Microbubbles and organic fouling in flat sheet ultrafiltration membranes

Proteins purified by ultrafiltration (UF) membranes are crucial in biotechnological and pharmaceutical applications. Membrane fouling occurs inevitably during the purification and results in reducing transmembrane flux or raising operational expenses. Periodical cleaning is imperative to prevent or eliminate fouling, enhance protein yield, and maintain membrane selectivity. Membranes are largely cleaned chemically despite the method’s inherent drawbacks, such as additional cost, environmental sustainability, and potential damage of membrane surface. Membrane surface modification, flow manipulation (turbulence promotion, back-flushing, and pulsing), ultrasonic treatment, feed pretreatment, and gas sparging can avoid or minimize the drawbacks. The air sparging is promising due to its relative efficiency and reduced impact on the environment due to the absence of cleaning chemicals.

At present, air sparging is used to minimize the biofouling of submerged UF hollow fiber in membrane bioreactors (MBRs). Typical sparging is performed via forcing compressed air through multiple nozzles at MBR bottom. Macrobubbles and air slugs flow upward, forming bubble and slug flow, respectively. Discrete gas bubbles usually characterize bubble flow in a continuous liquid phase. Slug flow contains large (Taylor) bubbles that span over the whole cross-section of the MBR. The mechanism of fouling prevention depends on the bubble size. Macrobubbles and air slugs are implemented in MBRs to shake hollow membrane fibers. Ellipsoidal or spherical-capped macrobubbles generate vortex rings and local eddies in the wake region formed as they are ascending through MBR. Those rings and eddies promote local circulation and resuspension of the fouling layer, followed by the back diffusion of foulants to the bulk. Slugs sweep over the membrane surface on its way up and shake the membrane due to the “Leonardo’s paradox.” The potential to generate and industrially implement microbubbles was never considered before. The effect of bubble size on the exact fouling mechanism remains largely underexplored.

On this account, Dr. Inna Levitsky and Professor Vitaly Gitis from the Ben-Gurion University of the Negev in collaboration with Professor Dorith Tavor from Shamoon College of Engineering studied the existing fouling prevention mechanisms and proposed an optimal bubble flow regime for preventing and controlling organic fouling in flat sheet ultrafiltration membranes. In their approach, a new air-water vortex device was used to generate oscillating and regular bubble flows to study different bubble regimes. A particle analyzer and laser doppler velocimeter were used to measure the size and velocity of the generated bubbles. Organic fouling was promoted in the UF polyethersulfone membranes using bovine serum albumin (BSA) suspension. The work is currently published in the journal, Separation and Purification Technology.

The research team findings revealed that the various mechanisms by which air bubble flows control or prevent organic fouling vary depending on the size of the bubbles. Small-sized air bubbles <0.05mm improved BSA retention but also led to flux decline. In contrast, larger bubbles of about 2mm minimized fouling despite not improving BSA retention. Interestingly, a flow of 0.5 – 1.0 mm air bubbles enabled continuous 3 hours run, suggesting the possibility of combining these fouling-combat mechanisms. Also, the authors reported that two-phase air-water flow could minimize organic fouling of UF membranes to simultaneously improve impurity retention while keeping them away from the membrane surface. It was worth noting that the effects of bubbles on flat sheet UF membranes differed from that of submerged hollow fibers.

In summary, the Israeli researchers investigated the effects of microbubbles on organic fouling prevention and control in flat sheet UF membranes. Though the exact mechanisms for preventing organic fouling were rather complicated, it involved multiple processes, including the adsorption of impurities by small bubbles and fouling layer distortion by large bubbles. Given the strong influence of filtration mode and bubble size on the fouling and retention in UF membrane-based systems, knowledge of bubbles sizes and their related effects is vital in evaluating the effectiveness of different fouling prevention scenarios. In a statement to Advances in Engineering, the authors explained their study advance the optimization of fouling combat mechanisms to minimize fouling while reducing energy and air demands for such operations.