Hydrodynamic Techniques to Enhance Membrane Filtration

This article reviews the use of various techniques for membrane filtration, such as Dean and Taylor vortices, pulsatile flows, and dynamic filtration, which can generate high shear rates more efficiently than cross-flow filtration. In dynamic filtration, shear rates are generated not by a pump, but by moving parts or by vibrations. The most successful application of Taylor vortices has been plasma collection from donors in transfusion centers by microfiltration (MF), using small rotating cylindrical filters. Industrial dynamic filtration modules consist of metal disks with vanes or blades rotating between circular flat membranes or rotating ceramic membrane disks. These systems can be operated at high rotation speeds in order to produce very high permeate fluxes, or they can be operated at low speeds and save energy as compared with cross-flow filtration for the same flux. Vibrating modules (i.e., vibratory shear-enhanced processing) consist of a stack of circular membranes oscillating around a vertical shaft at its resonant frequency. While instabilities created by Dean vortices and pulsatile flows are mostly efficient in laminar flow and in MF and ultrafiltration, the benefits of high shear dynamic filtration are even more impressive in nanofiltration and reverse osmosis, as the reduction in concentration polarization not only increases permeate flux as compared with cross-flow filtration, but also decreases microsolute transmission.