EFFECT OF CALCIUM-ION ON THE FOULING OF NANOFILTER BY HUMIC-ACID IN DRINKING-WATER PRODUCTION

The influence of the charge of humic acid (HA) on the fouling of nanof ilters was investigated as a Function of pH and concentration of calci um ions. The charge of the humic acid as well as the zeta-potential of the membrane surface were measured to elucidate the mechanism of humi c acid deposition on the membrane surface. The negative charge of humi c acid as well as the negative zeta-potential of the membrane surface fouled with humic acid increased with higher pH. As a result the furth er deposition of humic acid on the membrane surface was expected to de crease with higher pH because of larger repulsive forces. With moderat e calcium ion concentration, however, the adsorption of humic acid ont o the membrane surface decreased until neutral pH and then increased a gain. It was attributed to calcium ion bridging between two free funct ional groups of humic acids. Calcium ion rejection decreased in the pr esence of humic acid deposited on membrane surface. This could be expl ained by the effect of humic acid on the net charge of the membrane. T he addition of calcium chelating agent, EDTA, improved flux, especiall y at alkaline conditions. The diagram of Ca-EDTA vs pH was used to exp lain the flux improvement. (C) 1998 Elsevier Science Ltd. All rights r eserved.