Study On Water Quality Improvement And Process Optimization Of Sewage Treatment Plant Tail Water By Cascaded Ecological Wetland

Sewage treatment plant generally implement the discharge standards of grade A,which had characteristics of relatively good tail water yield.However,the tail water has high concentration of nitrogen,phosphorus nutrients and refractory organic nutrients.The direct discharge of tail water will cause ecological risks of surface water environment.Wetland technology is one of the common ways to purify water quality.However,the application of ecological wetland in tail water quality improvement is limited by the factors such as easy clogging,high construction cost and large occupation area of subsurface flow and surface flow wetland.Therefore,the integrated wetland technology of tail water quality characteristics optimization should be applied.Cascaded ecological wetland integrates advantages of ecological pond,surface flow and subsurface flow wetland.It not only improves water quality,but also has the characteristics of small footprint,good economy and strong landscape plasticity.Based on long-term measured water quality data of Wuxi tail water ecological wetland,MIKE21’s Two-dimensional flow field module（HD）was used to conduct cascaded ecological wetland’s hydrodynamic and water quality model coupled with"WQ with nutrients”module in ECO Lab.The water purification mechanism of cascaded ecological wetland was analyzed,the key parameters of water purification were determined and the water purification function was identified.Meanwhile,the reduction approaches of BOD,NH₃-N,NO₃^--N and PO₄^3-were proposed through simulation.The optimization scheme was processed based on the results of working conditions,which was put forward to provide technical support for optimal design and promotion of cascade ecological wetland.The area of Wuxi tail water cascaded ecological wetland is 4420m²,with a treated water volume of 1200m³/d.The wetland was composed of ecological pond,shallow-water plant wetland and deep-water plant wetland.The designed pollutant removal loads of COD,NH₃-N and PO₄^3-were 5.43,0.95 and 0.05 g/（m²·d）,respectively.During the wetland operation,the effective removal of pollutants was realized.The annual average removal rates of COD,BOD,NH₃-N,NO₃^--N and PO₄^3-were 21.95%,25.14%,43.02%,28.20%and 53.73%,respectively.The effluent COD,BOD,NH₃-N and PO₄^3-of the wetland were superior to the standard of surface water gradeⅣ.Based on the designed parameters and long series measured water quality data of Wuxi tail water cascaded ecological wetland,the hydrodynamic and water quality models of each unit in cascaded ecological wetland were constructed by coupling CAD’s dxf2xyz and MIKE21.The accuracy of simulation results had high precision after corrected.The relative errors of water level simulation were less than 5%and the relative errors of effluent water quality were basically within 15%.The key water quality purification parameters of ecological pond,shallow-water plant wetland and deep-water plant were calibrated.The highest first-order reaction degradation coefficient of nitrification in ecological pond was 0.18d^-1.The first-order reaction degradation coefficient of BOD,first-order reaction degradation coefficient of denitrification,and the amount of NH₃-N absorbed by plants and microorganisms were the highest,which were 0.38 d^-1,0.22 d^-1,0.05 g N/g DO and 0.06 g N/g DO,respectively.The amount of PO₄^3-absorbed by plants and microorganisms in deep-water plant wetland were 0.055 g P/g DO and 0.045 g P/g DO,respectively.Various pollutant reduction quantity approaches in each unit of cascaded ecological wetland were also determined.The first-order reaction removal amount of BOD in ecological pond and shallow-water plant wetland were the highest,which were 360.09 g/d and 1353.30 g/d,respectively.The amount of PO₄^3-absorbed by plants and microorganisms in deep-water plant wetland were the highest,which were 564.30 g/d and 461.70 g/d,respectively.The hydrodynamic and water quality parameters of Wuxi tail water cascaded ecological wetland were calibrated by using similar cascaded ecological wetland.The relative errors of water level simulation were less than 5%.Through the simulation of modified water quality based on the experimental results,the relative errors of effluent water quality were also basically within 15%.The calibrated parameters were compared with the parameter results of similar researches,which indicated that parameters were within an order of magnitude and the setting was reasonable,universal and reliable.Conditions simulation and process optimization were carried out based on the hydrodynamic and water quality model.Under conventional effluent water quality conditions of sewage treatment plants,the effluent COD,BOD,NH₃-N and PO₄^3-of the wetland were superior to the standards of surface water gradeⅣ.On this condition,the redundancies of pollutant reduction were 668.11 g/d,1179.92 g/d and 1319.94 g/d,respectively.Under the design effluent water quality of sewage treatment plant,the aeration coefficient and centralized aeration time of each wetland unit were added to the model.The first-order reaction degradation coefficient of BOD in each unit were increased to 0.25 d^-1,0.58 d^-1 and 0.22 d^-1,respectively.The amount of NH₃-N absorbed by plants were increased to 0.088 g N/g DO,0.12 g N/g DO and 0.054 g N/g DO,respectively.In the project,it was recommended to increase the DO content in the water body of each wetland unit.The integrated plant density of ecological pond,shallow-water plant wetland and deep-water plant wetland were increased by 1.8 times,2 times and 4 times,respectively or selected plant species with good nitrogen removal effect.