Fluoride in Nature

Fluoride is a chemical which occurs naturally in most water supplies in concentrations ranging from 0.1 ppm to 10 ppm.  The chemical originates in several minerals, such as the one shown below.


As groundwater passes through the earth and comes into contact with these minerals, fluoride is dissolved and enters the water.  The deeper the water flows through the earth, the more fluoride-containing minerals it will come in contact with, and the greater the fluoride concentration in the water will be. 



Purpose of Fluoridation

Fluoridation is the process of adjusting the concentration of fluoride in public water supplies for the prevention of dental decay.  Fluoride has been added to drinking water in the United States since about 1945 and it has been estimated that every dollar spent on fluoridation has saved $50 in dentists' bills. 

Fluoride in water has been proven to prevent tooth decay among children and to prevent root tip rot.  The chemical acts by strengthening the tooth enamel and by making the enamel more resistant to decay.  This is a long-term process, with results usually being noticeable only after about 4 to 6 years. 



Fluorosis and Other Problems

Although fluoride is safe at the concentrations used in water treatment, an excess amount of fluoride in water can result in mottled brown stains on teeth. 


These stains are known as fluorosis.  Fluorosis results from fluoride concentrations of 2 to 13 ppm in drinking water.  Although fluorosis is only an aesthetic problem, treatment plants strive to prevent fluorosis by setting the recommended fluoride level at about 1 ppm.  Fluoride levels above 4 ppm are regulated by the Safe Drinking Water Act.

Some people have suggested that excessive fluoride consumption can cause cancer and increased rates of bone fractures, but studies have shown no relationship between fluoride and these problems.  However, extreme concentrations of fluoride can cause skeletal fluorosis, which is of concern to the water treatment plant operator. 




In the Treatment Plant

Overview of the Treatment Process

Fluoridation will consist of one of three possible processes.  In a few cases, water from two sources can be mixed to achieve the proper concentration of fluoride.  In other cases, fluoride must be removed from water by defluoridation, which is a special ion-exchange method using alumin and bone char.  However, in most treatment plants, the correct amount of fluoride must be added to water.  In this lesson, we will be concerned solely with the addition of fluoride to water.

With a few exceptions, fluoride can be added to water at any point between the raw water intake and the clear well.  However, if fluoride is fed before filtration, about 10% of the fluoride sticks to the floc and is lost, so fluoride is usually added after filtration.  In addition, if lime is fed, the fluoride should be added as far away from the lime injection point as possible. 



Concentration

Most dosages of water treatment chemicals are tailored to achieve the optimal concentration of the chemical in the treated water.  You will remember, for example, that the required chlorine residual is 0.5 ppm, which helps determine the chlorine dose. 

In fluoridation, we also set an optimal fluoride concentration, which is about 1 ppm in drinking water.  However, fluoridation has a different goal from chlorination and from other instances of chemical addition in water treatment.  In chlorination, the chlorine must react with substances in the water, so the optimal chlorine concentration depends primarily on water characteristics.  Fluoride, in contrast, is not meant to react with substances in water.  Instead, the goal when adding fluoride to water is to control the amount of fluoride which each customer will ingest per day.  You can think of fluoride as being similar to a vitamin or mineral for which each person has a recommended daily allowance. 

The average temperature at each water treatment plant will determine the amount of water which an average customer will drink per day.  People tend to drink more water when it's hot, so the optimal fluoride concentration in warm climates will be lower than in cool climates.  The map below shows optimal fluoride concentration in drinking water throughout the continental United States. 






Dosage

The amount of fluoride to be fed into water is influenced by several factors.  The climate of the region will determine the optimal concentration in the water, as discussed above.  But dosage will also be influenced by the amount of fluoride already existing in the raw water.  For example, if raw water contains 0.3 ppm fluoride and the recommended concentration is 0.9 ppm, then it will only be necessary to add 0.6 ppm of fluoride to the water being treated. 

Dosage also depends on the type of chemical used to fluoridate the water.  Several chemicals can be used to supply fluoride to water, and each chemical has a different fluoride concentration.  We will discuss fluoridation chemicals in the next section. 



Chemicals

There are three main chemicals used for fluoridation of drinking water - hydrofluosilicic acid, sodium silicofluoride, and sodium fluoride.  In addition, a few plants use other fluoride sources such as hydrofluoric acid and ammonium silicofluoride. 

Hydrofluosilicic acid is the most commonly used fluoridating chemical.  This acid, also known as fluorosilicic acid, hexafluosilicic acid, and silicofluoric acid, is a liquid with the formula H₂SiF₆.  The liquid may be fed directly into the raw water or may be diluted.  Hydrofluosilicic acid is a popular choice in many water treatment plants because it is usually the least expensive fluoridation chemical and is the easiest to feed.  However, it can be expensive to ship since it is a liquid and is heavier than the other fluoridation chemicals.

The other two commonly used fluoridation chemicals are dry powders.  Sodium silicofluoride, also known as sodium fluorosilicate and characterized by the formula Na₂SiF₆, has limited solubility which makes it difficult to dissolve and use.  Sodium fluoride, NaF, is also dry, but is easier to feed than other powdered fluoridation chemicals because it is more soluble in water.  Sodium fluoride was the first chemical used for fluoridation and is still used in small installations, but it is not generally used in large plants because of the high cost of chemicals and bulky saturators.