Intensifying mainstream nitrogen removal by coupling acidophilic ammonia oxidation with anammox

Partial nitritation and anammox (PNA) in membrane-aerated biofilm reactors (MABRs) offers a sustainable pathway for mainstream wastewater treatment; however, its application is often limited by the persistence of nitrite-oxidizing bacteria (NOB). This study demonstrated the feasibility of achieving the acidic PNA process in a lab-scale MABR treating synthetic mainstream wastewater. Long-term operation achieved stable and efficient nitrogen removal at a self-maintained bulk pH of ∼6.5, with a total nitrogen removal efficiency of 89.6 ± 0.9% and a nitrogen removal rate of 1.16±0.03 kg-N/(m³·d) (0.39 g-N/(m²·d)). Acidophilic ammonia-oxidizing bacteria (AOB, 'Candidatus Nitrosoglobus'), predominantly colonized the inner biofilm layer with a lower pH, while anammox bacteria ('Candidatus Brocadia') were mainly distributed in the outer layer. The continuous free nitrous acid (FNA) exposure in the inner layer contributed to NOB suppression, whereas nitrite competition by anammox bacteria in the outer layer further constrained NOB activity. This robust NOB suppression eliminated the need for strict dissolved oxygen limitation, thereby enabling higher oxygen supply and enhancing the acidic PN rate. Meanwhile, anammox bacteria achieved efficient performance that well matched with acidic PN within the mildly acidic microenvironment of the outer biofilm. Overall, this one-stage MABR-based acidic PNA process provides a new avenue towards stable and intensified mainstream nitrogen removal.