High ammonia concentrations have been shown to select AOB over NOB, resulting in nitrite accumulation. Generally,AOB have lower ammonia inhibition constants and can survive at elevated ammonia concentrations (42). Since influent feeding of the reactors was semicontinuous, ammonia accumulation occurred during nonaeration periods and biomass in reactors with different aeration cycles was subjected to unique ammonia concentrations. However, ammonia inhibition of NOB was likely not important, since usually only free ammonia is taken into consideration in the context of inhibition and free ammonia is negligible at the pH range (7.6 to 7.8) and total ammonia concentrations (1.2 to 64.0 mg NH3-N) maintained in this study.