Fluoride in Nature
Fluoride is a
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,
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
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
Fluoride in water has been proven to prevent tooth decay
children and to prevent root tip rot.
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
In the Treatment Plant
Overview of the Treatment Process
Fluoridation will consist of one of three possible processes. In
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
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.
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.
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
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.
Optimal fluoride concentration
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.
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
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.