Raising wastewater collection and discharge standards will reduce greenhouse gas emissions from metropolitan rivers

Urban rivers are increasingly recognized as significant sources for greenhouse gas (GHG) emissions. Few studies, however, quantify emissions of all three GHG from rehabilitating urban rivers that receive treated wastewater. This study analyzed carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) concentrations and diffusive fluxes from the Suzhou Creek in Shanghai that were investigated across the four seasons in 2021. Our results show that Suzhou Creek behaves as a source of atmospheric GHG emissions. The mean concentrations of CO₂ CH₄ and N₂O, in the main and tributaries were 80.62 ± 37.81 versus 82.07 ± 50.77 μmol L⁻¹, 0.38 ± 0.31 versus 0.73 ± 0.87 μmol L⁻¹, and 37.33 ± 17.70 versus 51.26 ± 35.84 nmol L⁻¹, respectively. The corresponding fluxes of CO₂, CH₄, and N₂O were 3.04 ± 2.36 versus 2.78 ± 2.25 mmol m⁻² h⁻¹, 17.82 ± 18.91 versus 35.35 ± 49.51 μmol m⁻² h⁻¹, and 1.44 ± 1.25 versus 2.2 ± 2.95 μmol m⁻² h⁻¹, respectively. GHG emissions from Suzhou Creek are lower than global urban rivers. N₂O generation in the nitrate-rich mainstem may primarily be attributed to denitrification and nitrification, and ammonium-rich tributaries may mainly associate with nitrification and coupling nitrification-denitrification. Tributaries are more suitable for CH₄ generation. CO₂ in the basin comes mainly from heterotrophic respiration of organic matter, and the high nutrient load and Chlorophyll a concentration in tributaries support photosynthesis. Although wastewater treatment plants and sewage treatment stations provide direct inputs of GHG and nutrient substrates, respectively, their input load (including GHG and nutrient substrates) is lower than that of other urban rivers. The study highlights that with the improvement of sewage collection capacity and treatment discharge standards in large cities, the input load and water pollution situation in urbanized areas will be greatly improved, thus reducing GHG emissions from urban rivers.