Initial free ammonia is critical for robust acidic partial nitrification in sequencing batch reactor.

The full implementation of partial nitrification (PN) is still limited by low influent NH-N concentrations. The stability and boundary conditions for nitrite-oxidizing bacteria (NOB) inhibition in the sequencing batch reactor (SBR) mode for low-strength wastewater with dynamic environments remain ambiguous. This study attempted to identify the critical factors and extended boundary conditions for the stable PN in SBR. In this study, based on the long-term operation (580 days) of a conventionally configured SBR, we investigated the robustness of the more complex combined inhibition by dynamic free ammonia (FA) and free nitrous acid conversion on different NOB in SBR. The experimental results revealed that the initial FA concentration was critical for establishing PN under low-alkalinity influent conditions. The NOB genus Nitrospira, which grows in periodically acidic environments, was extremely sensitive to FA inhibition (K=0.35 mg NH-N/L) and significantly reduced the FA concentration required for NOB inhibition. When the pH was lowered to 5.4, the acid-intolerant NOB genus Ca. Nitrotoga was effectively inhibited, which was essential for maintaining PN. Further, the influent concentration boundaries and corresponding discharge ratios for achieving PN in the SBR mode were predicted by measuring kinetic parameters and model development, and the results suggest that the initial FA concentration in the reactor is critical for achieving acidic PN. The results of this study can provide guidance for achieving robust acidic PN in conventionally configured SBR for low strength wastewater.