Insights into Nitrous Oxide Mitigation Strategies in Wastewater Treatment and Challenges for Wider Implementation.

Nitrous oxide (NO) emissions account for the majority of the carbon footprint of wastewater treatment plants (WWTPs). Many NO mitigation strategies have since been developed while a holistic view is still missing. This article reviews the state-of-the-art of NO mitigation studies in wastewater treatment. Through analyzing existing studies, this article presents the essential knowledge to guide NO mitigations, and the logics behind mitigation strategies. In practice, mitigations are mainly carried out by aeration control, feed scheme optimization, and process optimization. Despite increasingly more studies, real implementation remains rare, which is a combined result of unclear climate change policies/incentives, as well as technical challenges. Five critical technical challenges, as well as opportunities, of NO mitigations were identified. It is proposed that (i) quantification methods for overall NO emissions and pathway contributions need improvement; (ii) a reliable while straightforward mathematical model is required to quantify benefits and compare mitigation strategies; (iii) tailored risk assessment needs to be conducted for WWTPs, in which more long-term full-scale trials of NO mitigation are urgently needed to enable robust assessments of the resulting operational costs and impact on nutrient removal performance; (iv) current mitigation strategies focus on centralized WWTPs, more investigations are warranted for decentralised systems, especially decentralized activated sludge WWTPs; and (v) NO may be mitigated by adopting novel strategies promoting NO reduction denitrification or microorganisms that emit less NO. Overall, we conclude NO mitigation research is reaching a maturity while challenges still exist for a wider implementation, especially in relation to the reliability of NO mitigation strategies and potential risks to nutrient removal performances of WWTPs.