Urban constructed wetlands:Assessing ecosystem services and disservices for safe,resilient,and sustainable cities

Climate change and rapid urbanization are pressing environmental and social concerns,with approximately 56% of the global population living in urban areas.This number is expected to rise to 68% by 2050,leading to the expansion of cities and encroachment upon natural areas,including wetlands,causing their degradation and fragmentation.To mitigate these challenges,green and blue infrastructures （GBIs）,such as constructed wetlands,have been proposed to emulate and replace the functions of natural wetlands.This study evaluates the potential of eight constructed wetlands near Beijing,China,focusing on their ecosystem services （ESs）,cost savings related to human health,growing/maintenance expenses,and disservices using an emergy-based assessment procedure.The results indicate that all constructed wetlands effectively purify wastewater,reducing nutrient concentrations （e.g.,total nitrogen,total phosphorus,and total suspended solids）.Among the studied wetlands,the integrated vertical subsurface flow constructed wetland （CW-4） demonstrates the highest wastewater purification capability （1.63E+14 sej/m2/yr） compared to other types （6.78E+13 and 2.08E+13 sej/m2/yr）.Additionally,constructed wetlands contribute to flood mitigation,groundwater recharge,wildlife habitat protection,and carbon sequestration,resembling the functions of natural wetlands.However,the implementation of constructed wetlands in cities is not without challenges,including greenhouse gas emissions,green waste management,mosquito issues,and disturbances in the surrounding urban areas,negatively impacting residents.The ternary phase diagram reveals that all constructed wetlands provide more benefits than costs and impacts.CW-4 shows the highest benefit-cost ratio,reaching 50%,while free water surface constructed wetland（CW-3） exhibits the lowest benefits （approximately 38%）,higher impacts （approximately 25%）,and lower costs（approximately 37%） compared to other wetlands.The study advocates the use of an emergy approach as a reliable method to assess the quality of constructed wetlands,providing valuable insights for policymakers in selecting suitable constructed wetlands for effective urban ecological management.