. The sanitation of cities. BALTIMORE, MARYLANDImhofF tank sewage treatment plant vent the gases arising from the digesting mattersfrom entering the upper chamber. The lowerchamber is provided with vents or chimneyswhich reach the surface to permit the escape ofgases into the air. Many tanks of this form have been built,especially in Germany, and when properly de-signed and operated with a very short detentionperiod this has proved to be the best type of tank. The septic process is essentially a fermentationand rotting out process. It is an attempt to letnature take her course, in carrying out
. The sanitation of cities. BALTIMORE, MARYLANDImhofF tank sewage treatment plant vent the gases arising from the digesting mattersfrom entering the upper chamber. The lowerchamber is provided with vents or chimneyswhich reach the surface to permit the escape ofgases into the air. Many tanks of this form have been built,especially in Germany, and when properly de-signed and operated with a very short detentionperiod this has proved to be the best type of tank. The septic process is essentially a fermentationand rotting out process. It is an attempt to letnature take her course, in carrying out the variouschanges that are required, for the partial or com- 87 ZbQ Sanitation of Cities plete decay of organic solids; but confining thenuisance, and odorous process, within the limitof a tank. It now is well understood that thisprocess requires a long time to accomplish muchof value in the reduction of organic solids; andthat the danger of a nuisance is present, in thetanks so far !_ONG BEACH, CALIFORNIA R-W screen sewage treatment plant adjacent to bathing beach shown in small insert 88 CHAPTER IX THE MECHANICAL TREATMENTOF SEWAGE The natural agencies employed under operatingconditions sometimes escape from the controlunder which the septic type of plant attempts toplace them, and cause odors and a more or lessactive nuisance. Since the first tank was used,objections have been made on the part of thepublic, and sanitary engineers have sought forother methods of freeing sewage from matters insuspension. It was noted soon after tanks beganto be used that they did not give a very highremoval of total solids from the sewage, and thatmuch suspended matter passed from suspensioninto solution, making the liquid part more foulthan before, especially if subjected to longperiods of detention. An obvious principle of sewage treatment is,that the quicker the polluting organic solid mat-ters are removed from the liquid part the better,as this prevents solution of the solid
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