Characterizing the permeability of a foulant layer under a transient applied pressure

Pressure-driven membrane filtration systems are commonly operated at constant pressure, under which the presence of polymeric foulants deposited on the membrane surface leads to a decline in permeate flux. In the absence of further deposition, the polymeric foulant layer would typically compact in response to the permeate flow, resulting in a constant hydraulic resistance and a steady-state permeate flux lower than that of the clean membrane. However, the transient permeability of these soft, deformable porous layers remains poorly understood. Using a customized microfluidic nanofiltration system, we experimentally investigate the transient behavior of a porous deformable film under pressure steps and pressure waveforms. We demonstrate that a membrane fouled with a hydrogel layer can exhibit a transient permeability shift immediately after a sudden change in applied pressure. Moreover, this transient state suggests that compaction and swelling of the soft porous layer follow two different processes, involving distinct relaxation times and permeate volumes during the transient window induced by pressure increases and decreases, respectively. In addition, by imposing pressure waveforms, we show that a non-linear flow response arises as a result of the asymmetric dynamics of swelling and compaction of the soft material. Lastly, we demonstrate that such observations can have practical implications, as the non-linear flow response to oscillatory pressure can, in some cases, generate an enhanced overall permeability when compared to the averaged value of the pressure periodic function.