Methane emissions features from China's municipal wastewater management, based on the whole-process evaluation of collection, treatment and discharge

Municipal wastewater system, significantly influencing water environment via centralized treatment and discharge, becomes a major methane emission source. Anthropogenic organic matter degrades and transfers into methane throughout whole processes of wastewater collection, treatment, discharge and sludge disposal, yet the overall emissions, major contributors, spatial variations, and the synergy between pollutant elimination and emission mitigation remain rarely discussed. In this study we report a nationwide city-resolved estimate of methane emission from municipal wastewater system, involving (1) sewer networks and uncollected wastewater; (2) municipal wastewater treatment plants; (3) treated effluents; (4) sludge disposal, based on bottom-up analysis with firm-level active data and localized emission factor. We demonstrated that methane emissions from municipal wastewater system in China was 911 Gg, while wastewater collection (56.8 %) and sludge disposal (34.6 %) were the dominant contributors to nationwide emissions, which were generally excluded from previous estimations. Driven by regional climate, economic and infrastructure differences, major contributors for regions and cities showed spatial discrepancies. A negative linear relationship was identified between the sewage collection rate and per capita methane emissions in wastewater sector at city-level, which was explained by sewer management features linked to influent COD concentration, wastewater transportation and treatment efficiencies. Fine maintenance of sewer pipelines, appropriate match of sewers and treatment facilities, transition in sludge disposal methods may serve to realize synergistic mitigation of pollution and methane emission. This study highlighted the potential to achieve environment and climate co-benefits within municipal wastewater system, providing scientific basis and policy foresight for sustainable urbanization progress.