Influence of filter age on Fe, Mn and NH 4 + removal in dual media rapid sand filters used for drinking water production

Rapid sand filtration is a common method for removal of iron (Fe), manganese (Mn) and ammonium (NH ) 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 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 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 Mn and provide surfaces for the establishment of a microbial community. 16S rRNA gene amplicon sequencing shows that Gallionella, which are known Fe oxidizers, are present after 2 months, yet Fe removal is mostly chemical. Efficient NH removal (> 90%) establishes within 3 months of operation but leakage occurs upon high NH loading (> 160 µM). Two-step nitrification by Nitrosomonas and Candidatus Nitrotoga is likely the most important NH 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 Mn but also NH 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 Mn . Use of age-specific flow rates may increase the contact time with the filter medium in older filters and improve Mn and NH removal.