Impact of various coagulation technologies on membrane fouling in coagulation/ultrafiltration process

• Coagulation technologies were related to the property of flocs and membrane fouling.
• Membrane fouling was the most serious under the traditional coagulation process.
• Highly positively charged flocs induced the least serious membrane fouling.

To evaluate the membrane fouling under traditional and enhanced coagulation, polyferric chloride (PFC) and polyferric chloride–polydiallyldimethylammonium chloride (PFC–PDMDACC) were applied in fulvic acid (FA)–kaolin treatment by a coagulation–flocculation/ultrafiltration process. Optimum dosages of PFC and PFC–PDMDAAC were ascertained in traditional and enhanced coagulation region by jar tests. Subsequently, the influences of the characteristics of flocs pre-formed in traditional and enhanced coagulation process on ultrafiltration (UF) membrane fouling were studied. The results of the study indicated that traditional coagulation, the first enhanced coagulation and the second enhanced coagulation occurred at the dosage of 15 mg/L, 44 mg/L and 72 mg/L respectively in the PFC dosage range investigated in the experiments. And the coagulation mechanism was precipitation charge neutralization, charge neutralization, and sweep flocculation, respectively. For PFC–PDMDAAC, 10 mg/L, 21 mg/L and 41 mg/L was the optimum dosage in traditional coagulation (charge neutralization), the first enhanced coagulation (bridging and charge neutralization) and the second enhanced coagulation (sweep flocculation) respectively. Flocs formed in traditional coagulation region were the biggest. The floc structure was the most compact in the first enhanced coagulation region. Flocs of the second enhanced coagulation region were highly positively charged and flocs of traditional coagulation region were positively charged for PFC and uncharged for PFC–PDMDAAC. For both PFC and PFC–PDMDAAC, the hierarchy of membrane fouling under traditional and enhanced coagulation was traditional coagulation > the first enhanced coagulation > the second enhanced coagulation. Enhanced coagulation could reduce the UF membrane fouling effectively.