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wateronline.com ■ Water Online The Magazine residual (waste) stream, in which most contaminants are concentrated to four to five times their background level, is often problematic. SBA exchange removes Cr(VI) by passing the contaminated water through a bed of polymeric resin with chloride ions attached to charged functional groups integrated into the resin. The negatively charged Cr(VI) ions displace the chloride ions, attaching the Cr(VI) ions to the resin while releasing chloride ions into the treated water. Once the resin is exhausted, and there are no additional sites on the resin to take up Cr(VI), the resin is regenerated with a high concentration sodium chloride solution. The chloride ions reattach to the resin, and Cr(VI) is released into the salt solution for disposal. A similar process is used to treat arsenic and nitrate in drinking water. Pilot testing has found that SBA is more effective in removing Cr(VI) and less sensitive to the presence of co-occurring ions, like sulfate, than when used for arsenic or nitrate treatment. Several thousand bed volumes of throughput may be obtained when treating Cr(VI) before regeneration is needed. Multiple regenerations with the same sodium chloride solution are also possible. Hydraulically, SBA is very efficient, obtaining greater than 99 percent recovery. The waste stream produced by SBA is classified as a hazardous liquid waste. It consists of a highly concentrated saline solution containing Cr(VI) and other ions removed by the SBA process. Additional processing of the residuals stream is required if a hazardous liquid classification is to be avoided. WBA exchange also removes Cr(VI) by passing the contaminated water through a bed of polymeric resin. Unlike SBA, the WBA technology is not regenerated and operates as a single-use disposable medium. Throughputs of well over 100,000 bed volumes have been demonstrated by pilot tests. The term ion exchange is somewhat of a misnomer for this technology, since the throughput far exceeds the ion exchange capacity of the resin. In fact, it appears Cr(VI) is removed by reduction and precipitation of chromium on the resin. An important drawback of this technology is its sensitivity to pH. To be effective, the technology must operate at pH 6 or less. Generally, the pH of the water must be depressed for treatment and then raised to produce a non-corrosive stable water for distribution. Given the quality of most drinking water sources, substantial quantities of chemicals are needed to make these adjustments. Operating as a single- use disposable medium greatly simplifies problems with disposing of the treatment residuals, but this advantage is off-set by the technology's need for chemical handling facilities along with its considerable chemical consumption. RCF is a multi-step treatment process in which Cr(VI) is converted to Cr(III), and the Cr(III) is then removed by filtration. Specifically, ferrous iron (Fe(II)) in the form of ferrous sulfate or ferrous chloride is used to reduce Cr(VI) to Cr(III). At moderate pH, the Cr(III) precipitates as chromium hydroxide, Cr(OH) 3 by the following partial reaction: CrO 4 2- + 3Fe 2+ + 8H 2 O => Cr(OH) 3 + 3Fe(OH) 3 + 4H + A coagulant is used to aggregate Cr(III) into flocs suitable for removal via sedimentation followed by filtration with deep bed media or low-pressure membranes. The RCF process is widely used for industrial chromium treatment and is similar to the conventional treatment process used by many drinking water utilities. Hence, drinking water utilities are quite familiar with the basic design concepts, equipment, and chemicals used by this process. The RCF process produces a non-hazardous residuals stream that can be handled and disposed of in the same manner as residuals from a conventional treatment plant. However, the RCF technology uses multiple unit processes and chemical feeds that by necessity take up a large footprint. The RCF process also requires a good deal of operator attention. For these reasons, the technology is better suited for surface water systems, which typically treat a small number of sources at a central location, rather than groundwater systems, which typically consist of multiple, widely distributed wells located on small sites. Looking Ahead The State of California has established an MCL for Cr(VI) in drinking water of 0.010 mg/L. The MCL applies only to California, and there is no national MCL for Cr(VI). The U.S. EPA is in the process of determining if a national Cr(VI) regulation is justified. No date has been set by the U.S. EPA to announce a regulatory determination for Cr(VI). If Cr(VI) is regulated by the U.S. EPA at a level similar to California's, about 2 to 4 percent of all utilities nationally will be impacted. While the drinking water industry has little experience in treating Cr(VI), a number of studies have been performed investigating Cr(VI) treatability. Four technologies, NF/RO, SBA exchange, WBA exchange, and RCF, are effective for controlling Cr(VI) to the California MCL. The most suitable technology for any individual treatment situation will vary, but it is greatly influenced by the disposal scenario for the treatment residual. Regulatory Analysis 22 Dr. Philip Brandhuber is a professional associate at HDR Engineering in Denver, CO. Brandhuber is HDR's expert in the treatment of inorganic contaminants in drinking water using advanced technologies. He can be reached at Philip.Brandhuber@hdrinc.com.