Reactor performance. The influent wastewater was the same for all reactors. However, biomass was subjected to unique NH3 and oxygen concentrations in each reactor, as influent feeding was semicontinuous and NH3 accumulation occurred during nonaerated periods. The nitrogen, pH, DO, and oxidation-reduction potential profiles for the reactors described in this study have been previously discussed (M. A. Head et al.,submitted for publication). A summary of the data for nitrogen and DO profiles is presented in Table 2. Reactors B (ANA,1 h:3 h) and E (ANA, 1 h:4 h), showed nitrite accumulation of up to 5.4 mg NO2-N liter1 and 7.8 mg NO2-N liter1,respectively, while the other reactors exhibited negligible nitrite accumulation during aeration periods. Only reactor E,with the longest nonaeration period, showed substantial nitrite reduction during nonaeration periods as indicated by an effluent nitrite concentration of 0.8 mg NO2-N liter1 at the end of nonaeration periods. Nitrite reduction in reactor B was minor, as indicated by an effluent nitrite concentration of 4.9 mg NO2-N liter1 at the end of nonaeration periods.