Analyses of Sewage and Effluent from Chemical Precipitation Plant and Filter
Beds at Madison, Wis.
Parts per 1,000,000.
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¦ ¦ November 23, 1900 ¦
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¦ ¦ Chemical Precipitation ¦ Filter-beds ¦
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¦ ¦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 ¦ ¦
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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, 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:—
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¦ ¦ ¦ ¦ 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%¦
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_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.