Comparison of Methods of Sewage Purification

by Theodore Clifford Phillips; Edward John Schneider

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.