Abstract: Permeable reactive barriers are a promising new approach to the treatment of dissolved contaminants in aquifers. This technology has progressed rapidly from laboratory studies to full-scale implementation over the past decade. Laboratory treatability studies indicate the potential for treatment of a large number of inorganic contaminants, including As, Cd, Cr, Cu, Hg, Fe, Mn, Mo, Ni, Pb, Se, Tc, U, V, NO3, PO4, and SO4. Small scale field studies have indicated the potential for treatment of Cd, Cr, Cu, Fe, Ni, Pb, NO3, PO4, and SO4. Permeable reactive barriers have been used in full-scale installations for the treatment of hexavalent chromium, dissolved constituents associated with acid-mine drainage, including SO4, Fe, Ni, Co and Zn, and dissolved nutrients, including nitrate and phosphate. A full-scale barrier designed to prevent the release of contaminants associated with inactive mine tailings impoundment was installed at the Nickel Rim mine site in Canada in August 1995. This reactive barrier removes Fe, SO,, Ni and other metals. The effluent from the barrier is neutral in pH and contains no acid-generating potential, and dissolved metal concentrations are below regulatory guidelines. A full-scale reactive barrier was installed to treat Cr(VI) and halogenated hydrocarbons at the US Coast Guard site in Elizabeth City, North Carolina, USA in June 1996. This barrier removes Cr(VI) from >8 mg l(-1) to <0.01 mg l(-1).

Abstract: For the past 40 years various approaches have been developed to treat acid waters coming from abandoned as well as operating mining operations. System designs have evolved to meet increasingly stringent discharge permit limits for treated water, as well as to provide solid disposal within economic constraints. A treatment system for remediation of acid mine drainage (AMD) or acid groundwater (AG) requires two main steps: 1. The addition of chemicals to precipitate dissolved metals contained in the waters, and if necessary, to coagulate the precipitated solids ahead of physical separation. 2. Physical separation of the precipitated solids from the water so the water can be lawfully discharged from the site. Choosing the appropriate technology and equipment results in the most efficient plant design, the lowest capital outlay, and minimum operating cost. The goal of these plants is to discharge liquids and solids able to meet standards. The separation of solids from liquids can be achieved through various means, including gravity settling, flotation, mechanical dewatering, filtration and evaporation. As important as the liquid solids separation unit operations are, they are driven by the chemistry of the water to be treated. The content of the dissolved solids will influence the quality and quantity of the solids produced during precipitation. Thus the two aspects must be integrated, with chemistry first, then mechanical engineering. This presentation will provide an overview of a number of liquid solids separation tools currently being used to treat AMD-AG at several sites in the USA. It will also discuss how their operations are impacted by the chemistry of their particular acid water feeds. The tools used include clarifier-thickeners, solids contact clarifiers, dissolved air flotation, polishing filters, membrane filters, and mechanical dewatering devices (belt and filter presses, vacuum filters, and driers).