Chemical Precipitation
By using some coagulant such as copperas, lime, sulphate of alumina or perchloride of iron, the subsidence in basins of between 40 and 55 per cent. of the total organic matter and between 60 and 95 per cent. of the total suspended matter can be obtained. The bacterial removal is between 80 and 90 per cent., depending upon the character of the sewage. The objections to this process are great cost of chemicals and labor required and the difficulty of disposing of a large amount of sludge. There are a few plants of this kind in operation at the present time and there seems to be a general agreement among authorities that the process is now a back number. Fowler says, “It may be doubted whether dilute sewages resulting from the lavish use of water in American cities lend themselves generally to economical treatment by this process.” Metcalf and Eddy in their “American Sewerage Practice” express the opinion that the quantity of chemicals required for results would be a prohibitive expense. The sewerage commission report of New Jersey contains the statement that “on the standpoint of the officials in charge of the experimental station at Lawrence, Massachusetts, chemical precipitation is a process of the past.” The experiments of the Massachusetts State Board of Health showed that it is quite impossible to obtain effluents by chemical precipitation which compare in organic purity with those obtained by intermittent sand filtration. About the only plants of any importance in the United States are those at Worcester, Massachusetts, and Providence, Rhode Island. According to the report of the Superintendent of Sewers of Worcester, the experimental plant in that city has shown that “the cost of operation of Imhoff tanks and sprinkling filters per million gallons of sewage treated would be much less than the cost of operation of chemical precipitation or sand filtration as carried on in Worcester.”