In groundwater treatment, after aeration, iron(II) is precipitated in rapid sand filtration (RSF) by homogeneous, heterogeneous and biological oxidation. The contribution of homogeneous iron(II) oxidation may be calculated from equations and constants available in the literature. Heterogeneous iron(II) oxidation produces hydrous ferric oxides coated filter sand, resulting in a growing filter bed height, from which the contribution of heterogeneous iron(II) oxidation may be estimated. The complement is contributed by biological iron(II) oxidation. At present this contribution may also be estimated by Gallionella spp. counts by quantitative real-time polymerase chain reaction. Based on field data of drinking water treatment plants from the Netherlands and Belgium, it appears that at pH ≈ 7.5 biological iron(II) oxidation is the main iron(II) removal process. At higher pH homogeneous iron(II) oxidation becomes dominant, while at lower pH heterogeneous iron(II) oxidation delivers a relevant contribution. The distribution of these oxidation processes is influenced by RSF operation such as presence of supernatant water, wet or dry (trickle) filtration, oxygen concentration, filter velocity, etc. Experience shows that the distribution between these three iron(II) oxidation processes may change over time. These results are important for RSF operation, iron sludge production, and fields like chemical well clogging.

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