Manganese in groundwater is a common problem in many countries. The WHO guideline value for manganese in drinking water is 400 μg/L, while the limit for iron in water, 0.3 mg/l (ppm). The presence of both these compounds are often associated to aesthetic problems, such as, look, smell and taste.
In general, the treatment of slightly anaerobic groundwater consists of aeration and submerged granular filtration, which is primarily designed for the removal of dissolved iron (Fe), manganese (Mn) and ammonium (NH4). Aeration is needed to increase the concentration of oxygen, increase the pH level in the water and remove carbon dioxide. In groundwater filters, various physical, chemical and biological processes take place, which all relate to the oxidation of dissolved iron (Fe), manganese (Mn) and ammonium (NH4). These oxidised compounds are then physically captured in the filter bed.
Oxidation occurring in the filter depends on parameters, such as the supernatant water level, filtration velocity, pH and O2 concentration. Iron and manganese chemical oxidation and the subsequent hydrolysis into rusty flakes predominantly occurs in the supernatant water layer (A). The chemical oxidation of manganese is slow, but once it begins to oxidise into a coating on the filter media. This starts a catalytic process for more manganese to adsorb to the oxide coating (B). Finally, the removal of iron and manganese by biological oxidation occurs in the filter bed (C).
Iron: Iron-oxidising bacteria (IOB) derive energy by oxidising dissolved ferrous iron (Fe2+) inside the bacterial cell, rendering it insoluble. Most notable is, Leptothrix: a line of cylindrical cells enclosed in a sheath and Gallionella: a single cell attached to a spiral stalk. They are both characterised by a reddish- brown colour.
Manganese: Leptothrix (except Gallionelia) is also capable of converting manganese into MnO2 through oxidation providing the iron oxidation has been completed beforehand. Other previously studied bacteria include Bacillus and Pseudomonas. The action exercised by these microorganisms constitutes the basis of biological treatment.
We have included an illustrated instruction manual that explains how to construct the system using few resources, followed by maintenance protocols and more detail on the background and filtration processes involved.
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Illustrations by Iris van Driel
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