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For many years, conventional treatment processes employing chemical precipitation and physical separation have been used to reduce the concentrations of heavy metals and other contaminants in metal finishing wastewater. Although these processes may be effective, the chemical additions can be costly, and the effluent is usually not reusable. The Boeing Company generates large volumes of relatively dilute electroplating waste at their production facility in Kent, Washington, with the major contaminant of concern being hexavalent chromium (Cr(VI)). The current treatment scheme incorporates reduction of Cr (VI) to Cr(III) with sulfite, precipitation of Cr(OH)3 (s), and microfiltration, with the treated water having contaminant levels low enough to be discharged to the city sewer system. In an effort to reclaim some of this water and to aid in treatment of the waste, Boeing has installed a high-flow reverse osmosis system. Initial tests of the unit operating under conditions of high water recovery revealed that the concentrations of nitrate and fluoride in the permeate were too high to allow its reuse within the plant. The purpose of this research was to evaluate the performance of the RO unit for various wastewater compositions and operating parameters. Bench-scale tests showed that fluoride and nitrate rejection by the RO membrane was correlated to the pH of the influent. At pH lower than the pKa of F-/HF (3.15), fluoride rejection was essentially zero due to the majority of the fluoride in the influent being present as HF (unaffected by the repulsive electrical charge of the membrane). At higher pH, fluoride rejection improved dramatically, with permeate levels of

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Edition: Vol. - No. Published: 01/01/2002 Number of Pages: 20File Size: 1 file , 250 KB