Ultrasonic control of UF membrane fouling by natural waters: Effects of calcium, pH, and fractionated natural organic matter

Ultrasound at a frequency of 20 kHz and a power of 16 W was applied to a cross-flow ultrafiltration system to investigate the ultrasonic control of surface-water fouled ceramic membranes. Ultrasound was effective in improving the normalized permeate flux of a surface water, from 0.21 in the absence of ultrasound to 0.70 with the aid of ultrasound. Moreover, with ultrasound a stable permeate flux was obtained throughout the 2 h filtration process. Calcium and pH had little impact on the extent of membrane fouling in the absence of ultrasound; however, ultrasound was more effective at minimizing fouling at lower calcium concentration and moderate pH values, presumably due to weaker foulant–foulant and foulant–membrane interactions at these conditions. Fractionation of natural organic matter (NOM) showed that, in the absence of ultrasound, the hydrophilic fraction contributed greatest to fouling while the hydrophobic fraction contributed less. Also, the extent and rate of fouling caused by the hydrophilic NOM was not sensitive to the change of pH or calcium concentration in feed water. The ultrasonic control of membrane fouling caused by hydrophilic NOM was effective at both calcium concentrations (40 and 180 mg/L) investigated. Ultrasonic control of fouling by hydrophilic NOM was more effective at higher pH, presumably due to greater charge repulsion between the membrane and amino acids and peptides in the hydrophilic fraction.