Efficient Pollutant Removal and Low-Carbon Emission Mechanisms in Constructed Wetlands Synergistically Driven by Low COD/N Ratio and Coastal Location

Quantifying the variation in wetland greenhouse gas fluxes across large spatial scales and accurately assessing source–sink effects is crucial. However, there remains a limited understanding of the combined impacts of influent COD/N ratios and geographical distribution conditions on pollutant removal and GHG emissions. In this study, five typical constructed wetlands from across the country were selected to evaluate GHG emissions, pollutant removal efficiencies, and the main influencing factors for each wetland. The results showed that temperature, ammonia nitrogen concentration, COD, COD/N ratio, and geographical location were the main regulators of GHG emissions, with complex interactions among the factors. Overall, GHG emissions were higher in the coastal region than in the inland region, highlighting the importance of geographic distribution conditions on wetland operation. In addition, wetlands with a COD/N of 3 showed the best overall performance in terms of pollutant removal and GHG emission reduction. Moreover, COD/N had an important effect on the emission fluxes of all three greenhouse gases, which was an important influencing factor on the emission fluxes of greenhouse gases from constructed wetlands. Wetlands with lower COD/N ratios, especially coastal wetlands, showed stronger performance in pollutant removal and GHG emission reduction. This study emphasizes the need to fully consider the potential influence of influent COD/N ratio on GHG emissions when designing constructed wetlands for municipal wastewater treatment, providing valuable insights for future wetland design and GHG abatement strategies.