BACTERIOLOGY OF SEWAGE 233 



11,487,500 in September, 3,693,000 in October, 587,100 

 in November, and 712,000 in December (Winslow, 

 1905). There is also a marked diurnal variation 

 in the bacterial content of sewage, since the flow con- 

 tains a smaller proportion of intestinal matter at night 

 than at other times. For example, a series of hourly 

 samples at the Sewage Experiment Station of the 

 Massachusetts Institute of Technology showed the 

 following results: 



BACTERIA IN BOSTON SEWAGE AVERAGES FOR EACH 

 FOUR-HOUR PERIOD. AUGUST 13-14, 1903 



(WINSLOW AND PHELPS, 1905) 



It is evident that many published results of bacterial 

 examinations of sewage are in excess of the average 

 values, since they refer in most cases to day samples 

 only. 



Bacterial Content of Sewage Effluents. The bacterial 

 content of sewage effluents varies widely according to 

 the process of purification adopted and the efficiency 

 of the particular plant. The only process which 

 yields a notably purified effluent from the bacteri- 

 ological standpoint is that of filtration through sand. 

 Processes of this type when operated with care may 

 give a bacterial purification well over 99 per cent as 



