AWWA SOURCES55716

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 <1.0 mg/L at pH 6.0. Nitrate rejection was also improved at higher pH, though the reason for such improvement was not clear. Additional tests showed that by dosing the influent with aluminum such that the molar concentration ratio of aluminum to fluoride was 1:2 with the majority of the fluoride in solution present as AlFx 3-x complexes, fluoride rejection in a low pH system was enhanced greatly. These results provided two possible modifications to the influent that could lead to greater RO rejection efficiency. Further work was done to optimize the overall water recovery of the tested RO unit. A two stage process was developed in which unaltered, low pH influent was sent through the RO unit in "recycle mode" until ~95% of the water had been recovered as permeate. The goal of this first stage was to concentrate a majority of the dissolved solids in an extremely small volume. The permeate, still containing high concentrations of fluoride and nitrate, was stored, adjusted to pH 7.0 and sent through the RO unit again. The second stage resulted in extremely high quality permeate with fluoride, nitrate, and TDS concentrations of <1.0, <5.0, and ~10 mg/L, respectively. Virtually all of the water was recovered from this second stage, resulting in an overall water recovery efficiency of ~98% for the two stages. Includes 9 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

01/01/2002

Number of Pages:

20

File Size:

1 file , 250 KB