of fluoride is primarily of concern in domestic water supplies. Data indicate that an average of 1 mg/l of fluoride is beneficial for the prevention of dental caries (the allowable level of fluoride is determined by the annual average of the maximum daily air temperature) (U.S. Public Health Service 1962). Higher fluoride levels have been responsible for mottling of teeth. The level of fluoride must also be controlled for other uses, such as industrial water supply, irrigation water, stock watering, and aquatic life. The limits for these uses in mg/l (McKee and Wolf 1963) are industrial water (1.0), stock watering (1.0), irrigation (10), and aquatic life (1.5).
Wastewater effluents may contain some fluoride, as long as adequate dilution is assured in the receiving stream. However, fluoride concentration in effluent is frequently too great to be decreased by diluting waters, requiring treatment of the waste stream prior to discharge.
Principal flouride removal methods are precipitation by lime, absorption on activated alumina, or removal by an ion exchange process. The addition of lime results in the precipitation of fluoride as calcium fluoride:
Precipitated calcium fluoride can be settled out of solution by thickening and clarification. The settled chemical sludge can then be treated as other sludges and dewatered utilizing vacuum filtration or centrifugation. The limiting factor for this process is the solubility of calcium fluoride, which is 7.8 mg/l (as F). There are indications that lime high in magnesium can further reduce the fluoride solubility concentration (Rohrer 1971).
Another method of fluoride removal is the use of an aluminum compound to bind the aluminum and fluoride as a complex. While filter alum (aluminum sulfate) has been investigated, it has not been effective, as other anions in the water tend to reduce effectiveness. Activated alumina can be used to reduce fluoride concentration to the 1-2 mg/l range. The capacity of activated alumina for storing fluoride is about 0.1 lb/cu ft. Flowrates on the order of 3-5 gpm/sq ft are possible. The activated alumina can be regenerated with caustic soda, aluminum sulfate, or sul-furic acid with little apparent loss of capacity or activated alumina volume.
Ion exchange materials have also been investigated for defluoridation of water. Anion exchange materials regenerated with a caustic soda solution have been utilized, but this is an expensive process if fluoride removal is the only requirement.
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