Ultrasonic Control of Ceramic Membrane Fouling Caused by Synthetic and Surface Waters: Effects of NOM, pH and Calcium

The ultrasonic control of membrane fouling caused by a synthetic water made of purified Aldrich natural organic matter (NOM) and a surface water was investigated under various solution conditions. 20 kHz ultrasound was applied to a cross-flow ultrafiltration system with hydrophilic gamma-alumina ultrafiltration membranes. Experimental results indicated that the type of NOM and solution chemistry affected the ability of ultrasound to control membrane fouling. For purified Aldrich NOM, less fouling and more effective ultrasonic effect occurred at high pH, low ionic strength, and in the absence of divalent cations, consistent with the expectation of stronger electrostatic repulsion between foulant-foulant and foulant-membrane components. However, for the surface water containing 40 mg/L Ca, pH and additional Ca had little impact on the extent of membrane fouling in the absence of ultrasound; while ultrasound was more effective at fouling control at lower calcium concentration and moderate pH values. Fractionation of NOM from the surface water showed that in the absence of ultrasound, fouling was mainly from the hydrophilic fraction, which is composed of amino acids, peptides and sugars. The extent and rate of fouling caused by the hydrophilic NOM was not sensitive to pH or calcium concentration in the feed water, presumably due to weaker electrostatic interactions between foulant-foulant and foulant-membrane. As a result, ultrasound was effective to control fouling of hydrophilic fraction of NOM. However, the hydrophobic acids mainly composed of fulvic acids had similar fouling behaviors with purified Aldrich NOM with respect to pH and Ca. The different fouling behaviors between the unfractionated and fractionated NOM suggest the interactions among the various fractions and the membrane played an important role.