Combined Precipitative and Colloidal Fouling of Reverse Osmosis Membranes

Abstract

Reverse osmosis experiments with synthetic feed waters were carried out to simulate combined fouling of salt-rejecting membranes by colloids and CaSO4 scale. Two gypsum crystallization regimes were studied: (1) surface crystallization with the saturation index smaller than 1 in the bulk and higher than 1 near the membrane surface and (2) concomitant surface and bulk crystallization with supersaturated solutions. Rapid flux decline was observed for both undersaturated and supersaturated feeds. For the supersaturated feed, scaling resulted in a dramatic flux decline only marginally affected by SiO2 colloids. For the undersaturated feed, the flux decline was significantly higher than the summation of flux declines due to colloidal fouling alone and gypsum scaling alone. Colloidal silica did not have a statistically significant effect on the amount of gypsum crystals formed on membrane, whereas the deposition of colloids was enhanced in the presence of CaSO4 scale. The synergistic increase in fouling is attributed to changes in the structure of the fouling layer. The results point to the importance of removing colloidal species at the pretreatment stage.