Advanced nitrogen removal in a single return anaerobic/aerobic/anoxic/aerobic (AOAO) bioreactor treating municipal wastewater through hydroxylamine addition: Performance and microbial community.

The NHOH dosing strategy for nitrogen removal was investigated in a single return continuous-flow anaerobic/aerobic/anoxic/aerobic (AOAO) reactor fed with real municipal wastewater. A high nitrite accumulation ratio of 98% was achieved in only two days by continuously adding 10 mg/L NHOH. When gradually reducing dosing frequency to one day every four days, effluent total nitrogen was as low as 4.8 ± 2.2 mg N/L with removal efficiency of 88.7 ± 5.3%, under aerobic HRT of 4.6 h, DO below 1.0 mg/L, and C/N of 2.8 without external carbon sources. Batch test showed that nitrite oxidizing bacteria (NOB) activity decreased by 81% after adding NHOH, while ammonia oxidizing bacteria (AOB) activity remained stable. qPCR confirmed that NOB abundance decreased, and 16S rRNA sequencing further showed that g_Nitrospira belonging to NOB decreased significantly (P < 0.001). Overall, this study provides a novel strategy for advanced nitrogen removal from municipal wastewater in continuous flow systems.