Understanding the fouling mitigation mechanisms of alternating crossflow during cell-protein fractionation by microfiltration

Alternating crossflow filtration is an emerging technology to mitigate fouling in crossflow microfiltration of cell-protein mixtures by periodically reversing the direction of flow. So far, it is understood that the anti-fouling effects are mostly related to the pressure fluctuations upon changing from forward to reversed flow direction and to a smaller extent to the flow reversal along the membrane as such. However, it is not yet clear how pressure fluctuations and flow reversal influence the axial distribution of fouling and the prevailing types of fouling. We conducted alternating crossflow filtrations of a model suspension containing yeast cells (4.2% dry matter) and bovine serum albumin (0.8 g l⁻¹), where the duration and crossflow velocity of the forward and the backwards phase were varied independently. We found that the material, which deposits in the forward phase, can be better removed or loosened during the backwards phase, and vice versa, if forward and backwards flow conditions are equal in terms of time, crossflow velocity and overall transmembrane pressure. This leads to a detectable overall lower resistance due to deposit layer formation. Despite the positive effects of alternating crossflow filtration on filtration resistance, flux, and solute transmission, the less compact deposit layer also allows for an easier penetration of solutes and small particles into the membrane pores, which was detectable in terms of an increased ratio of irreversible fouling resistance.