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 (EF_5r) 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.