Fluoride Removal by Lime Precipitation and
Alum and Polyelectrolyte Coagulation
JOSEPH G. RABOSKY, Project Manager
Penn Environmental Consultants, Inc.
Pittsburgh, Pennsylvania 15220
JAMES P. MILLER, JR., Associate Professor
Civil Engineering Department
University of Pittsburgh
Pittsburgh, Pennsylvania 15261
INTRODUCTION
Fluoride-laden wastewaters originate from a variety of industrial activities such as
etching operations during the production of color television tubes (1), glass surface
preparation processes in the manufacture of frosted surfaces for light bulbs (2), as well as in
various other operations in the insecticide, chemical and fertilizer industries (2, 3).
The most well known effect of excessive concentrations of fluoride in water involves
dental health. If fluoride concentrations range from 0.7-1.5 mg/1 (3), incidence of tooth
decay is reduced. Excessive tooth decay results when fluoride concentrations are below 0.7
mg/1 (3, 4). Concentrations slightly greater than 1.5 mg/1 cause mottling of tooth enamel
and discoloration of teeth occurs at even higher conentrations (4). More severe effects
involve the loss of teeth at an early age because of extremely high fluoride concentrations
(5).
Bone disease, which is another affliction caused by excessive fluroides, has occurred in
patients when fluoridated tap water was used in kidney machines for dialysis (6).
Because of the above reasons, the control of fluoride in wastewater must be closely
maintained. If limitations are placed on fluoride concentrations, then reduction methods
must be devised and implemented when concentrations become excessive. One method by
which fluoride can be reduced involves two-stage chemical addition using lime and
coagulants. The investigation presented details this method.
THEORY
During the first stage of chemical addition, lime is used to supply the divalent calcium
ion, Ca+2, necessary to combine with the fluoride anion, F", to form the calcium fluoride
precipitate, CaF,. This concept is illustrated by the following relationship:
2F" + Ca(OH), - CaF2 I + 2 (OH)"
If calcium and fluoride are present in quantities greater than minimum solubility
values, then fluoride can be precipitated from solution as calcium fluoride according to the
solubility product expression. Because the solubility product is constant, the concentration
of the fluoride anion must decrease as the concentration of the calcium cation increases.
Upon removal of the calcium fluoride precipitate by filtration, sulfuric acid, alum, and
polyelectrolyte were added sequentially. This represents the second stage of chemical
treatment. Sufficient sulfuric acid was added only to adjust the pH to the desired range for
coagulation.
In contrast, coagulation does not involve chemical precipitation reactions.
Coagulation is primarily a phenomenon dealing with overcoming the surface charge of
suspended matter. The alum (aluminum sulfate) dissociates in water to provide the A1+3 ion
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