Application of composite flocculants for removing organic matter and mitigating ultrafiltration membrane fouling in surface water treatment: the role of composite ratio

Coagulation prior to the ultrafiltration (UF) process was implemented to improve natural organic matter (NOM) removal and membrane permeability. Polyaluminum chloride (PAC), polyferric chloride (PFC), and composite flocculants polyaluminum chloride-polydimethyldiallylammonium chloride (PAC-PDMDAAC) and PFC-PDMDAAC were used for comparison for treating HA-simulated water, FA-simulated water, Yellow River water and Yangtze River water. The comparative experiments showed that PAC-PDMDAAC achieved the best C-UF performance among the flocculants PAC, PFC, PFC-PDMDAAC and PFC-PDMDAAC due to its highest charge density. With an increase in the PDMDAAC content, the ability of the charge neutralization and adsorption bridging of PAC-PDMDAAC improved. PAC-PDMDAAC (5 : 1) contributed to the highest NOM removal efficiency and the formation of amorphous flocs with the lowest density and the most open structure because PDMDAAC as high positively charged organic matter quickly neutralized the negative charges on the micro-flocs and swept them to form highly branched flocs. In membrane experiments, PAC-PDMDAAC with a high content of PDMDAAC mitigated membrane fouling due to the formation of a cake layer with high porosity and less blockage in the membrane pores. With an increase in the PDMDAAC content in PAC-PDMDAAC, R-cs decreased and the cake layer resistance still accounted for the majority of the total resistance when an RC membrane was used. These results demonstrate that PAC-PDMDAAC with a high PDMDAAC content is effective in alleviating membrane fouling by reducing pore blockage and forming a porous cake layer, which is attributed to flocs with a large size and open structure.