Abstract
Membrane fouling caused by natural organic matter (NOM) hinders widespread application of microfiltration (MF) and ultrafiltration (UF) to water treatment. In this study, we focused on biopolymers, which are hydrophilic organic matter with large molecular weights and have been reported as primary foulants in recent studies. Influence of size of biopolymers and pore sizes of MF/UF membranes on evolution of membrane fouling was investigated. It was found that coagulation by polyaluminum chloride and anion exchange removed different fractions of biopolymers: coagulation removed biopolymers with large molecular weights (>1,000,000 Da) whereas anion exchange removed biopolymers with small molecular weights (<100,000 Da). Fouling in MF was significantly mitigated when the feed water was treated by coagulation, indicating that biopolymers with large molecular weight were the primary foulants in MF. Pretreatment by anion exchange followed by coagulation substantially removed biopolymers, leading to efficient control of membrane fouling in MF. The high removal of biopolymers by anion exchange followed by coagulation could be attributed to the fact that humic substances were significantly removed by anion exchange: more amounts of coagulant became available to react with biopolymers as a result of elimination of humics. When UF membrane was examined, evolution of irreversible fouling hardly occurred. This could be explained by the size of biopolymers. They could not intrude into pores of the UF membrane. However, small particles rejected by the UF membrane could cause reversible fouling.
