| Membrane fouling by natural organic matter (NOM) is one of the major problems in membrane filtration. Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by purified AldrichRTM humic acid (PAHA), a model NOM foulant, was systematically investigated by performing bench-scale fouling tests under constant pressure. Severe flux reduction and foulant accumulation occurred at high initial flux, [H+], and [Ca2+]. High flux promoted fouling probably through increased hydrodynamic drag and concentration polarization. On the other hand, flux was severely affected at low pH and high calcium concentration probably due to reduced electrostatic repulsion between foul ant-membrane and foulant-deposited-foulant.; Transmission electron micrographs of fouled membranes show that membrane coupons, except those with very low initial flux, were completely covered by a layer of humic acid whose thickness and density were greatly affected by feedwater composition ([H+] and [Ca2+]) and initial flux. A low density humic layer (about 0.1 g PAHA/cm 3) was formed at low initial flux (2 m/day or less) at pH 7 without calcium. It was several times denser at higher initial flux, pH 4.5, or 1 mM Ca2+. The denser foulant layers together with the greater PAHA accumulations were responsible for the severe flux reductions at high initial flux, [H+], and [Ca2+]. Streaming potential measurements show that the zeta potential of the fouled membranes was controlled by the humic acid layer due to its complete coverage on the membrane surfaces.; For high initial flux and severe fouling conditions (high [H+ ], [Ca2+], and ionic strength), membrane flux decreased to a limiting value that was independent of membrane properties and the initial flux. Flux reduction was much milder when the initial flux was lower than the limiting flux. The limiting flux was independent of membrane properties, probably due to the dominance of foul ant-deposited-foul ant interaction upon complete foulant coverage over membrane surfaces. On the other hand, the limiting flux depended strongly on the feedwater composition. The limiting flux was reduced at higher proton, calcium, and background electrolytes concentrations, likely due to reduced electrostatic repulsion under these conditions. |
