Simulation And Optimal Regulation Of AAO Process-Constructed Wetland For Municipal Wastewater Treatment

With the increasing perfection and strictness of the country’s environmental protection requirements,sewage discharge standards have been gradually improved in recent years.However,the tail water of sewage treatment plants still has the characteristics of large water volume,high pollutant concentration and poor biodegradability,especially the high concentration of nitrogen and phosphorus in the tail water,and the directly discharged into the environment has ecological risks such as water eutrophication.Therefore,it has become an inevitable trend for future construction and renovation of sewage plants to increase the treatment efficiency of sewage plants while adding advanced treatment units to further treat tail water.As a deep purification process for ecological safety buffer,constructed wetland can effectively utilize the highly coordinated biological/ecological operation mode to improve the efficiency and quality of sewage treatment plant tail water.In this study,a mathematical model was established based on the anaerobic-anoxic-oxic（AAO）biological treatment system of the actual urban sewage plant,and the numerical simulation of the actual AAO system was realized;Additionally,the boundary response model of the key operating parameters of the AAO process was built,and the effective control area of the key operating parameters of the AAO process under different discharge standards of the sewage treatment plant were determined.Finally,combined with the different characteristics of the biological treatment process and the ecological treatment process,the simulation and operation parameter regulation of the AAO process series constructed wetland treatment system was carried out,which provided a theoretical basis for the practical engineering application of the combined system.The main research contents of this paper were as follows:The one-dimensional hydrodynamics（O-D WQM）model of the AAO process system of the urban sewage treatment plant and the coupled model of the improved ASM2 were established.The improved gradient descent algorithm（GDA）was used to calibrate the model parameters of ASM2,and the mathematical simulation of the actual AAO process was realized.The research results show that the change of the extracellular polymer（EPS）of the activated sludge in the AAO process plays an important role in the process of biological phosphorus removal in the system.Based on this,an improved ASM2d model was established,which includes the tightly bound EPS(TB-EPS,_TB-EPS)formed in the inner layer,the loosely bound EPS(LB-EPS,_LB-EPS)diffused in the outer layer and the soluble EPS(_EPS)produced by the hydrolysis of EPS.The results show that the removal rate of phosphorus by TB-EPS is 9.37～9.64%,and the removal rate of phosphorus by LB-EPS is 4.17～4.29%.Besides,the O-D WQM+improved ASM2coupling model of the AAO process was further constructed,and the fitting effect of the simulation results of the treatment plant in the whole year,four seasons and the actual operation data were analyzed.The results show that the simulation results of the whole year can well fit the actual operation data of the AAO system,and the simulation results of each season are better than the simulation results of the whole year.Therefore,the established coupling model can better describe the sewage treatment process of the actual AAO system and realize the numerical simulation of the AAO sewage treatment process of the actual urban sewage treatment plant.The boundary response model of the key control parameters of the total oxygen transfer coefficient(k _La)and the internal reflux ratio（r）of the AAO process system were established,and the first-level A and first-level B standards were used as constraints to determine the effective regulation area of key operation regulation parameters of AAO system in annual and quarterly under the actual discharge standard of effluent.The results show that the multi-objective optimization realized the boundary optimization response of control parameters of AAO system under different influent pollutant changes.The sensitive range of total oxygen transfer coefficient（k _La）and denitrification efficiency（η）can be obtained.When the AAO system reaches the first-level A emission standard,the k _Lacorresponding to the same r increases.And when the r is lower than 1.8,the theoretically calculated denitrification efficiency is only69.70%,and no matter how the k _Laincreases in this system,it cannot meet the first-level A standard.In comparison,the influent total nitrogen（TN）concentration has a more significant impact on the two key control parameters,while fluctuations in influent total phosphorus（TP）concentration have little effect on the boundaries of k _Laand r parameters,and the fitted curve always presents a stacked state.Additionally,the energy consumption analysis of the sewage treatment process shows that the annual control parameters after optimizing the operating parameters can achieve 16.48%energy saving effect compared with before optimization,while when the AAO system optimizes and adjusts key control parameters according to the water inlet conditions in different seasons,the system can achieve 18.32%energy saving and the effluent quality and energy saving effect of the AAO system are significantly improved.A constructed wetland mathematical model driven by the tailwater treatment data of the AAO system was constructed,and the simulation and operation parameter regulation of the AAO process series constructed wetland treatment system were carried out.Besides,the sewage treatment effect was evaluated based on the constructed wetland model and the Z-point comprehensive evaluation method,and the best constructed wetland type and parameter combination based on the AAO tailwater treatment in this study were determined:the water distribution method is vertical subsurface flow;the matrix material is gravel-modified zeolite-fly ash;the plant types are reed and cattail;the surface area of the constructed wetland is 300,000 m²,the height is 0.8 m,and the water depth is 0.24 m;the surface hydraulic load is 0.1[m³/（m²·d）];the hydraulic retention time is 2.4 d.In addition,the multi-objective optimal control model of the AAO process series constructed wetland treatment system was established,and the stable control interval,optimal energy consumption,and control parameters of the combined process to meet the first-level A standard were determined.Also,the effluent quality of the combined process was identified at multiple levels and divided into red,yellow,and green zones.The results show that when the control parameters of the combined system are regulated in the corresponding operating parameters of the effluent in the green zone,the final effluent quality of the combined system can reach the first-level A standard stably.At the same time,when the control parameters of the combined system are regulated in the operation parameters corresponding to the effluent in the green zone,the optimal energy consumption per day is 2664.17 kw·h,which saves 40.16%of the energy consumption compared with the sewage treatment plant before optimization.Besides,when the control parameters of the combined system are regulated in the operation parameters corresponding to the effluent in the green zone and the yellow zone,the internal reflux flow of the AAO process in the combined system is 92050 m³/d,the minimum k _Lavalue is 52.1209 d-1,and the daily optimal energy is 92050 m³/d.The energy consumption is 2664.17 kw·h,which saves 28.32%of the energy consumption before optimization.