CHAPTER XI. 



BACTERIOLOGY OF SEWAGE AND SEWAGE EFFLUENTS. 



THE first object of modern sewage disposal is the 

 oxidation of putrescible organic matter. Chemical, rather 

 than bacterial, purification is 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 

 two-foot rule the saw will do the cutting, but the 

 rule will measure the work cut." (W. J. Dibdin.) 



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 prob- 

 lem assumes great importance. Where bacterially im- 



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