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Abstract Rapid sand filters are established and widely applied technologies for groundwater treatment. Conventionally, intensive aeration is employed to provide oxygen for the oxidation and removal of the main groundwater contaminants. While effective, intensive aeration promotes flocculent iron removal, which results in iron 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 O 2 pH 6.9) promoted biological iron removal and enabled iron oxidation at twice the rate compared to the intensively aerated HIGH filter (>10mg/L O 2, pH 7.4). ESEM images showed distinctive twisted stalk-like Fe solids, 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 FeOx was slower than by chemical FeOx flocs, resulting in lower backwash frequencies and yielding four times more water per run. Ultimately, our results reveal that biological Fe 2+ oxidation can be actively controlled and favoured over competing physico-chemical routes. The resulting operational benefits are only starting to be appreciated, with the counterintuitive higher oxidation rates and water yields at lower aeration regimes, and the production of more compact and practically valuable FeOx solids being of outmost interest.