Sewage and Sewage Effluents. 187 



mus and note the number of red colonies, as a check on 

 the enumeration of B. coli. 



The determination of the number of colon bacilli in 

 sewage and effluents should furnish an integral part of 

 bacteriological sewage analysis since it is important to 

 know whether the decrease of intestinal bacteria in the 

 process of purification is proportional to the reduction of 

 total bacteria. The State Sewerage Commission of New 

 Jersey has adopted this procedure in its supervision of 

 the disposal plants in that state; and the results seem 

 amply commensurate with the labor involved. As in the 

 case of polluted waters the enumeration of B. coli may be 

 carried out, either by the study of the red colonies which 

 appear on litmus-lactose-agar plates inoculated with the 

 sample directly, or by the use of a preliminary enrich- 

 ment process. The complete identification of B. coli 

 seems unnecessarily tedious, however, where the organ- 

 isms are present in such abundance. Some approximate 

 presumptive method is indicated here, if anywhere; and the 

 experience with polluted water, reviewed in Chapter VIII, 

 points to the Jackson bile medium as the most prom- 

 ising one. More than a year's experience at the Sewage 

 Experiment Station of the Massachusetts Institute of 

 Technology has shown that this presumptive test in general 

 yields good results. As pointed out above, a 72-hour 

 incubation period at 37 degrees is required. All tubes 

 showing 20 per cent gas at the end of this time may be 

 considered positive tests for B. coli without serious error. 



