360 



BACTERIA IN THE TREATMENT OF SEWAGE 



affected the operation of the Milwaukee and Chicago plants. The Calumet (Chicago) 

 plant has received paint-factory wastes containing acids, sulphur dioxide, arsenic, 

 lead, and copper. The cost of treatment (number of tanks and amount of air re- 

 quired) has been doubled by the presence of these wastes. Iron-pickling wastes at 

 Worcester (Mass.) interfered so seriously with the operation of the experimental ac- 

 tivated-sludge plant that the activated -sludge process could not be recommended 

 for the treatment of the city sewage. 



In general, it appears that one of the most serious problems in biological sewage 

 treatment in the large cities is the effect of inhibitory industrial wastes. 



REMOVAL OF BACTERIA 



Most processes of sewage treatment have not been designed primarily to remove 

 pathogenic bacteria or Bad. coli, the main function being to stimulate and promote 

 the growth of saprophytic forms. Nevertheless, a considerable reduction of Bad. 

 coli is effected by settling tanks, filters, and activated-sludge treatment. Clark {loc. 



TABLE II 



cit.) in 1904 reported very high percentage reductions of Bad. coli for experimental 

 devices at the Lawrence Experiment Station, averaging from o to 74 per cent for 

 septic tanks, from 49 to 76 per cent for contact beds, from 97 to 99 per cent for trick- 

 ling filters (stone \-i in. diameter), and from 98 to 99 per cent for sand filters. The 

 removal by trickling filters was undoubtedly greater than could be obtained by more 

 modern, coarser-grained filters. Johnson reported a reduction of 88 per cent of Bad. 

 coli by trickling filters at Columbus, with 35,000 per cubic centimeter remaining in 

 the effluent. 



Bacterial reductions are subject to such wide variations that a few results have 

 little meaning. Although a continuous record is kept at many treatment plants of 

 the bacterial counts on agar at 37° C, very few data are available over a period of 

 months or years concerning the reduction in Bad. coli. A conservative estimate of 

 the reduction in Bad. coli would show about 



30-5C per cent for Imhoff tanks 

 75-90 per cent for trickling filters 

 90-98 per cent for activated sludge 

 95-99 per cent for sand filters 



based on the count in the raw sewage. 



A year's analyses at the Des Plaines activated-sludge plant showed results as in 

 Table 11. 



