Permanganate-assisted pilot-scale gravity-driven membrane (GDM) filtration in treating Mn(II)-containing groundwater: Fast startup and mechanism

Groundwater containing Mn(II) can be effectively treated by utilizing the gravity-driven membrane (GDM) filtration method. However, reducing the startup period for Mn(II) removal remains challenging. This study presents the intermittent addition of permanganate (PM) to the pilot-scale GDM systems configured with varying membrane pore sizes to analyze its impact on the water quality and production while treating groundwater with high concentration of Mn(II) (1.49-1.87 mg/L). The addition of PM improves Mn(II) removal during the startup period because of its ability to oxidize Mn(II), the adsorption ability of the generated MnO2, and efficient transformation from the adsorption-saturated MnO2 to the birnessite-type manganese oxides (MnOx) having autocatalytic properties. Although the PM-assisted startup GDM systems demonstrated a higher efficiency in removing Mn(II) (97.2 +/- 0.57%), a temporary rebound in effluent Mn(II) was observed in the absence of PM. The freshly generated MnOx contribute to establishing a more heterogeneous biofouling layer on the membrane surface with PM assistance, resulting in shorter flux stabilization periods and significantly improving stable flux by 233% to 20.1 +/- 0.95 L/m2 h. The difference in pore size slightly impacts the membrane permeability performance because of the rapid formation of the biofouling layer which contains much MnOx. Furthermore, the addition of PM has little influence on Mn(II)-oxidizing bacteria (MnOB) in sustaining Mn(II) removal while reducing diversity and richness of eukaryotic community dominated by oxidant-resistant genus Apiotrichum. These findings offer comprehensive insights into the mechanisms involved in the GDM process with PM-assisted startup for treating Mn(II)-containing groundwater, prompting reconsideration of PM applicability during startup.