Stage-specific nitrous oxide emission responses to different oxygen-supply regimes in anaerobic/oxic/anoxic process...

Nitrous oxide (N₂O), a potent greenhouse gas, poses a challenge to low-carbon wastewater treatment because of its release during biological nitrogen removal. Aeration regulates nitrogen transformation and N₂O emissions in anaerobic/oxic/anoxic (AOA) systems, yet the stage-specific partitioning of these emissions under different oxygen-supply regimes remains unclear. Here, a gas-tight sequencing batch reactor treating real municipal wastewater was successively operated under three combined oxygen-supply regimes: 0.5 L/min for 180 min, 1.0 L/min for 120 min, and 1.5 L/min for 90 min. The best combined performance was obtained under the 0.5 L/min regime, with total inorganic nitrogen (TIN) removal efficiency of 75.5% and the lowest N₂O-N emission factor of 5.4 × 10^-4 kg N₂O-N/kg TIN_removed. Compared with this regime, the 1.5 L/min regime had 56% and 109% higher oxic-stage and total N₂O-N emission factors, respectively, while its TIN removal efficiency decreased to 70.0%. Although the anoxic-stage contribution increased from 2.4% to 19.4%, the oxic stage remained the dominant emission source, accounting for more than 60% under all three regimes. The higher contribution of the anoxic stage coincided with lower simultaneous nitrification and denitrification efficiency and greater nitrite and nitrate carryover from the oxic stage. The 0.5 L/min regime also exhibited higher nosZ gene abundance and nitrous oxide reductase-to-nitric oxide reductase ratios than the 1.5 L/min regime, consistent with its lower N₂O-N emission factor. These findings indicate that low-intensity aeration combined with sufficient oxic duration can maintain nitrogen removal while limiting oxic-stage N₂O emissions and oxidized nitrogen transfer to the subsequent anoxic stage.