TREATMENT OF CITY SEWAGE. 85 
chemical and bacterial agents, so that it flows out below quite 
changed in its nature. Sewage treated in this way does not look 
clear after treatment, but the organic products in it have undergone 
a change that makes them non-putrescible, and they will not undergo 
any further putrefactive changes. This method of treating sewage 
is recent and as yet not fully understood. 
Effect on Organic Products. As a result of these two types of 
decomposition the various organic bodies in the sewage are very 
largely destroyed by processes similar to those that occur in the 
manure heap. Various gases are liberated (HN 3 , N, CO 2 , CH 4 , 
H 2 S, etc.), and the total amount of solid matter is thus greatly re- 
duced. Later in the process, especially in the contact beds where 
oxygen is abundant, a vigorous oxidation of the nitrogen compounds 
begins (nitrification) which results in the formation of nitrates. 
These nitrates are, however, thoroughly soluble and become at once 
dissolved in the water of the sewage, which consequently clears up. 
In this way nearly all of the nitrogen which was held in high com- 
pounds in the original sewage, has either become dissipated into the 
air as ammonia or free nitrogen, or has become converted into 
nitrates and has dissolved in the water to form a clear solution which 
is not objectionable when discharged into streams. 
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- 
