140 UNIVERSITY OF WISCONSIN STUDIES 



of bacteria do chemically when allowed to act on se\vage in 

 pure cultures singly or mixed. It was hoped that something 1 

 could be shown which would justify us in trying to control 

 the bacterial flora in large masses of sewage under treatment. 

 It was also expected that the results would enable us to say 

 what we might expect if certain groups or species of bacteria 

 proved to be present in predominant numbers. 



It is reasonable to assume that mere numbers of micro-or- 

 ganisms do not guarantee any definite chemical action. It is 

 only when certain species or groups are predominant that a 

 definite chemical change may be predicted or expected. At 

 the present time we can make no prediction of that sort, be- 

 cause it is not known what bacteria or groups of bacteria are 

 responsible for those changes which are considered desirable. 



In the past, work on putrefaction and decomposition lias 

 been confined largety to the study of the nitrogenous material, 

 although there are certain other cycles which, as Fuller 1 points 

 out, must be quite as carefully studied. In conformity with 

 past work along this line, it was decided to study the nitrogen 

 cycle. 



Clark, 2 in his report on a somewhat similar investigation 

 done at the Lawrence Experiment Station in Massachusetts, 

 points out that there are five lines of action, i. e., putrefaction, 

 nitrification, de-nitrification, nitrogen liberation, and nitro- 

 gen fixation. While undoubtedly all five of the processes go 

 on simultaneously, the process which predominates depends 

 entirely upon conditions present. His work was done in con- 

 nection with the nitrification in filter beds, but it is reasonable 

 to suppose that much the same thing would hold true for 

 septic tanks, except that the conditions are such, usually, that 

 putrefactive action is more likeh^'to predominate, especially 

 in the one story type of tank. 



At the beginning of this work it was assumed that the most 

 likely change in composition would be an increase in the free 

 ammonia content, and a corresponding decrease in those de- 

 composition products represented by the organic nitrogen. 



1 Fuller, Sewage Disposal, p. 47. 



2 Clark, Report of the Mass. State Board of Health, p. 244, 1904. 



