Effect of silica fouling on fluoride removal by ultra low pressure reverse osmosis membrane

Abstract

The effects of silica fouling on fluoride removal by an ultra low pressure reverse osmosis (ULPRO) membrane were studied in cross-flow membrane test unit. The UTC-70, ULPRO membrane was studied under two operating conditions, namely, the concentration polarization experiment and silica fouling experiment at the operating transmembrane pressures of 0.1, 0.3, and 0.5 MPa. In the concentration polarization experiment, water containing various fluoride concentration of 0, 10, 25, and 50 mM were prepared as feed solutions under the controlled temperature of 25oC. The results showed that permeate flux of the membrane declined with the decreasing of operating transmembrane pressure and the increasing of fluoride concentration in feed water. The average mass transfer coefficient values of 0.1, 0.3, and 0.5 MPa were 4.79 x 10-5, 4.12 x 10-5, and 3.24 x 10-5 m/s, respectively. In the silica fouling experiment, water containing fluoride concentration of 15 mg/L combined with the different amount of silica concentration was prepared as feed solution. It was found that permeate flux slightly decreased with the increment of silica concentration in feed solution. However, fluoride rejection was higher during an increment of silica concentration between 0 and 100 mg/L. In contrast, a decrease in fluoride rejection was observed as silica concentration was increased from 100 to 300 mg/L. However, efficiencies of ULPRO membrane for defluoridation of between 87.0 and 98.7% were obtained. At 0.3 and 0.5 MPa, the formation of a polymerized silica fouled layer was found at the low silica concentration of 100 mg/L in feed water while the formation of colloidal silica fouled layer occurred at the high silica concentration of 300 mg/L. At 0.1 MPa, the polymerized silica fouled layer was present at both the low silica concentration and high silica concentration.