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January 9. 1996 -4- 933 271 nn5 <br /> reduction reaction is as follows (Lapakko and Eger, 1981; Dvorak and McIntine, 1992; <br /> Maree and Strydom, 1985): <br /> ( 1) SO'-2 + 2 C3H403-' (lactate) = 2 C2H402'' (acetate) + 2 CO3-2 + S"2 <br /> The carbonate or bicarbonate produced will buffer the solution pH and neutralize acidity if <br /> present. The short chain compounds needed as substrate and a source of carbon will be <br /> provided by the organic material. In addition to sulfate reduction, the same organisms can <br /> through a similar biochemical process reduce the oxidized forms of selenium (i.e., selenite <br /> and selenate) to its elemental form (Baldwin, et al., 1985; Levine, 1924). The removal of <br /> selenium through the use of anaerobic bacterial reduction has been investigated by Times <br /> Limited and more recently studied by the U.S. Bureau of Mines in Salt Lake City (Adams, <br /> et al., 1993; Altringer, et al., 1991). The sulfide and carbonate produced can combine with <br /> free and complexed monovalent and divalent cations, such as cadmium, lead, nickel, <br /> mercury, silver, and zinc, to form very insoluble compounds according to the following <br /> reactions: <br /> (2) S -2 + M+2 = MS(s) <br /> (3) CO'-2 + M+2 = MC03(s) <br /> With the organic layer within the biological passive treatment system being in an anoxic and <br /> chemically reducing environment, the metal sulfides will remain stable and insoluble. <br /> Process Performance and Limitations <br /> As discussed in the previous section, the primary biochemical processes at work in the <br /> biological passive treatment system include anaerobic bacteria sulfate reduction coupled with <br /> chemical precipitation and physical sorption of the metals. All of these processes have been <br /> examined in bench scale, continuous flow pilot plant evaluations, and full scale natural and <br /> manmade wetlands. In the present system, only partial sulfate reduction is necessary to <br /> form sulfide and carbonate, which in turn will precipitate the residual metals as insoluble <br /> compounds. All of the biological processes would be classified and would function as fixed <br /> film or attached growth systems, similar to the trickling filters used in domestic wastewater <br /> treatment plants. As a result, the discussion of the design, performance, and limitations of <br /> these systems has been based upon the principles affecting attached growth biological <br /> treatment processes. <br /> A review of the sulfate reducing efficiency of 25 constructed wetlands, provided a mean <br /> removal efficiency of 22 percent (Kleinman and Girtis, 1986). In a study conducted by the <br /> U.S. Bureau of Mines, an average of 24 percent sulfate removal efficiency was obtained <br /> Golder Associates <br />