Small-Portion Replacement with MnO _<i>x</i> Media Promotes Biofilm Development, Leading to Rapid Biological Maturation and Sustained Mn(II) Removal in Sand Filtration

Stringent control of Mn is critical for drinking water safety. However, conventional rapid sand filters exhibit slow biological maturation for Mn(II) removal, limiting their performance in engineered Mn(II) removal. This study examined the effectiveness of partially replacing sand with natural manganese ore (NMO) to enhance Mn(II) removal and elucidated the underlying mechanisms. The results showed that replacing only 1/8 of sand with NMO enabled consistently effective Mn(II) removal (residual <5 μg/L) from start-up through long-term operation, even under stressful conditions; its performance was comparable to columns packed with 100% NMO. In 1/8 NMO columns, early Mn(II) removal driven by MnO_<i>x</i>-mediated chemical processes rapidly diminished, after which MnO_<i>x</i>-enhanced biological processes promptly took over. Compared with sand columns, 1/8 NMO columns biologically matured nearly 100 days earlier, and biomass on the rough, porous MnO_<i>x</i> surface was 1-2 orders of magnitude higher than that on sand. Although NMO partly altered microbial communities, the relative abundance of Mn(II)-oxidizing bacteria did not increase; the larger absolute biofilm biomass primarily drove sustained efficient Mn(II) removal. These findings highlighted the important but previously overlooked role of MnO_<i>x</i> in promoting biofilm accumulation and accelerating the maturation of biological Mn(II) removal systems and provided a practical and cost-effective upgrade for conventional sand filtration.