Kinds of Germs in Beds 121 



of the sewage and the kind of germs native to the water 

 and soil. It thus comes about that the best results from 

 septic tanks, or contact and filter beds, are not obtained 

 until after a more or less prolonged period of preparation. 

 In order to facilitate the process, septic tanks have been 

 inoculated with sewage from old tanks in active opera- 

 tion, and gratifying results have been secured thereby. 

 Kinds of bacteria in filter beds and septic tanks. — While 

 there is thus a natural variation in the kinds and pro- 

 portions of bacteria occurring in different sewage- 

 purification plants, and an accompanying variation in 

 the amounts and composition of the products formed 

 by them, it still remains true that certain definite groups 

 of bacteria may be found in all septic tanks and filter 

 beds. The septic tanks all contain rod-shaped, spore- 

 forming cellulose ferments that can destroy woody tissue 

 with the formation of the combustible gases, hydrogen 

 and marsh gas. They also contain several kinds of rod- 

 shaped putrefaction-bacteria, and a small proportion of 

 spherical organisms. The contact and filter beds con- 

 tain, among others, a number of species of small rod- 

 shaped bacteria forming no spores and developing 

 preferably in the presence of atmospheric oxygen. They 

 contain, also, the nitrous and nitric ferments whose 

 function it is to change ammonia into nitrites and ni- 

 trates, as will be described more fully in the discussion 

 of soil bacteria. Moreover, the bacterial beds Hkewise 

 contain denitrifying bacteria — organisms that have the 

 power to destroy the nitrates already formed. In the 

 filter beds, a thorough aeration and comparatively small 

 proportion of ammonia may favor the activities of the 



