Improving ultrafiltration of algae-laden water with chitosan quaternary ammonium salt enhanced by sodium percarbonate

Ultrafiltration (UF) is extensively used for algae removal because of its ability to retain algal cells with high efficiency, but it still faces the problem of membrane fouling and low retention capacity of dissolved organics. Hence, a strategy of coagulation with chitosan quaternary ammonium salt (HTCC) enhanced by sodium percarbonate (SPC) pre-oxidation was proposed to improve the UF performance. The fouling resistances were calculated by a resistance-in-series model based on Darcy's formula, and the membrane fouling mechanism was evaluated using a pore plugging-cake filtration model. The effect of SPC-HTCC treatment on the properties of algal foulants was explored, and the result showed that the water quality was improved with the maximum removal rates of 78.8 %, 52.4 % and 79.5 % for algal cells, dissolved organic carbon and turbidity, respectively. The SPC could achieve a mild oxidation effect that degraded the electronegative organics attached to algal cells without destroying the cell integrity, making the algal pollutants easier to agglomerate through subsequent HTCC coagulation by forming larger flocs. In terms of membrane filtration, the terminal normalized flux was increased from 0.25 to 0.71, with the reversible and irreversible resistances reduced by 90.8 % and 40.2 %, individually. The synergistic treatment reduced the accumulation of algal cells and algae-derived organics on the membrane surface as inferred from the interface fouling characteristics. The interfacial free energy analysis showed that the synergistic treatment reduced the adhesion of contaminants to the membrane surface, as well as the attraction among pollutants. Overall, the proposed process has high application prospects for algae-laden water purification.