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)

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.