Sewage and Sewage Effluents. 189 



587,100 in November, and 712,000 in December (Wins- 

 low, 1905). There is also a marked diurnal variation 

 in the bacterial content of sewage, since the flow contains 

 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 



4-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 true values, 

 since they refer in most cases to day samples only. 



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. The aver- 

 age numbers obtained from the outdoor experimental 

 niters at Lawrence (each &-$ acre) in 1905 are tabu- 

 lated below. 



