New insights into the influence of ultrafiltration pretreatment on reverse osmosis membrane fouling during urban sewage reclamation: Interaction between extracellular polymeric substances and inorganic foulants

Ultrafiltration (UF) has been frequently employed as pretreatment to mitigate reverse osmosis (RO) membrane fouling. The organic and inorganic substances outflowing from UF pretreatment collectively determine the subsequent RO membrane fouling behavior. However, most of the previous studies were focused on single organic compounds as well as CaSO4 4 scaling, yet complex organic matter such as extracellular polymeric substances (EPS) and various inorganic scaling ( i.e. CaCO3, 3 , Ca3(PO4)2) 3 (PO 4 ) 2 ) coexisted during urban sewage reclamation. In this study, the impact of UF pretreatment on RO membrane fouling behavior, especially the interaction between EPS and CaCO3/Ca3(PO4)2 3 /Ca 3 (PO 4 ) 2 scaling, were systematically investigated. Results showed that the presence of UF significantly reduced the normalized flux decline (9.3% vs. 14.3%) and enhanced cleaning recovery efficiency (84.6 % vs. 64.4 %) of RO membrane, but increased scattered crystals (CaCO3 3 and Ca3(PO4)2) 3 (PO 4 ) 2 ) formation on RO membrane surface. Further investigation showed that the binding energy of the Ca2p1/2 1/2 and Ca2p3/2 2 p 3/2 peaks on fouled RO membrane exhibited a shift towards lower energy, which indicated a weak complexation between EPS and Ca2+, 2 + , thereby increasing inorganic scaling and confirming the two-sides role of EPS retained by UF in RO membrane fouling. Moreover, static experiments indicated that EPS inhibited CaCO3 3 scaling rather than Ca3(PO4)2 3 (PO 4 ) 2 scaling with the inhibition rates of 7.3% and 0.7%, respectively. To the best of our knowledge, it is the first to reveal the different effect of EPS on CaCO3 3 and Ca3(PO4)2 3 (PO 4 ) 2 scaling. The results may provide a new insight into the effect of UF pretreatment on RO membrane fouling during urban sewage reclamation process.