. Preventive medicine and hygiene. ge,according to the fineness of the screens. It is pressed and burned undera boiler or buried in land. Screening has attained its greatest develop-ment in Germany. Sedimentation.—Sedimentation is the most important of the pre-paratory processes. By allowing the sewage to flow slowly throughbasins in which the velocity is checked some of the suspended matter isdeposited and the sewage clarified accordingly. There are five typesof sedimentation basins: (1) grit chambers or detritus tanks, (2) plainsettling tanks, (3) septic tanks, (4) digestion tanks, and (5) c
. Preventive medicine and hygiene. ge,according to the fineness of the screens. It is pressed and burned undera boiler or buried in land. Screening has attained its greatest develop-ment in Germany. Sedimentation.—Sedimentation is the most important of the pre-paratory processes. By allowing the sewage to flow slowly throughbasins in which the velocity is checked some of the suspended matter isdeposited and the sewage clarified accordingly. There are five typesof sedimentation basins: (1) grit chambers or detritus tanks, (2) plainsettling tanks, (3) septic tanks, (4) digestion tanks, and (5) chemicalprecipitation tanks. (1) Grit Chambers.—Grit chambers are small settling basins inwhich the sewage remains for a brief interval, often not more than afew minutes, and where the velocity is commonly between 10 and 30 TEEATMENT OF SEWAGE 965 inches per minute. They require frequent cleaning. The material col-lected consists largely of sand and gravel, but usually with enough or-ganic matter to make the sludge offensive. 5. Fig. 121.—Cross Section of Septic Tank. (2) Plain Settling Tanks.—Plain settling basins retain the sewagefrom one to twelve hours. The velocity of flow is commonly from inch per minute. Sludge is removed at frequent intervals inorder to prevent bacterial decomposition. (3) Septic Tanks.—Septic tanks are settling tanks large enough toretain the flow of sewage from eight to twenty-four hours or longer, thevelocity of flow varying from 0,3 inch or more per sludge is allowed to remain inthe tanks for long periods, givingopportunity for intensified bac-terial action .to take place in theabsence of oxygen; that is, underanaerobic conditions. As a resultsome of the solid organic matter isliquefied or gasified and theamount of sludge reduced. Thisprocess is spoken of as is accompanied by the presenceof a scum on the surface of thetank and a continual rising andfalling of sludge through theliquid. The amount of so
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Keywords: ., bookauthorwh, bookcentury1900, bookdecade1910, booksubjecthygiene
