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 nanofilters was investigated as a function of pH and concentration of calcium ions. The charge of the humic acid as well as the zeta-potential of the membrane surface were measured to elucidate the mechanism of humic acid deposition on the membrane surface. The negative charge of humic acid as well as the negative zeta-potential of the membrane surface fouled with humic acid increased with higher pH. As a result the further deposition of humic acid on the membrane surface was expected to decrease with higher pH because of larger repulsive forces. With moderate calcium ion concentration, however, the adsorption of humic acid onto the membrane surface decreased until neutral pH and then increased again. It was attributed to calcium ion bridging between two free functional groups of humic acids. Calcium ion rejection decreased in the presence of humic acid deposited on membrane surface. This could be explained by the effect of humic acid on the net charge of the membrane. The addition of calcium chelating agent, EDTA, improved flux, especially at alkaline conditions. The diagram of Ca–EDTA vs pH was used to explain the flux improvement.