtered through such sands, no increase in hardness results. When,
however, water is filtered through sand containing lime, some of it
is usually dissolved and the water is made harder. The amount of lime
taken up in this way depends both upon the character of the sand, and
upon the solvent power of the water; and it does not necessarily follow
that a sand containing lime cannot be used for filtration, but a sand
nearly free from lime is to be preferred.

The presence of lime in sand can usually be detected by moistening it
with hydrochloric acid. The evolution of gas shows the presence of
lime. Some idea of the amount of lime can be obtained from the amount
of gas given off, and the appearance of the sample after the treatment,
but chemical analysis is necessary to determine correctly the amount.

Experiments with filters at Pittsburg were made with sand containing
1.3 per cent of lime, the result being that the hardness of the water
was increased about one part in 100,000; but the amount of lime in the
sand was so small that it would be washed out after a time, and then
the hardening effect would cease. Larger amounts of lime would continue
their action for a number of years and would be more objectionable.

Turning to the circumstances which influence the selection of the
sand size, we find that both the quality of the effluent obtained by
filtration and the cost of filtration depend upon the size of the
sand-grains.

With a fine sand the sediment layer forms more quickly and the removal
of bacteria is more complete, but, on the other hand, the filter clogs
quicker and the dirty sand is more difficult to wash, so that the
expense is increased.

EFFECT OF SIZE OF GRAIN UPON EFFICIENCY OF FILTRATION.

It is frequently stated that it is only the sediment layer which
performs the work of filtration, and that the sand which supports
it plays hardly a larger part than does the gravel which carries
the sand, and under some circumstances this is undoubtedly the case.
Nevertheless sand in itself, without any sediment layer, especially
when not too coarse and not in too thin layers, has very great
purifying powers, and, in addition, acts as a safeguard by positively
preventing excessive rates of filtration on account of its frictional
resistance. As an illustration take the case of a filter of sand with
an effective size of 0.35 mm. and the minimum thickness of sand allowed
by the German Board of Health, namely, one foot, and let us suppose
that with clogging the loss of head has reached two feet to produce
the desired velocity of 2.57 million gallons per acre daily. Suppose
now that by some accident the sediment layer is suddenly broken or
removed from a small area, the water will rush through this area,
until a new sediment layer is formed, at a rate corresponding to the
size, pressure, and depth of the sand, or 260 million gallons per
acre daily—a hundred times the standard rate. Under these conditions
the passing water will not be purified, but will pollute the entire
effluent from the filter. Under corresponding conditions, with a deep
filter of fine sand, say with an effective size of 0.20 mm. and 5 feet
deep, the resulting rate would be only 17 million gallons per acre
daily, or less than seven times the normal, and with the water passing
through the full depth of fine sand, the resulting deterioration in the
effluent before the sand again became so clogged as to reduce the rate
to nearly the normal, would be hardly appreciable.

The results at Lawrence have shown that with very fine sands 0.09 and
0.14 mm., and 4 to 5 feet deep, with the quantity of water which can
practically be made to pass through them, it is almost impossible to
drive more than an insignificant fraction of the bacteria into the
effluent. Even when the sands are entirely new, or have been scraped or
disturbed in the most violent way, the first effluent passing, before
the sediment layer could have been formed, is of good quality. Still
finer materials, 0.04 to 0.06 mm., as far as could be determined,
secured the absolute removal of all bacteria, but the rates of
filtration which were possible were so low as to preclude their
practical application.

With coarser sands, as long as the filter is kept at a steady rate of
filtration, without interruptions of any kind, entirely satisfactory
results are often obtained, although never quite so good as with
the finer sands. Thus at Lawrence the percentages of bacteria (_B.
prodigiosus_) appearing in the effluents under comparable conditions
were as follows:

1892 1893
With effective grain size 0.38 mm .... 0.16
With effective grain size 0.29 mm .... 0.16
With effective grain size 0.26 mm .... 0.10
With effective grain size 0.20 mm 0.13 0.01
With effective grain size 0.14 mm 0.04 0.03
With effective grain size 0.09 mm 0.02 0.02

We may thus conclude that fine sands give normally somewhat better
effluents than coarser ones, and that they are much more likely to
give at least a tolerably good purification under unusual or improper
conditions.

EFFECT OF GRAIN SIZE UPON FREQUENCY OF SCRAPING.

The practical objection to the use of fine sand is that it becomes
rapidly clogged, so that filters require to be scraped at shorter
intervals, and the sand washing is much more difficult and expensive.
The quantities of water filtered between successive scrapings at
Lawrence in millions of gallons per acre under comparable conditions
have been as follows:

1892 1893
Effective size of sand grain 0.38 mm .... 79
Effective size of sand grain 0.29 mm .... 70
Effective size of sand grain 0.26 mm .... 57
Effective size of sand grain 0.20 mm 58 ....
Effective size of sand grain 0.14 mm 45 49
Effective size of sand grain 0.09 mm 24 14

The increase in the quantities passed between scrapings with increasing
grain size is very marked.

With the fine sands, the depth to which the sand becomes dirty is much
less than with the coarse sands, but as it is not generally practicable
to remove a layer of sand less than about 0.6 inch thick, even when the
actual clogged layer is thinner than this, the full quantity of sand
has to be removed; and the quantities of sand to be removed and washed
are inversely proportional to the quantities of water filtered between
scrapings. On the other hand, with very coarse sands the sediment
penetrates the sand to a greater depth than the 0.6 inch necessarily
removed, so that a thicker layer of sand has to be removed, which
may more than offset the longer interval. This happens occasionally
in water-works, and a sand coarse enough to allow it occur is always
disliked by superintendents, and is replaced with finer sand as soon as
possible. It is obvious that the minimum expense for cleaning will be
secured with a sand which just does not allow this deep penetration,
and I am inclined to think that the sizes of the sands in use have
actually been determined more often than otherwise in this way, and
that the coarsest samples found, having effective sizes of about 0.40
mm., represent the practical limit to the coarseness of the sand,
and that any increase above this size would be followed by increased
expense for cleaning as well as by decreased efficiency.

SELECTION OF SAND.

In selecting a sand for filtration, when it is considered that repeated
washings will remove some of the finest particles, and so increase
slightly the effective size, a new sand coarser than 0.35 mm. would
hardly be selected. Perhaps 0.20 might be given as a suitable lower
limit. For comparatively clear lake- or reservoir-waters a finer
sand could probably be used than would be the case with a turbid
river-water. A mixed sand having a uniformity coefficient above 3.0
would be difficult to wash without separating it into portions of
different sizes, and, in general, the lower the coefficient, that is,
the more uniform the grain sizes, the better. Great pains should be
taken to have the sand of the same quality throughout, especially in
the same filter, as any variations in the grain sizes would lead to
important variations in the velocity of filtration, the coarser sands
passing more than their share of water (in proportion to the square of
the effective sizes) and with reduced efficiency.

At Lawrence a sufficient quantity of natural sand was found of the
grade required; but where suitable material cannot be so obtained it
is necessary to use other methods. A mixed material can be screened
from particles which are too large, and can be washed to free it from
its finer portions, and in this way a good sand can be prepared, if
necessary, from what might seem to be quite unpromising material. The
methods of sand-washing will be described in Chapter V.

THICKNESS OF THE SAND LAYER.

The thickness of the sand layer is made so great that when it is
repeatedly scraped in cleaning the sand will not become too thin for
good filtration for a considerable time. When this occurs the removed
sand must be replaced with clean sand. The original thickness of the
sand in European filters is usually from 24 to 48 inches, thicknesses
between 30 and 40 inches being extremely common, and this is reduced
before refilling to from 12 to 24 inches. The Imperial Board of Health
of Germany has fixed 12 inches as a limit below which the sand should
never be scraped, and a higher limit is recommended wherever possible.

A thick sand layer has the same steadying action as a fine sand, and
tends to prevent irregularities in the rate of filtration in proportion
to its frictional resistance, and that without increasing the frequency
of cleaning; but, on the other hand, it increases the necessary height
of the filter, throughout, and consequently the cost of construction.

In addition to the steadying effect of a deep sand layer, some
purification takes place in the lower part of the sand even with a good
sediment layer on the surface, and the efficiency of deep filters is
greater than that of shallow ones.

Layers of finer materials, as fine sand or loam, in the lower part
of a filter, which would otherwise give increased efficiency without
increasing the operating expenses, cannot be used. Their presence
invariably gives rise sooner or later to sub-surface clogging at the
point of junction with the coarser sand, as has been found by repeated
tests at Lawrence as well as in some of the Dutch filters where such
layers were tried; and as there is no object in putting a coarser sand
under a finer, the filter sand is best all of the same size and quality
from top to bottom.

UNDERDRAINING.

The underdrains of a filter are simply useful for collecting the
filtered water; they play no part in the purification. One of the first
requirements of successful filtration is that the rate of filtration
shall be practically the same in all parts of the filter. This is most
difficult to secure when the filter has just been cleaned and the
friction of the sand layer is at a minimum. If the friction of the
water in entering and passing through the underdrains is considerable,
the more remote parts of the filters will work under less pressure,
and will thus do less than their share of the work, while the parts
near the outlet will be overtaxed, and filtering at too high rates will
yield poor effluents.

To avoid this condition the underdrains must have such a capacity
that their frictional resistance will be only a small fraction of the
friction in the sand itself just after cleaning.

GRAVEL LAYERS.

The early filters contained an enormous quantity of gravel, but the
quantity has been steadily reduced in successive plants. Thus in 1866
Kirkwood, as a result of his observations, recommended the use of a
layer four feet thick, and in addition a foot of coarse sand, while
at the present time new filters rarely have more than two feet of
gravel. Even this quantity seems quite superfluous, when calculations
of its frictional resistance are made. Thus a layer of gravel with an
effective size of 20 mm.[5] (which is much finer than that generally
employed) only 6 inches thick will carry the effluent from a filter
working at a rate of 2.57 million gallons per acre daily for a distance
of 8 feet (that is, with underdrains 16 feet apart), with a loss of
head of only 0.001 foot, and for longer distances tile drains are
cheaper than gravel. To prevent the sand from sinking into the coarse
gravel, intermediate sizes of gravel must be placed between, each grade
being coarse enough so that there is no possibility of its sinking into
the layer below. The necessary thickness of these intermediate layers
is very small, the principal point being to have a layer of each grade
at every point. Thus on the 6 inches of 20 mm. gravel mentioned above,
three layers of two inches each, of 8 and 3 mm. gravel and coarse
sand, with a total height of six inches, or other corresponding and
convenient depths and sizes, would, if carefully placed, as effectually
prevent the sinking of the filter sand into the coarse gravel as the
much thicker layers used in the older plants.

The gravel around the drains should receive special attention. Larger
stones can be here used with advantage, taking care that adequate
spaces are left for the entrance of the water into the drains at a low
velocity, and to make everything so solid in this neighborhood that
there will be no chance for the stones to settle which might allow the
sand to reach the drains.

[Illustration: RECONSTRUCTING THE UNDERDRAINAGE SYSTEM OF A FILTER
AFTER 25 YEARS OF USE, BREMEN.]

[Illustration: PLACING SAND IN A FILTER, CHOISY LE ROI (PARIS).

[_To face page 36._]]

At the Lawrence filter, at Königsberg in Prussia, at Amsterdam and
other places, the quantity of gravel is reduced by putting the drains
in trenches, so that the gravel is reduced from a maximum thickness
at the drain to nothing half way between drains. The economy of the
arrangement, however, as far as friction is concerned is not so great
as would appear at first sight, and the cost of the bottom may be
increased; but on the other hand it gives a greater depth of gravel for
covering the drains with a small total amount of gravel.

As even a very small percentage of fine material is capable of
getting in the narrow places and reducing the carrying power of the
gravel, it is important that all such matters should be carefully
removed by washing before putting the gravel in place. In England and
Germany gravel is commonly screened for use in revolving cylinders of
wire-cloth of the desired sizes, on which water is freely played from
numerous jets, thus securing perfectly clean gravel. In getting gravel
for the Lawrence filter, an apparatus was used, in which advantage was
taken of the natural slope of the gravel bank to do the work, and the
use of power was avoided. The respective grades of gravel obtained were
even in size, and reasonably free from fine material, but it was deemed
best to wash them with a hose before putting them in the filter.

To calculate the frictional resistance of water in passing gravel, we
may assume that for the very low velocities which are actually found in
filters the quantity of water passing varies directly with the head,
which for these velocities is substantially correct, although it would
not be true for higher rates, especially with the coarser gravels.[6]
In the case of parallel underdrains the friction from the middle point
between drains to the drains may be calculated by the formula:

Total head = (1/2)[(Rate of filtration × (1/2 distance between
drains)^2)/(Average depth of gravel × discharge coefficient)].

The discharge coefficient for any gravel is 1000 times the quantity

of water which will pass when _h_/_l_ is 1/1000 expressed in million
gallons per acre daily. The approximate values of this coefficient for
different-sized gravels are as follows:

VALUES OF DISCHARGE COEFFICIENT.

For gravel with effective size 5 mm _c_ = 23,000
For gravel with effective size10 mm _c_ = 65,000
For gravel with effective size15 mm _c_ = 110,000
For gravel with effective size20 mm _c_ = 160,000
For gravel with effective size25 mm _c_ = 230,000
For gravel with effective size30 mm _c_ = 300,000
For gravel with effective size35 mm _c_ = 390,000
For gravel with effective size40 mm _c_ = 480,000

Example: What is the loss of head in the gravel at a rate of filtration
of 2 million gallons per acre daily, with underdrains 20 feet apart,
where the supporting gravel has an effective size of 35 millimeters,
and is uniformly 1 ft. deep?

Total head = (1/2)[(2 × 10^2)/(1 × 390,000)] = .000256 ft.

The total friction would be the same with the same average depth of
gravel whether it was uniformly 1 foot deep, or decreasing from 1.5 at
the drains to 0.5 in the middle, or from 2.0 to 0. The reverse case
with the gravel layer thicker in the middle than at the drains does not
occur and need not be discussed.

The depth of gravel likely to be adopted as a result of this
calculation, when the drains are not too far apart, will be much less
than that actually used in most European works, but as the two feet or
more there employed are, I believe, simply the result of speculation,
there is no reason for following the precedent where calculations show
that a smaller quantity is adequate.

The reason for recommending a thin lower layer of coarse gravel, which
alone is assumed to provide for the lateral movement of the water,
is that if more than about six inches of gravel is required to give a
satisfactory resistance, it will almost always be cheaper to use more
drains instead of more gravel; and the reason for recommending thinner
upper layers for preventing the sand from settling into the coarse
gravel is that no failures of this portion of filters are on record,
and in the few instances where really thin layers have been used the
results have been entirely satisfactory. In Königsberg filters were
built by Frühling,[7] in which the sand was supported by five layers
of gravel of increasing sizes, respectively 1.2, 1.2, 1.6, 2.0, 3.2,
or, together, 9.2 inches thick, below which there were an average of
five inches of coarse gravel. These were examined after eight years of
operation and found to be in perfect order.

At the Lawrence Experiment Station filters have been repeatedly
constructed with a total depth of supporting gravel layers not
exceeding six inches, and among the scores of such filters there has
not been a single failure, and so far as they have been dug up there
has never been found to have been any movement whatever of the sand
into the gravel. The Lawrence city filter, built with corresponding
layers, has shown no signs of being inadequately supported. In
arranging the Lawrence gravel layers care has always been taken that no
material should rest on another material more than three or four times
as coarse as itself, and that each layer should be complete at every
point, so that by no possibility could two layers of greater difference
in size come together. And it is believed that if this is carefully
attended to, no trouble need be anticipated, however thin the single
layers may be.

UNDERDRAINS.

The most common arrangement, in other than very small filters, is to
have a main drain through the middle of the filter, with lateral
drains at regular intervals from it to the sides. The sides of the main
drain are of brick, laid with open joints to admit water freely, and
the top is usually covered with stone slabs. The lateral drains may be
built in the same way, but tile drains are also used and are cheaper.
Care must be taken with the latter that ample openings are left for the
admission of water at very low velocities. It is considered desirable
to have these drains go no higher than the top of the coarsest gravel;
and this will often control the depth of gravel used. If they go
higher, the top must be made tight to prevent the entrance of the fine
gravels or sand. Sometimes they are sunk in part or wholly (especially
the main drain) below the floor of the filter. With gravel placed in
waves, that is, thicker over the drains than elsewhere, as mentioned
above, the drains are covered more easily than with an entirely
horizontal arrangement. When this is done, the floor of the filter is
trenched to meet the varying thickness of gravel, so that the top of
the latter is level, and the sand has a uniform thickness.

Many filters (Lambeth, Brunswick, etc.) are built with a double bottom
of brick, the upper layer of which, with open joints, supports the
gravel and sand, and is itself supported by numerous small arches or
other arrangements of brick, which serve to carry the water to the
outlet without other drains. This arrangement allows the use of a
minimum quantity of gravel, but is undoubtedly more expensive than the
usual form, with only the necessary quantity of gravel; and I am unable
to find that it has any corresponding advantages.

The frictional resistance of underdrains requires to be carefully
calculated; and in doing this quite different standards must be
followed from those usually employed in determining the sizes
of water-pipes, as a total frictional resistance of only a few
hundredths of a foot, including the velocity head, may cause serious
irregularities in the rate of filtration in different parts of the
filter.

The sizes of the underdrains differ very widely in proportion to the
sizes of the filters in European works, some of them being excessively
large, while in other cases they are so small as to suggest a doubt as
to their allowing uniform rates of filtration, especially just after
cleaning.

I would suggest the following rules as reasonably sure to lead to
satisfactory results without making an altogether too lavish provision:
In the absence of a definite determination to run filters at some
other rate, calculate the drains for the German standard rate of a
daily column of 2.40 meters, equal to 2.57 million gallons per acre
daily. This will insure satisfactory work at all lower rates, and
no difficulty on account of the capacity of the underdrains need be
then anticipated if the rate is somewhat exceeded. The area for a
certain distance from the main drain depending upon the gravel may be
calculated as draining directly into it, provided there are suitable
openings, and the rest of the area is supposed to drain to the nearest
lateral drain.

In case the laterals are round-tile drains I would suggest the
following limits to the areas which they should be allowed to drain:

Diameter of Drain. To Drain an Area not Corresponding Velocity of
Exceeding Water in Drain.
4 inches 290 square feet. 0.30 foot.
6 inches 750 square feet. 0.35 foot.
8 inches 1530 square feet. 0.40 foot.
10 inches 2780 square feet. 0.46 foot.
12 inches 4400 square feet. 0.51 foot.

And for larger drains, including the main drains, their cross-sections
at any point should be at least 1/6000 of the area drained, giving a
velocity of 0.55 foot per second with the rate of filtration mentioned
above.

The total friction of the underdrains from the most remote points
to the outlet will be friction in the gravel, plus friction in the
lateral drains, plus the friction in main drain, plus the velocity head.

[Illustration: FIG. 4.—PLAN OF ONE OF THE HAMBURG FILTERS, SHOWING
FRICTIONAL RESISTANCE OF THE UNDERDRAINS.]

I have calculated in this way the friction of one of the Hamburg
filters for the rate of 1,600,000 gallons per acre daily at which it
is used. The friction was calculated for each section of the drains
separately, so that the friction from intermediate points was also
known. Kutter’s formula was used throughout with _n_ = 0.013. On
the accompanying plan of the filter I have drawn the lines of equal
frictional resistance from the junction of the main drain with the last
laterals. My information was incomplete in regard to one or two points,
so that the calculation may not be strictly accurate, but it is nearly
so and will illustrate the principles involved.

[Illustration: CONSTRUCTING THE UNDERDRAINAGE SYSTEM OF A FILTER,
HAMBURG.

[_To face page 42._]
]

The extreme friction of the underdrains is 11 millimeters = 0.036 foot.

The frictional resistance of the sand 39 inches thick, effective size
0.32 mm. and rate 1.60 million gallons per acre daily, when absolutely
free from clogging, is by the formula, page 21, 15mm., or .0490
foot, when the temperature is 50°. Practically there is some matter
deposited upon the surface of the sand before filtration starts, and
further, after the first scraping, there is some slight clogging in the
sand below the layer removed by scraping. We can thus safely take the
minimum frictional resistance of the sand including the surface layer
at .07 foot. The average friction of the underdrains for all points
is about .023 foot and the friction at starting will be .07 + .023 =
.093 foot (including the friction in the last section to the effluent
well where the head is measured, .100 foot, but the friction beyond the
last lateral does not affect the uniformity of filtration). The actual
head on the sand close to the outlet will be .093 and the rate of
filtration .093/.070 · 1.60 = 2.12. The actual head at the most remote
point will be .093 - .036 = .057, and the rate of filtration will there
be .057/.070 · 160 = 1.30 million gallons per acre daily. The extreme
rates of filtration are thus 2.12 and 1.30, instead of the average rate
of 1.60. As can be seen from the diagram, only very small areas work
at these extreme rates, the great bulk of the area working at rates
much nearer the average. Actually the filter is started at a rate below
1.60, and the nearest portion never filters so rapidly as 2.12, for
when the rate is increased to the standard, the sand has become so far
clogged that the loss of head is more than the .07 foot assumed, and
the differences in the rates are correspondingly reduced. Taking this
into account, it would not seem that the irregularities in the rate of
filtration are sufficient to affect seriously the action of the filter.
They could evidently have been largely reduced by moderately increasing
the sizes of the lower ends of the underdrains, where most of the
friction occurs with the high velocities (up to .97 foot) which there
result.

The underdrains of the Warsaw filters were designed by Lindley to have
a maximum loss of head of only .0164 foot when filtering at a rate of
2.57, which gives a variation of only 10 per cent in the rates with the
minimum loss of head of .169 foot in the entire filter assumed by him.
The underdrains of the Berlin filters, according to my calculations,
have .020 to .030 foot friction, of which an unusually large proportion
is in the gravel, owing to the excessive distances, in some cases over
80 feet, which the gravel is required to carry the water. In this case,
using less or finer gravel would obviously have been fatal, but the
friction as well as the expense of construction would be much reduced
by using more drains and less gravel.

The underdrains might appropriately be made slightly smaller, with a
deep layer of fine sand, than under opposite conditions, as in this
case the increased friction in the drains would be no greater in
proportion to the increased friction in the sand itself.

The underdrains of a majority of European filters have water-tight
pipes connecting with them at intervals, and going up through the sand
and above the water, where they are open to the air. These pipes were
intended to ventilate the underdrains and allow the escape of air when
the filter is filled with water introduced from below. It may be said,
however, that in case the drains are surrounded by gravel and there is
an opportunity for the air to pass from the top of the drain into the
gravel, it will so escape without special provision being made for it,
and go up through the sand with the much larger quantity of air in the
upper part of the gravel which is incapable of being removed by pipes
connecting with the drains.

These ventilator pipes where they are used are a source of much
trouble, as unfiltered water is apt to run down through cracks in the
sand beside them, and, under bad management, unfiltered water may even
go down through the pipes themselves. I am unable to find that they
are necessary, except with underdrains so constructed that there is
no other chance for the escape of air from the tops of them, or that
they serve any useful purpose, while there are positive objections to
their use. In some of the newer filters they have been omitted with
satisfactory results.

DEPTH OF WATER ON THE FILTERS.

In the older works with but crude appliances for regulating the rate of
filtration and admission of raw water, a considerable depth of water
was necessary upon the filter to balance irregularities in the rates
of filtration; the filter was made to be, to a certain extent, its own
storage reservoir. When, however, appliances of the character to be
described in Chapter IV are used for the regulation of the incoming
water, and with a steady rate of filtration, this provision becomes
quite superfluous.

With open filters a depth of water in excess of the thickness of any
ice likely to be formed is required to prevent disturbance or freezing
of the sand in winter. It is also frequently urged that with a deep
water layer on the filter the water does not become so much heated in
summer, but this point is not believed to be well taken, for in any
given case the total amount of heat coming from the sun to a given area
is constant, and the quantity of water heated in the whole day—that
is, the amount filtered—is constant, and variations in the quantity
exposed at one time will not affect the average resulting increase in
temperature. If the same water remained upon the filter without change
it would of course be true that a thin layer would be heated more than
a deep one, but this is not the case.

It is also sometimes recommended that the depth of water should be
sufficient to form a sediment layer before filtration starts, but this
point would seem to be of doubtful value, especially where the filter
is not allowed to stand a considerable time with the raw water upon it
before starting filtration.

It is also customary to have a depth of water on the filter in excess
of the maximum loss of head, so that there can never be a suction in
the sand just below the sediment layer. It may be said in regard to
this, however, that a suction below is just as effective in making the
water pass the sand as an equal head above. At the Lawrence Experiment
Station filters have been repeatedly used with a water depth of only
from 6 to 12 inches, with losses of head reaching 6 feet, without the
slightest inconvenience. The suction only commences to exist as the
increasing head becomes greater than the depth of water, and there is
no way in which air from outside can get in to relieve it. In these
experimental filters in winter, when the water is completely saturated
with air, a small part of the air comes out of the water just as it
passes the sediment layer and gets into reduced pressure, and this
air prevents the satisfactory operation of the filters. But this is
believed to be due more to the warming and consequent supersaturation
of the water in the comparatively warm places in which the filters
stand than to the lack of pressure, and as not the slightest trouble
is experienced at other seasons of the year, it may be questioned
whether there would be any disadvantage at any time in a corresponding
arrangement on a large scale where warming could not occur.

The depths of water actually used in European filters with the full
depth of sand are usually from 36 to 52 inches. In only a very few
unimportant cases is less than the above used, and only a few of the
older works use a greater depth, which is not followed in any of the
modern plants. As the sand becomes reduced in thickness by scraping,
the depth of water is correspondingly increased above the figures given
until the sand is replaced. The depth of water on the German covered
filters is quite as great as upon corresponding open filters. Thus the
Berlin covered filters have 51, while the new open filters at Hamburg
have only 43 inches.

CHAPTER IV.

RATE OF FILTRATION AND LOSS OF HEAD.

The rate of filtration recommended and used has been gradually reduced
during the past thirty years. In 1866 Kirkwood found that 12 vertical
feet per day, or 3.90 million gallons per acre daily, was recommended
by the best engineers, and was commonly followed as an average rate.
In 1868 the London filters averaged a yield of 2.18 million gallons[8]
per acre daily, including areas temporarily out of use, while in 1885
the quantity had been reduced to 1.61. Since that time the rate has
apparently been slightly increased.

The Berlin filters at Stralau constructed in 1874 were built to filter
at a rate of 3.21 million gallons per acre daily. The first filters at
Tegel were built for a corresponding rate, but have been used only at
a rate of 2.57, while the more recent filters were calculated for this
rate. The new Hamburg filters, 1892-3, were only intended to filter
at a rate of 1.60 million gallons per acre daily. These in each case
(except the London figures) are the standard rates for the filter-beds
actually in service.

In practice the area of filters is larger than is calculated from
these figures, as filters must be built to meet maximum instead of
average daily consumptions, and a portion of the filtering area usually
estimated at from 5 to 15 per cent, but in extreme cases reaching 50
per cent, is usually being cleaned, and so is for the time out of
service. In some works also the rate of filtration on starting a filter
is kept lower than the standard rate for a day or two, or the first
portion of the effluent, supposed to be of inferior quality, is

wasted, the amount so lost reaching in an extreme case 9 to 14 per
cent of the total quantity of water filtered.[9] In many of the older
works also, there is not storage capacity enough for filtered water to
balance the hourly fluctuations in consumption, and the filters must be
large enough to meet the maximum hourly as well as the maximum daily
requirements. For these reasons the actual quantity of water filtered
in a year is only from 50 to 75 per cent of what would be the case if
the entire area of the filters worked constantly at the full rate. A
statement of the actual yields of a number of filter plants is given in
Appendix IV. The figures for the average annual yields can be taken as
quite reliable. The figures given for rate, in many cases, have little
value, owing to the different ways in which they are calculated at
different places. In addition most of the old works have no adequate
means of determining what the rate at any particular time and for a
single filter really is, and statements of average rates have only
limited value. The filters at Hamburg are not allowed to filter faster
than 1.60 or those at Berlin faster than 2.57 million gallons per acre
daily, and adequate means are provided to secure this condition. Other
German works aim to keep within the latter limit. Beyond this, unless
detailed information in regard to methods is presented, statements of
rate must be taken with some allowance.

EFFECT OF RATE UPON COST OF FILTRATION.

The size of the filters required, and consequently the first cost,
depends upon the rate of filtration, but with increasing rates the cost
is not reduced in the same proportion as the increase in rate, since
the allowance for area out of use is sensibly the same for high and low
rates, and in addition the operating expenses depend upon the quantity
filtered and not upon the filtering area. Thus, to supply 10 million
gallons at a maximum rate of 2 million gallons per acre daily we should
require 10 ÷ 2 = 5 acres + 1 acre reserve for cleaning = 6 acres, while
with a rate twice

as great, and with the same reserve (since the same amount of cleaning
must be done, as will be shown below), we should require 10 ÷ 4 + 1
= 3.5 acres, or 58 per cent of the area required for the lower rate.
Thus beyond a certain point increasing the rate does not effect a
corresponding reduction in the first cost.

The operating cost for the same quantity of water filtered does not
appear to be appreciably affected by the rate. It is obvious that
at high rates filters will became clogged more rapidly, and will
so require to be scraped oftener than at low rates, and it might
naturally be supposed that the clogging would increase more rapidly
than the rates, but this does not seem to be the case. At the Lawrence
Experiment Station, under strictly parallel conditions and with
identically the same water, filters running at various rates became
clogged with a rapidity directly proportional to the rates, so that
the quantities of water filtered between scrapings under any given
conditions are the same whether the rate is high or low.

The statistics bearing upon this point are interesting, if not entirely
conclusive. There were eleven places in Germany filtering river waters,
from which statistics were available for the year 1891-92. Of these
there were four places with high rates, Lübeck, Stettin, Stuttgart, and
Magdeburg, yielding 3.70 million gallons per acre daily, which filtered
on an average 59 million gallons per acre between scrapings. Three
other places, Breslau, Altona, and Frankfurt, yielding 1.85, passed
on an average 55 million gallons per acre between scrapings, and four
other places, Bremen, Königsberg, Brunswick and Posen, yielding 1.34
million gallons per acre daily, passed only 40 million gallons per
acre between scrapings. The works filtering at the highest rates thus
filtered more water in proportion to the sand clogged than did those
filtering more slowly, but I cannot think that this was the result of
the rate. It is more likely that some of the places have clearer waters
than others, and that this both allows the higher rate and causes less
clogging than the more turbid waters.

EFFECT OF RATE UPON EFFICIENCY OF FILTRATION.

The effect of the rate of filtration upon the quality of the effluent
has been repeatedly investigated. The efficiency almost uniformly
decreases rapidly with increasing rate. Fränkel and Piefke[10] first
found that with the high rates the number of bacteria passing some
experimental filters was greatly increased. Piefke[11] afterward
repeated these experiments, eliminating some of the features of the
first series to which objection was made, and confirmed the first
results. The results were so marked that Piefke was led to recommend
the extremely low limit of 1.28 million gallons per acre daily as the
safe maximum rate of filtration, but he has since repeatedly used 2.57
million gallons.

Kümmel,[12] on the other hand, in a somewhat limited series of
experiments, was unable to find any marked connection between the rate
and the efficiency, a rate of 2.57 giving slightly better results than
rates of either 1.28 or 5.14.

The admirably executed experiments made at Zürich in 1886-8 upon this
point, which gave throughout negative results, have but little value in
this connection, owing to the extremely low number of bacteria in the
original water.

At Lawrence in 1892 the following percentages of bacteria (_B.
prodigiosus_) passed at the respective rates:

--------+--------+-----------+---------------------------------------
No. of | Depth. | Effective | Rate. Millions gallons per acre daily.
Filter. | | Size of +-------+-------+-------+-------+-------
| | Sand. | 0.5 | 1.0 | 1.5 | 2.0 | 3.0
--------+--------+-----------+-------+-------+-------+-------+-------
| | | | | | |
33A | 60 | 0.14 | 0.002 | | | 0.040 |
34A | 60 | 0.09 | 0.001 | 0.005 | | 0.020 |
36A | 60 | 0.20 | | | 0.050 | | 0.050
37 | 60 | 0.20 | | | 0.010 | 0.130 |
38 | 24 | 0.20 | 0.018 | | 0.140 | 0.110 | 0.310
39 | 12 | 0.20 | 0.014 | 0.070 | | 0.080 | 0.520
40 | 12 | 0.20 | | 0.070 | | 0.090 |
42 | 12 | 0.20 | 0.016 | | | 0.150 | 0.550
--------+--------+-----------+-------+-------+-------+-------+-------
Average | 0.010 | 0.048 | 0.067 | 0.088 | 0.356
-----------------------------+-------+-------+-------+-------+-------

These results show a very marked decrease in efficiency with
increasing rates, the number of bacteria passing increasing in general
as rapidly as the square of the rate. The 1893 results also showed
decreased efficiency with high rates, but the range in the rates
under comparable conditions was less than in 1892, and the bacterial
differences were less sharply marked.

While the average results at Lawrence, as well as most of the European
experiments, show greatly decreased efficiency with high rates, there
are many single cases, particularly with deep layers of not too coarse
sand, where, as in Kümmel’s experiments, there seems to be little
connection between the rate and efficiency. An explanation of these
apparently abnormal results will be given in Chapter VI.

It is commonly stated[13] that every water has its own special rate of
filtration, which must be determined by local experiments, and that
this rate may vary widely in different cases. Thus it is possible that
the rate of 1.60 adopted at Hamburg for the turbid Elbe water, the rate
of 2.57 used at Berlin, and about the same at London for much clearer
river-waters, and the rate of 7.50 used at Zürich for the almost
perfectly clear lake-water are in each case the most suitable for the
respective waters. In other cases however, where rates much above 2.57
are used for river-waters, as at Lübeck and Stettin, there is a decided
opinion that these rates are excessive, and in these instances steps
are now being taken to so increase the filtering areas as to bring the
rates within the limit of 2.57 million gallons per acre daily.

From the trend of European practice it would seem that for American
river-waters the rate of filtration should not exceed 2.57 in place of
the 3.90 million gallons per acre daily recommended by Kirkwood, or
even that a somewhat lower rate might be desirable in some cases. Of
course, in addition to the area

necessary to give this rate, a reserve for fluctuating rates and for
cleaning should be provided, reducing the average yield to 2.00, 1.50,
or even less. In the case of water from clear lakes, ponds, or storage
reservoirs, especially when they are not subject to excessive sewage
pollution or to strong algæ growths, it would seem that rates somewhat
and perhaps in some cases very much higher (as at Zürich) could be
satisfactorily used.

THE LOSS OF HEAD.

The loss of head is the difference between the heads of the waters
above and below the sand layer, and represents the frictional
resistance of that layer. When a filter is quite free from clogging
this frictional resistance is small, but gradually increases with the
deposit of a sediment layer from the water filtered until it becomes so
great that the clogging must be removed by scraping before the process
can be continued. After scraping the loss of head is reduced to, or
nearly to, its original amount. With any given amount of clogging the
loss of head is directly proportional to the rate of filtration; that
is, if a filter partially clogged, filtering at a rate of 1.0, has a
frictional resistance of 0.5 ft., the resistance will be doubled by
increasing the rate to 2.00 million gallons per acre daily, provided
no disturbance of the sediment layer is allowed. This law for the
frictional resistance of water in sand alone also applies to the
sediment layer, as I have found by repeated tests, although in so
violent a change as that mentioned above, the utmost care is required
to make the change gradually and prevent compression or breaking of the
sediment layer. From this relation between the rate of filtration and
the loss of head it is seen that the regulation of either involves the
regulation of the other, and it is a matter of indifference which is
directly and which indirectly controlled.

REGULATION OF THE RATE AND LOSS OF HEAD IN THE OLDER FILTERS.

In the older works, and in fact in all but a few of the newest works,
the underdrains of the filters connect directly through a pipe with a
single gate with the pure-water reservoir or pump-well, which is so
built that the water in it may rise nearly or quite as high as that
standing upon the filter.

[Illustration: FIG. 5.—SIMPLEST FORM OF REGULATION: STRALAU FILTERS AT
BERLIN.]

A typical arrangement of this sort was used at the Stralau works at
Berlin (now discontinued), Fig. 5. With this arrangement the rate of
filtration is dependent upon the height of water in the reservoir or
pump-well, and so upon the varying consumption. When the water in the
receptacle falls with increasing consumption the head is increased,
and with it the rate of filtration, while, on the other hand, with
decreasing draft and rising water in the reservoir, the rate of
filtration decreases and would eventually be stopped if no water were
used. This very simple arrangement thus automatically, within limits,
adjusts the rate of filtration to the consumption, and at the same time
always gives the highest possible level of water in the pump-well, thus
also economizing the coal required for pumping.

In plants of this type the loss of head may be measured by floats
on little reservoirs built for that purpose, connected with the
underdrains; but more often there is no means of determining it,
although the maximum loss of head at any time is the difference between
the levels of the water on the filter and in the reservoir, or the
outlet of the drain-pipe, in case the latter is above the water-line
in the reservoir. The rate of filtration can only be measured with this
arrangement by shutting off the incoming water for a definite interval,
and observing the distance that the water on the filter sinks. The
incoming water is regulated simply by a gate, which a workman opens or
closes from time to time to hold the required height of water on the
filter.

The only possible regulation of the rate and loss of head is effected
by a partial closing of the gate on the outlet-pipe, by which the
freshly-cleaned filters with nearly-closed gates are kept from
filtering more rapidly than the clogged filters, the gates of which
are opened wide. Often, however, this is not done, and then the fresh
filters filter many times as rapidly as those which are partially
clogged.

A majority of the filters now in use are built more or less upon this
plan, including most of those in London and also the Altona works,
which had such a favorable record with cholera in 1892.

The invention and application of methods of bacterial examination in
the last years have led to different ideas of filtration from those
which influenced the construction of the earlier plants. As a result
it is now regarded as essential by most German engineers[14] that
each filter shall be provided with devices for measuring accurately
and at any time both the rate of filtration and the loss of head, and
for controlling them, and also for making the rate independent of
consumption by reservoirs for filtered water large enough to balance
hourly variations (capacity 1/4 to 1/3 maximum daily quantity) and low
enough so that they can never limit the rate of filtration by causing
back-water on the filters. These points are now insisted upon by the
German Imperial Board of Health,[15] and all new filters are built in
accordance with them, while most of the old works are being built over
to conform to the requirements.

APPARATUS FOR REGULATING THE RATE AND LOSS of HEAD.

Many appliances have been invented for the regulation of the rate and
loss of head. In the apparatus designed by Gill and used at both Tegel
and Müggel at Berlin the regulation is effected by partially closing
a gate through which the effluent passes into a chamber in which the
water-level is practically constant (Fig. 6). The rate is measured
by the height of water on the weir which serves as the outlet for
this second chamber into a third connecting with the main reservoir,
while the loss of head is shown by the difference in height of floats
upon water in the first chamber, representing the pressure in the
underdrains, and upon water in connection with the raw water on the
filter. From the respective heights of the three floats the attendant
can at any time see the rate of filtration and the loss of head, and
when a change is required it is effected by moving the gate.

[Illustration: FIG. 6.—REGULATION APPARATUS AT BERLIN (TEGEL).]

In the apparatus designed in 1866 by Kirkwood for St. Louis and never
built (Fig. 7) the loss of head was directly, and the rate indirectly,
regulated by a movable weir, which was to have been lowered from time
to time by the attendant to secure the required results. This plan is
especially remarkable as it meets the modern requirements of a regular
rate independent of rate of consumption and of the water-level in the
reservoir, and also allows continual measurements of both rate (height
of water on the weir) and head (difference in water-levels on filter
and in effluent chamber) to be made, and control of the same by the
position of the weir. Mr. Kirkwood found no filters in Europe with such
appliances, and it was many years after his report was published before
similar devices were used, but they are now regarded as essential.

[Illustration: FIG. 7.—REGULATION APPARATUS AND SECTION OF FILTER
RECOMMENDED FOR ST. LOUIS BY KIRKWOOD IN 1866.]

[Illustration: FIG. 8.—REGULATION APPARATUS USED AT HAMBURG.]

The regulators for new filters at Hamburg (Fig. 8) are built upon the
principle of Kirkwood’s device, but provision is made for a second
measurement of the water if desired by the loss of head in passing a
submerged orifice. Both the rate and loss of head are indicated by a
float on the first chamber connecting directly with the underdrain,
which at the same time indicates the head on a fixed scale, the zero of
which corresponds to the height of the water above the filter, and the
rate upon a scale moving with the weir, the zero of which corresponds
with the edge of the weir. The water on the filter is held at a
perfectly constant level.

The regulators in use at Worms and those recently introduced at
Magdeburg act upon the same principle, but the levels of the water on
the filters are allowed to fluctuate, and the weirs and in fact, the
whole regulating appliances are mounted on big floats in surrounding
chambers of water connecting with the unfiltered water on the filters.
I am unable to find any advantages in these appliances, and they are
much more complicated than the forms shown by the cuts.

APPARATUS FOR REGULATING THE RATE DIRECTLY.

[Illustration: FIG. 9.—LINDLEY’S REGULATION APPARATUS AT WARSAW,
RUSSIA.]

The above-mentioned regulators control directly the loss of head,
and only indirectly the rate of filtration. The regulators at Warsaw
were designed by Lindley to regulate the rate directly and make it
independent of the loss of head. The quantity of water flowing away
is regulated by a float upon the water in the effluent chamber,
which holds the top of the telescope outlet-pipe a constant distance
below the surface and so secures a constant rate. As the friction of
the filter increases the float sinks with the water until it reaches
bottom, when the filter must be scraped. A counter-weight reduces the
weight on the float, and at the same time allows a change in the rate
when desired. This apparatus is automatic. All of the other forms
described require to be occasionally adjusted by the attendant, but
the attention they require is very slight, and watchmen are always on
duty at large plants, who can easily watch the regulators. The Warsaw
apparatus is reported to work very satisfactorily, no trouble being
experienced either by leaking or sticking of the telescope-joint,
which is obviously the weakest point of the device, but fortunately a
perfectly tight joint is not essential to the success of the apparatus.
Regulators acting upon the same principle have recently been installed
at Zürich, where they are operating successfully.

Burton[16] has described an ingenious device designed by him for the
filters at Tokyo, Japan. It consists of a double acting valve of gun
metal (similar to that shown by Fig. 11), through which the effluent
must pass. This valve is opened and closed by a rod connecting with
a piston in a cylinder, the opposite sides of which connect with the
effluent pipe above and below a point where the latter is partially
closed, so that the valve is opened and closed according as the loss
of head in passing this obstruction is below or above the amount
corresponding to the desired rate of filtration.

The use of the Venturi meter in connection with the regulation of
filters would make an interesting study, and has, I believe, never been
considered.

[Illustration: REGULATOR-HOUSE, SHOWING RATE OF FILTRATION AND LOSS OF
HEAD ON THE OUTSIDE, BREMEN.]

[Illustration: INLET FOR ADMISSION OF RAW WATER TO A FILTER, EAST
LONDON.

[_To face page 58._]
]

APPARATUS FOR REGULATING THE HEIGHT OF WATER UPON FILTERS.

It will be seen by reference to the diagrams of the Berlin and Hamburg
effluent regulators (Figs. 6 and 8) that their perfect operation
is dependent upon the maintenance of a constant water-level upon
the filters. The old-fashioned adjustment of the inlet-gate by the
attendant is hardly accurate enough.

The first apparatus for accurately and automatically regulating the
level of the water upon the filters was constructed at Leeuwarden,
Holland, by the engineer, Mr. Halbertsma, who has since used a similar
device at other places, and improved forms of which are now used at
Berlin and at Hamburg.

At Berlin (Müggel) the water-level is regulated by a float upon the
water in the filter which opens or shuts a balanced double valve on
the inlet-pipe directly beneath, as shown in Fig. 10. It is not at all
necessary that this valve should shut water-tight; it is only necessary
that it should prevent the continuous inflow from becoming so great as
to raise the water-level, and for this reason loose, easily-working
joints are employed. The apparatus is placed in a little pit next to
the side of the filter, and the overflowing water is prevented from
washing the sand by paving the sand around it for a few feet.

[Illustration: FIG. 10.—REGULATION OF INFLOW USED AT MÜGGEL, BERLIN.]

At Hamburg the same result is obtained by putting the valve in a
special chamber outside of the filter and connected with the float by a
walking-beam (Fig. 11).

[Illustration: FIG. 11.—REGULATION OF INFLOW USED AT HAMBURG.]

The various regulators require to be protected from cold and ice by
special houses, except in the case of covered filters, where they can
usually be arranged with advantage in the filter itself. In regard to
the choice of the form of regulator for both the inlets and outlets of
filters, so far as I have been able to ascertain, each of the modern
forms described as in use performs its functions satisfactorily, and in
special cases any of them could properly be selected which would in the
local conditions be the simplest in construction and operation.

LIMIT TO THE LOSS OF HEAD.

The extent to which the loss of head is allowed to go before filters
are cleaned differs widely in the different works, some of the newer
works limiting it sharply because it is believed that low bacterial
efficiency results when the pressure is too great, although the
frequency of cleaning and consequently the cost of operation are
thereby increased.

At Darlington, England, I believe as a result of the German theories,
the loss of head is limited to about 18 inches by a masonry weir built
within the last few years. At Berlin, both at Tegel and Müggel, the
limit is 24 inches, while at the new Hamburg works 28 inches are
allowed. At Stralau in 1893 an effort was made to not exceed a limit
of 40 inches, but previously heads up to 60 inches were used, which
corresponds with the 56 inches used at Altona; and, in the other old
works, while exact information is not easily obtained because of
imperfect records, I am convinced that heads of 60 or even 80 inches
are not uncommon. At the Lawrence Experiment Station heads of 70 inches
have generally been used, although some filters have been limited to 36
and 24 inches.

In 1866 Kirkwood became convinced that the loss of head should not go
much above 30 inches, first, because high heads would, by bringing
extra weight upon the sand, make it too compact, and, second, because
when the pressure became too great the sediment layer on the surface of
the sand, in which most of the loss of head occurs, would no longer be
able to support the weight and, becoming broken, would allow the water
to pour through the comparatively large resulting openings at greatly
increased rates and with reduced efficiency.

In regard to the first point, a straight, even pressure many times
that of the water on the filter is incapable of compressing the sand.
It is much more the effect of the boots of the workmen when scraping
that makes the sand compact. I have found sand in natural banks at
Lawrence 70 or 80 feet below the surface, where it had been subjected
to corresponding pressure for thousands of years, to be quite as porous
as when packed in water in experimental filters in the usual way.

The second reason mentioned, or, as I may call it, the breaking-through
theory, is very generally if not universally accepted by German
engineers, and this is the reason for the low limit commonly adopted by
them.

A careful study of the results at Lawrence fails to show the slightest
deterioration of the effluents up to the limit used, 72 inches. Thus
in 1892, taking only the results of the continuous filters of full
height (Nos. 33A, 34A, 36A, and 37), we find that for the three days
before scraping, when the head was nearly 72 inches, the average
number of bacteria in the effluents was 31 per cc., while for the
three days after scraping, with very low heads, the number was 47. The
corresponding numbers of _B. prodigiosus_[17] were 1.1 and 2.7. This
shows better work with the highest heads, but is open to the objection
that the period just after scraping, owing to the disturbance of the
surface, is commonly supposed to be a period of low efficiency.

To avoid this criticism in calculating the corresponding results for
1893, the numbers of the bacteria for the intermediate days which could
not have been influenced either by scraping or by excessive head are
put side by side with the others. Taking these results as before for
continuous filters 72 inches high, and excluding those with extremely
fine sands and a filter which was only in operation a short time toward
the end of the year, we obtain the following results:

Water B.
Bacteria Prodigiosus
per cc. per cc.
Average 1st day after scraping, low heads 79 6.1
Average 2d day after scraping, low heads 44 4.1
Average 3d day after scraping, low heads 45 3.6
Intermediate days, medium heads 59 4.5
Second from last day, heads of nearly 72 inches 66 2.7
Next to the last day, heads of nearly 72 inches 56 3.2
Last day, heads of nearly 72 inches 83 2.5

These figures show a very slight increase of the water bacteria in the
effluent as the head approaches the limit, but no such increase as
might be expected from a breaking through of the sediment layer, and
the _B. prodigiosus_ which is believed to better indicate the removal
of the bacteria of the original water, actually shows a decrease, the
last day being the best day of the whole period.

The Lawrence results, then, uniformly and clearly point to a conclusion
directly opposite to the commonly accepted view, and I have thus
been led to examine somewhat closely the grounds upon which the
breaking-through theory rests.

The two works which have perhaps contributed most to the theories
of filtration are the Stralau and Altona works. After examining the
available records of these works, I am quite convinced that at these
places there has been, at times at least, decreased efficiency with
high heads. For the Stralau works this is well shown by Piefke’s
plates in the _Zeitschrift für Hygiene_, 1894, after page 188. In both
of these works, however, the apparatus (or lack of apparatus) for
regulating the rate is that shown by Fig. 5, page 49, and the rate
of filtration is thus dependent upon the rate of consumption and the
height of water in the reservoir. At the Stralau works, at the time
covered by the above-mentioned diagrams, the daily quantity of water
filtered was 27 times the capacity of the reservoir, and the rate
of filtration must consequently have adapted itself to the hourly
consumptions. The data which formed the basis of Kirkwood’s conclusions
are not given in detail, but it is quite safe to assume that they were
obtained from filters regulated as those at Altona and Stralau are
regulated, and what is said in regard to the latter will apply equally
to his results.

Piefke[18] shows that among the separate filters at Stralau, all
connected with the same pure-water reservoir, those connected through
the shorter pipes gave poorer effluents than the more remote filters,
and he attributes the difference to the frictional resistance of the
connecting pipes, which helped to prevent excessive rates in the
filters farthest away when the water in the reservoir became low, and
thus the fluctuations in the rates in these filters were less than in
those close to the reservoir. He

does not, however, notice, in speaking of the filters in which the
decreased efficiencies with high heads were specially marked, that they
follow in nearly the same order, and that of the four open filters
mentioned three were near the reservoir and only one was separated by
a comparatively long pipe, indicating that the deterioration with high
heads was only noticeable, or at least was much more conspicuous, in
those filters where the rates fluctuated most violently.

It requires no elaborate calculation to show that of two filters
connected with the same pure-water reservoir, as shown by Fig. 5,
with only simple gates on the connecting pipes, one of them clean and
throttled by a nearly closed gate, so that the normal pressure behind
the gate is above the highest level of water in the reservoir, and the
other clogged so that the normal pressure of the water in the drain is
considerably below the highest level of the water in the reservoir,
the latter will suffer much the more severe shocks with fluctuating
water-levels; and the fact being admitted that fluctuating levels are
unfavorable, we must go farther and conclude that the detrimental
action will increase with increasing loss of head. I am inclined to
think that this theory is adequate to explain the Stralau and Altona
results without resource to the breaking-through theory.

While the above does not at all prove the breaking-through theory to be
false, it explains the results upon which it rests in another way, and
can hardly fail to throw so much doubt upon it as to make us refuse to
allow its application to those works where a regular rate of filtration
is maintained regardless of variations in the consumption, until proof
is furnished that it is applicable to them.

I have been totally unable to find satisfactory European results in
regard to this point. The English works can furnish nothing, both on
account of the lack of regulating appliances and because the monthly
bacterial examinations are inadequate for a discussion of hourly or
daily changes. The results from the older Continental works are also
excluded for one or the other, or more often for both, of the above
reasons. The Hamburg, Tegel, and Müggel results, so far as they go,
show no deterioration with increased heads, but the heads are limited
to 24 or 28 inches by the construction of the filters, and the results
thus entirely fail to show what would be obtained with heads more than
twice as high.

I am thus forced to conclude that there is no adequate evidence of
inferior efficiency with high heads in filters where the rates are
independent of the water-level in the pure-water reservoir, the
only results directly to the point—the Lawrence results mentioned
above—indicating that the full efficiency is maintained with heads
reaching at least 72 inches.

The principal reason for desiring to allow a considerable loss of head
is an economical one; the period will then be lengthened, while the
frequency of scraping and the volume of sand to be washed and replaced
will be correspondingly reduced. There may be other advantages in long
periods, such as less trouble with scraping and better work in cold
winter weather, but the cost is the most important consideration.

It is the prevalent idea among the German engineers that the loss of
head after reaching 24 to 30 inches would increase very rapidly, so
that the quantity of water filtered, in case a much higher head was
allowed, would not be materially increased. No careful investigations,
however, have been made, and indeed they are hardly possible with
existing arrangements, as in the older filters the loss of head
fluctuates with varying rates of filtration in such a way that only
results of very doubtful value can be obtained, and in the newer works
the loss of head is too closely limited, and the curves which can be
drawn by extrapolation are evidently no safe indications of what would
actually happen if the process was carried farther.

On the other hand, I was told by the attendant at Darlington, England,
that since the building of the weir a few years ago, which now limits
the loss of head to about 18 inches instead of the 5 feet or more
formerly used, the quantity of sand to be removed has been three times
as great as formerly. No records are kept, and this can only be given
as the general impression of the man who superintends the work.

At Lawrence the average quantities of water filtered between scrapings
with sand of an effective size of 0.20 mm. have been as follows:

Maximum Loss of Million Gallons per Acre filtered
Head. between Scrapings.

1892. 1893. Average.

70 inches 58 88 73
34 foot 32 22 27
22 foot 17 16 16

With sand of an effective size of 0.29 mm. the results were:

1893.

70 inches 70
22 foot 29

These results indicate a great increase in the quantity of water
filtered between scrapings with increasing heads, the figures being
nearly proportional to the maximum heads used in the respective
cases. It is, of course, quite possible that the results would differ
in different places with the character of the raw water and of the
filtering material.

The depth of sand to be removed by scraping at one time is, within
limits, practically independent of the quantity of dirt which it has
accumulated, and any lengthening of the period means a corresponding
reduction in the quantity of sand to be removed, washed and replaced
and consequently an important reduction in the operating cost, as well
as a reduction in the area of filters out of use while being cleaned,
and so, in the capital cost.

Among the minor objections to an increased loss of head are the
greater head against which the water must be pumped, and the possible
increased difficulty of filling filters with filtered water from below
after scraping, but these would hardly have much weight against the
economy indicated by the Lawrence experiments for the higher heads.

High heads will also drive an increased quantity of water through any
cracks or passages in the filter. Such leaks have at last been found to
be the cause of the inferior work of the covered filters at Stralau,
the water going down unfiltered in certain corners, especially at high
heads; but with careful construction there should be no cracks, and
with the aid of bacteriology to find the possible leaks this ought not
to be a valid objection.

In conclusion: the trend of opinion is strongly in favor of limiting
the loss of head to about 24 to 30 inches as was suggested by Kirkwood,
but I am forced to conclude that there is reason to believe that
equally good results can be obtained with lower operating expenses by
allowing higher heads to be used, at least in the case of filters with
modern regulating appliances, and, I would suggest that filters should
be built so as not to exclude the use of moderately high heads, and
that the limit to be permanently used should be determined by actual
tests of efficiency and length of period with various losses of head
after starting the works.

CHAPTER V.

CLEANING FILTERS.

When a filter has become so far clogged that it will no longer pass
a satisfactory quantity of water with the allowable head it must be
cleaned by scraping off and removing the upper layer of dirty sand.

To do this without unnecessary loss of time the unfiltered water
standing upon the filter is removed by a drain above the sand provided
for that purpose. The water in the sand must then be lowered below the
surface of the sand by drawing water from the underdrains until the
sand is firm enough to bear the weight of the workmen. By the time
that this is accomplished the last water on the surface should have
soaked away, and the filter is ready to be scraped. This is done by
workmen with wide, sharp shovels, and the sand removed is taken to
the sand-washing apparatus to be washed and used again. Special pains
are given to securing rapid and cheap transportation of the sand. In
some cases it is wheeled out of the filter on an inclined plane to the
washer. In other cases a movable crane is provided which lifts the sand
in special receptacles and allows it to fall into cars on a tram-line
on which the crane also moves. The cars as filled are run to the washer
and also serve to bring back the washed sand. When the dirty sand has
been removed, the surface of the sand is carefully smoothed and raked.
This is especially necessary to remove the effects of the workmen’s
boots.

It is customary in the most carefully managed works to fill the sand
with filtered water from below, introduced through the underdrains. In
case the ordinary level of the water in the pure-water canal is higher
than the surface of the sand in the filters, this is accomplished
by simply opening a gate provided for the purpose, which allows the
water to pass around the regulating apparatus. Otherwise filters can
be filled from a special pipe taking its water from any filter which
at that time can deliver its effluent high enough for that purpose.
The quantity of water required for filling the sand from below is
ordinarily but a fraction of one per cent of the quantity filtered.

Formerly, instead of filling from below, after cleaning, the raw water
was brought directly onto the surface of the filter. This was said to
only imperfectly fill the sand-pores, which still contained much air.
If, however, the water is not brought on too rapidly it will sink into
the sand near the point where it is applied, pass laterally through the
sand or underlying gravel to other parts of the filter, and then rise,
so that even in this case all but a little of the filter will be really
filled from below. This is, however, open to the objection that however
slowly the water is introduced, the sand which absorbs it around the
inlet filters it at a very high rate and presumably imperfectly, so
that the water in the underdrains at the start will be poor quality
and the sand around the inlet will be unduly clogged. The practice of
filling from below is therefore well founded.

As soon as the surface of the sand is covered with the water from
below, raw water is introduced from above, filling the filter to
the standard height, care being taken at first that no currents
are produced which might wash the surface of the sand. It has been
recommended by Piefke and others that this water should be allowed
to stand for a time up to twenty-four hours before starting the
filtration, to allow the formation of a sediment layer, and in some
places, especially at Berlin and the works of some of the London
companies, this is done; but varying importance is attached to the
procedure, and it is invariably omitted, so far as I can learn, when
the demand for water is heavy.

The depth of sand removed by scraping must at least equal the
depth of the discolored layer, but there is no sharp dividing line,
the impurities gradually decreasing from the surface downward.
Fig. 12 shows the relative number of bacteria found in the sand at
various depths in one of the Lawrence experimental filters, and is a
representative result, although the actual numbers vary at different
times. In general it may be said that the bulk of the sediment is
retained in the upper quarter inch, but it is desirable to remove also
the less dirty sand below and, in fact, it is apparently impossible
with the method of scraping in use to remove so thin a layer as one
fourth inch. Practically the depth to which sand is removed is stated
to be from 0.40 to 1.20 inch. Exact statistics are not easily obtained,
but I think that 2 centimeters or 0.79 inch may be safely taken as
about the average depth usually removed in European filters, and it is
this depth which is indicated on Fig. 12.

[Illustration: FIG. 12.—DIAGRAM SHOWING ACCUMULATION OF BACTERIA NEAR
THE SURFACE OF THE SAND.]

At the Lawrence Experiment Station, the depth removed is often much
less than this, and depends upon the size of grain of the sand
employed, the coarser sands requiring to be more deeply scraped than
the finer ones. The method of scraping, however, which allows the
removal of very thin sand layers, is only possible because of the
small size of the filters, and as it is incapable of application on a
large scale, the depths thus removed are only interesting as showing
the results which might be obtained in practice with a more perfect
method of scraping.

The replacing of the washed sand is usually delayed until the filter
has been scraped quite a number of times—commonly for a year. The last
scraping before refilling is much deeper than usual, because the sand
below the depth of the ordinary scraping is somewhat dirty, and might
cause trouble if left below the clean sand.

In England it is the usual if not the universal practice to replace the
washed sand at the bottom between the old sand and the gravel. This is
done by digging up the entire filter in sections about six feet wide.
The old sand in the first section is removed clear down to the gravel,
and the depth of washed sand which is to be replaced is put in its
place. The old sand from the next six-foot section is then shovelled
upon the first section of clean sand, and its place is in turn filled
with fresh sand. With this practice the workmen’s boots are likely to
disturb the gravel each year, necessitating a thicker layer of the
upper and finest grade than would otherwise be required.

In Germany this is also sometimes done, but more frequently the upper
layer of slightly clogged sand below the regular scraping is removed
as far as the slightest discoloration can be seen, perhaps 6 inches
deep. The sand below is loosened for another 6 inches and allowed to
stand dry, if possible, for some days; afterwards the washed sand is
brought on and placed above. The washed sand is never replaced without
some such treatment, because the slightly clogged sand below the layer
removed would act as if finer than the freshly washed sand,[19] and
there would be a tendency to sub-surface clogging.

FREQUENCY OF SCRAPING.

The frequency of scraping depends upon the character of the raw water,
the thoroughness of the preliminary sedimentation, the grain-size of
the filter sand, the rate of filtration, and the maximum loss of head
allowed. With suitable conditions the period between scrapings should
never be less than one week, and will but rarely exceed two months.
Under exceptional conditions, however, periods have been recorded as
low as one day and as high as one hundred and ten days. Periods of less
than a week’s duration are almost conclusive evidence that something
is radically wrong, and the periods of one day mentioned were actually
accompanied by very inadequate filtration. In 1892 the average periods
at the German works varied from 9.5 days at Stettin (with an excessive
rate) to 40 days at Brunswick, the average of all being 25 days.[20]

The quantity of water per acre filtered between scrapings forms the
most convenient basis for calculation. The effect of rate (page
49), loss of head (page 65), and size of sand grain (page 32) have
already been discussed, and it will suffice to say here that the total
quantity filtered between scrapings is apparently independent of the
rate of filtration, but varies with the maximum loss of head and with
the grain-size of the sand, and apparently nearly in proportion to
them. Eleven German filter-works in 1892, drawing their waters from
rivers, filtered on an average 51 million gallons of water per acre
between scrapings, the single results ranging from 28 at Bremen to 71
at Stuttgart, while Zürich, drawing its water from a lake which is
but very rarely turbid, filtered 260 million gallons per acre between
scrapings. Unfortunately, the quantities at Berlin, where (in 1892 two
thirds and now all) the water is drawn from comparatively large ponds
on the rivers, are not available for comparison.

At London, in 1884, the average quantities of water filtered

between scrapings varied from 43 to 136 million gallons per acre with
the different companies, averaging 85, and in 1892 the quantities
ranged from 73 to 157, averaging 90 million gallons per acre. The
greater quantity filtered at London may be due to the greater sizes of
the sedimentation-basins, which for all the companies together hold a
nine days’ supply at London against probably less than one day’s supply
for the German works.

There is little information available in regard to the frequency
of scraping with water drawn from impounding reservoirs. In some
experiments made by Mr. FitzGerald at the Chestnut Hill reservoir,
Boston, the results of which are as yet unpublished, a filter with
sand of an effective size of only .09 mm. averaged 58 million gallons
per acre between scrapings for nine periods, the rate of filtration
being 1.50 million gallons per acre daily, while another filter, with
sand of an effective size of .18 mm., passed an average of 93 million
gallons per acre for ten periods at the same rate. These experiments
extended through all seasons of the year, and taking into account the
comparative fineness of the sands they show rather high quantities of
water filtered between scrapings.

The quantity of water filtered between scrapings is usually greatest
in winter, owing to the smaller quantity of sediment in the raw water
at this season, and is lowest in times of flood, regardless of season.
In summer the quantity is often reduced to a very low figure in waters
supporting algæ growths, especially when the filters are not covered.
Thus at Stralau in 1893 during the algæ period the quantity was reduced
to 14 million gallons per acre for open filters,[21] but this was quite
exceptional, the much-polluted, though comparatively clear, Spree water
furnishing unusually favorable conditions for the algæ.

QUANTITY OF SAND TO BE REMOVED.

In regard to the quantity of sand to be removed and washed, if we
take the average result given above for the German works filtering
river-waters of 51,000,000 gallons per acre filtered between scrapings,
and the depth of sand removed at two centimeters or 0.79 inch, we
find that one volume of sand is required for every 2375 volumes of
water filtered, or 2.10 cubic yards per million gallons. At Bremen,
the highest average result, the quantity would be 3.80 yards, and at
Stralau during the algæ season 7.70 yards. At Zürich, on the other
hand, the quantity is only 0.41 yard, and at London, with 87,000,000
gallons per acre filtered between scrapings, the quantity of sand
washed would be 1.24 yards per million gallons; assuming always that
the layer removed is 0.79 inch thick.

These estimates are for the regular scrapings only, and do not include
the annual deeper scraping before replacing the sand, which would
increase them by about one third.

WASTING THE EFFLUENTS AFTER SCRAPING.

It has already been stated that an important part of the filtration
takes place in the sediment layer deposited on top of the sand from
the water. When this layer is removed by scraping its influence is
temporarily removed, and reduced efficiency of filtration may result.
The significance of this reduced efficiency became apparent when the
bacteria in the water were studied in their relations to disease, and
Piefke suggested[22] that the first effluent after scraping should
be rejected for one day after ordinary scrapings and for one week
after replacing the sand. In a more recent paper[23] he reduces these
estimates to the first million gallons of water per acre filtered after
scraping

for open and twice as great a quantity for covered filters, and to six
days after replacing the sand, which last he estimates will occur only
once a year. Taking the quantity of water filtered between scrapings at
13.9 million gallons per acre, the quantity observed at Stralau in the
summer of 1893, he finds that it is necessary to waste 9 per cent of
the total quantity of effluent from open and 13.8 per cent of that from
covered filters.

The eleven German water-works[24] filtering river-waters, however,
filtered on an average 51.0 instead of 13.9 million gallons per acre
between scrapings, and applying Piefke’s figures to them the quantities
of water to be wasted would be only about one fourth of his estimates
for Stralau.

The rules of the Imperial Board of Health[25] require that every German
filter shall be so constructed “that when an inferior effluent results
it can be disconnected from the pure-water pipes and the filtrate
allowed to be wasted.” The drain-pipe for removing the rejected water
should be connected below the apparatus for regulating the rate and
loss of head, so that the filter can be operated exactly as usual,
and the effluent can be turned back to the pure-water pipes without
stopping or changing the rate. The works at Berlin and at Hamburg
conform to this requirement, and most of the older German works have
been or are being built over to make them do so.

In regard to the extent of deterioration after scraping, Piefke’s
experiments have always shown much larger numbers of bacteria both of
the ordinary forms and of special applied forms on the first day after
scraping, the numbers frequently being many times as high as at other
times.

At the Lawrence Experiment Station it was found in 1892 that on an
average the number of water bacteria was increased by 70 per cent
(continuous filters only) for the three days following scraping, while
_B. prodigiosus_ when applied was increased 140 per

cent, the increase being most marked where the depth of sand was
least, and with the highest rate of filtration.

The same tendency was found in 1893, when the increase in the water
bacteria on the first day after scraping was only 19 per cent and _B.
prodigiosus_ 64 per cent, but for a portion of the year the difference
was greater, averaging 132 and 262 per cent, respectively. These
differences are much less than those recorded by Piefke, and with the
high efficiencies regularly obtained at Lawrence they would hardly
justify the expensive practice of wasting the effluent.

The reduction in efficiency following scraping is much less at low
rates, and if a filter is started at much less than its normal rate
after scraping, and then gradually increased to the standard after
the sediment layer is formed, the poor work will be largely avoided.
Practically this is done at Berlin and at Hamburg. The filters are
started at a fourth or less of the usual rates and are gradually
increased, as past experience with bacterial results has shown it can
be safely done, and the effluent is then even at first so well purified
that it need not be wasted.

Practically in building new filters the provision of a suitable
connection for wasting the effluents into the drain which is necessary
for emptying them involves no serious expense and should be provided,
but it may be questioned how often it should be used for wasting the
effluents. If the raw water is so bad that a good effluent cannot be
obtained by careful manipulation even just after scraping, the course
of the Berlin authorities in closing the Stralau works and seeking a
less polluted supply would seem to be the only really safe procedure.

SAND-WASHING.

[Illustration: CLEANING A FILTER, EAST LONDON.]

[Illustration: WASHING DIRTY SAND WITH HOSE, ANTWERP.]

[_To face page 76._]

The sand-washing apparatus is an important part of most European
filtering plants. It seldom happens that a natural sand can be found
clean enough and sufficiently free from fine particles although such
a sand was found and used for the Lawrence filter. Most of the sand in
use for filtration in Europe was originally washed. In the operation of
the filters also, sand-washing is used for the dirty sand, which can
then be used over and over at a much lower cost than would be the case
if fresh sand was used for refilling. The methods used for washing sand
at the different works present a great variety both in their details
and in the underlying principles. Formerly boxes with double perforated
bottoms in which the sand was placed and stirred by a man as water from
below rose through them, and other similar arrangements were commonly
used, but they are at present only retained, so far as I know, in some
of the smaller English works. The cleansing obtained is apparently
considerably less thorough than with some of the modern devices.

[Illustration: FIG. 13.—HOSE-WASHING FOR DIRTY SAND.]

Hose-washing is used in London by the Southwark and Vauxhall, Lambeth
and Chelsea companies, and also at Antwerp. For this a platform is
constructed about 15 feet long by 8 feet wide, with a pitch lengthwise
of 6 to 8 inches (Fig. 13). The platform is surrounded by a wall
rising from one foot at the bottom to three feet high at the top,
except the lower end, which is closed by a removable plank weir 5 or 6
inches high. From two to four cubic yards of the sand are placed upon
this platform and a stream of water from a hose with a 3/4 or 7/8-inch
nozzle is played upon it, moving it about from place to place. The sand
itself is always kept toward the upper end of the platform, while the
water with the dirt removed flows down into the pond made by the weir,
where the sand settles out and the dirt overflows with the water. When
the water comes off clear, which is usually after an hour or a little
less, the weir is removed, and, after draining, the sand is removed.
These arrangements are built in pairs so that the hose can be used in
one while the sand is being changed in the other. They are usually
built of brick laid in cement, but plank and iron are also used. The
corners are sometimes carried out square as in the figure, but are more
often rounded. The washing is apparently fairly well done.

In Germany the so-called “drum” washing-machine, drawings of which have
been several times published,[26] has come to be almost universally
used. It consists of a large revolving cylinder, on the bottom of the
inside of which the sand is slowly pushed up toward the higher end by
endless screw-blades attached to the cylinder, while water is freely
played upon it all the way. The machine requires a special house for
its accommodation and from 2 to 4 horse-power for its operation. It
washes from 2.5 to 4 yards of sand per hour most thoroughly, with a
consumption of from 11 to 14 times as large a volume of water. The
apparatus is not patented or made for sale, but full plans can be
easily secured.

A machine made by Samuel Pegg & Sons, Leicester, Eng., pushes the
sand up a slight incline down which water flows. It is very heavy and
requires power to operate it. The patent has

expired. A machine much like it but lighter and more convenient and
moved by water-power derived from the water used for washing instead of
steam-power is used at Zürich with good results.

In Greenway’s machine the sand is forced by a screw through a long
narrow cylinder in which there is a current of water in the opposite
direction. The power required is furnished by a water-motor, as with
the machine at Zürich. The apparatus is mounted on wheels and is
portable; it has an appliance for piling up the washed sand or loading
it onto cars. It is patented and is manufactured by James Gibb & Co.,
London.

Several of the London water companies are now using ejector washers,
and such an apparatus has been placed by the side of the “drum” washers
at Hamburg. This apparatus was made by Körting Brothers in Hannover,
and combines the ejectors long made by that firm with hoppers from
designs by Mr. Bryan, engineer of the East London Water Company. An
apparatus differing from this only in the shape of the ejectors and
some minor details has been patented in England, and is for sale by
Messrs. Hunter, Frazer & Goodman, Bow, London.

Both of these forms consist of a series of conical hoppers, from the
bottom of each of which the sand and water are forced into the top of
the next by means of ejectors, the excess of dirty water overflowing
from the top of each hopper. The apparatus is compact and not likely
to get out of order, but is not portable. It can be easily arranged to
take the sand at the level of the ground, or even lower if desired, and
deliver it washed at some little elevation, thus minimizing hand-labor.
The washing is regular and thorough. The objection most frequently
raised against its use is the quantity of water required, but at
Hamburg I was informed that the volume of water required was only about
15 times that of the sand, while almost as much (13-14 volumes) were
required for the “drum” washers, and the saving in power much more
than offset the extra cost for water.

In addition to the above processes of sand-washing, Piefke’s method
of cleaning without scraping[27] might be mentioned, although as
yet it has hardly passed the experimental stage, and has only been
used on extremely small filters. The process consists of stirring
the surface sand of the filter with “waltzers” while a thin sheet of
water rapidly flows over the surface. This arrangement necessitates a
special construction of the filters, providing for rapidly removing
the unfiltered water from the surface, and for producing a regular and
rapid movement of a thin sheet of water over the surface. In the little
filters now in use, one of which I saw in a brewery in Berlin, the
cleaning is rapidly, cheaply, and apparently well done.

In washing dirty sand it is obvious that any small sand-grains will
be removed with the dirt, and in washing new sand the main object is
to remove the grains below a certain size. It is also apparent that
the sizes of grains which will and those which will not be removed
are dependent upon the mechanical arrangements of the washer, as, for
example, with the ejectors, upon the sizes of the hoppers, and the
quantity of water passing through them, and care should be taken to
make them correspond with the size of grain selected for the filter
sand. This can only be done by experiment, as no results are available
on this point.

In some places filtered water is used for sand-washing, although this
seems quite unnecessary, as ordinary river-water answers very well.
It is, however, often cheaper, especially in small works, to use the
filtered water from the mains rather than provide a separate supply for
the washers.

The quantity of water required for washing may be estimated at 15 times
the volume of the sand and the sand as 0.04 per cent of the volume of
the water filtered (page 74), so that

0.6 per cent of the total quantity of water filtered will be required
for sand-washing.

The cost of sand-washing in Germany with the “drum” washers is said
to be from 14 to 20 cents per cubic yard, including labor, power, and
water. In America the water would cost no more, but the labor would be
perhaps twice as dear. With an ejector apparatus I should estimate the
cost of washing dirty sand as follows: The sand would be brought and
dumped near to the washer, and one man could easily feed it in, as no
lifting is required. Two men would probably be required to shovel the
washed sand into barrows or carts with the present arrangements, but I
think with a little ingenuity this handling could be made easier.

ESTIMATED COST OF OPERATING EJECTOR WASHERS 9 HOURS.

Wages of 3 men at $2.00 $6.00
110,000 gals, water (15 times the volume of sand)
at 0.05 a thousand gals. 5.50
-----
Total cost of washing 36 cubic yards $11.50
or 32 cents a cubic yard.

The cost of washing new sand might be somewhat less. The other costs of
cleaning filters, scraping, transporting, and replacing the sand are
much greater than the washing itself. Lindley states that at Warsaw 29
days’ labor of 10 hours for one man are required to scrape an acre of
filter surface, and four times as much for the annual deep scraping,
digging up, and replacing the sand. The first expense occurs in general
monthly, and the second only once a year. At other places where I have
secured corresponding data the figures range from 19 to 40 days’ labor
to scrape one acre, and average about the same as Lindley estimates.

Under some conditions sand-washing does not pay, and in still others
it is almost impossible. No apparatus has yet been devised which will
wash the dirt out of the fine dune-sands used in Holland without
washing a large part of the sand itself away, and in these works fresh
sand, which is available in unlimited quantities and close to the
works, is always used. At Breslau the dirty sand is sold for building
purposes for one third of the price paid for new sand dredged from the
river, delivered at the works, and no sand is ever washed. Budapest,
Warsaw, and Rotterdam also use fresh river-sand without washing, except
a very crude washing to remove clay at Budapest.

CHAPTER VI.

THEORY AND EFFICIENCY OF CONTINUOUS FILTRATION.

The first filters for a public water-supply were built by James
Simpson, engineer of the Chelsea Water Company at London in 1829. They
were apparently intended to remove dirt from the water in imitation
of natural processes, and without any very clear conception of either
the exact extent of purification or the way in which it was to be
accomplished. The removal of turbidity was the most obvious result, and
a clear effluent was the single test of the efficiency of filtration,
as it remains the legal criterion of the work of the London filters
even to-day, notwithstanding the discovery and use of other and more
delicate tests.

The invention and use of methods for determining the organic matters
in water by Wanklyn and Frankland, about 1870, led to the discovery
that the proportion of organic matters removed by filtration was
disappointingly low, and as, at the time, and for many years afterward,
an exaggerated importance was given to the mere quantities of organic
matters in water, it was concluded that filtration had only a limited
influence upon the healthfulness of the filtered water, and that
practically as much care must be given to securing an unpolluted water
as would be the case if it were delivered direct without filtration.
This theory, although not confirmed by more recent investigation,
undoubtedly has had a good influence upon the English works by causing
the selection of raw waters free from excessive pollutions, and, in
cases like the London supplies, drawn from the Thames and the Lea, in
stimulating a most jealous care of the watersheds and the purification
of sewage by the towns upon them.

It was only after the discovery of the bacteria in water and their
relations to health that the hygenic significance of filtration
commenced to be really understood. Investigations of the bacteria in
the waters before and after filtration were carried out at Berlin
by Plagge and Proskauer, at London by Dr. Percy Frankland, and also
at Zürich, Altona, and on a smaller scale at other places. These
investigations showed that the bacteria were mainly removed by
filtration, the numbers in the effluents rarely exceeding two or three
per cent of those in the raw water. This gave a new aspect to the
problem.

It was further observed, especially at Berlin and Zürich, that the
numbers of bacteria in effluents were apparently quite independent
of the numbers in the raw water, and the theory was formed that all
of the bacteria were stopped by the filters, and that those found in
the effluents were the result of contamination from the air and of
growths in the underdrains. The logical conclusion from this theory was
that filtered water was quite suitable for drinking regardless of the
pollution of its source.

It was, however, found that the numbers of bacteria in the effluents
were higher immediately after scraping than at other times, and it was
concluded that before the formation of the sediment layer some bacteria
were able to pass the sand, and it was therefore recommended that the
first water filtered after scraping should be rejected.

Piefke at Berlin gave the subject careful study, and came to the
conclusion that it was almost entirely the sediment layer which
stopped the bacteria, and that the bacteria themselves in the sediment
layer formed a slimy mass which completely intercepted those in the
passing water. When this layer was removed by scraping, the action
was stopped until a new crop of bacteria had accumulated. In support
of this idea he stated that he had taken ordinary good filter-sand
and killed the bacteria in it by heating it, and that on passing
water through, no purification was effected—in fact, the effluent
contained more bacteria than the raw water. After a little, bacteria
established themselves in the sand, and then the usual purification
was obtained. Piefke concluded that the action of the filter was a
biological one; that simple straining was quite inadequate to produce
the results obtained; that the action of the filter was mainly confined
to the sediment layer, and that the depth of sand beyond the slight
depth necessary for the support of this layer had no appreciable
influence upon the results. The effect of this theory is still seen in
the shallow sand layers used at Berlin and some other German works,
although at London the tendency is rather toward thicker sand layers.

Piefke’s deductions, however, are not entirely supported by his data
as we understand them in the light of more recent investigation.
The experiment with sterilized sand has been repeatedly tried at
the Lawrence Experiment Station with results which quite agree with
Piefke’s, but it has also been found that the high numbers, often many
times as high as in the raw water, do not represent bacteria which pass
in the ordinary course of filtration, but instead enormous growths of
bacteria throughout the sand supported by the cooked organic matter in
it. It has been repeatedly found that ordinary sand quite incapable of
supporting bacterial growths, after heating to a temperature capable
of killing the bacteria will afterwards furnish the food for most
extraordinary numbers. A filter of such sand may stop the bacteria of
the passing water quite as effectually as any other filter, but if so,
the fact cannot be determined without recourse to special methods, on
account of the enormous numbers of bacteria in the sand, a small part
of which are carried forward by the passing water, and completely mask
the normal action of the filter.

The theory that all or practically all of the bacteria are intercepted
by the sediment layer, and that those in the effluent are the result
of growths in the sand or underdrains, received two hard blows in 1889
and 1891, when mild epidemics of typhoid fever followed unusually
high numbers of bacteria in the effluents at Altona and at Stralau in
Berlin, with good evidence in each case that the fever was directly due
to the water. Both of these cases came during, and as the result of,
severe winter weather with open filters and under conditions which are
now recognized as extremely unfavorable for good filtration.

As a result of the first of these epidemics a series of experiments
were made at Stralau by Fränkel and Piefke in 1890. Small filters were
constructed, and water passed exactly as in the ordinary filters.
Bacteria of special kinds not existing in the raw water or effluents
were then applied, and the presence of a very small fraction of them
in the effluents demonstrated beyond a doubt that they had passed
through the filters under the ordinary conditions of filtration. These
experiments were afterwards repeated by Piefke alone under somewhat
different conditions with similar results. The numbers of bacteria
passing, although large enough to establish the point that some do
pass, were nevertheless in general but a small fraction of one per cent
of the many thousands applied.

This method of testing the efficiency of filters had already been
used quite independently by Prof. Sedgwick at the Lawrence Experiment
Station in connection with the purification of sewage, and has since
been extensively used there for experiments with water-filtration.

Kümmel also found at Altona that while in the regular samples for
bacterial examination, all taken at the same time in the day, there
was no apparent connection between the numbers of bacteria in the raw
water and effluents, by taking samples at frequent intervals throughout
the twenty-four hours, as has been done in a more recent series of
experiments, and allowing for the time required for the water to pass
the filters, a well-marked connection was found to exist between the
numbers of bacteria in the raw water and in the effluents.

[Illustration: FIG. 14.—SHOWING BACTERIA SUPPOSED TO COME THROUGH
FILTERS AND FROM THE UNDERDRAINS.]

The subject has more recently been studied in much detail at the
Lawrence Experiment Station, and it now appears that the bacteria
in the effluent from a filter are from two sources: directly from
the filtered water, and from the lower layers of the filter and
underdrains. Thus we may say:

Bacteria in effluent = Bacteria from underdrains + _a_/100 × bacteria
in raw water,

where _a_ is the per cent of bacteria actually passing the filter.

Both of these terms depend upon a whole series of complex and but
imperfectly understood conditions. In general the bacteria from the
underdrains are low in cold winter weather, often almost _nil_, while
at Lawrence with water temperatures of 70 to 75 degrees, and over, in
July and August, the numbers from this source may reach 200 or 300,
but for the other ten months of the year rarely exceed 50 under normal
conditions. In summer especially it seems to be greater at low than
at high rates of filtration (although a high rate for a short time
only increases it), and so varies in the opposite way from the numbers
actually passing the filters. This subject is by no means clearly
understood; it is difficult, almost impossible, to separate the numbers
of bacteria into the two parts—those which come directly through and
may be dangerous, and those which have other origins and are harmless.
The sketch, Fig. 14, is drawn to represent my idea of the way they may
be divided. It has no statistical basis whatever. The light unshaded
section shows the percentage number of bacteria which I conceive to
be coming through a filter under given conditions at various rates of
filtration, while the shaded section above represents the bacteria from
other sources, and the upper line represents the sum of the two, or
the total number of bacteria in the effluent. The relative importance
of the two parts would probably vary widely with various conditions.
With the conditions indicated by the sketch the number of bacteria in
the effluent is almost constant: for a variation of only from 1.4 to
2.5 per cent of the number applied for the whole range is not a wide
fluctuation for bacterial results, but the number in the lower and
dangerous section is always rapidly increasing with increasing rate.

This theory of filtration accounts for many otherwise perplexing facts.
The conclusion reached at Zürich and elsewhere that the efficiency of
filtration is independent of rate may be explained in this way. This is
especially probable at Zürich, where the number of bacteria in the raw
water was only about 200, and an extremely large proportion relatively
would have to pass to make a well-marked impression upon the total
number in the effluent.

These underdrain bacteria are, so far as we know, entirely harmless;
we are only interested in them to determine how far they are capable
of decreasing the apparent efficiency of filtration below the actual
efficiency, or the per cent of bacteria really removed by the filter.

This efficiency is dependent upon a large number of conditions many
of which have already been discussed in connection with grain-size
of filter sand, underdrains, rate of filtration, loss of head, etc.,
and a mere reference to them here will suffice. Perhaps the most
important single condition is the rate, the numbers of bacteria passing
increase rapidly with it. Next, fine sand and in moderately deep
layers tends to give high efficiency. The influence of the loss of
head, often mentioned, is not shown to be important by the Lawrence
results, nor can I find satisfactory European results in support of
it. Uniformity in the rate of filtration on all parts of the filtering
area and a constant rate throughout the twenty-four hours are regarded
as essential conditions for the best results. Severe winter weather
has indirectly, by disturbing the regular action of open filters, an
injurious influence, and has been the cause of most of the cases where
filtered waters have been known to injure the health of those who have
drunk them. This action is excluded in filters covered with masonry
arches and soil, and such construction is apparently necessary for the
best results in places subject to cold winters.

The efficiency of filtration under various conditions has been studied
by a most elaborate series of experiments at Lawrence with small
filters to which water has been applied containing a bacterium (_B.
prodigiosus_) which does not occur naturally in this country and is
not capable of growing in the filter, so that the results should
represent only the bacteria coming through the filter and not include
any additions from the underdrains. These results, which have been
published in full in the reports of the Massachusetts State Board of
Health, especially for the years 1892 and 1893, show that the number
of bacteria passing increases rapidly with increasing rate, and slowly
with decreasing sand thickness and increased size of sand-grain.

Assuming that the number of bacteria passing is expressed by the formula

1 [(rate)^2 × effective size of sand]
Per cent bacteria passing = — ————————————————————————————————————————
2 ([sqrt](thickness of the sand in inches))

where the rate is expressed in million gallons per acre daily, and
calculating by it the numbers of bacteria for the seventy-three months
for which satisfactory data are available from 11 filters in 1892 and
1893, we find that

In 14 cases the numbers observed were 4 to 9 times as great as the
calculated numbers;

In 6 cases they were 2 to 3 times as great;

In 35 cases they were between 1/2 and 2 times the calculated numbers.

In 17 cases they were 1/2 to 1/3 of them.

In 11 cases they were less than 1/3 the calculated numbers.

The agreement is only moderately good, and in fact no such formula
could be expected to give more than very rough approximations, because
it does not take into consideration the numerous other elements, such
as uniformity and regularity of filtration, the influence of scraping,
the character of the sediment in the raw water, etc., which are known
to affect the results. Perhaps the most marked general difference is
the tendency of new or freshly-filled filters to give higher, and
of old and well-compacted filters to give lower, results than those
indicated by the formula.

Comparing this formula with Piefke’s results given in his “Neue
Ermittelungen”[28] the formula gives in the first series (0.34 mm.
sand, 0.50 m. thick, and rate 100 mm. per hour), 0.25 per cent passing,
while the average number of _B. violacious_ reported, excluding the
first day of decreased efficiency after scraping, was 0.26 per cent. In
the second series, with half as high a rate the numbers checked exactly
the calculated 0.06 per cent.

In other experiments,[29] however, in 1893, when the calculated per
cent was also 0.25, only 0.03, 0.04, and 0.07 per cent were observed in
the effluents.

Comparing the results from the actual filters, (which numbers also
include the bacteria from the underdrains and should therefore be
somewhat higher) with the numbers calculated as passing through,
I find that for the 46 days, Aug. 20 to Oct. 4, 1893, for which
detailed results of the Stralau works are given by Piefke, the average
calculated number passing is 0.20 per

cent, while twice as many were observed in the effluents; although
three of the filters gave better effluents than the other eight, and
the numbers from them approximated closely the calculated numbers. If
we calculate the percentages of bacteria passing a number of filters,
using the maximum rate of filtration allowed for the German filters
where this is accurately determined, and for the English filters
taking the maximum rate at one and one-half times the rate obtained by
dividing the daily quantity by the area of filters actually in use, we
obtain:

----------------------+--------+---------+-----------+--------------
|Average |Effective| Maximum | Per cent
|Depth of| Size of | Rate of | Bacteria
| Sand, | Sand- |Filtration.| passing
| Inches.| grain. | | 1 r^2d
| | | |= - ----------
| | | | 2 [sqrt]sand
----------------------+--------+---------+-----------+--------------
Hamburg | 32 | 0.31 | 1.60 | 0.07
Altona | 28 | 0.34 | 2.57 | 0.21
Berlin, Stralau | 20 | 0.34 | 2.57 | 0.25
Berlin, Müggel | 20 | 0.34 | 2.57 | 0.25
Berlin, Tegel | 20 | 0.37 | 2.57 | 0.27
London, Southwark & | | | |
Vauxhall | 36 | 0.34 | 2.81 | 0.22
London, West Middlesex| 39 | 0.37 | 2.81 | 0.23
London, Chelsea | 54 | 0.36 | 3.27 | 0.26
London, Grand Junction| 30 | 0.40 | 3.27 | 0.39
London, Lambeth | 36 | 0.36 | 3.75 | 0.42
Middlesborough | 20 | 0.42 | 5.85 | 1.58
Zürich | 26 | 0.35 | 7.50 | 1.90
----------------------+--------+---------+-----------+--------------

The numbers actually observed are in every case higher than the
calculated per cents passing, as indeed they should be on account of
those coming from the underdrains, accidental contamination of the
samples, etc.

It may be said that filtration now practised in European works under
ordinary conditions never allows over 1 or 2 per cent bacteria of the
raw water to pass, and ordinarily not over one fourth to one half
of one per cent, although exact data cannot be obtained owing to
masking effect of the bacteria which come from below and which bear
no relation to those of the raw water. By increasing the size of the
filters, fineness and depth of sand (as at Hamburg), the efficiency
can be materially increased above these figures. At the same time
it must be borne in mind that the effectiveness of a filter may be
greatly impaired by inadequate underdraining, by fluctuating rates of
filtration where these are allowed, by freezing in winter in the case
of open filters in cold climates, and by other irregularities, all of
which can be prevented by careful attention to the respective points.

The action of a continuous filter throughout is mainly that of an
exceedingly fine strainer, and like a strainer is mainly confined to
the suspended or insoluble matters in the raw water. The turbidity,
sediment, and bacteria of the raw water are largely or entirely
removed, while hardness, organic matter, and color, so far as they are
in solution, are removed to only a slight extent, if at all. Hardness
can be removed by the addition of lime in carefully determined quantity
before filtration (Clark’s process), by means of which the excess of
carbonic acid in the water is absorbed and the lime added, together
with that previously in the water, is precipitated.

Ordinary filtration will remove from one fourth to one third of the
yellow-brown color of peaty water. A larger proportion can be removed
by the addition of alum, which by decomposing forms an insoluble
compound of alumina with the coloring matter, while the acid of the
alum goes into the effluent either as free acid, or in combination with
the lime or other base in the water, according to their respective
quantities. Freshly precipitated alumina can be substituted for the
alum at increased expense and trouble, and tends to remove the color
without adding acid to the water. These will be discussed more in
detail in connection with mechanical filters. Alum is but rarely used
in slow sand filtration, the most important works where it is used
being in Holland with peaty waters.

After all, the most conclusive test of the efficiency of filtration is
the healthfulness of the people who drink the filtered water; and the
fact that many European cities take water-supplies from sources which
would not be considered fit for use in the United States and, after
filtering them, deliver them to populations having death-rates from
water-carried diseases which are so low as to be the objects of our
admiration, is the best proof of the efficiency of carefully conducted
filtration.

It is only necessary to refer to London, drawing its water from the
two small and polluted rivers, the Thames and the Lea; to Altona,
drawing its water from the Elbe, polluted by the sewage of 6,000,000
people, 700,000 of them within ten miles above the intakes; to Berlin,
using the waters of the Havel and the Spree; to Breslau, taking its
water from the Oder charged with the sewage of mining districts in
Silicia and Galicia, where cholera is so common; to Lawrence, with its
greatly decreased death-rate since it has had filtered water, and to
the hundred other places which protect themselves from the infectious
matters in their raw waters by means of filtration. A few of these
cases are described more in detail in Appendices V to IX, and many
others in the literature mentioned in Appendix X.

An adequate presentation of even those data which have been already
worked up and published would occupy too much space. I think every one
who has carefully studied the recent history of water filtration in
its relation to disease has been convinced that filtration carefully
executed under suitable and normal conditions, even if not an absolute,
is at least a very substantial protection against water-carried
diseases, and the few apparent failures to remove objectionable
qualities have been without exception due to abnormal conditions which
are now understood and in future can be prevented.

BACTERIAL EXAMINATION OF WATERS.

Every large filter-plant should have arrangements for the systematic
bacterial examination of the water before and after filtration,
especially where the raw water is subject to serious pollution. Such
examinations need not be excessively expensive, and they will not
only show the efficiency of the plant as a whole, but may be made to
show the relative efficiencies of the separate filters, the relative
efficiencies at different parts of the periods of operation, the effect
of cold weather, etc., and will then be a substantial aid to the
superintendent in always securing good effluents at the minimum cost.

In addition a complete record of the bacteria in the water at different
times may aid in determining definitely whether the water was connected
with outbreaks of disease. Thus if an outbreak of disease of any
kind were preceded at a certain interval by a great increase in the
number of bacteria,—as has been the case, for example, with the
typhoid epidemics at Altona and Berlin (see Appendices II and VII),—a
presumption would arise that they might have been connected with
each other, and each time it was repeated the presumption would be
strengthened, while, on the other hand, outbreaks occurring while the
bacteria remained constantly low would tend to discredit such a theory.

Bacterial investigations inaugurated after an epidemic is recognized,
as has frequently been done, seldom lead to results of value, both
because the local normal bacterial conditions are generally unknown at
the commencement of the investigation, and because the most important
time, the time of infection, is already long past before the first
samples are taken. The fact that such sporadic activities have led
to few definite results should throw no discredit upon continued
observations, which have repeatedly proved of inestimable value.

Considerable misconception of the use of bacterial examinations
exists. The simple bacterial count ordinarily used, and of which I
am now speaking, does not and cannot show whether a water contains
disease-germs or not. I object to the Chicago water, not so much
because a glass of it contains a hundred thousand bacteria more
or less, as because I am convinced, by a study of its source in
connection with the city’s death-rate, that it actually carries
disease-germs which prove injurious to thousands of those who drink
it. Now the fact being admitted that the water is injurious to health,
variations in the numbers of bacteria in the water drawn from different
intakes and at different times probably correspond roughly with varying
proportions of fresh sewage, and indicate roughly the relative dangers
from the use of the respective waters. If filters should be introduced,
the numbers of bacteria in the effluents under various conditions would
be an index of the respective efficiencies of filtration, and would
serve to detect poor work, and would probably suggest the measures
necessary for better results.

I would suggest the desirability of such investigations where
mechanical filters are used, quite as much as in connection with
slow filtration; and it would also be most desirable in the case of
many water-supplies which are not filtered at all. Such continued
observations have been made at Berlin since 1884; at London since 1886;
at Boston and Lawrence since 1888; and recently at a large number
of places, including Chicago, where observations by the city were
commenced in 1894. They are now required by the German Government in
the case of all filtered public water-supplies in Germany, without
regard to the source of the raw water. The German standard requires
that the effluent from each single filter, as well as the mixed
effluent and raw water, shall be examined daily, making at some works
10 to 30 samples daily. This amount of work, however, can usually be
done by a single man; and when a laboratory is once started, the cost
of examining 20 samples a day will not be much greater than if only
20 a week are taken. In England and at some of the Continental works
drawing their waters from but slightly polluted sources, much smaller
numbers of samples are examined.

The question whether the examinations should be made under the
direction of the water-works company or department, or by an
independent body—as, for instance, by the Board of Health—will depend
upon local conditions. The former arrangement gives the superintendent
of the filters the best chance to study their action, as he can himself
control the collection of samples in connection with the operation
of the filters, and arrange them to throw light upon the points he
wishes to investigate; while examination by a separate authority
affords perhaps greater protection against the possible carelessness or
dishonesty of water-works officials. An arrangement being adopted in
many cases in Germany is to have a bacterial laboratory at the works
which is under the control of the superintendent, and in which the very
numerous compulsory observations are made, while the Board of Health
causes to be examined from time to time by its own representatives,
who have no connection with the water-works, samples taken to check
the water-works figures, as well as to show the character of the water
delivered.

It seems quite desirable to have a man whose principal business is to
make these examinations; as in case he also has numerous other duties,
the examinations may be found to have been neglected at some time when
they are most wanted. Such a man should have had thorough training in
the principles of bacterial manipulation, but it is quite unnecessary
that he should be an expert bacteriologist, especially if a competent
bacteriologist is retained for consultation in cases of doubt or
difficulty.

CHAPTER VII.

INTERMITTENT FILTRATION.

By intermittent nitration is understood that filtration in which the
filtering material is systematically and adequately ventilated, and
where the water during the course of filtration is brought in contact
with air in the pores of the sand. In continuous filtration, which
alone has been previously considered, the air is driven out of the sand
as completely as possible before the commencement of filtration, and
the sand is kept continuously covered with water until the sand becomes
clogged and a draining, with an incidental aeration, is necessary to
allow the filter to be scraped and again put in service.

In intermittent filtration, on the other hand, water is taken over the
top of the drained sand and settles into it, coming in contact with
the air in the pores of the sand, and passes freely through to the
bottom when the water-level is kept well down. After a limited time the
application of water is stopped, and the filter is allowed to again
drain and become thoroughly aerated preparatory to receiving another
dose of water.

This system of treating water was suggested by the unequalled
purification of sewage effected by a similar treatment. It has been
investigated at the Lawrence Experiment Station, and applied to the
construction of a filter for the city of Lawrence, both of which are
due to the indefatigable energy of Hiram F. Mills, C.E.

In its operation intermittent differs from continuous filtration in
that the straining action is less perfect, because the filters yield
no water while being aerated, and must therefore filter at a greater
velocity when in use to yield the same quantity of water in a given
time, and also on account of the mechanical disturbance which is
almost invariably caused by the application of the water; but, on the
other hand, the oxidizing powers of the filter, or the tendency to
nitrify and destroy the organic matters, are stronger, and in addition,
if the rate is not too high, the bacteria die more rapidly in the
thoroughly aerated sand than is the case with ordinary filters.

It was found at Lawrence in connection with sewage filters that when
nitrification was actively taking place the numbers of bacteria were
much lower than under opposite conditions, and it was thought that
nitrification in itself might cause the death of the bacteria. Later
experiments, however, with pure cultures of bacteria of various kinds
applied to intermittent filters with water to which ammonia and salts
suitable for nitrification were added, showed that bacteria of all
the species tried were able to pass the filter in the presence of
nitrification, producing at least one thousand times as much nitrates
as could result in any case of water-filtration, as freely as was
the case when the ammonia was not added and there was but little
nitrification. These results showed conclusively that nitrification
in itself is not an important factor in bacterial removal, although
nitrification and bacterial purification do to some extent go together;
perhaps in part because the nitrification destroys the food of the
bacteria and so starves them out, but probably much more because the
conditions of aeration, temperature, etc., which favor nitrification
also favor equally, and even in its absence, the death of the bacteria.

The rate at which water must pass through an intermittent filter
is, on account of the intervals of rest, considerably greater than
that required to give a corresponding total yield from a continuous
filter, and its straining effect is reduced to an extent comparable to
this increase in rate; and if other conditions did not come in, the
bacterial efficiency of an intermittent filter would remain below that
of a continuous one.

As a matter of fact the bacterial efficiency has usually been found
to be less with intermittent filters at the Lawrence Experiment
Station, when they have been run at rates such as are commonly used for
continuous filters in Europe, say from one and one half to two million
gallons and upwards per acre daily. With lower rates, and especially
with rather fine materials, the bacterial efficiency is much greater;
but it may be doubted whether it would ever be greater than that of a
continuous filter with the same filtering material and the same total
yield per acre. The number of bacteria coming from the underdrains is
apparently generally less, and with very high summer temperatures much
less, than in continuous filters, and this often gives an apparent
bacterial superiority to the intermittent filters.

The effluents from intermittent often contain less slightly organic
matter than those from continuous filters; but, on the other hand,
hardly any water proposed for a public water-supply has organic matter
enough to be of any sanitary significance whatever, apart from the
living bodies which often accompany it; and if the latter are removed
by straining or otherwise, we can safely disregard the organic matters.
In addition, the water filtered will in a great majority of cases have
enough air dissolved in itself to produce whatever oxidation there is
time for in the few hours required for it to pass the filter, and it is
only at very low rates of filtration that intermittent filters produce
effluents of greater chemical purity than by the ordinary process. The
yellow-brown coloring matter present in so many waters appears to be
quite incapable of rapid nitrification; and where it is to some extent
removed by filtration, the action is dependent upon other and but
imperfectly understood causes which seem to act equally in continuous
and intermittent filters.

The peculiarities of construction involved by this method of filtration
will be best illustrated by a discussion of the Lawrence city filter
designed by Hiram F. Mills, C.E., which is the only filter in existence
upon this plan.[30]

THE LAWRENCE FILTER.

The filter consists of a single bed 2-1/2 acres in area, the bottom of
which is 7 feet below low water in the river, and filled with gravel
and sand to an average depth of 4-1/2 feet. The filter is all in a
single bed instead of being divided into the three or four sections
which would probably have been used for a continuous filter of this
size. The water-tight bottom also was dispensed with, and the gravel
was prevented from sinking into the silt by thin intermediate layers
of graded materials. The saving in cost was considerable; but, on the
other hand, a considerable quantity of ground-water comes up through
the bottom and increases the hardness of the water from 1.5 to 2.6
parts of calcium carbonate in 100,000; and while the water when
compared with many other waters is still extremely soft, the addition
cannot be regarded as desirable. The ground-water also contains iron,
which increases the color of the water above what it would otherwise be.

The underdrains have a frictional resistance ten times as great as
would be desirable for a continuous filter, the idea being to check
extreme rates of filtration in case of unequal flooding, and also to
limit the quantity of water which could be gotten through the filter to
that corresponding to a moderate rate of filtration.

The sand, instead of being all of the same-sized grain, is of two
grades, with effective sizes respectively 0.25 and 0.30 mm., the
coarser sand being placed farthest away from the underdrains, where
its greater distance is intended to balance its reduced frictional
resistance and make all parts filter at an equal rate.

The surface instead of being level is waved, that is, there are ridges
thirty feet apart, sloping evenly to the valleys one foot deep half
way between them, to allow water to be brought on rapidly without
disturbing the sand surface. For the same reason, as well as to secure
equality of distribution, a system of concrete carriers for the raw
water goes to all parts of the filter, reducing the effective filtering
area by 4 or 5 per cent. The filter is scraped as necessary in
sections, the work being performed when the filter is having its daily
rest and aeration. Owing to the difference in frictional resistance
before and after scraping, and to the fact that it is impossible to
scrape the entire area in one day, considerable variations in the rate
of filtration in different parts of the filter must occur. The heavy
frictional resistance of the underdrains when more than the proper
quantity of water passes them tends to correct this tendency especially
for the more remote parts of the filter, but perhaps at the expense of
those near to the main drain.

The filter is not covered as the suggestions in Chapter II would
require, but this is hardly on account of its being an intermittent
filter.

The annual report of the Massachusetts State Board of Health for 1893
states that during the first half of December, 1893, the surface
remained covered, that is, it was used continuously, and after December
16th it was so used when the temperature was below 24°, and was drained
only when the temperature was 24° or above. The days on which the
filter was drained during the remainder of December are not given, but
during January and February, 1894, the filter remained covered 29 days
and was drained 30 days. Bacterial samples were taken on 44 of these
days, 22 days when it was drained and 22 when it was not. The average
number of bacteria on the days when it was not drained was 137 and on
those days when it was drained 252 per cubic centimeter.

From February 24th to March 12th the number of bacteria were unusually
high, averaging 492 per cubic centimeter, or 5.28 per cent of the 9308
applied. During this period the filter was used intermittently; there
was ice upon it, and parts of the surface were scraped under the ice,
and high rates of filtration undoubtedly resulted on the scraped areas.
After March 12th the ice had disappeared and very much better results
were obtained.

While there may be some question as to the direct cause of this
decreased efficiency with continued cold weather and ice, the results
certainly are not such as to show the advisability of building open
filters in the Lawrence climate.

The cost of building the filter in comparison with European filters
was extraordinarily low—only $67,000, or $27,000 per acre of filter
surface. To have constructed open continuous filters of the same area
with water-tight bottoms, divided into sections with separate drains
and regulating apparatus, with the necessary piping, would have cost at
least half as much more, and with the masonry cover which I regard as
most desirable in the Lawrence climate the cost would have been two or
three times the expenditure actually required.

It was no easy matter to secure the consent of the city government to
the expenditure of even the sum used; there was much skepticism as to
the process of filtration in general, and it was said that mechanical
filters could be put in for about the same cost. Insisting upon the
more complete and expensive form might have resulted either in an
indefinite postponement of action, or in the adoption of an inferior
and entirely inadequate process. Still I feel strongly that in the
end the greater expense would have proved an excellent investment in
securing softer water and in the greater facility and security of
operating the filter in winter.

In regard to the effect of the Lawrence filter upon the health of
the city, I can best quote from Mr. Mills’ paper in the Report of
the Massachusetts State Board of Health for 1893, and also published
in the Journal of the New England Water-works Association. Mr. Mills
says: “In the following diagram [Fig. 15] the average number of deaths
from typhoid fever at Lawrence for each month from October to May, in
the preceding five years, are given by the heavy dotted line; and the
number during the past eight months are given by the heavy full line.

“The total number for eight months in past years has been forty-three,
and in the present year seventeen, making a saving of twenty-six. Of
the seventeen who died nine were operatives in the mills, each of whom
was known to have drunk unfiltered canal water, which is used in the
factories at the sinks for washing.

[Illustration: FIG. 15.—TYPHOID FEVER IN LAWRENCE.]

“The finer full line shows the number of those who died month after
month who are not known to have used the poisoned canal water. The
whole number in the eight months is eight.

“It is evident from the previous diagram [not reproduced] that the
numbers above the fine full line, here, follow after those at Lowell in
the usual time, and were undoubtedly caused by the sickness at Lowell;
but we have satisfactory reason to conclude that the disease was not
propagated through the filter but that the germs were conveyed directly
into the canals and to those who drank of the unfiltered canal water.
Among the operatives of one of the large corporations not using the
canal water there was not a case of typhoid fever during this period.
Warnings have been placed in the mills where canal water is used to
prevent the operatives from drinking it.

“We find, then, that the mortality from typhoid fever has, during the
use of the filter, been reduced to 40 per cent of the former mortality,
and that the cases forming nearly one half of this 40 per cent were
undoubtedly due to the continued use of unfiltered river water drawn
from the canals.”

The records of typhoid fever in Lawrence before and after the
introduction of filters are as follows:

DEATHS FROM TYPHOID FEVER IN LAWRENCE, 1888-98.

--------+----------+-------------+------------------------------------
| | | Persons who are known to have been
Years. | Total | Deaths | exposed to infection.
| Number | per +--------------+---------------------
| of | 10,000 | | While living out
| Deaths. | of | By drinking | of town just before
| | Population. | Canal Water. | falling sick in
| | | | Lawrence.
--------+----------+-------------+--------------+---------------------
1888 | 48 | 11.36 | |
1889 | 55 | 12.66 | |
1890 | 60 | 13.44 | |
1891 | 55 | 11.94 | |
1892 | 50 | 10.52 | |
1893 | 39 | 7.96 | |
1894 | 24 | 4.75 | 12 |
1895 | 16 | 3.07 | 9 | 2
1896 | 10 | 1.86 | 2 | 4
1897 | 9 | 1.62 | |
1898 | 8 | 1.39 | 1 |
--------+----------+-------------+--------------+---------------------
Filter put in operation September, 1893.
Average rate before the introduction of filtered water (1888-92) 11.31
Average rate afterward (1894-98) 2.54

These results show a striking reduction in the deaths from typhoid
fever with the introduction of filtered water, which has been most
gratifying in every way.

The more recent history of the underdrains of the Lawrence filter
is particularly instructive. Owing to the absence of a water-tight
bottom to the filter, and its low position, a certain amount of water
constantly entered the filter from the ground below. This water
contained iron in solution as ferrous carbonate. When this water came
in contact with the filtered water in the gravel and underdrains, the
iron was oxidized by the dissolved oxygen carried in the filtered water
and precipitated. This was accompanied by a growth of crenothrix in
the gravel and underdrains, which gradually reduced their carrying
capacity. This reduction in carrying capacity first became apparent
in cold weather when the yield from the filter was less free than
formerly. There was difficulty in maintaining the supply during the
winter of 1896-7 and more difficulty in the following winter.

[Illustration: FIG. 16.—TYPHOID FEVER IN LAWRENCE, 1888 TO 1898.]

The sand of the filter was as capable of filtering the full supply
of water as it ever had been, and the efficiency was as good; but
the underdrains were no longer able to collect the filtered water
and deliver it. As the filtering area was ample for the supply, it
was desired to avoid construction of additional filtering area. The
underdrains were dug up and cleaned during the periods when the filter
was drained. As the filter is all in one bed, the times when the filter
could be allowed to remain drained, and when the work could proceed,
were limited. Great care was taken to leave the work in good condition,
and free from passages, at the end of each day’s work, but the numbers
of bacteria in the effluent nevertheless increased somewhat. Some
weeks afterward the number of cases of typhoid fever in the city
increased. The numbers did not become as high as they had been prior to
the introduction of filtered water, but they were much higher than they
had been since that time, and they pointed strongly to the disturbance
of the underdrains as the cause of the increase.

The numbers of bacteria in the applied water and in the effluent from
the Lawrence filter by months, from the time the filter was put in
operation, compiled from the reports of the State Board of Health, as
far as available, are as follows:

BACTERIA IN WATER APPLIED TO AND EFFLUENT FROM LAWRENCE FILTER.

RAW WATER.

----------------+--------+--------+--------+--------+--------+--------
| 1893. | 1894. | 1895. | 1896. | 1897. | 1898.
----------------+--------+--------+--------+--------+--------+--------
January | | 7,700 | 18,700 | 7,500 | 13,314 | 6,519
February | | 7,600 | 15,040 | 12,600 | 13,113 | 4,653
March | | 6,500 | 20,770 | 5,900 | 12,055 | 3,748
April | | 11,200 | 8,420 | 3,800 | 6,904 | 2,320
| | | | | |
May | | 6,000 | 7,000 | 9,600 | 4,625 | 2,050
June | | 8,300 | 9,000 | 6,400 | 4,650 | 6,775
July | | 2,400 | 10,000 | 3,900 | 6,240 | 2,840
August | | 3,100 | 5,000 | 2,700 | 10,700 | 8,575
| | | | | |
September | 57,500 | 6,500 | 5,000 | 12,300 | 27,300 | 6,100
October | 22,200 | 25,300 | 19,000 | 5,300 | 13,200 | 5,120
November | 10,800 | 16,600 | 8,700 | 5,600 | 6,644 | 4,310
December | 8,100 | 23,800 | 6,700 | 9,695 | 5,581 | 5,200
+--------+--------+--------+--------+--------+--------
Average | 24,650 | 10,417 | 11,111 | 7,108 | 10,360 | 4,850

EFFLUENT.

January | | 129 | 206 | 166 | 91 | 39
February | | 244 | 283 | 315 | 79 | 45
March | | 455 | 405 | 133 | 67 | 34
April | | 281 | 84 | 40 | 47 | 21
| | | | | |
May | | 134 | 68 | 56 | 35 | 48
June | | 110 | 68 | 22 | 56 | 50
July | | 25 | 50 | 39 | 106 | 22
August | | 36 | 38 | 146 | 72 | 28
| | | | | |
September | 6,850 | 42 | 40 | 37 | 98 | 67
October | 1,216 | 116 | 60 | 30 | 33 | 28
November | 161 | 175 | 64 | 37 | 27 | 122
December | 111 | 364 | 84 | 67 | 24 |
+--------+--------+--------+--------+--------+--------
Average | 2,084 | 176 | 121 | 91 | 61 | 46
| | | | | |
Average | | | | | |
efficiency | 91.55 | 98.31 | 98.91 | 98.72 | 99.41 | 98.95

CHEMNITZ WATER-WORKS.

The only other place which I have found where anything approaching
intermittent filtration of water is systematically employed is
Chemnitz, Germany. The method there used bears the same relation to
intermittent filtration as does broad irrigation of sewage to the
corresponding method of sewage treatment; that is, the principles
involved are mainly the same, but a much larger filtering area is used,
and the processes take place at a lower rate and under less close
control.

[Illustration: FIG. 17.—PLAN OF AREA USED FOR INTERMITTENT FILTRATION
AT CHEMNITZ.]

The water-works were built about twenty years ago by placing
thirty-nine wells along the Zwönitz River, connected by siphon pipes,
with a pumping-station which forced the water to an elevated reservoir
near the city (Fig. 17). The wells are built of masonry, 5 or 6 feet in
diameter and 10 or 12 feet deep, and are on the rather low bank of the
river. The material, with the exception of the surface soil, and loam
about 3 feet deep, is a somewhat mixed gravel with an effective size of
probably from 0.25 to 0.50 mm., so that water is able to pass through
it freely. The wells are, on an average, about 120 feet apart, and the
line is seven eighths of a mile long.

It was found that in dry times the ground-water level in the entire
neighborhood was lowered some feet below the level of the river without
either furnishing water enough or stopping the flow of the river below.
The channel of the river was so silted that, notwithstanding the porous
material, the water could not penetrate it to go toward the wells.

A dam was now built across the river near the pumping-station, and
a canal was dug from above the dam, crossing the line of wells and
running parallel to it on the back side for about half a mile. Later a
similar canal was dug back of the remaining upper wells. Owing to the
difference in level in the river above and below, the canals can be
emptied and filled at pleasure. They are built with carefully prepared
sand bottoms, and the sand sides are protected by an open paving, to
allow the percolation of as much water as possible, and the sand is
cleaned by scraping, as is usual with ordinary sand filters, once a
year or oftener.

The yield from the wells was much increased by these canals, but the
water of the river is polluted to an extent which would ordinarily
quite prevent even the thought of its being used for water-supply, and
it was found that the water going into the ground from the canals,
and passing through the always saturated gravel to the wells, without
coming in contact with air at any point, after a time contained iron
and had an objectionable odor.

To avoid this disagreeable result the meadow below the pumping-station
was laid out as an irrigation field (Fig. 16). The water from above the
dam was taken by a canal on the opposite side of the river through a
sedimentation pond (which, however, is not now believed to be necessary
and is not always used), and then under the river by a siphon to a
slightly elevated point on the meadow, from which it is distributed
by a system of open ditches, exactly as in sewage irrigation. The
area irrigated is not exactly defined and varies somewhat from time
to time; the rate of filtration may be roughly estimated at from
100,000 to 150,000 gallons per acre daily, although limited portions
may occasionally get five times these quantities for a single day. The
water passes through the three feet of soil and loam, and afterward
through an average of six feet of drained coarse sand or gravel in
which it meets air, and afterward filters laterally through the
saturated gravel to the wells. The water so obtained is invariably of
good quality in every way, colorless, free from odor and from bacteria.
The surface of the irrigated land is covered with grass and has
fruit-trees (mostly apple) at intervals over its entire area.

This first system of irrigation is entirely by gravity. On account
of natural limits to the land it could not be conveniently extended
at this point, and to secure more area, the higher land above the
pumping-station was being made into an irrigation field in 1894. This
is too high to be flooded by gravity, and will be used only for short
periods in extremely dry weather. The water is elevated the few feet
necessary by a gas-engine on the river-bank. In times of wet weather
enough water is obtained from the wells without irrigation, and the
land is only irrigated when the ground-water level is too low.

During December, January, and February irrigation is usually impossible
on account of temperature, and the canals are then used, keeping them
filled with water so that freezing to the bottom is impossible; but
trouble with bad odors in the filtered water drawn from the wells is
experienced at these times.

The drainage area of the Zwönitz River is only about 44 square miles,
and upon it are a large number of villages and factories, so that the
water is excessively polluted. The water in the wells, however, whether
coming from natural sources, or from irrigation, or from the canals,
has never had as many as 100 bacteria per cubic centimeter, and is
regarded as entirely wholesome.

In extremely dry weather the river, even when it is all used for
irrigation so that hardly any flows away below, cannot be made to
supply the necessary daily quantity of 2,650,000 gallons, and to supply
the deficiency at such times, as well as to avoid the use of the canals
in winter, a storage reservoir holding 95,000,000 gallons has recently
been built on a feeder of the river. This water, which is from an
uninhabited drainage area, is filtered through ordinary continuous
filters and flows to the city by gravity. Owing to the small area of
the watershed it is incapable of supplying more than a fraction of the
water for the city, and will be used to supplement the older works.

This Chemnitz plant is of especial interest as showing the successful
utilization of a river-water so grossly polluted as to be incapable of
treatment by the ordinary methods. Results obtained at the Lawrence
Experiment Station have shown that sewage is incapable of being
purified by continuous filtration, the action of air being essential
for a satisfactory result. With ordinary waters only moderately
polluted this is not so; for they carry enough dissolved air to effect
their own purification. In Chemnitz, however, as shown by the results
with the canals, the pollution is so great that continuous filtration
is inadequate to purify the water, and the intermittent filtration
adopted is the only method likely to yield satisfactory results in such
cases.

Intermittent filtration is now being adopted for purifying brooks
draining certain villages and discharging into the ponds or reservoirs
from which Boston draws its water-supply. The water of Pegan Brook
below Natick has been so filtered since 1893 with most satisfactory
results, and affords almost absolute protection to Boston from any
infection which might otherwise enter the water from that town. A
similar treatment is soon to be given to a brook draining the city of
Marlborough. The sewage from these places is not discharged into the
brooks, but is otherwise provided for, but nevertheless they receive
many polluting matters from the houses and streets upon their banks.

The filtration used resembles in a measure that at Chemnitz, and I am
informed by the engineer, Mr. Desmond FitzGerald, that it was adopted
on account of its convenience for this particular problem, and not
because he attaches any special virtue to the intermittent feature.

APPLICATION OF INTERMITTENT FILTRATION.

In regard to the use of waters as grossly polluted as the Zwönitz,
the tendency is strongly to avoid their use, no matter how complete
the process of purification may be; but in case it should be deemed
necessary to use so impure a water for a public supply, intermittent
filtration is the only process known which would adequately purify
it. And it should be used at comparatively low rates of filtration.
I believe that an attempt to filter the Zwönitz at the rate used for
the Merrimac water at Lawrence, which is by comparison but slightly
polluted, would result disastrously.

The operation in winter must also be considered. Intermittent
filtration of sewage on open fields in Massachusetts winters is only
possible because of the comparatively high temperature of the sewage
(usually 40° to 50°), and would be a dismal failure with sewage at
the freezing-point, the temperature to be expected in river-waters in
winter.

It is impossible to draw a sharp line between those waters which are
so badly polluted as to require intermittent filtration for their
treatment and those which are susceptible to the ordinary continuous
filtration. Examples of river-waters polluted probably beyond the
limits reached in any American waters used for drinking purposes and
successfully filtered with continuous filters are furnished by Altona,
Breslau, and London.

Intermittent filtration may be considered in those cases where it
is proposed to use a water polluted entirely beyond the ordinary
limits, and for waters containing large quantities of decomposable
organic matters and microscopical organisms; but in those cases where
a certain and expeditious removal of mud is desired, and where
waters are only moderately polluted by sewage, but still in their raw
state are unhealthy, it is not apparent that intermittent filtration
has any advantages commensurate with the disadvantages of increased
rate to produce the same total yield and of the increased difficulty
of operation, particularly in winter; and in such cases continuous
filtration is to be preferred.

In the removal of tastes and odors from pond or reservoir waters which
are not muddy, but which are subject to the growths of low forms of
plants, which either by their growth or decomposition impart to the
water disagreeable tastes and odors, intermittent filtration may have a
distinct advantage. In such cases there is often an excess of organic
matter to be disposed of by oxidation, and the additional aeration
secured by intermittent filtration is of substantial assistance in
disposing of these matters.

CHAPTER VIII.

TURBIDITY AND COLOR, AND THE EFFECT OF MUD UPON SAND-FILTERS.

The ideal water in appearance is distilled water, which is perfectly
clear and limpid, and has a slight blue color. When other waters are
compared with it, the divergences in color from the color of distilled
water are measured, and not the absolute colors of the waters. Many
spring waters and filtered waters are indistinguishable in appearance
from distilled water.

Public water-supplies from surface sources contain two substances or
classes of substances which injure their appearance, namely, peaty
coloring matters, and mud. Waters discolored by peaty matters are most
common in New England and in certain parts of the Northwest, while
muddy waters are found almost everywhere, but of different degrees of
muddiness, according to the physical conditions of the water-sheds from
which they are obtained.

Muddy waters are often spoken of as colored waters, and in a sense this
is correct where the mud consists of clays or other materials having
distinct colors; but it is more convenient to classify impurities of
this kind as turbidities only, and to limit the term colored waters to
those waters containing in solution vegetable matters which color them.

The removal of either color or turbidity may be called clarification.

Colored waters are usually drawn from water-sheds where the underlying
rock is hard and does not rapidly disintegrate, and where the soils are
firm and sandy, and especially from swamps. The water here comes in
contact with peat or muck, which colors it, but is so firm as not to
be washed by flood flows, and so does not cause turbidity.

Large parts of the United States have for rock foundations shales or
other soft materials which readily disintegrate when exposed, and
which form clayey soils readily washed by hard rains. Waters from
such watersheds are generally turbid and very rarely colored. In fact
a water carrying much clay in suspension is usually found colorless
when the clay is removed, even if it were originally colored. It thus
happens that waters which are colored and turbid at the same time
hardly exist in nature.

Color-producing matters and turbidity-producing matters are different
in their natures, and the methods which must be used to remove them are
different.

THE MEASUREMENT OF COLOR.

The colors of waters are measured and recorded by comparing them with
colors of solutions or substances which are permanent, or which can be
reproduced at will. One of the earliest methods of measuring colors of
waters was to compare them with the colors of the Nessler standards
used for the estimation of ammonia in water analysis. The Nessler
standards were similar in appearance to yellow waters, and their colors
depended upon the amounts of ammonia which had been used in preparing
them, and a record was made of the standard which most closely
resembled the water under examination.

The method was open to the serious objections that the hues of the
standards did not match closely the hues of the waters; that the colors
produced with different lots of Nessler reagent differed considerably,
and therefore the exact values of results were more or less uncertain;
and further, that the numbers obtained for color were not even
approximately proportional to the amounts of coloring matter present.
Because of this peculiarity, in filtration the percentage of color
removal, as determined by the use of these standards, is not even
approximately correct, but is much above the truth.

In the Lovibond tintometer, which has been extensively used in England,
the standards of color are based upon the colors of certain glass
slips, which are in turn compared with standard originals kept for
that purpose. This process answers quite well, but is open to some
objections because of possible uncertainties in the standardization of
the units.

Another method of measuring colors is to compare them with dilute
solutions of platinum and cobalt. The ratio of cobalt to platinum can
be varied to make the hue correspond very closely with the hues of
natural waters, and the amount of platinum required to match a water
affords a measure of its color, one part of metallic platinum in 10,000
parts of water forming the unit of color.

This standard has the advantages that it can be readily prepared with
absolute accuracy in any laboratory, and that by varying the ratio of
platinum to cobalt the hues of various waters can be most perfectly
matched. It is important that the observations should not be made in
too great a depth, as the discrepancy in hues increases much more
rapidly than the depth of color.

For further information regarding colors the reader is referred to
articles in the American Chemical Journal, 1892, vol. xiv, page 300;
Journal of the American Chemical Society, vol. ii, page 8; vol. xviii,
1896, pp. 68, 264, and 484; Journal of the Franklin Institute, Dec.
1894, p. 402; Journal of the New England Water Works Association, vol.
xiii, 1898, p. 94.

AMOUNT OF COLOR IN AMERICAN WATERS.

New England surface-waters have colors ranging from almost nothing
up to 2.00. The colors of the public water-supplies of Massachusetts
cities have been recorded in the reports of the State Board of Health
for some ten years. The figures given were recorded first upon the
Nessler standard, and afterwards upon a modification of the same, known
as the natural water standard. The figures given are approximately
equal to those for the platinum color standard, the relations between
the two having been frequently determined by various observers and
published in the above-mentioned papers. The accompanying diagram shows
the colors in several Massachusetts supplies, as plotted from the
figures given in the published reports.

[Illustration: FIG. 18.—COLORS OF WATERS.

(Analyses of the Mass. State Board of Health.)]

In Connecticut also the colors of many public water-supplies have been
recorded in the reports of the State Board of Health on the platinum
color-standard.

The waters of the Middle States, with rare exceptions, are almost free
from color. In the Northwest waters are obtained often with very high
colors, even considerably higher than the New England waters, and some
of the Southern swamps also yield highly colored waters.

REMOVAL OF COLOR.

Peaty coloring-matter is almost perfectly in solution, and only a
portion of it is capable of being removed by any form of simple
filtration. In order to remove the coloring-matter it is necessary to
change it chemically, or to bring it into contact with some substance
capable of absorbing it. For this reason sand filtration with ordinary
sands, having no absorptive power for color, commonly removes only from
one fourth to one third of the color of the raw water.

MEASUREMENT OF TURBIDITY.

The amount of mud or turbidity in a water is often expressed as the
weight of the suspended matters in a given weight of the water. Most
of the data relating to turbidities of waters are stated in this
way, because this was the only method recognized by the earlier
investigators.

This method of statement has some disadvantages: it fails to take
into account the different sizes of particles which are carried in
suspension by different waters, and at different times. Thus the
Merrimac River in a great flood may carry 100 parts in 100,000 of
fine sand in suspension, and still it could hardly be called muddy;
while another stream carrying only a fraction of this amount of fine
clay would be extremely muddy. Further, an accurate determination of
suspended matters is a very troublesome and tedious operation, and
cannot be undertaken as frequently as is necessary for an adequate
study of the mud question.

Turbidity is principally important as it affects the appearance of
water, and it would seem that optical rather than gravimetric methods
should be used for its determination. Various optical methods of
measuring turbidity have been proposed. The general method employed
is to measure the thickness of the layer of water through which some
object can be seen under definite conditions of lighting. The most
accurate results can probably be obtained in closed receptacles and
with artificial light. Such a method has been used by Mr. G. W. Fuller
at Louisville and Cincinnati in connection with his experiments, and is
described by Parmelee and Ellms in the Technology Quarterly for June,
1899. This apparatus is called by Mr. Fuller a diaphanometer.

At the Lawrence Experiment Station of the Massachusetts State Board of
Health as early as 1889 it became necessary to express the turbidities
of various waters approximately, and the very simple device of
sticking a pin into a stick, and pushing it down into the water under
examination as far as it could be seen, was adopted. Afterwards a
platinum wire 0.04 of an inch in diameter was substituted for the pin,
and the stick was graduated so that the turbidities could be read from
it directly. The figures on the stick were inversely proportional to
their distances from the wire. When the wire could be seen one inch
below the surface, the turbidity was reported as 1.00; when the wire
could be seen two inches, the turbidity was 0.50, and when it could
be seen ten inches the turbidity was 0.10, etc. This scale is much
more convenient than a scale showing the depth at which the wire can
be seen; and within certain limits the figures obtained with it are
directly proportional to the amount of the elements which obstruct
light in the water. Thus, if a water having a turbidity of 1.00 is
mixed with an equal volume of clear water, the mixture will have a
turbidity of 0.50. Advantage is taken of this fact for the measurement
of turbidities so great that they cannot be accurately determined
by direct observation. For turbidities much above 1.00 it is very
difficult to read the depth of wire with sufficient accuracy, and such
waters are diluted with one, two, or more times their volume of clear
water in a pail or other receptacle, the turbidity of the diluted water
is taken, and multiplied by the appropriate factor.

For the greatest accuracy it is necessary that the observations should
be taken in the open air and not under a roof. They should preferably
be made in the middle of the day when the light is strongest, and in
case the sun is shining, the wire must be kept in shadow and not in
direct sunlight.

The turbidities of effluents are usually so slight that they cannot
be taken in this manner; in fact, turbidities of less than 0.02, with
the wire visible 50 inches below the surface, cannot be conveniently
read in this way. For the estimation of lower turbidities a water is
taken having a turbidity of 0.03 or 0.04 and as free as possible from
large suspended particles. The turbidity of this water is measured by
a platinum wire in the usual way, and the water is then diluted with
clear water to make standards for the lower turbidities.

The comparisons between standards and waters are best made in bottles
of perfectly clear glass, holding at least a gallon, and the comparison
is facilitated by surrounding the bottles with black cloth except at
the point of observation, and lighting the water by electric lights so
arranged that the light passes through the water but is hidden from
the observer. In case the water under examination is colored, the
comparison is rendered difficult, and it is often advisable to add a
small amount of methyl orange to the standards to make the colors equal.

Instead of diluting a water of known turbidity for the standards, a
standard can be made by precipitating a known amount of silver chloride
in the water. For this purpose about one per cent of common salt is
dissolved in clear water and small measured amounts of silver nitrate
added, until the turbidity produced is equal to that of the water under
examination. The relation of the amount of silver nitrate used to the
turbidity is entirely arbitrary, and is established by comparisons of
standards made in this way with waters having turbidities from 0.02 to
0.04, the turbidities of which are measured with the platinum wire, and
which afterwards serve to rate the standards. The silver chloride has a
slight color, which is an objection to its use, and perhaps some other
substance could be substituted for it with advantage. The standards
have to be made freshly each day.

One disadvantage of the platinum-wire method of observing turbidities
in the open air, as compared with the diaphanometric method using
artificial light, is that observations cannot be made in the night. To
get the general character of the water in a stream, daily observations
taken about noon will generally be sufficient; but for some purposes it
is important to know the turbidity at different hours of the day, and
in such cases the platinum-wire method is at a distinct disadvantage.
Variations in the amount of light, within reasonable limits, do not
affect the results materially, although extreme variations are to be
avoided. The size of the wire also influences the results somewhat. The
wire commonly used is 0.04 of an inch or one millimeter in diameter.
A wire only four tenths of this size in some experiments at Pittsburg
gave results 25 per cent higher; with a wire twice as large the
results were lower, but the differences were much less. Wire 0.04 of
an inch in diameter was adopted as being very well adapted to rather
turbid river-waters. For very clear lake or reservoir waters, usually
transparent to a great depth, a much larger object is preferable.
Within certain limits the results obtained with an object of any size
can be converted into corresponding figures for another object, or
another light, by the use of a constant factor. Thus the turbidities
obtained with a platinum wire always have approximately the same ratio
to the turbidities of the same waters determined by the diaphanometer.

The platinum-wire method has been used in many cases with most
satisfactory results. If it lacks something in theoretical accuracy
as compared with more elaborate methods, it more than makes up for it
by its simplicity; and reliable observations can be taken with it by
people who would be entirely incompetent to operate more elaborate
apparatus; and it can thus be used in many cases where other methods
would be impossible.

Upon this scale the most turbid waters which have come under the
observation of the author have turbidities of about 2.50, although
waters much more turbid than this undoubtedly exist. A water with
a turbidity of 1.00 is extremely muddy, and only one tenth of this
turbidity would cause remark and complaint among those who use it for
domestic purposes. In an ordinary pressed-glass tumbler a turbidity of
0.02 is just visible to an ordinary observer who looks at the water
closely, but it is not conspicuous, nor would it be likely to cause
general complaint; and this amount may be taken as approximately the
allowable limit of turbidity in a good public water-supply. In a
carefully polished, and perfectly transparent glass a turbidity of 0.01
will be visible, and in larger receptacles still lower turbidities may
be seen if the water is examined carefully. In gallon bottles of very
clear glass, under electric light and surrounded by black cloth, a
turbidity of 0.001 can be distinguished, but a turbidity even several
times as large as this could hardly be detected except by the use of
special appliances, or where water is seen in a depth of several feet.

RELATION OF PLATINUM-WIRE TURBIDITIES TO SUSPENDED MATTERS.

The relation of turbidity to the weight of suspended matters is
approximately constant for waters from which the coarser matters have
been entirely removed by sedimentation. For these waters the suspended
matters in parts per 100,000 are about 16 times the turbidity. For
river-waters the ratios are always larger. With very sluggish rivers
the ratio is only a little larger than for settled waters. For average
river-waters the ratio is considerably higher, and increases with the
turbidity, and for very rapid rivers and torrents the ratio is much
wider, as the suspended matters consist largely of particles which are
heavy but do not increase very much the turbidity.

The following table gives the amounts of suspended matters for various
classes of waters corresponding to the turbidities stated, which have
been deduced from the experience of the author. It is very likely that
ratios different from the above would be obtained with waters in which
the sediment was of different character.

--------------+-----------------------------------------------
| Suspended Matters: Parts in 100,000.
Turbidity, +-----------------------------------------------
Platinum-wire | | River | River | River
Standard. | Settled | Waters, | Waters, | Waters,
| Waters. | Finest | Average | Coarsest
| | Sediment. | Sediment. | Sediment.
--------------+-----------+-----------+-----------+-----------
0.01 | 0.16 | | |
0.05 | 0.80 | 0.85 | 1.30 | 2.40
0.10 | 1.60 | 1.75 | 2.60 | 4.90
0.20 | 3.20 | 3.60 | 5.50 | 10.00
0.30 | 4.80 | 5.70 | 8.50 | 15.00
0.40 | 6.40 | 7.80 | 11.60 | 21.00
0.50 | 8.00 | 10.00 | 15.00 | 26.00
1.00 | 16.00 | 23.00 | 36.00 | 59.00
1.50 | 24.00 | 40.00 | 62.00 | 97.00
2.00 | 32.00 | 61.00 | 94.00 | 140.00
3.00 | 48.00 | 110.00 | 175.00 | 250.00
--------------+-----------+-----------+-----------+-----------

SOURCE OF TURBIDITY.

Much turbidity originates in plowed fields of clayey soil, or in
fields upon which crops are growing. If it has not rained for some
days, and the surface-soil is comparatively dry, the first rain that
falls upon such land is absorbed by the pores of the soil until they
are filled. If the rain is not heavy, but little runs off over the
surface. If, however, the rain continues rapidly after the surface-soil
is saturated, the excess runs off over the surface to the nearest
watercourse. The impact of the rain-drops upon the soil loosens
the particles, and the water flowing off carries some of them in
suspension, and the water is said to be muddy.

The particles carried off in this way are extremely small. Mr. George
W. Fuller, in his report upon water purification at Louisville,
estimates that many of them are not more than a hundred thousandth of
an inch in diameter, and not more than a tenth as large as common water
bacteria.

The turbidity of the water flowing from a field of loose soil may be
2.00 or more; that is to say, the wire is hidden by a depth of half an
inch of water or less. When the water reaches the nearest watercourse
it meets with water from other kinds of land, such as woodlands and
grassed fields, and these waters are less turbid. The water in the
first little watercourse is thus a mixture and has a turbidity of
perhaps 1.00.

The conditions which control the turbidity of any brook are numerous
and complicated. The turbidity of a stream receiving various brooks
depends upon the turbidities of all the waters coming into it.
Generally speaking, the turbidity of a river depends directly upon the
turbidities of its feeders, and is not affected materially by erosion
of its bed or by sedimentation in it. There are, of course, some
streams which in times of great floods cut their banks, and all streams
pick up and move about from place to place more or less of the sand and
other coarse materials upon their bottoms. The materials thus moved,
however, have but little influence upon the turbidity.

After the rain is over some of the water held by the soil will find
its way to the watercourses by underground channels, and will prevent
the stream from drying up between rains, but the average volume of the
stream-flows between rains will be much less than the volumes during
the rains when the water is most turbid.

[Illustration: FIG. 19.—FLUCTUATIONS IN TURBIDITY OF THE WATER OF THE
ALLEGHENY RIVER AT PITTSBURG DURING 1898.]

These conditions are well illustrated by a few data upon the turbidity
of three Pennsylvania streams, recently collected by the author. One
of these streams is a small brook having a drainage area of less than
three square miles. The observations extended over a period of 47 days.
During this time there were five floods, or an average of one flood in
ten days. The duration of floods was less than twenty-four hours in
each case. Selecting the days when the turbidity was the highest, to
the number of one tenth of the whole number of days, the sum of the
turbidities for these days was 67 per cent of the aggregate turbidities
for the whole period. That is to say, 67 per cent of the whole amount
of mud was in the water of only a tenth of the days; the water of the
other nine tenths of the days contained only 33 per cent of the whole
amount of turbidity. The average turbidity of the water for the flood
days was eighteen times as great as the average turbidity for the
remaining days.

The next stream is a considerable creek having a drainage area of
350 square miles. The observations extended over 117 days, during
which time there were seven floods, or an average of one flood in 19
days. The floods lasted in each case one or two days, and the sum of
the turbidities for the one tenth of the whole number of days when
the water was muddiest was 55 per cent of the aggregate of all the
turbidities for the period.

The last case is that of a large river, with a drainage area of over
11,000 square miles. The observations extended over a full year. In
this period there were sixteen floods, each lasting from one to six
days, and the sum of the turbidities for the one tenth of the whole
number of days when the water was muddiest is 45 per cent of the
aggregate turbidities for the year. The floods occurred on an average
of once in 22 days, and the average duration was two and one half days.

The results are very striking as showing that a very large proportion
of the mud is carried by the water in flood flows of comparatively
short duration. They also show that in small streams the proportion of
mud in the flood-flows is greater, and the average duration of floods
is shorter, than in larger streams. In other words, the differences
between flood- and low-water flows are greatest in small streams, and
gradually become less as the size of the stream increases.

When a stream is used for water-works purposes in the usual way, a
certain quantity of water is taken from the stream each day, which
quantity is nearly constant, and is not dependent upon the condition
of the stream, or the volume of its flow. The proportions of the
total flows taken at high- and low-water stages are very different,
and it thus happens that the average quality of the water taken for
water-works purposes is different from the average quality of all the
water flowing in the stream.

Let us assume, for example, a stream having a watershed of such a
size that in times of moderate floods water from the most distant
points reaches the water-works intake in twenty-four hours. Let us
assume further that rainfalls of sufficient intensity to cause floods
and muddy water occur, on an average, once in ten days, and that the
turbidity of the water at these times reaches 1.00, and that for the
rest of the time the turbidity averages 0.10. Let us assume further
that at times of storms the average flow of the stream is 100 units
of volume, and for the nine days between storms the average flow is
10 units of volume. We shall then have in a ten days’ period, for one
day, 100 volumes of water with a turbidity of 1.00, and nine days with
10 volumes each, or a total of 90 volumes of water with a turbidity
of 0.10. The total discharge of the stream will then be 190 volumes,
and the average turbidity 0.57. The turbidity of 0.57 represents the
average turbidity all the water flowing in the stream, or, in other
words, the turbidity which would be found in a lake if all the water
for ten days should flow into it and become thoroughly mixed without
other change.

Now let us compute the average turbidity of the water taken from the
stream for water-works purposes. The water-works require, let us
say, one volume each day, and we have for the first day water with a
turbidity of 1.00, and then for nine days water with a turbidity of
0.10. The average turbidity of the water taken by the water-works for
the period is thus only 0.19 in place of 0.57, the average turbidity of
the whole run-off.

The average turbidity of all the water flowing in the stream is thus
three times as great as that of the water taken from the stream for
water-works purposes.

It is often noted that with long streams the water becomes muddier
farther down, and it may naturally be thought that it is because of the
added erosion of the stream upon its bed in its longer course. This, of
course, may be a cause, or the lower tributaries may be muddier than
the upper ones, but the fact that the water taken at the lower point is
more muddy than farther up is not an indication of this.

Let us take, for example, a watershed of twice the size of that assumed
above, that is, so long that 48 hours will be required for the water
from the most remote feeders to reach the water-works intake. Let us
divide this shed into two parts, which we will assume to be equal, one
of which furnishes water reaching the intake within 24 hours, and the
other water reaching the intake between 24 and 48 hours. Now suppose
a storm upon the watershed producing turbidities equal to those just
assumed for the smaller stream. On the first day the water from the
lower half of the shed, namely, 100 volumes having a turbidity of 1.00,
passes the intake, but this is mixed with 10 volumes of water from
the upper half of the watershed, having a turbidity of 0.10, and the
total flow is thus 110 volumes of water having a turbidity of 0.92.
On the second day the water from the lower half of the watershed has
returned to its normal condition, and the flood-flow of the upper half
of the watershed, 100 volumes with a turbidity of 1.00, is passing,
and mingles with the 10 volumes from the lower half with a turbidity
of 0.10, and the total flow is again 110 volumes having a turbidity of
0.92. The following eight days, until the next rain, will have flows
of 20 volumes each, with turbidities of 0.10. The average turbidity of
all of the water flowing off is 0.57 as before, but the water taken
for water-works purposes will consist of 2 volumes of water with
turbidities of 0.92, and 8 volumes with turbidities of 0.10 making 10
volumes with an average turbidity of 0.26.

By doubling the length of the watershed we have thus doubled the length
of time during which the water is turbid, and have increased the
average turbidity of the water taken for water-works purposes from 0.19
to 0.26, although the average turbidity of all the water running off
remains exactly the same.

If now we assume a watershed so long that three days are required
for the water from the most remote points to reach the intake, with
computations as above, water taken for water-works purposes will have
an average turbidity of 0.32; and with still longer watersheds this
amount will increase, until with a watershed so long that ten days,
or the interval between rains, are required for the water from the
upper portions to reach the intake, the average turbidity of the water
taken for water-works purposes will reach the average turbidity of the
run-off, namely, 0.57.

In the above computations the numbers taken are round ones, and of
course do not represent closely actual conditions. They do serve,
however, to illustrate clearly the principle that the larger the
watershed, other things being equal, the more muddy will be the water
obtained from it for water-works purposes, and the longer will be the
periods of muddy water, and the shorter the periods of clear water
between them.

It cannot be too strongly emphasized that the period of duration of
muddy water is, in general, dependent upon the length of time necessary
for the muddy water to run out of the stream system after it is once in
it, and be replaced by clear water; and that the settling out of the
mud in the river has very little to do with it.

Muddy waters result principally from the action of rains upon the
surface of ground capable of being washed, and the turbidities of the
stream at any point below will occur at the times when the muddy waters
reach it in the natural course of flow, and will disappear again when
the muddy waters present in the stream system at the end of the rain
have run out, and have been replaced with clear water from underground
sources, or from clearer surface sources.

THE AMOUNTS OF SUSPENDED MATTERS IN WATER.

There is a large class of waters, including most lake and reservoir
waters, and surface-waters from certain geological formations, which
are almost free from suspended matters and turbidities. That is to say,
the average turbidities are less than 0.10, and the average suspended
matters are less than 2 parts in 100,000, and are often only small
fractions of these figures. This class includes the raw waters of the
supplies of many English cities drawn from impounding reservoirs, and
also the waters of the rivers Thames and Lea at London, and the raw
waters used by both of the Berlin water-works, and in the United States
the waters of the great lakes except at special points near the mouths
of rivers, nearly all New England waters, and many other waters along
the Atlantic coast and elsewhere where the geological formations are
favorable.

Data regarding the suspended matters in these waters are extremely
meagre. The official examinations of the London waters contain no
records of suspended matters, although the clearness of filtered
waters is daily reported. Dibden, in his analytical investigations of
the London water-supply, mentioned in his book upon “The Purification
of Sewage and Water,” reports the average suspended matters in the
water of the Thames near the water-works intakes as 0.77 part in
100,000. No figures are available for the raw waters used by the Berlin
water-works, but both are taken from lakes, and are generally quite
clear. Even in times of floods of the rivers feeding the lakes, the
turbidities are not very high, because the gathering grounds for the
waters are almost entirely of a sandy nature, yielding waters with low
turbidities, and further, the streams flow through successions of lakes
before finally reaching the lakes from which the waters are taken. It
is safe to assume that the suspended matters and turbidities do not
exceed those of the London waters. Even at times when somewhat turbid
water is obtained, due to agitation by heavy winds, the suspended
matter is mainly of a sandy nature, readily removed by settling, and
it does not seriously interfere with filtration.

The examinations of the Massachusetts State Board of Health, with a
very few exceptions, contain no statements of suspended matters. This
is due to the fact that the suspended matters, in most of the waters,
are so small in amount as to make them hardly capable of determination
by the ordinary gravimetric processes, and the determinations if made
would have but little value. The Merrimac River at Lawrence, at the
time of the greatest flood in fifty years, carried silt to the amount
of about 111 parts in 100,000. This was for a very short time, and the
suspended matter consisted almost entirely of sand, which deposited
in banks, the deposited sand having an effective size of 0.04 or 0.05
millimeter. No clayey matter is ever carried in quantity by the river.

The reports of the Connecticut State Board of Health also contain no
records of suspended matters for the same reason. It may be safely said
that the average suspended matters of New England waters are almost
always less than 1 part in 100,000.

Lake waters are generally almost entirely free from sediment. At
Chicago the city water drawn from Lake Michigan has slightly more than
1 part in 100,000 of suspended matters, as determined by Professor Long
in 1888-9, and by Professor Palmer in 1896. The suspended matter in
this case is probably due to the nearness of the intake to the mouth of
the Chicago River, and to mud brought up from the bottom in times of
storms. The lake-water further away from the shore would probably give
much lower results.

Turning now to waters having considerable turbidities, at Pittsburg the
average suspended matters in the Allegheny River water, as shown by
the weekly or semi-weekly analyses of the Filtration Commission during
1897-8, were 4 parts in 100,000. During a large part of the time the
suspended matters were so small that it was not deemed worth while
to determine them, and the results are returned as zero. This is not
quite correct, and a recomputation of the amount of suspended matters,
based on the observed amounts, and the amounts calculated from the
turbidities when they were very low, leads to an average of a little
less than 5 parts in 100,000, which is probably more accurate than the
direct average. The average turbidity on the platinum-wire scale was
0.16.

At Cincinnati the suspended matters are about 23 parts in 100,000,
and at Louisville about 35 parts, both of these figures being from
Mr. Fuller’s reports. In all these cases the enormous and rapid
fluctuations in the turbidity of the water is a most striking feature
of the results.

Observations on the Mississippi River above the Ohio have been
made by Professor Long in 1888-9, and by Professor Palmer in 1896.
These results are not as full and systematic as could be desired,
but indicate averages of 20 to 30 parts in 100,000 at the different
points. Professor William Ripley Nichols, in his work on water-supply,
states the amount of suspended matter in the water of the Mississippi,
probably referring to the lower river, as 66.66 parts.

Investigations of Professor Long and Professor Palmer for numerous
interior Illinois streams extending over considerable periods give
average results ranging from 1 to 8 parts in 100,000. The very much
lower results for the interior streams as compared with the Mississippi
and Ohio rivers may be due to the relative sizes and lengths of the
streams, or in part to other causes.

Regarding muddy European rivers there are but few data. The Maas, used
for the water-supply of Rotterdam, is reported by Professor Nichols as
having from 1.40 to 47.61 and averaging 10 parts of suspended matters
in 100,000. More recent information is to the effect that the raw water
has at most 30 parts of suspended matters, and that that quantity is
very seldom reached.

At Bremen the Weser often becomes quite turbid. The turbidity of the
water is noted every day by taking the depth at which a black line on a
white surface can be seen. Assuming that this procedure is equivalent
to the platinum-wire procedure, the depths at which the wire can be
seen, namely, from 15 to 600 millimeters, correspond to turbidities of
from 0.04 to 1.70, a result not very different from the conditions at
Pittsburg.

At Hamburg and Altona the water is generally tolerably clear, but at
times of flood the Elbe becomes very turbid, and the amount of mud
deposited in the sedimentation-basins is considerable. At Dresden,
several hundred miles up the river, I have repeatedly seen the
river-water extremely turbid with clayey matter, the color of the clay
varying from day to day, corresponding to the color of the earth from
which it had been washed.

At Budapest, where filters were used temporarily, the Danube water
was excessively muddy with clayey material. At first very high rates
of filtration were employed and the results were not satisfactory.
Afterward the rate of filtration was limited to 1.07 million gallons
per acre daily, and good results were secured. There was no preliminary
sedimentation. Professor Nichols reports the average suspended matters
in the Danube at 32.68 parts in 100,000, but does not state at what
place.

Many of the French and German rivers drain prairie country not
different in its general aspect from the Mississippi basin, and the
soil is probably in many places similar. There is no reason to suppose
that the turbidities of these streams in general are materially
different from those of corresponding streams in the United States,
although it is true that, other things being equal, the average
turbidity of water taken for water-works purposes will increase with
the size of the stream; and it may be that some American streams,
especially the Ohio, Missouri, and Mississippi rivers, are of larger
size than European streams, and consequently that the turbidity of the
water taken from them for water-works purposes may be greater.

The following are the drainage areas of a number of European and
American streams yielding more or less muddy waters at points where
they are used for public water-supplies after filtration, with a few
other American points for comparison. The results are obtained in most
cases from measurements of the best available maps.

PRELIMINARY PROCESSES TO REMOVE MUD.

With both sand and mechanical filtration the difficulty and expense
of treatment of a water increase nearly in direct proportion to the
turbidity of the water as applied to the filter; and it is thus highly
important to secure a water for filtration with as little turbidity
as possible, and thus to develop to their economical limits the
preliminary processes for the removal of mud. One of the most important
of these processes is the use of reservoirs.

Reservoirs serve two purposes in connection with waters drawn
from streams: they allow sedimentation, and they afford storage.
If a water having a turbidity of 1.00 is allowed to remain in a
sedimentation-basin for 24 hours, its turbidity may be reduced by
as much as 40 per cent, or to 0.60. If it is held a second day the
additional reduction is much less.

If samples are taken of the water in the reservoir before and after
settling and sent to the chemist for analysis, he will probably report
that from 70 to 80 per cent of the suspended matters have been removed
by the process. The suspended matters are removed in much larger ratio
than the turbidity. This arises from the fact that there is a certain
proportion of comparatively coarse material in the water as it is
taken from the river. This coarse material increases the weight of the
suspended matters without increasing the turbidity in a corresponding
degree. In 24 hours the coarser materials are removed completely, and
at the end of that time only the clayey or finer particles remain in
suspension. It is these clayey particles, however, that constitute the
turbidity, which are most objectionable in appearance, and which are
most difficult of removal by filtration or otherwise.

Sedimentation thus removes the heavier matters from the water,
but it does not remove the finer matters which principally affect
the appearance of the water and are otherwise most troublesome. A
sedimentation of 24 hours removes practically all of the coarser
matters, and the clayey material remaining at the end of that time
can hardly be removed by further sedimentation. The economic limit of
sedimentation is about 24 hours.

Sedimentation has practically no effect upon the clearer waters between
flood periods.

Let us consider the effect of a sedimentation-basin, or reservoir
holding a 24-hours’ supply of water, into which water is constantly
pumped at one end, and from which an equal quantity is constantly
withdrawn from the other, upon the water of a stream of such size
that the time of passage of water from the feeders to the intake is
less than 24 hours. During the period between storms the water is
comparatively clear and passes through the sedimentation basin without
change. When a storm comes the water in the stream promptly becomes
muddy, and muddy water is supplied to the reservoir; but owing to
the time required for water to pass through it, the outflowing water
remains clear for some hours. There is a gradual mixing, however, and
long before the expiration of 24 hours somewhat muddy water appears
at the outlet. The turbid-water period rarely lasts in streams of
this size more than 24 hours, and at the expiration of that time the
water in the sedimentation-basin is as muddy or muddier than the water
flowing in the stream. After the height of the flood the stream clears
itself by the flowing away of the turbid water much more rapidly than
the water clears itself by sedimentation in the reservoir. That is to
say, if at the time of maximum turbidity we take a certain quantity of
water from the stream and put it aside to settle, at no time will the
improvement by settling equal the improvement which has taken place in
the stream from natural causes. Generally the improvement in the stream
is several times as rapid as in the sedimentation-basin, and the water
from it will at times have only a fraction of the turbidity of the
water in the basin.

Let us now consider what the sedimentation has done to improve the
water. During the period of clear water, that is for most of the
time, it has done nothing. For the first day of each flood period
very much clearer water has been obtained from it than was flowing
in the stream. For the first days following floods the water in the
sedimentation-basin has been more muddy than the water in the stream.
The only time when the sedimentation-basin has been of use is during
the first part of floods, that is, when the turbidity of the water in
the stream is increasing. During this period it has been of service
principally because of its storage capacity, yielding up water received
from the stream previously, when it was less muddy. Such sedimentation
as has been secured is merely incidental and generally not important in
amount.

It will be obvious from the above that for these conditions storage
is much more important than sedimentation. This brings us back to
the old English idea of having storage-reservoirs large enough to
carry water-works over flood periods without the use of flood-waters.
Reservoirs of this kind were, and still are, considered necessary for
the successful utilization of waters of many English rivers, although
these waters do not approach in turbidity the waters of some American
streams. This idea of storage has been but little used in the United
States.

In the above case, if we use our reservoir for storage instead of as a
sedimentation-basin, the average quality of the water can be greatly
improved. The reservoir should ordinarily be kept full, and pumping to
it should be stopped whenever the turbidity exceeds a certain limit,
to be determined by experience; and the reservoir is then to be drawn
upon for the supply until the turbidity again falls to the normal. In
the case assumed above, with a stream in which all of the water reaches
the intake in 24 hours, a reservoir holding a 24-hours’ supply, or in
practice, to be safe, a somewhat larger one, would yield a water having
a very much lower average turbidity than would be obtained with water
pumped constantly from the stream without a reservoir.

With a river having a watershed so long that 48 hours are required to
bring the water down from the most remote feeders, a reservoir twice as
large would be required, and would result in a still greater reduction
in the average turbidity.

As the stream becomes larger, and the turbid periods longer, the size
of a reservoir necessary to utilize this action rapidly becomes larger,
and the times during which it can be filled are shortened, and thus the
engineering difficulties of the problem are increased. For moderately
short streams, cost for cost, storage is far more effective than
sedimentation, and we must come back to the old English practice of
stopping our pumps during periods of maximum turbidity.

EFFECT OF MUD UPON SAND FILTERS.

There are two aspects of the effect of mud upon the operation of sand
filters which require particular consideration. The first relates to
the rapidity of clogging, and consequently the frequency of scraping
and the cost of operation; while the second relates to the ability of
the filters to yield well-clarified effluents.

EFFECT OF TURBIDITY UPON THE LENGTH OF PERIOD.

The amount of water which can be filtered between scrapings is directly
dependent upon the turbidity of the raw water. The greater the
turbidity, the more frequently will filters require to be scraped. In
the experiments of the Pittsburg Filtration Commission, with 4 feet of
sand of an effective size of about 0.30 millimeter, and with rates of
filtration of about three million gallons per acre daily, and with the
loss of head limited to 4 feet, sand filters were operated as follows:
For five periods the turbidities of the raw water ranged from 0.035 to
0.062, and averaged 0.051, and the corresponding periods ranged from
102 to 136, and averaged 113 million gallons per acre filtered between
scrapings. For ten periods the turbidities of the raw water ranged from
0.079 to 0.128, and averaged 0.102, and the periods averaged 78 million
gallons per acre between scrapings. For fifteen other periods the
turbidities of the raw water ranged from 0.134 to 0.269, and averaged
0.195, and the periods averaged 52 million gallons per acre between
scrapings. In two other periods the turbidities of the raw water
averaged 0.67, and the periods between scrapings averaged 16 million
gallons. In all cases the turbidity is taken as that of the water
applied to the filter. Usually this was the turbidity of the settled
water, but in some cases raw water was applied, and in these case the
turbidity of the raw water is taken. These results are approximately
represented by the formula

Period between scrapings, } = 12/(turbidity + 0.05).
million gallons per acre }

Except for very clear waters the amount of water passed between
scrapings is nearly inversely proportional to the turbidity. With twice
as great an amount of turbidity, filters will have to be cleaned twice
as often, the reserve area for cleaning will require to be twice as
great, and the cost of scraping filters and of washing and replacing
sand, which is the most important element in the cost of operation,
will be doubled.

With waters having turbidities of 0.20 upon this basis, the average
period will be about 51 million gallons per acre between scrapings.
This is about the average result obtained at the German works filtering
river waters, and there is no serious difficulty in operating filters
which require to be scraped with this frequency. With more turbid
waters the period is decreased. With an average turbidity of 0.50 the
average period is only 24 million gallons per acre between scrapings, a
condition which means very difficult operation and a very high cost of
cleaning. With much more turbid waters the difficulties are increased,
and if the duration of turbid water should be long-continued, the
operation of sand filters would clearly be impracticable, and the
expense, also, would be prohibitive.

In applying these figures to actual cases it must be borne in mind
that the turbidity is only one of the several factors which control
the length of period; and that the turbidity of a water of a given
stream is never constant, but fluctuates within wide limits; and that
raw water can be applied to filters for a short time without injurious
results, even though it is so turbid that its continued application
would be fatal.

It is very likely also that the suspended matters in different streams
differ in their natures to such an extent that equal turbidities would
give quite different periods, although the Pittsburg results were so
regular as to give confidence in their application to other conditions
within reasonable limits, and when so applied they afford a most
convenient method of computing the approximate cost of operation of
filters for waters of known or estimated turbidities.

POWER OF SAND FILTERS TO PRODUCE CLEAR EFFLUENTS FROM MUDDY WATER.

When the turbidity of the applied water is not too great it is entirely
removed in the course of filtration. With extremely muddy raw waters,
however, turbid effluents are often produced with sand filters.
The conditions which control the passage of the finest suspended
matters through filters have been studied by Mr. Fuller at Cincinnati
at considerable length. They are similar in a general way to the
conditions which control the removal of bacteria. That is to say, the
removal is more complete with fine filter sand than with coarse sand;
with a deep sand layer than with a shallow sand layer; and with low
rates of filtration than with high rates. The practicable limits to
the size of sand grain, depth of sand layer, and rate of filtration
are established by other conditions, and the question remains whether
within these limits a clear effluent can be produced.

At Pittsburg the turbidity of the effluent from a sand filter operated
as mentioned above, which received water which had passed through a
sedimentation-basin holding about a 24-hours’ supply, but without
taking any advantage of storage to avoid the use of muddy water, was
nearly always less than 0.02, which may be taken as the admissible
limit of turbidity in a public water-supply. This limit was exceeded on
less than 20 days out of 365, these days being during the winter and
spring freshets, and on these days the excess was not such as would be
likely to be particularly objectionable. For the water of the Allegheny
River, then, sand filtration with one day’s sedimentation is capable
of yielding a water not absolutely clear, but sufficiently clear to be
quite satisfactory for the purpose of municipal water-supply.

At Cincinnati, on the other hand, where the amount of suspended matters
was five times as great as at Pittsburg, the effluents which could be
obtained by sand filtration without recourse to the use of alum, even
under most favorable conditions, were very much more turbid than those
obtained at Pittsburg, and were, in fact, so turbid as to be seriously
objectionable for the purpose of public water-supply.

With rivers no more turbid than the Allegheny River at Pittsburg, and
rivers having floods of such short duration that the use of flood-flows
can be avoided by the use of reservoirs, sand filters are adequate for
clarification. For waters which are much muddier than the Allegheny,
as, for instance, the Ohio at Cincinnati and at Louisville, sand
filtration alone is inadequate. Mr. Fuller,[31] as a result of his
Cincinnati experiments, has stated the case as follows:

“For the sake of explicitness it is desired to show, with the data
of the fairly normal year of 1898, the proportion of the time when
English filters (that is, sand filters) would be inapplicable in the
purification of the unsubsided Ohio River water at Cincinnati. This
necessitates fixing an average limit of permissible suspended matter in
this river water, and is a difficult matter from present evidence.

“In part this is due to variations in the character and in the relative
amounts of the suspended silt, clay, and organic matter; and in part
it is due to different amounts of clay stored in the sand layer, which
affects materially the capacity of the filter to retain the clay of the
applied water. During these investigations the unsubsided river-water
was not regularly applied to filters; and, with the exception of
the results of tests for a few days only, it is necessary to depend
upon general information obtained with reference to this point. So
far as the information goes, it appears that an average of 125 parts
per million is a conservative estimate of the amount of suspended
matters in the unsubsided river-water, which could be regularly and
satisfactorily handled by English filters. But at times this estimated
average would be too low, and at other times too high....

“While English filters are able to remove satisfactorily on an

average about 125 parts of silt and clay of the unsubsided water,
actual experience shows that they can regularly handle suspended
clay in subsided water in amounts ranging only as high as from 30 to
70 parts (depending upon the amount of the clay stored in the sand
layer), and averaging about 50 parts per million. But it is true that
for two or three days on short rises in the river, or at the beginning
of long freshets, the retentive capacity of the sand layer allows of
satisfactory results with the clay in the applied water considerably in
excess of 70 parts. If this capacity is greatly overtaxed, however, the
advantage is merely temporary, as the stored clay is washed out later,
producing markedly turbid effluents.”

Translating Mr. Fuller’s results into terms of turbidity, the 125
parts per million of suspended matters in the raw water represent a
turbidity of about 0.40, and the 30 to 70 parts of suspended matters in
the settled water represent turbidities from 0.20 to 0.40, the average
of 50 parts of suspended matters corresponding to a turbidity of about
0.30.

Upon this basis, then, sand filters are capable of treating raw waters
with average turbidities up to 0.40, or settled waters with average
turbidities up to 0.30, but waters more turbid than this are incapable
of being successfully treated without the use of coagulants or other
aids to the process. These results are in general accordance with
the results of the experiments at Pittsburg, and demonstrate that
while sand filters as generally used in Europe are adequate for the
clarification of many, if not most, river waters in the United States,
there are other waters carrying mud in such quantities as to make the
process inapplicable to them.

EFFECT OF MUD UPON BACTERIAL EFFICIENCY OF FILTERS.

The question is naturally raised as to whether or not the presence of
large quantities of mud in the raw water will not seriously interfere
with the bacterial efficiency of filters. Experiments at Cincinnati
and Pittsburg have given most conclusive and satisfactory information
upon this point. Up to the point where the effluents become quite
turbid, the mud in the raw water has no influence upon the bacterial
efficiency; and even somewhat beyond this point, with effluents so
turbid that they would hardly be suitable for the purpose of a public
water-supply, the bacterial efficiency remains substantially equal to
that obtained with the clearest waters. Only in the case of excessive
quantities of mud, where, for other reasons, sand filters can hardly
be considered applicable, is there a moderate reduction in bacterial
efficiency. As mentioned above, particles constituting turbidity are
often much smaller than the bacteria, and in addition, the bacteria
probably have an adhesive power far in excess of that of the clay
particles. For these reasons clay particles are able to pass filters
under conditions which almost entirely prevent the passage of bacteria.

On the other hand, it does not necessarily follow that the removal
of turbidity is accompanied by high bacterial efficiency. Although
this is often the case, there are marked exceptions, particularly in
connection with the use of coagulants, where very good clarification is
obtained, and notwithstanding this, effluents are produced containing
comparatively large numbers of bacteria.

LIMITS TO THE USE OF SUBSIDENCE FOR THE PRELIMINARY TREATMENT OF MUDDY
WATERS.

When water is too muddy to be applied directly to filters, the most
obvious treatment is to remove as much of the sediment as possible by
sedimentation. Sedimentation-basins are considered as essential parts
of filtration plants for the treatment of muddy waters. The effect of
sedimentation, as noted above, is to remove principally the larger
particles in the raw water. By doing this the deposit upon the surface
of the filters and the cost of operation are greatly reduced.

These larger particles are mainly removed by a comparatively short
period of sedimentation, and the improvement effected after the first
24 hours is comparatively slight. The particles remaining in suspension
at the end of this time consist almost entirely of very fine clay, and
the rate of their settlement through the water is extremely slow; and
currents in the basin, due to temperature changes, winds, etc., almost
entirely offset the natural tendency of the sediment to fall to the
bottom.

There is thus a practical limit to the effect of sedimentation which is
soon reached, and it has not been found feasible to extend the process
so as to allow much more turbid waters to be brought within the range
which can be economically treated by sand filtration.

CHAPTER IX.

THE COAGULATION OF WATERS.

The coagulation of water consists in the addition to it of some
substance which forms an inorganic precipitate in the water, the
presence of which has a physical action upon the suspended matters, and
allows them to be more readily removed by subsidence or filtration.

The most common coagulant is sulphate of alumina. When this substance
is added to water it is decomposed into its component parts, sulphuric
acid and alumina, the former of which combines with the lime or other
base present in the water, or in case enough of this is lacking, it
remains partly as free acid and partly undecomposed in its original
condition; while the alumina forms a gelatinous precipitate which draws
together and surrounds the suspended matters present in the water,
including the bacteria, and allows them to be much more easily removed
by filtration than would otherwise be the case. In addition, the
alumina has a chemical attraction for dissolved organic matters, and
the chemical purification may be more complete at very high rates than
would be possible with sand filtration without coagulant at any rate,
however low.

Coagulants have been employed in connection with filtration from
very early times. As early as 1831 D’Arcet published in the “Annales
d’hygiène publique,”[32] an account of the purification of Nile water
in Egypt by adding alum to the water, and afterwards filtering it
through small household filters. More recently alum has been repeatedly
used in connection with sand filters, particularly

at Leeuwarden, Groningen, and Schiedam in Holland, where the river
waters used for public supplies are colored by peaty matter which
cannot be removed by simple filtration.

SUBSTANCES USED FOR COAGULATION.

Mr. Fuller[33] has given a very full account of the substances which
can be used for the clarification of waters. Without taking up all of
the unusual substances which have been suggested, the most important of
the coagulants will be briefly described below.

_Lime._—Lime has been extensively used in connection with the
purification of sewage, and also for softening water. Lime is first
slaked and converted into calcium hydrate, which is afterwards
dissolved in water, and applied to the water under treatment. The
amount of lime to be used is fixed by the amount of carbonic acid in
the water. So much lime is always used as will exactly convert the
whole of the carbonic acid of the water into normal carbonate of lime.
This substance is but slightly soluble in water and it precipitates.
The precipitate is crystalline rather than flocculent, and is not as
well adapted to aid in the removal of clayey matters as some other
substances, although its action in this respect is considerable. The
precipitate is quite heavy, and is largely removed by sedimentation,
although filtration must be used to complete the process. Water which
has been treated with lime is slightly caustic; that is to say, there
is a deficiency of carbonic acid in it, and it deposits lime in the
pipes, in pumps, etc.; and although the precipitated calcium carbonate
is much softer than steel, it rapidly destroys pumps used for lifting
it.

Principally for these reasons it is necessary to supply carbonic acid
to water which has been treated in this way, and this is done by
bringing it in contact with flue-gases, or by the direct addition of
carbonic acid.

The use of lime for softening waters is known as Clark’s process. It
was patented in England many years ago, and the

patent has now expired. Various ingenious devices have been constructed
for facilitating various parts of the operation. The process has hardly
been used in the United States, but there is a large field for it in
connection with the softening of very hard waters, and where such
waters also contain iron or clay, these substances will be incidentally
removed by the process.

Larger quantities of lime have an action upon the suspended matters
which is entirely different from that secured in Clark’s process, and
the action upon bacteria is particularly noteworthy. This action was
noted in experiments at Lawrence,[34] where it was found that sewage
was almost completely sterilized by the application of considerable
quantities of lime. An extremely interesting series of experiments upon
the application of large quantities of lime to water was made by Mr.
Fuller in 1899.[35] The bacterial results were extremely favorable,
although the necessity for removing the excess of lime afterward is a
somewhat serious matter, and in these experiments it was not entirely
accomplished.

_Aluminum Compounds._—Sulphate of alumina is most commonly employed.
It can be obtained in a state of considerable purity at a very
moderate price, and important improvements in the methods used for its
manufacture have been recently introduced. Potash and soda alums have
no advantage over sulphate of alumina, and, in fact, are less efficient
per pound, while their costs are greater. Chloride of alumina is
practically equivalent to the sulphate in purifying power, but is more
expensive.

_Sodium Aluminate_ has been examined by Mr. Fuller, who states that
experience has shown that its use is impracticable in the case of the
Ohio River water.

_Compounds of Iron._—Iron forms two classes of compounds, namely,
ferrous and ferric salts. When the ferrous salts are applied to water,
under certain conditions, ferrous hydrate is precipitated, but this
substance is not entirely insoluble in water containing carbonic acid.
Under some conditions the precipitated ferrous hydrate is oxidized
by oxygen present in the water to ferric hydrate, and so far as this
is the case, good results can be obtained. Ferrous sulphate is not
as readily oxidized when applied to water as is the ferric carbonate
present in many natural waters, and for this reason ferrous sulphate
has not been successfully used in water purification. In the treatment
of sewage, where the requirements are somewhat different, it has been
one of the most satisfactory coagulants.

Ferric sulphate acts in much the same way as sulphate of alumina,
and is entirely suitable for use where sulphate of alumina could be
employed, but it has not been used in practice, due probably to its
increased cost as compared with its effect, and to the practical
difficulties of applying it in the desired quantities due to its
physical condition.

_Metallic Iron: The Anderson Process._—The use of metallic iron for
water purification in connection with a moderately slow filtration
through filters of the usual form is known as Anderson’s process
(patented), and has been used at Antwerp and elsewhere on a large
scale, and has been experimentally examined at a number of other places.

The process consists in agitating the water in contact with metallic
iron, a portion of which is taken into solution as ferrous carbonate.
Upon subsequent aeration this is supposed to become oxidized and
precipitate out as ferric hydrate, with all the good and none of the
bad effects which follow the use of alum. The precipitate is partially
removed by sedimentation, while filtration completes the process.
The process is admirable theoretically, and in an experimental way
upon a very small scale often gives most satisfactory results, muddy
waters very difficult of filtration, and colored peaty waters yielding
promptly clear and colorless effluents.

In applying the process on a larger scale, however, with peaty waters
at least, it seems impossible to get enough iron to go into solution
in the time which can be allowed, and the small quantity which is
taken up either remains in solution or else slowly and incompletely
precipitates out, without the good effects which follow the sudden and
complete precipitation of a larger quantity, and in this case the color
is seldom reduced, and may even be increased above the color of the raw
water by the iron remaining in solution.

The ingenuity of those who have studied the process has not yet found
any adequate means of avoiding these important practical objections;
and even at Antwerp a great extension of the filtering area, as well
as the use of alum at times of unusual pollution, is good evidence
that simple filtration, in distinction from the effect of the iron, is
relied upon much more than formerly.

At Dordrecht also, where the process has been long in use, the rate of
filtration does not exceed the ordinary limits; nor is the result, so
far as I could ascertain, in any way superior to that obtained a few
miles away at Rotterdam, by ordinary filtration, with substantially the
same raw water.

The results obtained at Boulogne-sur-Seine, near Paris, have been
closely watched by the public chemist and bacteriologist of Paris,
and have been very favorable, and a number of new plants of very
considerable capacity have been built, to supply some of the suburbs of
Paris, but even in these cases only moderate rates of filtration are
employed which would yield excellent effluents without the iron.

_Compounds of Manganese._—Manganese forms compounds similar to those
of iron, that is to say manganous and manganic salts, but their use
in connection with water filtration has not been found possible. In
addition, manganese forms a series of compounds, known as manganates
and permanganates, quite different in their structure and action from
the others. These compounds contain an excess of oxygen which they
give up very readily to organic matters capable of absorbing oxygen,
and because of this power, they have been extensively used in the
treatment of sewage. Applied to the treatment of waters their action is
very slight, and the compounds are so expensive that they have not been
employed for this purpose. Theoretically the action is very attractive,
as the oxygen liberated by their decomposition oxidizes some of the
organic matter of the water, thereby purifying it in part, while the
manganese is precipitated as a flocculent precipitate having all of
the advantages pertaining to a precipitate of hydrate of alumina, and
without the disadvantage of adding acid to the water, as is the case
with the compounds of alumina and iron. These chemicals, when used in
comparatively concentrated condition, have powerful germicidal actions,
but in water purification the amounts which can be used are so small
that no action of this kind results. The amount which can be applied to
a water is limited to the amount which can be decomposed by the organic
matters present in the water, and is not large.

_The Use of Metallic Iron and Aluminum, with the Aid of
Electricity._—Elaborate experiments were made at Louisville with
metallic iron and aluminum oxidized and made available by the aid of
electric currents. The use of iron with electric currents was tried
in sewage purification some years ago, under the name of the Webster
process, but was never put to practical use. The theory is to oxidize
the iron or aluminum in contact with the water, with the formation
of flocculent hydrates, by the aid of an electric current, thereby
securing the advantages of the application of salts of these metals to
the water without the disadvantage of the addition of acid.

_Other Chemicals Employed._—A solution containing chlorine produced
by electrical action has been suggested. Chlorine is a powerful
disinfectant, and when used in large quantities kills bacteria. It is
not possible to use enough chlorine to kill the bacteria in the water
without rendering it unfit for human use. The nature of this treatment
has been concisely described by Dr. Drown,[36] who shows that the
electrically prepared fluids do not differ in their action in any way
from well-known chemicals, the use of which would be hardly considered.

The use of ozone and peroxide of hydrogen have also been suggested, but
I do not know that they have been successfully used on a large scale.
The same is true of many other chemicals, the consideration of which is
hardly necessary in this connection.

COAGULANTS WHICH HAVE BEEN USED.

In actual work sulphate of alumina is practically the only coagulant
which has been employed, excepting the alums, which are practically its
equivalent in action, differing only in strength. Nearly all important
experiments upon the coagulation of water have been made with sulphate
of alumina, and in the further discussion of this subject only this
coagulant will be considered.

AMOUNT OF COAGULANT REQUIRED TO REMOVE TURBIDITY.

In the coagulation of turbid waters a certain definite amount of
coagulant must be employed. If less than this amount is used either no
precipitate will be formed, or it will not be formed in sufficient bulk
to effect the desired results. It is necessary that the precipitate
should be sufficient, and that it should be formed practically all at
one time. The amount of coagulant necessary to accomplish this purpose
is dependent upon the turbidity of the raw water. With practically
clear waters sulphate of alumina of the ordinary commercial strength,
that is to say, with about 17 per cent soluble oxide of aluminum,
used in quantities as small as 0.3 or 0.4 of a grain per gallon, will
produce coagulation. As the turbidity increases larger amounts must be
employed.

A special study was made of this point in connection with the Pittsburg
experiments.[37] As an average of these results it was found that two
grains per gallon of sulphate of alumina were

required to properly coagulate waters having turbidities of 1.00, so
that they could be filtered by the Jewell filter, and 2.75 grains were
required for the Warren filter.

[Illustration: FIG. 20.—AMOUNT OF COAGULANT REQUIRED TO REMOVE
TURBIDITY.]

Aside from the amount required to produce a precipitate in the clearest
waters, the amount of coagulant required was proportional to the
turbidity. As an average for the two filters the required quantity was
approximately 0.30 of a grain, and in addition 0.02 of a grain for each
0.01 of turbidity. Thus a water having a turbidity of 0.20 requires
0.70 of a grain per gallon; a water having a turbidity of 0.50 requires
1.30 grains; of 1.00, 2.30 grains; of 2.00, 4.30 grains, etc. These are
average minimum results. Occasionally clear effluents were produced
with smaller quantities of coagulant, while at other times larger
quantities were necessary for satisfactory results.

The amount of coagulant required for clarification at Cincinnati has
been stated by Mr. Fuller in his report. A number of his results are
brought together in the following table, to which has also been added a
column showing approximately the corresponding results at Pittsburg.

ESTIMATED AVERAGE AMOUNTS OF REQUIRED CHEMICAL FOR DIFFERENT GRADES OF
WATER.

----------+---------------------------------------------
| Chemical Required, Grains per Gallon.
+----------+----------+----------+------------
Suspended | Raw | Subsided | Subsided |Minimum for
Matter, |Water for |Water for |Water for |Raw Water
Parts in | Sand | Sand |Mechanical| for
100,000. | Filters. | Filters. | Filters. |Mechanical
|Cincinnati|Cincinnati|Cincinnati| Filters.
| Report, | Report, | Report, |
|Page 290. |Page 290. |Page 341. | Pittsburg.
----------+----------+----------+----------+------------
1.0 | 0 | 0 | 0.75 | 0.40
2.5 | 0 | 0 | 1.25 | 0.50
5.0 | 0 | 0 | 1.50 | 0.70
7.5 | 0 | 1.30 | 1.95 | 0.90
10.0 | 1.50 | 1.60 | 2.20 | 1.00
12.5 | 1.60 | 1.80 | 2.45 | 1.15
15.0 | 1.70 | 2.00 | 2.65 | 1.30
17.5 | 1.80 | 2.10 | 2.85 | 1.40
20.0 | 1.95 | 2.20 | 3.00 | 1.60
30.0 | 2.25 | 2.45 | 3.80 | 2.00
40.0 | 2.50 | 2.75 | 4.40 | 2.50
50.0 | 2.80 | | |
60.0 | 3.05 | | |
75.0 | 3.40 | | |
100.0 | 4.00 | | |
120.0 | 4.75 | | |
----------+----------+----------+----------+------------

Mr. Fuller’s results seem to show that a greater amount of coagulant
is required for the preparation of water for mechanical filters than
is necessary in connection with sand filters. The results with sand
filters indicate that settled waters and raw waters containing equal
amounts of suspended matters are about equally difficult to treat. The
results at Pittsburg indicate that the raw waters required much smaller
quantities of coagulant for given amounts of suspended matters than was
the case with subsided waters at Cincinnati, the results agreeing more
closely with the amounts required to prepare raw water for sand filters
at Cincinnati.

AMOUNT OF COAGULANT REQUIRED TO REMOVE COLOR.

The information upon this point is, unfortunately, very inadequate. In
some experiments made by Mr. E. B. Weston at Providence in 1893 with a
mechanical filter,[38] with quantities of sulphate of alumina averaging
0.6 or 0.7 of a grain per gallon, the removal of color was usually
from 70 to 90 per cent. The standard used for the measurement of color
is not stated, and there is no statement of the basis of the scale,
consequently no means of determining the absolute color of the raw
water upon standards commonly used.

At Westerly, R. I., with a New York filter, the actual quantity of
potash alum employed from Oct. 10, 1896, to March 1, 1897, was 1.94
grains per gallon, the amount being regulated to as low a figure as it
was possible to use to secure satisfactory decolorization. There is no
record of the color of the raw water. A very rough estimate would place
it at 0.50 upon the platinum scale. The chemical employed in this case
was alum, and two thirds as large a quantity of sulphate of alumina
would probably have done corresponding work, had suitable apparatus for
applying it been at hand.

At Superior, Wisconsin, the water in the bay coming from the St.
Louis River, having a color of 2.40 platinum scale, was treated
experimentally with quantities of sulphate of alumina up to 4 grains
per gallon, by Mr. R. S. Weston in January, 1899, but even this
quantity of coagulant utterly failed to coagulate and decolorize it.

At Greenwich, Conn., during 1898 the average amount of sulphate of
alumina employed, as computed from quantities stated in the annual
report of the Connecticut State Board of Health for 1898, was about
0.44 of a grain per gallon, and this quantity sufficed to reduce the
color of the raw water from 0.40 to 0.30, platinum standard. This
reduction is very slight, and it is obvious that this quantity of
coagulant was not enough for decolorization.

Some experiments bearing on color removal were made at East Providence,
R. I., by Mr. E. B. Weston, and are described in the Proceedings of the
American Society of Civil Engineers for September, 1899. In this case
the color is reported to have been reduced from 0.58 to 0.10 platinum
standard by the use of one grain of sulphate of alumina, containing 22
per cent of effective alumina, equivalent to about 1.30 grains of the
ordinary article per gallon.

The various experiments seem to indicate that a removal from 80 to
90 per cent of the color can be effected by the use of a quantity of
sulphate of alumina equal to rather more than two grains per gallon
for waters having colors of 1.00, platinum standard, and proportionate
quantities for more and less deeply colored waters. With much less
sulphate of alumina decolorization is not effected, and even larger
quantities do not remove all of the color.

The data are much less complete than could be desired, and it is to be
hoped that experiments will be undertaken to throw more light upon this
important subject.

SUCCESSIVE APPLICATION OF COAGULANT.

Mr. Fuller, in his experiments at Louisville, has ascertained that when
sulphate of alumina is added to extremely muddy water the sediment
absorbs some of the chemical before it has time to decompose, and
carries it to the bottom, and so far as this is the case, no benefit
is derived from that part of the coagulant which is absorbed. In other
words, it is necessary to add more coagulant than would otherwise be
necessary because of this action. The data showed that different kinds
of suspended matters took up very different amounts of coagulant in
this way. With only moderately turbid waters the loss of chemical
from this source is unimportant. Hardly any trace of it was found at
Pittsburg with the Allegheny River water. At Louisville, however, it
was an important factor, as shown by Mr. Fuller’s results.

To avoid this loss of chemical Mr. Fuller has suggested the removal of
the greater part of the suspended matters by sedimentation, without
chemicals, or with the aid of a small quantity of chemical, followed by
the application of the final coagulant prior to filtration. With the
worst waters encountered at Louisville the saving in coagulant to be
effected in this way is very great.

Mr. Fuller states in “Water Purification at Louisville,” p. 417: “The
practical conclusions to be drawn from this experience are that with
preliminary coagulation, followed by subsidence for a period of about
three hours, the application of coagulants may be divided to advantage,
and a considerable portion of the suspended matter kept off the filter,
when the total amount of required coagulant ranges from 2 to 2.5
grains or more of ordinary sulphate of alumina per gallon. In the case
of a water requiring more than this amount of coagulating treatment,
a proper division of the application would increase the saving of
coagulants and would diminish the frequency of washing the filter.”

In his final summary and conclusions, page 441, Mr. Fuller estimates
the amount of sulphate of alumina required for the clarification of the
Ohio River at Louisville at 3.00 grains per gallon of water filtered
if all applied at one point, or at 1.75 grains by taking advantage of
subsidence to its economical limit prior to the final coagulation. The
saving to be effected in this way is sufficient to justify the works
necessary to allow it to be carried out. With less turbid waters, or
waters highly turbid for only short intervals, the advantages of double
coagulation would be less apparent.

THE AMOUNT OF COAGULANT WHICH VARIOUS WATERS WILL RECEIVE.

The amount of coagulant which can be safely used is dependent upon
the alkalinity of the raw water. When sulphate of alumina is added
to water it is decomposed, as explained above, with the formation
of alumina, which is alone useful in the work of purification, and
sulphuric acid, which combines with the calcium carbonate or lime
present in the water. There should always be an excess of alkalinity or
lime in the raw water. If for any reason there is not, there is nothing
to combine with the liberated sulphuric acid, and the decomposition of
the coagulant is not complete, and a portion of it goes undecomposed
into the effluent. The effluent then has an acid reaction, and is unfit
for domestic supply. When distributed through iron pipes, it attacks
the iron, rusting the pipes, and giving rise to all the disagreeable
consequences of an iron containing water.

The amount of lime in a water available to combine with the sulphuric
acid can be determined by a very simple chemical operation, namely,
by titration with standard acid with a suitable indicator. The amount
of coagulant corresponding to a given quantity of lime can be readily
and accurately calculated, but it is not regarded safe to use as
much sulphate of alumina as corresponds to the lime. The quantity of
coagulant used is not susceptible to exact control, but fluctuates
somewhat, and if the exact theoretical quantity should be employed
during 24 hours, there would surely be an excess during some portion of
that time from which bad results would be experienced. It is therefore
considered only prudent to use three quarters as much sulphate of
alumina as corresponds to the lime in the water. With sulphate of
alumina containing 17 per cent of soluble aluminum oxide and the
corresponding amount of sulphuric acid, the amount which can be applied
to a water in grains per gallon is slightly less than the alkalinity
expressed in terms of parts in 100,000 of calcium carbonate.

Many waters contain sufficient lime to combine with the acid of all
the coagulant which is necessary for their coagulation. Others will
not, and it thus becomes an important matter to determine whether a
given water is capable of decomposing sufficient coagulant for its
treatment. It is usually the flood-flows of rivers which control in
this respect. The water at such times requires much larger quantities
of coagulant for its clarification, and it also usually contains much
less lime than the low-water flows. The reason for this is obviously
that the water of the flood-flows is largely rain-water which has come
over the surface without coming into very intimate contact with the
soil, and consequently without having taken from it much lime, while
the low-water flows contain a considerable proportion of water which
has percolated through the soil and has thus become charged with lime.

In some parts of the country, as, for instance, in New England, the
soil and underlying rock are almost entirely free from lime, and
rivers from such watersheds are capable of receiving only very small
quantities of coagulant without injurious results.

The deficiency of alkalinity in raw water can be corrected by the
addition to it of lime or of soda-ash. Lime has been used for this
purpose in many cases. When used only in moderate amounts it hardens
the water, and is thus seriously objectionable. The use of so large a
quantity as would precipitate out, as in Clark’s process, has not been
employed in practice. If it should be attempted, the amount of lime
would require to be very accurately controlled, and the effluent would
have to be treated with carbonic acid to make it suitable for supply.

Waters so hard as to require the use of the Clark process almost always
have sufficient alkalinity, and do not require to be treated with lime
in connection with the use of sulphate of alumina.

The use of soda-ash is free from the objections to the use of lime,
but is more expensive, and would require to be used with caution. Its
use has often been suggested, but I do not know that it has ever been
employed in practice. In small works the use of a filtering material
containing marble-dust, or other calcareous matter, would seem to have
some advantages in case of deficiency of alkalinity, although it would
harden the water so treated.

The alkalinities of a number of waters computed as parts in 100,000 of
calcium carbonate (approximately equal to the safe doses of sulphate to
alumina in grains per gallon) are as follows:

-------------------------------------+--------+--------+--------
|Maximum.|Minimum.|Average.
-------------------------------------+--------+--------+--------
Boston water, 1898 | 2.87 | 0.33 | 1.08
Conestoga Creek, Lancaster, Penn. | 12.20 | 3.70 | 6.80
Allegheny River, Pittsburg | 8.00 | 1.02 | 2.90
Mahoning River and tributaries, 1897 | 20.00 | 2.20 | 10.00
Scioto River and tributaries, 1897 | 35.00 | 10.00 | 20.00
Ohio River, Cincinnati, 1898 | 7.00 | 2.00 | 4.50
Ohio River, Louisville | 10.87 | 2.12 | 6.70
Lake Erie, Lorain, Ohio | | | 9.50
Lake Michigan, Chicago | | | 11.50
-------------------------------------+--------+--------+--------

CHAPTER X.

MECHANICAL FILTERS.

The term mechanical filters is used to designate a general class of
filters differing in many respects quite radically from the sand
filters previously described. They had their origin in the United
States, and consisted originally of iron or wooden cylinders filled
with sand through which the water was forced at rates of one to two
hundred million gallons per acre daily, or from fifty to one hundred
times the rates usually employed with sand filters. These filters were
first used in paper-mills to remove from the large volumes of water
required the comparatively large particles, which would otherwise
affect the appearance and texture of the paper; and in their earlier
forms they were entirely inadequate to remove the finer particles,
such as the bacteria, and the clay particles which constitute the
turbidity of river waters. Various improvements in construction have
since been made, and, in connection with the use of coagulants, much
more satisfactory results can now be obtained with filters of this
class; and their use has been extended from manufacturing operations to
municipal supplies, in many cases with most satisfactory results.

The information gathered in regard to the conditions essential to the
successful design and operation of these filters in the last few years
is very great, and may be briefly reviewed.

PROVIDENCE EXPERIMENTS.[39]

The first data of importance were secured from a series of experiments
conducted by Mr. Edmund B. Weston of Providence, R. I., in 1893 and
1894, upon the Pawtuxet river water used by

that city. The experimental filter was 30 inches in diameter, and had
a layer of sand 2 feet 10 inches deep. The sand was washed by the use
of a reverse current, the sand being stirred by a revolving rake at the
same time. The amount of coagulant employed was about 0.7 of a grain
per gallon. The raw water was practically free from turbidity, and the
filter was operated to remove color and bacteria.

The removal of color, as stated in Mr. Weston’s report, amounted to
from 70 to 90 per cent. The experiments extended over a period of
ten months. The rate of filtration employed was about 128 million
gallons per acre daily. The bacterial results of the first six months’
operations were rejected by Mr. Weston on account of defective methods
of manipulation.

During the period from November 17, 1893, to January 30, 1894, the
average bacterial efficiency of filtration was about 95 per cent, and
the manipulation was considered to be in every respect satisfactory.
The efficiency was occasionally below 90 per cent, but for four
selected weeks was as high as 98.6 per cent. The average amount of
sulphate of alumina used, as calculated from Mr. Weston’s tables, was
two thirds of a grain per gallon. The highest efficiency followed the
application of a solution of caustic soda to the filtering material.
The first day following this treatment the bacterial efficiency was
above 99 per cent. Afterwards it decreased until January 30, when the
experiments were stopped. The high bacterial efficiency following the
use of caustic soda was of such short duration as to suggest very
grave doubts as to its practical value. It is extremely unfortunate
that the experiments stopped only a week after this experiment, and
the results were never repeated. I consider that the average bacterial
efficiency of about 95 per cent obtained for the period of October 17
to January 30, when the manipulation was considered to be in every way
satisfactory, more nearly represents what can be obtained under these
conditions than the results for certain periods, particularly after the
use of the caustic soda.

LOUISVILLE EXPERIMENTS.[40]

These experiments were inaugurated by the Louisville Water Company
in connection with the manufacturers of certain patented filters.
Mr. Charles Hermany, Chief Engineer of the Company, had general
charge of the experiments. Mr. George W. Fuller was Chief Chemist and
Bacteriologist and had direct charge of the work and has made a most
elaborate report upon the same. In these examinations many devices were
investigated; but the two which particularly deserve our attention are
the filters known as the Warren Filter and the Jewell Filter.

These filters were operated for two periods, namely, from October
18, 1895, to July 30, 1896, and from April 5 to July 24, 1897. The
investigations were directed toward the clarification of the river
water from the mud, and to the removal of bacteria. The water was
substantially free from color. The character of the water at this
point was such that in its best condition at least three fourths of
a grain of sulphate of alumina were necessary for its coagulation,
and with this and with larger quantities of coagulant fair bacterial
purification was nearly always obtained. The problem studied
therefore was principally that of clarification from mud. The average
efficiencies, as shown by the total averages, (page 248,) were as
follows: Warren filter, bacterial efficiency, 96.7 per cent; Jewell
filter, 96.0 per cent.

LORAIN TESTS.[41]

These tests were made by the author of a set of Jewell filters at
Lorain, Ohio. The filters were six in number, each 17 feet in diameter,
having an effective filtering area of 226 square feet each, or 1356
square feet in all. The construction of the filters was in all respects
similar to the Jewell filter used at Louisville. The raw water was from
Lake Erie, and during the examination was

always comparatively clear, but contained considerable numbers of
bacteria. The problem was thus entirely one of bacterial efficiency.
The question of clarification hardly presented itself. Although
the water became turbid at times it did not approach in muddiness
the condition of the Ohio River water, and an amount of coagulant
sufficient for a tolerable bacterial efficiency in all cases was more
than sufficient for clarification.

A summary of the results obtained is as follows:

-----------+--------------+-----------+--------+---------+----------
| Average Rate |Sulphate of| | |Bacterial
Week Ending|of Filtration,| Alumina, |Bacteria|Bacteria |Efficiency
6:00 P.M. | Gallons per |Grains per | in Lake| in |per cent.
| Sq. Ft. Min. |Gallon. | Water. |Effluent.|
-----------+--------------+-----------+--------+---------+----------
June 19 | 1.06 | 2.58 | 1441 | 16 | 98.9
26 | 1.10 | 2.50 | 385 | 6 | 98.4
July 3 | 1.11 | 2.27 | 367 | 9 | 97.5
10 | 1.28 | 1.07 | 154 | 14 | 90.9
17 | 1.14 | 0.94 | 189 | 26 | 86.3
+--------------+-----------+--------+---------+----------
Average | 1.14 | 1.83 | 507 | 14 | 96.4
-----------+--------------+-----------+--------+---------+----------

The average bacterial efficiency was 96.4 per cent with 1.83 grains of
sulphate of alumina per gallon.

PITTSBURG EXPERIMENTS.[42]

The Pittsburg experiments were inaugurated by the Pittsburg Filtration
Commission. The operation of the filters extended from January to
August, 1898. A Jewell and a Warren filter were used similar in design
to those used at Louisville. The raw water contained large numbers of
bacteria, and was also often very turbid, although less turbid than at
Louisville. At times more coagulant was necessary for clarification
than was required for bacterial efficiency; while as a rule more was
required for satisfactory bacterial purification than was necessary for
clarification. The opportunities were therefore favorable for the study
of both of these conditions. The amount of coagulant necessary for
clarification has been mentioned in connection with coagulation.

The results secured upon the relation of the quantity of

coagulant to the number of bacteria in the effluent were more complete
than any other experiments available, and are therefore here reproduced
from the Pittsburg report nearly in full.

It was found that the amount of sulphate of alumina employed was
more important than any other factor in determining the bacterial
efficiency, and special experiments were made to establish the effect
of more and of less coagulant than used in the ordinary work. These
experiments were made upon the Warren filter during May, and with
the Jewell filter during June. The monthly averages for these months
are thus abnormal and are not to be considered. The remaining six
months for each filter may be taken as normal and as representing
approximately the work of these filters under ordinary careful working
conditions.

During the six months when the Warren filter was in normal order the
raw water contained 11,531 bacteria and the effluent 201, the average
bacterial efficiency being 98.26 per cent. The bacterial efficiency was
very constant, ranging only, by months, from 97.48 to 98.96 per cent.
During the same period a sand filter receiving the same water yielded
an effluent having an average of 105 bacteria per cubic centimeter.

The Jewell filter, for the six months in which it was in normal order,
received raw water containing an average of 11,481 bacteria and yielded
an effluent containing an average of 293, the bacterial efficiency
being 97.45 per cent, and ranging, in different months, from 93.23 to
98.61 per cent.

WASTING EFFLUENT AFTER WASHING FILTERS.

After washing a mechanical filter the effluent for the first few
minutes is often inferior in quality to that obtained at other times,
and if samples are taken at these times and averaged with other samples
taken during the run, an apparent efficiency may be obtained inferior
to the true efficiency. To guard against this source of error, whenever
samples have been taken at such times, the average work for the day
has been taken, not as the numerical average of the results, but each
sample has been given weight in proportion to the amount of time which
it could be taken as representing; so that the results represent as
nearly as possible the average number of bacteria in the effluent for
the whole run. As a matter of fact, however, comparatively few samples
were taken during these periods of reduced efficiency, and thus most of
the results represent the normal efficiency exclusive of this period. A
study has been made, however, of the results of examinations of samples
taken directly after washing, somewhat in detail. The following is a
tabular statement of the average results obtained from each filter by
months, including only the results obtained on those days when samples
were taken within twenty minutes after washing, the results of other
days being excluded.

AVERAGE NUMBER OF BACTERIA IN EFFLUENT.

--------------+------------+-------------+-------------+-------------
| Shown by | Within Ten | 11 to 20 | More than
| Record |Minutes after|Minutes after| Twenty
| Sheets. | Washing. | Washing. |Minutes after
| | | | Washing.
--------------+------------+-------------+-------------+-------------
WARREN FILTER.| | | |
February | 115 | | 118 | 114
March | 316 | 50 | 515 | 301
April | 79 | 417 | 207 | 75
May | (Special experiments, omitted.)
June | 197 | 493 | 272 | 170
July | 300 | | 546 | 207
August | 174 | 356 | 601 | 223
| | | |
JEWELL FILTER.| | | |
February | 2453 | 2425 | | 2099
March | 455 | 657 | 958 | 354
April | 99 | 665 | 462 | 165
May | 144 | 998 | 346 | 127
June | (Special experiments, omitted.)
July | 279 | 1330 | 272 | 274
August | 344 | 612 | 323 | 376
--------------+------------+-------------+-------------+-------------

The time of inferior work very rarely exceeded twenty minutes. It
will be seen from the tables that the results as shown by the record
sheets are never very much higher, and are occasionally lower than the
results of samples taken on corresponding days more than twenty minutes
after washing; and thus while a decrease in bacterial efficiency was
noted after washing, no material increase in the average bacterial
efficiency of the mechanical filters would have been obtained if these
results had been excluded. The results for the whole time would be
affected much less than is indicated by the table, because the table
includes only results of those days when samples were taken just after
washing, while the much larger number of days when no such samples were
taken would show no change whatever.

It has been suggested that these inferior effluents after washing
should be wasted. Such a procedure would mean wasting probably on
an average two per cent of the water filtered, and a corresponding
increase in the cost of filtering. Mr. Fuller[43] in his Louisville
report comes to the conclusion that with adequate washing and
coagulation it is unnecessary to waste any effluent, and that inferior
results after washing usually indicate incomplete washing. While our
experiments certainly indicate a reduction in efficiency after washing
so regular and persistent as to make it doubtful whether incomplete
washing can be the cause of it, it may be questioned whether or
not wasting the effluent would be necessary or desirable in actual
operation. At any rate the results as given in this report are not
materially influenced by this factor.

INFLUENCE OF AMOUNT OF SULPHATE OF ALUMINA ON BACTERIAL EFFICIENCY OF
MECHANICAL FILTERS.

The number of bacteria passing a mechanical filter is dependent
principally upon the amount of sulphate of alumina used; and by using
a larger quantity of sulphate of alumina than was actually used in the
experiments the bacterial efficiency could be considerably increased.
To investigate this point, the results obtained each day with each
of the mechanical filters were arranged in the order of the sulphate
of alumina quantities used, and averaged by classes. In this and the
following tables a few abnormal results were omitted.[44] A summary of
the results is as follows:

SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING TO
SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
Number | | Bacteria. | | |Sulphate of
of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina
Represented.| |Raw Water.|Effluent.|remaining.|removed.|used Grains
| | | | | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
7 | 0.05 | 4,773 | 1713 | 35.89 | 64.11 | 0.00
2 | 0.08 | 2,785 | 850 | 30.52 | 69.48 | 0.12
4 | 0.10 | 5,109 | 726 | 14.21 | 85.79 | 0.26
2 | 0.20 | 8,713 | 214 | 2.45 | 97.55 | 0.36
8 | 0.06 | 3,224 | 112 | 3.47 | 96.53 | 0.44
19 | 0.06 | 3,488 | 123 | 3.53 | 96.47 | 0.55
11 | 0.06 | 5,673 | 154 | 2.71 | 97.29 | 0.64
10 | 0.10 | 6,100 | 112 | 1.84 | 98.16 | 0.74
8 | 0.09 | 8,647 | 148 | 1.71 | 98.29 | 0.85
5 | 0.16 | 5,645 | 142 | 2.52 | 97.48 | 0.93
13 | 0.12 | 10,397 | 200 | 1.92 | 98.08 | 1.07
10 | 0.08 | 12,778 | 121 | 0.95 | 99.05 | 1.13
13 | 0.14 | 13,397 | 164 | 1.22 | 98.78 | 1.25
19 | 0.13 | 10,462 | 160 | 1.53 | 98.47 | 1.34
10 | 0.12 | 12,851 | 107 | 0.83 | 99.17 | 1.46
4 | 0.27 | 16,015 | 77 | 0.48 | 99.52 | 1.57
7 | 0.53 | 12,262 | 191 | 1.18 | 98.82 | 1.64
4 | 0.58 | 26,950 | 347 | 1.29 | 98.71 | 1.74
5 | 0.29 | 14,570 | 86 | 0.59 | 99.41 | 1.84
3 | 0.23 | 13,833 | 153 | 1.11 | 98.89 | 1.92
19 | 0.40 | 18,222 | 92 | 0.50 | 99.50 | 2.48
5 | 0.45 | 29,300 | 1119 | 3.82 | 96.18 | 3.37
5 | 1.06 | 33,030 | 535 | 1.62 | 98.38 | 8.06
------------+----------+----------+---------+----------+--------+-----------

SUMMARY OF RESULTS WITH JEWELL MECHANICAL FILTER, ARRANGED ACCORDING TO
SULPHATE OF ALUMINA QUANTITIES.

------------+----------+----------+---------+----------+--------+-----------
Number | | Bacteria. | | |Sulphate of
of Days |Turbidity.+----------+---------+ Per cent |Per cent| Alumina
Represented.| |Raw Water.|Effluent.|remaining.|removed.|used Grains
| | | | | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
6 | 0.03 | 14,037 | 6217 | 44.29 | 55.71 | 0.00
5 | 0.07 | 4,267 | 680 | 15.93 | 84.07 | 0.24
14 | 0.06 | 2,613 | 170 | 6.50 | 93.50 | 0.35
10 | 0.06 | 2,446 | 113 | 4.62 | 95.38 | 0.44
9 | 0.11 | 7,303 | 234 | 3.20 | 96.80 | 0.55
20 | 0.09 | 6,979 | 220 | 3.15 | 96.85 | 0.65
9 | 0.08 | 5,191 | 130 | 2.50 | 97.50 | 0.75
16 | 0.12 | 8,504 | 242 | 2.84 | 97.16 | 0.83
22 | 0.16 | 8,506 | 99 | 1.16 | 98.84 | 0.96
12 | 0.11 | 11,998 | 246 | 2.05 | 97.95 | 1.05
14 | 0.18 | 18,982 | 423 | 2.23 | 97.77 | 1.16
5 | 0.14 | 13,981 | 224 | 1.60 | 98.40 | 1.23
9 | 0.27 | 19,806 | 325 | 1.64 | 98.36 | 1.34
14 | 0.27 | 16,549 | 324 | 1.96 | 98.04 | 1.45
9 | 0.29 | 12,194 | 96 | 0.79 | 99.21 | 1.54
6 | 0.25 | 13,483 | 51 | 0.38 | 99.62 | 1.65
7 | 0.53 | 24,243 | 220 | 0.91 | 99.09 | 1.72
3 | 0.90 | 20,953 | 602 | 2.88 | 97.12 | 1.90
5 | 0.43 | 25,958 | 307 | 1.19 | 98.81 | 2.19
4 | 0.84 | 21,017 | 228 | 1.09 | 98.91 | 3.71
------------+----------+----------+---------+----------+--------+-----------

These results are shown graphically by Fig. 21.

[Illustration: FIG. 21.—BACTERIAL EFFICIENCIES OF MECHANICAL FILTERS.]

INFLUENCE OF DEGREE OF TURBIDITY UPON BACTERIAL EFFICIENCY OF
MECHANICAL FILTERS.

It will be noticed by referring to the tables that as the sulphate of
alumina quantities increased the turbidities increased and the numbers
of bacteria increased, as well as the bacterial efficiencies. That
is to say, with the less turbid waters, small sulphate of alumina
quantities have been used, the numbers of bacteria in the raw water
have been low, and the bacterial efficiencies have also been low. With
turbid waters much larger quantities of sulphate of alumina have been
used, the raw water has contained more bacteria, and the bacterial
efficiencies have been higher. It may be then that the increased
efficiencies with increased quantities of sulphate of alumina are not
due alone to the increased sulphate of alumina, but in part also to
other conditions. Thus it may be easier to remove a large percentage of
bacteria from a water containing many than from a water containing only
a few.

To investigate this matter and eliminate the influence of turbidity and
numbers of bacteria in the raw water, the results were first classified
with reference to turbidity. The results with waters having turbidities
of 0.10 or less, and called for convenience turbid waters, are arranged
by alum quantities as before. Afterwards the results obtained with
turbidities from 0.11 to 0.50, and called for convenience muddy
waters, are grouped; and finally the results with turbid water having
turbidities of 0.51 and over, and called for convenience thick waters.
The results thus arranged are as follows:

SUMMARY OF RESULTS WITH WARREN MECHANICAL FILTER, ARRANGED ACCORDING
TO TURBIDITIES AND SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate of
Number | | Bacteria. | Per cent |Per cent| Alumina
of Days |Turbidity.+----------+---------+remaining.|removed.|used Grains
Represented.| |Raw Water.|Effluent.| | |per Gallon.
------------+----------+----------+---------+----------+--------+-----------
7 | 0.05 | 4,773 | 1713 | 35.89 | 64.11 | 0.00
2 | 0.07 | 2,785 | 850 | 30.52 | 69.48 | 0.12
12 | 0.06 | 3,209 | 224 | 7.00 | 93.00 | 0.42
31 | 0.06 | 4,238 | 119 | 2.81 | 97.19 | 0.60
9 | 0.06 | 7,953 | 130 | 1.64 | 98.36 | 0.84
16 | 0.04 | 11,265 | 137 | 1.22 | 98.78 | 1.11
29 | 0.06 | 11,500 | 158 | 1.37 | 98.63 | 1.58
| | | | | |
5 | 0.17 | 8,783 | 416 | 4.73 | 95.27 | 0.36
10 | 0.16 | 6,535 | 165 | 2.54 | 97.46 | 0.85
13 | 0.19 | 13,253 | 186 | 1.40 | 98.60 | 1.13
15 | 0.22 | 10,944 | 93 | 0.85 | 99.15 | 1.36
13 | 0.29 | 14,089 | 112 | 0.80 | 99.20 | 1.73
10 | 0.35 | 18,088 | 102 | 0.57 | 99.43 | 2.38
5 | 0.29 | 25,580 | 540 | 2.11 | 97.89 | 4.30
| | | | | |
6 | 0.87 | 25,433 | 369 | 1.45 | 98.55 | 1.74
6 | 0.73 | 26,566 | 79 | 0.30 | 99.70 | 2.64
4 | 1.35 | 42,037 | 1388 | 3.30 | 96.70 | 8.16
------------+----------+----------+---------+----------+--------+-----------

SUMMARY OF RESULTS WITH JEWELL MECHANICAL FILTER, ARRANGED ACCORDING
TO TURBIDITIES AND SULPHATE OF ALUMINA QUANTITIES.

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate of
Number | | Bacteria. | | | Alumina
of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains
Represented.| |Raw Water.|Effluent.|remaining.|removed.|per Gallon.
------------+----------+----------+---------+----------+--------+-----------
6 | 0.03 | 14,037 | 6217 | 44.29 | 55.71 | 0.00
3 | 0.07 | 5,170 | 991 | 19.15 | 80.85 | 0.21
25 | 0.05 | 2,403 | 143 | 5.95 | 94.05 | 0.38
20 | 0.06 | 6,531 | 185 | 2.84 | 97.16 | 0.64
27 | 0.06 | 5,811 | 122 | 2.10 | 97.90 | 0.88
14 | 0.06 | 14,978 | 412 | 2.75 | 97.25 | 1.11
10 | 0.06 | 15,787 | 390 | 2.47 | 97.53 | 1.37
10 | 0.05 | 10,847 | 47 | 0.43 | 99.57 | 2.17
| | | | | |
14 | 0.16 | 7,525 | 256 | 3.40 | 96.60 | 0.60
17 | 0.24 | 11,310 | 208 | 1.84 | 98.16 | 0.91
15 | 0.24 | 15,441 | 262 | 1.70 | 98.30 | 1.13
10 | 0.28 | 17,842 | 232 | 1.30 | 98.70 | 1.43
8 | 0.29 | 9,556 | 59 | 0.62 | 99.38 | 1.59
4 | 0.29 | 20,212 | 135 | 0.67 | 99.33 | 2.00
| | | | | |
5 | 0.66 | 23,680 | 336 | 1.42 | 98.58 | 1.42
7 | 0.96 | 30,200 | 475 | 1.57 | 98.43 | 1.74
4 | 1.25 | 37,587 | 496 | 1.32 | 98.68 | 2.81
------------+----------+----------+---------+----------+--------+-----------

The following table shows the bacterial efficiencies with turbid,
muddy, and thick waters, with substantially equal quantities of
sulphate of alumina:

-------------------------------+-------------------------------------
Grains of Sulphate of Alumina.|Corresponding Bacterial Efficiencies.
----------+----------+---------+-----------+----------+--------------
Turbid. | Muddy. | Thick. | Turbid. | Muddy. | Thick.
----------+----------+---------+-----------+----------+--------------
WARREN FILTER.
0.42 | 0.36 | | 93.00 | 95.27 |
0.84 | 0.85 | | 98.36 | 97.46 |
1.11 | 1.13 | | 98.78 | 98.60 |
1.58 | 1.73 | 1.74 | 98.63 | 99.20 | 98.55
| 2.38 | 2.64 | | 99.43 | 99.70
| 4.30 | 8.16 | | 97.89 | 96.70

JEWELL FILTER.
0.64 | 0.60 | | 97.16 | 96.60 |
0.88 | 0.91 | | 97.90 | 98.16 |
1.11 | 1.13 | | 97.25 | 98.30 |
1.37 | 1.43 | 1.42 | 97.53 | 98.70 | 98.58
2.17 | 1.59 | 1.74 | 99.57 | 99.38 | 98.43
| 2.00 | 2.81 | | 99.33 | 98.68
----------+----------+---------+-----------+-------------------------

It appears from this table that waters of various degrees of turbidity
give substantially equal bacterial efficiencies with equal quantities
of sulphate of alumina, the results varying as often in one direction
as the other. Within certain limits it may thus be said that turbidity
is without influence upon the bacterial efficiency obtained in
mechanical filtration.

It must be borne in mind, however, that the quantities of sulphate
of alumina, with very few exceptions, were sufficient to produce
full coagulation. Mr. Fuller has shown in his Louisville report that
considerable quantities of sulphate of alumina may be added to turbid
waters without producing appreciable coagulation; and therefore if
a quantity of sulphate of alumina sufficient to produce a certain
bacterial efficiency in a clear water should be added to a water so
turbid that it was unable to coagulate it, scarcely any effect would
be produced. The above statement therefore only applies in those cases
where sufficient sulphate of alumina is used to adequately coagulate
the water.

As the numbers of bacteria often vary with the turbidity, the variation
in the numbers of bacteria in the different classes is much less than
in the first tables; but to further investigate the question of whether
the numbers of bacteria in the raw water have an important influence
upon the bacterial efficiencies, each of the two largest classes in the
foregoing tables was divided into two parts, according to the bacterial
numbers in the raw water, namely, the results from the Jewell filter
with turbid waters and with sulphate of alumina quantities ranging from
0.75 to 1.00 grain per gallon, and the results from the Warren filter
with turbid waters and with sulphate of alumina quantities of 1.25
grains per gallon and upward. The results are as follows:

------------+----------+--------------------+----------+--------+-----------
| | | | |Sulphate
Number | | Bacteria. | | |of Alumina
of Days |Turbidity.+----------+---------+ Per cent |Per cent|used Grains
Represented.| |Raw Water.|Effluent.|remaining.|removed.|per Gallon.
------------+----------+----------+---------+----------+--------+-----------
JEWELL FILTER.
14 | 0.05 | 3,938 | 81 | 2.06 | 97.94 | 0.88
13 | 0.07 | 7,827 | 167 | 2.13 | 97.87 | 0.87
WARREN FILTER.
15 | 0.06 | 3,545 | 59 | 1.66 | 98.34 | 1.67
14 | 0.06 | 20,022 | 265 | 1.32 | 98.68 | 1.48
------------+----------+----------+---------+----------+--------+-----------

It will be observed that the bacterial efficiencies are substantially
the same, with the lower and with the higher numbers of bacteria in
the raw water. That is to say, other things being equal, as the number
of bacteria increase in the raw water the number of bacteria in the
effluent increase in the same ratio. A further analysis of other groups
of results would perhaps show variations in one direction or the other,
but on the whole it is believed that the comparison is a fair one, and
that there is no well-marked tendency for bacterial efficiencies of
mechanical filters to increase or decrease with increasing numbers of
bacteria.

AVERAGE RESULTS OBTAINED WITH VARIOUS QUANTITIES OF SULPHATE OF ALUMINA.

As it appears that neither the turbidity nor the number of bacteria in
the raw water has a material influence upon the percentage bacterial
efficiency obtained, we can take the results given above, which
include all the results obtained (except a very few abnormal ones) for
computing the various efficiencies obtained with various quantities of
sulphate of alumina. These results are graphically shown by Fig. 21, p.
167, on which lines have been drawn indicating the normal efficiencies
from various quantities of sulphate of alumina as deduced from our
experiments.

In computing the amount of sulphate of alumina which it would be
necessary to use in operating a plant at a given place to give these
efficiencies, the quantities of sulphate of alumina shown by the
diagram can be taken as those which it would be necessary to use during
those days in the year when the raw water was clear, or sufficiently
clear, so that the amounts of sulphate of alumina mentioned would
suffice to properly coagulate it.

TYPES OF MECHANICAL FILTERS.

Sections of the Warren and Jewell filters used at Pittsburg are
presented herewith. The filters here shown are practically identical
with those used at Lorain and Louisville, and nearly all the exact
information regarding mechanical filters relates to filters of these
types. These sections show clearly the constructions used at Pittsburg
and Louisville, but there are some points in connection with the
designs of these filters which require to be considered more in detail.

The simplest idea of a mechanical filter is a tub, with sand in the
bottom and some form of drainage system. Water is run over the sand,
passes through it, and is collected by the drainage system. When the
sand becomes clogged it is washed by the use of a reverse current of
water. This reverse current of water is so rapid as to preclude the use
of a drainage system consisting of gravel, tile-drains, etc., such as
are used in sand filters operated at lower rates, and instead metallic
strainers in some form are used. The sand comes directly against these
strainers, which are made as coarse as it is possible to have them,
without allowing the sand to pass.

The rate of washing is usually from five to seven gallons per square
foot per minute. In the Warren filter the openings in the strainers at
the bottom are 6 to 8 per cent of the total area, and during washing
the water has an average velocity of 0.20 foot per second upward
through them. This velocity is so slow that the friction of the water
in passing through the openings in the screen is practically nothing.
A result of this is that if there is any unequal resistance of the
sand to the water, the bulk of the water goes up at the points of least
resistance in the sand.

[Illustration: FIG. 22.—SECTION OF JEWELL MECHANICAL FILTER USED IN
PITTSBURG EXPERIMENTS.]

This tendency would be fatal were it not for the revolving rake which
loosens and mixes the sand and largely corrects it. The correction,
however, is imperfect, and some parts of the filter are washed more
than others.

The rake is also necessary to prevent the separation of sand into
coarser and finer particles. It is practically impossible to get
filter sand the grains of which are all of the same size. When a filter
is washed the tendency is for the wash water to go up in limited areas.
The larger sand grains tend to collect at these points while the
finer grains collect in places where there is no upward current, or
where it is less rapid. In many filters this tendency is very strong.
The revolving rake is necessary to correct it, and to keep the sand
thoroughly mixed, otherwise when a filter is put in operation after
washing, the frictional resistance through the coarse sand being less,
the bulk of the water goes through it, with the result that a part of
the area, and the part which is least efficient as a filter, passes
nearly all of the water, and with inferior results.

In the Jewell filter provision is made for the distribution of the wash
water over the whole area in another way. The strainers have areas at
the surface amounting to 1.2 to 1.4 per cent of the whole area, but
the water before reaching them passes through throats much smaller in
size than the strainer outlets, and amounting in the aggregate to only
about 0.07 per cent of the filter area. When washing at a rate of seven
gallons per square foot per minute, water passes through these necks
at a velocity of 22 feet per second. The friction and velocity head in
passing these necks is estimated to be about 30 vertical feet, and is
so much greater than the friction of the outlets proper, and of the
sand, that the water passes through each strainer with approximately
the same velocity, and the wash water is equally distributed over the
whole area of the bottom of the filter.

This result is accomplished, however, at a great loss of head in the
wash water. When a filter is washed from the pressure-mains without
separate pumping, the pressure is usually sufficient and there is no
disadvantage in the arrangement. When, however, the water is specially
pumped for washing, the required head is much greater than would
otherwise be necessary.

[Illustration: MECHANICAL FILTERS AT ELMIRA, N. Y. OUTLET TO FILTERS
WITH CONTROLLER AND PURE-WATER FLUME.

[_To face page 174._]
]

It would not be possible to increase the size of the necks, thereby
decreasing the friction, without increasing very largely the size
of the pipes in the underdrainage system into which the strainers are
fastened. These pipes are so small that during washing the velocity in
them is about 13 feet per second, and if the throats of the necks were
increased without also enlarging these pipes, the friction would be so
reduced that most of the water would go through the necks nearest the
supply, thus failing to reach the object to be attained.

A more rational system would be to increase the sizes of all the
waterways in the outlet and wash-water system. The Jewell filter is
also provided with a rake to keep the sand mixed during washing, as
this is necessary even with the complete distribution of wash-water
over the area of the filter.

Both the Warren and the Jewell filters are provided with receptacles
through which the water passes after receiving the coagulant, and
before entering the filter. In the Jewell filter the receptacle, called
a sedimentation-basin, is of such size as to hold as much water as is
filtered in 15 minutes. In the Warren filter the receptacle is entirely
independent and larger, holding about an hour’s supply.

The rates of filtration used in the experiments have ranged from less
than 100 to about 130 million gallons per acre daily. To employ a rate
much higher than this involves the use of a much coarser sand, or an
increase in the height of water upon the filter to an impracticable
extent. There would seem to be no material advantage in the use of
lower rates within certain limits, while the cost of filters would be
greatly increased.

The sand used in the Warren filters has been crushed quartz. In the
Jewell filters a silicious sand from Red Wing, Minn., with rounded
grains has been used. These sands are somewhat coarser than are
commonly used in sand filters, and the uniformity coefficients are
very low. It is necessary to use sand with the very lowest uniformity
coefficients to avoid the separation of sand particles according to
sizes as mentioned above, and for this reason the sand must be
selected with much greater care than is required for sand filters.

[Illustration: PLAN JUST ABOVE COPPER.

SECTION SHOWING FILTER DURING ORDINARY OPERATION.

FIG. 23.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 1.]

[Illustration: PLAN OF AGITATOR, GUTTER CASTINGS, ETC.

SECTION SHOWING FILTER DURING OPERATION OF WASHING.

FIG. 24.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 2.]

The round-grained sand is more readily and completely washed than the
angular crushed quartz. It has been claimed that the crushed quartz is
more efficient as a filtering material, but the evidence of this is not
very clear.

The amount of water filtered by a filter between washings is, in a
general way, about the same as that filtered by a sand filter between
scrapings, in relation to its area. The amount of water required for
washing is, on an average, about equal to a vertical column 5 or 6
feet high equal in area to the area of the filter, exclusive of water
on the top of the filter wasted before the current is reversed. With
clear waters, as for instance, the Allegheny at low water, the amount
of washing is almost directly proportional to the amount of sulphate
of alumina used. With muddy waters the sulphate of alumina required is
proportional to the mud, and the frequency of washing and the amount of
wash-water are proportional to both. The amount of wash-water required
averages about five per cent; with very muddy waters more is required.
At Louisville, with the worst waters, the per cents of wash-water rose
at times to 30 per cent of the total quantity of water filtered.

The rate of filtration with mechanical filters should be kept as
constant as possible, and can be regulated by devices similar to
those described in connection with sand filters. Owing to the smaller
areas and capacities, the amounts of water to be handled in the units
are smaller, and the regulating devices are thus smaller, and have
always been made of metal, either cast iron or copper. None of the
devices employed in the above-mentioned experiments has been entirely
satisfactory in this respect. The devices employed have been too small,
and the water has gone through at too high velocities to allow close
adjustment.

[Illustration: MECHANICAL FILTERS AT ELMIRA, N. Y. UPPER PLATFORM AND
GENERAL ARRANGEMENT OF FILTERS.

[_To face page 178._]
]

As between the two types of filters, the Jewell filter requires a large
loss of head. The water has to be pumped at a sufficient elevation
to reach the top of a tank about 18 feet high, while the effluent
must be drawn off at the extreme bottom. The Warren filter is much
more economical in head, the plants at Pittsburg and Louisville only
requiring about 9 feet from the inlet to the outlet.

The earlier mechanical filters were usually constructed of wrought
iron or steel plates. More recently wooden tanks have been commonly
employed, although steel is regarded as preferable. Concrete or masonry
tanks have been suggested, but they have not as yet been employed.

EFFICIENCY OF MECHANICAL FILTERS.

The efficiency of mechanical filters depends entirely upon the use of
coagulants. Without coagulants they can only be used to remove very
large particles. The efficiency of the filtration depends much more
upon the kind, and amount, and method of application of coagulant than
upon the arrangement of the filter. In fact, the arrangements of the
filter are more directed to the convenience and economy of operation
and washing than towards the efficiency of the results.

The conditions which control the efficiency of mechanical filters
have been discussed in connection with coagulation. With sufficient
coagulant the removal of turbidity or mud is complete. Color also can
be removed with these filters. The bacterial efficiencies secured with
them have been discussed at length in connection with the Pittsburg
experiments.

With careful coagulation and manipulation it is possible to get 98 per
cent bacterial efficiency without difficulty. The results are somewhat
irregular, for reasons not as yet fully understood. On some occasions
higher bacterial efficiencies are secured with smaller quantities of
coagulant, while at other times the efficiencies are less without
apparent reason. There seems to be a limit to the bacterial efficiency
which can be secured with any amount of sulphate of alumina and
rapid filtration, and it is doubtful if a plant could be operated to
regularly secure as high a bacterial efficiency as 99 per cent with any
amount of sulphate of alumina.

PRESSURE FILTERS.

Pressure mechanical filters are constructed in entirely closed
receptacles, through which the water is forced under pressure and not
by gravity. Many of the earlier mechanical filters were of this type.
In small plants this system has the distinct advantage that the water
can be pumped from a river or other source of supply through a filter
direct to the reservoir or into the mains, while any other system would
involve a second pumping. Pressure filters are extensively used for
hotel supplies, etc., where, from the conditions, gravity filters are
impossible. The practical objections to this system have been found
to be so great that it is rarely used under other conditions. Some
experiments were made at Louisville with a filter of this type, but
they were not long continued, and aside from them there is no precise
information as to what can be accomplished with filters of this type.

CHAPTER XI.

OTHER METHODS OF FILTRATION.

WORMS TILE SYSTEM.

This system, invented and patented by Director Fischer of the Worms
water-works, consists of the filtration of water through artificial
hollow sandstone tiles, made by heating a mixture of broken glass
and sand, sifted to determined sizes, to a point just below the
melting-point of the glass, in suitable moulds or forms. The glass
softens and adheres to the sand, forming a strong porous substance
through which water can be passed. These tiles are made hollow and
are immersed in the water to be treated, the effluent being removed
from the centre of each tile. They are connected together in groups
corresponding in size to the units of a sand-filtration plant. They
are washed by a reverse current of filtered water. These tiles have
been used for some years at Worms, Germany, and at a number of smaller
places, and were investigated experimentally at Pittsburg. Some
difficulty has been experienced in getting tiles with pores small
enough to yield an effluent of the desired purity, and at the same time
large enough to allow a reasonable quantity of water to pass. In fact,
with other than quite clear waters, it has not been found feasible to
accomplish both objects at the same time, and it has been necessary
to treat the water with coagulants and preliminary sedimentation or
filtration before applying it to the tiles. The problem of making the
joints between the tiles and the collection-pipes water-tight when
surrounded by the raw water also is a matter of some difficulty.

THE USE OF ASBESTOS.

It has been suggested by Mr. P. A. Maignen that the surface of sand
filters should be covered with a thin layer of asbestos, applied in
the form of a pulp, with the first water put onto the filter after
scraping. The asbestos forms a sort of a paper on the sand which
intercepts the sediment of the passing water. The advantage of the
process is in the cleaning. When dried to the right consistency this
asbestos can be rolled up like a carpet, and taken from the filter
without removing any of the sand.

This procedure is almost identical with that which has occurred
naturally in iron-removal plants, where algæ grow in the water upon the
filters, and form a fibrous substance with the ferric oxide removed
from the water, which can be rolled up and removed in the same way
as the asbestos. The advantages of the process, from an economical
standpoint, are less clear.

FILTERS USING HIGH RATES OF FILTRATION WITHOUT COAGULANTS.

Numerous filters have been suggested, and a few have been constructed
for the use of much higher rates of filtration than are usually
employed with sand filters, but without the use of coagulants. The
results obtained depend upon the requirements and upon the character
of the raw water. If a reservoir water contains an algæ growth, it can
often be removed by a coarse and rapid filter. The organisms in this
case are many times larger than the bacteria, and many times larger
than the clay particles which constitute turbidity. The requirements in
this case are rather in the nature of straining than of filtration.

The conditions necessary for the removal of bacteria and turbidity are
very well understood, and it can be stated with the utmost confidence
that no system of filtration through sand at rates many times as
high as are used in ordinary sand filtration, and without the use of
coagulants, will be satisfactory where either bacterial efficiency
or clarification is required. The application of such systems of
filtration would therefore seem to be somewhat limited.

[Illustration: REMOVING DIRTY ASBESTOS COVERING FROM AN EXPERIMENTAL
FILTER. MAIGNEN SYSTEM.

[_To face page 182._]
]

HOUSEHOLD FILTERS.

The subject of household filters is a somewhat broad one, as the
variety in these filters is even greater than in the larger filters,
and the range in the results to be expected from them is at least as
great. I shall only attempt to indicate here some of the leading points
in regard to them.

Household filters may be used to remove mud or iron rust from the tap
water, or to remove the bacteria in case the latter is sewage-polluted,
or to do both at once. Perhaps oftener they are used simply because
it is believed to be the proper thing, and without any clear
conception either of the desired result or the way in which it can
be accomplished. I shall consider them only in their relations to
the removal of bacteria, as I credit the people who employ them with
being sufficiently good judges of their efficiency in removing visible
sediment.

In the first place, as a general rule, which has very few if any
exceptions, we may say that all small filters which allow a good stream
of water to pass do not remove the bacteria. The reason for this is
simply that a material open enough to allow water to pass through it
rapidly is not fine enough to stop such small bodies as the bacteria.
The filters which are so often sold as “germ-proof,” consisting of
sand, animal charcoal, wire-cloth, filter-paper, etc., do not afford
protection against any unhealthy qualities which there may be in the
raw water. Animal charcoal removes color without retaining the far more
objectionable bacteria.

The other household filters have filtering materials of much finer
grain, unglazed porcelain and natural sandstone being the most
prominent materials, while infusorial earth is also used. The smaller
sizes of these filters allow water to pass only drop by drop, and when
a fair stream passes them the filters have considerable filtering
area (as a series of filter-tubes connected together). On account of
their slow action, filters of this class are, as a rule, provided with
storage reservoirs so that filtered water to the capacity of the
reservoir can be drawn rapidly (provided the calls do not come too
often). Some of these filters are nearly germ-proof, and are comparable
in their efficiency to large sand-filters. There is no sharp line
between the filters which stop and which do not stop the bacteria; but
in general the rule that a filter which works rapidly in proportion to
its size does not do so, and _vice versa_, will be found correct.

In thinking of the efficiency of household filters we must distinguish
between the filter carefully prepared for an award at an exhibition
and the filter of the same kind doing its average daily work in the
kitchen. If we could be sure in the latter case that an unbroken layer
of fine sandstone or porcelain was always between ourselves and the raw
tap-water we could feel comparatively safe. The manufacturers of the
filters claim that leaky joints, cracked tubes, etc., are impossible;
but I would urge upon the people using water filtered in this way that
they personally assure themselves that this is actually the case with
their own filters, for in case any such accident should happen the
consequences might be most unpleasant. The increased yield of a filter
due to a leaky joint is sure not to decrease it in favor with the cook,
who is probably quite out of patience with it because it works so
slowly, that is, in case it is good for anything.

The operation of household filters is necessarily, with rare
exceptions, left to the kitchen-girl and luck. Scientific supervision
is practically impossible. With a large filter, on the other hand,
concentrating all the filters for the city at a single point, a
competent man can be employed to run them in the best-known way; and
if desired, and as is actually done in very many places, an entirely
independent bacteriologist can be employed to determine the efficiency
of filtration. With the methods of examination now available, and
a little care in selecting the times and places of collecting the
samples, it is quite impossible for a filter-superintendent to
deliver a poor effluent very often or for any considerable length of
time without being caught. The safety of properly-conducted central
filtration is thus infinitely greater than that from even the best
household filters. Further, it may be doubted whether an infected water
can be sent into every house in the city to be used for washing and all
the purposes to which water is put except drinking, without causing
disease, although less than it would if it were also used for drinking.

The use of household filters must be regarded as a somewhat desperate
method of avoiding some of the bad consequences of a polluted
water-supply, and they are adopted for the most part by citizens who
in some measure realize the dangers from bad water, but who cannot
persuade their fellow-citizens to a more thorough and adequate solution
of the problem. Such citizens, by the use of the best filters, and by
carefully watching their action, or by having their drinking-water
boiled, can avoid the principal dangers from bad water, but their
vigilance does not protect their more careless neighbors.

CHAPTER XII.

REMOVAL OF IRON FROM GROUND-WATERS.

The filtration of ground-waters is a comparatively recent development.
Ground-waters are filtered by their passage through soil generally
much more perfectly than it is possible to filter other waters, and
any further filtration of them is useless. Such waters, however,
occasionally contain iron in solution as ferrous carbonate.

Waters containing iron have been used as mineral waters for a very long
time. Such waters have an astringent taste, and have been esteemed
for some purposes. As ordinary water-supplies, however, they are
objectionable. The iron deposits in the pipes when the current is
slow, and is flushed out when it is rapid, and makes the water turbid
and disagreeable; and still worse, the iron often gets through the
pipe-system in solution, and deposits in the wash-tub, coloring the
linen a rusty brown and quite spoiling it.

An organism called crenothrix grows in pipes carrying waters containing
iron, and after a while this organism dies, and decomposes, and
gives rise to very disagreeable tastes and odors. It thus happens
that ground-waters containing iron are unsatisfactory as public
water-supplies, and are sources of serious complaint.

AMOUNT OF IRON REQUIRED TO RENDER WATER OBJECTIONABLE.

Three hundredths of a part in 100,000 of metallic iron very rarely
precipitate or cause any trouble. Five hundredths occasionally
precipitate, and this amount may be taken as about the allowable
limit of iron in a satisfactory water. One tenth of a part is quite
sure to precipitate and give rise to serious complaint. Two or three
tenths make the water entirely unsuitable for laundry purposes, and
are otherwise seriously objectionable, and will hardly be tolerated
by a community. Under some conditions ground-waters carry as much
as 1 part in 100,000 of iron, and such waters are hardly usable. In
iron-removal plants an effluent containing less than 0.05 is regarded
as satisfactory. One containing less than 0.02, as is the case with
many plants, is all that can be desired. The percentage of removal is
of no significance, but only the amount left in the effluent.

CAUSE OF IRON IN GROUND-WATERS.

Natural sands, gravels, and rocks almost always contain iron, often in
considerable amount. The iron is usually combined with oxygen as ferric
oxide, and in this condition it is insoluble in water. Water passing
through iron containing materials will not ordinarily take up iron.
When, however, the water contains a large amount of organic matter in
solution, this organic matter takes part of the oxygen away from the
iron, and reduces the ferric oxide to ferrous oxide. The ferrous oxide
combines with carbonic acid, always present under these conditions,
forming ferrous carbonate, which is soluble and which goes into
solution.

Surface-waters nearly always carry free oxygen, and when such waters
enter the ground they carry oxygen with them, and the organic matters
in the water use up the free oxygen before they commence to take oxygen
away from the iron of the ground. It is thus only in the presence of
organic matters, and in the absence of free oxygen, that the solution
of iron is possible. It sometimes happens that the organic matters
which reduce the iron are contained in the soil itself, in which
case iron may be taken up even by water originally very pure, as for
instance, by rain-water.

Generally speaking, iron is everywhere present in sufficient quantity
in the strata from which ground-waters are obtained, and wherever the
conditions of the organic matters and oxygen necessary for solution
occur, iron-containing waters are secured, and the iron is usually
present in the earth in such quantity that the water can dissolve as
much as it will take up for a long series of years, or for centuries,
without exhausting the supply. There is thus little prospect of
improvement of such waters from exhaustion of the supply of iron.

The circumstances which control the solution of iron are very
complicated and difficult to determine. Wells near a river, and drawing
their water largely from it by seepage, are apt to yield a water
containing iron sooner or later, especially where the river-water
carries a large amount of organic matter in solution. Waters drawn from
extensive gravel deposits, in which the water is renewed principally by
the rainfall upon the surface of the deposits themselves, often remain
entirely free from iron indefinitely. The rain-water is almost free
from organic matter, and the air is able to take care of decomposing
organic matters in the surface soil, and below this there are no
accumulations of organic matter sufficient to cause the solution of
iron. Under other conditions there are subterranean sources of organic
matter which result in the solution of iron under conditions which, on
the surface, appear most favorable for securing good water. Wells are
often used for many years without developing iron, when suddenly iron
will appear. This appearance of iron is often connected with increasing
consumption of water. In some cases it may result from drawing water
from areas not previously drawn upon.

When iron once makes its appearance in a water, it seldom disappears
completely afterward, although it often fluctuates widely at different
seasons of the year and under different conditions of pumping. In some
cases a decrease in the quantity of iron is noted after a number of
years, but in other cases this does not happen.

In a few cases manganese has been found in ground-waters. Manganese in
water behaves much like iron, but there are some points of difference,
so that the possibility of the presence of this substance should be
borne in mind.

Iron-containing waters are generally entirely free from oxygen, and
when first drawn from the ground they are bright and clear and do not
differ in appearance from other ground-waters. On exposure to the air
they quickly become turbid from the oxidation of the iron, and its
precipitation as ferric hydrate. At West Superior, Wisconsin, a water
was found containing both iron and dissolved oxygen. It was turbid
as pumped from the well. This condition of affairs seemed abnormal,
but was repeatedly checked, and the theory was advanced by Mr. R. S.
Weston, who made the observations, that it resulted from a mixture in
the wells of two entirely different waters, namely, a water resulting
from the rainfall on sand deposits back of the wells, containing
dissolved oxygen and no iron, and water from the lake which had seeped
through the sand, and which contained a considerable amount of iron
in solution but no dissolved oxygen. The wells thus drew water from
opposite directions, and the two waters were entirely different in
character, and the mixture thus had a composition which would not have
been possible in a water all of which came from a single source.

TREATMENT OF IRON-CONTAINING WATERS.

The removal of iron from ground-water is ordinarily a very simple
procedure. It is simply necessary to aerate the water, by which process
the ferrous carbonate is decomposed, and oxidized with the formation
of ferric hydrate, which forms a flocculent precipitate and is readily
removed by filtration. The aeration required varies in different cases.
The quantity of oxygen required to oxidize the iron is only a small
fraction of the amount which water will dissolve, and allowing water to
simply fall through the air for a few feet in fine streams will usually
supply several times as much oxygen as is necessary for this purpose.

Aerating devices of this kind have proved sufficient in a number of
cases, as at Far Rockaway, L. I., and at Red Bank, N. J. In some cases,
however, a further aeration is necessary, not for the purpose of
getting more oxygen into the water, but to get the excess of carbonic
acid out of it. Carbonic acid seems to retard in some way the oxidation
of the iron, and it is occasionally present in ground-waters in
considerable quantity, and quite seriously interferes with the process.
It can be removed sufficiently by aeration, but the necessary amount of
exposure to air is much greater than that required to simply introduce
oxygen.

Coke-towers have sometimes been used for this purpose. The towers are
filled with coarse coke and have open sides, and water is sprinkled
over the tops of them and allowed to drip through to the bottoms. In
general the simple exposure of water to the air for a sufficient length
of time, in any form of apparatus or simply in open channels, will
accomplish the desired results.

Mr. H. W. Clark[45] has called attention to the fact that in some cases
coke seems to have a direct chemical action upon the water which is
entirely independent of its aerating effect. In his experiments there
seemed to be some property in the coke which caused the iron to oxidize
and flocculate in many cases when it refused to do so with simple
aeration and filtration.

When the right conditions are reached the oxidation of the iron is
very rapid, and it separates out in flakes of such size that they can
be removed by filtration at almost any practicable rate. Mechanical
filters have been used for this purpose, with rates of filtration
of 100 million gallons per acre daily. In Germany, where plants for
the removal of iron are quite common, modified forms of sand filters
have usually been employed which have been operated at rates up to 25
million gallons per acre daily.

In experiments made by the Massachusetts State Board of Health rates
from 10 to 25 million gallons per acre daily have been employed.

The sand used for filtration may appropriately be somewhat coarser than
would be used for treating surface-waters, and the thickness of the
sand layer may be reduced. Owing to the higher

rates the underdrainage system must be more ample than is otherwise
necessary.

The rate of filtration employed is usually not a matter of vital
importance, but by selecting a rate that is not too high it is possible
to use a moderate loss of head. It is thus not necessary to clean the
filters too often, and the expenses of operation are not as high as
with an extreme rate. In some cases it is desired to accomplish other
results than the removal of iron by filtration, and this may lead to
the selection of a rate lower than would otherwise be used.

Under normal conditions of operation all of the iron separates on the
top of the sand. No appreciable amount of it penetrates the sand at
all. With open filters at Far Rockaway and at Red Bank there is an algæ
growth in the water upon the filters which, with the iron, forms a
mat upon the surface of the filter; and when the filter is put out of
service and allowed to partially dry, this mat can be rolled up like a
carpet and thrown off without removing any sand, and the filters have
been in use for several years without renewing any sand and without any
important decrease in the thickness of the sand layer.

Some waters contain iron in such a form that it cannot be successfully
removed in this manner. Thus at Reading, Mass., it was reported by
Dr. Thomas M. Drown that the iron was present in the form of ferrous
sulphate instead of ferrous carbonate, and that it was not capable of
being separated by simple aeration and filtration. A Warren mechanical
filter was installed, and the water is treated by aeration and with
the addition of lime and alum. The cost of the process is thereby much
increased, and the hardness of the water is increased threefold.

Several other cases have been reported where it was believed that
simple aeration and filtration were inadequate; but the advantages of
the simple procedure are so great as to make it worth a very careful
study to determine if more complete aeration, or the use of coke-towers
and perhaps slower filtration, would not serve in these cases without
resorting to the use of chemicals and their attendant disadvantages.

IRON-REMOVAL PLANTS IN OPERATION.

Iron-removal plants are now in use at Amsterdam and The Hague in
Holland, at Copenhagen in Denmark, at Kiel, Charlottenburg, Leipzig,
Halle, and many other places in Germany; at Reading, Mass.; Far
Rockaway, L. I.; Red Bank, Asbury Park, Atlantic Highlands, and
Keyport, N. J.

Among the earliest plants for the removal of iron were the filters
constructed at Amsterdam and The Hague in Holland. At Amsterdam the
water is derived from open canals in the dunes draining a large area.
The water has its origin in the rain-water falling upon the sand. The
sand is very fine and contains organic matter in sufficient amount
so that the ground-water is impregnated with iron. In flowing to a
central point in the open canals the water becomes aerated and the iron
oxidized. There are also algæ growths in the water which perhaps aid
the process. Sand filters of ordinary construction are used, and remove
both the iron and the algæ, and the rate of filtration is not higher
than is usually used in the treatment of river-waters, although it
could probably be largely increased without detriment to the supply.

The works at The Hague are very similar to those at Amsterdam, but
covered collectors are used to supplement the open canals. Both
of these plants were built before much was known about iron in
ground-waters and the means for its removal, but they have performed
their work with uniformly satisfactory results. In the more recent
German works various aerating devices are employed, and filters similar
in general construction to ordinary sand filters, but with larger
connections suited to very high rates of filtration, are employed.

The plant at Asbury Park was the first of importance constructed in
America. The water is raised from wells from 400 to 1100 feet deep
by compressed air by a Pohle lift. It is delivered into a square
masonry receiving-basin holding some hours’ supply. The aeration of
the water by this means is very complete. It is afterwards pumped
through Continental pressure filters direct into the service-pipes. The
reservoir for the aerated water was not a part of the original plant,
but was added afterwards to facilitate operation, and to give more
complete aeration before filtration.

At Far Rockaway, L. I., the water is lifted from wells by a Worthington
Pump, and is discharged over the bell of a vertical 16-inch pipe,
from which it falls through the air to the water in a receiving
chamber around it. The simple fall through the air aerates the water
sufficiently. From the receiving-chamber the water is taken to either
or both of two filters, each with an area of 20,000 square feet. These
filters are open, with brick walls and concrete bottoms, three feet
of sand and one foot of gravel, and the underdrains are of the usual
type. The water flows through regulator-chambers to a well 25 feet in
diameter and 12 feet deep, from which it is pumped to a stand-pipe in
the town. The plant was built to treat easily three million gallons
per day, and has occasionally treated a larger quantity. Either filter
yields the whole supply while the other is being cleaned. The rate of
filtration in this case was made lower than would have otherwise been
necessary, as there was an alternate supply, namely, the water from
two brooks, which could be used on occasions, and to purify which a
lower rate of filtration was regarded necessary, than would have been
required for the well-water. The removal of iron is complete.

[Illustration: FIG. 25.]

The plant of the Rumson Improvement Company at Red Bank, N. J., is
quite similar to that at Far Rockaway, but is much smaller. The outlet
is a 6-inch pipe perforated with 1/4-inch holes which throws the water
out in a pine-tree shape to the receiving-tank, thoroughly aerating
it. Each of the two filters has 770 square feet of area. The filtering
material is three feet of beach sand. From the regulator-chamber the
water flows to a circular well 18 feet in diameter, covered by a brick
dome and holding 17,000 gallons, from which it is pumped to the
stand-pipe. Either of the filters will treat ten thousand gallons of
water per hour, which is equal to the capacity of the pumps; and as
the consumption is considerably less than this figure, they are only
in use for a part of each day, the number of hours depending upon the
consumption. These filters are shown by the accompanying plan. The cost
of the work was as follows:

Filters and pure-water reservoir, with piping
and drains complete $3,799.47
New pump and connections 492.68
Engineering and superintendence 992.91
---------
Total cost of plant $5,285.06

The engineer who operates the pumps takes care of the filters, and no
additional labor has been required. The entire cost of operation is
thus represented by the additional coal required for the preliminary
lift from the wells to the filters. The effluent is always free from
iron.

The plant at Reading,[46] Mass., was installed by the Cumberland
Manufacturing Company, and combines aeration, treatment with lime and
sulphate of alumina and rapid filtration. The aeration is effected
by pumping air through the water, after the water has received the
lime. It afterwards receives sulphate of alumina and passes to a
settling-tank holding 40,000 gallons, in which the water remains for
about an hour. There are six filters of the Warren type, each with an
effective filtering area of 54 square feet.

The cost of coagulant is considerable. The chief disadvantage of the
process is that it hardens the water, which is naturally soft. From
the completion of the plant in July, 1896, to the end of the year the
hardness of the water was increased, according to analyses of the State
Board of Health, from 4.1 to 11.3 parts in 100,000, and for the year
1897 the increase was from 4.0 to 12.7. The iron, which is present in
the raw water to the extent of about 0.26 part in 100,000, is removed
sufficiently at all times.

Prior to the erection of this plant Mr. Desmond FitzGerald advised
aeration followed by sedimentation in two reservoirs holding half a
million gallons each, and by rapid filtration. Mr. Bancroft states that
in his opinion, if the reservoir recommended by Mr. FitzGerald had been
built, the filters could be run with very little or no coagulation, and
consequently without increase in hardness, which is the most obvious
disadvantage to the procedure. The nominal capacity of the plant is
one million gallons, and the average consumption about 200,000 gallons
daily.

The plant at Keyport, N. J., is similar, but smaller.

CHAPTER XIII.

TREATMENT OF WATERS.

Having now reviewed the most important methods in use for the treatment
of waters, we may take a general view of their application to various
classes of waters. Different raw waters vary so much, and the
requirements of filtration are so different, that it is not possible
to outline any general procedure or combination of procedures, but
each problem must be taken up by itself. Nevertheless, some general
suggestions may be of service.

In the first place, we may consider the case of waters containing very
large quantities of oxidizable organic matter. Such waters are obtained
from some reservoirs containing very active vegetable and animal
growths, or from rivers receiving large amounts of sewage. Waters
of both of these classes are, if possible, to be avoided for public
water-supplies. When circumstances require their use, they can best be
treated by intermittent filtration, this process being best adapted to
the destruction by oxygen of excessive quantities of organic matter.

Where the pollution is less, so that the dissolved oxygen contained
in the raw water is sufficient for the oxidation of the organic
matters, continuous filtration will give substantially as good results
as intermittent filtration, and in other respects it has important
advantages. The application of intermittent filtration for the
treatment of public water-supplies is thus somewhat limited, and, as a
matter of fact, it has been used in only a few cases.

For the treatment of very highly polluted waters double filtration has
been used in a number of cases, notably by the Grand Junction Company
at London, at Schiedam in Holland, and at Bremen and Altona in Germany.
At the two first-mentioned places two separate systems of filters are
provided differing somewhat in construction, the first filters being at
a higher level than the after filters. The first filters supply water
of comparative purity, and very constant composition, to the after
filters, which are able to treat it with great efficiency and at very
low operating cost.

This procedure is probably the most perfect which has been used for the
removal of disease-producing qualities from highly polluted waters; and
the cost of the process may not be as much greater than that of simple
filtration as would at first appear, because the cost of cleaning the
after filters is merely nominal, and the attendance, pumping, etc.,
are practically common to both sets of filters, and are not materially
greater than they would be for a single set.

For very bad waters the first filters might appropriately be
intermittent, while the after filters should be continuous. This was
the procedure originally intended for Lawrence, but the intermittent
filter first constructed yielded such very good results that it has not
been considered necessary to complete the plant as originally projected.

At Bremen and at Altona a different procedure has been adopted. The
filters are all upon the same level, and of the same construction.
When a filter is put in service the effluent from it, instead of being
taken to the pure-water reservoir, is taken to another filter which
has already been some time in service. After the first filter has been
in operation for some time its effluent is taken to the pure-water
reservoir, and in turn it is supplied with the effluent from a filter
more recently cleaned. The loss of head of water passing a freshly
cleaned filter is comparatively slight, and the water of the second
filter is allowed to fall a few inches below the high-water mark,
at which level it will take the effluent from the other filter. The
connections between the filters are made by siphons of large pipe, the
summits of which are considerably above the high-water line. These
siphons are filled by exhausting the air, and when opened to the air
there is no possibility of a flow of water through them. The process
has given extremely good results in practice, yielding effluents of the
very greatest purity and at a quite moderate cost of operation.

An objection to the method is the possible filling of a siphon some
time when the water standing upon the after-filter is higher than that
in the pure-water well of the fore-filter, and while the fore-filter is
connected with the pure-water reservoir. Such a connection would send
unfiltered water into the pure-water reservoir direct. I do not know
that any trouble of this kind has ever been experienced at Bremen or at
Altona; and the objection to this system is perhaps not well founded
where the management is careful and conscientious. The fact that an
unscrupulous attendant can make the connection at any time to help out
a deficiency of supply, or simply through carelessness, is certainly
objectionable.

For the treatment of river-waters and lake-waters containing only
a small quantity of sediment, and where the removal of bacteria or
disease-producing qualities is the most important object of filtration,
sand filters can be used. Where the rivers are subject to floods and
moderate amounts of muddy water, sedimentation-basins or storage
reservoirs for raw water will often be found advantageous.

For the treatment of extremely muddy waters, and waters which are
continuously muddy for long periods of time, and for the removal
of color from very highly colored waters, resource must be had to
coagulants. The coagulants which are necessary in each special case and
which can be used without injury to the water must be determined by
most careful investigation of the raw water.

For the filtration of these waters after coagulation either sand or
mechanical filters can be employed. As the principal work in this
case is done by the coagulant, the kind of filtration employed is
of less consequence than where filtration alone is relied upon,
and the cheapest form of filter will naturally be employed. Under
present conditions mechanical filters will usually be cheaper than
sand filters for use in this way; but where waters, in addition to the
mud, carry bacteria in such large numbers as to make high bacterial
efficiency a matter of importance, sand filters may be selected, as the
bacterial efficiency obtained with them is not dependent upon the use
of coagulant; and is therefore less subject to interruptions from the
failure to apply coagulant in the right proportion.

Mechanical filters have also been used for the treatment of
comparatively clear waters where bacterial efficiency was the principal
object of filtration. For this purpose the efficiencies obtained with
them are usually inferior to those obtained with sand filters, while
the cost of coagulants is so great as to make their use often more
expensive than that of sand filters.

In the case of many streams which are comparatively clear for a part of
the year, but occasionally are quite turbid, the use of sand filters
has this advantage, that the use of coagulants can be stopped and the
cost of operation reduced whenever the water is clear enough to allow
of satisfactory treatment by them; and that coagulant can be employed
on those days when otherwise insufficient clarification would be
obtained.

In this case the high bacterial efficiency is secured at all times,
while the cost of coagulant is saved during the greater part of the
time. In such cases, also, the preliminary process of sedimentation and
storage should be developed as far as possible.

The application of other processes of filtration to special problems
are not sufficiently well understood to allow general discussion, and
must be taken up separately with reference to the requirements of each
special situation.

COST OF FILTRATION.

The cost of filtration of water depends upon the character of the raw
water, upon the nature of the plant employed, upon its size, and
upon the skill and economy of manipulation. These conditions affect
the cost to such an extent as to make any accurate general estimate
quite impossible. Nevertheless a little consideration of the subject,
although not leading to exact results, may be helpful as furnishing a
rough idea of the probable cost before estimates for local conditions
are made.

Open sand filters, with masonry walls, with reasonably favorable
conditions of construction, and not too small in area, have averaged
to cost in the United States within the last few years perhaps about
thirty thousand dollars per acre. The relative cost of small plants is
somewhat greater, and with embankments instead of masonry walls, the
cost is somewhat reduced. The cost is less where natural deposits of
sand can be made use of practically in their original condition, and is
increased where the filtering materials have to be transported by rail
for long distances, or where the sites are difficult to build upon.
Covered filters cost about a half more than open filters. Mechanical
filters at current prices cost about $20 per square foot of filtering
area, to which must be added the cost of foundations and buildings,
which perhaps average to cost half as much more, but are dependent upon
local conditions and the character of the buildings.

To these figures must be added the costs of pumps, reservoirs,
sedimentation-basins, and pipe-connections, which are often greater
than the costs of the filters, but which differ so widely in different
cases as to make any general estimate impossible.

Filters must be provided sufficient to meet the maximum and not the
average consumption. The excess of maximum over average requirements
varies greatly in different cities, and depends largely upon reservoir
capacities and arrangements.

As a result of a considerable number of estimates made by the author
for average American conditions, the cost of installing filters may
be taken very roughly as five dollars per inhabitant, but the amounts
differ widely in various cases.

The cost of operation of sand filters in England probably averages
about one dollar per million gallons of water filtered. The following
table shows the costs of operation of the filters of the seven London
companies for fifteen years, compiled in the office of Mr. W. B. Bryan,
Chief Engineer of the East London Water Company. The results have been
computed to dollars per million U. S. gallons, and include the cost of
all labor, sand, and supplies for the filters, but do not include any
pumping or interest costs.

COST OF FILTRATION, LONDON WATER COMPANIES.

(Computed from data furnished Wm. B. Bryan, C.E., East London Water
Works.)

Dollars per Million U. S. Gallons.

--------+-------+------+--------+-------+-----+---------+---------+--------
| | East | Grand | | New |Southwark| West |
|Chelsea|London|Junction|Lambeth|River| & |Middlesex|Average.
| Co. | Co. | Co. | Co. | Co. |Vauxhall | Co. |
| | | | | | Co. | |
--------+-------+------+--------+-------+-----+---------+---------+--------
1880-1 | 1.16 | 1.16 | 1.00 | 0.83 |1.34 | 1.16 | 1.67 | 1.19
1881-2 | 1.19 | 1.39 | 0.95 | 0.82 |1.15 | 1.37 | 1.54 | 1.20
1882-3 | 1.10 | 1.23 | 1.39 | 0.96 |1.40 | 1.47 | 1.74 | 1.33
1883-4 | 1.00 | 1.06 | 1.73 | 0.92 |1.11 | 1.62 | 1.67 | 1.30
1884-5 | 1.06 | 1.06 | 1.82 | 0.90 |1.02 | 1.40 | 1.30 | 1.22
1885-6 | 1.15 | 1.16 | 1.35 | 0.90 |1.00 | 1.15 | 1.07 | 1.11
1886-7 | 0.80 | 0.96 | 1.39 | 0.87 |0.98 | 1.43 | 1.70 | 1.16
1887-8 | 1.07 | 1.22 | 1.74 | 0.90 |0.92 | 1.28 | 1.00 | 1.16
1888-9 | 0.83 | 1.28 | 1.55 | 0.95 |0.98 | 1.52 | 0.83 | 1.13
1889-90 | 0.66 | 1.50 | 1.22 | 0.88 |0.90 | 1.70 | 3.56 | 1.49
1890-1 | 0.72 | 1.42 | 1.32 | 0.85 |1.02 | 1.16 | 1.00 | 1.07
1891-2 | 0.75 | 1.54 | 1.23 | 1.00 |0.92 | 1.15 | 0.96 | 1.08
1892-3 | 0.67 | 1.42 | 1.30 | 1.19 |1.16 | 1.26 | 1.42 | 1.20
1893-4 | 1.15 | 2.63 | 2.00 | 1.46 |1.43 | 1.52 | 0.95 | 1.59
1894-5 | 0.60 | 1.68 | 1.67 | 2.53 |1.03 | 1.34 | 0.96 | 1.40
--------+-------+------+--------+-------+-----+---------+---------+--------
Average | 0.93 | 1.38 | 1.44 | 1.06 |1.09 | 1.37 | 1.43 | 1.24
--------+-------+------+--------+-------+-----+---------+---------+--------

Average of seven companies for 15 years, $1.24 per million gallons.

Variations from year to year are caused by differences in the amounts
of ice, and in the quantities of new sand purchased. Wages average
about $1.00 per day. At Liverpool for 1896 the cost was $1.08 per
million U. S. gallons.

In Germany, with more turbid river-waters, the costs of operation are
somewhat higher than the London figures, while at Zürich, where the
water is very clear, they are lower.

In the United States the data regarding the cost of operation of sand
filters are less complete. At Mt. Vernon, N. Y., with reservoir-water,
the cost has averaged about two dollars per million gallons. At
Poughkeepsie, N. Y., with the Hudson River water, which is occasionally
moderately turbid, the cost for twenty years has averaged three dollars
per million gallons. This cost includes the cost of handling ice, and
as the average winter temperature is considerably below that suggested
for open filters, the expense of this work has been considerable, and
has increased considerably the total cost of operation.

At Far Rockaway, L. I., and Red Bank N. J., for iron-removal plants,
the cost of operation has hardly been appreciable. The plants are both
close to the pumping-stations, and it has been possible to operate
them with the labor necessarily engaged at the pumping-station without
additional cost, except a very small amount of labor on the sand at Far
Rockaway. No computation has been made in these cases of the additional
coal required for pumping.

At Lawrence, Mass., the cost of operation for 1895 was as follows:

Cost of scraping and replacing sand $3,467
Cost of care of ice 2,903
------
Total cost of operation $6,370
Water filtered, millions of gallons 1,097
Cost per million gallons $5.80

The cost of care of ice has been excessive at Lawrence, and it has
been repeatedly recommended to cover the filter to avoid this expense.
The cost of handling sand has been very greatly increased, because the
filter is built in one bed, and all work upon it has to be done during
the comparatively short intervals when the filter is not in use, an
arrangement which is not at all economical in the use of labor. The
cost of operation is thus much higher than it would be had the plant
been constructed in several units, each of which could be disconnected
for the purpose of being cleaned in the ordinary manner. As against
this the first cost of construction was extremely low, and the saving
in interest charges should be credited against the increased cost of
labor in cleaning.

The cost of operating filters at Ashland, Wis., has been estimated by
Mr. William Wheeler at $2.26 per million gallons. This estimate is
based upon the performance for the first year that they were in service.

In the operation of mechanical filters one of the largest items of
expense is for the coagulant, and the amount of this depends entirely
upon the character of the raw water and the thoroughness of the
treatment required. The data regarding the other or general costs of
operation of mechanical filters are few and unsatisfactory.

I recently made some estimates of cost of clarifying waters of various
degrees of turbidity by sand and mechanical filters. These estimates
were made for a special set of conditions, and I do not know that
they will fit others, but they have at least a suggestive value. The
results shown by Fig. 26 include only the cost of operation, and not
interest and depreciation charges. These figures, when used for plants
in connection with which preliminary treatments are used, should be
applied to the turbidity of the water as applied to the filters, and
not to the raw water, and the costs of the preliminary processes should
be added.

With sand filters the frequency of scraping is nearly proportional
to the turbidity; and as scraping represents most of the expenses,
the costs of operation are proportional to the turbidity, except
the general costs, and the cost of the amount of scraping, which is
necessary with even the clearest waters.

With mechanical filters the amount of sulphate of alumina required for
clarification increases with the turbidity, and most of the costs of
operation increase in the same ratio. The diagram shows the amount of
sulphate of alumina in grains per gallon necessary for clarification
with different degrees of turbidity.

With the clearest waters the costs of operation on the two systems are
substantially equal. With muddy waters, the expense of operating sand
filters increases more rapidly than the expense of operating mechanical
filters.

[Illustration: TURBIDITY
FIG. 26.—COST OF OPERATION OF FILTERS.]

There is another element which often comes into the comparison, namely,
the question of purification from the effects of sewage-pollution.
Nearly all rivers used for public water-supplies receive more or less
sewage, and in filtering such waters it is regarded as necessary to
remove as completely as possible the bacteria.

The quantities of sulphate of alumina required for the clarification
of the least turbid waters are not sufficient to give even tolerably
good bacterial efficiencies. To secure a reasonably complete removal of
bacteria with mechanical filters, the use of a considerable quantity
of sulphate of alumina is required. Let us assume that 98 per cent
bacterial efficiency is required, and that to produce this efficiency
it is necessary to use one grain of coagulant to the gallon. With water
requiring less than this quantity of coagulant for clarification this
quantity must nevertheless be used, and the costs will be controlled
by it, and not by the lower quantities which would suffice for
clarification, but would not give the required bacterial efficiency.

I have added this line to the diagram, and this, combined with the
upper portion of the line showing cost of clarification, represents the
cost of treating waters with mechanical filters, where both bacterial
efficiency and clarification are required.

This line, considered as a whole, increases much less rapidly with
increasing turbidity than does the corresponding line for sand filters,
and the two lines cross each other. With the clearest waters sand
filters are cheaper than mechanical filters, and for the muddiest
waters they are more expensive. It does not appear from the diagram,
but it is also true in each case, that the cheaper system is also the
more efficient. Sand filters are more efficient in removing bacteria
from clear waters than are mechanical filters, and mechanical filters
are more efficient in clarifying very muddy waters than are sand
filters.

WHAT WATERS REQUIRE FILTRATION?

From the nature of the case a satisfactory general answer to this
question cannot be given, but a few suggestions may be useful.

In the first place, ground-waters obviously do not require filtration:
they have already in most cases been thoroughly filtered in the ground
through which they have passed, and in the exceptional cases, as, for
instance, an artesian well drawing water through fissures in a ledge
from a polluted origin, a new supply will generally be chosen rather
than to attempt to improve so doubtful a raw material.

River-waters should be filtered. It cannot be asserted that there
are no rivers in mountainous districts in which the water is at once
clear and free from pollution, and suitable in its natural state
for water-supply; but if so, they are not common, least of all in
the regions where water-supplies are usually required. The use of
river-waters in their natural state or after sedimentation only,
drawn from such rivers as the Merrimac, Hudson, Potomac, Delaware,
Schuylkill, Ohio, and Mississippi, is a filthy as well as an unhealthy
practice, which ought to be abandoned.

The question is more difficult in the case of supplies drawn from
lakes or storage reservoirs. Many such supplies are grossly polluted
and should be either abandoned or filtered. Others are subject to algæ
growths, or are muddy, and would be much improved by filtration. Still
others are drawn either from unpolluted water-sheds, or the pollution
is so greatly diluted and reduced by storage that no known disadvantage
results from their use.

In measuring the effects of the pollution of water-supplies, the
typhoid-fever death-rate is a most important aid. Not that typhoid
fever is the sole evil resulting from polluted water, but because it
is also a very useful index of other evils for which corresponding
statistics cannot be obtained, as, for instance, the causation of
diarrhœal diseases or the danger from invasion by cholera.

I think we shall not go far wrong at the start to confine our attention
to those cities where there are over 25 deaths from typhoid fever per
100,000 of population. This will at once throw out of consideration
a large number of relatively good supplies, including those of New
York and Brooklyn. It is not my idea that none of these supplies
cause disease. Many of them, as for instance that of New York, are
known to receive sewage, and it is an interesting question worthy of
most careful study whether there are cases of sickness resulting from
this pollution. The point that I wish to make now is simply that in
those cases the death-rate itself is evidence that, with existing
conditions of dilution and storage, the resulting damage of which we
have knowledge is not great enough to justify the expense involved by
filtration.

In this connection it should not be forgotten that, especially with
very small watersheds, there may be a danger as distinct from present
damage which requires consideration. Thus a single house or groups of
houses draining into a supply may not appreciably affect it for years,
until an outbreak of fever on the water-shed results in infecting the
water with the germs of disease and in an epidemic in the city below.
This danger decreases with increasing size of the water-shed and volume
of the water with which any such pollution would be mixed, and also
with the population draining into the water, as there is a probability
that the amount of infection continually added from a considerable town
will not be subject to as violent fluctuation as that from only a few
houses.

Thus in Plymouth, Pa., in 1885, there were 1104 cases of typhoid
fever and 114 deaths among a population of 8000, as the result of the
discharge of the dejecta from a single typhoid patient into the water
of a relatively small impounding reservoir. The cost of this epidemic
was calculated with unusual care. The care of the sick cost in cash
$67,100.17, and the loss of wages for those who recovered amounted to
$30,020.08. The 114 persons who died were earning before their sickness
at the rate of $18,419.52 annually.

Such an outbreak would hardly be possible with the Croton water-shed
of the New York water-supply, on account of the great dilution and
delay in the reservoirs, but it must be guarded against in small
supplies.

Of the cities having more than 25 deaths per 100,000 from typhoid
fever, some will no doubt be found where milk epidemics or other
special circumstances were the cause; but I believe in a majority
of them, and in nearly all cases where the rate is year after year
considerably above that figure, the cause will be found in the
water-supply. Investigation should be made of this point; and if the
water is not at fault, the responsibility should be located. If the
water is guilty, it should be either purified or a new supply obtained.

CHAPTER XIV.

WATER-SUPPLY AND DISEASE—CONCLUSIONS.

One of the most characteristic and uniform results of the direct
pollution of public water-supplies is the typhoid fever which results
among the users of the water. In the English and German cities with
almost uniformly good drinking-water, typhoid fever is already nearly
exterminated, and is decreasing from year to year. American cities
having unpolluted water-supplies have comparatively few deaths from
this cause, although the figures never go so low as in Europe, perhaps
on account of the fresh cases which are always coming in from less
healthy neighborhoods in ever-moving American communities. In other
American cities the death-rates from typhoid fever are many times what
they ought to be and what they actually are in other cities, and the
rates in various places, and in the same place at different times,
bear in general a close relation to the extent of the pollution of the
drinking-water. The power of suitable filtration to protect a city from
typhoid fever is amply shown by the very low death-rates from this
cause in London, Berlin, Breslau, and large numbers of other cities
drawing their raw water from sources more contaminated than those of
any but the very worst American supplies, and by the marked and great
reductions in the typhoid-fever death rates which have followed at once
the installation of filters at Zürich, Switzerland; Hamburg, Germany;
Lawrence, Mass., and other places.

The following is a list of the cities of 50,000 inhabitants and upward
in the United States, with deaths from typhoid fever and the sources
of their water-supplies. The deaths and populations are from the
U. S. Census for 1890; the sources of the water-supplies, from the
_American Water-Works Manual_ for the same year. Four cities of this
size—Grand Rapids, Lincoln, St. Joseph, and Des Moines—are not included
in the census returns of mortality. Two cities with less than 50,000
inhabitants with exceptionally high death-rates have been included, and
at the foot of the list are given corresponding data for some large
European cities for 1893.

TYPHOID FEVER DEATH-RATES AND WATER-SUPPLIES OF CITIES.

-----------------+-----------+--------------+-------------------------
| | Deaths from |
| | Typhoid |
| | Fever. |
City. |Population.+------+-------+ Water-supply.
| | | Per |
| |Total.|100,000|
| | |living.|
-----------------+-----------+------+-------+-------------------------
Birmingham | 26,178 | 69 | 264 |Five Mile Creek
1. Denver | 106,713 | 232 | 217 |North Platte River and
| | | | wells
2. Allegheny | 105,287 | 192 | 182 |Allegheny River
3. Camden | 58,313 | 77 | 132 |Delaware River
4. Pittsburg | 238,617 | 304 | 127 |Allegheny and Monongahela
| | | | rivers
Lawrence | 44,654 | 54 | 121 |Merrimac River
5. Newark | 181,830 | 181 | 100 |Passaic River
6. Charleston | 54,955 | 54 | 98 |Artesian wells yielding
| | | | 1,600,000 gallons daily
7. Washington | 230,392 | 200 | 87 |Potomac River
8. Lowell | 77,696 | 64 | 82 |Merrimac River
9. Jersey City | 163,003 | 134 | 82 |Passaic River
10. Louisville | 161,129 | 122 | 76 |Ohio River
11. Philadelphia | 1,046,964 | 770 | 74 |Delaware and Schuylkill
| | | | rivers
12. Chicago | 1,099,850 | 794 | 72 |Lake Michigan
13. Atlanta | 65,533 | 47 | 72 |South River
14. Albany | 94,923 | 67 | 71 |Hudson River
15. Wilmington | 61,431 | 43 | 70 |Brandywine Creek
16. St. Paul | 133,156 | 92 | 69 |Lakes
17. Troy | 60,956 | 42 | 69 |Hudson River and
| | | | impounding reservoirs
18. Los Angeles | 50,395 | 34 | 67 |Los Angeles River and
| | | | springs
19. Nashville | 76,168 | 49 | 64 |Cumberland River
20. Cleveland | 261,353 | 164 | 63 |Lake Erie
21. Richmond | 81,388 | 50 | 61 |James River
22. Hartford | 53,230 | 32 | 60 |Connecticut River and
| | | | impounding reservoir
23. Fall River | 74,398 | 44 | 59 |Watupa Lake
24. Minneapolis | 164,738 | 94 | 57 |Mississippi River
25. San Francisco| 298,997 | 166 | 56 |Lobus Creek, Lake Merced,
| | | | and mountain streams
26. Indianapolis | 105,436 | 57 | 54 |White River
27. Cincinnati | 296,908 | 151 | 51 |Ohio River
28. Memphis | 64,495 | 33 | 51 |Artesian Wells
29. Reading | 58,661 | 29 | 49 |Maiden Creek and Springs
30. Baltimore | 434,439 | 202 | 47 |Impounding reservoir
31. Omaha | 140,452 | 63 | 45 |Missouri River
32. Columbus | 88,150 | 38 | 43 |Surface-water and wells
33. Providence | 132,146 | 53 | 40 |Pawtuxet River
34. Kansas City | 132,716 | 53 | 40 |Missouri River
35. Rochester | 133,896 | 53 | 39 |Hemlock and Candice lakes
36. Evansville | 50,756 | 20 | 39 |Ohio River
37. Boston | 448,477 | 174 | 39 |Impounding reservoirs
38. Toledo | 81,434 | 29 | 36 |Maumee River
39. Cambridge | 70,028 | 24 | 34 |Impounding reservoir
40. St. Louis | 451,770 | 145 | 32 |Mississippi River
41. Scranton | 75,215 | 24 | 32 |Impounding reservoir
42. Buffalo | 255,664 | 80 | 31 |Niagara River
43. Milwaukee | 204,468 | 61 | 30 |Lake Michigan
44. New Haven | 81,298 | 22 | 27 |Impounding reservoir
45. Worcester | 84,655 | 22 | 26 |Impounding reservoir
46. Paterson | 78,347 | 20 | 26 |Passaic River
| | | | (higher up)
47. Dayton | 61,220 | 15 | 25 |Wells
48. Brooklyn | 806,343 | 194 | 24 |Wells, ponds, and
| | | | impounding reservoirs
49. New York | 1,515,301 | 348 | 23 |Impounding reservoir
50. Syracuse | 88,143 | 18 | 20 |Impounding reservoir
| | | | and springs
51. New Orleans | 242,039 | 45 | 19 |Mississippi River
52. Detroit | 205,876 | 40 | 19 |Detroit River
53. Lynn | 55,727 | 9 | 16 |Impounding reservoir
54. Trenton | 57,458 | 9 | 16 |Delaware River
| | | |
London | 4,306,411 | 719 | 17 |Filtered Thames and Lea
| | | | rivers and 1/4 from
| | | | wells
Glasgow | 667,883 | 138 | 20 |Loch Katrine
Paris | 2,424,705 | 609 | 25 |Spring water
Amsterdam | 437,892 | 69 | 16 |Filtered dune-water
Rotterdam | 222,233 | 12 | 5 |Filtered Maas River
Hague | 169,828 | 3 | 2 |Filtered dune-water
Berlin | 1,714,938 | 161 | 9 |Filtered Havel and Spree
| | | | rivers
Hamburg | 634,878 | 115 | 18 |Filtered Elbe River
Breslau | 353,551 | 37 | 11 |Filtered Oder River
Dresden | 308,930 | 14 | 5 |Ground-water
Vienna | 1,435,931 | 104 | 7 |Spring-water
-----------------+-----------+------+-------+-------------------------

Any full discussion of these data would require intimate acquaintances
with the various local conditions which it is impossible to take
up in detail here, but some of the leading facts cannot fail to be
instructive.

Each of the places having over 100 deaths per 100,000 from typhoid
fever used unfiltered river-water. Lower in the list, but still very
high, Charleston, said to have been supplied only from artesian wells,
had an excessive rate; but the reported water-consumption is so low as
to suggest that private wells or other means of supply were in common
use. Chicago and Cleveland both drew their water from lakes where they
were contaminated by their own sewage. St. Paul’s supply came from
ponds, of which I do not know the character. With these exceptions all
of the 22 cities with over 50,000 inhabitants, at the head of the list,
had unfiltered river-water.

The cities supplied from impounding reservoirs as a rule had lower
death rates and are at the lower end of the list, together with some
cities taking their water supplies from rivers or lakes at points where
they were subject to only smaller or more remote infection. Only three
of the American cities in the list were reported as being supplied
entirely with ground-water.

It is not my purpose to make too close comparisons between the various
cities on the list; some of them may have been influenced by unusual
local conditions in 1890. Others have in one way or another improved
their water-supplies since that date, and there are several cities in
which I know the present typhoid-fever death-rates to be materially
lower than those of 1890 given in the table. On the other hand, it is
equally true that a number of cities, including some of the larger
ones, have since had severe epidemics of typhoid fever which have given
very much higher rates than those for 1890.

These fluctuations would change the order of cities in the list from
year to year; they would not change the general facts, which are as
true to-day as they were in 1890. Nearly all of the great cities of the
United States are supplied with unfiltered surface-waters, and a great
majority of the waters are taken from rivers and lakes at points where
they are polluted by sewage. The death-rates from typhoid fever in
those cities, whether they are compared with better supplied cities of
this country, or with European cities, are enormously high.

Such rates were formerly common in European cities, but they have
disappeared with better sanitary conditions. The introduction of
filters has often worked marvellous changes in Europe, and in Lawrence
the improvement in the city’s health with filtered water was prompt
and unquestionable. There is every reason to believe that the general
introduction of better water in American cities will work corresponding
revolutions; and looking at it from a merely money standpoint, the
value of the lives and the saving of the expenses of sickness will pay
handsomely when compared with the cost of good water.

The reasons for believing that cholera is caused by polluted water
are entirely similar to those in the case of typhoid fever. It was no
accident that the epidemic of cholera which caused the death of 3400
persons followed the temporary supply of unfiltered water by the East
London Water Company in 1866, while the rest of London remained nearly
free, or that the only serious outbreak of cholera in Western Europe
in 1892 was at Hamburg, which was also the only city in Germany which
used raw river-water. This latter caused the sickness of 20,000 and the
death of over 8000 people within a month, and an amount of suffering
and financial loss, with the panics which resulted, that cannot be
estimated, but that exceeded many times the cost of the filters which
have since been put in operation. Hamburg had several times before
suffered severely from cholera, and the removal of this danger was a
leading, although not the sole, motive for the construction of filters.

How little cities supplied with pure water have to dread from
cholera is shown by the experience of Altona and other suburbs of
Hamburg with good water-supplies, which had but few cases of cholera
not directly brought from the latter place, and by the experience
of England, which maintained uninterrupted commercial intercourse
with the plague-stricken city, absolutely without quarantine, and,
notwithstanding a few cases which were directly imported, the disease
gained no foothold in England.

I do not know of a single modern European instance where a city with a
good water-supply not directly infected by sewage has suffered severely
from cholera. I shall leave to others more familiar with the facts the
discussion of what happened before the introduction of modern sanitary
methods, as well as of the present conditions in Asia; although I
believe that in these cases also there is plenty of evidence as to the
part water plays in the spread of the disease.

A considerable proportion of the water-supplies of the cities of the
United States are so polluted that in case cholera should gain a
foothold upon our shores we have no ground for hoping for the favorable
experience of the English cities rather than the plague of Hamburg in
1892.

The fæces from a man contain on an average perhaps 1,000,000,000
bacteria per gram,[47] most of them being the normal bacilli of
the intestines, _Bacillus coli communis_. Assuming that a man
discharges 200 grams or about 7 ounces of fæces daily, this would give
200,000,000,000 bacteria discharged daily per person. The number of
bacteria actually found in American sewage is usually higher, often
double this number per person; but there are other sources of bacteria
in sewage, and in addition growths or the reverse may take place in the
sewers, according to circumstances.

This number of bacteria in sewage is so enormously large that the
addition of the sewage from a village or city to even a large river
is capable of affecting its entire bacterial composition. Thus taking
the population of Lowell in 1892 at 85,000, and the average daily
flow of the Merrimac at 6000 cubic feet per second, and assuming that
200,000,000,000 bacteria are discharged daily in the sewage from each
person, they would increase the number in the river by 1160

per cubic centimeter, or about 300,000 in an ordinary glass of water.
The average number found in the water eight miles below, at the intake
of the Lawrence water-works, was more than six times as great as this,
due in part to the sewage of other cities higher up.

There is every reason to believe that the bulk of these bacteria were
harmless to the people of Lawrence, who drank them; but some of them
were not. Fæces of people suffering from typhoid fever contain the
germs of that disease. What proportion of the total number of bacteria
in such fæces are injurious is not known; but assuming that one fourth
only of the total number are typhoid germs, and supposing the fæces of
one man to be evenly mixed with the whole daily average flow of the
river, it would put one typhoid germ into every glass of water at the
Lawrence intake, and at low water several times as many proportionately
would be added. This gives some conception of the dilution required to
make a polluted water safe.

One often hears of the growth of disease-germs in water, but as far
as the northern United States and Europe are concerned there is no
evidence whatever that this ever takes place. There are harmless forms
of bacteria which are capable of growing upon less food than the
disease-germs require and they often multiply in badly-polluted waters.
Typhoid-fever germs live for a longer or shorter period, and finally
die without growth. The few laboratory experiments which have seemed
to show an increase of typhoid germs in water have been made under
conditions so widely different from those of natural watercourses that
they have no value.[48]

The proportionate number of cases of typhoid fever among the users
of a polluted water varies with the number of typhoid germs in the
water. Excessive pollution causes severe epidemics or continued high
death-rates according as the infection is continued or intermittent.
Slight infection causes relatively few cases of fever. Pittsburg and
Allegheny, taking their water-supplies from below the outlets of some
of their own sewers, have suffered severely (103.2 and 127.4 deaths
from typhoid fever annually per 100,000, respectively, from 1888 to
1892). Wheeling, W. Va., with similar conditions in 1890, was even
worse, a death rate of 345 per 100,000 from this cause being reported,
while Albany had only comparatively mild epidemics from the less
directly and grossly polluted Hudson. Lawrence and Lowell, taking their
water from the Merrimac, both had for many years continued excessive
rates, increasing gradually with increasing pollution; and the city
having the most polluted source had the higher rate.

In Berlin and Altona, in winter, with open filters, epidemics of
typhoid fever followed decreased efficiency of filtration, but the
epidemics were often so mild that they would have entirely escaped
observation under present American conditions. Chicago has for years
suffered from typhoid fever, and the rate has fluctuated, as far as
reliable information can be obtained, with the fluctuations in the
pollution of the lake water. An unusual discharge of the Chicago River
results in a higher death-rate. Abandoning the shore inlet near the
mouth of the Chicago River in 1892, resulted in the following year in
a reduction of 60 per cent in the typhoid fever death-rate.[49] This
reduction shows, not that the present intakes are safe, but simply that
they are less polluted than the old ones to an extent measured by the
reduction in the death-rate.

It is not supposed that in an epidemic of typhoid fever caused by
polluted water every single person contracts the disease directly by

drinking the water. On the contrary, typhoid fever is often
communicated in other ways. If we have in the first place a thousand
cases in a city caused directly by the water, they will be followed
by a large number of other cases resulting directly from the presence
in the city of the first thousand cases. The conditions favoring this
spread may vary in different wards, resulting in considerable local
variations in the death-rates. Some persons also will suffer who did
not drink any tap-water. These facts, always noted in epidemics, afford
no ground for refusing to believe, in the presence of direct evidence,
that the water was the cause of the fever. These additional cases are
the indirect if not the direct result of the water. The broad fact that
cities with polluted water-supplies as a rule have high typhoid-fever
death-rates, and cities with good water-supplies do not (except in the
occasional cases of milk epidemics, or where they are overrun by cases
contracted in neighboring cities with bad water, as is the case with
some of Chicago’s suburbs), is at once the best evidence of the damage
from bad water and measure of its extent.

The conditions which remove or destroy the sewage bacteria in a water
tend to make it safe. The most important of them are: (1) dilution; (2)
time, allowing the bacteria to die (sunlight may aid in this process,
although effective sunshine cannot reach the lower layer of turbid
waters or through ice); (3) sedimentation, allowing them to go to
the bottom, where they eventually die; and (4) natural or artificial
filtration. In rivers, distance is mainly useful in affording time,
and also, under some conditions, in allowing opportunities for
sedimentation. Thus a distance of 500 miles requires a week for water
travelling three miles an hour to pass, and will allow very important
changes to take place. The old theory that water purifies itself
in running a certain distance has no adequate foundation as far as
bacteria are concerned. Some purification takes place with the time
involved in the passage, but its extent has been greatly overestimated.

The time required for the bacteria to die simply from natural causes
is considerable; certainly not less than three or four weeks can
be depended upon with any confidence. In storage reservoirs this
action is often considerable, and it is for this reason that American
water-supplies from large storage reservoirs are, as a rule, much more
healthy than those drawn from rivers or polluted lakes, even when the
sources of the former are somewhat polluted. The water-supplies of New
York and Boston may be cited as examples. In many other water-works
operations the entire time from the pollution to the consumption of
the water is but a few days or even less, and time does not materially
improve water in this period.

Sedimentation removes bacteria only slowly, as might be expected from
their exceedingly small size; and in addition their specific gravity
probably is but slightly greater than that of water. The Lawrence
reservoir, holding from 10 to 14 days’ supply, effected, by the
combined effect of time and sedimentation, a reduction of 90 per cent
of the bacteria in the raw water. In spite of this the city suffered
severely and continuously from fever. It would probably have suffered
even more, however, had it not been for this reduction. Nothing is
known of the removal of bacteria by sedimentation from flowing rivers,
but, considering the slowness with which the process takes place in
standing water, it is evident that we cannot hope for very much in
streams, and especially rapid streams, where the opportunities for
sedimentation are still less favorable.

Filtration as practiced in Europe removes promptly and certainly a very
large proportion of the bacteria—probably, under all proper conditions,
over 99 per cent, and is thus much more effective in purification
than even weeks of storage or long flows in rivers. The places using
filtered water have, in general, extremely low death-rates from typhoid
fever. The fever which has occurred at a few places drawing their
raw water from greatly polluted sources has resulted from improper
conditions which can be avoided, and affords no ground for doubt of the
efficiency of properly conducted filtration.

Corresponding evidence has not yet been produced in connection with
the mechanical filters which have been largely used in the United
States; but the bacterial efficiencies secured with them, under proper
conditions, and with enough coagulant, have been such as to warrant the
belief that they also will serve to greatly diminish the danger from
such infection, although they have not shown themselves equal in this
respect to sand filters.

The main point is that disease-germs shall not be present in our
drinking-water. If they can be kept out in the first place at
reasonable expense, that is the thing to do. Innocence is better
than repentance. If they cannot be kept out, we must take them out
afterwards; it does not matter much how this is done, so long as the
work is thorough. Sedimentation and storage may accomplish much, but
their action is too slow and often uncertain. Filtration properly
carried out removes bacteria promptly and thoroughly and at a
reasonable expense.

APPENDICES.

Appendix I.

RULES OF THE GERMAN GOVERNMENT IN REGARD TO THE FILTRATION OF
SURFACE-WATERS USED FOR PUBLIC WATER-SUPPLIES.

Rules somewhat similar to those of which a translation is given
below were first issued by the Imperial Board of Health in 1892.
These rules were regarded as unnecessarily rigid, and a petition was
presented to the government signed by 37 water-works engineers and
directors requesting a revision.[50] As a result a conference was
organized consisting of 14 members.[51] Köhler presided, and Koch,
Gaffsky, Werner, Günther, and Reincke represented the Imperial Board
of Health. The bacteriologists were represented by Flügge, Wolffhügel,
and Fränkel, while Beer, Fischer, Lindley, Meyer, and Piefke were the
engineer members.

This conference prepared the 17 articles given below in the first
days of January, 1894. A little later the first 16 articles were
issued to all German local authorities, signed by Bosse, minister of
the “Geistlichen,” and Haase, minister of the interior, and they are
considered as binding upon all water-works using surface-water. The
bacterial examinations were commenced April 1, 1894, by most of the
cities which had not previously had them.

Although the articles do not deal with rate of filtration, or the
precautions against snow and ice, they have a very great interest both
because they are an official expression, and on account of the personal
standing of the men who prepared them.

* * * * *

§ 1. In judging of the quality of a filtered surface-water the
following points should be especially observed:

_a_. The operation of a filter is to be regarded as satisfactory
when the filtrate contains the smallest possible number of bacteria,
not exceeding the number which practical experience has shown to be
attainable with good filtration at the works in question. In those
cases where there are no previous records showing the possibilities of
the works and the influence of the local conditions, especially the
character of the raw water, and until such information is obtained,
it is to be taken as the rule that a satisfactory filtration will
never yield an effluent with more than about 100 bacteria per cubic
centimeter.

_b_. The filtrate must be as clear as possible, and, in regard to
color, taste, temperature, and chemical composition, must be no worse
than the raw water.

§ 2. To allow a complete and constant control of the bacterial
efficiency of filtration, the filtrate from each single filter must be
examined daily. Any sudden increase in the number of bacteria should
cause a suspicion of some unusual disturbance in the filter, and should
make the superintendent more attentive to the possible causes of it.

§ 3. Filters must be so constructed that samples of the effluent
from any one of them can be taken at any desired time for the
bacteriological examination mentioned in § 1.

§ 4. In order to secure uniformity of method, the following is
recommended as the standard method for bacterial examination:

The nutrient medium consists of 10 per cent meat extract gelatine with
peptone, 10 cc. of which is used for each experiment. Two samples of
the water under examination are to be taken, one of 1 cc. and one
of 1/2 cc. The gelatine is melted at a temperature of 30° to 35° C.,
and mixed with the water as thoroughly as possible in the test-tube
by tipping back and forth, and is then poured upon a sterile glass
plate. The plates are put under a bell-jar which stands upon a piece
of blotting-paper saturated with water, and in a room in which the
temperature is about 20° C.

The resulting colonies are counted after 48 hours, and with the aid of
a lens.

If the temperature of the room in which the plates are kept is lower
than the above, the development of the colonies is slower, and the
counting must be correspondingly postponed.

If the number of colonies in 1 cc. of the water is greater than about
100, the counting must be done with the help of the Wolffhügel’s
apparatus.

§ 5. The person entrusted with the carrying-out of the bacterial
examinations must present a certificate that he possesses the necessary
qualifications, and wherever possible he shall be a regular employé of
the water-works.

§ 6. When the effluent from a filter does not correspond to the
hygienic requirements it must not be used, unless the cause of the
unsatisfactory work has already been removed during the period covered
by the bacterial examinations.

In case a filter for more than a very short time yields a poor
effluent, it is to be put out of service until the cause of the trouble
is found and corrected.

It is, however, recognized from past experience that sometimes
unavoidable conditions (high water, etc.) make it impossible, from an
engineering standpoint, to secure an effluent of the quality stated
in § 1. In such cases it will be necessary to get along with a poorer
quality of water; but at the same time, if the conditions demand it
(outbreak of an epidemic, etc.), a suitable notice should be issued.

§ 7. Every single filter must be so built that, when an inferior
effluent results, which does not conform to the requirements, it can be
disconnected from the pure-water pipes and the filtrate allowed to be
wasted, as mentioned in § 6. This wasting should in general take place,
so far as the arrangement of the works will permit it:

(1) Immediately after scraping a filter; and

(2) After replacing the sand to the original depth.

The superintendent must himself judge, from previous experience with
the continual bacterial examinations, whether it is necessary to waste
the water after these operations, and, if so, how long a time will
probably elapse before the water reaches the standard purity.

§ 8. The best sand-filtration requires a liberal area of
filter-surface, allowing plenty of reserve, to secure, under all local
conditions, a moderate rate of filtration adapted to the character of
the raw water.

§ 9. Every single filter shall be independently regulated, and the
rate of filtration, loss of head, and character of the effluent shall
be known. Also each filter shall, by itself, be capable of being
completely emptied, and, after scraping, of having filtered water
introduced from below until the sand is filled to the surface.

§ 10. The velocity of filtration in each single filter shall be capable
of being arranged to give the most favorable results, and shall be as
regular as possible, quite free from sudden changes or interruptions.
On this account reservoirs must be provided large enough to balance the
hourly fluctuation in the consumption of water.

§ 11. The filters shall be so arranged that their working shall not be
influenced by the fluctuating level of the water in the filtered-water
reservoir or pump-well.

§ 12. The loss of head shall not be allowed to become so great as
to cause a breaking through of the upper layer on the surface of
the filter. The limit to which the loss of head can be allowed to
go without damage is to be determined for each works by bacterial
examinations.

§ 13. Filters shall be constructed throughout in such a way as to
insure the equal action of every part of their area.

§ 14. The sides and bottoms of filters must be made water-tight, and
special pains must be taken to avoid the danger of passages or loose
places through which the unfiltered water on the filter might find its
way to the filtered-water channels. To this end special pains should be
taken to make and keep the ventilators for the filtered-water channels
absolutely tight.

§ 15. The thickness of the sand-layer shall be so great that under no
circumstances shall it be reduced by scraping to less than 30 cm. (=
12 inches), and it is desirable, so far as local conditions allow, to
increase this minimum limit.

Special attention must be given to the upper layer of sand, which must
be arranged and continually kept in the condition most favorable for
filtration. For this reason it is desirable that, after a filter has
been reduced in thickness by scraping and is about to be refilled, the
sand below the surface, as far as it is discolored, should be removed
before bringing on the new sand.

§ 16. Every city in the German empire using sand-filtered water is
requested to make a quarterly report of its working results, especially
of the bacterial character of the water before and after filtration,
to the Imperial Board of Health (Kaiserlichen Gesundheitsamt), which
will keep itself in communication with the commission chosen by the
water-works engineers in regard to these questions; and it is believed
that after such statistical information is obtained for a period of
about two years some farther judgments can be reached.

§ 17. The question as to the establishment of a permanent inspection
of public water-works, and, if so, under what conditions, can be best
answered after the receipt of the information indicated in § 16.

APPENDIX II.

EXTRACTS FROM “BERICHT DES MEDICINAL-INSPECTORATS DES HAMBURGISCHEN
STAATES FÜR DAS JAHR 1892.”

The following are translations from Dr. Reincke’s most valuable report
upon the vital statistics of Hamburg for 1892. I much regret that I am
unable to reproduce in full the very complete and instructive tables
and diagrams which accompany the report.

=Diarrhœa and Cholera Infantum= (page 10). “It is usually assumed that
the increase of diarrhœal diseases in summer is to be explained by
the high temperature, especially by the action of the heat upon the
principal food of infants—milk. Our observations, however, indicate
that a deeper cause must be sought.” (Tables and diagrams of deaths
from cholera infantum by months for Hamburg and for Altona with the
mean temperatures, 1871-1892.)

“From these it appears that the highest monthly mortality of each year
in Hamburg occurred 7 times in July, 13 times in August, and 3 times
in September, and substantially the same in Altona. If one compares
the corresponding temperatures, it is found that in the three years
1886, 1891, and 1892, with high September mortalities, especially the
first two of them, had their maximum temperature much earlier, in fact
earlier than usual. Throughout, the correspondence between deaths and
temperatures is not well marked. Repeated high temperatures in May and
June have never been followed by a notable amount of cholera infantum,
although such periods have lasted for a considerable time. For example,
toward the end of May, 1892, for a long time the temperature was higher
than in the following August, when the cholera infantum appeared.

“The following observations are still more interesting. As is seen
from the diagram, in addition to the annual rise in summer there is
also a smaller increase in the winter, which is especially marked
in Altona. In 1892 this winter outbreak was greater than the summer
one, and nearly as great in 1880 and in 1888. The few years when
this winter increase was not marked, 1876-7, 1877-8, 1881-2, 1883-4,
were warm winters in which the mean temperature did not go below the
freezing-point. It is also to be noted that the time of this winter
outbreak is much more variable than that of the summer one. In 1887 the
greatest mortality was in November; in 1889 in February; in other years
in December or January, and in Altona, in 1886 and 1888, in March,
which is sufficient evidence that it was not the result of Christmas
festivities.

“Farther, the winter diarrhœa of Hamburg and of Altona are not parallel
as is the case in summer. In Hamburg the greatest mortality generally
comes before New Year’s; in Altona one to two months later.

“In Bockendahl’s Generalbericht über das öffentliche Gesundheitswesen
der Provinz Schleswig-Holstein für das Jahr 1870, page 10, we read:
‘Yet more remarkable was an epidemic of cholera infantum in Altona
in February which proved fatal to 43 children. These cases were
distributed in every part of the city, and could not be explained
by the health officer until he ascertained that the water company
had supplied unfiltered water to the city. This occurred for a few
days only in January, and was the only time in the whole year that
unfiltered Elbe water was delivered. However little reason there may
be to believe that there was a connection between these circumstances,
future interruptions of the service of filtered water should be most
critically watched, as only in this way can reliable conclusions be
reached. Without attempting to draw any scientific conclusions from
the fact, I cannot do less than record that, prior to the outbreak
of cholera on August 20, 1871, unfiltered together with filtered
water had been supplied to the city August 11 to 18. The action of
the authorities was then justified when they forbade in future the
supply of unfiltered water except in cases of most urgent necessity,
as in case of general conflagration; and in such a case, or in case of
interruption due to broken pipes, that the public should be suitably
warned.’

“The author of this paragraph, Dr. Kraus, became later the health
officer of Hamburg, and in an opinion written by him in 1874, and now
before me, he most earnestly urged the adoption of sand-filtration in
Hamburg, and cites the above observations in support of his position.
In the annual report of vital statistics of Hamburg for 1875 he says
that it is quite possible that the addition of unfiltered Elbe water
to milk is the cause of the high mortality from cholera infantum, as
compared with London, and this idea was often afterward expressed by
him. Since then so much evidence has accumulated that his view may
fairly be considered proved.

“For the information of readers not familiar with local conditions,
a mention of the sources of the water-supplies up to the present
time used by Hamburg and Altona will be useful. Both cities take
their entire water-supplies from the Elbe—Altona from a point about 7
miles below the discharge of the sewage of both cities, Hamburg from
about 7 miles above. The raw water at Altona is thus polluted by the
sewage from the population of both cities, having now together over
700,000 inhabitants, and contains in general 20,000 to 40,000 or more
bacteria per cubic centimeter. The raw water of Hamburg has, however,
according to the time of year and tide, from 200 to 5000, but here also
occasionally much higher numbers are obtained when the ebb tide carries
sewage up to the intake. How often this takes place is not accurately
known, but most frequently in summer when the river is low, more rarely
in winter and in times of flood. Recent bacterial examinations show
that it occurs much more frequently than was formerly assumed from
float experiments. This water is pumped directly to the city raw, while
that for Altona is carefully filtered.

“Years ago I expressed the opinion that the repeated typhoid epidemics
in Altona stood in direct connection with disturbances of the action
of the filters by frost, which result in the supply of insufficiently
purified water. Wallichs in Altona has also come to this conclusion
as a result of extended observation, and recently Robert Koch has
explained the little winter epidemic of cholera in Altona in the same
way, thus supporting our theory. When open filters are cleaned in cold,
frosty weather the bacteria in the water are not sufficiently held back
by the filters. Such disturbances of filtration not only preceded the
explosive epidemics of typhoid fever of 1886, 1887, 1888, 1891, and
1892, and the cholera outbreaks of 1871 and 1893, but also the winter
outbreaks of cholera infantum which have been so often repeated. It
cannot be doubted that these phenomena bear the relation to each other
of cause and effect. It is thus explained why in the warm winters no
such outbreaks have taken place, and also why the cholera infantum in
winter is not parallel in Hamburg and Altona.

* * * * *

“A farther support of this idea is furnished by Berlin, where in the
same way frost has repeatedly interfered with filtration. In the
following table are shown the deaths from diarrhœa and cholera infantum
for a few winter periods having unusual increases in mortality in
comparison with the bacteria in the water-supply.” (These tables show
that in March, 1886, March, 1888, February-March, 1889, and February,
1891, high numbers of bacteria resulted from frost disturbance at
the Stralau works, and in every case they were followed by greatly
increased death-rates from diarrhœal diseases.—A. H.)

“No one who sees this exhibition can doubt that here also the supply
of inadequately purified water has every time cost the lives of many
children.” (100 to 400 or more each time.—A. H.) “Even more conclusive
is the evidence, published by the Berlin Health Office, that this
increase was confined to those parts of the city supplied from Stralau”
(with open filters.—A. H.), “and that the parts supplied from the
better Tegel works took no part in the outbreaks, which was exactly
the case with the well-known typhoid epidemic of February and March,
1889.... It was also found that those children nursed by their mothers
or by wet-nurses did not suffer, but only those fed on the milk of
animals or other substitutes, and which in any case were mixed with
more or less water.”

Under =Cholera=, page 28, he says: “The revised statistics here given
differ slightly from preliminary figures previously issued and widely
published.” (The full tables, which cannot be here reproduced, show
16,956 cases and 8605 deaths. 8146 of the deaths occurred in the month
ending September 21. Of these, 1799 were under 5 years old; 776 were 5
to 15; 744, 15 to 25; 3520, 25 to 50; 1369, 50 to 70; and 397 over 70
or of unknown age. The bulk of the cases were thus among mature people,
children, except very young children, suffering the least severely of
any age class.)

“The epidemic began on August 16, in the port where earlier outbreaks
have also had their origin. The original source of the infection
has not been ascertained with certainty, but was probably from one
of two sources. Either it came from certain Jews, just arrived from
cholera-stricken Russia, who were encamped in large numbers near the
American pier, or the infection came from Havre, where cholera had been
present from the middle of July. Perhaps the germs came in ships in
water-ballast which was discharged at Hamburg, which is so much more
probable, as the sewage of Havre is discharged directly into the docks.

“It is remarkable that in Altona, compared to the total number of
cases, very few children had cholera, while in the epidemic of 1871 the
children suffered severely. This may be explained by supposing that the
cholera of 1892 in Altona was not introduced by water, but by other
means of infection....

“It is well known that the drinking-water (of Hamburg) is supposed to
have been from the first the carrier of the cholera-germs. In support
of this view the following points are especially to be noted:

“1. The explosive rapidity of attack. The often-compared epidemic
in Munich in 1854, which could not have come from the water is
characteristically different in that its rise was much slower and was
followed by a gradual decline. In Hamburg, with six times as large a
population, the height of the epidemic was reached August 27, only 12
days after the first cases of sickness, while in Munich 25 days were
required. In Hamburg also the bulk of the cases were confined to 12
days, from August 25 to September 5, while in Munich the time was twice
as long.

“2. The exact limit of the epidemic to the political boundary between
Hamburg and Altona and Wandsbeck, which also agrees with the boundary
between the respective water-supplies, while other differences were
entirely absent. Hamburg had for 1000 inhabitants 26.31 cases and 13.39
deaths, but Altona only 3.81 cases and 2.13 deaths, and Wandsbeck 3.06
cases and 2.09 deaths.

“3. The old experience of cholera in fresh-water ports, and the analogy
of many earlier epidemics. In this connection the above-mentioned
epidemic of 1871 in Altona has a special interest, even though some
of the conclusions of Bockendahl’s in his report of 1871 are open to
objection. First there were 3 deaths August 3, which were not at once
followed by others. Then unfiltered Elbe water was supplied August 11
to 18. On the 19th an outbreak of cholera extended to all parts of
the city, which reached its height August 25 and 26, and afterwards
gradually decreased. In all 105 persons died of cholera and 186 (179 of
them children) of diarrhœa. In Hamburg, four times as large, only 141
persons died of cholera at this time, thus proportionately a smaller
number. The conditions were then the reverse of those of 1892, an
infection of the Altona water and a comparative immunity in Hamburg.

“It is objected that the cholera-germs were not found in the water
in 1892. To my knowledge they were first looked for, and then
with imperfect methods, in the second half of September. In the
after-epidemics at Altona, they were found in the river-water by R.
Koch by the use of better methods.

“It is quite evident that the germs were also distributed by other
methods than by the city water, especially by dock-laborers who became
infected while at their work and thus set up little secondary epidemics
where they went or lived.... These laborers and sailors, especially on
the smaller river-boats, had an enormously greater proportionate amount
of cholera than others.... These laborers do not live exclusively near
the water, but to a measure in all parts of the city.” (And in Altona
and Wandsbeck.—A. H.)

“Altona had 5 deaths from cholera December 25 to January 4, and 19
January 23 to February 11, and no more. As noted above, this is
attributed to the water-supply, and to defective filtration in presence
of frost....

“The cholera could never have reached the proportion which it did, had
the improvements in the drinking-water been earlier completed.”

Further accounts of the water-supplies of Altona and of Hamburg and of
the new filtration works at the latter city are given in Appendices VII
and VIII.

APPENDIX III.

METHODS OF SAND-ANALYSIS.

(From the Annual Report of the Massachusetts State Board of Health for
1892.)

A knowledge of the sizes of the sand-grains forms the basis of many of
the computations. This information is obtained by means of mechanical
analyses. The sand sample is separated into portions having grains
of definite sizes, and from the weight of the several portions the
relative quantities of grains of any size can be computed.

=Collection of Samples.=—In shipping and handling, samples of sand
are best kept in their natural moist condition, as there is then no
tendency to separation into portions of unequal-sized grains. Under no
circumstances should different materials be mixed in the same sample.
If the material under examination is not homogeneous, samples of each
grade should be taken in separate bottles, with proper notes in regard
to location, quantity, etc. Eight-ounce wide-necked bottles are most
convenient for sand samples, but with gravels a larger quantity is
often required. Duplicate samples for comparison after obtaining the
results of analyses are often useful.

=Separation into Portions having Grains of Definite Sizes.=—Three
methods are employed for particles of different sizes—hand-picking
for the stones, sieves for the sands, and water elutriation for the
extremely fine particles. Ignition, or determination of albuminoid
ammonia, might be added for determining the quantity of organic matter,
which, as a matter of convenience, is assumed to consist of particles
less than 0.01 millimeter in diameter.

The method of hand-picking is ordinarily applied only to particles
which remain on a sieve two meshes to an inch. The stones of this size
are spread out so that all are in sight, and a definite number of the
largest are selected and weighed. The diameter is calculated from the
average weight by the method to be described, while the percentage is
reckoned from the total weight. Another set of the largest remaining
stones is then picked out and weighed as before, and so on until the
sample is exhausted. With a little practice the eye enables one to pick
out the largest stones quite accurately.

With smaller particles this process becomes too laborious, on account
of the large number of particles, and sieves are therefore used
instead. The sand for sifting must be entirely free from moisture, and
is ordinarily dried in an oven at a temperature somewhat above the
boiling-point. The quantity taken for analysis should rarely exceed
100-200 grams. The sieves are made from carefully-selected brass-wire
gauze, having, as nearly as possible, square and even-sized meshes. The
frames are of metal, fitting into each other so that several sieves can
be used at once without loss of material. It is a great convenience to
have a mechanical shaker, which will take a series of sieves and give
them a uniform and sufficient shaking in a short time; but without this
good results can be obtained by hand-shaking. A series which has proved
very satisfactory has sieves with approximately 2, 4, 6, 10, 20, 40,
70, 100, 140, and 200 meshes to an inch; but the exact numbers are of
no consequence, as the actual sizes of the particles are relied upon,
and not the number of meshes to an inch.

It can be easily shown by experiment that when a mixed sand is shaken
upon a sieve the smaller particles pass first, and as the shaking
is continued larger and larger particles pass, until the limit is
reached when almost nothing will pass. The last and largest particles
passing are collected and measured, and they represent the separation
of that sieve. The size of separation of a sieve bears a tolerably
definite relation to the size of the mesh, but the relation is not to
be depended upon, owing to the irregularities in the meshes and also
to the fact that the finer sieves are woven on a different pattern
from the coarser ones, and the particles passing the finer sieves are
somewhat larger in proportion to the mesh than is the case with the
coarser sieves. For these reasons the sizes of the sand-grains are
determined by actual measurements, regardless of the size of the mesh
of the sieve.

It has not been found practicable to extend the sieve-separations to
particles below 0.10 millimeter in diameter (corresponding to a sieve
with about 200 meshes to an inch), and for such particles elutriation
is used. The portion passing the finest sieve contains the greater
part of the organic matter of the sample, with the exception of roots
and other large undecomposed matters, and it is usually best to remove
this organic matter by ignition at the lowest possible heat before
proceeding to the water-separations. The loss in weight is regarded as
organic matter, and calculated as below 0.01 millimeter in diameter.
In case the mineral matter is decomposed by the necessary heat, the
ignition must be omitted, and an approximate equivalent can be obtained
by multiplying the albuminoid ammonia of the sample by 50.[52] In this
case it is necessary to deduct an equivalent amount from the other fine
portions, as otherwise the analyses when expressed in percentages would
add up to more than one hundred.

Five grams of the ignited fine particles are put in a beaker 90
millimeters high and holding about 230 cubic centimeters. The beaker
is then nearly filled with distilled water at a temperature of 20° C.,
and thoroughly mixed by blowing into it air through a glass tube. A
larger quantity of sand than 5 grams will not settle uniformly in the
quantity of water given, but less can be used if desired. The rapidity
of settlement depends upon the temperature of the water, so that it is
quite important that no material variation in temperature should occur.
The mixed sand and water is allowed to stand for fifteen seconds, when
most of the supernatant liquid, carrying with it the greater part of
the particles less than 0.08 millimeter, is rapidly decanted into a
suitable vessel, and the remaining sand is again mixed with an equal
amount of fresh water, which is again poured off after fifteen seconds,
carrying with it most of the remaining fine particles. This process is
once more repeated, after which the remaining sand is allowed to drain,
and is then dried and weighed, and calculated as above 0.08 millimeter
in diameter. The finer decanted sand will have sufficiently settled
in a few minutes, and the coarser parts at the bottom are washed back
into the beaker and treated with water exactly as before, except that
one minute interval is now allowed for settling. The sand remaining
is calculated as above 0.04 millimeter, and the portion below 0.04 is
estimated by difference, as its direct determination is very tedious,
and no more accurate than the estimation by difference when sufficient
care is used.

=Determination of the Sizes of the Sand-grains.=—The sizes of the
sand-grains can be determined in either of two ways—from the weight of
the particles or from micrometer measurements. For convenience the size
of each particle is considered to be the diameter of a sphere of equal
volume. When the weight and specific gravity of a particle are known,
the diameter can be readily calculated. The volume of a sphere is
1/6π_d_³, and is also equal to the weight divided by the specific
gravity. With the Lawrence materials the specific gravity is uniformly
2.65 within very narrow limits, and we have _w_/2.65 = 1/6π_d_³.
Solving for _d_ we obtain the formula _d_ =.9∛_w_, where _d_
is the diameter of a particle in millimeters and _w_ its weight in
milligrams. As the average weight of particles, when not too small,
can be determined with precision, this method is very accurate, and
altogether the most satisfactory for particles above 0.10 millimeter;
that is, for all sieve separations. For the finer particles the method
is inapplicable, on account of the vast number of particles to be
counted in the smallest portion which can be accurately weighed, and
in these cases the sizes are determined by micrometer measurements.
As the sand-grains are not spherical or even regular in shape,
considerable care is required to ascertain the true mean diameter. The
most accurate method is to measure the long diameter and the middle
diameter at right angles to it, as seen by a microscope. The short
diameter is obtained by a micrometer screw, focussing first upon the
glass upon which the particle rests and then upon the highest point to
be found. The mean diameter is then the cube root of the product of the
three observed diameters. The middle diameter is usually about equal
to the mean diameter, and can generally be used for it, avoiding the
troublesome measurement of the short diameters.

The sizes of the separations of the sieves are always determined from
the very last sand which passes through in the course of an analysis,
and the results so obtained are quite accurate. With the elutriations
average samples are inspected, and estimates made of the range in
size of particles in each portion. Some stray particles both above
and below the normal sizes are usually present, and even with the
greatest care the result is only an approximation to the truth; still,
a series of results made in strictly the same way should be thoroughly
satisfactory, notwithstanding possible moderate errors in the absolute
sizes.

=Calculation of Results.=—When a material has been separated into
portions, each of which is accurately weighed, and the range in the
sizes of grains in each portion determined, the weight of the particles
finer than each size of separation can be calculated, and with enough
properly selected separations the results can be plotted in the form of
a diagram, and measurements of the curve taken for intermediate points
with a fair degree of accuracy. This curve of results may be drawn upon
a uniform scale, using the actual figures of sizes and of per cents by
weight, or the logarithms of the figures may be used in one or both
directions. The method of plotting is not of vital importance, and
the method for any set of materials which gives the most easily and
accurately drawn curves is to be preferred. In the diagram published
in the Report of the Mass. State Board of Health for 1891, page 430,
the logarithmic scale was used in one direction, but in many instances
the logarithmic scale can be used to advantage in both directions. With
this method it has been found that the curve is often almost a straight
line through the lower and most important section, and very accurate
results are obtained even with a smaller number of separations.

=Examples of Calculation of Results.=—Following are examples of
representative analyses, showing the method of calculation used with
the different methods of separation employed with various materials.

I. ANALYSIS OF A GRAVEL BY HAND-PICKING, 11,870 GRAMS TAKEN FOR
ANALYSIS.

--------+---------+-----------+-----------+-------------+-------+--------
Number | Total | Average | Estimated |Corresponding| Total |Per Cent
of |Weight of| Weight of | Weight of | Size. | Weight| of
Stones | Portion.| Stones. | Smallest | Millimeters.| of |Total
in | Grams. |Milligrams.| Stones. | |Stones |Weight
Portion.| | |Milligrams.| |Smaller|Smaller
(Largest| | | | | than | than
Selected| | | | | this | this
Stones.)| | | | | Size. | Size.
--------+---------+-----------+-----------+-------------+-------+--------
| .... | .... | .... | .... |11,870 |=100=
10 | 3,320 | 332,000 | 250,000 | =56= | 8,550 | =72=
10 | 1,930 | 193,000 | 165,000 | =49= | 6,620 | =56=
10 | 1,380 | 138,000 | 124,000 | =45= | 5,240 | =44=
20 | 2,200 | 110,000 | 93,000 | =41= | 3,040 | =26=
20 | 1,520 | 76,000 | 64,000 | =36= | 1,520 | =13=
20 | 1,000 | 50,000 | 36,000 | =30= | 520 | =4.4=
20 | 460 | 23,000 | 10,000 | =20= | 60 | =.5=
10 | 40 | 4,000 | 2,000 | =11= | 20 | =.2=
Dust | 20 | .... | .... | .... | .... | ....
--------+---------+-----------+-----------+-------------+-------+--------

The weight of the smallest stones in a portion given in the fourth
column is estimated in general as about half-way between the average
weight of all the stones in that portion and the average weight of the
stones in the next finer portion.

The final results are shown by the figures in full-faced type in the
last and third from the last columns. By plotting these figures we
find that 10 per cent of the stones are less than 35 millimeters in
diameter, and 60 per cent are less than 51 millimeters. The “uniformity
coefficient,” as described below, is the ratios of these numbers, or
1.46, while the “effective size” is 35 millimeters.

II. ANALYSIS OF A SAND BY MEANS OF SIEVES.

A portion of the sample was dried in a porcelain dish in an air-bath.
Weight dry, 110.9 grams. It was put into a series of sieves in a
mechanical shaker, and given one hundred turns (equal to about seven
hundred single shakes). The sieves were then taken apart, and the
portion passing the finest sieve weighed. After noting the weight, the
sand remaining on the finest sieve, but passing all the coarser sieves,
was added to the first and again weighed, this process being repeated
until all the sample was upon the scale, weighing 110.7 grams, showing
a loss by handling of only 0.2 gram. The figures were as follows:

-------+------------+--------+---------
| Size of | |
| Separation |Quantity|Per Cent
Sieve | of this | of Sand| of
Marked.| Sieve. |Passing.| Total
|Millimeters.|Grams. | Weight.
-------+------------+--------+---------
190 | =.105= | .5 | =.5=
140 | =.135= | 1.3 | =1.2=
100 | =.182= | 4.1 | =3.7=
60 | =.320= | 23.2 | =21.0=
40 | =.46= | 56.7 | =51.2=
20 | =.93= | 89.1 | =80.5=
10 | =2.04= | 104.6 | =94.3=
6 | =3.90= | 110.7 | =100.0=
-------+------------+--------+--------

Plotting the figures in heavy-faced type, we find from the curve that
10 and 60 per cent respectively are finer than .25 and .62 millimeter,
and we have for effective size, as described above, .25, and for
uniformity coefficient 2.5.

III. ANALYSIS OF A FINE MATERIAL WITH ELUTRIATION.

The entire sample, 74 grams, was taken for analysis. The sieves used
were not the same as those in the previous analysis, and instead of
mixing the various portions on the scale they were separately weighed.
The siftings were as follows:

Remaining on sieve marked 10, above 2.2 millimeters 1.5 grams
Remaining on sieve marked 20, above .98 millimeters 7.0 grams
Remaining on sieve marked 40, above .46 millimeters 22.0 grams
Remaining on sieve marked 70, above .24 millimeters 20.2 grams
Remaining on sieve marked 140, above .13 millimeters 9.2 grams
Passing sieve 140, below .13 millimeters 14.1 grams

The 14.1 grams passing the 140 sieve were thoroughly mixed, and one
third, 4.7 grams, taken for analysis. After ignition just below a red
heat in a radiator, the weight was diminished by 0.47 gram. The portion
above .08 millimeter and between .04 and .08 millimeter, separated as
described above, weighed respectively 1.27 and 1.71 grams, and the
portion below .04 millimeter was estimated by difference [4.7 - (0.47
+ 1.27 + 1.71)] to be 1.25 grams. Multiplying these quantities by 3,
we obtain the corresponding quantities for the entire sample, and the
calculation of quantities finer than the various sizes can be made as
follows:

-----------------------+-------+------------+-------------+-----------
| | Size of |Weight of all|Per Cent by
|Weight.| Largest | the Finer | Weight of
Size of Grain. | Grams.| Particles. | Particles. | all Finer
| |Millimeters.| Grams. | Particles.
-----------------------+-------+------------+-------------+-----------
Above 2.20 millimeters | 1.50 | .... | 74.00 | =100=
.98-2.20 millimeters | 7.00 | =2.20= | 72.50 | =98=
.46- .98 millimeters | 22.00 | =.98= | 65.50 | =89=
.24- .46 millimeters | 20.20 | =.46= | 43.50 | =60=
.13- .24 millimeters | 9.20 | =.24= | 23.30 | =32=
.08- .13 millimeters | 3.81 | =.13= | 14.10 | =19=
.04- .08 millimeters | 5.13 | =.08= | 10.29 | =14=
.01- .04 millimeters | 3.75 | =.04= | 5.16 | =7=
Loss on ignition | | | |
(assumed to be less | | | |
than .01 millimeter)| 1.41 | =.01= | 1.41 | =1.9=
-----------------------+-------+------------+-------------+-----------

By plotting the heavy-faced figures we find that 10 and 60 per cent are
respectively finer than .055 and .46 millimeter, and we have effective
size .055 millimeter and uniformity coefficient 8.

* * * * *

The effective size and uniformity coefficient calculated in this way
have proved to be most useful in various calculations, particularly
in estimating the friction between the sands and gravels and water.
The remainder of the article in the Report of the Mass. State Board
of Health is devoted to a discussion of these relations which were
mentioned in Chapter III of this volume.

APPENDIX IV.

FILTER STATISTICS.

STATISTICS OF OPERATION OF SAND FILTERS.

------------+----------+---------+-------+-------+---------+-------+----------
| |Total | | Area | Average |Area of|Period,
| |Quantity | | of | Daily |Filter |
| |of Water | |Filters| Yield, |Surface|Million
| |filtered |Million|in use,| |cleaned|Gallons
Place. | Year | for |Gallons| | Million |in One |per Acre
| Ending. |One Year.| Daily.| Acres.| Gallons | Year, |filtered
| | Million | | |per Acre.| |between
| | Gallons.| | | |Acres. |Scrapings.
------------+----------+---------+-------+-------+---------+-------+----------
Altona |Mar., 1895| 1,620 | 4.44 | 3.08 | 1.45 | 31.0 | 52
|Mar., 1896| 1,730 | 4.75 | 3.08 | 1.55 | 48.5 | 36
|Mar., 1897| 1,960 | 5.40 | 3.08 | 1.75 | 44.0 | 45
|Mar., 1898| 1,940 | 5.30 | 3.08 | 1.72 | 36.5 | 53
Amsterdam, |Dec., 1894| 1,390 | 3.80 | 5.43 | 0.71 | 23 | 62
River |Dec., 1896| 1,490 | 4.08 | 5.43 | 0.75 | 48 | 31
|Dec., 1897| 1,600 | 4.40 | 5.43 | 0.81 | 30 | 53
Amsterdam, |Dec., 1894| 2,330 | 6.40 | 4.94 | 1.29 |116 | 20
Dunes |Dec., 1896| 2,360 | 6.50 | 4.75 | 1.37 | 90 | 26
|Dec., 1897| 2,290 | 6.25 | 4.75 | 1.31 |109 | 21
Ashland, |Feb., 1897| 398 | 1.09 | 0.50 | 2.18 | 4.83 | 83
Wis. | | | | | | |
Berlin, |Mar., 1896| 13,000 | 35.60 | 25.10 | 1.42 | |
total |Mar., 1897| 12,900 | 35.40 | 25.10 | 1.40 | |
|Mar., 1898| 13,200 | 36.20 | 27.00 | 1.34 | |
Bremen |Mar., 1895| 1,190 | 3.27 | 2.51 | 1.31 | 50 | 24
|Mar., 1896| 1,220 | 3.34 | 3.21 | 1.04 | 32.5 | 38
|Mar., 1897| 1,280 | 3.50 | 3.21 | 1.09 | 25.2 | 50
|Mar., 1898| 1,400 | 4.10 | 3.21 | 1.28 | 34.0 | 41
Breslau |Mar., 1895| 2,840 | 7.80 | 5.12 | 1.52 | 45 | 64
|Mar., 1896| 2,960 | 8.10 | 5.12 | 1.58 | 40.0 | 74
|Mar., 1897| 2,990 | 8.20 | 5.12 | 1.60 | 37 | 81
|Mar., 1898| 3,060 | 8.40 | 5.12 | 1.64 | 43 | 71
Brunn |Dec., 1896| 1,110 | 3.04 | 1.62 | 1.87 | 8.6 | 128
|Dec., 1897| 1,190 | 3.25 | 1.62 | 2.00 | 9.1 | 131
Brunswick |Mar., 1895| 815 | 2.23 | 1.48 | 1.51 | 14.8 | 55
|Mar., 1896| 840 | 2.30 | 1.48 | 1.56 | 13.3 | 63
|Mar., 1897| 820 | 2.25 | 1.48 | 1.52 | 13.7 | 60
|Mar., 1898| 870 | 2.38 | 1.48 | 1.61 | 11.9 | 73
Budapest |Dec., 1892| 7,360 | 20.20 | 3.00 | 6.70 |254 | 29
Copenhagen |Dec., 1895| 2,330 | 6.40 | 2.88 | 2.22 | 45 | 52
|Dec., 1896| 2,490 | 6.80 | 2.88 | 2.35 | 52 | 48
|Dec., 1897| 2,580 | 7.10 | 2.88 | 2.47 | 54 | 48
Dordrecht |Dec., 1894| 365 | 1.00 | 0.56 | 1.79 | |
Frankfort |Dec., 1895| 310 | 0.85 | 0.37 | 2.28 | 2.9 | 107
on Oder |Dec., 1896| 325 | 0.89 | 0.37 | 2.40 | 7.4 | 44
|Dec., 1897| 356 | 0.98 | 0.37 | 2.65 | 8.8 | 41
Hamburg |Dec., 1894| 11,450 | 31.40 | 34.0 | 0.92 |350 | 33
|Dec., 1895| 11,700 | 32.10 | 34.0 | 0.94 |275 | 43
|Dec., 1896| 11,500 | 31.70 | 34.0 | 0.93 |266 | 43
|Dec., 1897| 12,000 | 32.70 | 34.0 | 0.96 |285 | 42
|Dec., 1898| 11,900 | 32.60 | 43.0 | 0.76 |246 | 48
Hudson, |Dec., 1892| 697 | 1.91 | 0.74 | 2.58 | |
N. Y. |Dec., 1893| 543 | 1.49 | 0.74 | 2.01 | |
|Dec., 1895| 535 | 1.46 | 0.74 | 1.98 | |
Ilion, N. Y.|Feb., 1899| 182 | 0.50 | 0.14 | 3.57 | 1.40 | 130
Königsberg |Mar., 1895| 1,060 | 2.90 | 2.70 | 1.07 | 38.5 | 27
|Mar., 1896| 1,085 | 2.97 | 2.70 | 1.10 | 35.0 | 31
|Mar., 1897| 1,085 | 2.97 | 2.70 | 1.10 | 41.0 | 27
|Mar., 1898| 1,140 | 3.12 | 2.70 | 1.16 | 44.0 | 26
Lawrence |Dec., 1894| 1,050 | 2.88 | 2.50 | 1.15 | 10 | 105
|Dec., 1895| 1,097 | 3.00 | 2.50 | 1.20 | 27 | 41
|Dec., 1896| 1,101 | 3.02 | 2.50 | 1.20 | 30 | 37
|Dec., 1897| 1,114 | 3.06 | 2.50 | 1.22 | 41 | 27
Liverpool |Dec., 1896| 8,520 | 23.40 | 10.92 | 2.14 |158 | 54[53]
London, all |Dec., 1892| 65,783 |180 |109.75 | 1.64 | 90 |
filters |Dec., 1893| |195 |116.00 | 1.68 | |
but not |Dec., 1894| 68,700 |188 |117.00 | 1.60 | |
including |Dec., 1895| 76,900 |210 |123.75 | 1.70 | |
ground |Dec., 1896| 72,482 |198 |123.75 | 1.60 | |
water |Dec., 1897| 73,340 |201 |125.00 | 1.61 | |
London, |Dec., 1897| 5,370 | 14.70 | 8.00 | 1.85 | |
Chelsea | | | | | | |
E. London |Dec., 1897| 18,000 | 49.00 | 31.00 | 1.58 | |
Grand |Dec., 1897| 8,560 | 23.40 | 21.75 | 1.07 | |
Junction | | | | | | |
Lambeth |Dec., 1897| 10,370 | 28.40 | 12.25 | 2.30 | |
New River |Dec., 1897| 15,750 | 43.00 | 16.50 | 2.60 | |
Southwark & |Dec., 1897| 14,800 | 40.50 | 20.50 | 1.98 | |
Vauxhall | | | | | | |
West |Dec., 1897| 8,910 | 24.30 | 15.00 | 1.61 | |
Middlesex | | | | | | |
Lübeck |Mar., 1895| 1,520 | 4.15 | 1.40 | 2.95 | 16.2 | 94
|Mar., 1896| 1,600 | 4.38 | 1.40 | 3.13 | 24.4 | 66
|Mar., 1897| 1,650 | 4.50 | 1.40 | 3.22 | 27.0 | 61
|Mar., 1898| 1,750 | 4.80 | 1.40 | 3.42 | 38.5 | 45
Magdeburg |Mar., 1895| 1880 | 5.15 | 3.76 | 1.37 | 47.5 | 40
|Mar., 1896| 1950 | 5.35 | 3.76 | 1.42 | 65.0 | 30
|Mar., 1897| 1880 | 5.15 | 3.76 | 1.37 | 59.0 | 32
|Mar., 1898| 2070 | 5.66 | 3.76 | 1.50 | 63.0 | 33
Mt. Vernon, |Dec., 1895| 493 | 1.35 | 1.10 | 1.22 | 7.3 | 68
N. Y |Dec., 1896| 608 | 1.66 | 1.10 | 1.51 | 9.2 | 66
|Dec., 1897| 808 | 2.21 | 1.10 | 2.00 | 16.6 | 49
|Dec., 1898| 933 | 2.56 | 1.10 | 2.34 | 18.4 | 51
Posen |Mar., 1895| 305 | 0.84 | 0.70 | 1.20 | 10.3 | 30
|Mar., 1896| 346 | 0.94 | 0.70 | 1.35 | 10.4 | 33
|Mar., 1897| 325 | 0.89 | 0.70 | 1.27 | 10.1 | 32
|Mar., 1898| 360 | 0.99 | 0.70 | 1.42 | 9.6 | 38
Poughkeepsie|Dec., 1892| 696 | 1.91 | 0.68 | 2.81 | 14.0 | 50
|Dec., 1893| 667 | 1.83 | 0.68 | 2.70 | 12.0 | 56
|Dec., 1894| 633 | 1.73 | 0.68 | 2.55 | 14 | 45
|Dec., 1895| 686 | 1.88 | 0.68 | 2.77 | 14 | 49
|Dec., 1896| 664 | 1.82 | 0.68 | 2.68 | 9 | 73
|Dec., 1897| 615 | 1.69 | 1.36 | 1.24 | |
|Dec., 1898| 611 | 1.67 | 1.36 | 1.23 | 10.88 | 57
Rostock |June, 1897| 560 | 1.54 | 1.11 | 1.38 | 9.3 | 60
|June, 1898| 625 | 1.71 | 1.11 | 1.55 | 9.0 | 70
Rotterdam |Dec., 1893| 4850 | 13.30 | 6.30 | 2.11 | |
Stettin |Mar., 1895| 1130 | 3.10 | 2.26 | 1.37 | 26.5 | 43
|Mar., 1896| 1030 | 2.83 | 2.26 | 1.25 | 15.5 | 66
|Mar., 1897| 980 | 2.70 | 2.26 | 1.19 | 16.1 | 61
|Mar., 1898| 1020 | 2.80 | 2.26 | 1.24 | 20.3 | 50
Stockholm |Dec., 1895| 2375 | 6.50 | 2.78 | 2.33 | 70.0 | 34
|Dec., 1896| 2500 | 6.85 | 2.78 | 2.45 | 68.0 | 37
|Dec., 1897| 2750 | 7.50 | 3.60 | 2.08 | 76.0 | 36
Stralsund |Mar., 1897| 215 | 0.59 | 1.11 | 0.53 | 16.0 | 13
|Mar., 1898| 210 | 0.58 | 1.11 | 0.51 | 17.3 | 12
Stuttgart |Mar., 1895| 1040 | 2.85 | 1.46 | 1.96 | 13.7 | 76
|Mar., 1896| 1220 | 3.34 | 1.66 | 2.04 | 17.7 | 69
|Mar., 1897| 1270 | 3.48 | 2.32 | 1.50 | 18.7 | 68
|Mar., 1898| 1320 | 3.60 | 2.32 | 1.54 | 20.2 | 65
Utrecht |Dec., 1896| 510 | 1.40 | 0.60 | 2.33 | 31 | 16
Zürich |Dec., 1891| 2010 | 5.50 | 0.84 | 6.50 | 8 | 250
|Dec., 1892| 2150 | 5.90 | 0.84 | 7.00 | 10 | 215
|Dec., 1893| 2310 | 6.38 | 1.19 | 5.35 | 13 | 177
|Dec., 1894| 2250 | 6.15 | 1.19 | 5.18 | 17 | 133
|Dec., 1895| 2460 | 6.70 | 1.19 | 5.62 | 27 | 91
|Dec., 1896| 2360 | 6.45 | 1.66 | 3.88 | 30 | 79
|Dec., 1897| 2500 | 6.84 | 1.66 | 4.13 | 35 | 71
|Dec., 1898| 2730 | 7.50 | 1.66 | 4.50 | 47 | 58
--------------+----------+---------+-------+-------+---------+-------+----------

PARTIAL LIST OF CITIES USING SAND FILTERS.

-------------------+------------------+--------+--------+------------
| When |Population.| Area | Number | Average
Place. |Built.| 1890. | of | of | Daily
| | |Filters.|Filters.|Consumption.
-------------------+------+-----------+--------+--------+------------
UNITED STATES.
Poughkeepsie. N. Y.| 1872 | 24,000 | 1.36 | 3 | 1.67
Hudson, N. Y. | 1874 | 9,970 | 0.74 | 2 | 1.50
St. Johnsbury, Vt. |187(?)| 3,857 | 0.14 | 3 | 0.70
Nantucket, Mass. | 1893 | 3,268 | 0.11 | 1 | 0.09
Lawrence, Mass. | 1893 | 44,654 | 2.50 | 1 | 3.00
Ilion, N. Y. | 1893 | 4,057 | 0.14 | 2 | 0.50
Mount Vernon, N. Y.| 1894 | 10,830 | 1.10 | 3 | 1.66
Grand Forks, N. D. | 1894 | 4,979 | 0.42 | 1 | ....
Milford, Mass. | 1895 | 9,956 | 0.25 | 1 | 0.70
Ashland, Wis. | 1895 | 9,956 | 0.50 | 3 | 1.09
Hamilton, N. Y. | 1895 | 1,744 | 0.12 | 1 | 0.03
Lambertville, N. J.| 1896 | 4,142 | 0.28 | 2 | 0.25
Far Rockaway, N. Y.| 1896 | 2,288 | 0.92 | 2 | 0.93
Red Bank, N. J. | 1897 | 500 | 0.03 | 2 | 0.10
Somersworth, N. H. | 1897 | 6,207 | 0.50 | 1 | ....
Little Falls, N. Y.| 1898 | 8,783 | 0.76 | 1 | ....
Berwyn, Penna. | 1898 | 826 | 0.52 | 3 | ....
Harrisburg, Penna. | 1899 | 1,200 | 0.12 | 2 | 0.15
Albany, N. Y. | 1899 | 94,923 | 5.60 | 8 | 11.00[54]
Rock Island, | 1899 | 13,634 | 1.20 | 3 | 3.50
Illinois | | | | |
+------+-----------+--------+--------+------------
Total | | 259,774 | 17.31 | 45 | 26.87
BRITISH COLUMBIA.
Victoria | | 16,841 | 0.82 | 3 | 1.80
SOUTH AMERICA.
Buenos Ayres | | 500,000 | 4.15 | 3 | ....
Montevidio | | | Filters reported ....
HOLLAND.
Amsterdam | | 555,821 | 10.18 | 12 | 11.20
Rotterdam | | 290,000 | 6.30 | 18 | 13.00
The Hague | | 191,000 | 2.88 | 6 | 4.20
Schiedam | | 25,300 | 1.33 | 5 | 0.68
Utrecht | | 140,000 | 0.60 | .... | 1.40
Groningen | | 57,900 | 0.59 | 2 | ....
Dordrecht | | 34,100 | 0.56 | 2 | 1.00
Leeuwarden | | 30,700 | 0.31 | 2 | ....
Vlaardingen | | .... | .... | .... | ....
Sliedrecht | | .... | .... | .... | ....
Gorinchem | | 10,000 | .... | .... | ....
Zutphen | | 18,000 | .... | .... | ....
Leyden | | 44,200 | .... | .... | ....
Enschede | | .... | .... | .... | ....
Middelburg | | 17,000 | .... | .... | ....
+------+-----------+--------+--------+------------
Total | | 1,414,021 | 22.75 | 47 | 31.48
-------------------+------+-----------+--------+--------+------------

GREAT BRITAIN.
London | | 5,030,267 | 125.00 | 120 | 200.00
Liverpool | | 790,000 | 10.92 | .... | 26.67
Dublin | | 349,000 | 5.00 | 10 | 18.00
Leeds | | 420,000 | 6.00 | 8 | 17.99
Bradford | | 436,260 | 4.62 | 6 | 13.31
Leicester | | 220,005 | 2.50 | .... | 4.75
York | | 72,083 | 2.04 | 6 | 3.00
Edinburgh | | 292,364 | 2.00 | 4 | 18.00
Darlington | | 43,000 | 1.32 | 7 | ....
Wakefield | | 36,815 | 1.25 | .... | ....
Carlisle | | 40,000 | 0.90 | .... | ....
Dumfries | | 17,821 | 0.25 | .... | ....
Accrington | | 42,000 | .... | .... | ....
Birmingham | | 680,140 | .... | .... | 19.05
Blackburn | | 130,000 | .... | .... | 4.10
Bolton | | 250,000 | .... | .... | 6.60
Chester | | 40,000 | .... | .... | ....
Halifax | | 217,000 | .... | .... | 5.18
Hereford | | 20,000 | .... | .... | ....
Middlesborough | | 187,331 | .... | .... | 11.39
Newcastle | | 320,000 | .... | .... | 14.00
Oldham | | 145,800 | .... | .... | 5.30
Oxford | | 53,000 | .... | .... | 1.59
Preston | | 113,864 | .... | .... | 4.20
Reading | | 71,558 | .... | .... | 3.00
Southampton | | 76,430 | .... | .... | 3.45
Wigan | | 60,000 | .... | .... | 1.22
Worcester | | 45,000 | .... | .... | 1.93
| +-----------+--------+--------+------------
Total | |10,199,738 | 161.80 | 161 | 382.73
GERMANY.
Hamburg | | 661,200 | 42.00 | 22 | 33.00
Berlin | | 1,746,424 | 31.45 | 55 | 36.00
Breslau | | 380,000 | 5.12 | 5 | 8.20
Magdeburg | | 217,067 | 3.76 | 11 | 5.66
Bremen | | 157,500 | 3.21 | 12 | 3.50
Altona | | 162,427 | 3.08 | 13 | 5.40
Königsberg | | 176,000 | 2.70 | 7 | 3.00
Stuttgart | | 162,516 | 2.32 | .... | 4.00
Stettin | | 145,000 | 2.26 | 9 | 3.00
Lübeck | | 70,000 | 1.40 | 6 | 4.50
Brunswick | | 100,883 | 1.48 | 4 | 2.30
Stralsund | | 30,105 | 1.11 | 6 | 0.60
Rostock | | 49,891 | 1.11 | 3 | 1.54
Lignitz | | 46,852 | 0.96 | 6 | 1.40
Posen | | 75,000 | 0.70 | 4 | 0.90
Schwerin | | 36,000 | 0.65 | 4 | 0.50
Chemnitz | | 164,743 | 0.59 | 3 | ....
Worms | | 30,000 | 0.50 | 3 | 0.64
Ratibor | | 20,729 | 0.42 | 3 |
Frankfort on Oder | | 59,161 | 0.37 | 5 | 0.89
Kiel | | 69,214 | 0.31 | | 1.50
Tilsit | | 30,000 | 0.25 | | 0.20
Brieg | | 20,154 | 0.20 | 4 |
Gluckstadt | | 6,214 | 0.14 | | 0.10
Wandsbeck | | 22,000 | 0.13 | | 0.30
| +-----------+--------+--------+------------
Total | | 4,639,080 |106.22 | 185 | 117.13

OTHER EUROPEAN FILTERS.
Warsaw | | 500,000 | 6.20 | 12 | 6.00
St. Petersburg | | 954,000 | 5.85 | 11 | 39.00
Odessa | | 380,000 | 4.75 | 5 | 8.00
Choisy le Roi and | | } 200,000 |{ 3.85 | 25 | 10.00
Neuilly sur Marne | | } |{ 2.31 | 15 |
Copenhagen | | 340,000 | 2.88 | 9 | 6.80
Stockholm | | 274,000 | 2.78 | | 7.00
Antwerp | | 240,000 | 2.10 | 8 | 2.00
Zürich | | 96,839 | 1.66 | | 7.00
Brunn | | | 1.62 | | 3.04
Constantinople, | | | 0.74 | 3 |
South side | | | | |
| +-----------+--------+--------+------------
Total | | 2,984,839 | 34.74 | 88 | 88.84

ASIA.
Blandarwada, India | | | 1.97 | 6 |
Agra, India | | | 1.37 | |
Bombay, India | | 821,000 | 1.22 | 4 |
Shanghai, China | | | 0.88 | 4 |
Hong Kong | | | 0.67 | 6 |
Yokohama, Japan | | 110,000 | 0.58 | 3 |
Calcutta, India | | 466,000 | | |
Tokyo, Japan | | | | |
Baroda, India | | | | |
Allahabad, India | | | | |
| +-----------+--------+--------+------------
Total | | 1,397,000 | 6.69 | 23 |

SUMMARY.
United States | | 259,774 | 17.31 | 45 | 26.87
British Columbia | | 16,841 | 0.82 | 3 | 1.80
South America | | 500,000 | 4.15 | 3 |
Holland | | 1,414,021 | 22.75 | 47 | 31.48
Great Britain | |10,199,738 |161.80 | 161 | 382.73
Germany | | 4,639,080 |106.22 | 185 | 117.13
Other European | | 2,984,839 | 34.74 | 88 | 88.84
countries | | | | |
Asia | | 1,397,000 | 6.69 | 23 |
| +-----------+--------+--------+------------
Total | |21,411,293 |354.48 | 555 | 648.85
-------------------+------+-----------+--------+--------+------------

LIST OF CITIES AND TOWNS USING MECHANICAL FILTERS. ARRANGED BY
POPULATIONS.

Abbreviations.--P., Pressure filters; G., Gravity filters; J., Jewell
system; N. Y., New York system; W., Warren system; C., Continental
system; Am., American system.

--------------------------+-----------+----------+-----------+
| | | Nominal |
| | | Capacity |
Place. |Population,| Filters |of Filters,|
| 1890. | First | 1899. |
| |Installed.| Million |
| | | Gallons. |
--------------------------+-----------+----------+-----------+
Denver, Col. | 108,204 | | |
Atlanta, Ga. | 65,533 | 1887 | 8 |
St. Joseph, Mo. | 52,324 | 1898 | 10.2 |
Oakland, Cal. | 48,682 | 1891 | 5 |
Kansas City, Kan. | 38,316 | 1898 | 6 |
Wilkesbarre, Pa.[55] | 37,718 | | 10 |
Norfolk, Va. | 34,871 | 1899 | 6 |
Augusta, Ga. | 33,300 | 1899 | 6 |
Quincy. Ill. | 30,494 | 1892 | 4 |
Dubuque, Iowa[56] | 30,311 | 1899 | 2 |
Terre Haute, Ind. | 30,217 | 1890 | 4 |
Elmira, N. Y. | 29,708 | 1897 | 6 |
Chattanooga, Tenn. | 29,100 | 1887 | 9 |
Davenport, Iowa | 26,872 | 1891 | 7.5 |
Little Rock, Ark. | 25,874 | 1891 | 5.5 |
Winnipeg, Mann. | 25,642 | 1887 | 1.5 |
Oshkosh, Wis. | 22,836 | 1891 | 2.4 |
Macon, Ga. | 22,746 | 1893 | 4 |
Burlington, Ia. | 22,565 | 1894 | 3.5 |
Knoxville, Tenn. | 22,535 | 1894 | 5 |
Lexington, Ky. | 21,567 | 1895 | 2 |
Kingston, N. Y. | 21,261 | 1897 | 4 |
York, Penna. | 20,793 | 1899 | 4 |
Biddeford, Maine | 20,500 | 1896 | 3 |
Newport, R. I. | 19,467 | | 4 |
Bangor, Maine | 19,103 | 1897 | 5 |
Cedar Rapids, Ia. | 18,020 | 1896 | 2.5 |
Elgin, Ill. | 17,823 | 1888 | 4.3 |
Decatur, Ill. | 16,841 | 1893 | 3 |
Belleville, Ill. | 15,361 | | 1 |
Columbia, S. C. | 15,353 | 1892 | 3 |
Keokuk, Ia. | 14,101 | 1893 | 3 |
Ottumwa, Ia. | 14,001 | 1895 | 2 |
Rock Island, Ill.[55] | 13,634 | | 2 |
Raleigh, N. C. | 12,678 | 1887 | 1 |
Shreveport, La. | 11,979 | 1889 | 1 |
New Castle, Penna | 11,600 | 1898 | 4 |
Charlotte, N. C. | 11,557 | 1896 | 1 |
Nebraska City, Neb. | 11,494 | 1891 | 0.4 |
Streator, Ill. | 11,414 | | 2 |
Hornelsville, N. Y.[58] | 10,966 | 1899 | 3 |
Augusta, Maine | 10,527 | 1887 | 0.6 |
St. Thomas, Ont. | 10,370 | 1891 | 2.5 |
Cairo, Ill. | 10,324 | 1889 | 0.8 |
Alton, Ill. | 10,294 | 1898 | 3 |
Asheville, N. C. | 10,235 | 1889 | 1 |
Greenwich, Conn. | 10,131 | 1887 | 2 |
Huntington, W. Va. | 10,108 | 1899 | 2 |
Beaver Falls, Pa. | 9,735 | | 2 |
Champaign, Ill.[57] | 9,719 | | |
Chatham, Ont. | 9,052 | 1895 | 1 |
Adrian, Mich. | 8,756 | 1899 | 1.75 |
Athens, Ga. | 8,639 | 1893 | 1 |
East Providence, R. I. | 8,422 | 1899 | 0.5 |
Winston, N. C. | 8,018 | 1895 | 0.5 |
Danville, Penna. | 7,998 | 1896 | 1 |
Clarksville, Tenn.[58] | 7,924 | 1899 | 1.5 |
Stevens Point, Wis. | 7,896 | 1889 | 0.5 |
Carlisle, Pa. | 7,620 | 1896 | 1.5 |
Calais, Me. | 7,290 | 1893 | 1.5 |
Long Branch, N. J. | 7,231 | 1888 | 3 |
Creston, Ia. | 7,200 | 1891 | 0.5 |
St. Hyacinthe, Que. | 7,016 | 1898 | 1 |
Rome, Ga.[58] | 6,957 | 1899 | 1.5 |
Westerly, R. I. | 6,813 | 1896 | 1.5 |
Merrill, Wis. | 6,809 | 1897 | 1 |
Dennison, Ohio[58] | 6,767 | 1899 | 1.25 |
Parsons, Kan. | 6,736 | 1894 | 2 |
Waterloo, Iowa | 6,674 | 1891 | 1.5 |
Somerville, N. J. | 6,417 | 1885 | 1.9 |
Athol, Mass. | 6,319 | 1888 | 1.5 |
Owego, N. Y. | 6,200 | 1887 | 1 |
Brunswick, Maine | 6,012 | 1887 | 0.6 |
Bucyrus, Ohio | 5,974 | 1887 | 0.5 |
Warren, Ohio | 5,973 | 1896 | 1.5 |
Hopkinsville, Ky. | 5,833 | 1895 | 0.5 |
Brainerd, Minn. | 5,703 | 1897 | 0.5 |
New Brighton, Pa. | 5,616 | 1889 | 0.5 |
Niagara Falls, N. Y. | 5,502 | 1896 | 4.5 |
Durham, N. C. | 5485 | 1893 | 0.9 |
Winfield, Kan. | 5184 | 1894 | 1 |
Louisiana, Mo. | 5090 | 1888 | 0.8 |
Trenton, Mo. | 5039 | 1889 | 0.4 |
Lorain, Ohio | 4863 | 1896 | 3 |
Sidney, Ohio[59] | 4850 | | |
Mexico, Mo. | 4789 | 1889 | 0.3 |
Mt. Clemens, Mich. | 4748 | 1888 | 1 |
Riverside, Cal. | 4683 | 1892 | 0.09 |
Columbus, Miss.[60] | 4559 | 1899 | 0.5 |
Winchester, Ky. | 4519 | 1894 | 0.75 |
Salisbury, N. C. | 4418 | 1889 | 0.5 |
Eufaula, Ala. | 4394 | 1897 | 0.5 |
Greenville, Tex. | 4330 | 1888 | 0.8 |
Exeter, N. H. | 4284 | 1887 | 0.114 |
Bordentown, N. J. | 4232 | 1890 | 0.5 |
Lake Forest, Ill. | 4203 | 1892 | 1 |
Henderson, N. C. | 4191 | 1899 | 0.25 |
Reading, Mass. | 4088 | 1896 | 1 |
Goldsboro, N. C. | 4017 | 1896 | 0.5 |
Rich Hill, Mo. | 4008 | 1893 | 0.5 |
Mt. Pleasant, Ia. | 3997 | 1888 | 0.5 |
Murphysboro, Ill. | 3880 | 1890 | 0.2 |
Brandon, Manitoba | 3778 | 1893 | 1 |
Danville, Ky. | 3766 | 1894 | 0.5 |
Royersford, Pa. | 3612 | 1893 | 1 |
Warsaw, Ind. | 3514 | 1896 | 0.5 |
Asbury Park, N. J. | 3500 | | 2 |
Keyport, N. J. | 3411 | 1895 | 0.5 |
Deseronto, Ont. | 3338 | 1896 | 0.5 |
Milledgeville, Ga. | 3322 | 1893 | 0.5 |
Carlinville, Ill. | 3293 | | 0.1 |
Gettysburg, Pa. | 3221 | 1894 | 0.3 |
Independence, Kan. | 3127 | 1891 | 0.75 |
LaGrange, Ga. | 3090 | 1893 | 0.25 |
Paola, Kan. | 2943 | 1887 | 0.25 |
Benwood, W. Va.[60] | 2934 | 1899 | 1 |
Gadsden, Ala. | 2901 | 1887 | 1.325 |
Lamar, Mo. | 2860 | 1891 | 0.25 |
Longueuil, Que. | 2757 | 1895 | 0.4 |
Washington, Mo. | 2725 | 1888 | 0.2 |
Renfrew, Ont. | 2611 | 1897 | 0.432 |
Oswego, Kan. | 2574 | 1893 | 0.5 |
Holden, Mo. | 2520 | 1893 | 0.2 |
Burlington, Kan. | 2239 | | 0.5 |
Council Grove, Kan. | 2211 | 1898 | 0.5 |
Wakefield, R. I.[61] | 2170 | | 0.15 |
Catonsville, Md. | 2115 | 1890 | 0.25 |
Attica, N. Y. | 1994 | 1896 | 0.4 |
Hightstown, N. J. | 1875 | 1899 | 0.25 |
No. Berwick, Me. | 1803 | 1896 | 0.3 |
Dunnville, Ont. | 1776 | 1899 | 0.5 |
Rogers Park, Ill. | 1708 | 1889 | 0.4 |
Eatonton, Ga. | 1682 | 1897 | 0.5 |
Caldwell, Kan. | 1642 | 1890 | 0.5 |
LaGrange, Tex. | 1626 | 1891 | 0.25 |
Richfield Springs, N. Y. | 1623 | 1889 | 0.35 |
Valatie, N. Y. | 1437 | 1894 | 0.15 |
Tunkhannock, Pa. | 1253 | | 0.1 |
Mechanics Falls, Me. | 1030 | 1898 | 0.72 |
New Bethlehem, Pa. | 1026 | 1899 | 0.1 |
Fairmount, W. Va. | 1023 | 1898 | 1 |
Atlantic Highlands, N. J. | 945 | | 0.3 |
Rumford Falls, Me. | 898 | 1897 | 0.5 |
Lakewood, N. J. | 730 | 1889 | 0.5 |
Veazie, Me. | 650 | 1889 | 1 |
Portersville, Cal. | 606 | 1890 | 0.151 |
Holmesburg, Pa. | | 1896 | 1 |
Pickering Creek, Pa. | | 1896 | 0.75 |
Overbrook, Penna. | | 1895 | 0.25 |
Vandergrift, Pa. | | 1897 | 0.5 |
Frazerville, P. Q.[62] | | 1899 | 0.2 |
Arnate, Pa. | | 1899 | 0.12 |
Chihuahua, Mex.[62] | | 1899 | 1 |
West Reading, Pa. | | | |
+-----------+ +-----------+
Totals | 1,565,881 | | 252 |
--------------------------+-----------+----------+-----------+
--------------------------+------------+--------+---------------------
| Average | |
|Consumption,|Area of |
Place. | Million |Filters,| Filter
| Gallons: |Sq. Ft. | System.
|Water Works | 1899. |
| Manual. | |
--------------------------+------------+--------+---------------------
Denver, Col. | | 2260 | Special.
Atlanta, Ga. | 4.54 | 2056 | N. Y. P.
St. Joseph, Mo. | | 3842 | J. G.
Oakland, Cal. | 10 | 1960 | N. Y. P.
Kansas City, Kan. | 2 | 2260 | J. G.
Wilkesbarre, Pa.[55] | | 3166 | J. G.
Norfolk, Va. | 3.5 | 2112 | J. G.
Augusta, Ga. | 3.8 | 2112 | N. Y. G.
Quincy. Ill. | 1.2 | 1582 | J. G.
Dubuque, Iowa[56] | | 880 | J. G.
Terre Haute, Ind. | 3 { | 1076 | N. Y. P.
| { | 226 | J. G.
Elmira, N. Y. | 3 | 2034 | J. G.
Chattanooga, Tenn. | | 2080 | J. & N. Y. P.
Davenport, Iowa | 3 | 2380 | Am. P.
Little Rock, Ark. | | 1544 | Am., J., & N. Y. P.
Winnipeg, Mann. | | 390 | N. Y. P.
Oshkosh, Wis. | 2.1 | 550 | W. G.
Macon, Ga. | 1.65 | 1437 | J., W., & N. Y.
Burlington, Ia. | | 1243 | J. G.
Knoxville, Tenn. | 1.93 | 1404 | W. G.
Lexington, Ky. | 1.2 | 678 | J. G.
Kingston, N. Y. | 1.5 | 1120 | N. Y. P.
York, Penna. | 2.37 | 1408 | J. G.
Biddeford, Maine | 2 | 780 | W. G.
Newport, R. I. | 2.1 | | Special.
Bangor, Maine | 3 | 1404 | W. G.
Cedar Rapids, Ia. | 2 | 905 | J. G.
Elgin, Ill. | 1 | 780 | Am. P.
Decatur, Ill. | 2 | 1008 | W. G.
Belleville, Ill. | 0.6 | 339 | J. G.
Columbia, S. C. | | 678 | J. G.
Keokuk, Ia. | | 980 | N. Y. P.
Ottumwa, Ia. | 1.2 | 678 | J. G.
Rock Island, Ill.[55] | 3.5 | 452 | J. G.
Raleigh, N. C. | 1 | 296 | N. Y. P.
Shreveport, La. | | 312 | N. Y. P.
New Castle, Penna | 2 | 1408 | N. Y. G.
Charlotte, N. C. | 0.5 | 530 | N. Y. G.
Nebraska City, Neb. | 0.7 | 116 | N. Y. P.
Streator, Ill. | 1.3 | 100 | Western & Am. P.
Hornelsville, N. Y.[58] | | 700 | N. Y. P.
Augusta, Maine | 1.6 | 100 | W.
St. Thomas, Ont. | 0.6 | 700 | N. Y. P.
Cairo, Ill. | 2.5 | 197 | N. Y. P.
Alton, Ill. | 1 | 1056 | N. Y. G.
Asheville, N. C. | 0.35 | 312 | N. Y. P.
Greenwich, Conn. | 0.4 | 592 | N. Y. P.
Huntington, W. Va. | | 704 | N. Y. G.
Beaver Falls, Pa. | 4.5 | | N. Y.
Champaign, Ill.[57] | 0.75 | | N. Y.
Chatham, Ont. | 0.4 | 280 | N. Y. P.
Adrian, Mich. | 0.45 | 565 | J. G.
Athens, Ga. | 0.45 | 420 | W. G.
East Providence, R. I. | | 176 | J. G.
Winston, N. C. | 0.3 | 156 | W. G.
Danville, Penna. | 1 | 226 | J. G.
Clarksville, Tenn.[58] | 0.5 | 704 | J. G.
Stevens Point, Wis. | 0.25 | 156 | N. Y. P.
Carlisle, Pa. | | 339 | J. G.
Calais, Me. | 0.85 | 275 | W. G.
Long Branch, N. J. | 1.3 | 904 | N. Y. P.
Creston, Ia. | 0.3 | 150 | J.
St. Hyacinthe, Que. | 0.84 | 294 | J. P.
Rome, Ga.[58] | 1.3 | 528 | J. G.
Westerly, R. I. | 0.375 | 396 | N. Y. G.
Merrill, Wis. | | 339 | J. G.
Dennison, Ohio[58] | 1 | 528 | J. G.
Parsons, Kan. | 0.6 | 452 | J. G.
Waterloo, Iowa | 0.7 | 565 | J. G.
Somerville, N. J. | 0.75 | 552 | N. Y. P.
Athol, Mass. | 0.5 | 350 | N. Y. P.
Owego, N. Y. | 0.75 | 234 | N. Y. P.
Brunswick, Maine | 0.33 | 100 | W.
Bucyrus, Ohio | 0.55 | 156 | N. Y. P.
Warren, Ohio | 1.5 | 462 | W. G.
Hopkinsville, Ky. | 0.15 | 140 | N. Y. P.
Brainerd, Minn. | | 156 | N. Y. P.
New Brighton, Pa. | | 156 | N. Y. P.
Niagara Falls, N. Y. | 2.62 | 1019 | J. G.
Durham, N. C. | 0.7 | 252 | W. G.
Winfield, Kan. | | 336 | W. G.
Louisiana, Mo. | | 242 | N. Y. P. & G.
Trenton, Mo. | | 128 | N. Y. P.
Lorain, Ohio | 1.5 | 1356 | J. G.
Sidney, Ohio[59] | 0.5 | | N. Y.
Mexico, Mo. | 0.4 | 66 | N. Y. P.
Mt. Clemens, Mich. | 0.6 | 251 | N. Y. P.
Riverside, Cal. | | 20 | N. Y. P.
Columbus, Miss.[60] | 0.175 | 176 | J. G.
Winchester, Ky. | 0.107 | 152 | J. P.
Salisbury, N. C. | 0.35 | 156 | N. Y. P.
Eufaula, Ala. | | 140 | N. Y. P.
Greenville, Tex. | 0.175 | 156 | N. Y. P.
Exeter, N. H. | 0.179 | 34 | N. Y. P.
Bordentown, N. J. | 0.5 | 156 | N. Y. P.
Lake Forest, Ill. | | 168 | J. P.
Henderson, N. C. | | 118 | W. G.
Reading, Mass. | 0.198 | 336 | W. G.
Goldsboro, N. C. | 0.1 | 156 | W. G.
Rich Hill, Mo. | 0.24 | 140 | N. Y. P.
Mt. Pleasant, Ia. | | 156 | N. Y. P.
Murphysboro, Ill. | | 60 | N. Y. P.
Brandon, Manitoba | 0.36 | 240 | N. Y. P.
Danville, Ky. | 0.1 | 140 | N. Y. P.
Royersford, Pa. | 0.08 | 226 | J. G.
Warsaw, Ind. | 0.5 | 156 | N. Y. P.
Asbury Park, N. J. | 0.5 | 670 | C.
Keyport, N. J. | 0.06 | 156 | W. G.
Deseronto, Ont. | 0.84 | 147 | J. P.
Milledgeville, Ga. | | 156 | N. Y. P.
Carlinville, Ill. | | 38 | Am. or Jackson.
Gettysburg, Pa. | 0.075 | 78 | W. G.
Independence, Kan. | 0.25 | 129 | Am. P.
LaGrange, Ga. | | 34 | N. Y. P.
Paola, Kan. | 0.45 | 66 | N. Y. P.
Benwood, W. Va.[60] | | 306 | J. G.
Gadsden, Ala. | | 430 | N. Y. P. & G.
Lamar, Mo. | | 78 | N. Y. P.
Longueuil, Que. | 0.3 | 100 | N. Y. P.
Washington, Mo. | 0.075 | 50 | N. Y. P.
Renfrew, Ont. | | 100 | N. Y. P.
Oswego, Kan. | 0.3 | 140 | N. Y. P.
Holden, Mo. | 0.05 | 100 | N. Y. P.
Burlington, Kan. | | 79 | J.
Council Grove, Kan. | 0.08 | 78 | N. Y.
Wakefield, R. I.[61] | 0.25 | | N. Y.
Catonsville, Md. | | 78 | N. Y. P.
Attica, N. Y. | | 100 | N. Y. P.
Hightstown, N. J. | 0.025 | 78 | N. Y. G.
No. Berwick, Me. | | 78 | W. G.
Dunnville, Ont. | | 140 | N. Y. P.
Rogers Park, Ill. | 0.35 | 100 | N. Y. P.
Eatonton, Ga. | | 132 | N. Y. G.
Caldwell, Kan. | | 156 | N. Y. P.
LaGrange, Tex. | | 34 | N. Y. P.
Richfield Springs, N. Y. | | 100 | N. Y. P.
Valatie, N. Y. | | 50 | N. Y.
Tunkhannock, Pa. | | | N. Y.
Mechanics Falls, Me. | | 176 | W. G.
New Bethlehem, Pa. | | 50 | J. G.
Fairmount, W. Va. | | 280 | N. Y. P.
Atlantic Highlands, N. J. | 0.109 | 130 | C.
Rumford Falls, Me. | | 113 | W. G.
Lakewood, N. J. | | 156 | N. Y. P.
Veazie, Me. | 0.1 | 176 | W. G.
Portersville, Cal. | 0.060 | 34 | N. Y. P.
Holmesburg, Pa. | 0.046 | 280 | N. Y. P.
Pickering Creek, Pa. | | 234 | W. G.
Overbrook, Penna. | | 78 | W. G.
Vandergrift, Pa. | | 156 | W. G.
Frazerville, P. Q.[62] | | 78 | N. Y. G.
Arnate, Pa. | | 50 | N. Y. G.
Chihuahua, Mex.[62] | | 612 | J. G.
West Reading, Pa. | 0.07 | | W. G.
+------------+--------+
Totals | 108 |77,806 |
--------------------------+------------+--------+---------------------

Special filters, neither sand nor mechanical: Wilmington, Del.; Pop.,
61,431; area, 10,000 sq. ft.; nominal capacity, 10 million gallons. See
Eng. News, Vol. 40, p. 146.

NOTES REGARDING SAND FILTERS IN THE UNITED STATES.

POUGHKEEPSIE, N. Y. Designed by James P. Kirkwood, built in 1872, was
the earliest of its kind in the United States. It was enlarged by the
Superintendent, Charles E. Fowler, in 1896. The walls of the original
filters were of rubble, and in course of time developed cracks and
leaked badly. The walls of the new filter are of rubble, faced with
vitrified brick. The filters treat the water of the Hudson River, which
is sewage-polluted and more or less muddy. Description: Jour. N. E.
Water Works Assoc., Vol. 12, p. 209.

HUDSON, N. Y. Designed by James P. Kirkwood, built in 1874. enlarged in
1888. The filters are open and are used for treating the Hudson River
water, which is sewage-polluted and more or less muddy. Description:
Eng. News, Vol. 31, p. 487.

ST. JOHNSBURY, VT. (E. & T. Fairbanks & Co.) These filters were built
about 30 years ago, and have been recently enlarged. The filters were
originally open, but were afterwards covered with a roof. The single
roof proved inadequate to keep them from freezing, and a second roof
was added inside and under the main roof. They are used for filtering
pond water, which is quite clear and not subject to much pollution. The
water supply is one of two, the other is the town supply and is taken
from the Passumpsic River. No published description.

NANTUCKET, MASS. Designed by J. B. Rider, built in 1892. This filter
is used to remove organisms from the reservoir water supply. It is
only used when the organisms are troublesome, and is satisfactory in
preventing the tastes and odors which formerly resulted from their
presence. Description: Jour. N. E. Water Works Assoc., Vol. 8, p. 171;
Eng. News, Vol. 31, p. 336.

LAWRENCE, MASS. Designed by Hiram F. Mills, built in 1892-3, and put
in operation September, 1893. It is used for treating the water of the
Merrimac River, which contains a large amount of sewage. Description:
Report of the Mass. State Board of Health, 1893, p. 543; Jour. N. E.
Water Works Assoc., Vol. 9, p. 44; Eng. News, Vol. 30, p. 97.

ILION, N. Y. Designed by the Stanwix Engineering Company and are used
for treating reservoir water, which is generally clear and not subject
to pollution. Description: Eng. News, Vol. 31, p. 466.

MOUNT VERNON, N. Y. (New York Suburban Water Company.) Designed by
J. N. Chester, built in 1894. These filters are similar in general
construction to the Lawrence filter, although the dimensions both
vertical and horizontal are reduced, and the area is divided into three
parts. The filters are used for treating reservoir water, which is
generally quite clear, but which is polluted by a considerable amount
of sewage. Since the use of filters the reduction in the typhoid fever
death-rate has been very great. Description: Eng. News, Vol. 32, p. 155.

MILFORD, MASS. Designed by F. L. Northrop. This filter is very simple
in construction, and is used for filtering Charles River water as an
auxiliary supply. Description: Jour. N. E. Water Works Assoc., Vol. 10,
p. 262.

GRAND FORKS, N. D. Designed by W. S. Russell. These filters are covered
with roofs. They treat the water from the Red River, which is very
muddy, and also sewage-polluted, and which formerly caused typhoid
fever. Description: Eng. News, Vol. 33, p. 341.

ASHLAND, WIS. Designed by William Wheeler, built in 1895. The Ashland
filters were the first vaulted masonry filters to be constructed in
the United States, and are used for treating the bay water, which
is polluted with sewage, and is at times muddy from the river water
discharging into the bay near the intake. The filters are below the bay
level, and receive water from it by gravity. Description: Jour. N. E.
Water Works Assoc., Vol. 11, p. 301; Eng. News, Vol. 38, p. 338.

LAMBERTVILLE, N. J. Designed by Churchill Hungerford, and built in
1896. These are open filters with earth embankments, for filtration of
reservoir water. Description: Eng. News, Vol. 36, p. 4.

FAR ROCKAWAY, L. I. (Queens County Water Company.) Designed by Charles
R. Bettes, Engineer in Charge; Charles B. Brush & Co., Chief Engineers;
and Allen Hazen, Consulting Engineer. Constructed in 1896. These
masonry filters were used for the removal of iron from well waters.
They are also designed to be suitable for the filtration of certain
brook waters which are available as auxiliary supplies, but the brook
water has been but rarely used. Description: Eng. Record, Vol. 40, p.
412.

RED BANK, N. J. (Rumson Improvement Company.) Designed by Allen Hazen,
built in 1897. They are similar in construction to the Far Rockaway
filters, and are used for iron removal only. Description: Eng. Record,
Vol. 40, p. 412.

HAMILTON, N. Y. Designed by the Stanwix Engineering Company, and were
built in 1895 to filter lake water. Description: Eng. News, Vol. 39, p.
254.

LITTLE FALLS, N. Y. Designed by Stephen E. Babcock. These filters are
open, and were built in 1898, and are used for filtering river water.
Description: Eng. Record, Vol. 38, p. 7.

SOMERSWORTH, N. H. Designed by William Wheeler. These were the second
vaulted filters to be built in the United States. The supply is from
the Salmon Falls River and flows to the filters by gravity, the filters
being below the river level. Description: Eng. News, Vol 40, p. 358;
Eng. Record, Vol. 38, p. 270.

BERWYN, PENNA. Designed by J. W. Ledoux. These open filters are used
for filtering creek water. Description: Eng. News, Vol. 41, p. 150.

HARRISBURG, PENNA. (State Lunatic Hospital.) Designed by Allen Hazen;
open masonry filters, used for treating the water from a small creek
which is often muddy and is subject to pollution. No published
description.

ALBANY, N. Y. Designed by Allen Hazen. Constructed 1898-99. This
was the third and is the largest vaulted masonry filter plant yet
constructed in the United States. It is used for filtering the Hudson
River water, which is slightly muddy and much polluted by sewage.
Description: Eng. News, Vol. 39, p. 91; Vol. 40, p. 254.

ROCK ISLAND, ILL. Designed by Jacob A. Harman. Open filters with
embankments, used for filtering the Mississippi River water, which is
very muddy and also polluted by sewage. No published description.

* * * * *

CAPACITY OF FILTERS.

Estimating the total additional area of sand filters for which figures
are not available at 100 acres, and the maximum capacity of sand
filters at three million gallons per acre daily, and of mechanical
filters at three million gallons per thousand square feet of filtering
area, the total filtering capacity of all the filters in the world used
for public water supplies in 1899 is nearly 1600 million gallons daily,
of which 15 per cent is represented by mechanical filters and 85 per
cent by sand filters. In the United States, including Wilmington, the
total filtering capacity is nearly 300 million gallons daily, of which
18 per cent is represented by sand filters, 79 per cent by mechanical
filters, and 3 per cent by a special type of filters.

APPENDIX V.

LONDON’S WATER-SUPPLY.

London alone among great capitals is supplied with water by private
companies. They are, however, under government supervision, and
the rates charged for water are regulated by law. There are eight
companies, each of which supplies its own separate district, so that
there is no competition whatever. One of the companies supplying
460,000 people uses only ground-water drawn from deep wells in the
chalk, but the other seven companies depend mainly upon the rivers
Thames and Lea for their water. All water so drawn is filtered, and
must be satisfactory to the water examiner, who is required to inspect
the water supplied by each company at frequent intervals, and the
results of the examinations are published each month.

In 1893 the average daily supply was 235,000,000 gallons, of which
about 40,000,000 were drawn from the chalk, 125,000,000 from the
Thames, and 70,000,000 from the Lea. Formerly some of the water
companies drew water from the Thames within the city where it was
grossly polluted, and the plagues and cholera which formerly ravaged
London were in part due to this fact. These intakes were abandoned
many years ago, and all the companies now draw their water from points
outside of the city and its immediate suburbs.

The area of the watershed of the Thames above the intakes of the water
companies is 3548 square miles, and the population living upon it in
1891 was 1,056,415. The Thames Conservancy Board has control of the
main river for its whole length, and of all tributaries within ten
miles in a straight line of the main river, but has no jurisdiction
over the more remote feeders. The area drained is essentially
agricultural, with but little manufacturing, and there are but few
large towns. In the area coming under the conservators there are but
six towns with populations above 10,000 and an aggregate population
of 170,000, and there are but two or three other large towns on the
remaining area more than ten miles from the river. These principal
towns are as follows:

Town. Population 1891. Distance above
Water Intakes.
Reading 60,054 49 miles
Oxford 45,791 87 miles
New Swindon 27,295 116 miles
High Wycomb 13,435 33 miles
Windsor 12,327 18 miles
Maidenhead 10,607 25 miles
Guildford 14,319 20 miles

Guildford is outside of the conservators’ area. All of the above towns
treat their sewage by irrigation.

Among the places that are regarded as the most dangerous are Chertsey
and Staines, with populations of 9215 and 5060, only 8 and 11 miles
above the intakes respectively. These towns are only partially sewered
and still depend mainly on cesspools. An attempt is made to treat the
little sewage which they produce upon land, but the work has not as yet
been systematically carried out. There are also several small towns of
3000 inhabitants or less upon the upper river which do not treat their
sewage so far as they have any, but, owing to their great distance,
the danger from them is much less than from Chertsey and Staines.
Twenty-one of the principal towns upon the watershed have sewage farms,
and there are no chemical precipitation plants now in use.

Boats upon the river are not allowed to drain into it, but are
compelled to provide receptacles for their sewage, and facilities
are provided for removing and disposing of it; and as an additional
precaution no boat is allowed to anchor within five miles of the
intakes.

The conservators of the river Lea have control of its entire drainage
area, which is about 460 square miles, measured from the East London
water intakes, and has a population of 189,287. On this watershed there
is but a single town with more than 10,000 inhabitants, this being
Lutton near the headwaters of the river, with a population of 30,005.
The sewage from Lutton and from seventeen smaller places is treated
upon land. No crude sewage is known to be ordinarily discharged into
the river. At Hereford, eleven miles above the East London intakes,
there is a chemical precipitation plant. The conservators do not
regard this treatment as satisfactory, and have recently conducted
an expensive lawsuit against the local authorities to compel them to
further treat their effluent. The suit was lost, the court holding that
no actual injury to health had been shown. It is especially interesting
to note that of the thirty-nine places on the Thames and the Lea giving
their sewage systematic treatment there is but a single place using
chemical precipitation, and there it is not considered satisfactory.
Formerly quite a number of these towns used other processes than land
treatment, but in every case but Hereford land treatment has been
substituted.

In regard to the efficiency of the sewage farms, it is believed that
in ordinary weather the whole of the sewage percolates through the
land, and the inspectors of the Conservancy Boards strongly object to
its being allowed to pass over the surface into the streams. The land,
however, is for the most part impervious, as compared to Massachusetts
and German sewage farms, and in times of heavy storms the land often
has all the water it can take without receiving even the ordinary flow
of sewage, and much less the increased storm-flow. At such times the
sewage either does go over the surface, or perhaps more frequently
is discharged directly into the rivers without even a pretence of
treatment. The conservators apparently regard this as an unavoidable
evil and do not vigorously oppose it. It is the theory that, owing
to the increased dilution with the storm-flows, the matter is
comparatively harmless, although it would seem that the reduced time
required for it to reach the water-works intakes might largely offset
the effect of increased dilution.

The water companies have large storage and sedimentation basins with
an aggregate capacity equal to nine days’ supply, but the proportion
varies widely with the different companies. It is desired that the
water held in reserve shall be alone used while the river is in flood,
as, owing to its increased pollution, it is regarded as far more
dangerous than the water at other times; but as no record is kept of
the times when raw sewage is discharged, and no exact information is
available in regard to the times when the companies do not take in
raw water, it can safely be assumed that a considerable amount of raw
sewage does become mixed with the water which is drawn by the companies.

The water drawn from the river is filtered through 113 filters having
an area of 116 acres. None of the filters are covered, and with an
average January temperature of 39° but little trouble with ice is
experienced. A few new filters are provided with appliances for
regulating the rate on each filter separately and securing regular and
determined rates of filtration, but nearly all of the filters are of
the simple type described on page 48, and the rates of filtration are
subject to more or less violent fluctuation, the extent of which cannot
be determined.

The area of filters is being continually increased to meet increasing
consumption; the approximate areas of filters in use having been as
follows:

1839 First filters built
1855 37 acres
1866 47 acres
1876 77 acres
1886 104 acres
1894 116 acres

There has been a tendency to reduce somewhat the rate of filtration. In
1868, with 51 acres of filters, the average daily quantity of water
filtered was 111,000,000 gallons, or 2,180,000 gallons per acre. In
1884, with 97 acres of filter surface, the daily quantity filtered was
157,000,000 gallons, or 1,620,000 gallons per acre; and in 1893, with
116 acres of filter surface and 195,000,000 gallons daily, the yield
per acre was 1,680,000 gallons.

Owing to the area of filter surface out of use while being cleaned,
the variations in consumption of water, and the imperfections of the
regulating apparatus, the actual rates of filtration are often very
much higher and at times may easily be double the figures given.

Evidence regarding the healthfulness of the filtered river-water was
collected and examined in a most exhaustive manner in 1893 by a Royal
Commission appointed to consider the water-supply of the metropolis in
all its aspects with reference to future needs. This commission was
unable to obtain any evidence whatever that the water as then supplied
was unhealthy or likely to become so, and they report that the rivers
can safely be depended upon for many years to come.

The numbers of deaths from all causes and from typhoid fever annually
per million of inhabitants for the years 1885-1891 in the populations
receiving their waters from different sources in London were as follows:

Water used. Deaths from Deaths from
All Causes. Typhoid Fever.
Filtered Thames water only 19,501 125
Filtered Lea water only 21,334 167
Kent wells only 18,001 123
Thames and Lea jointly 18,945 138
Thames and Kent jointly 18,577 133

The population supplied exclusively from the Lea by the East London
Company is of a poorer class than that of the rest of London, and this
may account for the slightly higher death-rate in this section. Aside
from this the rate is remarkably uniform and shows no great difference
between the section drinking ground-water only and those drinking
filtered river-waters. The death-rate from typhoid fever is also very
uniform and, although higher than that of some Continental cities with
excellent water-supplies (Berlin, Vienna, Munich, Dresden), is very
low—lower than in any American city of which I have records.

In this connection, it was shown by the Registrar-General that there
is only a very small amount of typhoid fever on the watersheds of
the Thames and Lea, so that the danger of infection of the water as
distinct from pollution is less than would otherwise be the case. Thus
for the seven years above mentioned the numbers of deaths from typhoid
fever per million of population were only 105 and 120 on the watersheds
of the Thames and the Lea respectively, as against 176 for the whole of
England and Wales.

LONDON FILTERS, 1896.

Twenty-sixth Annual Report of the Local Government Board, pages 206-213.

--------------+-------+---------+-----------------+--------------------------
|Amount |Average | Average Rate | Bacterial Efficiency.
| of |Thickness| of Filtration. |
|Storage| of +--------+--------+--------+--------+--------
| Raw | Sand, |Imperial|Millions|Maximum.|Minimum.|Average.
Company. |Water, | Feet. |Gallons | U. S. | | |
| Days. | | per |Gallons | | |
| | | Square |per Acre| | |
| | |Foot per| Daily. | | |
| | | Hour. | | | |
--------------+-------+---------+--------+--------+--------+--------+--------
Chelsea | 12.0 | 4.0 | 1.75 | 2.19 | 99.92 | 99.62 | 99.86
West Middlesex| 5.6 | 2.75 | 1.25 | 1.56 | 99.94 | 91.48 | 99.79
Southwark & | | | | | | |
Vauxhall | 4.1 | 2.5 | 1.5 | 1.88 | 100.00 | 84.33 | 97.77
Grand Junction| 3.3 | 2.25 | 1.63 | 2.05 | 99.98 | 84.03 | 99.31
Lambeth | 6.0 | 2.8 | 2.08 | 2.60 | 99.97 | 96.45 | 99.81
New River | 2.2 | 4.4 | 1.89 | 2.37 | 100.00 | 77.14 | 99.07
East London | 15.0 | 2.0 | 1.33 | 1.67 | 99.93 | 97.03 | 99.56
--------------+-------+---------+--------+--------+--------+--------+--------

APPENDIX VI.

THE BERLIN WATER-WORKS.

The original works were built by an English company in 1856, and were
sold to the city in 1873 for $7,200,000.

The water was taken from the river Spree at the Stralau Gate, which
was then above, but is now surrounded by, the growing city. The water
was always filtered, and the original filters remained in use until
1893, when they were supplanted by the new works at Lake Müggel. Soon
after acquiring the works the city introduced water from wells by Lake
Tegel as a supplementary supply, but much trouble was experienced from
crenothrix, an organism growing in ground-waters containing iron, and
in 1883 this supply was replaced by filtered water from Lake Tegel.
With rapidly-increasing pollution of the Spree at Stralau the purity
of this source was questioned, and in 1893 it was abandoned (although
still held as a reserve in case of urgent necessity), the supply now
being taken from the river ten miles higher up, at Müggel.

The watershed of the Spree above Stralau, as I found by map
measurement, is about 3800 square miles; the average rainfall is about
25 inches yearly. At extreme low water the river discharges 457 cubic
feet per second, or 295 million gallons daily, and when in flood 5700
cubic feet per second may be discharged. The city is allowed by law to
take 46 million gallons daily for water-supply, and this quantity can
be drawn either at Stralau or at Müggel.

Above Stralau the river is polluted by numerous manufactories and
washing establishments, and by the effluent from a considerable part
of the city’s extensive sewage farms. The shipping on this part of the
river also is heavy, and sewage from the boats is discharged directly
into the river. The average number of bacteria in the Spree at this
point is something over ten thousand per cubic centimeter, and 99.6 per
cent of them were removed by the filters in 1893.

The watershed of the Spree above the new water-works at Müggel I found
by map measurement to be 2800 square miles, and the low water-discharge
is said to be 269 million gallons daily. The river at this point flows
through Lake Müggel, which forms a natural sedimentation-basin, and the
raw water is quite clear except in windy weather.

There were 16 towns on the watershed with populations above 2000 each
in 1890, and an aggregate population of 132,000, which does not include
the population of the smaller places or country districts. None of
these places purify their sewage so far as they have any. Fürstenwalde
with a population of 12,935, and 22 miles above Müggel, has surface
sewers discharging directly into the river. Above Fürstenwalde the
river runs through numerous lakes which probably remove the effect
of the pollution from the more distant cities. There is considerable
shipping on the river for some miles above Fürstenwalde (which forms a
section of the Friedrich Wilhelm Canal), but hardly any between Müggel
and Fürstenwalde. The raw water at Müggel contains two or three hundred
bacteria per cubic centimeter, and is thus a comparatively pure water
before filtration. It is slightly peaty and the filtered water has a
light straw color.

Lake Tegel, which supplies the other part of the city’s supply, is an
enlargement of the river Havel. The watershed above Tegel I find to be
about 1350 square miles, and the annual rainfall is about 22 inches.
The low water-discharge is said to be 182 million gallons daily, and
the city is allowed by law to take 23 million gallons for water-supply.

There were ten towns upon the watershed with populations above 2000
each in 1890, and with an aggregate population of 44,000. Of these
Tegel is directly upon the lake with a population of 3000, and
Oranienburg, 14 miles above, has a population of 6000 and is rapidly
increasing. The shipping on the lake and river is heavy. The lake water
ordinarily contains two or three hundred bacteria per cubic centimeter.
The lake is shallow and becomes turbid in windy weather.

There are 21 filter-beds at Tegel with a combined area of 12.40 acres
to furnish a maximum of 23 million gallons of water daily, and 22
filters at Müggel with a combined area of 12.7 acres to deliver the
same quantity. Twenty-two more filters will be built at Müggel within
a few years to purify the full quantity which can be taken from the
river. All of these filters are covered with brick arches supported by
pillars about 16 feet apart from centre to centre in each direction,
and the whole is covered by nearly 3 feet of earth, making them quite
frost-proof. The original filters at Stralau were open, but much
difficulty was experienced with them in winter.

The bottom of the filters at Tegel consists of 8 inches of concrete
above 20 inches of packed clay and with 2 inches of cement above, and
slopes slightly from each side to the centre. The central drain goes
the whole length of the filters and has a uniform cross-section of
about 1/7300 of the area of the whole bed. There are no lateral drains,
but the water is brought to the central drain by a twelve-inch layer
of stones as large as a man’s fist; above this there is another foot
of gravel of graded sizes supporting two feet of fine sand, which is
reduced by scraping to half its thickness before the sand is replaced.
The average depth of water above the sand is nearly 5 feet. The filters
are not allowed to filter at a rate above 2.57 million gallons per
acre daily, and at this rate with 70 per cent of the area in service
the whole legal quantity of water can be filtered. The filters work
at precisely the same rate day and night, and the filtered water
is continuously pumped as filtered to ample storage reservoirs at
Charlottenburg. The pumps which lift the water from the lake to the
filters work against a head of 14 feet. The apparatus for regulating
the rate of filtration was described on page 51.

As yet no full description of the Müggel works has been published, but
they resemble closely the Tegel works. Both were designed by or under
the direction of the late director of the water-works, Mr. Henry Gill.

The average daily quantity of water supplied for the fiscal year ending
March 31, 1893, was 29,000,000 gallons daily, which estimate allows
10 percent for the slip of the pumps. Of this quantity 9,650,000 was
furnished by Stralau and 19,350,000 by Tegel. The greatest consumption
in a single day was 43,300,000 gallons, or 26.6 gallons per head,
while the average quantity for the year was 18.4 gallons per head. All
water without exception is sold by meter, the prices ranging from 27.2
cents a thousand gallons for small consumers to 13.6 cents for large
consumers and manufacturers. The average receipts for all water pumped,
including that used for public purposes and not paid for, were 15.4
cents a thousand gallons, against the cost of production, 9.8 cents,
which covers operating expenses, interest on capital, and provision for
sinking fund. This leaves a handsome net profit to the city. On account
of the comparatively high price of the city water and the ease with
which well-water is obtained, the latter is almost exclusively used
for running engines, manufacturing purposes, etc., and this in part
explains the very low per-capita consumption.

The volume of sewage, however, for the same year, including rain-water,
except during heavy showers, was only 29 gallons per head, showing even
with the private water-supplies an extraordinarily low consumption.

The friction of the water in the 4.75 miles of 3-foot pipe between
Tegel and the reservoir at Charlottenburg presents an interesting
point. When well-water with crenothrix was pumped, the friction rose
to 34.5 feet, when the velocity was 2.46 feet per second. According to
Herr Anklamm, who had charge of the works at the time, the friction was
reduced to 19.7 feet when filtered water was used and after the pipe
had been flushed, and this has not increased with continued use. He
calculated the friction for the velocity according to Darcy 15.0 feet,
Lampe 17.8 feet, Weisbach 18.7 feet, and Prony 21.5 feet.

APPENDIX VII.

ALTONA WATER-WORKS.

The Altona water-works are specially interesting as an example of
a water drawn from a source polluted to a most unusual extent: the
sewage from cities with a population of 770,000, including its own, is
discharged into the river Elbe within ten miles above the intake and
upon the same side.

The area of the watershed of the Elbe above Altona is about 52,000
square miles, and the average rainfall is estimated to be about
28 inches, varying from 24 or less near its mouth to much higher
quantities in the mountains far to the south. On this watershed there
are 46 cities, which in 1890 had populations of over 20,000 each,
and in addition there is a permanent population upon the river-boats
estimated at 20,000, making in all 5,894,000 inhabitants, without
including either country districts or the numberless cities with less
than 20,000 inhabitants each. The sewage from about 1,700,000 of these
people is purified before being discharged; and assuming that as many
people living in cities smaller than 20,000 are connected with sewers
as live in larger places without being so connected, the sewage of
over four million people is discharged untreated into the Elbe and its
tributaries.

The more important of these sources of pollution are the following:

City Population On what Approximate
in 1890. River. Distance, Miles.
Shipping 20,000
Altona 143,353 Elbe 6
Hamburg 570,534 Elbe 7
Wandsbeck 20,586 Elbe 8
Harburg 35,101 Elbe 11
Magdeburg 202,325 Elbe 185
Dresden 276,085 Elbe 354
Berlin and suburbs 1,787,859 Havel 243
Halle 101,401 Saale 272
Leipzig 355,485 Elster 305
Chemnitz 138,955 Mulde 340
Prague 310,483 Moldau 500

The sewage of Berlin and of most of its suburbs is treated before being
discharged, and in addition the Havel flows through a series of lakes
below the city, allowing better opportunities for natural purification
than in the case of any of the other cities. Halle treats less than a
tenth of its sewage. Magdeburg will treat its sewage in the course of
a few years. Leipzig, Chemnitz, and other places are thinking more or
less seriously of purification.

The number of bacteria in the raw water at Altona fluctuates with the
tide and is extremely variable; numbers of 50,000 and 100,000 are not
infrequent, but 10,000 to 40,000 is perhaps about the usual range.

The works were originally built by an English company in 1860, and have
since been greatly extended. They were bought by the city some years
ago. The water is pumped directly from the river to a settling-basin
upon a hill 280 feet above the river. From this it flows by gravity
through the filters to the slightly lower pure-water reservoir and
to the city without further pumping. The filters are open, with
nearly vertical masonry walls, as described in Kirkwood’s report. The
cross-section of the main underdrain is 1/2800 of the area of the beds.

Considerable trouble has been experienced from frost. With continued
cold weather it is extremely difficult to satisfactorily scrape the
filters, and very irregular rates of filtration may result at such
times. In the last few years, with systematic bacterial investigation,
it has been found that greatly decreased efficiency frequently follows
continued cold weather, and the mild epidemics of typhoid fever
from which the city has long suffered have generally occurred after
these times. Thus a light epidemic of typhoid in 1886 came in March,
following a light epidemic in Hamburg. In 1887 a severe epidemic in
February followed a severe epidemic in Hamburg in December and January.
In 1888 a severe epidemic in March followed an epidemic in Hamburg
lasting from November to January. Hamburg’s epidemic of 1889, coming in
warm weather, September and October, was followed by only a very slight
increase in Altona. In 1891 Altona suffered again in February from a
severe epidemic, although very little typhoid had been in Hamburg. A
less severe outbreak also came in February, 1892, and a still slighter
one in February, 1893. In the ten years 1882-1892, of five well-marked
epidemics, three broke out in February and two in March, while two
smaller outbreaks came in December and January. No important outbreak
has ever occurred in summer or in the fall months, when typhoid
is usually most prevalent, thus showing clearly the bad effect of
frost upon open filters (see Appendix II). With steadily increasing
consumption the sedimentation-basin capacity of late years has become
insufficient as well as the filtering area, and it is not unlikely that
with better conditions a much better result could be obtained in winter
even with open filters.[63]

The brilliant achievement of the Altona filters was in the summer of
1892, when they protected the city from the cholera which

so ravaged Hamburg, although the raw water at Altona must have
contained a vastly greater quantity of infectious matter than that
which worked such havoc in Hamburg.

From these records it appears that for about nine months of the year
the Altona filters protect the city from the impurities of the Elbe
water, but that during cold weather, with continued mean temperatures
below the freezing-point, such protection is not completely afforded,
and bad effects have occasionally resulted. Notwithstanding the recent
construction of open filters in Hamburg it appears to me that there
must always be more or less danger from open filters in such a climate.
Hamburg’s danger, however, will be much less than Altona’s on account
of its better intake above the outlets of the sewers of Hamburg and
Altona, which are the most important points of pollution at Altona.

APPENDIX VIII.

HAMBURG WATER-WORKS.

The source and quality of the water previously supplied has been
sufficiently indicated in Appendix II. It was originally intended to
filter the water, but the construction of filters was postponed from
time to time until the fall of 1890, when the project was seriously
taken up, and work was commenced in the spring of 1891. Three years
were allowed for construction. In 1892, however, the epidemic of
cholera came, killing 8605 residents and doing incalculable damage to
the business interests of the city. The health authorities found that
the principal cause of this epidemic was the polluted water-supply.
To prevent a possible recurrence of cholera in 1893, the work of
construction of the filters was pressed forward much more rapidly than
had been intended. Electric lights were provided to allow the work to
proceed nights as well as days, and as a result the plant was put in
operation May 27, 1893, a full year before the intended time. Owing to
the forced construction the cost was materially increased.

The new works take the raw water from a point one and a half miles
farther up-stream, where it is believed the tide can never carry the
city’s own sewage, as it did frequently to the old intake. The water
is pumped from the river to settling-basins against heads varying with
tide and the water-level in the basins from 8 to 22 feet. Each of the
four settling-basins has an area of about 10 acres, and, with the water
6.56 feet deep, holds 20,500,000 gallons, or 82,000,000 gallons in
all. The works are intended to supply a maximum of 48,000,000 gallons
daily, but the present average consumption is only about 35,000,000
gallons (1892), or 59 gallons per head for 600,000 population.
This consumption is regarded as excessive, and it is hoped that it
will be reduced materially by the more general use of meters. The
sedimentation-basins are surrounded by earthen embankments with slopes
of 1:3, the inner sides being paved with brick above a clay layer. The
water flows by gravity from these basins to the filters, a distance
of 1-1/2 miles, through a conduit 8-1/2 feet in diameter. The flow of
the water out of the basins and from the lower end of the conduit is
regulated by automatic gates connected with floats, shown by Fig. 11,
page 60.

The filters are 18 in number, and each has an effective area of 1.89,
or 34 acres in all. They are planned to filter at a rate of 1.60
million gallons per acre daily, which with 16 filters in use gives a
daily quantity of 48,000,000 gallons as the present limit of the works.
The sides of the filters are embankments with 1:2 slopes. Both sides
and bottoms have 20 inches of packed clay, above which are 4 inches of
puddle, supporting a brick pavement laid in cement. The bricks are laid
flat on the bottom, but edge-wise on the sides where they will come in
contact with ice.

The main effluent-drain has a cross-section for the whole length of
the filter of 4.73 square feet, or 1/17000 of the area of the filter;
and even at the low rate of filtration proposed, the velocity in the
drain will reach 0.97 foot. The drain has brick sides, 1.80 feet
high, covered with granite slabs. The lateral drains are all of brick
with numerous large openings for admission of water. They are not
ventilated, and I am unable to learn that any bad results follow this
omission.

The filling of the filters consists of 2 feet of gravel, the top being
of course finer than the bottom layers, above which are 40 inches of
sand, which are to be reduced to 24 inches by scraping before being
refilled. The water over the sand, when the latter is of full depth,
is 43 inches deep, and will be increased to 59 inches with the minimum
sand-thickness. The apparatus for regulating the rate of filtration was
described page 52. The cost of the entire plant, including 34 acres
effective filter-surface, 40 acres of sedimentation-basins, over 2
miles of 8-1/2-foot conduit, pumping-machinery, sand-washing apparatus,
laboratory, etc., was about 9,500,000 marks, or $2,280,000. This all
reckoned on the effective filter area is $67,000 per acre, or $3.80 per
head for a population of 600,000.

The death-rate since the introduction of filtered water has been lower
than ever before in the history of the city, but as it is thought that
other conditions may help to this result, no conclusions are as yet
drawn.

DEATHS IN HAMBURG FROM ALL CAUSES, AND FROM TYPHOID FEVER, BEFORE AND
AFTER THE INTRODUCTION OF FILTERS.

--------------------+-----------+-----------+--------------
|Deaths from|Deaths from|
|all Causes | Typhoid |
Year. | per 1000 | Fever per |
| Living. | 100,000 |
| | Living. |
--------------------+-----------+-----------+--------------
1880 | 24.9 | 26 |
1881 | 24.1 | 30 |
1882 | 23.7 | 27 |
1883 | 25.2 | 25 |
1884 | 25.1 | 26 |
1885 | 25.3 | 42 |
1886 | 29.0 | 71 |
1887 | 26.6 | 88 |
1888 | 24.5 | 54 |
1889 | 23.5 | 43 |
1890 | 22.0 | 27 |
1891 | 23.4 | 24 |
1892 | 41.1 | 34 |Cholera year.
1893 | 20.2 | 18 |Filtered water
| | | from May 28.
1894 | 17.9 | 7 |
1895 | 19.0 | 11 |
1896 | 17.3 | 6 |
1897 | 17.0 | 7 |
1898 | 17.5 | 5 |
Average for 5 years,| | |
excluding cholera | | |
year, before | | |
filtration, | | |
1887 to 1891 | 24.0 | 47.2 |
Average for 5 years | | |
with filtration, | | |
1894 to 1898 | 17.7 | 7.2 |
--------------------+-----------+-----------+--------------

APPENDIX IX.

NOTES ON SOME OTHER EUROPEAN WATER-SUPPLIES.

=Amsterdam.=—The water is derived from open canals in the dunes. These
canals have an aggregate length of about 15 miles, and drain about 6200
acres. The water, as it enters the canals from the fine dune-sand,
contains iron, but this is oxidized and deposited in the canals. The
water after collection is filtered. It has been suggested that by using
covered drains instead of open canals for collecting the water, the
filtration would be unnecessary; but, on the other hand, the cost of
building and maintaining covered drains in the very fine sand would be
much greater than that of the canals, and it is believed, also, that
the water so collected would contain iron, the removal of which might
prove as expensive as the present filtration. In 1887 filters were
built to take water from the river Vecht, but the city has refused to
allow the English company which owns the water-works to sell this water
for domestic purposes, and it is only used for public and manufacturing
purposes, only a fraction of the available supply being required.
Leyden, the Hague, and some other Dutch cities have supplies like the
dune supply of Amsterdam, and they are invariably filtered.

=Antwerp= is also supplied by an English company. The raw water is
drawn from a small tidal river, which at times is polluted by the
sewage of Brussels. It is treated by metallic iron in Anderson revolver
purifiers, and is afterward filtered at a rather low average rate. The
hygienic results are closely watched by the city authorities, and are
said to be satisfactory.

=Rotterdam.=—The raw water is drawn from the Maas, as the Dutch
call the main stream of the Rhine after it crosses their border. The
population upon the river and its tributaries in Switzerland, Germany,
Holland, France, and Belgium is very great; but the flow is also great,
and the low water flow is exceptionally large in proportion to the
average flow, on account of the melting snow in summer in Switzerland,
where it has its origin.

The original filters had wooden under-drains, and there was constant
trouble with crenothrix until the filters were reconstructed without
wood, since which time there has been no farther trouble. The present
filters are large and well managed. There is ample preliminary
sedimentation.

=Schiedam.=—The filters at Schiedam are comparatively small, but are
of unusual interest on account of the way in which they are operated.
The intake is from the Maas just below Rotterdam. The city was unable
to raise the money to seek a more distant source of supply, and the
engineer, H. P. N. Halbertsma, was unwilling to recommend a supply
from so doubtful a source without more thorough treatment than simple
sand-filtration was then thought to be. The plan adopted is to filter
the supply after preliminary sedimentation through two filters of 0.265
acre each, and the resulting effluent is then passed through three
other filters of the same size. River sand is used for the first, and
the very fine dune sand for the second filtration. The cost both of
construction and operation was satisfactory to the city, and much below
that of any other available source; and the hygienic results have been
equally satisfactory, notwithstanding the unfavorable position of the
intake.

=Magdeburg.=—The supply is drawn from the Elbe, and is filtered through
vaulted filters after preliminary sedimentation. The pollution of
the river is considerable, although less than at Altona or even at
Hamburg. The city has been troubled at times by enormous discharges of
salt solution from salt-works farther up, which at extreme low water
have sometimes rendered the whole river brackish and unpleasant to the
taste; but arrangements have now been made which, it is hoped, will
prevent the recurrence of this trouble.

=Breslau= is supplied with filtered water from the river Oder,
which has a watershed of 8200 square miles above the intake, and is
polluted by the sewage from cities with an aggregate population of
about 200,000, some of which are in Galicia, where cholera is often
prevalent. In recent years the city has been free from cholera, and
from more than a very limited number of typhoid-fever cases; but the
pollution is so great as to cause some anxiety, notwithstanding the
favorable record of the filters, and there is talk of the desirability
of securing another supply. Until 1893 there were four filter-beds,
with areas of 1.03 acres each, and not covered. In 1893 a fifth bed was
added. This is covered by vaulting and is divided into four sections,
which are separately operated, so that it is really four beds of 0.25
acre each. The vaulting is concrete arches, supported by steel I beams
in one direction.

=Budapest.=—A great variety of temporary water-supplies have at
different times been used by this rapidly growing city. The filters
which for some years have supplied a portion of the supply have not
been altogether satisfactory; but perhaps this was due to lack of
preliminary sedimentation for the extremely turbid Danube water, and
also to inadequate filter-area. The city is rapidly building and
extending works for a supply of ground-water, and in 1894 the filters
were only used as was necessary to supplement this supply, and it was
hoped that enough well-water would be obtained to allow the filters to
be abandoned in the near future. The Danube above the intake receives
the sewage of Vienna and innumerable smaller cities, but the volume of
the river is very great compared to other European streams, so that the
relative pollution is not so great as in many other places.

=Zürich.=—The raw water is drawn by the city from the Lake of Zürich
near its outlet, and but a few hundred feet from the heart of the city.
Although no public sewers discharge into the lake, there is some
pollution from boats and bathers and other sources, and, judging by
the number of bacteria in the raw water, this pollution is increasing.
The raw water is extremely free from sediment, and the filters only
become clogged very slowly. The rate of filtration is high, habitually
reaching 7,000,000 gallons per acre daily; but, with the clear lake
water and long periods between scrapings, the results are excellent
even at this rate. The filters are all covered with concrete groined
arches.

Filtration was commenced in 1886, and was followed by a sharp decline
in the amount of typhoid fever, which, up to that time, had been rather
increasing; for the six years before the change there were sixty-nine
deaths from this cause annually per 100,000 living, and for the six
years after only ten, or one seventh as many; and this reduction is
attributed by the local authorities to the filtration.[64]

=St. Petersburg.=—The supply is drawn from the Neva River by an English
company, and is filtered through vaulted filters at a very high rate.

=Warsaw.=—The supply is drawn from the Weichsel River by the city, and
is filtered through vaulted filters after preliminary sedimentation at
a rate never exceeding 2,570,000 gallons per acre daily.

THE USE OF UNFILTERED SURFACE-WATERS.

The use of surface-water without filtration in Europe is comparatively
limited. In Germany this use is now prohibited by the Imperial Board
of Health. In Great Britain, Glasgow draws its supply unfiltered from
Loch Katrine; and Manchester and some other towns use unfiltered
waters from lakes or impounding reservoirs the watersheds of which are
entirely free from population. The best English practice, however, as
in Germany, requires the filtration of such waters even if they are not
known to receive sewage, and the

unpolluted supplies of Liverpool, Bradford, Dublin, and many other
cities are filtered before use.

THE USE OF GROUND-WATER.[65]

Ground-waters are extensively used in Europe, and apparently in
some localities the geological formations are unusually favorable
to this kind of supply. Paris derives all the water it now uses for
domestic purposes from springs, but has a supplementary supply from
the river for other purposes. Vienna and Munich also obtain their
entire supplies from springs, while Budapest, Cologne, Leipzig,
Dresden, Frankfurt, many of the great French cities, Brussels, a part
of London, and many other English cities derive their supplies from
wells or filter-galleries, and among the smaller cities all over Europe
ground-water supplies are more numerous than other kinds.

APPENDIX X.

LITERATURE OF FILTRATION.

The following is a list of a number of articles on filtration. The
list is not complete, but it is believed that it contains the greater
part of articles upon slow sand-filtration, and that it will prove
serviceable to those who wish to study the subject more in detail.

ANKLAMM. Glasers Annalen, 1886, p. 48.

A description of the Tegel filters at Berlin, with excellent plans.

BAKER. Engineering News.

Water purification in America: a series of descriptions of filters, as
follows: Aug. 3, 1893, Lawrence filter and description of apparatus of
screening sand and gravel; Apr. 26, 1894, filter at Nantucket, Mass.;
June 7, 1894, filters at Ilion, N. Y., plans; June 14, 1894, filters
at Hudson, N. Y.; July 12, 1894, filters at Zürich, Switzerland,
plans; Aug. 23, 1894, filters at Mt. Vernon, N. Y., plans.

BERTSCHINGER. Journal für Gas- und Wasserversorgung, 1889, p. 1126.

A record of experiments made at Zürich upon the effect of rate of
filtration, scraping, and the influence of vaulting. Rate and vaulting
were found to be without effect, but poorer results followed scraping.
The numbers of bacteria in the lake-water were too low to allow
conclusive results.

—— Journal für Gas- und Wasserversorgung, 1891, p. 684.

A farther account of the Zürich results, with full analyses and a
criticism of Fränkel and Piefke’s experiments.

BOLTON. Pamphlet, 1884.

Descriptions and statistics of London filters.

BÖTTCHER and OHNESORGE. Zeitschrift für Bauwesen, 1876, p. 343.

A description of the Bremen works, with full plans.

BURTON. Water-supply of Towns. London, 1894.

Pages 94-115 are upon filtration and mention a novel method of
regulating the rate.

CODD. Engineering News, Apr. 26, 1894.

A description of a filter at Nantucket, Mass.

CRAMER. Centralblatt für Bauwesen, 1886, p. 42.

A description of filters built at Brieg, Germany.

CROOK. London Water-supply. London, 1883.

DELBRUCK. Allgemeine Bauzeitung, 1853, p. 103.

A general article on filtration; particularly valuable for notices of
early attempts at filtration and of the use of alum.

Deutsche Verein von Gas- und Wasserfachmänner.

Stenographic reports of the proceedings of this society are printed
regularly in the _Journal für Gas- und Wasserversorgung_, and the
discussions of papers are often most interesting.

DROWN. Journal Association Eng. Societies, 1890, p. 356.

Filtration of natural waters.

FISCHER. Vierteljahresschrift für Gesundheitspflege, 1891, p. 82.

Discussion of papers on water-filtration.

FRÄNKEL. Vierteljahresschrift für Gesundheitspflege, 1891, p. 38.

On filters for city water-works.

FRÄNKEL and PIEFKE. Zeitschrift für Hygiene, 1891, p. 38, Leistungen
der Sandfiltern.

E. FRANKLAND. Report in regard to the London filters for 1893
in the Annual Summary of Births, Deaths, and Causes of Death in
London and Other Great Towns, 1893. Published by authority of the
Registrar-General.

P. FRANKLAND. Proc. Royal Society, 1885, p. 379.

The removal of micro-organisms from water.

—— Proceedings Inst. Civil Engineers, 1886, lxxxv. p. 197.

Water-purification; its biological and chemical basis.

—— Trans. of Sanitary Institute of Great Britain, 1886.

Filtration of water for town supply.

FRÜHLING. Handbuch der Ingenieurwissenschaften, vol. ii.

Chapter on water-filtration gives general account of filtration, with
details of Königsberg filters built by the author and not elsewhere
published.

FULLER. Report Mass. State Board of Health, 1892, p. 449.
Report Mass. State Board of Health, 1893, p. 453.

Accounts of the Lawrence experiments upon water-filtration for 1892
and 1893.

—— American Public Health Association, 1893, p. 152.

On the removal of pathogenic bacteria from water by sand filtration.

—— American Public Health Association, 1894, p. 64.

Sand filtration of water with special reference to results obtained at
Lawrence, Mass.

GILL. Deutsche Bauzeitung, 1881, p. 567.

On American rapid filters. The author shows that they are not to be
thought of for Berlin, as they would be more expensive as well as
probably less efficient than the usual procedure.

—— Journal für Gas- und Wasserversorgung, 1892, p. 596.

A general account of the extension of the Berlin filters at Müggel. No
drawings.

GRAHN. Journal für Gas- und Wasserversorgung, 1877, p. 543.

On the filtration of river-waters.

—— Journal für Gas- und Wasserversorgung, 1890, p. 511.

Filters for city water-works.

—— Vierteljahresschrift für Gesundsheitpflege, 1891, p. 76.

Discussion of papers presented on filtration.

—— Journal für Gas- und Wasserversorgung, 1894, p. 185.

A history of the “Rules for Water-filtration” (Appendix I), with some
discussion of them.

GRAHN and MEYER. Reiseberichte über künstliche central Sandfiltration.
Hamburg, 1876.

An account of the observations of the authors in numerous cities,
especially in England.

GRENZMER. Centralblatt der Bauverwaltung, 1888, p. 148.

A description of new filters at Amsterdam, with plans.

GRUBER. Centralblatt für Bakteriologie, 1893, p. 488.

Salient points in judging of the work of sand-filters.

HALBERTSMA. Journal für Gas- und Wasserversorgung, 1892, p. 43.

Filter-works in Holland. Gives sand, gravel, and water thickness, with
diagrams.

—— Journal für Gas- und Wasserversorgung, 1892, p. 686.

Description of filters built by the author at Leeuwarden, Holland,
with plans.

HART. Proceedings Inst. of Civil Engineers, 1890, c. p. 217.

Description of filters at Shanghai.

HAUSEN. Journal für Gas- und Wasserversorgung, 1892, p. 332.

An account of experiments made for one year with three 16-inch filters
at Helsingfors, Finland, with weekly analyses of effluents.

HAZEN. Report of Mass. State Board of Health, 1891, p. 601.

Experiments upon the filtration of water.

—— Report of Mass. State Board of Health, 1892, p. 539.

Physical properties of sands and gravels with reference to their use
in filtration. (Appendix III.)

HUNTER. Engineering, 1892, vol. 53, p. 621.

Description of author’s sand-washing apparatus.

KIRKWOOD. Filtration of River-waters. New York, 1869.

A report upon European filters for the St. Louis Water Board in 1866.
Contains a full account of thirteen filtration-works visited by the
author, and of a number of filter-galleries, with a project for
filters for St. Louis. This project was never executed, but the report
is a wonderful work which appeared a generation before the American
public was able to appreciate it. It was translated into German, and
the German edition was widely circulated and known.

KOCH. Zeitschrift für Hygiene, 1893.

Water-filtration and Cholera: a discussion of the Hamburg epidemic of
1892 in reference to the effect of filtration.

KRÖHNKE. Journal für Gas- und Wasserversorgung, 1893, p. 513.

An account of experiments made at Hamburg, as a result of which the
author recommends the addition of cuprous chloride to the water before
filtration to secure greater bacterial efficiency.

KÜMMEL. Journal für Gas- und Wasserversorgung, 1877, p. 452.

Operation of the Altona filters, with analyses.

—— Vierteljahresschrift für Gesundheitspflege, 1881, p. 92.

The water-works of the city of Altona.

—— Journal für Gas- und Wasserversorgung, 1887, p. 522.

An article opposing the use of rapid filters (David’s process).

—— Journal für Gas- und Wasserversorgung, 1890, p. 531.

A criticism of Fränkel and Piefke’s results, with some statistics of
German and English filters. (The English results are taken without
credit from Kirkwood.)

—— Vierteljahresschrift für Gesundheitspflege, 1891, p. 87.

Discussion of papers on filtration, with some statistics.

—— Vierteljahresschrift für Gesundheitspflege, 1892, p. 385.

The epidemic of typhoid-fever in Altona in 1891.

—— Journal für Gas- und Wasserversorgung, 1893, p. 161.

Results of experiments upon filtration made at Altona, and bacterial
results of the Altona filters in connection with typhoid death-rates.

—— Trans. Am. Society of Civil Engineers, 1893, xxx. p. 330.

Questions of water-filtration.

LESLIE. Trans. Inst. Civil Engineers, 1883, lxxiv. p. 110.

A short description of filters at Edinburgh.

LINDLEY. A report for the commissioners of the Paris Exposition of
1889 upon the purification of river-waters, and published in French
or German in a number of journals, among them _Journal für Gas- und
Wasserversorgung_, 1890, p. 501.

This is a most satisfactory discussion of the conditions which modern
experience has shown to be essential to successful filtration.

MASON. Engineering News, Dec. 7, 1893.

Filters at Stuttgart, Germany, with plans.

MEYER and SAMUELSON. Deutsche Bauzeitung, 1881, p. 340.

Project for filters for Hamburg, with diagrams. Except in detail, this
project is the same as that executed twelve years later.

MEYER. Deutsche Bauzeitung, 1892, p. 519.

Description of the proposed Hamburg filters, with diagrams.

—— The Water-works of Hamburg.

A paper presented to the International Health Congress at Rome, March
1894, and published as a monograph. It contains a full description of
the filters as built, with drawings and views in greater detail than
the preceding paper.

MILLS. Special Report Mass. State Board of Health on the Purification
of Sewage and Water, 1890, p. 601.

An account of the Lawrence experiments, 1888-1890.

—— Report Mass. State Board of Health, 1893, p. 543.

The Filter of the Water-supply of the City of Lawrence and its Results.

—— Trans. Am. Society of Civil Engineers, 1893, xxx. p. 350.

Purification of Sewage and Water by Filtration.

NEVILLE. Engineering, 1878, xxvi. p. 324.

A description of the Dublin filters, with plans.

NICHOLS. Report Mass. State Board of Health, 1878, p. 137.

The filtration of potable water.

OESTER. Gesundheits-Ingenieur, 1893, p. 505.

What is the Rate of Filtration? A purely theoretical discussion.

ORANGE. Trans. Inst. Civil Engineers, 1890, c. p. 268.

Filters at Hong Kong.

PFEFFER. Deutsche Bauzeitung, 1880, p. 399.

A description of filters at Liegnitz, Germany.

PIEFKE. Results of Natural and Artificial Filtration. Berlin, 1881.

Pamphlet.

—— Journal für Gas- und Wasserversorgung, 1887, p. 595. Die Principien
der Reinwassergewinnung vermittelst Filtration.

A sketch of the theory and practical application of filtration.

—— Zeitschrift für Hygiene, 1889, p. 128. Aphorismen über
Wasserversorgung.

A discussion of the theory of filtration, with a number of experiments
on the thickness of sand-layers, etc.

PIEFKE. Vierteljahresschrift für Gesundheitspflege, 1891, p. 59.

On filters for city water-works.

FRÄNKEL and PIEFKE. Zeitschrift für Hygiene, 1891, p. 38.

Leistungen der Sandfiltern. An account of the partial obstruction of
the Stralau filters by ice, and a typhoid epidemic which followed.
Experiments were then made upon the passage of cholera and typhoid
germs through small filters.

PIEFKE. Journal für Gas- und Wasserversorgung, 1891, p. 208. Neue
Ermittelungen über Sandfiltration.

The above mentioned experiments being objected to on certain
grounds, they were repeated by Piefke alone, confirming the previous
observations on the passage of bacteria through filters, but under
other conditions.

—— Zeitschrift für Hygiene, 1894, p. 151, Über Betriebsführung von
Sandfiltern.

A full account of the operation of the Stralau filters in 1893, with
discussion of the efficiency of filtration, etc.

PLAGGE AND PROSKAUER. Zeitschrift für Hygiene, 11. p. 403.

Examination of water before and after filtration at Berlin, with
theory of filtration.

REINCKE. Bericht über die Medicinische Statistik des Hamburgischen
Staates für 1892.

Contains a most valuable discussion of the relations of filtration to
cholera, typhoid fever, and diarrhœa, with numerous tables and charts.
(Abstract in Appendix II.)

REINSCH. Centralblatt für Bakteriologie, 1895, p. 881.

An account of the operation of the Altona filters. High numbers of
bacteria in the effluents have often resulted from the discharge of
sludge from the sedimentation-basins onto the filters, due to the
interference of ice on the action of the floating outlet for the
basins, and this, rather than the direct effect of cold, is believed
to be the direct cause of the low winter efficiency. The author urges
the necessity of a deeper sand-layers in no case less than 18 inches
thick.

RENK. Gesundheits-Ingenieur, 1886, p. 54.

—— Über die Ziele der künstliche Wasserfiltration.

RUHLMANN. Wochenblatt für Baukunde, 1887, p. 409.

A description of filters at Zürich.

SALBACH. Glaser’s Annalen, 1882.

Filters at Groningen, Holland, built in 1880. Alum used.

SAMUELSON. Translation of Kirkwood’s “Filtration of River-waters” into
German, with additional notes especially on the theory of filtration
and the sand to be employed. Hamburg, 1876.

SAMUELSON. Filtration and constant water-supply. Pamphlet. Hamburg,
1882.

—— Journal f. Gas- und Wasserversorgung, 1892, p. 660.

A discussion of the best materials and arrangement for sand-filters.

SCHMETZEN. Deutsche Bauzeitung, 1878, p. 314.

Notice and extended criticism of Samuelson’s translation of Kirkwood.

SEDDEN. Jour. Asso. Eng. Soc., 1889, p. 477.

In regard to the sedimentation of river-waters.

SEDGWICK. New England Water-works Association, 1892, p. 103.

European methods of Filtration with Reference to American Needs.

SOKAL. Wochenschrift der östreichen Ingenieur-Verein, 1890, p. 386.

A short description of the filters at St. Petersburg, and a comparison
with those at Warsaw.

STURMHÖFEL. Zeitschrift f. Bauwesen, 1880, p. 34.

A description of the Magdeburg filters, with plans.

TOMLINSON. American Water-works Association, 1888.

A paper on filters at Bombay and elsewhere.

TURNER. Proc. Inst. Civil Engineers, 1890, c. p. 285.

Filters at Yokohama.

VAN DER TAK. Tijdschrift van de Maatschapping van Bouwkunde, 1875(?).

A description (in Dutch) of the Rotterdam water-works, including the
wooden drains which caused the trouble with crenothrix, and which have
since been removed. Diagrams.

VAN IJSSELSTEYN. Tijdschrift van het Koninklijk Instituut van
Ingenieurs, 1892-5, p. 173.

A description of the new Rotterdam filters, with full drawings.

VEITMEYER. Verhandlungen d. polyt. Gesell. zu Berlin, April, 1880.

Filtration and purification of water.

WOLFFHÜGEL. Arbeiten aus dem Kaiserliche Gesundheitsamt, 1886, p. 1.

Examinations of Berlin water for 1884-5, with remarks showing superior
bacterial efficiency with open filters.

—— Journal für Gas- u. Wasserversorgung, 1890, p. 516.

On the bacterial efficiency of the Berlin filters, with diagrams.

ZOBEL. Zeitschrift des Vereins deutsche Ingenieure, 1884, p. 537.

Description of filters at Stuttgart.

OTHER LITERATURE.

Many scientific and engineering journals publish from time to time
short articles or notices on filtration which are not included in the
above list. Among such journals none gives more attention to filtration
than the _Journal für Gasbeleuchtung und Wasserversorgung_, which
publishes regularly reports upon the operation of many German filters,
and gives short notices of new construction. The first articles upon
filtration in this journal were a series of descriptions of German
water-works in 1870-73, including descriptions of filters at Altona,
Brunswick, Lübeck, etc. Stenographic reports of many scientific
meetings have been published, particularly since 1890, and since 1892
there has been much discussion in regard to the “Rules for Filtration”
given in Appendix I.

A Report of a Royal Commission to inquire into the water-supply of the
metropolis, with minutes of evidence, appendices, and maps (London,
1893-4), contains much valuable material in regard to filtration.

The monthly reports of the water examiner, and other papers published
by the Local Government Board, London, are often of interest.

The German “Verein von Gas- u. Wasserfachmänner” prints without
publishing a most useful annual summary of German water-works
statistics for distribution to members. Many of the statistics given in
this volume are from this source.

Description of the filters at Worms was given in the _Deutsche
Bauzeitung_, 1892, p. 508; of the filters at Liverpool in
_Engineering_, 1889, p. 152, and 1892, p. 739. The latter journal also
has given a number of descriptions of filters built in various parts of
the world by English engineers, but, excepting the articles mentioned
in the above list, the descriptions are not given in detail.