Inoculated rapid sand filter start-up 16S rRNA gene amplicon sequencing

Long start-up times to achieve manganese removal in rapid sand filters can pose challenges for drinking water companies. This study assessed the start-up dynamics of manganese removal in two full-scale dual-media rapid sand filters treating groundwater containing iron, ammonium, and manganese. After inoculation with 20% biologically active coated sand, ammonium and manganese removal efficiencies of approximately 60-70% and 30-50% were achieved, respectively. Complete removal of ammonium occurred after approximately 8 weeks but required around 17 and 25 weeks for manganese in the two filters. Full manganese removal, accompanied by manganese oxide formation on new grains, was achieved when about 50% of the ammonium was removed within the anthracite layer. X-ray spectroscopy of manganese oxides in the mineral coatings indicates a dominance of biologically produced manganese oxide with a structure similar to delta-MnO2, suggesting continuous microbial manganese oxidation in inoculated rapid sand filters. Concomitant changes in 16S rRNA gene profiles combined with qPCR and solute profiles suggest a key role for Nitrospira in both nitrification and manganese oxidation. We show that inoculation with biologically active filter medium enhances the efficiency of manganese and ammonium removal during filter start-up, offering a promising improvement strategy for rapid sand filter.