Fluoridation

Fluoridation

• Fluoridation is adjustment of water fluoride
• Used for optimum oral health benefits
• One of ten great public health achievements of the twentieth century (CDC)
• Water fluoridation has a 60-year history of success
• Tooth decay is the most prevalent chronic disease among humans, especially children
• Water fluoridation is the most economical and effective method of reducing tooth decay

Optimal Fluoride Dose

• Varies depending on climate - how much water people drink in a day
• Too little fluoride doesn’t protect against cavities
• Too much fluoride can cause fluorosis

Fluorosis

Dental fluorosis

• Mottling of the teeth
• Pitting of teeth, greater chance of tooth decay
• Can occur at twice the optimal dosage

Skeletal fluorosis

• Crippling disease with rheumatic attacks, pain, and stiffness in the joints
• Can occur at 20 mg per day for many years

Fluoride Consumption

• If a person drinks one liter of water containing 1 mg/L of fluoride, that person has consumed 1 mg of fluoride.
• If a person drinks two liters of water containing 1 mg/L of fluoride, that person has consumed 2 mg of fluoride.

Optimal fluoride concentration in drinking water.

General Practices

• There are no state regulations requiring water systems to fluoridate
• USEPA sets MCLs on fluoride, but there is no minimum level
• Primary MCL is 4.0 mg/L
• Secondary MCL is 2.0 mg/L
• The minimum fatal dose of fluoride for a 150-lb person is 2 grams.

Sampling Requirements

• Daily sampling and analysis
• One sample to state required monthly

Fluoride Chemicals

Fluoride is not in pure form when purchased. It is combined with other elements for stability and ease of handling.

• Sodium Fluoride - (NaF) 98%
• Sodium Fluorosilicate - (Na₂SiF₆) 98.5%
• Fluorosilicic Acid - (H₂SiF₆) 23%

Sodium Fluoride - NaF

• Fist compound used for fluoridation
• White, odorless crystals or powder
• Solubility constant at 4 grams per 100 mL
• Solutions have pH near neutral
• 98% pure
• Principle hazard is dust

Sodium Fluorosilicate - Na₂SiF₆

• Salt of fluorosilicic acid
• Generated as by-product of fertilizer
• White, odorless crystalline powder
• Solubility varies with temperature
• 98% or higher purity
• Principle hazard is dust
• Most expensive

Fluorosilicic Acid - H₂SiF₆

• Once referred to as “silly acid”
• Also known as hydrofluosilicic acid
• Clear, colorless to slightly yellow liquid
• Pungent odor, fuming, will etch glass
• Corrosive, pH 1.0 – 1.5
• 18-23% pure
• Can cause burns, respiratory irritation
• More costly, easier to handle than dry chemicals
•  

Fluoridation Systems

Dry Feeders

• Measure a given amount of dry chemical into a mixing tank where it forms a solution
• Requires a stirrer to provide good mixing
• Solution pumped or gravity-fed to clearwell

Types of Dry Feeders

Volumetric Feeders:

• Easy to operate
• Less expensive
• Less accurate
• Feeds measured volume per revolution (unit time)

Gravimetric Feeders:

• Very accurate
• More expensive
• Feed by weight, not volume
• Belt-type
• Loss-of-weight type

Solution Feeders

• Small pumps feed solution from tank
• Diaphragm pumps - cam speed or stroke is adjusted
• Piston pump - adjusting stroke length; can pump at very low rates
• Both are accurate, can pump against pressure
• Positive displacement
• Must ensure that chemical is completely dissolved to avoid clogging of feed lines and inconsistent dosages - 5 minute mixing time
• Must have vacuum breaker between tank and inlet valve as backflow preventer; no cross-connections

Saturators

• Used for sodium fluoride
• Saturated solution will result if water trickles through a bed of NaF crystals
• Constant 4% at normal temperatures
• Pumped by metering pump

Upflow saturators

• Water introduced at bottom
• Bed of undissolved NaF on bottom of tank

Downflow saturators

• Bed of granular NaF sits on layer of sand and gravel, water flows from top to bottom

Acid Feed Systems

• Can be fed from shipping container
• Container set on scale to record usage
• Small systems may have to dilute
• Large systems can purchase tank truck, feed from day tank

Safety Equipment Needed

• Shower and eye wash facilities
• Chemical aprons
• Gauntlets
• Goggles
• Respirators or dust masks

Scales

• Required for all systems except saturators
• Must have capacity to weigh full tank
• Measure to nearest pound (½ pound for acid)
• Connections to water lines, discharge lines must be flexible

Dissolving Tanks

• Also known as solution tanks
• Dry chemicals from a feeder must be dissolved in a tank
• Minimum of 35 gallon tank
• Chemicals must be completely dissolved to avoid clogging or inconsistent feed rates

Mixers

• Mechanical mixer with stainless steel shaft and propeller
• In-line mixers also available

Day Tanks (Acid)

• Hold enough chemical for 1 day or 1 shift
• Required to be set on a scale for weight measurement
• Polyethylene tanks most common
• Should be vented to outdoors
• All connections sealed to prevent corrosion

Hoppers

• On dry feeders, hold chemical over feeder
• Should hold at least one bag of chemical to avoid spillage and excess dust
• May be installed with agitator to promote smooth and even flow

Bag Loaders

• Helps operator lift chemical to fill hopper
• Holds one bag of chemical
• Door swings down, bag is attached by a rod through bottom of bag
• Bag is cut open and swung into position
• Requires operator to wear dust mask

Operation of Fluoridation Systems

Dry Feeders

• Inspect and clean regularly to prevent breakdowns
• Inspect feed mechanism for signs of wear
• Inspect solution tank for precipitate buildup
• Calibrate feeder occasionally

Saturators

• Maintain at least 6” of chemical at all times
• Saturators treating over 1000 gpm need 10”
• Use crystalline NaF - less dust, does not compact
• Metering pumps periodically cleaned, worn parts replaced

Fluoride Injection Point

• Apply fluoride after all treatment is complete - after filtration, before clearwell
• Should be as far as possible from lime or calcium hypochlorite application
• Injectors should be cleaned periodically to prevent precipitate buildup
• If injected into a pipe, inject into lower part

Chemical Storage

• Chemical storage areas must be kept clean and orderly
• Improper storage can cause chemical loss and safety hazards

Dry Chemical Storage

• Dry chemicals must be kept dry
• Bags should be stored as close to feeder as possible to minimize handling and dust
• Whole bags should be emptied into hopper to minimize spillage and dust
• Cut the top of the bag; do not tear
• Pour contents gently; do not toss the bags
• Good ventilation is required even if dust isn’t bad

Fluorosilicic Acid Storage

• Containers should be kept tightly closed or vented to the outdoors due to corrosive vapors
• Should be stored away from switches, contacts, and control panels
• Never re-use empty fluoride containers. They may still contain high levels of fluoride even after repeated rinsing.

Analysis of Fluorides in Drinking Water

Ion Selective Electrode method

• Preferred because of fewer interferences

SPADNS method

• Interferences from phosphates

When Running Fluoride Tests:

• Make sure glassware is clean and rinsed with distilled water
• The use of phosphate-based detergents can interfere with test results (with SPADNS)

Determination of Fluoride With Ion-Selective Electrode Method

This method can be used for measuring total solubilized fluoride in drinking waters, natural surface waters and ground waters.  ISEs must be used carefully and results must be interpreted cautiously, since an ISE may be affected by numerous analytical interferences which may either increase or decrease the apparent analyte concentration, or which may damage the ISE. Effects of most interference can be minimized or eliminated by adding appropriate chemical reagents to the sample. Obtaining the most accurate results, therefore, requires some knowledge of the sample composition. ISE manufacturers usually include a list of interferences in the instruction manual accompanying an ISE, along with recommended methods for minimizing or eliminating effects of these interferences.

Summary of Method

Total solubilized fluoride is determined potentiometrically using a fluoride ion-selective electrode (ISE) in conjunction with a standard single-junction reference electrode, or a fluoride combination ISE, and a pH meter with an expanded millivolt scale or an ISE meter capable of being calibrated directly in terms of fluoride concentration.

Standards and samples are mixed 1:1 with a total ionic strength adjustment buffer (TISAB). TISAB adjusts ionic strength, buffers pH to 5-5.5, and contains a chelating agent to break up metal-fluoride complexes. Calibration is performed by analyzing a series of standards and plotting mV vs. fluoride concentration on semilog paper or by calibrating the ion meter directly in terms of fluoride concentration.

Interferences

Polyvalent cations (e.g., Fe⁺³ and Al⁺³) interfere by forming complexes with fluoride which are not measured by the fluoride ISE.  Aluminum-fluoride complex ions are approximately ten times more stable than corresponding iron(III)-fluoride complex ions. As the aluminum concentration increases, more fluoride is consumed to form the metal-fluoride complex. Adding TISAB, which contains a strong chelating agent, eliminates this interference by complexing polyvalent cations.

Sample pH is critical. Hydroxide is a significant interferant at concentrations ten times the fluoride concentration. This interference is avoided by adding TISAB which buffers the sample at a pH of 5-5.5. At low pH values, fluoride forms bifluoride (HF ) which is not detected by the 2 - fluoride ISE. Again, adding TISAB prevents this interference by buffering the pH. Temperature changes affect electrode potentials.  Therefore, standards and samples must be equilibrated at the same temperature.

The user should be aware of the potential of interferences from colloidal substances and that, if necessary, the samples may be filtered.

Apparatus and Materials

• pH/mV meter capable of reading to 0.1 mV or an ISE meter.
• Combination fluoride ISE, or separate fluoride ISE  and reference electrode  prepared for use as described in owner's manual.
• Thermally isolated magnetic stirrer, polytetrafluoroethylene (PTFE)-coated stir bar, and stopwatch.
• Volumetric flask, 1,000 mL.
• Polyethylene labware.

Reagents

• TISAB solution
• Fluoride calibration standards

Sample Collection, Preservation, and Handling

• All samples must have been collected using a sampling plan.
• Samples must be stored and handled in polyethylene containers. Samples should be stored at 4oC.

Procedure

Calibration

If using a fluoride combination ISE, ensure that the ISE is filled with the solution recommended by the manufacturer. Change the solution if the ISE has not been used for a week. If using a fluoride ISE and a separate reference electrode, ensure that the reference electrode is filled with the solution recommended by the manufacturer. In either case, equilibrate the electrodes for 24 hours in a 10.0 mg/L fluoride standard before use.

Calibrate the fluoride ISE using standards that narrowly bracket the expected sample concentration. If the sample concentration is unknown, calibrate with 1.00 mg/L and 10.0 mg/L fluoride standards. Add 20.0 mL of standard and 20.0 mL of TISAB to a 50 mL polyethylene beaker. Add a PTFE-coated magnetic stir bar, place the beaker on a magnetic stir plate, and stir at slow speed (no visible vortex). Immerse the electrode tips to just above the rotating stir bar. If using an ISE meter, calibrate the meter in terms of fluoride concentration following the manufacturer's instructions. If using a pH/mV meter, record the meter reading (mV) as soon as the reading is stable, but in no case should the time exceed five minutes after immersing the electrode tips. Prepare a calibration curve by plotting measured potential (mV) as a function of the logarithm of fluoride concentration. The slope must be 54-60 mV per decade of fluoride concentration. If the slope is not acceptable, the ISE may not be working properly. For corrective action, consult the ISE operating manual. Use only plastic ware (polyethylene) when preparing and analyzing fluoride samples.

Allow samples and standards to equilibrate to room temperature.

Prior to and between analyses, rinse the electrode thoroughly with reagent water and gently shake off excess water. Low-level measurements are faster if the electrode tips are first immersed for five minutes in reagent water.

Add 20.0 mL of sample and 20.0 mL of TISAB to a 50 mL polyethylene beaker. Add a PTFE-coated magnetic stir bar. Place the beaker on a magnetic stir plate and stir at a slow speed (no visible vortex). Immerse the electrode tips to just above the rotating stir bar. Record the meter reading (mV or concentration) as soon as the reading is stable, but in no case should the time exceed five minutes after immersing the electrode tips. If reading mV, determine fluoride concentration from the calibration curve.

When analyses have been completed, rinse the electrodes thoroughly and store them in a 10.0 mg/L fluoride standard solution. If the electrodes will not be used more than one day, drain the internal filling solution, rinse with reagent water, and store dry.

Operating Problems

Varying Concentrations

• Takes time for fluoride concentration in distribution system to stabilize
• Variations caused by inaccurate calibration, improper functioning of feeders, equipment
• Unfluoridated water introduced into the system

Low Concentrations

• Interferences with lab tests
• Inadequate chemical depth in a saturator
• Improper mixing in a solution tank
• Improper injection point

High Concentrations

• Phosphates cause high readings with SPADNS test
• Naturally occurring fluoride levels can vary

Fluoride Poisoning

• Can occur from malfunctioning equipment or failure to turn off feed system when flow is stopped
• Illnesses and at least one death blamed on high fluoride levels in public water supply

Control Tests

• Check raw fluoride level
• Check finished fluoride level
• Automatic monitors continuously record level, have alarm if max concentration is exceeded

Symptoms of Overexposure

Chronic Toxic Exposure

• Prolonged exposure to 2-8 times recommended level
• Mottled teeth, calcified ligaments & tendons
• Vertebrae consolidation

Acute Toxic Exposure

• Results from a single, massive dose
• Inhalation causes sharp pains in nose, nosebleed, respiratory distress
• Ingestion causes vomiting, abdominal pain
• Acid spilled on skin can cause burns, splashed in eyes causes severe irritation

Fluoride Math

Determining Feed Rate

• Fluoride is not in pure form when purchased. It is combined with other elements for stability and ease of handling.
• To calculate the correct dosage, you must know the following:
• Ion concentration
• Percent purity

Fluoride Chemicals Purity

• Sodium Fluoride (NaF)                   =       98% purity
• Sodium Fluorosilicate (Na2SiF6)   =       98.5% purity
• Fluorosilicic Acid (H2SiF6)           =       18-23% purity

Fluoride Chemicals

Ion Concentration Percent (AFI)

• Sodium Fluoride = 45.2%
• Sodium Fluorosilicate = 60.7%
• Fluorosilicic Acid = 79.2%

Optimal Dosage

• The optimal dosage for fluoride in drinking water  is 0.8 - 1.0 mg/L.
• To determine fluoride dosage, subtract the naturally occurring fluoride concentration from the desired concentration.

Fluoride Dose

A water plant wants a fluoride concentration of 1.00 mg/L in the finished water. If the raw fluoride content is 0.25 mg/L, how much fluoride needs to be added?

Fluoride dose = Total fluoride, mg/L - raw fluoride, mg/L

Fluoride dose = 1.00 mg/L - 0.25 mg/L

Fluoride dose = 0.75 mg/L

Calculating Feed Rate

A water plant treats 750,000 gpd and wants to add 0.9 mg/L of fluoride to the water using sodium fluoride. How many pounds per day will be used?

Calculating Feed Rate

A water plant treats 200,000 gpd and wants to add 0.82 mg/L of fluoride to the water using sodium fluorosilicate. How many pounds per day will they use?

Calculating Feed Rate

A water plant treats 12.0 MGD and wants to add 0.75 mg/L of fluoride using fluorosilicic acid to the water. How many pounds per day will be used?

Feed Rate for Saturator

A water plant produces 250 gpm. Determine the feed rate for a saturator in gal/day if the dosage is 1.0 mg/L?

Dosages

A total of 25 lbs of sodium fluorosilicate is used to treat 1.75 MGD. What is the concentration of fluoride in the water in mg/L?

Dosages

A total of 5 lbs of sodium fluoride is used to treat 0.2 MGD. What is the concentration of fluoride in the water in mg/L?

Dosages

A water plant uses 284 lbs of fluorosilicic acid to treat 6.2 MGD. What is the concentration of fluoride in the water in mg/L?

Saturator Dosages

A water plant uses 9 gallons of sodium fluoride from a saturator to treat 200,000 gpd. There is 0.18 mg/L fluoride in the raw water. What is the total concentration of fluoride in mg/L?

*You can only use NaF in saturators because it has a constant saturation point.