The limit for manganese in drinking water is 0.05 ppm. Above that concentration it stains fixtures and interferes with laundering and chemical processing. Manganese usually occurs with iron in ground and surface waters, and many iron-removing processes (Table 8.3.3) will also remove manganese. The treatment process usually oxidizes the water-soluble manganous salts to insoluble manganese dioxide by catalytic air oxidation in the pH range of 8.5-10, by chlorination at a pH of 9-10, or by ozonation at neutral pH. Ion exchange is also effective, but it removes other salts that may or may not be desirable.
Manganese, like iron, is extracted from the bottom of deep reservoirs or from the ground by carbon dioxide-rich, oxygen-poor water as manganous bicarbonate.
Catalytic air oxidation at an alkaline pH is the most economical method for large treatment plants. Aeration occurs on contact beds of coke or stone coated with manganese dioxide or on beds of pyrolusite. Oxidation is more rapid when the pH is adjusted between 8.5 and 10.0 by lime or caustic (Table 8.3.6). One ppm dissolved oxygen oxidizes approximately 7 ppm manganese. The insoluble manganese dioxide is usually removed by settling and filtration.
Oxidation of manganese can also be carried out at a neutral pH using ozone as oxidant, and excessive ozone dosages can oxidize the manganese to pink permanganate. Chlorine oxidation of manganese requires no catalyst.
Ion exchange processes using sodium or hydrogen exchange resin remove manganous salts effectively, together with other salts. The exchange resin has to be regenerated
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