Reducing fluoride in industrial wastewater

0 Fluoride contamination occurs in a wide range of industrial effluents-wastewater from aluminum and steel production, metal finishing and electroplating, glass and semiconductor manufacturing, ore-beneficiation, and fertilizer operations. Concentration limits for fluoride of 1.0 to 1.5 mg/L have been established from time to time for surface waters, based upon two premises: (1) that these waters will serve as sources of potable water, and (2) that processes used in water-treatment plants ,would normally have no effect on the fluoride concentration. Various studies on the toxicity of fluoride to aquatic life have produced conflicting results, but apparently this toxicity decreases with an increase in dissolved solids and in hardness of the water. The requirement to meet drinking-water standards, and not the toxic effect on aquatic life, becomes the limiting factor. It would, for any localized site, call for establishment of fluoride levels in a receiving stream based upon the impact of fluoride on local fish and other aquatic life. Standard analytical methods are available for determining fluoride content of water. The most commonly used technique involves distillation of the sample, followed by colorimetric determination of the fluoride using SPADNS reagent on the distillate. Development of the specific-ion electrode started a trend toward direct determination of fluoride in the waste sample, by use of a TISAB XI (total ionic strength adjustment buffer) to eliminate the effects of complexing ions such as aluminum. Recently, some analysts have been using the specific-ion electrode instead of the SPADNS reagent to minimize the possibility of interference from entrainment of sulfate during the distillation step. Most of the development work on these procedure3 has involved relatively low fluoride concentrations. Concentrated spent process solutions, containing very