Analyses of Sewage and of Effluent from Intermittent Downward Filtration Plant
at Mendota, Ills.
Parts per 1,000,000
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¦ ¦ April 20, 1900 ¦ May 4, 1900 ¦
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¦ ¦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 ¦ ¦
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_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)