140 UNIVERSITY OF WISCONSIN STUDIES 
of bacteria do chemically when allowed to act on sewage in 
pure cultures singly or mixed. It was hoped that something 
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 has 
been confined largely to the study of the nitrogenous material, 
although there are certain other cycles which, as Fuller’ 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,” in his report on a somewhat similar investigation 
done at the Lawrenee 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 likely 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, 
1Fuller, Sewage Disposal, p. 47. 
?Clark, Report of the Mass. State Board of Health, p. 244, 1904. 
