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T he study o f three membrane processes w hich are widely used in water treatment, MF, U F, and NF, showed that fouling significantly i nfluences membrane rejection behaviour. A 0 FA 0 HA 0 NOM 25 FA 25 HA 25 NOM 100 FA 100 HA NOM 72.4 69.4 68.8 45.1 45.9 47.6 Calcium, Sodium, % % 94.6 93.8 96.7 99.7 98.4 96.8 44.3 71.2 63.4 96.1 93.2 93.7 U V254rtm, 31.2 31.4 21.9 r eduction in pore size by pore adsorption and p ore plugging increased rejection, while deposits o n NF membranes can also decrease rejection o f s ome compounds. A pore size to foulant size r atio was important in the determination o f the m echanisms involved in rejection and fouling.