

'-^T^.f-^^^V^Va^.W^ W^^«^ -''^^-^ -- .: 



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Fig. 40. (A) Primary settling tanks. Solids settle out as the 

 sewage flows slowly through these tanks. (B) Trickling filter. The 

 effluent from the settling tanks is sprayed intermittently onto the bed of 

 rocks where aerobic microorganisms oxidize the organic matter in the 

 liquid. {From Microbiology, W. B. Sarles, W. C. Frazier, J. B. Wilson, 

 and S, D. Knight. Copyright 1951, Harper and Brothers, New York.) 



244 



