Low DO at Full Scale: Parallel Comparison of Airflow Reduction, Effluent Quality, and N ₂ O Emissions

Lowering dissolved oxygen (DO) may reduce aeration demand while maintaining biological nutrient removal in activated sludge, but parallel full-scale comparisons remain limited. This study compared two side-by-side basins at a municipal water resource recovery facility (WRRF): a conventional anaerobic-oxic control basin operated at 3 mg L–1 DO and a modified anaerobic-anoxic-oxic test basin with internal mixed-liquor recycle and adaptive low-DO control (0.3–1.0 mg L–1). Airflow, DO, and nitrous oxide (N2O) were monitored in multiple subzones, and grab-profile data were used to assess nitrogen pathway partitioning, including simultaneous nitrification-denitrification (SND). The test basin reduced airflow by an average of 27%. Effluent nitrate was 2–5 mg L–1 lower than in the control, while ammonia oxidation remained complete, and profile-based estimates indicated that SND contributed about 16% of total nitrogen removal. Total phosphorus performance was maintained despite a 64% reduction in ferric dosing, consistent with process indicators of stronger biological phosphorus removal. N2O increased transiently during the initial step-down to low DO but returned toward baseline after stabilization. These results show that controlled low-DO operation can reduce aeration demand while improving nitrogen removal and maintaining stable phosphorus performance at full scale.