Insights into the control mechanism of different coagulation pretreatment on ultrafiltration membrane fouling for oily wastewater treatment

[Display omitted] •Coagulation pretreatment can effectively alleviate membrane fouling.•PAC had better coagulation performance than Fe salts in oily wastewater treatment.•PAC-PD composite coagulant performed well in reducing membrane fouling.•Different dosing modes of coagulants led to different coagulation mechanism.•Distribution of membrane resistance was related to coagulation behaviors. Oily wastewater treatment is necessary to reduce environmental pollution. Herein, a coagulation-ultrafiltration integrated process was used to remove emulsified oil. AlCl3, FeCl3, polyaluminum chloride (PAC), polyferric chloride (PFC), and polydimethyl diallyl ammonium chloride (PDMDAAC) were used in coagulation process. It was found that PAC was more effective than AlCl3 and Fe based coagulants in treating oil/water emulsions. Flocs formed in PAC+PDMDAAC, PDMDAAC+PAC, and PAC-PDMDAAC coagulation systems had different size, resistance, structure, and recovery ability, which was due to the changes in the dominant coagulation mechanism. The flocs formed by PAC-PDMDAAC composite coagulant were large and loose, indicating that bridging action and entrapment dominated the removal of oil droplets. PAC-PDMDAAC was efficient in reducing irreversible membrane fouling, while the PAC+PDMDAAC dual coagulation system performed well in controlling reversible fouling. The PDMDAAC+PAC system had a large adsorption resistance and a small cake layer resistance. The PAC-PDMDAAC system had a large polarization resistance. The different membrane resistance distribution was closely related to the discrepancy in coagulation behaviors. Therefore, the enhancement in coagulation performance is the key factor to alleviate membrane fouling. This study will provide theoretical guidance for the treatment of actual oily wastewater.