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Ion-exchange (IX) resin is perhaps the most promising INTRODUCTIONIn the southwestern United States, perchlorate (ClO 4 -) contamination has been detected in waters and soils near facilities that manufactured or utilized sodium, potassium, and ammonium perchlorate salts.The concentration of this contaminant varies depending on the site; with many measurements in the parts per million (ppm) range in locations where perchlorate was manufactured (Boralessa, 2001). In other sites, where perchlorate contamination is associated with the migratory plumes resulting from the use of perchlorate in rocket and explosive manufacturing, perchlorate levels in the parts per billion (ppb) range are found (Gingras & Batista, 2002).The presence of perchlorate in waters has become a concern because of its interference with the normal functioning of the human thyroid gland. In March 2002, the USEPA National Center for Environmental Assessment issued a report recommending a standard of 1 part per million (ppm) for perchlorate in drinking waters (EPA, 2002), making remediation of perchlorate contaminated waters necessary.With regulatory standards looming in the future, the search for effective means to remove perchlorate from waters is intensifying. Several technologies, including chemical reduction, carbon adsorption, membrane filtration (Yoon et al., 2000), biological reduction (Miller & Logan, 2000;Rittmann & Nerenberg, 2000;Liu & Batista, 2000), and ion exchange (Tripp & Clifford, 2000a;Batista et al., 2000;Gu et al., 2000a) have been investigated.Thus far, biological reduction and ion exchange seem to be the most promising.While the water industry is familiar with the use of ion exchange because of its historic use for hardness and nitrate removal, the industry is not very comfortable with the intentional introduction of microorganisms to the water treatment train.Thus it is likely that biological reduction of perchlorate will not be used for perchlorate removal from drinking waters. Ion Combining Ion-Exchange (IX) Technology and Biological Reduction for Perchlorate Removal exchange will be probably the technology of choice to remove perchlorate from several contaminated drinking waters. Ion-exchange, however, does not eliminate perchlorate from the environment; it transfers perchlorate from waters to the brine solutions used to regenerate the resins.The concentrated waste regenerant solutions contain high levels of perchlorate, and treatment technologies for these solutions are needed.This article reviews the latest knowledge on perchlorate removal and anion competition by ion-exchange resins; introduces the potential treatment of ion-exchange waste regenerant solutions by biological reduction; discusses the effects of brine composition on perchlorate biodegradation; and assess additional work that needs to be accomplished to fully develop this technology. PERCHLORATE REMOVAL BY ION-EXCHANGE RESINS Resin Characteristics Affecting Perchlorate RemovalStrong-base anionic resins (SBAX) and weak-base anionic resins (WBAX) with both st...