. Public works . ch change as soon as it occurs. The frequencyof such occurrences will depend upon local conditions;and as these vary so widely the attendant must make acareful study of his plant, the processes involved, and theresults to be accomplished. In general, the attendantshould know just how much chemical is required andthe exact strength of his solutions. Excesses of precipi-tants should be avoided, both on account of the waste ofthe material and for fear of injuring fish life in thestream. Most fish are extremely sensitive to chemicalsand an overdose is likely to cause the death of
. Public works . ch change as soon as it occurs. The frequencyof such occurrences will depend upon local conditions;and as these vary so widely the attendant must make acareful study of his plant, the processes involved, and theresults to be accomplished. In general, the attendantshould know just how much chemical is required andthe exact strength of his solutions. Excesses of precipi-tants should be avoided, both on account of the waste ofthe material and for fear of injuring fish life in thestream. Most fish are extremely sensitive to chemicalsand an overdose is likely to cause the death of the fishin the adjacent parts of the stream. After the sludge has settled, the clear liquids are drawnoff and the sludge removed and disposed of as any ordi-nary sludge. Frequently it is forced into specially con-structed sludge presses and pressed between layers ofcanvas or sack cloth. The pressed cake is more easilyhandled. In some instances it is used for land fillingand in others it is dumped into deep SLUDGE PRESSES AT WORCESTER. MASS. In order to make the status of chemical precipitationmore plain it may be well to quote the following resumefrom George W. Fullers excellent book on SewageDisposal: Chemical precipitation affords efficient clarification,but the removal of finely suspended matter, due to coag-ulation, does not permit a stable effluent to be obtainedfrom ordinary city sewage. The effluent is freer fromorganic matter than that obtained by plain sedimentation,and this allows a smaller degree of dilution when dis-persed in water, and of a higher rate of filtration, otherthings being equal. But present evidence shows stronglythat the improvement in the quality of the effluent overthat obtained by plain sedimentation is not commensuratewith the cost involved. For ordinary sewages the day of chemical precipitationplants is rapidly passing. In the case of certain tradewastes there may be a field of usefulness for this methodof clarification. With good managem
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