Interrelationship of photooxidation and microfiltration in drinking water treatment

Abstract

Photooxidation by vacuum ultraviolet radiation (VUV, 254 + 185 nm) was investigated as a pretreatment to break down the natural organic matter (NOM) in surface water and thus reduce fouling in microfiltration systems for drinking water treatment. The behaviour of variously treated waters, namely raw water, VUV-treated water and water with aluminium chlorohydrate as coagulant, were compared in a stirred cell using 0.22 μm hydrophobic and surface-modified hydrophilic polyvinylidene fluoride membranes. Significant flux improvements were obtained for the VUV-treated waters. VUV-treatment has two interrelated effects; it breaks down the hydrophobic aromatic components of NOM to smaller, more hydrophilic components, and reduces the DOC content by mineralisation. Over the course of a microfiltration run, the DOC content in the permeate initially decreased then gradually increased to approach the feed value. The initial decrease coincided with reduced concentrations of very hydrophobic acid (VHA) and neutral (NEU) fractions in the permeate. The reduction in slightly hydrophobic acid (SHA) concentration was lower, but over a longer period. We propose that the VHA and NEU fractions were strongly adsorbed, and that the SHA fraction was adsorbed until saturation of the adsorption sites in the membrane pores occurred which then enabled the passage of subsequent material. The adsorptive capacity of the hydrophobic membrane was reduced from 10.3 mg g −1 membrane for raw water to 6.0 mg g −1 following VUV-treatment.