Comprehensive Evaluation And Process Comparison Of Wastewater Treatment Plant Upgrade Based On Multi-objective Optimization

In recent years,in order to enhance the administration of water pollution,more and more stringent sewage discharge standards promote the upgrading of sewage plants,and bring higher requirements for the operation and management of sewage plants.Although high quality effluent can be obtained by improving the discharge standard of sewage treatment plants,more energy and material consumption are often needed,which will increase the operation cost of sewage treatment plants and produce potential environmental impact.Therefore,the upgrading of wastewater treatment plants is a complex process involving technology,economy,environment and other objectives,which needs to take into account both policy formulation and engineering program.In this paper,a multi-objective optimization analysis framework of environment-technology-economy was constructed.Through studying process simulation,statistical analysis,life cycle assessment and multi-objective optimization of sewage treatment plants,the impact of the enhanced discharge standards was evaluated,and the scheme selection,operation management and sustainable development in the process of upgrading sewage treatment plants were also evaluated.In terms of the impact analysis of discharge standards,a benchmark wastewater treatment plant model was established to meet the current situation of wastewater treatment in China.According to the discharge standards(Class A and Class IV),three modification schemes(dosing method,high-efficiency sedimentation tank and denitrification deep bed filter)were set up.Influent data include rainy and sunny days,which can simulate the effects of combined system and diversion system.Mathematical models were used to simulate different schemes,response surface methodology and statistical method were used to analyze the effluent quality,and life cycle assessment was used to calculate the environmental impact and operation cost.It could be found that nitrogen and phosphorus in effluent of sewage treatment plant were difficult to reach the effluent standard,but the existing technology could meet the requirements of the corresponding standards.The effluent quality of each scenario in sunny days(diversion system)was better than that of combined system.It showed that the improvement of the effluent standard of sewage treatment plant should be synchronized with the construction of pipeline network.The effluent of denitrification deep bed filter scheme was the best.(TN<10 mg/L,TP<0.2 mg/L).Improving emission standards will have potential environmental impacts,and environmental benefits could only be generated when appropriate schemes were selected,among which anaerobic/anoxic biochemical treatment and natural purification were appropriate.The results of multi-objective optimization showed that the water quality increased with the increase of energy consumption and material consumption,but the marginal benefit decreased gradually.For example,when scenario 3 reached the inflection point,the environmental impact and the cost of increasing the same water quality were 13.77 and 44.17 times higher.An upgraded sewage plant in north China was chosen as the study case,and the influent and effluent presented sinusoidal change law with seasonal fluctuation.The up-to-standard discharge of TN and TP in winter effluent is the difficulty in the reconstruction of sewage factory.Choosing denitrification deep bed filter and high-efficiency sedimentation tank,TN<10 mg/L in spring,summer and autumn,TN<15mg/L in winter.There were significant seasonal differences in environmental impact.The total environmental impact in winter was 1.5 times that in summer.After the upgrade,the total environmental impact both in winter and summer increased,while the total environmental impact in spring and autumn decreased from 0.6011 to 0.6005.This was consistent with the conclusion of the reference sewage plant.In terms of cost,the operation cost in winter was the highest,and increased by 20%after renovation.Multi-objective optimization for the case of sewage plant can improve the operation efficiency,compared with the original operation strategy.The electric consumption can be reduced by 20%,operation cost can be reduced by 10.5%,the total environmental impact dropped from 0.6005 to 0.582,as DO remained 0.4-0.5mg/L.The cycle than was 9.5,the mud volume was 622 m~3/d,the chemical for phosphorus removal was kg Fe/10~4·m~3 sewage,carbon source was 0.01 m~3/10~4 m~3sewage.The multi-objective framework constructed in this paper can complete the three-dimensional analysis of the upgrading of the sewage plant from the policy and engineering levels,which has certain practical significance for the policy analysis and the operation and management of the sewage plant.