National University of Singapore
Food Science & Technology
Concentration polarisation
Concentration polarisation is a phenomenon that happens when a semipermeable membrane is in contact with a liquid solution containing solutes. It results in an accumulation of solutes at the boundary layer next to the membrane, which can cause the boundary layer has higher concentration of solutes as compared to the bulk and this can cause a problem during membrane separation. Further details about concentration will be explained in Box 1.
(Left) As the fluid moves across the membrane, large solutes concentrate near the membrane wall as the pressure (red big arrow) pushes the fluid through. This results in the concentration at the walls of the membrane (cw) to be higher than the bulk fluid (cb). At the same time, diffusion occurs from the wall back to the bulk fluid (represented by the small orange arrows). Thus at steady state, back diffusion is equivalent to the convective flux.
(Right) In ultrafiltration and reverse osmosis systems, concentration polarization is a common problem. However, if the solute molecules have very high molecular weight, such as proteins, a gel layer forms and this causes greater problem in practical application.
Box 1. Brief description of concentration polarization occuring in either reverse osmosis or ultrafiltration membrane systems