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Title: Comparison of Methods of Sewage Purification
Author: Schneider, Edward John, Phillips, Theodore Clifford
Language: English
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COMPARISON OF METHODS
OF
SEWAGE PURIFICATION
BY
THEODORE CLIFFORD PHILLIPS
AND
EDWARD JOHN SCHNEIDER
THESIS
FOR DEGREE OF BACHELOR OF SCIENCE
IN MUNICIPAL AND SANITARY ENGINEERING
COLLEGE OF ENGINEERING
UNIVERSITY OF ILLINOIS
PRESENTED JUNE 1900
UNIVERSITY OF ILLINOIS
_May 31, 1900._
THIS IS TO CERTIFY THAT THE THESIS PREPARED UNDER MY SUPERVISION BY
_Theodore Clifford Phillips and Edward John Schneider_ ENTITLED
_Comparison of Methods of Sewage Purification_ IS APPROVED BY ME AS
FULFILLING THIS PART OF THE REQUIREMENTS FOR THE DEGREE OF _Bachelor of
Science in Municipal and Sanitary Engineering_.
_Arthur M. Talbot_
HEAD OF DEPARTMENT OF _Municipal and Sanitary Engineering_.
_Contents_
Comparison of Methods of Sewage Purification
Dilution
Irrigation
Intermittent Downward Filtration
Chemical Precipitation
Septic Tank
Contact Bed
Discussion
Conclusion
_Comparison of Methods of Sewage Purification._
The disposal of sewage is one of the most important problems that
confront the municipal and sanitary engineer to-day.
The study of this subject dates back to 1844 when the Healths of Towns
Commission of London made the report which first showed the real course
of the rapid increase in the death rate of the larger cities. In this
country practically nothing was done until 1872 when the Massachusetts
Legislature instructed the State Board of Health to investigate and
report on the subjects of sewerage, sewage disposal, and the pollution
of streams, and to-day Massachusetts leads in the investigation of the
best methods of sewage disposal.
Six different methods for the treatment of sewage are now in use. These
methods are used separately or in combination, the choice depending upon
local conditions and the amount of purification required:—
These methods are:
1— Dilution.
2— Irrigation.
3— Intermittent Downward Filtration.
4— Chemical Precipitation.
5— Septic Tank.
6— Contact Bed.
The purpose of this thesis is to investigate each method and make a
comparison along the following lines:
1— Purification.
2— Capacity.
3— Applicability.
4— Cost.
_Dilution_
Disposal of sewage by dilution is the most common method at the present
time, and no doubt will continue to be for some time to come where the
stream or body of water is not used for domestic purposes. Under certain
conditions this affords a satisfactory solution of the problem. However,
if the water into which the sewage is discharged is a source of water
supply, two very important questions arise. They are: 1—How large must
the dilution be? 2—How far must the polluted stream flow before the
water way may be used with safety to public health?
In 1867 Dr. Letheby and others in England made the statement that if
sewage is diluted at least twenty times its volume, it will not only be
made inoffensive but be thoroughly destroyed after flowing a dozen miles
or more.
The Rivers Pollution Commission of Great Britain in 1878 maintained that
no river in England was long enough to allow of a complete disappearance
of sewage matter discharged into it.
Mr. F. P. Stearns in an article on The Pollution and Self-Purification
of Streams in the Massachusetts State Board of Health Report 1890 gives
a table showing, “The calculated composition of sewage for different
degrees of dilution in a running stream”. A comparison was made of the
Blackstone and Merrimack rivers, and extended over three years. It was
from this investigation that the Commissioner of the City of Chicago
reported that the flow of the Chicago Drainage Canal should be four
cubic feet per second per 1000 inhabitants.
The purification by dilution may be due to three things:
1— Oxidation of organic matter.
2— Subsidence.
3— Destruction of the organic matter by small animals and plants.
The oxidation is a very slow process and depends mainly upon bacterial
life and conditions favorable to it. Subsidence depends upon the
difference in the specific gravity of the suspended matter, and the
clarification is increased as the velocity is diminished.
In regard to the destruction of the organic matter by small animals and
plants Dr. Sorby says, (_Journal Royal Microscopic Society, 1884 p.
988_.) “It appears to me that the removal of impurities from rivers is
more of a biolytic than a chemical question, and it is most important to
consider the action of minute animals and plants, which may be looked
upon as being indirectly most powerful agents.”
_Irrigation_
Although the method of broad irrigation has been carried on successfully
in several of the largest cities of the old world, it has not been used
to any great extent in the United States, except at a few state
institutions in the east, and in the arid districts of the west.
Several years ago this method of disposal was given much attention as it
was thought the sewage if used would yield large profits, but more
recent information shows conclusively that such is not the case.
Perhaps the most noticeable examples of this system that are now in
operation in the United States are the plants at the State Hospital of
the Insane, Worcester, Mass., at the Rhode Island State Institute, and
at the Reformatory at Concord, Mass. In all cases the labor was done by
the inmates so that it is not possible to get a fair statement of the
cost. General information regarding the use of sewage for irrigation in
the west may be obtained from Baker and Rafters’ “Sewage Disposal in the
United States”.
Undoubtedly the best example of sewage farming is at Berlin. The
following summary is given by James H. Fuentes, M. Am. Soc. C. E., in
No. 2. Vol. 40 of the Engineering Record. The average quantity of sewage
pumped to the farms and distributed amounts to from 6,000,000 to
7,000,000 gallons per acre per year, or in other words each acre serves
for 750 people. The area farmed is divided into small beds about 150 by
200 feet separated from each other by slight embankments and ditches for
distributing the sewage over the surface. The sewage is admitted into
the carriers from the force mains through checking chambers made of
woven willow and posts driven into the sand.
The farms are said to yield small profits over the expenses of working
them, consequently contribute scarcely anything toward the cost of
pumping the sewage.
The great reason why this method of disposal has been possible and
profitable in a degree at Berlin is that this large and beautiful city
lies in the midst of an extensive sandy area, the greater part of which
in the immediate neighborhood is quite sterile and therefore
comparatively thinly populated. Better conditions for such sewage farms
could not exist.
Statistics compiled by Chas. S. Swan, M. Am. Soc. C. E. in his paper,
Notes on European Practice in Sewage Disposal, in Jour. Assn. of Eng.
Soc. Vol. 7 No. 7 shows the volume of sewage per acre per day (yearly
average) to vary from 2155 to 29450 gallons, and the average depth per
annum varies from 2.4 to 32.8 feet.
The nineteenth annual report of the Massachusetts State Board of Health
1888 states that on an ordinary farm in Massachusetts 2500 gallons per
acre per day are as much as could be applied to any valuable grass crop.
This would require such a large area for large cities that irrigation
could not be depended upon for preventing the pollution of streams.
It is evident from the above report that the irrigation system can be
used to advantage only where large tracts of low land can be obtained at
a low cost, and where help and plenty of cheap labor can be obtained.
_Intermittent Downward Filtration_
Filtration means the concentration of sewage on an area of especially
chosen porous ground, or on an especially prepared bed as small as will
absorb and purify it. The intermittency of application is a necessary
requirement even in suitably constituted soils wherever complete success
is arrived at. The reason for this is that free oxygen is indispensable
to nitrification and the nitrifying organism.
This method has been in operation only during the last thirty years, and
the correct theory of its action has been found out within the last ten
years.
Dr. Frankland, in the first report of Rivers Pollution Commission of
England, shows that not only a chemical action but also a biolytic
action takes place with the assistance of minerals and the oxygen of the
air. This biolytic action is known as nitrification, and consists in the
oxidation of the nitrogen of ammonia and its ultimate conversion into
nitric acid. This change takes place in two stages, each characterized
by a distinct organism. The office of one of these organisms is to
convert ammonia into nitrite, while the other converts nitrites into
nitrate.
The bacteria are present or quickly develop in the sewage which may be
considered a nutrient medium for them by reason of containing a large
amount of their natural nitrogenous food.
These organisms, according to Warrington, may be separated by successive
cultivations in favorable media; from a potassium nitrate solution with
no ammonia we may obtain nitric organism alone; from an ammonium
carbonate solution we may obtain nitrous organisms in a pure state.
It is a noticeable fact that frost checks nitrification, but several
satisfactory methods have been used so that no serious objection can be
raised to intermittent filtration on this ground.
“The purification of sewage by this method depends upon the oxygen and
the time of action” (Hazen). To obtain the highest efficiency the sewage
must be applied intermittently and there must be a large percentage of
voids in the sand.
Experiments made on especially prepared beds at Lawrence, Mass. show
that by treating 85000 gallons per acre per day, 94 percent of the
organic matter and 98 percent of the bacteria are removed, and by
treating 60,000 gallons per acre per day, 97 to 99 percent of the
organic matter may be removed. The above data shows that 100,000 may be
successfully treated with a five foot bed of sand.
The writers inspected the intermittent downward filtration plant at
Mendota Illinois and the following data were obtained. The city has a
population of about 6000 with 8 miles of sewers and 210 house
connections. The average daily flow is about 350,000 gallons although
during the rainy season there is a considerable amount of ground water
present.
The disposal plant was constructed under the supervision of the Iowa
Engineering Company of Clinton, Iowa, on a 15 acre tract of land, one
and a half miles south of the city. Although original plans called for
18 beds with five feet of gravel, only four beds each 275 feet by 75
feet were finished, and instead of 5 feet of ground only 24 inches was
used. The plant was complete in July 1899 and was operated until
December, when it was discontinued on account of cold weather, it not
being considered necessary to operate it during the winter. From
December until April the sewage was discharged directly into a creek
which runs along one side of the land.
The beds have no automatic device for distributing the sewage at
definite intervals of time, but are connected with the main sewer by a
number of gate valves which were regulated by the attendant. The sewage
is distributed by means of a wooden sluice way which extended the entire
length of the bed and has six inch openings on the side every 16 feet
which gave a fairly even distribution.
Although no analysis of the sand and gravel was made, it was apparent
that the uniformity coefficient was very large.
The material was shipped a distance of 90 miles and resembles ordinary
ballast used for rail roads. The largest pieces were thrown along the
sluice way to prevent excessive washing. The finest of the sand was of a
size which would be considered too fine for good mortar sand. No attempt
was made to screen the material. The gravel would not be considered
first class for filtering beds.
Samples of the sewage and effluent were taken at two different times.
The first was between 9:00 and 10:30 A. M., April 20, 1900. Until the
day before no sewage had been applied to the beds since last fall. The
second collection was made two weeks later, when samples of the sewage
and effluent were collected at intervals of one hour during the day, and
mixed into composite samples. The sewage was taken from a manhole just
above the beds, and the effluent from the outlet of the underdrain about
900 feet below the beds.
The analysis is given in Table I. and the results show that the beds are
not working as efficiently as might be expected from a first class
plant. This may be accounted for from the fact that the filtration is
too rapid and that the material which is less than two feet in depth is
not suitable for such a purpose.
TABLE I.
Analyses of Sewage and of Effluent from Intermittent Downward Filtration Plant
at Mendota, Ills.
Parts per 1,000,000
┌──────────────────────┬──────────────────────────┬──────────────────────────┐
│ │ April 20, 1900 │ May 4, 1900 │
├──────────────────────┼──────┬────────┬──────────┼──────┬────────┬──────────┤
│ │Sewage│Effluent│Efficiency│Sewage│Effluent│Efficiency│
├──────────────────────┼──────┼────────┼──────────┼──────┼────────┼──────────┤
│Oxygen consumed: │ │ │ │ │ │ │
│ Total │ 48.5 │ 6.50 │ 86% │ 27.7 │ 12.0 │ 55% │
│ In suspension│ 33.4 │ │ │ 15.5 │ 4.6 │ 70% │
│ In solution │ 15.1 │ │ │ 12.2 │ 7.4 │ 40% │
│Nitrogen as albuminoid│ │ │ │ │ │ │
│ ammonia │ │ │ │ │ │ │
│ Total │ 10.4 │ .32 │ 97% │ 3.68 │ .768 │ 79% │
│ In suspension│ 9.28 │ │ │2.816 │ .416 │ 85% │
│ In solution │ 1.12 │ │ │ .864 │ .352 │ 59% │
│Total organic nitrogen│17.960│ .920 │ 95% │ 9.60 │ 1.860 │ 81% │
│ In suspension│15.04 │ │ │7.500 │ 1.152 │ 85% │
│ In solution │ 2.92 │ │ │2.100 │ .708 │ 66% │
│Nitrogen as free │ │ │ │ │ │ │
│ ammonia │ 7.20 │ 2.40 │ 67% │ 3.84 │ 1.28 │ 67% │
│Nitrogen as nitrites │ .675 │ .375 │ │ 000 │ 000 │ │
│Nitrogen as nitrates │ 4.00 │ 18.00 │ │ .120 │ .120 │ │
│Chlorine as chlorides │ 53.0 │ 49.0 │ │ 47.0 │ 44.0 │ │
└──────────────────────┴──────┴────────┴──────────┴──────┴────────┴──────────┘
_Chemical Precipitation_
When certain chemicals are added to sewage a precipitate is formed which
under favorable conditions may carry down with it all the suspended
matter as well as a portion of the dissolved organic matter. The
addition of the chemicals together with the working of the various
appliances for grinding and mixing the same, the decanting of the
effluent, and the caring for the sludge all constitute what is known as
the chemical treatment of sewage, the complete process being in reality
partly chemical and partly mechanical.
The following matter concerning the theory of precipitation is taken
from Baker and Rafter’s Sewage Disposal.
The reagents chiefly used at the present time are lime, sulphate of
alumina, and ferrous sulphate. In the case of lime, there is a
combination of some of the lime and with free and partially combined
carbon dioxide to form an insoluble carbonate of lime. There is probably
a further combination of an additional part of the lime with a portion
of the organic matter in solution. The insoluble substances so formed
sink to the bottom, carrying with them the major portion of the
suspended matter in the sewage in the form of sludge.
Sulphate of alumina exercises a precipitating effort by a combination of
the sulphuric acid with lime while alumina hydrate forms a flocculent
precipitate which entangles and carries down the suspended organic
matters.
On November 11, 1899 the writers visited the chemical precipitation
plant at Madison Wisconsin and through the kindness of Mr. McClellen
Dodge, City Engineer, were shown the plant at that time in operation.
The sewage was screened as it came from the city and emptied into a
well. Here it was dosed with lime and then pumped to one of the four
settling tanks. These are 15 feet deep and 25 feet in diameter. The
sewage enters near the bottom of the tanks. The sludge settles to the
bottom and the effluent rises to the top, where it is carried away to
three of the filter beds. These three beds are in continuous use. A
fourth bed is out of use while it is washed and allowed to rest for one
day.
The total area of the beds is 5550 square feet. The flow through the
beds is about 8,000,000 gallons per acre per day.
The company that constructed and operated the plant agreed that the
effluent on analysis should be found to be equal to the waters of Lake
Mendota and the analyses of Lake Mendota water which was adapted as a
standard of purity is reproduced in the Engineering News Vol. 42 No. 26
p. 414. Samples of the crude sewage tank effluent and filter effluent
were taken and analyzed as shown in Table II. A comparison of this table
with the standard shows that in no particulars does the effluent from
the plant come up to the standard.
The company has since abandoned the contract and the city is considering
other methods of disposal and purification “in order that there may be
no more fiascos in civil engineering at the seat of this well known
school.” (University of Wisconsin)
TABLE II.
Analyses of Sewage and Effluent from Chemical Precipitation Plant and Filter
Beds at Madison, Wis.
Parts per 1,000,000.
┌──────────────────────┬─────────────────────────────────────────────────────┐
│ │ November 23, 1900 │
├──────────────────────┼──────────────────────────┬──────────────────────────┤
│ │ Chemical Precipitation │ Filter-beds │
├──────────────────────┼──────┬────────┬──────────┼──────┬────────┬──────────┤
│ │Sewage│Effluent│Efficiency│Sewage│Effluent│Efficiency│
├──────────────────────┼──────┼────────┼──────────┼──────┼────────┼──────────┤
│Oxygen consumed: │ │ │ │ │ │ │
│ Total │ 38.3 │ 23.2 │ 40% │ 23.2 │ 14.3 │ 38% │
│ In suspension│ 27.2 │ 12.1 │ 55% │ 12.1 │ 3.5 │ 71% │
│ In solution │ 11.1 │ 11.1 │ 00% │ 11.1 │ 10.8 │ 3% │
│Nitrogen as albuminoid│ │ │ │ │ │ │
│ ammonia │ │ │ │ │ │ │
│ Total │ 8.32 │ 4.00 │ 52% │ 4.00 │ 2.46 │ 38% │
│ In suspension│ 7.12 │ 2.56 │ 64% │ 2.56 │ 1.60 │ 37% │
│ In solution │ 1.20 │ 1.44 │ –20% │ 1.44 │ .864 │ 40% │
│Total organic nitrogen│15.30 │ 5.32 │ 67% │ 5.32 │ 3.56 │ 33% │
│ In suspension│12.48 │ 1.86 │ 85% │ 1.86 │ 1.54 │ 17% │
│ In solution │ 2.82 │ 3.46 │ –23% │ 3.46 │ 2.02 │ 42% │
│Nitrogen as free │ │ │ │ │ │ │
│ ammonia │ 32.0 │ 32.0 │ 0% │ 32.0 │ 32.0 │ 0% │
│Nitrogen as nitrites │ │ │ │ │ │ │
│Nitrogen as nitrates │ .120 │ .240 │ │0.240 │ 0.60 │ │
│Chlorine as chlorides │ 134 │ 136 │ │ 136 │ 119 │ │
└──────────────────────┴──────┴────────┴──────────┴──────┴────────┴──────────┘
Another example of the use of chemical precipitation is at Alliance,
Ohio. There are three tanks each with a capacity of 144000 gallons. The
amount of sewage flowing into the tanks during the year 1897 averaged
about 300,000 gallons per day. During the year 1897 each day 180 pounds
of chemicals were added to the sewage and 650 pounds of sludge was
precipitated and pressed daily.
The total cost of the disposal plant was $20755.00. The cost of
operation and maintenance for the years 1897 and 1898 was $1290.00 and
$1567.43 respectively. The average amount of lime used during this time
was 95000 pounds yearly.
Two men perform all the labor necessary for the operation of the plant.
From the chemical analyses given in the Engineering Record Jan. 13, 1900
of the sewage and effluent, averages representing five months show the
efficiency in albuminoid ammonia to be 40 percent and oxygen required 30
percent.
_Septic Tank_
We will now discuss a very different process of treating sewage, the
septic method. The septic tank consists of a tank from which generally
the air and light are excluded and through which the sewage flows with a
slow velocity, thus allowing the matter in suspension to settle to the
bottom or rise to the top by reason of its specific gravity, there to be
further acted upon or decomposed by bacteria. The currents should be so
guided that they are distributed over a considerable depth and not
allowed to disturb the sediments at the bottom or the material at the
top. The darkened ill ventilated and modestly heated conditions in the
tank are all conducive to the propagation of a micro-organism known as
bacteria. The activity of these organisms causes a chemical
decomposition of the organic matter in the sewage a part passing off as
gas and a part passing off in the effluent as inorganic matter, while
another portion is deposited in the bottom of the tank as sludge.
So far only matter in suspension has been considered but it is also true
that chemical action takes place to some extent in the organic matter in
solution. This chemical action is a denitrifying action similar to that
which takes place in intermittent downward filtration.
An important fact in the septic tank effluent is that its compounds have
been broken down and it can now be more readily purified.
The septic tank process is self-regulating, self-acting, requires no
attention except the occasional cleaning out of the sludge.
As an illustration of this process the septic tank at Champaign Illinois
may be described.
This tank although designed in 1895, has only been in operation since
1897. The dimensions of this tank are 37 by 16 by 5 feet and it has a
cubic capacity of 22,000 gallons. It has taken care of a dry weather
flow of 300,000 gallons per day, about 13 times its cubic contents. The
tank has operated with a range of from 11 to 27 times its cubic capacity
with a high efficiency of purification though much of this was ground
water.
The reduction of the organic matter in suspension, as shown by the
reduction of oxygen consumed, nitrogen as albuminoid ammonia, and total
organic nitrogen in three analyses averaged 94 percent. However it is
not considered that an average of more than 80 to 90 percent may
generally be expected. The reduction of organic matter in solution is
considerable, averaging 17 percent.
This sludge at the bottom is a black muddy looking silt-like deposit
containing about 60 percent of water and 5 percent of organic matter.
Its value as a fertilizer is small.
Although the amount of sludge is relatively small the accumulation
requires removal occasionally, three or four times a year.
It is estimated that for every 1,000,000 gallons sewage there is about 3
cubic feet of dry matter. By chemical precipitation this would amount to
from 20 to 25 cubic feet per 1,000,000 gallons.
Another good example of the septic tank method is the one in use at
Exeter, England.
This plant has been in operation since 1896. It was designed for 98000
gallons capacity. The sewage is allowed to remain in the tank nearly 24
hours. The tank is used in conjunction with five beds in which further
purification is used. The beds are filled with coke breeze and one is
always resting for a week at a time. The tank is made nearly air tight
by an arched roof and any gasses collecting may be burned at a vent.
The rate of working this tank is much slower than the process at
Champaign Illinois.
At Southwold England, there is another good illustration of this method.
This has been called an open septic tank and is used in conjunction with
two beds, and an anaerobic, and an aerobic bed, the latter containing
polarite.
This plant is interesting from the fact that the effluent from the
anaerobic bed is distributed over the aerobic bed by means of a
revolving sprinkler which prevents the liquid from passing unequally
through the large grain, porous material. The action upon these beds is
continuous.
From a report of this system the analysis of gas found in three samples
are as follows:—
┌─────────────┬─────────────┬─────────────┬─────────────┬─────────────┐
│ │ │ │ Carbon │Sulphuretted │
│ │ Oxygen │ Nitrogen │ dioxide │ hydrogen │
├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤
│Septic tank │ 0.25%│ 28.46%│ 70.03%│ 1.26%│
│Anaerobic bed│ 7.26%│ 39.10%│ 52.37%│ 0.91%│
│Aerobic bed │ 20.62%│ 78.63%│ 0.75%│ 0.00%│
└─────────────┴─────────────┴─────────────┴─────────────┴─────────────┘
_Contact Bed_
The contact bed as distinguished from the bacteria bed may be said to be
made up of fine material, while the bacteria beds are built of coarse
material. The latter are used for taking out the rougher solids, and the
former for taking out the more finely divided material, and the organic
matter in solution. While this distinction is not commonly made it seems
to be growing into usage.
In the process of purification by means of contact beds the sewage is
applied intermittently by distributing pipes or troughs so as to slowly
fill the beds which are filled from a depth of from three to six feet
with any kind of hard, porous, jagged material. The change which takes
place is due to the action of certain bacteria in the presence of air.
Although this process is not a new one, the method by which the results
were obtained was not fully understood until the more recent discoveries
in the science of bacteriology were made.
The beds are made water tight to a depth of about four feet, the bottom
being channeled or tiled to drain the effluent either to a secondary bed
or into the effluent channel.
Many eminent men have advocated special material as coal, coke, clinker
slag, sand, and gravel, and even glass for filling material. The results
do not differ materially and Prof. L. P. Kennicutt in an article in the
Journal of the Association of Engineering Societies, February 1900 makes
the following statement, “The material should be more or less porous so
as to have a larger water absorbing area and have a jagged surface on
which the gelatinized micro-organisms can be easily retained.”
“The quantity of sewage that can be successfully treated by intermittent
filtration has been shown not to be over 100,000 gallons per acre per
day, a quantity so small as to be quite useless for towns and cities
which would be obliged to construct beds with sand not “in situ”. This
point was quickly perceived in England where sand “in situ” is not of
common occurrence. These bacteria beds were not used long before the
problem arose: Can the amount of land required by the intermittent
filtration method be so reduced that the construction of artificial
bacteria beds will be a practical possibility?
“The results of the investigation started by this problem have given us
what is known as the contact system of treatment and the septic tank
treatment and have apparently shown not only that by combining these two
methods the amount of area required for 100,000 gallons can be reduced
from one acre to about one seventh of an acre but also that the
bacterial treatment is possible with sewage containing manufacturing
refuse, and have outlined how sewage containing storm water may be
treated.”
It has been found by experiment that after two or three weeks there is a
marked reduction in the initial capacity of the tank due to breaking
down of the filling material and also to the filling material becoming
charged and coated over with a gelatinous slime consisting of living
organisms and organic matter in the process of transformation.
If the bed is not overworked the capacity of the tank will remain
constant after the first two or three weeks, showing that the durability
is unlimited.
The beds are usually filled in half an hour, and the sewage allowed to
remain on them about two hours when the effluent is run off and the beds
allowed to rest several hours before again being filled.
By contact beds in series, the number depending upon the kind of sewage,
almost any degree of purification can be obtained.
The Engineering Record of Jan. 27, 1900 gives a very interesting account
of the sewage disposal works at Sutton, England. The works were
constructed in 1891–93, and comprise an area of 28 acres, 18 acres only
being capable of irrigation. They were originally designed for chemical
precipitation and broad irrigation, but after giving these methods a two
years’ trial the local board found itself unable to satisfy the
requirements of the conservators of the River Thames, into which the
effluent flowed.
Mr. W. J. Dibdin advised the construction of filters or fine grained
bacteria beds. Two beds having a combined area of a quarter of an acre
and a capacity of 200,000 gallons, were built in 1895–96 for the
purification of the chemically treated sewage. In 1896 the first
bacteria bed was constructed for the treatment of crude sewage.
As previously stated, many tests were made to determine the best kind of
material for filling the beds.
The crude sewage, after being screened to intercept floating matter, is
run directly onto the filters without the addition of any chemicals.
After remaining in the beds for a period of two hours the effluent is in
a fit condition to be discharged into the brook leading to the Thames
and is uniformly superior to the effluent obtainable by local land
treatment.
All experiments with bacteria beds show that the objects for which they
were intended, to abolish sludge, has been realized and that sewage can
be purified without chemicals at a small cost, being but little more
than that incurred by the labor in attending to, or supervising the
filling and discharging, the filters, and that sewage purification can
be carried on with little or no nuisance.
_Discussion_
The amount of purification to be obtained by dilution depends upon the
size of the stream into which the sewage is discharged and also upon the
amount of oxygen contained in the stream. The latter condition is
controlled very largely by the rate of flow of the stream and its
previous condition of pollution. Mr. E. P. Stearns in his report to the
Massachusetts State Board of Health, 1890, on Pollution and
Self-Purification of Streams, gave a table showing the calculated amount
of free ammonia, dissolved solids and chlorine which sewage adds to
running streams.
Much interest is being taken in the effect of the discharge of the
sewage of Chicago, and the waters of Lake Michigan, into the Illinois
River, and the outcomes of the analyses of samples taken along the river
is awaited with interest. Published reports have not been given out, but
information from the most reliable sources seem to show that a
considerable purification takes place in the passage down the river.
No reliable data could be obtained giving the percent of purification by
irrigation. At Berlin the sewage is purified by this method, and the
effluent comes well within the requirements of the German law.
With chemical treatment about 90 percent of the matter in suspension and
a small percent of that in solution is removed, and the purification is
about 53 percent of the total organic matter.
The best examples of intermittent downward filtration show an efficiency
of 95 percent on the total organic matter. However, if this process is
used in connection with other methods the organic matter may be reduced
99 percent and the chemical analyses of the effluent may fill the
drinking water requirements.
Results of analyses of sewage and effluent from septic tanks show an
efficiency of from 85 to 90 percent in the organic matter in suspension.
Very little change takes place in the matter in solution.
The result of experiments with a single contact bed at Sutton, England,
from November 1896 to March 1898 shows a purification of 64 percent; if,
however, two beds are used in series, a further purification of 50
percent is obtained or a total purification of 82 percent of the crude
sewage.
The capacities of irrigation, and intermittent downward filtration
plants, and contact beds are usually stated in terms of the number of
gallons per acre per day.
TABLE III.
_RATE OF SEWAGE TREATMENT._
System. Location. Sewage treated in
gallons per acre per
day.
───────────────────────────────────────────────────────────────────────
IRRIGATION.
Berlin, Germany. 18,000.
Manchester, Eng.—Estimated but not
constructed. 18,500.
INTERMITTENT DOWNWARD FILTRATION.
Framingham, Mass. 19,000.
Leicester, Mass. 38,750.
Brockton, Mass. 44,000.
Marlborough, Mass. 54,000.
Gardner, Mass. 140,000.
Mendota, Ills. 184,000.
Worcester,[1] Mass. 322,000.
Worcester,[1] Mass. (Fletcher’s estimate). 500,000.
Madison,[1] Wis. 8,000,000.
CONTACT BEDS.
Manchester, Eng. (crude sewage) 500,000.
Manchester,[1] Eng. 700,000.
Manchester, Eng. (storm water sewage) 2,500,000.
Footnote 1:
Secondary filtration.
Table III shows the capacity of representative plants. Best authorities
consider 100,000 gallons per acre per day to be the maximum rate
permissible under the best conditions for the treatment of crude sewage
by the intermittent downward filtration system. With unfavorable
conditions the quantity of sewage should be limited to as low as 20,000
gallons. Wherever too large a dose is applied to the bed the sewage is
not properly purified and the beds soon become clogged up and unfit for
further use.
When the beds are used for secondary purification, 750,000 gallons may
be treated per acre per day.
The results of extensive experiments made at Manchester, England in 1898
and 1899 show that by means of contact beds crude sewage may be treated
at the rate of 500,000 gallons per acre per day. When the beds are used
for secondary purification, 750,000 gallons may be successfully treated,
and storm water sewage treated at the rate of 2,500,000 gallons per acre
per day.
The capacity of the chemical precipitation plant at Madison, Wisconsin
is 68 percent of the daily flow, which in 1899 was estimated at 300,000.
The company that built the plant agreed that the plant should have a
daily capacity of 1,200,000 for each and every day in the year. This
shows the estimated capacity of the tank to be 28 percent.
At Worcester, one of the best examples of this process is in operation.
The size of the tanks is equal to 28 percent of the total flow of 17.1
million gallons per day of which 10.1 million gallons is the out flow
from a comparatively clear pond. If the actual amount of sewage is
considered the tanks have a capacity of 65 percent of the total flow.
Rafter and Baker in their discussion of the size of tanks recommends
that the total capacity should be nearly 50 percent of the average daily
flow.
The two typical types of septic tanks are those at Exeter, England and
at Champaign Illinois. The first has a cubic capacity of 93 percent of
the total daily flow, while the second has a capacity of 7.5 percent of
the daily flow.
When the capacity of the tank is as small as at Champaign the bacteria
do not have time to act upon the sewage as they would if the flow was
not so rapid.
It is conceded now by the highest authorities on this subject that the
tank should have a capacity equal to from one fourth to one half of the
average daily flow.
Concerning the conditions for which the various processes are
applicable, it may be said that the dilution process is used when
advantage may be taken of natural resources. This method can be utilized
by most cities situated along rivers or streams large enough to
sufficiently dilute and carry away the sewage, and fulfill sanitary
requirements.
As an example of the use of dilution the Chicago River may be cited.
There the uncertain flow of the river was made to pass inward toward the
Illinois River with a speed of 2½ miles per hour and a discharge of
20,000 gallons per minute per 100,000 inhabitants. The Blackstone River
was used by the cities of Massachusetts until the pollution of the river
became unbearable and the state was compelled to pass a law forcing the
cities to purify their sewage.
Broad irrigation is a method which cannot generally be used on account
of the large amount of land and labor required. This method is
especially applicable to asylums, alms houses, and reformatories where
cost of labor is small. It is used in the West where the value of all
available water leads to the application of sewage to crops, and also on
account of the low stage of western streams during the summer which
renders sewage discharged into them an unbearable nuisance.
Intermittent downward filtration may be used where there is a
considerable area of sandy soil, and also where a high degree of
purification is necessary.
Chemical precipitation does not require a large area for its operation
and used alone does not give a high degree of purification. Where land
and material for beds is expensive, and partial purification is
sufficient, this system may be used.
The septic tank requires a small area. The odors are not offensive and
cannot be noticed 100 feet away. The effluent may be discharged into
small streams and soon loses its identity.
Contact beds are used where a high degree of purity is required; if the
effluent is emptied into rivers or sources of water supply. The beds do
not require a large area and the process may be recommended if suitable
material is at hand.
The cost of construction and maintenance of the different systems vary
so largely according to local conditions that a fair estimate is not
possible. As the most striking example of maximum and minimum cost the
disposal system at Chicago and at Lowell, Massachusetts may be cited.
The purification in both cases is by dilution, the cost being
practically nothing at Lowell while the Chicago Drainage Canal cost
$33,000,000. It may not be fair to charge this entire cost to the
sewerage system; nevertheless up to date it has been used for no other
purpose.
At Berlin, Germany where broad irrigation is used, the sewage has to be
pumped to the farms and involves a considerable cost. Aside from this
the receipts derived from the sale of farm products are enough to pay
the running expenses of the farm.
The first cost of the intermittent downward filtration plant is
estimated by the writers at $90,000 per million gallons per day of
sewage treated, where the beds are artificial. Where natural beds can be
used the first cost may be reduced to $7,500 per million gallons per
day. The cost of maintenance is about $2.00 per 1,000,000 gallons.
The average cost for operating a chemical disposal plant is about 58
cents per inhabitant per year, or $16.00 per 1,000,000 gallons treated.
At Manchester England where broad irrigation and chemical precipitation
are used in combination, the average cost was $63.00 per 10,000,000
gallons treated.
A septic tank for treating 1,000,000 gallons of sewage per day will cost
less than $10,000 and the cost of maintenance will be about $1.00 per
1,000,000 gallons treated.
_Conclusion_
In selecting a method of sewage disposal, the conditions, surroundings,
and requirements of the city should be carefully studied and analyzed,
and judgment and discretion must be used. A matter of so much importance
to the community should be placed in the hands of men qualified to make
a proper solution of the problem. While in general several methods of
purification may be applied to the requirements of the city, usually
local conditions and considerations will narrow the choice to two plans
or possibly to a single method.
The governing features of the dilution process are so distinct that it
is not usually difficult to determine where this method is applicable,
and likewise, the broad irrigation process has peculiar conditions. In
both, the plans involve only matters of construction.
The other methods have more in common and the determination of their
relative value is not so easy. As before stated, the recently developed
biolytic processes promise to displace chemical precipitation. Where a
suitable deposit of sand or gravel is conveniently located on cheap land
which may be made without great expense into filtration areas and the
sewage discharged upon them by gravity, intermittent downward filtration
may be the most satisfactory, especially if a highly purified effluent
is desired. The item of expense of attendance, and labor of maintenance
must be considered in connection with the cost of this method. In the
absence of such favorable conditions and especially where complete
purification is not required the septic tank may be the most suitable.
For higher purification the combination of the septic tank with filter
beds, or contact beds run at comparatively high rates, makes a
satisfactory purification and is applicable to a wide range of
conditions. A bacteria bed may be substituted for the septic tank for
the roughing process but its applicability in not so general.
In conclusion it may be said that if the next ten years gives as much
development in sewage purification as has the last decade, some of the
processes herein outlined will have been discarded and sanitary
engineering will have achieved still greater triumphs.
------------------------------------------------------------------------
TRANSCRIBER’S NOTES
1. Table of Contents added by transcriber.
2. All text after the Contents was handwritten.
3. Silently corrected typographical errors and variations in spelling.
4. Retained anachronistic, non-standard, and uncertain spellings as
printed.
5. Footnotes have been re-indexed using numbers.
6. Enclosed underlined text in _underscores_.
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