354 



BACTERIA IN THE TREATMENT OF SEWAGE 



possible. Chemical precipitation, now little used, is not bacterial, and the clarifica- 

 tion effected by activated sludge may be partly mechanical. 



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25 



ZO 



15 



20 

 Fig. I 



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The major work of sewage treatment, however, is done by bacterial processes, 

 including intermittent sand filtration, broad irrigation, contact beds, trickling filters, 

 activated sludge, and digestion of sludge. Septic tanks are biological only because 

 they are not designed to keep the sludge separate from the overflowing sewage. Im- 

 hoff tanks and frequently cleaned plain settling tanks do not depend on biological ac- 

 tion in the liquid, but only in the deposited solids after removal. It is, of course, true 



