TREATMENT OF CITY SEWAGE 93 
This whole topic is only a part of the general subject of the 
transformation of nitrogen. Whenever nitrogenous matter is 
mixed with water and allowed to stand for a time, decomposition 
changes begin which result in a more or less complete destruction 
of the compounds. This occurs in the soil, in the manure heap, in 
the privy vault, in the sink drain or in sewage, the phenomena 
being fundamentally the same in all cases, although differing in 
details with differences in the kind of compounds present, the 
amount of water, the temperature, the access of oxygen, the species 
of bacteria present, and, doubtless, other factors. It results in a 
purification of the soil or a purification of sewage from similar 
reasons. 
Effect on Bacteria.—It might be supposed that the bacterial 
treatment would increase the number of bacteria in the sewage. 
The rapid destruction of organic matter certainly points to active 
bacterial growth and we should expect to find bacteria more abun- 
dant at the end than at the beginning of the treatment. But for 
reasons as yet little understood, the reverse is the case. The 
number of bacteria in the treated sewage appears to be always less 
than in the raw sewage. The amount of reduction in bacteria is by 
no means constant. Sometimes it is comparatively small. Ina 
series of tests upon the sewage of London, treated in this way, a 
reduction of only about 32 per cent. was found (7,000,000 to 5,000,- 
ooo). In other cases the reduction is greater, and sometimes there 
is found a number as high as 9,000,000 per c.c. in the raw sewage, 
and only from 5,000 to 10,000 in the treated product. Something 
evidently is at work destroying the bacteria, but its efficiency 
varies widely in different instances. 
Whether these methods of treating sewage destroy its danger- 
ous nature as well as its offensiveness is not easy to answer. The 
danger in sewage comes primarily from the disease bacteria it may 
contain, foremost among which is the typhoid bacillus. The 
bacterial treatment greatly reduces the number of bacteria, but 
does not by any means eliminate them. Does it eliminate the 
