Study On The Bubbling And Backwashing Enhancement Of Microfiltration In Submerged Hollow Fiber Membrane Module

Hollow fiber membrane microfiltration has been widely applied in wastewater treatment. Recently, in this field, there is a strong trend to use submerged membranes as filtration medium. However, the pressure driven submerged membrane processes suffers from membrane fouling much more heavily than cross flow filtration, a very important drawback that remains one of the most challenging issues facing further development of this technology. Bubbling and backwashing are two methods which enhance microfiltration in order to deal with this problem effectively.This study proposed a novel submerged hollow fiber membrane module to better control membrane fouling in gas bubbling and backwashing enhanced microfiltration. Using yeast as model particle in water, the performance of this module was tested by measuring the transmembrane pressure (TMP) under constant flux.It was found that the nozzle size at the base end of the module and gas flow rate greatly affected the size and shape of created bubbles. At a fixed nozzle size, the gas flow pattern transformed from bubble flow to the single bullet-shaped slug flow with the increase gas flow rate after exceeding one threshold value. Meanwhile, at a fixed gas flow rate, gas flow pattern transformed from bubble flow to the single bullet-shaped slug flow with the increase nozzle size after exceeding one threshold value. Nevertheless, the relative efficiency of membrane fouling control by gas sparging depended on not only the nozzle size and gas flow rate, but also the membrane module and bubbling frequency.It was thought that three factors affect the efficiency of backwash, that was backwash duration, filtration duration and filtration flux. Longer backwash duration and shorter filtration was better for membrane fouling control. However, the permeate production was lower. From this point, it should weigh the membrane fouling control and permeate production to maximize the backwash efficiency.Based on the experiments, resistance analysis was carried out for microfiltration process with gas bubbling or backwashing. It was detected that gas bubbling was effective for cake resistance control, while backwashing was effective for the internal fouling resistance control. Meanwhile, the empirical equation between the irreversible membrane fouling resistance and filtration cycle number was set up.A mathematical model for backwashing using dead-end hollow fiber membranes has been developed by combining the Hagen-Poiseuille equation and the filtration equation to describe the time dependence of the filtration behavior of hollow fiber membranes experiencing particle sweeping away from their surface. Introducing two parametersλ_max andτ_b, they can characterize the influence of backwashing to the back diffusion of the accumulative particles on the membrane surface. The developed model has been used to simulate the transmembrane pressure decrease rate under varied operation conditions, and to simulate the distribution of TMP and flux along the hollow fiber membrane.