Enhanced separation and antifouling performance of ultrafiltration for surface water based on coagulation pretreatment by zirconium salts

Abstract

Membrane fouling is the main challenge that limits ultrafiltration (UF) membrane long-term application. Coagulation, as an effective pretreatment method, not only reduces the pollutants loaded on the membrane surface, but also transforms membrane-pollutant contact into pollutant-floc reaction. Zircon (Zr) salts have drawn wide attention as a novel metal-based coagulant because of their excellent coagulation performance. In present study, three types of Zr coagulants zirconium tetrachloride (ZrCl4) and polyzirconium chloride (PZC), polyzirconium chloride/Poly dimethyl diallyl ammonium chloride (PZC/PDMDAAC) were estimated in terms of the coagulation removal of organic pollutants and the mitigation of membrane fouling. Results showed that the combination of PZC with alkalinity of 1.5 and PDMDAAC with molecular weight of 2.0–3.5 × 105 (abbreviated as PZC1.5/P2) presented the best coagulation performance under acidic and neutral conditions due to its better bridging affinity and sweeping ability. 8 mg/L dosage of PZC1.5/P2 could effectively remove turbidity (98.27%) and dissolved organic carbon (DOC) (59.04%) during the coagulation process. Furthermore, pre-coagulation by PZC1.5/P2 induced the transformation from cake filtration into thoroughly intermediate blocking and the removal of particulate matter would prevent reversible fouling-induced flux decline, whereas elimination of organic matter was beneficial for promoting UF efficiency by mitigating irreversible fouling. Besides, Derjaguin-Landau-Verwey-Overbeek (XDLVO) analysis showed that the foulants became more hydrophilic coagulated by PZC1.5/P2, and the adhesion between foulants and membrane surface was greatly weakened. All the improvement originated from the flocs structure contributed to the better UF performance with the improved normalize flux (J/J0) from 0.50 to 0.75, larger decrease in reversible and irreversible fouling by 82.27% and 91.27%, respectively. More importantly, PZC1.5/P2 exhibited excellent coagulation and membrane performances in the treatment of actual water from Wolf Mushan reservoir, which demonstrated that Zr-based coagulant is a promising coagulant for practical application in surface water purification.