National-scale Environmental And Economic Comprehensive Assessment Of Tailwater Constructed Wetlands For Wastewater Treatment Plants

In order to protect and restore the water ecology,China has implemented increasingly stringent discharge standards for wastewater treatment plants(WWTPs).Constructed Wetland(CW)is an effective means to improve WWTP tailwater standards because of its low cost,easy operation and maintenance,and good ecological effects.However,the suitability,economic cost,and environmental cost and benefit of promoting CW nationwide are still unclear,and an environmental-economic simulation study based on national-scale high-resolution models is needed.In this study,three national and local wastewater discharge standards(Class IA,Quasi-IV,and Quasi-III)were selected as targets for WWTP improvement,while two land use scenarios with and without arable land were set in order to explore the scientific value of arable land conservation,and a total of six scenarios were combined.The CW area was calculated based on CW loading parameters in five climatic zones across the country,and a national-scale land suitability analysis method for CW construction was established to assess the construction potential of 8,054 WWTP tailwater CWs across the country under the six scenarios.A refined CW model was constructed and a CW construction and operation life cycle inventory was obtained.The environmental impacts and their costs of constructing CWs in 8,054 WWTPs across the country over a 20-year time horizon were quantified through life cycle assessment and life cycle costing individually,and the two were combined to obtain the total environmental economic cost.A benefit-cost analysis was also conducted to provide eco-friendly and sustainable development recommendations for the promotion of CWs.The results of the study show that:(1)When protecting arable land from being occupied,the CW construction potential of tailwater under Class IA,Quasi-III and Quasi-IV upgrading targets are:90.38%,62.36% and 61.55%,respectively;when considering the use of arable land,the CW construction potential of tailwater is: 98.78%,95.35% and 95.15%,respectively.With the gradual strict drainage standards,the number of WWTP incorporated into the upgrading demand rises,but the land cannot meet the construction demand of CW,the construction potential of CW decreases,and the conservation demand of arable land becomes a key limiting factor for the constructability of CW.(2)The environmental impact of CW mainly comes from the use of steel and gravel during construction and the emission of greenhouse gases during operation.The environmental impacts of tailwater CWs in the upgrade target Class IA scenario are small,and the magnitude of pollutant removal in the upgrade target Quasi III scenario is large.The pollutant reduction in the Class IA and Quasi III scenarios offset the negative environmental impacts associated with the CW’s ecosystem.Among the various climate zones,the most complex environmental impacts of CW construction are greater in colder regions.(3)The major costs of CW are land acquisition costs and substrate material costs.In the scenario with strict protection of arable land,the unit costs of CW water treatment for tailwater under the three objectives of upgrading are: 0.24,0.40 and 0.39yuan;in the scenario where arable land use is allowed,the unit costs of CW water treatment for tailwater are: 0.27,0.41 and 0.41 yuan.The benefit-cost ratios of the six scenarios were 2.47,1.49,1.52,2.25,1.44 and 1.45,respectively,illustrating the low cost and high return of CW.A comprehensive national-scale environmental-economic study of wastewater treatment plant tailwater CWs has deepened understanding of the potential costs of raising wastewater treatment plant discharge standards and provided a basis for developing regional policies for implementing pollutant discharge standards.