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- 



