Laserfiche WebLink
January 9. 1996 -3- 933 71 nn5 <br /> sump will be installed to collect drainage, and an organic substrate will be placed in the <br /> trench as shown in Figure 1. <br /> The passive biological treatment cell would be constructed as an upflow cell and consists of <br /> a geomembrane lined pond in which a layer of organic substrate is placed upon a set of <br /> evenly spaced perforated drain pipes covered by a drainage layer of sand or gravel. A <br /> typical cross section through a passive treatment cell is shown on Figure 2. The perforated <br /> drain pipes must be laid in a manner to receive and distribute seepage evenly across the <br /> bottom of the cell for contact with the organic substrate. The organic layer is then covered <br /> with a collection drainage layer and geomembrane cover for the necessary anaerobic <br /> conditions. The upper surface is covered with soil and reclaimed to promote runoff and <br /> evapotranspiration. The upper drain layer of the cell would discharge to an aerobic leach <br /> field for oxidation of the organic content in the treated solution. The cell would be sized to <br /> handle specific seepage rates to allow sufficient residence times for sulfate reduction and <br /> metals precipitation. <br /> Biological and Chemical Processes <br /> The most important feature of the biological passive treatment system is the organic layer <br /> into which the acid drainage will be directed, if it occurs. This layer will contain the <br /> necessary nutrients and organic substrates to support the growth of a diverse population of <br /> microorganisms, principally anaerobic and facultative bacteria. Since the layer will be <br /> placed many feet below the vegetated surface, anoxic or anaerobic conditions will be <br /> established along with a chemically reducing environment, as the organic matter is degraded <br /> and the residual oxygen is depleted. The anaerobic and chemically reducing environments <br /> promote the growth of the bacterial population capable of uptake, treatment, sorption, and/or <br /> precipitation of sulfate, and metals. <br /> The initial chemical process that will occur as the drainage enters the biological passive <br /> treatment system involves physical sorption of some of metals within the organic layer, for <br /> example arsenic and copper. The sorption of metals on both inactivated and living biomass, <br /> even from solutions containing elevated total dissolved solids levels, is well documented and <br /> forms the basis of several specialized treatment systems, such as AlgaSORB and BIO-FIX, a <br /> treatment process currently being evaluated by the U.S. Bureau of Mines and Department of <br /> Interior (Darnall and Gabel, 1989; Ferguson and Jeffers, 1991). <br /> The sulfate entering the organic layer, which contains the necessary organic substrate, <br /> nutrients, and bacterial inoculum, will slowly be converted to sulfide by one of several <br /> strictly anaerobic and chemoheterotrophic bacteria which utilize sulfate as a terminal <br /> electron acceptor and organic matter as their carbon source. The bacteria most commonly <br /> associated with sulfate reduction are species of the Desulfovibrio genus. The basic sulfate <br /> Golder Associates <br />