The introduction of an anaerobic zone before the aerobic zone of a conventional activated sludge system was previously found to result in less-than-theoretical oxygen use. The difference between actual and theoretical oxygen use was termed Anaerobic Stabilization (AnS). In this study, an improved method was developed for the determination of AnS, and was applied to a lab-scale Anaerobic/Oxic (A/O) system in which nitrification was chemically inhibited. 15-20 percent of the theoretical oxygen requirement was eliminated by AnS. Oxygen, COD, and sulfate were found to be the major species involved in redox reactions in the system, and therefore major components of AnS. Production of hydrogen, methane, or carbon monoxide, and limitations of the COD test were eliminated as possible explanations for AnS. Aeration-induced stripping of reduced volatile species produced in the anaerobic zone was identified as a possible mechanism requiring further investigation.

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