On the effect of flow reversal during crossflow microfiltration of a cell and protein mixture

Alternating tangential flow (ATF) filtration is a cell retention technique of increasing importance in continuous biomanufacturing. Despite its frequent application, the role of flow reversal in fouling mitigation remains unclear this far. We implemented a customized pilot plant comprising a low-shear centrifugal pump and fast switching valves, which enables us to create alternating crossflow, where crossflow velocities and cycle times can be varied independently, as well as conventional steady forward and pulsatile forward crossflow. We then used a model feed containing yeast cells and bovine serum albumin (BSA) in order to simulate a fermentation broth and investigated the impact of alternating crossflow on microfiltration performance at this customized filtration plant. By comparing the results with filtration trials conducted at forward only pulsatile crossflow as well as steady crossflow using the same plant, we observed the true effect of flow reversal, corrected for effects induced by crossflow velocity, transmembrane pressure as well as transmembrane pressure fluctuations. We found that alternating crossflow, i.e. crossflow with flow reversal, leads to a reduced fouling resistance and a higher solute transmission in comparison to steady crossflow filtration. However, the flux increase is lower than for pulsatile crossflow filtration, which might be attributed to length dependency effects. The transmission of BSA, on the other side, is even higher at alternating crossflow filtration than for pulsatile crossflow. Thus, we conclude that flow reversal plays a part in fouling mitigation, but a greater part of the anti-fouling effect of alternating crossflow, however, is caused by the transmembrane pressure fluctuations.