Effect of ionic strength and calcium ions on humic acid fouling of hollow-fiber ultrafiltration membrane

Effect of ionic strength and calcium ions addition on the molecular properties of humic acid (HA), as well as the HA rejection coefficient and fouling characteristics of ultrafiltration were investigated in this study. Experiments were performed with an immersed hollow-fiber polyvinyl chloride membrane at a laboratory scale. The results indicated that both ionic strength and calcium ions could effectively reduce zeta potential of the HA solution. Average size of HA was identical with various ionic strength (10, 20, and 40 mM) or low calcium ion concentration (0.5 mM), but a much larger HA size was obtained with higher calcium ion concentrations (1.0 mM or 2.0 mM) for the complexation interaction between HA molecules. The ionic strength significantly reduced the HA removal, and somewhat mitigated the membrane fouling for the reduction of electrostatic repulsive forces between HA molecules and the membrane surface. At a low calcium ion concentration, the surface complexation dominated the fouling behavior of the process, and led to an increase of cake fouling resistance. However, at higher calcium ion concentrations, the bulk complexation dominated the interaction fouling behavior of the process, leading to a reduction of both cake fouling and pore blocking for the larger size aggregations.