Novel insights on fouling mechanism of forward osmosis membrane during deep thickening waste activated sludge

Abstract

Application of forward osmosis (FO) membrane for sludge thickening (FO-MST process) has aroused growing interests owing to its high rejection, low footprint and low energy consumption. However, severe FO membrane fouling at an increasing sludge concentration condition remains a main obstacle for causing water flux decline and sludge thickening efficiency reduction. Here, fouling behaviors and mechanisms of FO membrane in the FO-MST process were comprehensively investigated by analyzing surface morphology, foulants composition, fouling layer structure and fouling reversibility of FO membrane at different sludge concentrations. Results indicated that the FO-MST process successfully achieved deeply thickening waste activated sludge from mixed liquor suspended solids (MLSS) concentration of approximately 5–50 g/L. And sludge concentration had a significant impact on FO membrane fouling in the FO-MST process. According to the tendency of sludge concentration in the FO-MST process, FO membrane fouling could be divided into three stages. At the first stage (i.e., MLSS concentration less than 15 g/L), only few sludge flocs and organic foulants deposited onto the FO membrane surface and inorganic scaling was the dominant fouling type. At the second stage (i.e., MLSS concentration ranged 15–30 g/L), the amount of foulants especially organic foulants and biofoulants significantly increased on the FO membrane surface and formed a loose fouling layer. In the first two stages, the flux decline was not significant. At the last stage (i.e., MLSS concentration ranged 30–50 g/L), more and more microorganisms and organic foulants deposited on the FO membrane surface. Meanwhile, the subsequent foulants kept compacting the former foulants that were already on the membrane surface, resulting in the formation of a dense fouling layer and a significant flux decline of FO membrane. Our finding is helpful for better understanding FO membrane fouling at an extreme sludge condition and for searching for some FO membrane fouling mitigations in the FO-MST process.