Selective fluoride removal by aluminum precipitation & membrane filtration
Fluoride is an anionic constituent of most natural surface and ground waters. At low concentrations it has the beneficial effect of improving dental health, but at high concentrations it causes skeletal and other problems. The drinking water standard for fluoride is 4.0 mg/L, but even at this level approximately 10% of exposed children will develop dental enamel fluorosis. Several small communities in New Mexico have source waters with fluoride concentrations higher than 4.0 mg/L. No commercial technologies selectively remove fluoride from drinking water in part because it is relatively non-reactive in aqueous solution and also because its chemistry is similar to chloride which is almost always present at much higher concentrations. The purpose of this project was to evaluate a new technology to selectively remove fluoride from drinking water. This technology is based on precipitation by aluminum hydroxide Al(OH)3 with subsequent removal of the floc by membrane ultrafiltration. A review of the published literature shows that using aluminum to defluoridate water may rely on aluminum dose, pH, flocculation time and sedimentation time. Aluminum coagulation and membrane filtration technology (Al-CMF) was shown to selectively remove fluoride from drinking water to concentrations below the drinking water standard. Laboratory experiments were done to characterize fluoride removal regarding pH, aluminum dosage and kinetics; this characterization was used to develop a pilot scale system. Laboratory testing was done using fluoride-spiked tap water in jar tests at pH levels of 5.5, 6.5, 7.5, 8.5, and 9.5 with aluminum doses of 0 to 50 mg/L (0 mmol/L to 1.85 mmol/L). Fluoride removal by the Al-CMF process was found to be independent of pH over the range of 5.5 to 9.5. Mixing intensity was found to be important with the best removal achieved using a blender with a mixing velocity gradient (G) in excess of 9,000 s-1. Laboratory testing found that 1 mmol/L of fluoride was removed for every 4.45 mmol/L aluminum added. A 0.3 gal/min pilot plant was constructed using the Al-CMF process. The pilot system used a Koch ABCOR®-ULTRA-COR® ultrafiltration membrane. It was used to treat tap water in the Village of San Ysidro, NM which has a natural fluoride concentration of 5.7 mg/L and arsenic concentration of 85 ug/L. The pilot testing demonstrated fluoride removal to a concentration of 3.5 mg/L at an aluminum dose of 30 mg/L. This produced a ratio of 1 mmol/L fluoride removed for every 9.53 mmol/L aluminum added. The pilot treatment system was also removed 80% of the arsenic, though the treated water did not meet the arsenic standard of 10 ug/L.