CHAPTER XXVII 

 THE ROLE OF BACTERIA IN THE TREATMENT OF SEWAGE 



F. W. MOHLMAN 

 Sanitary District of Chicago 



SEWAGE 



The heterogeneous nature of the components of human sewage insures the presence 

 of a complex bacterial life, primarily the major intestinal forms with occasional patho- 

 gens. A rich and varied food supply is presented to the organisms, but under new en- 

 vironment, as compared with the temperature and conditions of the intestinal tract. 

 The fate of the bacteria and of the organic and mineral solids depends upon the succes- 

 sive predominance of major groups of organisms and various conditions of symbiosis 

 so complex as to defy analysis. 



Because of this complexity there has been little attempt to isolate specific species. 

 Clark and Gage^ have listed the prevalence of some twenty organisms in Lawrence 

 (Mass.) sewage. The total count at 20° C. on gelatin, at 37° C. on agar, and the total 

 number of Bact.coli have been determined on so many sewages that they have become 

 standard procedures in sewage works operation. The 20° and 37° counts have little 

 significance unless the age of the sewage is known, since a sewage of low organic con- 

 tent might have an excessive count due to storage and septicity; on the other hand, 

 the Bad. coli count has more stability and is less subject to the vagaries of time and 

 temperature. 



Studies have been made by the United States PubHc Health Service on various 

 polluted rivers where the population contributing pollution has been known. The re- 

 sults shown in Table I have been summarized from their reports.^ These data indi- 

 cate the maximum quantitative units of total bacteria and Bad. coli per capita under 

 conditions of great dilution. The variation between winter and summer values is to 

 be expected for the gelatin and agar counts, but the variation in the Bad. coli count 

 is noteworthy. At the bottom of the table has been added the per capita count at 

 the Des Plaines River Sewage Treatment Works of the Sanitary District of Chicago, 

 where a moderately concentrated sewage, at 106 gallons per capita per day, is treated. 

 The agar and gelatin per capita counts are only 5 and t,.t, per cent of the river counts, 

 but Bad. coli is 43 per cent of the annual average. The constancy of Bad. coli counts 

 is apparently affected but little by dilution, but with a trend toward increased counts 

 with increased dilution, due either to growth or more effective dispersion. 



DECOMPOSITION OF SEWAGE 



The discharge of excreta into water dilutes the liquids and disperses the solids 

 more or less completely, dependent upon the time and degree of agitation. The water 



' Clark, H. W., and Gage, S. DeM.: Bacteriology and Biochemistry of Sewage Purification, 

 36th Ann. Rep., Mass. State Bd. of Health, pp. 231-33. 1904. 



' Hoskins, J. K.: "Quantitative Studies of Bacterial Pollution," Proc. Am. Soc. Civ. Engin., 

 SI, 9, 1810-55. Nov., 1925. 



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