Fouling of ultrafiltration membrane during secondary effluent filtration

Ultrafiltration (UF) membranes have been increasingly used for the pretreatment of secondary effluent in reverse osmosis wastewater reclamation plants. The fouling behavior of UF membranes during secondary effluent filtration was studied under constant pressure conditions in a cross flow filtration unit. The rate of flux decline was found to be strongly dependent on the permeate flux level, and drastic flux decline was observed at high flux during the initial period of filtration (within the first 30 min of each fouling test). Further flux decline was insignificant and a limiting flux behavior was observed. Membranes with high initial flux (>the limiting value) approached asymptotically to an identical stable flux, i.e., the limiting flux. The stable flux was strongly affected by solution chemistry (both pH and calcium concentration), with more severe membrane fouling occurred near the isoelectrical point of the foulant (pH 6) or at greater calcium concentration. Zeta potential measurements revealed that the stable flux may be correlated to the square of zeta potential of fouled membrane samples. The current study seems to suggest that foulant-foulant electrostatic repulsion plays a significant role in membrane fouling by effluent organic matter.