Study On Numerical Simulation And Optimization Of Operational Parameters For Oxidation Ditch And High-Rate Clarifier In Wastewater Treatment Plant

In recent years,with the rapid development of urbanization,China has increased the scale of municipal sewage treatment plants and improved the discharge standards of secondary wastewater effluent.Oxidation ditch(OD)with the advantages of long hydraulic retention time and durable resistance to shock loadings,is a commonly used wastewater treatment process.Due to the complex hydraulic characteristics of plug-flow and induced mixing,there is a lack of mature theoretical guidance on how to optimize the operation of aeration equipments and submerged impellers in the OD process,so as to achieve a balance between energy saving and fulfilment of emission standard.In addition,in order to further reduce the contents of phosphorus and suspended solids(SS)in effluent,the high-rate clarifier is adopted by some wastewater treatment plants(WWTPs)to upgrade their processes.However,due to the complex phosphorus species and unclear hydraulic characteristics after the addition of the high-rate clarifier following the OD and secondary settling tank,it still remains unclear about the removal efficiencies of total phosphorus and the optimization strategies of SS removal.Therefore,to address the above issues,this thesis focused on investigating the OD process of the Nantaizi-Lake WWTP in Wuhan,China.The computational fluid dynamics(CFD)and coupled biochemical models were established.The optimal operation of OD and the related field-test verifications were carried out.A comparative study on the phosphorus removal efficiencies of different phosphorus species in the newly added high-rate clarifiers after the upgradation of the ODs of the Nantaizi-Lake WWTP and the Tangxun-Lake WWTP was carried out.The CFD simulation of operational parameters and optimization of key structural parameters of high-rate clarifier in Nantaizi-Lake WWTP were carried out.The main research contents include:1.Established the coupled model of CFD simulation with ASM2 biochemical model and developed optimized energy saving operation strategies for full-scale ODBased on a laboratory-scale OD,a liquid-gas two-phase CFD model of the OD with surface aerators was explored by the model of fluid volume.The model was verified by measuring gas hold-up and flow field distribution.The simulation results showed that the flow velocity of down layer can be increased by 22.06%when the ratio of submerged impellers setting height to effective water depth was reduced from the current 0.545 to 0.273,which would be beneficial for minimizing sludge sedimentation.Subsequently,the full-scale Carrousel OD of Nantaizi-Lake WWTP in Wuhan was chosen as the model obejct.A three-dimensional(3D)CFD model was built and imported by the source term of the user defined function in ASM2(Activated Sludge Model No.2)in terms of the species transport model and oxygen injection inlet.The CFD-ASM2 model was successfully established and firstly applied in a full-scale OD.Then it was verified by the measured 3D flow velocities,the concentration of SS,the concentration of dissolved oxygen(DO),the concentration of chemical oxygen demand(COD),the concentration of total nitrogen(TN),the concentration of ammonium nitrogen(NH₄⁺-N),and the concentration of total phosphorus(TP).The corresponding normalized standard errors were less than 4.34%,0.23%,0.57%,5.32%,3.32%,3.20%,and 5.09%,respectively.The established CFD-ASM2coupled model was used to simulate and optimize different operation modes with different sets of surface aerators and submerged impellers.Compared with the current operation mode(9 surface aerators and 13 submerged impellers operated),the simulation scenarios II(7surface aerators and 10 submerged impellers operated)could reduce the SS concentrations in the second and fourth channels,which is beneficial to prevent sludge sedimentation.And these two modes both fulfilled the Class A effluent emission standard(GB 18918-2002).Meanwhile,an energy saving benefit can be thereby achieved with an operation energy consumption reduction by 22.3%of the full-scale OD,saving operational electricity consumption of554,600 yuan/year.2.Revealed the key roles of phosphorus species of the secondary effluent in enhanced phosphorus removal,and conducted the optimization of phosphorus removal efficiencies during the coagulation-flocculation treatment in the high-rate clarifier after upgradationThe phosphorus removal efficiencies in coagulation-flocculation process of secondary effluents in high-rate clarifiers from Nantaizi-Lake WWTP(mixed municipal and industrial wastewater)and Tangxun-Lake WWTP(municipal wastewater)with different initial phosphorus concentration were compared.A response surface methodology(RSM)was used to determine the optimal doses of cationic polyacrylamide(PAM)and polyaluminum chloride(PAC)in treating secondary effluents from the foregoing two WWTPs.For the secondary effluent from the Nantaizi-Lake WWTP,the maximum TP removal efficiency was 68.26%after adding 3 mg/L of cationic PAM and 25 mg/L of PAC.For the secondary effluent from the Tangxun-Lake WWTP,the maximum TP removal efficiency was 85.52%with adding 50mg/L of PAC.Moreover,a method to differentiate between organic and inorganic phosphorus contents(R=0.915,p<0.05)in secondary effluent samples was developed,which was more advantageous than the common phosphorus speciation method based on soluble and particulate phosphorus(p>0.05)to evaluate the effciencies of different coagulants and flocculants on the phosphorus removal.Further analysis by chemical analysis and liquid ³¹P nuclear magnetic resonance measurements results revealed that cationic PAM was more effective in removing organic phosphorus,such as phosphonates and orthophosphate monoesters.In contrast,PAC was more effective in removing inorganic phosphorus mainly based on orthophosphate.3.Established a three-dimensional liquid-solid two-phase CFD model to guide the optimization of the stable operation of the high-rate clarifierThe high-rate clarifier of the Nantaizi-Lake WWTP in Wuhan was chosen as the model object.A 3D liquid-solid two-phase CFD model has been developed for the high-rate clarifier by a mixture two-phase model.The developed CFD model was then successfully validated against the experimental results of distribution of velocities and SS concentrations.And the normalized standard errors of flow velocities and SS were less than 6.71%and 1.24%,respectively.The different geometrical configurations with different under-through channel heights and widths as well as overflow wall heights and baffle heights for the high-rate clarifier were simulated and compared.The height of under-through channel was found to more significantly affect flow field than its width.With the height of the under-through channel decreased from 2000 to 500 mm and the height of the baffle increased from 3300 to 5213 mm,the average SS concentration at the bottom of reaction tank would be decreased by 34.95%and the average residence time would be shortened by 4.77%,which can effectively prevent sludge sedimentation.This scheme can improve the SS removal efficiency,and reduce the amount of sludge dredging,thus avoiding unnecessary costs.This study provides both a theoretical basis and a technical approach for the efficient and stable operation of the OD and the high-rate clarifier process in WWTPs.