128 AGRICULTURAL BACTERIOLOGY. 



but no attempt has been made to utilize it. It is well for 

 the agriculturist to remember that such effluent water derived 

 from sewage treated by bacteria, contains useful material if he 

 can contrive to use it for irrigation. 



It might be supposed that the bacterial treatment would 

 greatly 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 far 

 more abundant at the end than at the beginning of the treat- 

 ment. 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. In a long 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 5,000,000). 

 In other cases the reduction is very much greater, and some- 

 times 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 bac- 

 teria in large numbers, but its efficiency varies widely in differ- 

 ent instances. 



It must be remembered that this method of treating sewage 

 has been designed for the purpose of producing a chemical 

 rather than a bacterial purification. As a chemical process 

 the method is a success even if the bacteria should be left in 

 large numbers. If, in addition, the bacteria can be as much 

 reduced as indicated in some of the experiments, the whole 

 plan may be made surprisingly efficacious as a means of dis- 

 pnsing of sewage. 



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, decomposi- 

 tion changes begin which result in a more or less complete 



