Aerobic nitrous oxide emission in anoxic/aerobic and intermittent aeration sequencing batch reactors

This study characterized nitrogen removal and nitrous oxide (N₂O) emissions from lab-scale anoxic/aerobic sequencing batch reactor (AOSBR) and intermittent aeration sequencing batch reactor (IASBR), respectively, for treating synthetic municipal wastewater. The N₂O emission was evaluated in a simulated cycle, and batch conditions of aerobic nitrification, simultaneous nitrification and denitrification (SND), and aerobic denitrification. The results show that nitrogen removal was enhanced in IASBR compared to AOSBR, with 94.2% and 67.9% of total inorganic nitrogen removal efficiency in IASBR and AOSBR, respectively. In the simulated cycle, the emission factors (of oxidized ammonium) were 4.9% and 0.6% in AOSBR and IASBR, respectively. Under batch conditions, the N₂O emission factors during SND were obviously higher than that during aerobic nitrification and denitrification. The N₂O emission factors during SND ranging 0.68-11.68% in AOSBR and 1.25-5.13% in IASBR. Furthermore, N₂O emission under batch conditions was affected by the aeration ratios. Moderate and high aeration ratios used in this study stimulated the N₂O emission from SND. The N₂O emission was enhanced with the nitrite accumulation during aerobic nitrification when the nitrite-oxidizing bacteria was inhibited by a chemical inhibitor. Aerobic denitrification via nitrite could be the main pathway of N₂O generation from SND processes. The findings from our study can help further understand N₂O emission mechanisms and guide the optimization of the current wastewater treatment process for minimizing N₂O emission.