Field Evaluation of Ammonia-Based Aeration Control and N₂O Response in a Full-Scale BNR Reactor

[b]Objectives and Status[/b] Wastewater treatment plants are under growing pressure to reduce aeration energy while also mitigating direct greenhouse gas emissions, particularly nitrous oxide (N₂O). Although ammonia-based aeration control (ABAC) is increasingly implemented to reduce energy demand, its combined impacts on aeration energy, nitrogen removal performance, and N₂O emissions have not been quantified comprehensively at full scale. Most previous studies have evaluated either energy savings or N₂O behavior in isolation, leaving their integrated effects under real operating conditions uncertain. This study presents a full-scale comparison of energy–N₂O–performance trade-offs under ABAC and conventional fixed dissolved oxygen (DO) control strategies in a mainstream biological nutrient removal (BNR) reactor. To the authors' knowledge, this is the first study to compare high-DO, reduced-DO, and ammonia-based aeration control within the same full-scale mainstream BNR reactor using continuous dissolved N₂O monitoring. [b]Methodology[/b] The investigation was conducted in a nine-cell BNR reactor serving approximately 1.37 million population equivalents in western Canada and treating 40–50 MLD. Three aeration strategies were implemented sequentially: (i) fixed high DO (1.5–3.0 mg L⁻¹), (ii) feedback-Feedforward ABAC using online ammonia measurements to dynamically adjust airflow to meet effluent ammonia targets, and (iii) fixed reduced DO (0.8–2.0 mg L⁻¹). Each strategy was operated for 3–4 weeks under steady-state conditions. Two online Clark-type dissolved N₂O sensors were installed along the reactor length to continuously measure dissolved N₂O, which was converted to gaseous emissions using established mass-transfer relationships. Blower airflow, bioreactor power, and specific aeration energy were extracted from the plant SCADA system, while routine grab samples and online sensors were used to monitor NH₄⁺–N, NO₂⁻–N, NO₃⁻–N, COD, DO, pH, temperature, and dissolved N₂O concentrations. [b]Findings[/b] All aeration strategies achieved stable organic matter removal, with COD removal ranging from 78–83% across all phases. Ammonia removal efficiencies were 98.2% under high DO, 96.9% under ABAC, and 89.0% under reduced DO, with ABAC consistently maintaining effluent ammonia within compliance limits despite operating at lower average DO. Relative to the high-DO baseline, ABAC reduced airflow by 16% and bioreactor power by 18%, lowering the specific aeration energy from 0.096 to 0.086 kWh m⁻³. Operation under reduced DO achieved larger energy savings, with airflow and power reductions of 24% and 26%, respectively, and specific energy decreasing to 0.074 kWh m⁻³. Despite these reductions in aeration demand, N₂O emission factors expressed as a percentage of total nitrogen removed remained statistically similar for high DO (0.50%) and ABAC (0.50%), demonstrating that ABAC delivered meaningful energy savings without increasing N₂O emissions. In contrast, reduced DO operation decreased the N₂O emission factor to 0.37% but was associated with reduced nitrification robustness, as reflected by lower ammonia removal efficiency. These results indicate that while more aggressive DO reduction can suppress N₂O formation, ABAC achieves a favorable balance by lowering energy consumption while preserving both treatment performance and N₂O stability. [b]Significance[/b] This full-scale dataset demonstrates that ammonia-based aeration control can achieve significant reductions in aeration energy while maintaining nitrogen removal performance and without increasing N₂O emissions relative to conventional high-DO operation. The findings provide rare full-scale evidence that dynamic ammonia-feedback-feedforward aeration represents an energy-efficiency strategy that is climate-neutral with respect to N₂O, whereas more aggressive DO minimization introduces trade-offs between energy savings, emissions, and process stability.