Flow Simulation And Optimization Research Of Oxidation Ditch With Fine Bubble Based On CFD-PBM Coupled Model

Oxidation ditch with fine bubble is widely used in the municipal wastewater treatment in our country due to its advantages such as high oxygenation efficiency,convenient operation and excellent effluent quality and so on.In view of the fact that the velocity distribution is unreasonable,the effect of aeration and flow driving is not good,and the energy consumption is too high,A CFD-PBM coupling model is used to simulate the flow field.On the basis of verifying the feasibility of the simulation,this paper makes a study on the optimization of the structure modification,the aerator and the arrangement of the submerged propellers.The main contents include:Based on the simulation of gas-liquid two phase flow,the CFD-PBM model is established by coupling the bubble population balance model.By investigating the distribution of flow velocity,gas holdup,bubble size,bubble coalescence and breakup,the effect of aeration on sewage in various regions is evaluated.Firstly,the feasibility of CFD-PBM model is verified.The results show that the simulation results obtained by the coupled model are better than the traditional simulation results and are closer to the actual values.The average relative deviations are less than ±8%,±8.5%,the convergence is stable.Therefore,the coupling model is suitable for the optimization of microporous aeration oxidation ditch.The flow field of aeration and anoxic zone under different area ratio(such as1:1,2:1,3:2)was studied on the basis of CFD-PBM model while keeping the total areation constant.When the ratio is 3:2,the flow rate of aeration zone is reduced by0.1-0.35m/s,the overall average flow velocity is 0.28m/s,which could reduce the energy consumption and meet the requirements of hydraulic circulation and mixing.The average diameter of the bubble obtained by PBM model is less than that of the former two,which is about 7.2mm,and the total mass transfer area is the highest;at the same time,the decrease of flow velocity in the aeration zone increases the residence time of the bubble,the average gas holdup in the aeration zone is increased to 0.45,and the overall oxygenation effect is improved.In the end,the transformation scheme of the section area ratio of 3:2 is selected,which can reduce the energy consumption during the process.The influence of the flow field on the arrangement of the impeller and aerator is further investigated.By changing the two impeller installation depth,it shows that when the upstream of the impeller is 1.5m and the downstream is 2m,the low velocity zone inthe middle of anoxic zone and the upper area of the aeration zone is reduced,thus the flow energy is reasonably distributed.The influence of water depth and aeration density on the micro aerator was then studied.When the depth of the equipment is increased from 250 mm to 500 mm,the bottom sewage flows under the influence of the horizontal and swirling flows,the average flow velocity of the trench is improved to 0.36m/s,the upper area velocity and gas holdup are improved.Finally,according to the idea to balance oxygen supply and demand balance,the aeration disk is divided into three stages according to the proportion of 59%?31% and 10%.At the same time,the speed of impeller is improved to meet the requirements of the mixed flow,the gas holdup distribution in the aeration zone is decreased along the flow direction,and the oxygen supply efficiency is optimized,and the energy consumption of the aeration device is reduced.