Assessment of methane and nitrous oxide emissions from urban community sewer networks: Field quantification and insights into environmental factors

Sewer networks are essential components of urban infrastructure, yet their contribution to greenhouse gas (GHG) emissions remains poorly understood. In this study, we deployed a new approach of in situ measurements to assess methane (CH₄) and nitrous oxide (N₂O) emissions across an urban sewer network, which spans 4769.43 m and receives about 750 m³ of domestic sewage per day. By monitoring at 248 and 151 sites for concentrations and fluxes respectively, we confirmed local GHG hotspots. Overall, the sewer network's total GHG emissions were estimated to be 763.3 g CO₂eq/h, with CH₄ accounting for 99.4 % of the emissions. The mean emission factor was estimated to be 1.05 kg CO₂eq/(m·yr). N₂O concentrations above the atmospheric background were detected in almost every manhole. Septic tanks (n = 19) were identified as the predominant sources, accounting for 92.5 % of emissions, while sewer pipes (n = 132) contributed the remaining 7.5 %. Emissions exhibited significant spatiotemporal variability, with daily fluctuations in CH₄ and N₂O ranging from 17- to 138-fold and 3- to 5-fold, respectively. Additionally, strong correlations were observed between CH₄ emissions and sewage temperature (R = 0.70, p = 0.017), as well as manhole depth (R = 0.67, p = 0.016). For N₂O, its emission strength was mostly related to the sewage temperature (R = 0.67, p = 0.024). These findings indicate that sewage temperature and sewer ventilation are critical factors influencing non-CO₂ GHG emissions. This study represents the first direct measurement of GHG emissions from an urban community sewer network in China, providing vital field evidence for regional GHG estimations and further management practices for GHG mitigation.