Iron usually occurs with manganese in groundwater. The presence of these metals in excess of 0.1 ppm and 0.05 ppm, respectively, is unacceptable for public water supplies and for most industrial uses. Above these concentra tions, precipitates are formed on contact with air; residues stain fixtures and interfere with clothes washing and most manufacturing processes. The iron may be a water soluble ferrous salt or iron bacteria, i.e., hydrated iron oxide enclosed in the cell structure of filamentous microorganisms, such as Crenothrix polyspora. Dissolved inorganic iron is usually removed by aeration, chemical precipitation, or ion exchange. Iron bacteria removal requires destruction of cell membranes by strong oxidizing agents such as ozone or chlorine (Table 8.3.3).

The oxygen-poor, carbon dioxide-rich lower layers of water reservoirs reduce and dissolve iron salts in the soil as ferrous salts. Similarly, the relatively oxygen-free acidic

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