SEWAGE TREATMENT — KEEFER 367 



nearly to the bottom of the tank. Porous diffusers are provided at the 

 bottom of the central compartment along the center line of the tank. 

 When air is blown through these diffusers, it carries any grease and 

 oil to the tank surface where these materials are removed manually. 



SEDIMENTATION TANKS 



After sewage has been screened and the grit has been removed, it is 

 processed to remove a considerable percentage of the suspended solids 

 by passing the sewage continuously through sedimentation tanks at a 

 greatly reduced velocity. The suspended solids settle out and are re- 

 moved in a variety of ways. This deposited material is called sewage 

 sludge. 



In the United States sedimentation tanks are of such size that the 

 sewage is retained in the tanks for about 2 to 3 hours. During this 

 detention period about 35 to 40 percent of the oxidizable materials 

 and 60 to 70 percent of the suspended solids are removed. 



Sedimentation tanks are of many different types and shapes. They 

 are all designed basically to obtain a reduced and uniform velocity of 

 flow throughout the tank, with the ratio of tank surface to tank volume 

 large, and to remove the sludge rapidly and in as concentrated a con- 

 sistency as possible. They are usually square, rectangular, or circular 

 in plan, with working depths varying from 5 to 6 feet for small units 

 to almost 14 feet for large ones. Various methods have been devised 

 for removing the sludge. In the older designs this was done manually. 

 After a tank had been in service for several days or weeks, the flow 

 was diverted to another tank and the supernatant was pumped or 

 drained off until the sludge was exposed. This material was then 

 squeegeed or flushed with water to a pit in or near the tank and then 

 removed, usually by pumping, for further treatment or disposal. 



Another type of tank that still has a limited use is provided with 

 steep floors that slope to a sump. From here there is a drain pipe 

 through which the sludge discharges to some point of disposal. 



The type that has come into almost universal use during the past 

 30 years is provided with mechanically operated equipment for remov- 

 ing the sludge either continuously or intermittently. One type of 

 equipment that is extensively used in circular tanks (text fig. 1) con- 

 sists of two trusses made of structural steel shapes suspended immedi- 

 ately above the tank floor and supported at the center of the tank. 

 Thin metal squeegees are attached to the bottoms of the trusses and 

 make contact with the tank floor, which slopes toward the center of the 

 tank. The whole mechanism is rotated slowly about the vertical center 

 line of the tank, and the sludge is moved by the squeegees toward the 

 tank center. From here the sludge discharges through a pipe as a 

 result of the hydrostatic pressure of the superimposed sewage. An- 

 other type of cleaning equipment adopted especially for rectangular 



