Numerical Simulation Analysis Of The External Pressure Immersion Hollow Fiber Membrane Cleaning Process

The membrane fouling has limited extensive applications of hollow fiber membrane in water treatment.Backwashing and air scouring can effectively solve this problem in submerged outside-in hollow fiber membrane system.In this study,variation of fouling layer on the membrane surface during backwashing and the impact of shear stress caused by air scouring on fouling removal were investigated through Computational Fluid Dynamics(CFD)simulation.The two processes involves the simulation of liquid-solid and gas-liquid two-phase flow,respectively.In the simulation process,the membrane module which reflect the true size of hollow fiber membrane is placed in the geometry,and there is simplified air sparger at the bottom of the reactor for the simulation of air scouring.The fibers are set as porous media rather than non-slip wall in the simulation,which can truly reflect the filtration and backwash characteristics of hollow fiber membrane.Firstly,variation of fouling layer on the membrane surface during hydraulic backwashing was studied by using CFD numerical simulation.It is assumed that particles form a very thin fouling layer on the membrane surface after the filtration process.As backwashing is processed,the peeling process of the fouling layer on the membrane surface was revealed by the change of the particle volume fraction.According to the results of simulation,the velocity profile inside the reactor were presented through three-dimensional graph,and visualization of the particle movement for illustrating the dynamic peeling process of the fouling layer on the membrane surface were also showed.The results of CFD simulation show that backwashing can effectively remove the contaminant particles on the membrane surface,and the fouling will be further removed as the backwashing process continues.The formation of uneven cleaning reveals that the upper region of the fibers has the excellent cleaning effect during backwashing.Secondly,the gas-liquid two-phase flow in the membrane reactor was numerically simulated by the CFD technique,and the movement of bubble flow in the reactor was compared under different air scouring rate of 0.167m/s,0.387m/s and 0.512m/s.The optimal value of air scouring rate was determined by the shear stress exerted on membrane surface and the effect of backwashing for different areas of the membrane.It is concluded that the lower part and the middle region of the fiber suffer greater shear stress by analyzing the velocity contours and vectors,and the analysis results indicated that air scouring can further remove membrane fouling.In addition,air scouring can effectively applied to areas where backwash effect is poor,and further remove the fouling in these areas,such as 0.25,0.275 and 0.425 m.This also indicates that the use of air scouring after hydrodynamic backwashing can further clean the hollow fiber membranes and mitigate the uneven distribution of membrane fouling.Finally,the backwashing and air scouring process were simulated using CFD and the results were verified by experimental studies.The membrane specific flux and transmembrane pressure difference were compared between two different cleaning methods in the experiment.It was found that the backwashing combined with air scouring could inhibit the growth of TMP,and the membrane specific flux decreased relatively slowly.The experimental results show that backwashing coupled with air scouring can effectively remove the fouling on the membrane surface and restore the membrane performance in a greater extent.The results of experimental studies are in accordance with the simulation results.