Rapid sand filters (RSF) are an established and widely applied technology for the removal of dissolved iron (Fe²⁺) and ammonium (NH₄⁺) among other contaminants in groundwater treatment. Most often, biological NH₄⁺oxidation is spatially delayed and starts only upon complete Fe²⁺ depletion. However, the mechanism(s) responsible for the inhibition of NH₄⁺oxidation by Fe²⁺ or its oxidation (by)products remains elusive, hindering further process control and optimization. We used batch assays, lab-scale columns, and full-scale filter characterizations to resolve the individual impact of the main Fe²⁺ oxidizing mechanisms and the resulting products on biological NH₄⁺ oxidation. modeling of the obtained datasets allowed to quantitatively assess the hydraulic implications of Fe²⁺ oxidation. Dissolved Fe²⁺ and the reactive oxygen species formed as byproducts during Fe²⁺ oxidation had no direct effect on ammonia oxidation. The Fe³⁺ oxides on the sand grain coating, commonly assumed to be the main cause for inhibited ammonia oxidation, seemed instead to enhance it. modeling allowed to exclude mass transfer limitations induced by accumulation of iron flocs and consequent filter clogging as the cause for delayed ammonia oxidation. We unequivocally identify the inhibition of NH₄⁺oxidizing organisms by the Fe³⁺ flocs generated during Fe²⁺ oxidation as the main cause for the commonly observed spatial delay in ammonia oxidation. The addition of Fe³⁺ flocs inhibited NH₄⁺oxidation both in batch and column tests, and the removal of Fe³⁺ flocs by backwashing completely re-established the NH₄⁺removal capacity, suggesting that the inhibition is reversible. In conclusion, our findings not only identify the iron form that causes the inhibition, albeit the biological mechanism remains to be identified, but also highlight the ecological importance of iron cycling in nitrifying environments.

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Rapid sand filters are established and widely applied technologies for groundwater treatment. In these filters, main groundwater contaminants such as iron, manganese, and ammonium are oxidized and removed. Conventionally, intensive aeration is employed to provide oxygen for these redox reactions. While effective, intensive aeration promotes flocculent iron removal, which results in iron oxide flocs that rapidly clog the filter. In this study, we operated two parallel full-scale sand filters at different aeration intensities to resolve the relative contribution of homogeneous, heterogeneous and biological iron removal pathways, and identify their operational controls. Our results show that mild aeration in the LOW filter (5 mg/L O2, pH 6.9) promoted biological iron removal and enabled iron oxidation at twice the rate compared to the intensively aerated HIGH filter (>10 mg/L O2, pH 7.4). Microscopy images showed distinctive twisted stalk-like iron solids, the biosignatures of Gallionella ferruginea, both in the LOW filter sand coatings as well as in its backwash solids. In accordance, 10 times higher DNA copy numbers of G. ferruginea were found in the LOW filter effluent. Clogging by biogenic iron solids was slower than by chemical iron flocs, resulting in lower backwash frequencies and yielding four times more water per run. Ultimately, our results reveal that biological iron oxidation can be actively controlled and favoured over competing physico-chemical routes. The production of more compact and practically valuable iron oxide solids is of outmost interest. We conclude that, although counterintuitive, slowing down iron oxidation in the water before filtration enables rapid iron removal in the biofilter.@en

Nitrate leaching from agricultural soils is increasingly found in groundwater, a primary source of drinking water worldwide. This nitrate influx can potentially stimulate the biological oxidation of iron in anoxic groundwater reservoirs. Nitrate-dependent iron-oxidizing (NDFO) bacteria have been extensively studied in laboratory settings, yet their ecophysiology in natural environments remains largely unknown. To this end, we established a pilot-scale filter on nitrate-rich groundwater to elucidate the structure and metabolism of nitrate-reducing iron-oxidizing microbiomes under oligotrophic conditions mimicking natural groundwaters. The enriched community stoichiometrically removed iron and nitrate consistently with the NDFO metabolism. Genome-resolved metagenomics revealed the underlying metabolic network between the dominant iron-dependent denitrifying autotrophs and the less abundant organoheterotrophs. The most abundant genome belonged to a new Candidate order, named Siderophiliales. This new species, “Candidatus Siderophilus nitratireducens,” carries genes central genes to iron oxidation (cytochrome c cyc2), carbon fixation (rbc), and for the sole periplasmic nitrate reductase (nap). Using thermodynamics, we demonstrate that iron oxidation coupled to nap based dissimilatory reduction of nitrate to nitrite is energetically favorable under realistic Fe3+/Fe2+ and NO3−/NO2− concentration ratios. Ultimately, by bridging the gap between laboratory investigations and nitrate real-world conditions, this study provides insights into the intricate interplay between nitrate and iron in groundwater ecosystems, and expands our understanding of NDFOs taxonomic diversity and ecological role.@en

Rapid sand filters (RSF) are an established and widely applied technology for groundwater treatment. Yet, the underlying interwoven biological and physical-chemical reactions controlling the sequential removal of iron, ammonia and manganese remain poorly understood. To resolve the contribution and interactions between the individual reactions, we studied two full-scale drinking water treatment plant configurations, namely (i) one dual-media (anthracite and quartz sand) filter and (ii) two single-media (quartz sand) filters in series. In situ and ex situ activity tests were combined with mineral coating characterization and metagenome-guided metaproteomics along the depth of each filter. Both plants exhibited comparable performances and process compartmentalization, with most of ammonium and manganese removal occurring only after complete iron depletion. The homogeneity of the media coating and genome-based microbial composition within each compartment highlighted the effect of backwashing, namely the complete vertical mixing of the filter media. In stark contrast to this homogeneity, the removal of the contaminants was strongly stratified within each compartment, and decreased along the filter height. This apparent and longstanding conflict was resolved by quantifying the expressed proteome at different filter heights, revealing a consistent stratification of proteins catalysing ammonia oxidation and protein-based relative abundances of nitrifying genera (up to 2 orders of magnitude difference between top and bottom samples). This implies that microorganisms adapt their protein pool to the available nutrient load at a faster rate than the backwash mixing frequency. Ultimately, these results show the unique and complementary potential of metaproteomics to understand metabolic adaptations and interactions in highly dynamic ecosystems.

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Drinking water treatment plants (DWTPs) are designed to remove physical, chemical, and biological contaminants. However, until recently, the role of DWTPs in minimizing the cycling of antibiotic resistance determinants has got limited attention. In particular, the risk of selecting antibiotic-resistant bacteria (ARB) is largely overlooked in chlorine-free DWTPs where biological processes are applied. Here, we combined high-throughput quantitative PCR and metagenomics to analyze the abundance and dynamics of microbial communities, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs) across the treatment trains of two chlorine-free DWTPs involving dune-based and reservoir-based systems. The microbial diversity of the water increased after all biological unit operations, namely rapid and slow sand filtration (SSF), and granular activated carbon filtration. Both DWTPs reduced the concentration of ARGs and MGEs in the water by circa 2.5 log gene copies mL−1, despite their relative increase in the disinfection sub-units (SSF in dune-based and UV treatment in reservoir-based DWTPs). The total microbial concentration was also reduced (2.5 log units), and none of the DWTPs enriched for bacteria containing genes linked to antibiotic resistance. Our findings highlight the effectiveness of chlorine-free DWTPs in supplying safe drinking water while reducing the concentration of antibiotic resistance determinants. To the best of our knowledge, this is the first study that monitors the presence and dynamics of antibiotic resistance determinants in chlorine-free DWTPs.@en