| For a long time,the focus of China’s environmental protection work was mainly in cities,and the construction of rural sewage treatment projects was relatively lagging behind.Compared with cities,villages have differences in economic level,technical strength,residents’lifestyle and many other aspects.Blindly copying urban schemes will lead to inadequate use of sewage treatment facilities.Therefore,how to choose the water treatment process that adapts to the actual situation in rural areas has become an important issue for the construction of new rural areas.The comprehensive and systematic evaluation of sewage treatment process can provide scientific basis and method support for the optimization of sewage treatment mode in rural areas.In this study,the coupling model of life cycle sustainability assessment(LCSA)and multi-criteria decision analysis(MCDA)was established.The optimization of rural sewage treatment process was carried out,and improvement suggestions were put forward according to the evaluation results.Five commonly used sewage treatment processes,including constructed wetland(CW),sequence batch reactor activated sludge process(SBR),anaerobic-anoxic-oxic process(A~2O),membrane bio-reactor and bio-contact oxidation(MBR),were taken as the research object under the conditions that the treatment capacity was 100 m~3/d,the service life was 20 years,and the effluent water quality reached the first grade A standard of“Urban Sewage Treatment Plant Pollutant Discharge Standard”(GB18918-2002).”The main research conclusions were as follows:(1)Life cycle assessment results showed that each system contributed most of the environmental impact(over 85%)of the whole life cycle during operation.According to the results of standardization,the total environmental impact of CW,SBR,A~2O,MBR and BCO per function unit(100 m~3)were 5.39,6.64,7.56,6.81 and7.75,respectively.Life cycle cost analysis was used to evaluate the economics of five processes,and the results showed that the DLCC of CW,SBR,A~2O,MBR and BCO were 345,400 yuan,359,400 yuan,373,200 yuan,576,800 yuan and 355,500 yuan,respectively.IRR were 7.08%,5.00%,2.70%,-27.35%and 5.78%,respectively.The results of social life cycle assessment showed that CW occupied a large area,but had good reliability,could provide more employment and had a certain social landscape value.MBR occupied the smallest area and operates more stably,but the system reliability was low.The evaluation results of the remaining programs were between them.(2)Based on the results of social evaluation,CW had the lowest environmental impact and total cost,but its advantages were reduced by its large footprint;SBR and BCO had certain advantages in environment and society due to their low environmental impact and simple maintenance.A~2O operation required more energy,so the performance of environmental indicators was poor,and the cost was high.MBR had the largest environmental impact and the highest economic cost(576,800yuan),but had a good social evaluation.(3)The combined weighting-TOPSIS method was used to evaluate the performance of five processes in environmental,economic and social dimensions.The results of multi-criteria decision analysis for four types of stakeholders,namely environmentalists,residents,investors and equilibrium,showed that CW was the best(0.59～0.70),while MBR,SBR and BCO had their own advantages under different preferences.However,when the weight of environmental,economic and social indicators continued to change,the optimal plan would also change.When the subjective weight of land area reached 65%,CW was no longer the optimal plan.(4)The evaluation results show that:CW has the lowest investment and can provide more jobs,and it is suitable to develop CW in rural areas with abundant land resources and shortage of funds;SBR and BCO systems are relatively balanced in all aspects,and are relatively mature and stable water treatment solutions;A~2O operation cost is high,the impact on the environment is large,and social evaluation is not dominant,so A~2O is usually improved or combined with other processes for sewage treatment.MBR cannot be widely used,but in specific areas with high social conditions,if decision makers pay more attention to stable operation and floor space,MBR is a suitable solution.(5)Through the identification of key factors and sensitivity analysis of five technologies,the corresponding optimization direction are proposed.First,to optimize the key factors of consumption in the operation stage,such as establishing a clean power structure based on new energy,strengthening the decontamination per unit area of CW,optimizing the operation under low temperature conditions for SBR and other processes,and replacing with long-life MBR membrane modulesand other ways,to solve the technical shortcomings of each solution.The second is to consider flexible adjustment of discharge standards according to the direction of the drainage.The production system outside the boundary of this study can be taken into consideration by means of sewage irrigation,etc.,so as to reduce unnecessary processes and use of resources and energy,and further optimize the efficiency of resource utilization.The optimization and improvement methods adopted by users according to the evaluation results can improve the sustainability of the sewage treatment process and reduce the negative impact of environmental,economic and social dimensions.The LCSA-MCDA coupling model comprehensively considers the environmental,economic and social impacts,and the evaluation results have good scientific and practical value,which can provide technical support and theoretical guidance for the optimization of rural sewage treatment technology scheme. |
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