Effects of organics concentration on the gravity-driven membrane (GDM) filtration in treating iron- and manganese-containing surface water

•Effects of organics concentration on flux stabilization and manganese removal of GDM were identified.•Moderate organics concentration endowing GDM an excellent stable flux (∼6.3 L m−2 h−1) and Mn removal (∼94.6%).•Increasing organics concentration benefited to the formation of heterogeneous and porous biocake layer.•Excessive organics would wrap the MnOx to result in reduction of manganese removal. Iron and manganese contamination in the surface water is posing great challenges to the drinking water treatment supply, especially in the complex cases of organics involvement. Gravity-driven membrane (GDM) filtration equipped with the dual functions of ultrafiltration and biocake layer, conferred promising potentials in the removals of iron and manganese. This study evaluated the effects of organics concentrations on the removal performance of iron and manganese, as well as on the flux stabilization during GDM long-term filtration. The results indicated that stable flux level and the removal efficiency of manganese initially increased with the increase of organics concentration in the feed water, and then decreased. The moderate concentration of organic compounds in the feed water would positively facilitate the microbial activities and benefit to engineering a heterogeneous and porous biocake layer on the membrane surface, contributing to the highest improvements of stable flux (6.3 L m−2 h−1), while high concentration of organic compounds in the feed water would result in the increase in the thickness and EPS concentration of the biocake layer, leading to a flux reduction. Furthermore, the moderate concentration of organic compounds in the feed water was also beneficial to the manganese removal (> 94.6%) due to the more accumulation of auto-catalytic oxidation manganese oxides (MnOx) within the biocake layer and the improved biological degradation, however, further increase of organics concentration would deliver a negative impact on the manganese removal owing to the wrapping of MnOx by the organic substances. Overall, these findings provide practical and acceptable strategies to the selections of pre-treatments prior to GDM and promote its extensive application in treating the iron- and manganese-containing surface water. [Display omitted]