Membrane fouling control by UV/persulfate in tertiary wastewater treatment with ultrafiltration: A comparison with UV/hydroperoxide and role of free radicals

Abstract

The membrane fouling caused by effluent organic matter (EfOM) is a bottleneck for application of ultrafiltration (UF) in tertiary wastewater treatment. In this study, two ultraviolet-based oxidation pretreatments, i.e., ultraviolet/persulfate (UV/PS) and ultraviolet/hydrogen peroxide (UV/H2O2), were used to alleviate EfOM fouling at a low UV dose of 2.36 × 103 mJ cm−2, in consideration of the widely use of UV radiation for disinfection in wastewater treatment plants. Effects of the pretreatments on characteristics of EfOM were investigated in terms of fluorescence spectrum, molecular weight distribution, and hydrophobicity. Moreover, filtration tests were performed to assess impacts of the oxidation pretreatments on flux decline, fouling reversibility, and fouling mechanisms. The results showed that the UV/PS pretreatment substantially reduced flux decline and improved fouling reversibility during filtration of the secondary effluent, attributed to efficient degradation (~67%, in terms of UV254) and partial mineralization (~39%, in terms of DOC) of EfOM. In particular, the irreversible resistance was decreased by nearly one order of magnitude at the PS dose of 1.0 mM. UV/H2O2 was not effective in EfOM fouling control under the conditions investigated. The UV/PS pretreatment resulted in a shift of mechanisms for EfOM fouling from successive pore blocking and cake filtration to single standard blocking, attributing to preferential degradation of macromolecular components in EfOM and a weakened role of cake interception. The radical quenching experiment showed that the sulfate radicals played a dominant role in EfOM fouling control by UV/PS, and that transformation of sulfate radicals into hydroxyl radicals negatively affected EfOM fouling control.