Microbubbles and organic fouling in flat sheet ultrafiltration membranes

Significance 

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.

About the author

Dr. Inna Levitsky received her Ph.D. in 2018 from Ben – Gurion University in Beer – Sheva, Israel. Dr. Levitsky is currently a senior lecturer at Sami Shamoon College of Engineering and the Head of Process Manufacturing Track at the Chemical Engineering Department. Her current research focuses on developing devices for two-phase flow, atomization liquid, and water treatment.

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About the author

Vitaly Gitis is a Professor of Energy Engineering at the Ben Gurion University of the Negev in Israel, where he teaches courses on chemistry, separation processes, and alternative energy sources. His research interests embrace granular and membrane filtration separation processes, multi-layered ceramic membranes, wastewater treatment processes, and colloidal chemistry. Gitis received his M.Sc. degree from the Moscow State Mining University and his Ph.D. from the Hebrew University of Jerusalem. He then worked as a researcher at the US EPA in Cincinnati and at the IWW in Mülheim a/d Ruhr, Germany. Gitis has published over 70 peer-reviewed papers in international journals, co-authored the textbook Ceramic Membranes – New Opportunities and Practical Applications and co-edited Handbook of Porous Materials, and invented several patents. Since joining BGU, he has mentored over 50 graduate students. Eight of his graduates hold academic positions. Gitis serves on several scientific and management committees, including the IWA Specialist Group on Particle Separation. In 2010 he was appointed as Marie Curie Visiting Professor at the University of Amsterdam, and in 2018 he was appointed as Visiting Professor at the University of Adelaide.

About the author

Prof. Dorith Tavor is The Presidents’ adviser for Gender Equity, Head of The Centre for Quality Assessment and Promoting Teaching, and The Chair of the Green Processes Center at Sami Shamoon College of Engineering. Prof. Tavor earned her Ph.D. in Chemical Engineering and MBA at Ben-Gurion University in Beer-Sheva, Israel. Prof. Tavor found the Chemical Engineering Department, the Green Processes Center, and the Center for Promotion of Learning and Teaching at Sami Shamoon College of Engineering. Prof. Tavor has worked in green engineering, mainly on catalytic applications in organic reactions and alternative environmentally friendly solvents and recycling industrial wastewater, two-phase flow for water treatment, and Life Cycle Assessment (LCA). Prof. Tavor is also imbuing sustainability and sustainable development in academia, community, and industry. Prof. Tavor is responsible in the college for the incentive of women and girls to learn science and technology.

References

Levitsky, I., Tavor, D., & Gitis, V. (2021). Microbubbles and organic fouling in flat sheet ultrafiltration membranes. Separation And Purification Technology, 268, 118710.

Go To Separation And Purification Technology

Levitsky, I., Tavor, D., & Gitis, V. (2022). Micro and nanobubbles in water and wastewater treatment: A state-of-the-art review. Journal of Water Process Engineering 47 (2022) 102688

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