Treated sewage discharge mitigates net greenhouse gas emissions from polluted urban rivers.

Urban rivers heavily influenced by anthropogenic activities serve as hotspots for greenhouse gas (GHG) emissions within river systems worldwide. However, the effects of the discharge of treated sewage from municipal wastewater treatment plants (WWTPs) into urban rivers on water quality and riverine GHG emission patterns are poorly understood. This study investigated the effects of treated sewage discharge on the emission fluxes of key GHGs (CH4, CO2, and N2O) in two heavily polluted urban rivers in Guangzhou, China. We found that treated sewage discharge increased riverine dissolved oxygen levels, which in turn increased nitrate concentrations. The total fluxes of GHGs in the urban rivers receiving and not receiving treated sewage were 29.76 ± 10.88 and 64.32 ± 20.86 mmol CO2-eq m-2 h-1, respectively. The introduction of treated sewage reduced the contribution of CH4 to total GHG emissions from 74.0% to 0.1%, indicating that CH4 suppression was the primary driver of the lower net GHG emissions observed in the receiving rivers. However, treated sewage discharge simultaneously increased riverine N2O and CO2 emissions, creating a trade-off among individual GHG species, although a reduction in the N2O emission factor (EF5r) was observed. Additionally, we observed that benthic N2O consumption could contribute to reducing N2O emissions from urban rivers. Overall, this study highlights the potential of treated sewage discharge to reduce net GHG emissions from polluted urban rivers, primarily through substantial suppression of CH4 emissions.