Community water fluoridation involves adjusting the fluoride concentration in drinking water supplies to support dental health. This article explains the compounds and detailed procedures water treatment plants follow to add fluoride to municipal water.
Common Fluoride Compounds
Water utilities primarily use three compounds for fluoridation: fluorosilicic acid, sodium fluorosilicate, and sodium fluoride. Fluorosilicic acid (H₂SiF₆) is the most widely adopted additive in the United States, often a liquid byproduct from phosphate fertilizer manufacturing. This acid is highly soluble in water and is considered corrosive, necessitating careful handling.
Sodium fluorosilicate (Na₂SiF₆) is another common compound, a white or yellowish-white crystalline powder. It is the sodium salt of fluorosilicic acid and dissolves readily in water. Sodium fluoride (NaF), a white powder or crystal, was historically the first compound used.
These compounds are selected for their solubility, safety, availability, and cost-effectiveness. When dissolved, they fully dissociate in water to release fluoride ions, which are the active component. The choice among them often depends on factors like cost, system size, and handling preferences at the treatment plant.
The Fluoridation Process
Fluoride compounds are almost always introduced in liquid form, requiring precise delivery. Liquid compounds like fluorosilicic acid are transferred from bulk storage to day tanks, then precisely pumped into the water using metering pumps to ensure controlled amounts.
Solid compounds like sodium fluoride or sodium fluorosilicate are first dissolved to create a solution. Sodium fluoride is often dissolved using a saturator to produce a consistent 4% fluoride solution, which is then pumped into the water supply.
For sodium fluorosilicate, dry feeders measure the solid compound, which is then dissolved in a solution tank before introduction into the water stream. Fluoride chemicals are added at a late stage in the water treatment process, usually after filtration and disinfection, but before the water enters the distribution system.
Maintaining Optimal Levels
Maintaining correct fluoride concentration in drinking water is important for effectiveness and safety. The U.S. Public Health Service recommends an optimal fluoride concentration of 0.7 milligrams per liter (mg/L) for community water systems. This level aims to balance cavity prevention benefits with minimizing the risk of dental fluorosis.
Water utilities continuously monitor fluoride levels to ensure they remain within the recommended range. Personnel take daily measurements, which are verified monthly by independent, state-certified laboratories. Regulatory bodies like the U.S. Environmental Protection Agency (EPA) set enforceable maximum contaminant levels (MCLs) for fluoride in drinking water, currently 4.0 mg/L, with a secondary standard of 2.0 mg/L.
Water systems are required to obtain permits and adhere to guidelines from state and local authorities to ensure safe and effective fluoridation. Precision dosing equipment, online fluoride analyzers, and safety interlocks adjust dosage rates to prevent under- or overdosing, maintaining consistent fluoride levels.