120 Bacteria in Relation to Country Life 



sewage is then almost complete and the effluent from 

 the filter beds is non-putrescible, and not 'injurious to 

 fish. 



Temperature and filter efficiency. — The efiiciency of 

 the aerobic filters, like that of the septic tank is affected 

 by changes in temperature. The aerobic decay bacteria, 

 and the nitrifying bacteria in the filters, multiply more 

 and more slowly, and cause less and less chemical 

 change as the. temperature falls. Still, their activity is 

 not entirely suspended, even after a coating of ice has 

 formed on the surface. It may not, however, be vigorous 

 enough to produce the desired transformation. In order 

 to remedy this retarded bacterial action in the winter 

 months, it has been proposed to raise the temperature 

 of the beds by artificial means. In some of the filters 

 designed for this purpose, the filter beds are provided 

 with steam-pipes. However, considerable expense is 

 involved in such treatment — expense not always justified 

 by the circumstances. 



It is well known that, among the aerobic as well as 

 among the anaerobic bacteria, there are races that can 

 grow at lower temperatures than others. This leads to 

 a natural selection and the establishment of certain 

 combinations of species under any given set of conditions. 

 It follows, therefore, that the breaking down of the 

 animal and vegetable materials in the septic tank is not 

 necessarily accomplished by the same species in the 

 different localities, or even in two different septic tanks. 



Inoculation. — Aside from the temperature, the estab- 

 lishment of definite kinds of bacteria in the tank or in the 

 beds is influenced by the composition and concentration 



