Analysis of Laminar flow precipitation fouling on reverse osmosis membranes

Abstract

Precipitation fouling on reverse osmosis membranes was analyzed by the boundary layer integral method for laminar flow in a parallel plate passage. Flux decline and concentration polarization levels are governed by a complex coupling between precipitation kinetics, mass transfer, and the deposit structure Precipitation at the membrane relieves the concentration polarization of the precipitating salt. Concentration polarization is also relieved by the flux decline accompanying the increased hydraulic resistance of the growing deposit. As precipitation rate increases with growing concentration salt convection into the boundary layer is balanced by the rate of precipitation. In the usual case where an inert salt is also present, the concentration polarization of the precipitating salt may pass through a maximum. Permeate flux is reduced by the increasing osmotic pressure of the inert salt, thereby reducing the concentration polarization of the precipitating salt. The analysis stresses the importance of distinguishing between those effects mainly attributable to precipitation kinetics and those which are primarily a function of the deposit structure. Flux decline data alone are inadequate to elucidate kinetic aspects of precipitation fouling phenomena.