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 



The 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 3 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 



3 Lederer and Hommon, Journal American Public Health Association, I 

 (New Series), p. 267, 1911. 



