Effects of characteristics of softening as a pretreatment for ultrafiltration

Softening was proposed as a pretreatment for ultrafiltration, which reduces foulants prior to membrane processes. The objectives of this research were to understand the nature of the fouling mechanisms for ultrafiltration when used for waters that either require softening or have been softened, and to use that understanding to determine promising options for the use of softening as a pretreatment before ultrafiltration (UF). Several characteristics of softening, including extent of softening, precipitation kinetics, and hydrophobicity of membrane materials, were investigated as independent variables. Because of softening, both the organic matter concentration and the particle concentration of feed water to ultrafiltration were reduced, thus reducing the degree of fouling. The slow kinetics of softening had little effect on membrane fouling although precipitation directly occurred on the membrane surface in the system during the operation. Detailed images of the membrane surface obtained by scanning electron microscopy confirmed substantial amounts of the precipitates on the surface. In contrast, the hydrophobicity of the membrane material showed a significant effect on the flux decline behavior. Overall, the integrated water treatment of softening and ultrafiltration is shown to be a promising option for hard waters.