Study On Mass Transfer Characteristics Of SMBR By The Porous Media Model

Membrane Bioreactor is a high-efficiency wastewater treatment technology that combines activated sludge and membrane technology.It has the advantages of excellent treatment effects,small footprint and low residual sludge what make it becoming a widely used technology in the field of wastewater treatment.However,the membrane modules are easily contaminated resulting in higher cost of MBR.Only solving the membrane fouling problem can effectively improve the utilization efficiency of modules and reduce the costs.In addition to experimental research,Computational Fluid Dynamics can also be used as a fluid simulation tool to quantify the flow field with high precision,and examined interior of modules.Compared with the experiments,the simulation can get more important hydraulic characteristics in flow field,such as the velocity inside the membrane,the penetration velocity,and so on.The MBR mass transfer characteristics can be understood from the microscopic level,and help MBR optimization.The ICEM software and FLUENT software in ANSYS14.5 were used in the study,and the most widely used submerged hollow fiber membrane reactor(SMBR)was the object of study.First,compare the simulation differences between the wall model and porous media model.Then verify the simulation by consulting literature and experiment.The results are as follows:Due to the lack of permeation of the wall model and the single outlet of the entire reactor,the internal turbulent flow rate of the reactor is large,showing greater fluctuations in the data and cloud diagrams.Although the porous media model is not easy to calculate,and the convergence is more difficult,it has the penetration rate,infiltration competition and intramembrane velocity.What's more,the numerical fluctuation is more small.The data from porous model is more in line with the real SMBR flow field.Therefore,the porous media model is more conducive to theexploration of the mass transfer characteristics within the SMBR.The simulation was based on three-phase flow.The sludge phase was introduced in as a quasi-fluid phase.The hydraulic characteristics of different membrane spacings and different membrane diameters were systematically developed,such as flow rate in or outer membrane,liquid flow velocity,and sludge accumulation.Through several experimental results,the simulation data is verified,and the main results are as follows:(1)The hydrodynamic characteristics of a membrane module reactor composed of nine membranes were investigated when the membrane spacing was 1mm and4 mm.It shows that when the spacing is small,more low-velocity and slow zones are formed,especially inside the membrane module,so that the fouling is more likely to occur.Expanding the distance between membranes to optimize the hydraulic conditions of the regional flow field,and the scouring effect on single membranes was improved to get better fouling resistance.The pollution was significantly better than the 1mm membrane spacing.The smaller distance,the more block each other and the more competitive they are.And the module likes a large porous medium area,forming a large difference between the internal and external flow fields.The corner and side membranes,which are close to the outer flow field,are less contaminated.The center membrane is poorly positioned and seriously contaminated.Appropriately increase the film spacing does not change this phenomenon.(2)Examine the case of 1mm and 2mm in diameter of the membrane with 1mm spacing.TheMLSS and the width of the flow field between membranes are the main reasons for the formation of different contaminants.With the increase of the diameter,the flow area between the membranes increases,the upflow is faster and stable surface scour action is provided to resistance to pollution.However the larger diameter,the greater the influence of the infiltration.When the MLSS is high,the pollution will increase accordingly.The increase of the surface curvature of the membrane makes the turbulent kinetic energy inside and outside more stable.Then the thickness of the film surface contamination layer is smaller than1 mm diameter.Increasing the diameter of the membrane can increase the flux of the membrane and the corner membrane have the greatest increase.Through the mesh verification,velocity verification and the scanning electron microscope(SEM)map to correspondence of the simulated data.Confirm theaccuracy of simulation.