Fouling and Natural Organic Matter Removal in Adsorbent/Membrane Systems for Drinking Water Treatment

Adsorbent particles added to ultrafiltration (UF) systems treating drinking water can remove natural organic matter (NOM) and some other contaminants from the water, but their effect on membrane fouling is inconsistent-in some cases, fouling is reduced, and in others, it is exacerbated. This research investigated the behavior of UF systems to which powdered activated carbon (PAC), heated iron oxide particles (HIOPs), or (nonadsorbent) SiO2 particles were added. On a mass basis, the PAC removed the most NOM from solution, the HIOPs removed less, and the SiO2 removed essentially none. However, in the case of both PAC and SiO2, increasing the dose of solids led to a steady increase in fouling, whereas the opposite trend applied when HIOPs were added. In the absence of NOM, none of the solids fouled the membrane significantly. Thus, even though NOM is a causative agent for fouling, removing it from solution does not necessarily reduce fouling; the mechanism of removal can be just as important as the absolute amount removed, if the removal occurs in a cake layer near the membrane surface. Scanning electron microscopy images of the cake layers formed in the three systems suggest that the NOM binds PAC or SiO2 particles to one another and to the membrane surface, so that the particles become part of the foulant in the system. By contrast, the NOM appears to bind HIOPs to one another but not to the membrane. This process leaves enough pore space in the cake layer for water to reach the membrane with minimal resistance, and it reduces the tendency for either the NOM or the HIOPs to foul the membrane surface.