Primary sewage was pressurized at 100 psi for one hour. The effects of this pressure on the growth patterns of general and nitrifying bacteria were examined daily for eighteen consecutive days using Standard Plate Counts and Most Probable Number (MPN) estimates. In conjunction with these tests, the degree of biodegradability was also determined by measuring both daily biochemical oxygen demands (BOD's) and dissolved oxygen levels.

Graphical analysis of experimental data compare the BOD's exterted [sic] by microorganisms to their respective numbers of colonies as determined by plate counts of both pressurized and non-pressurized sanitary sewage diluted to 3. 00%. The analysis of the combined data revealed the following: (1) pressurization increased the initial population of general bacteria by 55% and 100%, and (2) the second-stage BOD's of pressurized sewage which began three to four days earlier than that of the non-pressurized sewage was reduced by as much as 17.7%.

In addition, the effects of pressure on the population growth of nitrifying bacteria were examined. Serial 10-fold and 5-fold dilution-to-extinction techniques were use to obtain the MPN estimates of the NH₄⁺ oxidizers group. This is a common indirect method used to enumerate the population growth of nitrosomas bacteria.

The graphs of the MPN's indicate that the population and the rate of growth of nitrosomas bacteria were greater in the pressurized sewage. Similar tests to determine the growth patterns of nitrobacter bacteria failed.

It was concluded that pressurization initiates the propagation of accelerated bacterial growth, thereby increasing the assimilative capacity of the bacteria and reducing biodegradability of sanitary sewage.