Evaluating Precipitative Softening as a Pretreatment for Ultrafiltration of a Natural Water

Abstract

Membrane processes are widely considered for application in drinking water treatment, but fouling often impedes their use. Precipitative softening is traditionally designed to remove hardness ions from hard waters, but it can also remove particles and organic matter, which are major foulants of membranes. Therefore, 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. To accomplish the objectives, a natural water, Lake Austin water, was investigated with three levels of softening: "standard softening" (maximum Ca²⁺ removal), "enhanced softening" [maximum lime addition without massive precipitation of Mg(OH)₂], and "Mg softening" (lime dose to achieve approximately 95% Mg²⁺ removal). Each condition represents a different degree of pretreatment to ultrafiltration in terms of organic matter (i.e., NOM fouling) and inorganics (i.e., inorganic fouling by further precipitation). Results showed that NOM removal was increased, and thus fouling was decreased, as the levels of softening were increased. At the Mg softening condition, significant flux improvement was achieved. The cause of flux improvement likely stemmed from the preferential removal of the hydrophobic fraction of NOM in softening. Direct images of the surface of the membranes by scanning electron microscopy allowed observation of the larger materials caught. In addition, X-ray photoelectron spectroscopy was used to determine the relative composition of each fouling material on the membrane surfaces.