Oxidants-assisted gravity-driven membrane (GDM) process for manganese removal from surface water: Rapid maturation and manganese oxide regulation

The rapid maturation of the gravity-driven membrane (GDM) process for manganese removal from surface water is critical to its practical application. In this study, KMnO4 and NaClO were used to investigate their effects on the start-up period of the GDM process for treating manganese-containing surface water. The start-up period of the GDM system with KMnO4 (24 days) was shorter compared to that with NaClO (40 days). After the addition of KMnO4 and NaClO, the stable flux of the GDM system was significantly improved, and the bio-cake layer became more porous, which was ascribed to the reduction of proteins in the extracellular polymeric substances (EPS). The active manganese oxide (MnOx) in the bio-cake layer was identified as delta-MnO2. The addition of the oxidants increased the proportion of Mn(III) with high catalytic oxidation activity in delta-MnO2, where the assisted effect of KMnO4 (43.7%) was superior to that of NaClO (27.8%). The development of delta-MnO2 in the natural filtration state was ascribed to the enrichment of manganese oxidizing bacteria (MnOB). The biological action only accounted for 9.4%-12.3% of the manganese removal and was not dominant. The difference in the start-up period of the GDM process was dependent on the autocatalytic oxidation of Mn(III) in delta-MnO2 in the bio-cake layer. These findings provided acceptable and practical strategies for the rapid and safe manganese removal in the GDM process.