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• 4.3 Applicable Treatment Processes -Biological Treatment Biological treatment offers a second category of treatment process that has been used in industrial and mining settings. Recent industrial studies show the ability to treat sulfate-rich streams by stimulating the growth of sulfate reducing bacteria (SRB), which use sulfate to generate energy by transforming sulfate to sulfide (Hilton et al., 1986; Mazee et al., 1986). The amount of methane and/or sludge produced is variable and is part of the treatment options evaluation. Biological treatment has a long history of use for treatment of organic wastewaters. In aerobic treatment, air is added to the wastewater through mixing or diffusion to provide oxygen as an electron acceptor so that the bacteria can degrade the organic compounds (electron donors). For high-strength waste constituents, anaerobic treatment (under conditions of no oxygen present) is used. Under anoxic conditions, the concentration of oxygen is too low for use by bacteria. When oxygen is depleted, these facultative bacteria will then use other electron acceptors. In natural endvontnents, bacteria use different constituents in a speciftc sequence that depends directly on the amount of energy released at each step (Stamm and Morgan, 1981). • Oxygen will be used first, followed by nitrate, iron, manganese, sulfate, and finally by carbon dioxide under anaerobic and methanogenic conditions. Many of the sulfate-reducing bacteria are strict anaerobes and cannot live in an environment where oxygen is present. Therefore, to remove sulfate from the groundwater using biological treatment, it is necessary to add an electron donor (organic constituent) to the groundwater and either rely on the natural bacteria in the water or inoculate the water with bacteria. The bacteria will use each available electron acceptor in the order of energy benefit, until that constituent is depleted. This process can take place in situ or in an aboveground bioreactor. In either case, the biochemical mechanisms aze the same. The time-scale for implementation of biological treatment technologies is different than other water treatment methods (e.g., nanofiltration) because the mechanism for removal of constituents depends on a base level of biological activity. Microbes that can reduce sulfate and oxidize • manganese exist in all natural environments, but aze typically limited by adverse environmental conditions (aerobic versus anaerobic) or by an inadequate supply of nutrients (organic carbon). Ba!!!e Maun(ain Resources, /nc. p:1l002671reportslmarchrptlvl6wlrmngfmarch.doc 42 March 22, 1999