Membrane fouling by Fe-Humic cake layers in nano-scale: Effect of in-situ formed Fe(III) coagulant

The application of in-situ Fe(III) coagulant, formed by reacting Fe(II) and KMnO4 (Fe/Mn), before membrane ultrafiltration (Fe/Mn-CUF) has been compared with conventional dosing of an Fe(III) coagulant (Fe-CUF). The performance of the alternative coagulants has been evaluated in terms of membrane fouling using model waters containing humic acid. A lower development of trans-membrane pressure (TMP) suggested that in-situ formed Fe(III) caused a lower rate of fouling compared to the dosed Fe(III). The lower TMP development with in-situ formed Fe(III), compared to dosed Fe(III), was associated with smaller, fewer and less compact coagulant flocs and differences in the membrane cake layer; in both cases influent flocs were aggregates of nanometer-scale primary particles. Interestingly, TEM and SEM images showed that the size of nanometer-scale particles formed by in-situ Fe(III) was slightly larger than dosed Fe (III), and this contributed to a more porous cake layer. In addition, the Fe/Mn flocs formed a thinner cake layer than that formed by Fe flocs. Overall, the effect of decreasing the thickness and density of the cake layer and increasing the size of the nanometer-sized primary particles was significantly beneficial in reducing membrane fouling. Also NOM analysis by size exclusion chromatography (SEC) indicated that the in-situ Fe(III) coagulant achieved greater NOM removal than the dosed Fe(III), both in the UF influent and filtrate waters. In summary, the results have shown that in-situ Fe(III) is a superior pretreatment to conventional dosed Fe(III), in terms of significantly improving membrane performance (reduced fouling) with a little higher improvement in water quality.