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.

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 (KMnO) or persulfate/iron(II) (PMS/Fe) enhanced coagulation as pretreatment for ceramic membrane ultrafiltration. The results showed that KMnO pre-oxidation achieved a slightly more effective Mn removal, and was almost unaffected by the initial dissolved organic carbon (DOC) concentrations. PMS/Fe removed UV more efficiently (above 90% at a dose of 0.25 mmol/L), compared with KMnO (less than 60% UV removal). According to X-ray photoelectron spectroscopy (XPS) analysis of aggregates, both KMnO and Fe/PMS oxidation resulted in the formation of MnO precipitate. Electron paramagnetic resonance(EPR) analysis demonstrated that only the reactors dosed with PMS/Fe 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 KMnO (whereby insignificant cell breakage was observed). The cell rupture resulted in higher amounts of organic matter released in the systems containing PMS/Fe, as demonstrated by excitation-emission matrix (EEM) and protein analysis. Despite the elevated level of organic matter, adding PMS/Fe 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.