A Limestone Reactor for Fluoride Removal from Wastewaters

Abstract

A two-column limestone reactor has been designed to reduce fluoride concentrations from wastewaters to below the maximum contaminant level (MCL limit) of 4 mg/L. The reactor functions by forcing calcite (CaCO 3 ) to dissolve and fluorite (CaF 2 ) to precipitate in the first column. The second column is not necessary to remove fluoride but is used to precipitate the calcite dissolved in the first column. This returns the treated water to its approximate initial composition. In laboratory experiments, the fluoride concentration of the effluent from feedwaters containing initial F amounts of up to 109 mg/L were brought below the MCL limit of 4 mg/L with a porewater residence time within the column of 2 h. Profile sampling shows that fluoride is reduced from 109 to 8 mg/L after only 35 min within the reactor. The major advantage of this potential technology over existing liming and ion exchange methods is that system monitoring is minimal, regular column regeneration is not required, and chemicals are not permanently added to the water. Because the CaCO 3 dissolved in the first column is precipitated in the second, the reactor has potential application to reduce the concentrations from wastewaters of contaminants similar in charge and size to Ca 2+ and CO 3 2- through coprecipitation reactions. In a pilot experiment where fly ash leachate was spiked with mg/L levels of Cd, As, Cr, and Se and directed through the reactor, reductions in all elements except Cr occurred. Cd was the most notable. Influent concentrations from 2 to 30 mg/L were reduced to below detection (<0.01 mg/L).