Fluorides in water can be detrimental or beneficial. It all depends on the concentration. Surface water supplies are normally low in fluorides (less than 0.5 ppm). Some have no fluoride at all. Well waters may contain excessive amounts of fluoride. There are some wells which contain the recommended amount (1 mg/1) for drinking water.
Fluorides are important because they have a definite relation to dental health. Research has shown that a concentration of 1 mg/1 of fluoride in drinking water reduces tooth decay. On the other hand, some children under nine years of age exposed to levels of fluoride greater than about 2 mg/1 may develop a condition known as "endemic dental fluorosis." Sometimes called "Colorado Brown Stain," this condition appears as a dark brown mottling or spotting of the permanent teeth. In certain cases, the teeth become chalky white in appearance. Further, federal regulations require that fluoride not exceed a concentration of 4 mg/1 in drinking water. This is an enforceable maximum contaminant level standard, and it has been established to protect public health. Exposure to drinking water levels above 4 mg/1 for many years may result in cases of crippling skeletal fluorosis, which is a serious bone disorder.
Research studies indicate that fluoride concentrations of 1 mg/1 are optimum. Authorities generally agree:
- Where concentrations are greater than 4 mg/1, the excess fluorides must be removed from water;
- Where concentrations are less than 1 mg/1, fluorides should be added. As a result of studies, cities are presently required by some states to add fluorides in optimum concentrations to municipal water supplies. Where the fluoride concentration is too great, it is necessary to reduce the amount to acceptable limits.
Various methods have been suggested for reducing fluorides. These can be classified broadly in three groups:
- Reverse osmosis.
- Those involving treatment with chemicals, such as aluminum sulfate, magnesium or calcium phosphate, and others.
- Those involving percolation through a bed of material, such as activated carbon, activated alumina, granular tricalcium phosphate or ion exchange resins.
The first treatment method has obvious advantages. Methods in the second category have distinct disadvantages. They require use of elaborate treatment plants, careful control of chemical dosage and pH. In some cases further treatment is necessary to restore the pH of the treated water to normal.
Methods in the third category do not require such elaborate control. Of these, the only widely used method of reducing fluoride content involves the use of a tricalcium phosphate filter. Such a filter functions in much the same way as a carbon filter. As the water flows through a tri-calcium phosphate filter, the fluorides are absorbed.