THE ACTION OF CERTAIN BACTERIA ON SEWAGE 141 
with little or no change in either the nitrites or nitrates. 
In order to bring about conservation of the higher oxidation 
products of nitrogen, it is essential that sufficient oxygen be 
present so that the micro-organism is not forced to reduce 
these substances to support life. I believe that at the present 
time it is considered wise to have as much residual oxygen 
as possible in the septic tank. I found, as I will point out 
later, that the action of certain bacteria, in fact of nearly all 
of the particular species studied, was much more dependable 
under aerobic than anaerobic conditions. 
METHODS 
Tlie methods used simulated natural conditions as much as 
possible in laboratory experiments on a small scale. In order 
to use sewage as a culture medium it is necessary to sterilize 
it, and any method of sterilization changes the chemical com- 
position of the sample. It is therefore necessary to select 
that method which produces a minimum change. Chemical 
methods are naturally out of the question, because of their 
ineffectiveness in bringing about complete sterility without a 
great excess of the chemical, which would have an inhibiting 
action on the bacteria subsequently added. The incomplete- 
ness of sterilization by chemical means has been shown by 
Lederer and Hommon? in their paper before the American 
Public Health Association, at the meeting of 1910. 
If the gross solid material is filtered out, the sewage can 
be sterilized by heat, without greatly affecting the chemical 
composition. 
While there is a small difference in composition between 
the unsterilized and sterilized sewage, the differences in the 
flasks of sterilized sewage made up from the same large 
sample are so slight as to be negligible. 
The sewage used was filtered through cotton to remove 
the gross solid material, put into liter flasks and sterilized at 
120° C. (15 pounds pressure) in the autoclave for 45 minutes. 
Two sets of flasks were made up at a time for each series. To 
> Lederer and Hommon, Journal American Public Health Association, I 
(New Series), p. 267, 1911. 
