FLUORIDE ION REMOVAL FROM WASTEWATER EMPLOYING
CALCIUM PRECIPITATION AND IRON SALT COAGULATION
William E. Link, Vice President
Murdoch/Betz Engineers, Inc.
Pittsburgh, Pennsylvania 15219
Joseph G. Rabosky, Senior Environmental Specialist
Mobay Chemical Corporation
Pittsburgh, Pennsylvania 15205
INTRODUCTION
Fluoride-laden wastewaters originate from many different types of industries.   For
instance, fluoride wastewaters are produced in etching operations during the production
of color television tubes.   Fluoride is also used in glass surface preparation processes in
the manufacture of frosted surfaces for light bulbs and other glass manufacturing operations.   Some metal finishing and electroplating operations produce effluents containing
fluoride.   Other industries which use fluorides are steel, aluminum, insecticide, chemical
and fertilizer.
Dental health is probably the most important consideration for control of fluoride in
water.   Incidence of tooth decay as well as more severe effects are dependent upon the
concentration of fluoride in drinking water.   The National Interim Primary Drinking
Water Standards, which are a function of maximum daily air temperature, indicate that
fluoride concentrations should not exceed 1.4 to 2.4 mg/1.
Because drinking water standards exist, it is necessary that the amount of fluoride in
water supplies be controlled.   Future discharges containing high concentrations of fluoride will probably have to be treated for removal of this ion.   This paper presents the
results of a study in which fluoride ion was reduced to approximately 3.0 mg/1.   Laboratory studies were conducted using both sodium fluoride solutions and actual plant
wastewater from a manufacturing plant.   The process employed in this study required
lime for precipitation which was then followed by coagulation with iron salts.
Briefly, lime was used in lieu of calcium chloride or a mixture of both to provide
the necessary calcium ion for precipitation and alkalinity for neutralization of acidity.
Hence, the fluoride waste stream could be segregated from the main waste stream, treated
with excess lime, settled or filtered and reintroduced into the acidic waste flow to provide partial neutralization.
Since segregation of the waste streams was possible, two-stage treatment was a realistic
consideration.   After mixing the lime-treated fluoride waste stream with other plant
wastewaters, samples of the combined wastewater were coagulated with iron.   Based on
this treatment method, the resultant fluoride concentrations averaged 6 to 7 mg/1.
LITERATURE REVIEW
Fluoride Precipitation with Calcium
Removal of fluoride by precipitation with calcium has been successfully practiced in
wastewater treatment. When the cause of fluorosis was discovered, the important value
of lime as a precipitating agent was realized [ 1 ]. In 1935, an analysis of a lime-softening
plant effluent yielded lower fluoride concentrations than those found in the raw influent waters [2].   This observation led to lime precipitation experiments with sodium
fluoride solutions.
In 1954, Wadhwani [3] experimented with defluoridation of sodium fluoride solutions using calcium oxide, calcium sulfate, calcium carbonate, rock phosphate, fluorapatite,
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