The experience of the New Jersey State Board of Health with Imhoff tanks has been that if properly designed, constructed and operated, they are a valuable means of sewage clarification. The observation of its engineers has shown that under these conditions the tanks overcome a great deal of trouble due to odors and greatly simplify the sludge problem. “However, their proper operation is a considerable problem,” reads one of its reports. “And the cost of keeping them in working order is several times greater than for septic or sedimentation tanks.” In view of the initial cost of this form of tank as compared with the older single story types the New Jersey engineers believe that “in cases where the works are far removed from a populous community, so that the odor problem is not serious, it is doubtful whether the Imhoff tank has any material advantage over a properly constructed, well baffled sedimentation tank of the old type.”
The Cleveland Sewage Testing Station reports that the most consistent results were obtained from the operation of the Imhoff tank, an average suspended matter removal of 50 per cent. being secured. A recent city report says: “In general it may be said that a detention period of thirty minutes accomplished a removal of suspended matter from 40 to 45 per cent. as compared with a 50 per cent. removal effected by a detention period of two hours and fifteen minutes.”
In a bulletin of the California Board of Health, Charles G. Hyde sums up the importance of the septic process thus: “The septic process as carried out either in the Cameron or Imhoff type, but especially in the latter, has at present two distinct fields of usefulness; first, it constitutes an effective means of preparation for any final process which can be better conducted with a sewage from which the suspended solids are more or less completely removed; secondly, it may be employed when disposal by dilution is permissible if the source of unsightly sludge and scum is removed.” Another advantage may be added, the Imhoff tank produces a sludge that can be disposed of easily.
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.”