Numerical Simulation And Structure Optimization Of Membrane Bioreactor Based On Computational Fluid Dynamics

Membrane bioreactor(MBR)is a relatively new sewage treatment reactor,with high-quality and stable effluent,small area and good treatment effect.It has been used widely in wastewater treatment.However the development of MBR is limited by membrane pollution and high energy consumption.Computational fluid dynamics(CFD)method can describe the hydrodynamic characteristics in the reactor,explore the influence of operation parameters and structure parameters,in combination with theory and experiments.The experimental cost will be reduced and the efficiency will be improved.Therefore,we simulated aerated MBRs with CFD method and investigated their hydrodynamic characteristics.Firstly,the gas-liquid two-phase flow in a two-dimensional aerated membrane bioreactor with three membrane bundles was simulated.With the increase of bubble diameter,the time of gas passing through the membrane bundle became longer,with little effect on the gas distribution.The increment of the bubble diameter enlarged the liquid velocity and enhanced the gas-liquid mixing.The water velocity in the reactor increased with the gas velocity.This improved the diffusion of air in the MBR,while the flow and mixing in the reactor were not strengthened.With the increase of the length of membrane bundle,the gas holdup in the upper part of the reactor first increased significantly,then decreased.The length of membrane bundle is selected at70 mm to have a better contact between gas and liquid and higher gas utilization efficiency.Secondly,the gas-liquid two-phase flow in the three-dimensional membrane bioreactor was simulated,with liquid phase being the mixture of water and activated sludge.The literature started their simulation from laminar flow,other models added gradually.Their results were obtained after several times calculation.Our simulation included all the models,i.e.turbulence model,non-Newtonian fluid and porous model,in the beginning of the simulation,which shortened the calculation time greatly.Our results were in good agreement with the literature.The effects of different inlet velocity and bubble size were investigated,and the optimal operating conditions were obtained.The gas holdup in the membrane unit increased greatly with the aeration rates,with significant gas accumulation at the top of the reactor.A large amount of gas left the reactor without circulation.When the aeration rate was 3m/s,the waste of gas would be reduced and the aeration efficiency would be improved.With the increase of bubble diameter,the liquid velocity above the membrane unit increased first and then decreased.When the bubble diameter was 6mm,both the velocities above the membrane unit and outside the membrane unit reached the maximum.In a word,in the current investigated range,the aeration efficiency was the highest with the bubble diameter of 6mm,which improved the function of the membrane unit.Finally,the gas-water-sludge three-phase flow in MBR was simulated by CFD for the first time,and the sludge was treated as a single phase for the first time.The flow and sedimentation characteristics of sludge in MBR were obtained.The inclusion of sludge made the liquid flow in three-phase simulation different from that in gas-liquid(mixture of sludge and water)two-phase flow obviously.In three-phase flow,some liquid bypassed the membrane unit and flowed upward,rather than flowing into the membrane unit like two-phase flow.There were obvious differences between the liquid velocities inside and outside the membrane unit in three-phase flow,while little difference appeared in two-phase flow.With the increase of aeration bubble diameter,the liquid velocity entering the membrane unit decreased,raising the possibility of sludge sedimentation.Within the scope of this study,the risk of membrane fouling was minimized with the bubble of 4-6mm in diameter.When the width of aeration surface was 2mm,the possibility of membrane pollution was lower and the aeration efficiency was higher,which benefits to the stable operation of membrane bioreactor.The increasing distance between the aeration and the membrane unit enlarged the possibility of the sludge to bypass the membrane unit.Combined with the analysis of low-speed zone,the pollution possibility was minimal with the aeration at the initial location.The gas-liquid two-phase flow and gas-water-sludge three-phase flow were simulated with Euler model combined with turbulence model,non-Newtonian fluid and porous model,respectively.The hydrodynamics in the reactor was obtained,such as the flow field,the distribution of sludge.The effects of aeration speed,bubble diameter,aeration area and location were discussed and the structure of the MBR was optimized.The membrane pollution will be reduced and the aeration efficiency will be improved with reduced energy cost.