Achieving partial nitritation-anammox in membrane-aerated biofilm reactor by hydroxylamine addition.

The membrane-aerated biofilm reactor (MABR) is naturally suitable for partial nitritation-anammox (PN/A) because aerobic and anaerobic microorganisms can grow in different biofilm layers. However, suppressing nitrite-oxidizing bacteria (NOB) in the oxygen-rich inner MABR biofilms remains challenging, while anammox bacteria (AnAOB) in the outer layer are more vulnerable to many harsh treatments. This study demonstrated the strategy of applying hydroxylamine (NH₂OH) to achieve stable PN/A in MABR. Over 250 days, long-term experiments showed that low dissolved oxygen (0.06-0.20 mg L^-1) could not suppress NOB, while both continuous and intermittent 10 mg L^-1 NH₂OH addition not only achieved effective NOB suppression but also significantly promoted ammonia-oxidizing bacteria and AnAOB. Consequently, the nitrogen removal efficiency increased from 74.0 ± 1.1 % to 91.7 ± 1.6 %. Fundamentally, NH₂OH addition rapidly changed the transcription levels of key functional genes in membrane biofilms, resulting in a significant decrease in nxrB transcription level by 68.6 ± 3.8 % and an increase in those of amoA and hzsB by 1835.8 ± 307.2 % and 314.2 ± 112.7 %, respectively. NH₂OH addition resulted in the temporary accumulation of hydrazine and nitric oxide at low levels, which collectively contributed to NOB suppression. Particularly, Nitrospira, "Ca. Nitrotoga" and Nitrolancea (all three distinct NOB genera present in MABR biofilm) relative abundances decreased by 48.6 ± 7.5 %, 10.8 ± 2.5 %, and 75.0 ± 6.8 % respectively, which alleviated NOB adaptation risk. Therefore, NH₂OH can be used to support NOB suppression in MABR.