Inoculation Improves Microbial Manganese Removal during the Start-Up of Rapid Sand Filters

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 ∼60-70% and ∼30-50% were achieved, respectively. Complete removal of ammonium occurred after ∼8 weeks, but ∼17 and ∼25 weeks were required for manganese removal in the two filters. Full manganese removal, accompanied by manganese oxide formation on new grains, was achieved when ∼50% of the ammonium was removed within the anthracite layer. X-ray spectroscopy of manganese oxides in the mineral coatings indicated a dominance of biologically produced manganese oxide with a structure similar to that of δ-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 <i>Nitrospira</i> in both nitrification and manganese oxidation. We show that inoculation with biologically active filter medium enhances the efficiency of ammonium and manganese removal during filter start-up, offering a promising improvement strategy for rapid sand filters.