Recent advances in variability analysis of NO emissions from WWTPs and innovative mitigation/utilization approaches.

Nitrous oxide (NO) is a stringent greenhouse gas emitted from nitrogen removal processes in wastewater treatment plants (WWTPs), with emission factors (EFs) varying widely due to multi-dimensional factors like water quality, operational parameters, and biological processes. Current estimations often rely on the IPCC 2019 default EF of 1.1 ± 0.16 % of influent total nitrogen load, which may not capture process-specific variability. Additionally, it is critical to understand the mechanisms causing such diversity of NO emissions in WWTPs in order to mitigating the NO emissions. Herein, we comprehensively explored the mechanisms for the variability in NO EF including detection methods, water quality, operation parameters and biological treatment processes. The long-term multipoint semi continuous sampling combined with model prediction are recommended to improve the accuracy of the qualification of NO emissions. Moreover, three mitigation strategies are proposed: i) enhancing nitrogen recovery from influent sewage; ii) promoting functional bacteria such as complete ammonia oxidation (Comammox) bacteria, NO-reducing bacteria and microbes of oxygenic denitrification; and iii) adopting novel low-NO processes like anaerobic/oxic/anoxic (AOA) or membrane aerated biofilm reactors (MABR). Additionally, optimizing processes to capture and utilize NO as an resource is explored. Given the current lack of efficient NO recovery technologies, controlling influent nitrogen and optimizing operational conditions remain critical for minimizing NO emissions to achieve carbon neutrality.