Mapping and mitigating greenhouse gas emissions from urban sewer networks in China.

Urban underground sewer network is the backbone of modern cities, yet its role in global urban carbon metabolism remains a critical blind spot. Here, we establish a bottom-up, province-level accounting framework integrating multi-source socio-environmental datasets to quantify sewer-related greenhouse gas emissions across China's 30 provinces from 2010 to 2023, with projections to 2060 under five mitigation scenarios. The framework disaggregates emissions into five categories, namely leakage, in-sewer CH4 from gravity sewers and rising mains, N2O generation, and electricity consumption for pumping, each estimated using source-specific emission factors derived from the SeweX model and IPCC guidelines. A profound structural transition is revealed: driven by improved sewage collection, leakage-related emissions dropped from 90.3% to 58.3%; however, the expanding and maturing pipe networks inadvertently functioned as vast bioreactors, driving in-sewer process emissions from 9.3% to 38.8%. Spatially, this evolution follows an urban-development gradient, with eastern coastal agglomerations peaking earlier than rapidly developing inland regions. Sensitivity analysis identifies sewage collection rate as the most influential parameter. Emissions intensity strongly correlates with economic development, sewage volume, and pipeline length. Scenario analysis demonstrates that integrating end-of-pipe methane recovery with targeted leakage control offers the highest mitigation potential, reducing emissions by up to 79.2% by 2060 relative to business-as-usual. We further propose a regionally adaptive mitigation portfolio that prioritizes methane recovery in eastern coastal provinces with extensive rising mains and improves sewage collection in northwestern regions with low collection coverage, offering a transferable blueprint for decarbonizing underground infrastructure in rapidly urbanizing regions worldwide.