Groundwaters in Finland are soft and acidic, and the main substances that require treatment are iron and manganese. Iron removal is usually relatively easy by oxidizing dissolved iron into an undissolved form either by aeration or chemical oxidization and removing the formed precipitate by sand filtration, for example. If the raw water contains high amounts of organic matter, problems may arise when using the traditional methods for iron removal. In Finland it is quite common that groundwater contains both high levels of iron and humus. The groundwater of Kukkala intake plant in Liminka has been found problematic due to the high level of organic matter and therefore this research studied the removal of iron from this water by means of oxidation with hydrogen peroxide and filtration. Iron was oxidized with hydrogen peroxide and when the dosage reached 3 mg l−1, all iron was in trivalent form, which means that nearly all of it was in undissolved form, i.e. in fractions greater than 0.45 μm. Oxidized iron particles were, however, very fine and they could not be removed by sand/anthracite filtration. However sand/anthracite filtration was able to remove iron well without the feed of oxidation chemicals, and the iron was then led to the filter in bivalent dissolved form, thus the filter operated as an adsorption filter.

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