In this lesson we will answer the following questions:
- What are the advantages and disadvantages of fluoridation?
- How does fluoridation fit into the water treatment process?
- What equipment is used to fluoridate water?
Along with the online lecture, read chapter 13 in Simplified Procedures for Water Examination.
Introduction to Fluoridation
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. Safety considerations for the plant operator will be discussed in a later section.
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.
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.
Optimal fluoride concentration in drinking water.
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.
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.
Equipment used to feed fluoride into water comes in two categories - solution feeders and dry feeders. In both cases, the chemicals are added to water in a liquid form. The distinction is whether the chemical is measured as a liquid (in solution feeders) or as a solid (in dry feeders.) In general, solution feeders are more expensive than dry feeders and are used in smaller systems. Dry feeders are used in large treatment plants.
Acid Feed System
Despite the name, solution feeders can be used for either liquid or dry chemicals. If the fluoride source is a liquid, then the fluoride can be fed directly into the water main using an acid feed system. If the fluoride source is a solid, a saturated solution can be produced using a saturator and the solution can then be fed into the water main. In either case, positive displacement pumps are used to pump the appropriate quantity of the fluoride solution into the water being treated.
In this section, we will be concerned with acid feed systems, which are usually used for pumping hydrofluosilicic acid into water. The setup is known as an acid feed system since hydrofluosilicic acid is pumped into the water in the same state in which it was delivered.
The hydrofluosilicic acid begins in the shipping container, which is placed on a scales to measure the remaining liquid in the container (just as was done for chlorine cylinders.) A pump pulls acid from the shipping container into a physical break box, the purpose of which is to prevent a major overfeed of acid into the main flow of water. From the break box, the fluoride flows to the other side of the pump and is pumped to the water line.
Positive Displacement Pumps
In the acid feed setup shown in the previous section, the amount of fluoride solution added to water is measured by a pump. Positive displacement pumps are used in solution feeders to feed a measured volume of a liquid chemical during a specific time period. They have several advantages over other types of pumps, being accurate and capable of feeding a solution against pressure into a pipe or tank of water as well as into open tanks of water.
All pumps move liquids, thus creating a flow from one side of the pump to the other. Positive displacement pumps work by delivering a constant volume of solution with each stroke or pulse, so volumes of the solution are discharged at intervals. This differs from non-positive displacement pumps which create constant flows of solution which are more difficult to measure.
Positive displacement pumps are used for many purposes in the water treatment plant in addition to fluoridation. The only specific requirement for fluoride pumps is that the pumps deliver an accurate quantity of the fluoride at a constant rate.
The operation of a positive displacement pump is shown below. First, the solution enters a chamber in the pump. Since the chamber has a specific volume, it measures a set volume of solution.
Once the chamber has been filled, the inlet valve is closed and the outlet valve is opened. The solution is pushed out of the chamber by a piston, shown in pink in the picture below.
The volume of solution fed by the pump is determined by the piston stroke length and by the stroke frequency. As you can see in the pictures above, the length of the piston stroke determines the volume of solution which is fed with each pulse. If the piston is drawn back a shorter distance, the chamber is smaller and fills with less solution. If the piston is drawn back further, then the chamber is larger and fills with more solution. The stroke frequency also influences the amount of solution fed since faster strokes will feed more solution.
Some types of positive displacement pumps may use diaphragms, gears, or screws instead of pistons. Volume of solution fed can be monitored in these types of pumps in much the same way as for a piston pump.
Feeding a vastly excess amount of fluoride into water can cause illness in the customers. Break boxes are often used to prevent overfeeding. A diagram of a break box is shown below:
During normal flow conditions, the fluoride solution flows from the shipping container to the break box, entering through the inlet at the top left on the diagram above. The solution then flows out of the container through the outlet and to the water being treated.
If there is a malfunction in the pump which results in an excess amount of fluoride solution being fed, then the break box operates as shown below.
Some solution continues to flow out to the water being treated. However, once the solution level in the break box rises above the normal full level, the solution can enter the overflow line. The excess solution flows out of the break box through the overflow line and back to the shipping container.
As mentioned above, solution feeders can be used to feed dry fluoride chemicals if a saturator is part of the setup. A saturator is a device used in fluoride feed systems which produces a fluoride solution. The saturator dissolves dry fluoride chemicals so that they may be fed into the water line using a solution feeder.
Saturators can either be upflow, with water flowing from the bottom to the top of the saturator, or downflow, in which water flows down through the saturator. The upflow saturator is the preferred type because it is easier to clean and maintain, so we will consider an upflow saturator here.
The diagram above shows a typical upflow saturator. Some form of dry fluoride is placed in the bottom of the saturator. The dry fluoride is most typically sodium fluoride, but can also be sodium silicofluoride.
Once the fluoride is in place, water is pumped down to the bottom of the saturator and is then spread throughout the saturator bottom using a distributor. The water flows up through the sodium fluoride, dissolving some of the fluoride and producing a fluoride solution.
A pump intake floats at the top of the fluoride solution. The pump pulls the solution out of the saturator and pumps it to the water main just as it would in an acid feed system.
Saturators are often used to dissolve dry fluoride chemicals because no measuring is required. Whole bags of sodium fluoride can be dropped into the saturator. The saturator always produces a 4% solution of sodium fluoride (or a 0.76% solution of sodium silicofluoride) due to the solubility of the dry fluoride chemical. The saturator completely saturates the incoming water with the fluoride chemical.
Backflow is the reverse flow of water in a system. Backflow problems can occur at several locations in fluoridation systems, especially around saturators, so control devices are added to prevent the reverse flow. The simplest way to prevent backflow is by using an air gap, which is a vertical separation between the inlet line and the outlet line. However, in some cases vacuum breakers are required.
Up to this point, we have been concerned only with solution feeders. However, dry feeders are commonly used to feed solid fluoride chemicals in large plants. The most common fluoride chemical fed using a dry feeder is sodium fluorosilicate.
Like solution feeders, dry feeders are used for many purposes besides fluoridation in the water treatment plant. In all cases, the dry feeder measures a dry chemical and mixes it with water in a solution tank. Then the resulting solution is delivered into the main flow of water using a pump or injector.
There are two types of dry feeders - volumetric feeders and gravimetric feeders. We will discuss each type below.
Volumetric feeders feed a measured volume of dry chemical within a given time interval. The picture below shows one type of volumetric feeder, in which the chemical begins in the hopper and is measured by a rotating screw. The dry chemical is then dropped into a solution tank where it is mixed with water to produce a fluoride solution. The solution can then be pumped to the water supply.
Volumetric feeders can also measure chemicals using rotating disks, oscillating pans, vibratory pans, rotating rollers, or star wheels.
Gravimetric feeders measure a certain weight (rather than volume) of chemical within a given time period. These feeders are more expensive, more complicated, and require more space than volumetric feeders, but they also feed larger amounts of chemicals and are more accurate. A typical gravimetric feeder is shown below.
As mentioned previously, excess fluoride concentrations in water can cause illness. The operator should monitor the fluoride concentration at least once per day to prevent overfeeding. Underfeeding is less of a problem than overfeeding and fluoridation equipment can be shut down for cleaning and maintenance for short time periods without shutting down the entire water treatment plant.
In addition to monitoring the fluoride concentration manually, a type of flow meter called a pacing meter can be be used to match the feed rate of fluoride to the flow rate of the water being treated. Pacing meters measure the total water flow rate and then produce a signal which allows for automatic adjustment of the fluoride feed rate. This type of meter is only required in systems in which the flow rate is very variable. Even in systems containing pacing meters, the fluoride concentration in the water should be tested daily by the operator.
A second type of flow meter is also used in fluoride systems. This is a simpler water meter used to merely measure the flow rate of water within the plant. This type of meter is not connected to the chemical feeder and is used by the operator to determine what flow rates the feeder must accommodate.
If daily monitoring shows a change in the fluoride concentration of treated water, the operator will need to be able to determine and correct the problem. A variety of factors can influence fluoride concentrations, and some of the most frequent problems will be discussed below.
The most common cause of low or erratic fluoride concentration is improper equipment maintenance or operation. The following factors may cause low fluoride readings in treated water:
- Undersized solution tanks, detention time of less than five minutes, inadequate solution water, and inadequate mixing in dry feeders.
- Inadequate chemical depth in saturators.
- Fluoride feed ahead of the filters.
- Unfluoridated water mixing with treated water in the distribution system.
- Low chemical purity.
In contrast, high fluoride readings are often the result of improper testing procedures. If the high fluoride readings are accurate, they may result from not taking the initial fluoride content of the water into account when calculating optimal fluoride dosage.
The water quality can also influence fluoridation to some extent. When water with a hardness greater than 10 ppm is fluoridated using a saturator, scaling can form on the saturator walls. The scaling consists of low solubility calcium and magnesium fluoride compounds. Scaling can be prevented by water softening upstream.
Although fluoride is safe to drink at the concentration recommended for treating public water, the chemicals used to treat the water are handled by the operator in much higher concentrations. Rubber gloves, coveralls, and protective eyewear should be worn when handling fluoride.
Solid forms of fluoride are the most problematic to operators, since inhaling fluoride dust is very dangerous. A dust collector should be used and a respirator should be worn when handling fluoride powders.
Liquid forms, such as hydrofluosilicic acid, can also be dangerous. Hydrofluosilicic acid produces poisonous fumes which must be vented and which are irritating to the skin. The liquid itself can cause burns when allowed to touch skin.
The most extreme safety problem when dealing with fluoride is fluoride poisoning, which can be fatal. However, fluoride poisoning occurs only when a large amount of fluoride - approximately one tablespoon - is ingested. This is an amount much larger than would normally be inhaled while handling dry fluorides. Accidental ingestion of fluoride chemicals can occur through contaminated food and drink. The operator should always wash his hands after handling fluoride chemicals and should not eat, drink, or smoke in areas where fluorides are used or stored.
Fluoride is added to drinking water to prevent dental decay. Several chemicals can be used in fluoridation, but the most common are hydrofluosilicic acid, sodium silicofluoride, and sodium fluoride. The optimal concentration of fluoride in drinking water is approximately 1 ppm, with the exact amount depending on a region's average daily temperature.
Fluoridation chemicals may be fed using a solution feeder or a dry feeder. Solution feeders can feed liquid chemicals using an acid feed system or can feed a fluoride solution produced from dry chemicals in a saturator. In either case, the chemical is measured in a liquid form using a positive displacement pump. Dry feeders, in contrast, feed dry chemicals which are measured by weight or volume in a gravimetric feeder or volumetric feeder.
Fluoride concentration in water must be monitored daily. When handling fluoridation chemicals, operators must follow appropriate safety precautions.
Alabama Department of Environmental Management. 1989. Water Works Operator Manual.
Basic Hydraulics. 2002. Sweethaven Publishing Services, Inc.
Hargrave and Burdick. 2000. Fluoridation Design Manual for Water Systems in B.C. Region. Toronto, Canada.
Kerri, K.D. 2002. Water Treatment Plant Operation. California State University: Sacramento.
Ragsdale and Associates. Version III. New Mexico Water Systems Operator Certification Study Guide. NMED Surface Water Quality Bureau: Santa Fe.
South Africa Department of Health. 2003. Water Fluoridation - A Manual for Water Plant Operators.
Complete the questions for Assignment 18, which deals with the fluoride virtual lab. When you have gotten all the answers correct, print the page and either mail or fax it to the instructor. You may also take the quiz online and directly submit it into the database for a grade.
Read the Fluoride lab and complete the assignmnet listed above.