Influence of filter backwashing on iron, manganese, and ammonium removal in dual-media rapid sand filters used for drinking water production

Iron (Fe), manganese (Mn), and ammonium (NH₄⁺) removal from groundwater using rapid sand filtration is a widely employed method in drinking water production. Over time, Fe and Mn oxides accumulate in the filter, which necessitates frequent backwashing to avoid clogging. In this study, we investigated the impact of backwashing on the microbial community and filter chemistry in a dual-media filter comprising anthracite and sand layers. Specifically, we focused on the removal of Fe, Mn, and NH₄⁺ over the runtime of the filter. With increasing runtime, depth profiles of dissolved and particulate Fe revealed the buildup of Fe oxide flocs, causing Fe²⁺ and Mn²⁺ oxidation and nitrification to occur at greater depths within the filter. Towards the end of the filter runtime, breakthrough of suspended Fe oxides was observed, likely due to preferential flow. Backwashing effectively removed metal oxide flocs and restored the Fe removal efficiency in the top layer of the filter. While the two layers remained separate, the anthracite and sand layers themselves fully mixed during backwashing, leading to a homogenous distribution of the microbial community within each layer. Methyloglobulus and Gallionella were the predominant organisms in the anthracite layer, likely catalyzing methane and Fe²⁺ oxidation, respectively. The nitrifying community of the anthracite consisted of Nitrosomonas, Candidatus Nitrotoga, and Nitrospira. In contrast, the nitrifying community in the sand layer was dominated by Nitrospira. Backwashing minimally affected the microbial community composition of the filter medium except for Gallionella, which were preferentially washed out. In conclusion, our research offers a molecular and geochemical basis for understanding how backwashing influences the performance of rapid sand filters.