7O SOIL AND SEWAGE 



and examinations, while of great value, are no guide in regard 

 to the extent to which pathogenic bacteria have been removed 

 by any process of sewage purification. 



Bacterial content of crude sezvage. 



As might be anticipated the actual bacterial content of crude 

 sewage varies enormously, since sewages vary so greatly in 

 strength and the extent to which they become mixed with 

 surface water. Speaking generally the number of bacteria 

 present, as shown by the number of colonies on gelatine plates, 

 usually ranges from I million to 100 million per c.c. This 

 number will vary from hour to hour according to the strength 

 of the sewage. 



The organisms used as indicators of sewage and excretal 

 contamination are all very abundant in sewage, B. coli organisms 

 about 100,000, streptococci about 10,000, and spores of B. enteri- 

 tidis sporogenes about 100 to 1000 per c.c. 



The number of different kinds of organisms in sewage is 

 very great, and it is probable that many of them occur in all 

 specimens of ordinary sewage, but except for the above organisms 

 their presence has not been ascertained with sufficient constancy, 

 nor has their numerical occurrence been sufficiently investigated 

 to enable them to be used as indicators of sewage pollution. 

 Further investigations in this direction are very desirable. 



The organisms of typhoid fever and cholera have never been 

 isolated with certainty from sewage, although they must fre- 

 quently be added in large numbers when cases of these diseases 

 are present. Anthrax bacilli have been found by Houston in 

 Yeovil sewage, both in the septic tank and in the primary 

 and secondary coke beds, also in the mud of the banks of the 

 river Yeo. 



Effects of sewage treatment upon the bacterial content of sewage* 

 The reports of the Royal Commission on Sewage Disposal 

 have made it abundantly clear that while the different processes 

 for treating sewage may effect their immediate purpose of 

 producing a non-putrefying effluent, yet they never yield one 

 which is sterile or anything approaching it. The results obtained 



