Biological Sewage Disposal 243 



and allowed to remain for several hours to permit anaerobic action 

 to become established. Then the liquid is drained off through 

 the bottom of the bed, and the solids clinging to the stones and 

 other rough materials in the contact bed are permitted to under- 



Gas 



Clean Out 



Fig. 39. Section through an Imhoff tank. A typical tank may be 

 about 30 feet by 80 feet in size. (Froin General Bacteriology, D. B. 

 Swingle and W. G. Walter. Copyright 1947, D. Van Nostrand Company, 

 Inc., New York.) 



go aerobic action in which oxidizing bacteria play an important 

 role. The complete cycle of filling, standing, and emptying usually 

 entails about eight hours for an acre-sized bed processing up to 

 800,000 gallons of sewage a day. 



