86 THE MANURE HEAP AND SEWAGE. 



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. In a 

 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 dangerous 

 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 disease 

 germs? So far as evidence goes to-day it seems that the typhoid 

 bacillus is eliminated by the treatment, and the effluent from such 

 beds fails to show typhoid bacteria, even when they have been pur- 

 posely put in the sewage. Bacteriologists who have had confidence 

 in the efficacy of the purification have not hesitated to drink 

 freely of the water from such a sewage filter bed. It is certain, 

 therefore, that the treatment greatly improves the healthfulness 

 of the sewage. But that it removes all danger from it cannot be 

 positively stated. 



Such a disposal of sewage means, of course, a complete loss of the 

 nitrogenous material, for no method is adopted for utilizing the 

 wasted nitrates. But this fact is no longer regarded so seriously as 

 it was a few years ago. We have learned that there are efficient 

 forces in nature for bringing back from the atmosphere the nitrogen 

 dissipated from the soil, and it is a matter of less significance to throw 

 away the sewage nitrogen than it appeared to be when the only 

 known source of nitrogen was supposed to be the fixed nitrogen of 

 the soil. Since the soil can readily replace its lost nitrogen through 



