Electron-acceptor-driven redox reconfiguration enables nitrogen polishing in a bioelectrochemical reactor operated in...

Bioelectrochemical systems offer a promising approach for nitrogen polishing of low-strength, carbon-limited wastewater; however, how electron-acceptor availability governs nitrogen conversion and electrochemical behaviour remains unclear. In this study, a bioelectrochemical sequencing batch reactor (BeSBR) and a control reactor were operated in sequencing batch mode, in which electron-acceptor conditions were sequentially varied within the same reactor system. The reactors were subjected to nitrite-abundant, nitrite-limited, nitrite-free, and oxygen-exposed phases, each maintained until stable performance was achieved. Nitrogen conversion was evaluated using cycle-based concentration profiles under steady-state conditions. Stable nitrogen polishing was achieved without external organic carbon addition, and powdered activated carbon (PAC) served as a conductive mediator supporting microbial retention and extracellular electron transfer. Nitrite-abundant conditions exhibited the most favourable electrochemical characteristics, while ammonium removal persisted under nitrite-limited and nitrite-free conditions, indicating adaptive nitrogen conversion under alternative electron acceptors. Kinetic analysis showed that ammonium removal exhibited approximately linear, zero-order-like behaviour under most conditions, with rates in the range of 2.0-2.5 mg N L^-1 h^-1, corresponding to removal of ∼25 mg N L^-1 within 10-12 h. Biomass stabilised at approximately 3,000-3,500 mg/L after initial reduction, indicating microbial adaptation. Microbial and functional analyses suggested multiple coexisting pathways, including nitrite-associated processes, dissimilatory nitrate reduction to ammonium, and sulfur-linked reactions. Overall, nitrogen conversion was governed by dynamic redox reconfiguration driven by electron-acceptor availability, rather than a single dominant pathway, highlighting its potential as an energy-efficient nitrogen polishing strategy.