SEWAGE TREATMENT KEEFER 



377 



DRIVE CHAIN.- 



LINK-BELT 

 MECHANICAL AERATOR 



' \\ 1 1 ,(l WATER DePTO Mil '■•; 



LINK-BELT 

 MAXIMUM WIDTH OF TANKS " 



Figure 6. — Link-Belt mechanical aerators used in activated-sludge aeration tanks. 



their capacity can be varied by changing their speed, and their capacity 

 remains the same even though the frictional resistance to the flow of 

 the air may increase as air diffusers become clogged. 



Where air is blown through porous diffuser plates or tubes with 

 narrow air passages, it is necessary to clean the air to prevent the 

 clogging of the plates or tubes with foreign matter. Various methods 

 of cleaning the air are available, such as passing it through a multi- 

 plicity of overlapping oil-coated screen panels, canton flannel, or 

 cellulose tissue niters. The mixture of sewage and activated sludge 

 is aerated in long rectangular tanks, a typical cross section of which 

 is shown in text figure 7. Aeration tanks used in the United States 

 are generally designed to have a detention period of 4 to 6 hours. In 

 many instances they are several hundred feet long with a working 

 depth, which has become fairly standard in large installations, of 

 about 15 feet and widths varying from 20 to 30 feet or more. 



Many methods have been perfected for introducing air into the 

 sewage in fine bubbles, one of which is to blow the air through porous 

 tubes or plates. Porous plates, 12 inches square and 1 inch thick, are 

 made of crystalline alumina or a high-silica sand. A number of the 

 plates are set in a row in a horizontal position in shallow cast-iron or 

 concrete containers with space provided for the passage of air between 



