Sewerage and Sewage Treatment

by Harold E. Babbitt

Chemical Precipitation

241. The Process.—Chemical precipitation consists in adding to the sewage such chemicals as will, by reaction with each other and the constituents of the sewage, produce a flocculent precipitate and thus hasten sedimentation. The advantages of this process over plain sedimentation are a more rapid and thorough removal of suspended matter. Its disadvantages include the accumulation of a large amount of sludge, the necessity for skilled attendance, and the expense of chemicals. The process is not in extensive use as the conditions under which the advantages outweigh the disadvantages are unusual. Sewage containing large quantities of substances which will react with a small amount of an added chemical to produce the required precipitate are the most favorable for this method of treatment.

Chemical precipitation accomplishes the same result as plain sedimentation, although the effluent from the chemically precipitated sewage may be of better quality than that from a plain sedimentation basin.

242. Chemicals.—Lime is practically the only chemical used for the precipitation of the solid matter in sewage. Commercial lime used for precipitation consists of calcium oxide (CaO), with large quantities of impurities. It should be stored in a dry place and protected from undue exposure to the air to prevent the formation of calcium carbonate (CaCO₃), the formation of which is commonly known as air slacking. The active work in the formation of the precipitate is performed by the lime (CaO) or calcium hydroxide (Ca(OH)₂). The lime should therefore be purchased on the basis of available CaO, which may be as low as 10 to 15 per cent in some commercial products. The amount of lime necessary depends on the quality of the sewage, the period of retention in the sedimentation basin, the method of application, the required results, and other less easily measured factors. Full scale tests for the amount of lime needed to produce certain results are the most satisfactory. In practice the amount of lime necessary when lime alone is used as a precipitant has been found to be about 15 grains per gallon. This may be markedly different, dependent on the quality of the sewage. For acid sewages, lime alone is not suitable as a precipitant since it is necessary to add sufficient lime to neutralize the sewage before the calcium carbonate will be precipitated.

The use of copperas (FeSO₄) together with lime, leads to economy in the use of chemicals as the flocculent precipitate of ferrous hydroxide (Fe(OH)₂) is more voluminous than the precipitate of calcium carbonate. This is commonly known as the lime and iron process. The presence of iron in certain trade wastes may reduce the cost of chemical precipitation, as the necessary amount of copperas is reduced. Where 15 grains of lime alone will be needed per gallon of sewage, the total amount of chemicals used will be reduced to 8 to 10 grains per gallon with the use of lime and iron. This combination is less expensive than the use of lime alone, and is even cheaper where the iron is already present in the sewage. Such a condition is well illustrated by the sewage at Worcester, Mass., where the oldest and best known chemical precipitation plant in the United States is located. The amount of lime used at this plant has varied between 6 and 10 grains per gallon of sewage, the normal amount being about 7 grains. No iron is added because of the amount already in solution.

The results of a series of experiments on the chemical precipitation of sewage by Allen Hazen, are given in the 1890 Report of the Massachusetts State Board of Health, on p. 737 of the volume on the Purification of Water and Sewage. Hazen concludes as the result of his experiments: concerning lime,

There is a certain definite amount of lime ... which gives as good or better results than either more or less. This amount is that which exactly suffices to form normal carbonates with all the carbonic acid of the sewage. This amount can be determined in a few minutes by simple titration.

Concerning lime and iron (copperas) he states:

Ordinary house sewage is not sufficiently alkaline to precipitate copperas, and a small amount of lime must be added to obtain good results. The quantity of lime required depends both upon the composition of the sewage and the amount of copperas used, and can be calculated from titration of the sewage. Very imperfect results are obtained from too little lime, and, when too much is used, the excess is wasted, the result being the same as with a smaller quantity.

In precipitation by ferric sulphate and crude alum, the addition of lime was found unnecessary, as ordinary sewage contains enough alkali to decompose these salts. Within reasonable limits the more of these precipitants used the better is the result, but with very large quantities the improvement does not compare with the increased cost.