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<br />attenuation, coupled with chemical precipitation <br />and physical sorption of the metals. All of these <br />processes have been examined in bench scale, <br />continuous flow pilot plant evaluations, and full <br />scale natural and manmade wetlands. In the <br />present system, only partial sulfate reduction is <br />necessary to form sulfide and carbonate, which <br />in tum will precipitate the residual metals as <br />insoluble compounds. All of the biological <br />processes would be classified and would <br />function as fixed film or attached growth <br />systems, similar to the trickling filters used in <br />domestic wastewater treatment plants. As a <br />result, the discussion of the design, <br />performance, and limitations of these systems <br />has been based upon the principles affecting <br />attached growth biological treatment processes. <br />A review of the sulfate reducing efficiency of <br />twenty-five constructed wetlands, provided a <br />mean removal efficiency of 22 percent <br />(Kleinman and Girtis, 1986). Tn a study <br />conducted by the U.S. Bureau of Mines, an <br />average of 24 percent sulfate removal efficiency <br />was obtained from two sepazate continuous <br />flow systems (Dvorak and Mclntine, 1992). <br />The lower percent reductions in sulfate were <br />related partially to the exhaustion of organic <br />matter and short-circuiting through the wetland. <br />However, in a system designed especially to <br />maximize sulfate reduction, removal <br />efficiencies as high as 88 percen[ have been <br />reported using sludge from a domestic <br />wastewater treatment plant as the organic <br />carbon source (Maree and Strydom, ]985). The <br />untreated sulfate levels employed al the various <br />studies ranged from about 1,000-3,000 mg/l. L-t <br />the case of most heap leach operations, the <br />sulfate levels in the barren solutions from the <br />three heap leach pads range from about 2,100- <br />4,300 mg/1. <br />Assuming only a partial reduction in sulfate <br />levels, sufficient sulfide is produced [o <br />precipitate the residual metals present in the <br />untreated barren solutions. Although the <br />bacterial reduction N sulfate requires a reaction <br />time of hours, the initial growth and acclimation <br />period requires many days. In the studies <br />reviewed, the initial startup, growth, and <br /> <br />acclimation period ranged from about 9-35 days <br />to attain maximum sulfate removal. <br />With respect to biological denitrification, the <br />process is well defined and has been used in <br />municipal wastewater treatment applications for <br />many yeazs (Adams, et al, 1993; Alttinger, et <br />al, 1991; Water Pollution Control Federation, <br />1983). As with sulfate reduction, dettitrification <br />will proceed at temperatures down to 10 <br />degrees C, although a correction factor must be <br />applied to increase the capacity of the system to <br />accommodate the slower growth and reaction <br />rate of the organisms. <br />In order to achieve nitrate removals of 90 <br />percent or greater, a mass loading rate for <br />denitrification in the range of about 400 lbs <br />N/day/cu ft of reactor and a hydraulic loading <br />rate in the range of 2.0 gpd/sq ft would be <br />employed at the lowest temperatures in an <br />attached growth biological system. In the case <br />of solids retention times (SRT) or startup <br />periods for establishment and acclimation of <br />bacterial growth, the theoretical time required <br />ranges from about 3-9 days, with the actual <br />SRT's ranging from 6-18 days tiling a safety <br />factor of two. Since the Biopass System would <br />involve a less rigorously designed and operated <br />passive process, with less control and less <br />efficient organic food sources, the SRT is <br />increased by 25-50 percen[ above the upper <br />value of 18 days. Nitrate removal efficiencies <br />of about 75 percent or greater are anticipated <br />producing treated levels below the MCL of 10 <br />mg/I as N. <br />With respect to wad cyanide removal, the initial <br />levels remaining in the pore solution are much <br />lower following draindown due to namral <br />attenuation. Stoning with an initial wad cyanide <br />concentration during leaching of > 100 mg/l, <br />the resultant solution wad cyanide level <br />following three months of natural attenuation <br />assuming a pH of about 9.0, a conservative <br />decay coefficient, and a first order decay rate. <br />would usually be well under 50 mg/1 in <br />temperate climates. The anaerobic degradation <br />of free and complexed cyanides up to 1,000 <br />mg/l has been reported, although cyanide levels <br />-10- <br />