Behaviors of dissolved organic matter in membrane desalination

The significance of dissolved organic matter (DOMs) in membrane fouling has been also increasingly noted in desalination plants using reverse osmosis (RO) and nanofiltration (NF). Nevertheless, little information is available on the interactions between DOM and other foulants such as inorganic scale. This study sought to gain a fundamental understanding of the complicated fouling phenomenon by scale formation in the presence of DOM. Experimental studies with model solutions were conducted in a small batch filtration device. Humic acid and calcium sulfate were used as model DOM and scale-forming salts. The fouling of RO/NF membranes by scale formation was observed to be greatly affected by DOM and there appeared to be a strong link between the rate of fouling and DOM concentrations. Although DOM itself is a foulant to membranes, it acted like an antiscalant to lessen fouling due to scale formation. It is likely that the adsorption of DOM on crystal-growing sites on membranes as well as crystal surfaces diminished the sites for crystal growth and subsequently retarded the rate of crystallization. Image analysis of crystal particles grown in the presence of DOM also supported this hypothesis.

In this work, the effect of humic acid on scale formation in RO membrane system was investigated using a laboratory scale filtration device and synthetic feed solution. The following conclusions can be drawn from this work: C Fouling of RO/NF membrane by scale formation significantly was affected by humic acid. As the concentration of humic acid increases, the scale formation was retarded and higher flux was obtained. Not only humic acid but also fulvic acid exhibited the inhibition effect. C The dominant mechanism of scale inhibition by humic acid was not complexation of calcium but coverage of active sites on crystal particles by humic acid adsorption. C The image analysis results indicated that the morphology of crystals grown in the presence of humic acid was significantly changed. Plate-like crystal particles were obtained at high concentration of humic acid.