Research And Evaluation Of Agricultural Non-point Source Pollution Remediation And Recycling With Aquaculture Tailwater As Core

With the continuous increase of agricultural non-point source pollution control,rural agricultural water pollution control mode is developing continuously.Under the background of the development of the world-class ecological island of Chongming during the 13th Five-Year Plan period,developing a new ecological agriculture mode and improving the water environment quality of rural agriculture have become one of the key tasks and actions to promote the ecological quality of Chongming.According to a typical modern agricultural cooperative community area in Chongming,this paper designs a comprehensive treatment and recycling scheme for agricultural tailwater produced in its production process.The scheme conforms to the requirements of the agricultural development of Chongming and lays a foundation for building ecological circulation agriculture model while exploring new models of agricultural production and pollution control.On the basis of the construction and operation of the demonstration project,the long-term tracking and monitoring of operation effect is carried out,including water quality,microorganisms and so on.The system is also evaluated by means of emergy theory and the evaluation is carried out in combination with environment and economy.The main conclusions are as follows:（1）Scheme design of pollution control and recycling of demonstration projectThe design of the demonstration project adheres to the principle of adapting measures to local conditions and recycling utilization.It takes energy flow and material circulation law as core management concept,and comprehensively applies forest land,aquaculture area and farmland in the research area.The agricultural tail water can be discharged after meeting the standard through ecological catchment pretreatment ditch,farmland purification unit,suspended bed ecological ditch,ecological water conveyance corridor and stable regulation ditch.If it fails to meet the standard,it will be lifted to landscape wetland forest by pump for further purification and the water can be recharged after purification.It includes the design of tail water conversion,wetland effluent recharge formation cycle,secondary utilization of aquatic plants,sludge and so on,which plays a role in water purification and provides important ecosystem services.The designed treatment water quantity of demonstration project is 3215 m³/d.The designed treatment water quality is that COD_Cris 50 mg/L,NH₄⁺-N is 8.55 mg/L,TN is 15 mg/L and TP is 1.57 mg/L.Among them,design hydraulic retention time of ecological catchment pretreatment ditch,farmland purification unit,suspended bed ecological ditch,ecological water conveyance gallery,stable regulation ditch and landscape wetland forest are 2.5 days,2.5 days,5 days,1.5 days,1 day and 2 days respectively.（2）Tracking monitoring and analysis of operation effect of demonstration projectAfter completion and operation of the demonstration project,its effluent water quality has been monitored for 9 months.The physical and chemical indexes of the effluent water of the project can meet the primary discharge requirements of the corresponding indexes in the discharge standard of fresh water aquaculture tail water,which means that the effluent water meets the design objectives of the demonstration project.In the circulating purification system,the concentration of pollutants in each purification unit gradually decreases,and the efficiency of each purification unit in summer is basically better than that in winter.The result of average removal efficiencies of total nitrogen,total phosphorus and COD_Crby different purification units are as followings.These average removal efficiencies in the ecological catchment pretreatment ditch are 24.71%,33.59%and 28.24%respectively.These average removal efficiencies in the farmland purification unit are 26.73%,29.39%and 23.90%.These average removal efficiencies in the suspended bed ecological ditch are 31.20%,40.86%and 31.15%.These average removal efficiencies in the stable regulation ditch are 11.92%,10.35%and 9.03%.These average removal efficiencies in the submerged wetlands in landscape wetland forests are 23.89%,26.29%and27.35%,which in the surface wetlands are 28.73%,15.77%and 34.32%.Except for the ecological catchment pretreatment ditch,the effluent of each purification unit in summer and autumn is in medium nutrition state,and in winter it is in lean nutrition state,with no signs of algae outbreak.The microbial diversity in each purification unit in summer is higher than that in winter.There are differences in functional bacterium that participate in nitrogen and phosphorus removal process at the time and the composition of community is different in time and space.（3）Demonstration project evaluation based on Emergy TheoryThe entire eco-economic system represented by the demonstration project includes natural resources,purchased resources,product output and ecological benefit feedback in emergy analysis.The input energy value of natural resources is7.68E+161SEJ/yr.The input resource of social purchase is 1.86E+17 SEJ/yr.The total system energy value is 3.09E+17 SEJ/yr and the feedback energy value is1.00E+131SEJ/yr.By calculation,the Energy Yield Rate（EYR）is 1.67,which is higher than the national average level of agriculture.The Emergency Investment Ratio（EIR）is 2.41 and the system is more dependent on social resources.Environmental Load Ratio（ELR）is 1.58,and the environmental impact of the system is at a low level.The Emergency Sustainable Development Index（ESI）is 1.06.This research system has positive economic contribution to society and certain development potential.The results of emergy model evaluation show that the research system is effective,which is reflected in the reasonable utilization of environmental resources on the one hand and its economic sustainability on the other.Compared with traditional aquaculture mode,the recirculating aquaculture represented by this research system has more advantages.Based on the evaluation results,some suggestions are put forward to improve the overall output and sustainability of the system.