Comparison between permanganate pre-oxidation and persulfate/iron(II) enhanced coagulation as pretreatment for ceramic membrane ultrafiltration of surface water contaminated with manganese and algae

Abstract

Concurrent presence of algae and manganese (Mn) in water poses a significant challenge for water treatment. This study compared the treatment efficiency of Mn-containing and algae-laden water using either permanganate pre-oxidation (KMnO4) or persulfate/iron(II) (PMS/Fe2+) enhanced coagulation as pretreatment for ceramic membrane ultrafiltration. The results showed that KMnO4 pre-oxidation achieved a slightly more effective Mn removal, and was almost unaffected by the initial dissolved organic carbon (DOC) concentrations. PMS/Fe2+ removed UV254 more efficiently (above 90% at a dose of 0.25 mmol/L), compared with KMnO4 (less than 60% UV254 removal). According to X-ray photoelectron spectroscopy (XPS) analysis of aggregates, both KMnO4 and Fe2+/PMS oxidation resulted in the formation of MnO2 precipitate. Electron paramagnetic resonance(EPR) analysis demonstrated that only the reactors dosed with PMS/Fe2+ were able to generate the highly reactive hydroxyl radical(·OH). The production of ·OH caused significant rupture of algal cells and thus higher algal removal compared to the treatment with KMnO4 (whereby insignificant cell breakage was observed). The cell rupture resulted in higher amounts of organic matter released in the systems containing PMS/Fe2+, as demonstrated by excitation-emission matrix (EEM) and protein analysis. Despite the elevated level of organic matter, adding PMS/Fe2+ was found to notably mitigate membrane fouling due to the formation of large flocs (311–533 μm) as well as the elimination of major ceramic membrane foulants, i.e. humic substances.