72 SOIL AND SEWAGE 



raw sewage. The different kinds of bacteria and their relative 

 abundance appear to be very much the same in the effluents 

 as in the crude sewage. Thus, as regards undesirable bacteria, 

 the effluents frequently contain nearly as many B. coli, proteus- 

 like germs, spores of B. enteritidis sporogenes, and streptococci, 

 as crude sewage. In no case, seemingly, has the reduction of 

 these objectionable bacteria been so marked as to be very 

 material from the point of view of the epidemiologist. No 

 definite proof has been furnished that the effluents from bacteria 

 beds are conspicuously more safe in this sense in their possible 

 relation to disease than is crude sewage. Indeed, all the avail- 

 able evidence tends to show that they must be regarded as 

 nearly, if not quite, as dangerous to health as raw sewage.... 

 The inoculation of animals with the effluents from bacterial 

 beds seems to show that they are nearly as pathogenic as crude 

 sewage." 



Judged from the bacteriological standpoint the only process 

 which yields a markedly purified effluent is filtration of the 

 sewage through sand. This method is very little used in 

 England, but is employed to some extent in the United States. 

 Extended experiments were carried out by the Massachusetts 

 State Board of Health with sand filters. These experiments 

 showed that 97 to nearly 100 per cent, of the B. coli could be 

 removed by these filters, and this whether they were treated with 

 raw or septic sewage. 



Although the percentage purification is very great the actual 

 number of B. coli remaining is large. Thus the experimental 

 sand filters at Columbus, Ohio, are recorded by Johnson as 

 removing 98'$ per cent, of these bacilli, but 500 to 10,000 

 B. coli per c.c. remained in the effluent. 



There does not appear to be much data available to judge 

 as to how far ordinary chemical processes of sewage purification 

 are capable of removing and reducing the bacteria in sewage, 

 but in general it may be said that they are not very effective. 

 Sterilization or partial sterilization of the sewage can be obtained 

 by chemical means, e.g. by the use of chlorine evolved in different 

 ways, but these are processes specially applied for this purpose, 

 and are not ordinary chemical methods of purification. 



