Recent advances in variability analysis of N₂O emissions from WWTPs and innovative mitigation/utilization approaches

Nitrous oxide (N₂O) 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 N₂O emissions in WWTPs in order to mitigating the N₂O emissions. Herein, we comprehensively explored the mechanisms for the variability in N₂O 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 N₂O 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, N₂O-reducing bacteria and microbes of oxygenic denitrification; and iii) adopting novel low-N₂O processes like anaerobic/oxic/anoxic (AOA) or membrane aerated biofilm reactors (MABR). Additionally, optimizing processes to capture and utilize N₂O as an resource is explored. Given the current lack of efficient N₂O recovery technologies, controlling influent nitrogen and optimizing operational conditions remain critical for minimizing N₂O emissions to achieve carbon neutrality.