Use of ultrafiltration membranes for the separation of TiO₂ photocatalysts in drinking water treatment

This study investigated the ability of cross-flow ultrafiltration (UF), combined with photocatalytic reactions, to separate TiO₂ photocatalysts from treated water in photocatalytic drinking water treatment. The effect of natural organic matter (i.e., humic acids) and cross-flow velocities on UF fluxes and organic removal was explored with and without UV irradiation in the photocatalytic reactor. The interaction between the two solutes in the system, humic acids and TiO₂ photocatalysts, played a significant role in the formation of dense cake layers at the membrane surface, leading to a greater flux decline during ultrafiltration of TiO₂ particles. According to visual observations of the used membranes and the estimation of back-transport velocities of the solutes, a substantial amount of TiO₂ deposited on the membrane induces more humic acids to accumulate at the membrane through the adsorption of humic acids onto TiO₂ particles. The humic-acid-laden TiO₂ particles offered more than four times higher specific cake resistance with a substantially increased compressibility coefficient than TiO₂ particles alone. The higher the cross-flow velocities, the greater the UV₂₅₄ removal achieved. This was because the rise of cross-flow velocities contributed to the reduction of concentration polarization at the membrane surface, thereby resulting in a decrease of the driving force for humic acids to pass through the membrane. When photocatalytic reactions took place with UV illumination, UV₂₅₄ removal efficiencies of the permeate were improved markedly, and also the permeate flux was kept at a constant level without any sign of fouling. Although humic acids were not completely mineralized by photocatalysis, the degradation of the humic acids helped to enhance the UF flux, as they were transformed to less adsorbable compounds.