Comprehensive Evaluation And Simulation Optimization Of Township Sewage Treatment Plant

The rapid development of urban sewage treatment facilities in China has stimulated the implementation of domestic sewage treatment in rural areas.To determine the suitable township sewage treatment technology for the Yichang area,this study took the township sewage treatment plant as the research object and establishes a comprehensive evaluation system and assessment model of the township sewage treatment plant by using the fuzzy analytical hierarchy process.Moreover,five township sewage treatment plants with different treatment processes were evaluated the overall benefits to provide decision-making information for the investment and construction of township sewage treatment facilities in this region.Meantime,the BioWin software was used to simulate and optimize the operation of the improved oxidation ditch process in a township sewage treatment plant to provide guidance and suggestions for its stable operation and energy saving.The main results are summarized as follows:（1）Based on the process characteristics,this study selected 14 indexes,from the aspects of the economy,technology,environment,and management,to establish a comprehensive evaluation system for the township sewage treatment plant.The weight and evaluation criteria of each index were determined by an expert questionnaire survey and analytical hierarchy process.Finally,the comprehensive evaluation model of the treatment process in township sewage treatment plants was established by using the fuzzy comprehensive evaluation method.（2）The numerical analysis of the comprehensive evaluation model indicated that the overall benefit evaluation scores of A,B,C,D and E sewage treatment plants with different processes were 81.40,75.34,69.97,89.64 and 78.62,respectively.Among them,the CASS process of Town D sewage treatment plant and the improved A²/O process of Town A sewage treatment plant had better comprehensive benefit evaluation.（3）Given the high operating cost and unstable effluent quality in Town E,this study conducted a field investigation and the treatment process analysis,and the following problems have been identified:The short length of the anoxic zone in the oxidation ditch with insufficient influent carbon source led to the poor denitrification performance;The recirculation of nitrifying liquid in the anaerobic tank caused the poor phosphorus removal and high cost for chemical phosphorus removal;The adding point of carbon source was not reasonable with the low utilization rate of carbon source.（4）BioWin software was used to simulate and optimize the operating parameters of the sewage treatment plant in Town E.The simulation results showed that the external recirculation ratio of the sewage treatment plant should be maintained at about 70%with a sludge retention time of about 30 days.Dissolved oxygen（DO）control scheme 8 was the optimal DO control mode,and the DO concentration in the aeration zone should gradually increase from 0.3 mg/L to 0.9 mg/L.The simulation analysis of different influent phosphorus and Fe Cl₃ dosage determined the linear relationship between the optimal Fe Cl₃dosage and influent TP,i.e.,Y=53.48X-44.89.The dosage of Fe Cl₃ can be adjusted according to the influent phosphorus concentration to reduce chemical consumption.（5）The comparison between the simulated values and the measured value showed that the optimal operation scheme reduced the effluent TN concentration by 20.54%and the effluent TP concentration by 18.52%,significantly improving the nitrogen and phosphorus removal of the sewage treatment plant.Moreover,the energy consumption for aeration,nitrification liquid recirculation,and sludge recirculation was reduced by 7.5%,100%,and10.3%,respectively,thus effectively decreasing the total energy consumption in the sewage treatment plant.The optimal operation scheme could reduce the dosage of carbon source by60%and ferric chloride by 15%,while ensured the effluent quality of TN and TP within the first-class A discharge standard and effectively reduce the chemical consumption cost.