Membrane fouling control by Ca²⁺ during coagulation-ultrafiltration process for algal-rich water treatment

Seasonal algal bloom, a water supply issue worldwide, can be efficiently solved by membrane technology. However, membranes typically suffer from serious fouling, which hinders the wide application of this technology. In this study, the feasibility of adding Ca²⁺ to control membrane fouling in coagulation-membrane treatment of algal-rich water was investigated. According to the results obtained, the normalized membrane flux decreased by a lower extent upon increasing the concentration of Ca²⁺ from 0 to 10 mmol/L. Simultaneously, the floc particle size increased significantly with the concentration of Ca²⁺, which leads to a lower hydraulic resistance. The coagulation performance is also enhanced with the concentration of Ca²⁺, inducing a slight osmotic pressure-induced resistance. The formation of Ca²⁺ coagulation flocs resulted in a looser, thin, and permeable cake layer on the membrane surface. This cake layer rejected organic pollutants and could be easily removed by physical and chemical cleaning treatments, as revealed by scanning electron microscopy images. The hydraulic irreversible membrane resistance was significantly reduced upon addition of Ca²⁺. All these findings suggest that the addition of Ca²⁺ may provide a simple-operation, cost-effective, and environmentally friendly technology for controlling membrane fouling during coagulation-membrane process for algal-rich water treatment.