Iron concentrations as low as 0.3 mg/L and manganese concentrations as low as 0.05 mg/L can cause dirty water complaints. At these concentrations, the water may appear clear but imparts brownish colors to laundered goods. Iron also affects the taste of beverages such as tea and coffee. Manganese flavors tea and coffee with medicinal tastes.
Some types of bacteria derive their energy by utilizing soluble forms of iron and manganese. These organisms are usually found in waters that have high levels of iron and manganese in solution. The reaction changes the species from soluble forms into less soluble forms, thus causing precipitation and accumulation of black or reddish brown gelatinous slimes. Masses of mucous iron and manganese can clog plumbing and water treatment equipment. They also slough away in globs that become iron or manganese stains on laundry.
Standards for iron and manganese are based on levels that cause taste and staining problems and are set under the Environmental Protection Agency Secondary Drinking Water Standards (EPA SDWA). They are, respectively, 0.3 mg/L for iron and 0.05 mg/L for manganese. Iron and manganese are normally found in concentrations not exceeding 10 mg/L and 3 mg/L, respectively, in natural waters. Iron and manganese can be found at higher concentrations; however, these conditions are rare. Iron concentrations can go as high as 50 mg/L.
Iron and manganese may be removed by reverse osmosis and ion exchange. The unit operation of reverse osmosis was discussed in a previous chapter; the unit process of ion exchange is discussed in a later chapter. This chapter discusses the removal of iron and manganese by the unit process of chemical precipitation.
One manufacturer claimed that these elements could also be removed by biological processes. It claimed that a process called the bioferro process encourages the growth of naturally occurring iron assimilating bacteria, such as Gallionella ferruginea, thus reducing iron concentration. An experimental result shows a reduction of iron from 6.0 mg/L to less than 0.1 mg/L. They also claimed that a companion process called bioman could remove manganese down to 0.08 mg/L. This uses naturally occurring manganese bacteria to consume manganese. Figure 13.1 is a photograph showing growths of "bioferro" and "bioman" bacteria.
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