Mitigating nitrous oxide emissions at a full-scale wastewater treatment plant

Abstract

Mitigation of nitrous oxide (N<inf>2</inf>O) emissions is of primary importance to meet the targets of reducing carbon footprints of wastewater treatment plants (WWTPs). Despite of a large amount of N<inf>2</inf>O mitigation studies conducted in laboratories, full-scale implementation of N<inf>2</inf>O mitigation is scarce, mainly due to uncertainties of mitigation effectiveness, validation of N<inf>2</inf>O mathematical model, risks to nutrient removal performance and additional costs. This study aims to address the uncertainties by investigating the quantification, development and implementation of N<inf>2</inf>O mitigation strategies at a full-scale sequencing batch reactor (SBR). To achieve this, N<inf>2</inf>O emission dynamics, nutrient removal performance and operation of the SBR were monitored to quantify N<inf>2</inf>O emissions, and identify the N<inf>2</inf>O generation mechanisms. N<inf>2</inf>O mitigation strategies centered on reducing dissolved oxygen (DO) levels were consequently proposed and evaluated using a multi-pathway N<inf>2</inf>O production mathematical model before implementation. The implemented mitigation strategy resulted in a 35% reduction in N<inf>2</inf>O emissions (from the emission factor of 0.89 ± 0.05 to 0.58 ± 0.06%), which was equivalent to annual reduction of 2.35 tonne of N<inf>2</inf>O from the studied WWTP. This could be mainly attributed to reductions in N<inf>2</inf>O generated via the NH<inf>2</inf>OH oxidation pathway due to the lowering of DO level. As the first reported mitigation strategy permanently implemented at a full scale WWTP, it showcased that the mitigation of N<inf>2</inf>O emissions at full-scale is feasible and that widely accepted N<inf>2</inf>O mitigation strategies developed in laboratory studies are also likely effective in full-scale plants. Furthermore, the close agreement between the validated and predicted N<inf>2</inf>O emission factors (0.58% vs 0.55%, respectively), showed that the N<inf>2</inf>O mathematical model is a useful tool to evaluate N<inf>2</inf>O mitigation strategies at full-scale. Importantly this work demonstrated that N<inf>2</inf>O mitigation does not necessarily require additional operational cost to meet reduction targets. In contrast, the N<inf>2</inf>O mitigation applied here reduced energy requirements for aeration by 20%. Equally important, long-term monitoring identified that N<inf>2</inf>O mitigation did not affect the nutrient removal performance of the plant. Finally, with the knowledge acquired in this study, a standard approach for mitigating N<inf>2</inf>O emissions from full-scale treatment plants was proposed.