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• Microbial community composition is largely unaffected by backwashing. • Backwashing causes a preferential wash-out of Gallionella spp. • Removal of accumulated Fe flocs by backwashing reinstates nitrification. • The anthracite and sand layers remain separated but mix internally during backwash. • Continuous effluent turbidity monitoring helps to determine backwash frequency. Iron (Fe), manganese (Mn), and ammonium (NH 4 +) 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 NH 4 + 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• Newly started rapid sand filters fail to remove Mn and NH 4 +. • Filters of intermediate age with thin metal coatings remove Mn and NH 4 + optimally. • The nitrifying community changes with filter age. • Despite the presence of Gallionella , Fe2+ removal is mostly chemical. • Suboptimal Mn removal in old filters is attributed to preferential flow. Rapid sand filtration is a common method for removal of iron (Fe), manganese (Mn) and ammonium (NH 4 +) from anoxic groundwaters used for drinking water production. In this study, we combine geochemical and microbiological data to assess how filter age influences Fe, Mn and NH 4 + removal in dual media filters, consisting of anthracite overlying quartz sand, that have been in operation for between ∼2 months and ∼11 years. We show that the depth where dissolved Fe and Mn removal occurs is reflected in the filter medium coatings, with ferrihydrite forming in the anthracite in the top of the filters (< 1 m), while birnessite-type Mn oxides are mostly formed in the sand (> 1 m). Removal of NH 4 + occurs through nitrification in both the anthracite and sand and is the key driver of oxygen loss. Removal of Fe is independent of filter age and is always efficient (> 97% removal). In contrast, for Mn, the removal efficiency varies with filter age, ranging from 9 to 28% at ∼2–3 months after filter replacement to 100% after 8 months. After 11 years, removal reduces to 60–80%. The lack of Mn removal in the youngest filters (at 2–3 months) is likely the result of a relatively low abundance of mineral coatings that adsorb Mn2+ and provide surfaces for the establishment of a microbial community. 16S rRNA gene amplicon sequencing shows that Gallionella, which are known Fe2+ oxidizers, are present after 2 months, yet Fe2+ removal is mostly chemical. Efficient NH 4 + removal (> 90%) establishes within 3 months of operation but leakage occurs upon high NH 4 +loading (> 160 µM). Two-step nitrification by Nitrosomonas and Candidatus Nitrotoga is likely the most important NH 4 + removal mechanism in younger filters during ripening (2 months), after which complete ammonia oxidation by Nitrospira and canonical two-step nitrification occur simultaneously in older filters. Our results highlight the strong effect of filter age on especially Mn2+but also NH 4 + removal. We show that ageing of filter medium leads to the development of thick coatings, which we hypothesize leads to preferential flow, and breakthrough of Mn2+. Use of age-specific flow rates may increase the contact time with the filter medium in older filters and improve Mn2+ and NH 4 + removal. [Display omitted] [ABSTRACT FROM AUTHOR]