Hydrodynamics And Flux Uniformity Of Hollow Fiber Membrane Bioreactor

Membrane bioreactor(MBR)is a new type of water treatment technology that combines biological treatment and membrane separation technology,which has been extensively studied due to the great application prospects in wastewater treatment.However,membrane fouling is still a key factor restricting widespread application in practical engineering.In this study,the models characterized the fluid flow and mass transfer both in fiber-and module-scales of submerged hollow fiber membrane bioreactor were implemented based on computational fluid dynamics(CFD).Moreover,the simulation and optimization of hydrodynamic characteristics of MBR were carried out.The effect of the particle microscopic packed structures on the uniformity of fluid distribution and membrane fouling during membrane filtration process was investigated.The model simulated the accumulation process of pollutants on the membrane surface,and the results show that the interception of membrane surface on pollutants plays an important role in the flux distribution,the cake layer deposited with high porosity leads to the uniformity of flux and the cake layer distribution along the length of the fiber.This would reduce the energy utilization during the filtration process.However,the structure of the particles with high porosity can maintain relatively high filtration flux,and in the short term,its water production performance is better.Based on dynamic simulation of hollow fiber membrane microfiltration process,CFD simulation was performed to further study the effects of fiber geometry,including the inner diameter and length,on the uniformity of fluid distribution during filtration process.The model simulation indicates that the uneven flux distribution is the intrinsic characteristic of the hollow fiber membrane filtration,which is caused by the flow pressure drop in the inner and outer channels of the fibers.In order to obtain uniform flux distribution during filtration process,the reasonable range of the fiber size is proposed as follow: the inner diameter of the membrane is within the range from 0.4 mm to 1.0 mm.At the same time,increasing fiber length can aggravate the non-uniform flux distribution.However,the flux distribution of the hollow fiber membrane module also evolves with the filtration process.During the short-term filtration the uniformity of flux distribution will be improved by the self-regulation of membrane fouling.However,after long-term filtration the membrane modules are still affected by the uneven flux distribution and the necessary backwashing is required.From the macroscopic point of the whole membrane module,a three-dimensional CFD model based on gas-liquid two-phase flow was established.The simulation shows that the gas-liquid two-phase flow generated by aeration has some limitations on the effective cleaning of membrane fouling.The scouring effect in the central region of the membrane module is better,but the fibers membrane near the reactor wall cannot be effectively cleaned.Through the analysis of gas holdup,flow velocity and shearing force,when the fibers spacing decreases the whole membrane module has high fluid velocity and surface shear,which is beneficial to alleviate membrane fouling.However,with the decrease of the spacing of the fibers,the uniformity of the fluid distribution of the gas-liquid two-phase flow in the membrane module region becomes worse,so that membrane module cannot fully utilize the energy.At the same time,the non-uniform hydrodynamics condition of gas-liquid two-phase flow in the membrane module is one of the intrinsic reasons for the high energy consumption of MBR.