Concentration Polarization in Hyperfiltration Systems

Abstract

When using hyperfiltration membranes for brackish or seawater desalination, an enrichment of the solute (concentration polarization) always occurs at the membrane surface juxtaposed with the brine solution. This is due to the formation of a boundary layer separating the membrane surface from the bulk flow. The thickness of this boundary layer is dependent upon the stirring or streaming velocity. Because the salt transport within the boundary layer is not affected by the bulk flow, the rejected salt at the membrane surface must return to the bulk flow by means of diffusion only. This diffusion of salt into the well stirred or streaming solution is counterbalanced by the convective salt flux (associated with the permeation of water through the membrane), thereby forming a concentration gradient within the boundary layer. The enhanced salt concentration at the membrane surface results in a lower water flux and salt rejection. Thus, the “intrinsic water flux” and the “intrinsic salt rejection”, which would be observed without concentration polarization, are always greater than the observed water flux and salt rejection. The effect of concentration polarization upon volume flow, q, and salt rejection, robs, of a membrane was first investigated by Merten[1]. He showed that the volume flow is limited by concentration polarization because of the effect on local osmotic pressure.