Full-scale experiments to mitigate nitrous oxide emissions in wastewater treatment plants through optimizing aeration strategy

The quantification and mitigation of nitrous oxide (N₂O) emissions from wastewater treatment plants (WWTPs) are crucial for achieving climate-neutral wastewater treatment. Despite a solid understanding of potential N₂O mitigation, few full-scale studies have implemented and evaluated actual N₂O mitigation strategies. This study presents a long-term, full-scale monitoring campaign to quantify N₂O emissions from a specific WWTP, utilizing various measurement methods and devices in both the liquid and gas phases. Multiple measurement points (MPs) across the biological treatment stage allowed comprehensive tracking of emissions along the flow path. The use of the membrane inlet mass (MIMS) spectrometer, which enables a novel approach for simultaneous measurement of N2O in gas and liquid phases at three different MPs, should be emphasized. The study determined an overall N2O emission factor (EFN₂O) of 0.12 % ± 0.14 % for the investigation period, with short-term emission variations primarily linked to ammonium load and long-term trends driven by seasonal temperature transitions. To date, only a limited number of studies have monitored the impact of process adjustments on N2O emissions under full-scale conditions. Recognizing the significant influence of the aeration strategy on N₂O emissions, a series of full-scale experiments were conducted to examine the influence of different dissolved oxygen (DO) setpoints. Results showed that low DO concentrations (≤ 1.0 mg O₂/L) led to substantial increases in N₂O emissions, especially during high ammonium loading. Elevated DO setpoints (≥ 4.0 mg O₂/L) reduced emissions by up to 80 %, though with a marked increase in aeration energy demand. The study concludes that optimizing aeration through dynamic, demand-oriented DO control offers a promising pathway to reduce both direct N₂O emissions and energy consumption without compromising effluent quality.