122 Bacteria in Relation to Country Life 



nitrifying bacteria. In the same filter beds, under 

 different conditions, a less thorough aeration and a 

 larger supply of soluble organic matter, may encourage 

 the growth of the denitrifying bacteria and the destruc- 

 tion of the nitrates already formed. 



Loss of nitrogen. — The extent of denitrification, that 

 is, the extent of destruction to which the nitrates are 

 subjected, is naturally variable. The same may be said 

 of the bacterial processes in the septic tank and beds, 

 which involve losses of gaseous nitrogen in the course of 

 putrefaction and decay occurring there. Economically, 

 the differences in question are of some moment, especially 

 when the effluents are used for irrigation purposes. It 

 seems, therefore, that with the better understanding of 

 the bacteriological changes in sewage-purification, means 

 will be found to avoid unnecessary losses of nitrogen 

 without detracting from the efficiency of the process. 

 Nitrogen-fixing bacteria have also been found in the 

 bacteria-beds, and actual gains of combined nitrogen 

 observed. Information on this point, however, is very 

 meager. 



Bacterial efficiency in sewage -purification. — The ca- 

 pacity of bacteria-beds for the purification of sewage 

 depends on the numbers and kinds of bacteria present 

 there, as well as on the vigor of the organisms. But the 

 bacterial work accomplished in the filter beds is by no 

 means determined by the numbers alone. It may readily 

 happen that 1,000,000 bacteria in one filter bed will 

 perform as much work as 2,000,000 or 3,000,000 of the 

 same kind in another bed. The difference is in vigor, or, 

 to use a more exact term, in physiological efficiency. 



