A comparison of sludge properties and kinetic differences between a pure oxygen system and a conventional system

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Virginia Polytechnic Institute and State University


The objective of this research was to analyze a pure oxygen activated sludge process and its claimed advantages. The major emphasis of both the claims and this research was the settleability of the sludge and the biological kinetics. An identical conventional air activated sludge unit was also operated as a direct comparison.

Settling tests were run in three different cylinders with increasing diameters to collect data on the sidewall effects in addition to the settling rates. The settling velocities were also calculated and the average values for the oxygen and the air sludges were 95.0 ml/min and 104.0 ml/min, respectively. The results indicated there was no significant difference in the settling rates and the mixed liquor solids concentration did not affect these rates. The mass settling rate, a new parameter obtained by multiplying the settling velocity times the mixed liquor solids concentration, indicated the oxygen sludge settled more mass in a given time period than the air sludge and therefore compacted to a greater degree.

The substrate utilization rate of the oxygen process was found to be 2.8 times greater than that of the conventional air process. This finding is comparable to other published reports.

The oxygen utilization rate for the sludge from the air unit was slightly higher than that of the oxygen sludge. The coefficients of the kinetic formula did indicate that the oxygen system had a greater degree of endogenous stabilization occurring than did the air.

The oxygen unit used in this study consistently had better BOD removal efficiencies than did the air. The oxygen unit averaged 94%, while the air unit averaged 85%.

This investigation also indicates that an air unit will not operate at the identical parameters utilized in an oxygen process. This is due primarily to hydraulic difficulties encountered.