CHAPTER XI 



BACTERIOLOGY OF SEWAGE AND SEWAGE EFFLUENTS 



Bacteriological and Chemical Examination of Sewage. 

 The first object of modern sewage disposal is the oxida- 

 tion of putrescible organic matter. Chemical, rather 

 than bacterial, purification is usually the prime requisite; 

 and chemical tests therefore serve best as criteria of 

 the results obtained. Bacteria are the agents in the 

 process of sewage purification; but the most generally 

 useful measure of the work accomplished is the chemical 

 oxidation attained. " To employ a simile, it is a case 

 of the saw and the 2 -foot rule — the saw will do the 

 cutting, but the rule will measure the work cut." 

 (W. J. Didbin.) 



In certain cases, however, bacterial as well as chemical 

 purity must be effected, in view of special local require- 

 ments. The sewage from a contagious disease hospital, 

 for example, should be freed from infectious material as 

 a factor of safety. Sewage discharged into a body of 

 water adapted for bathing may well be so treated as to 

 protect those using the water. In the case of seaboard 

 cities where sewage effluents are likely to contaminate 

 oyster beds and other layings of edible shellfish the 

 problem assumes great importance. Where bacterially 

 impure effluents are discharged into streams used for 



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