9.2 Fluoridation Chemistry, Chemicals & Feed Systems

Public Health Principles and Regulatory Thresholds

Community water fluoridation is the controlled addition of a fluoride compound to public drinking water to achieve an optimal concentration that prevents dental caries (cavities). Fluoride ions ($F^-$) replace hydroxyl ions ($OH^-$) within the crystalline hydroxyapatite lattice of dental enamel, converting it into fluoroapatite:

$Ca_{10}(PO_4)_6(OH)_2 + 2F^- \rightarrow Ca_{10}(PO_4)_6F_2 + 2OH^-$

Fluoroapatite is significantly harder, structurally denser, and markedly more resistant to demineralization by organic acids produced by oral cariogenic bacteria (Streptococcus mutans).

   [ Low Fluoride (<0.5 mg/L) ]  ---> Increased Dental Caries (Cavities)
   [ Optimal Target: 0.7 mg/L ]  ---> Maximum Caries Reduction; No Fluorosis
   [ Secondary MCL: 2.0 mg/L ]   ---> Cosmetic Dental Fluorosis (Enamel Mottling)
   [ Primary MCL: 4.0 mg/L ]     ---> Crippling Skeletal Fluorosis Risk

Regulatory Concentration Standards

• Optimal Fluoride Concentration (0.7 mg/L): In 2015, the U.S. Department of Health and Human Services (HHS) and the U.S. Public Health Service (USPHS) updated the federal standard to a uniform 0.7 mg/L nationwide. This replaced the legacy 1962 standard (0.7 to 1.2 mg/L, which was varied based on outdoor air temperature and expected fluid consumption).
• Secondary Maximum Contaminant Level (SMCL = 2.0 mg/L): A non-enforceable federal aesthetic guideline (enforceable in many states) designed to prevent dental fluorosis—a cosmetic condition occurring during childhood enamel development (up to age 8) characterized by white spotting, chalky striations, or brown pitting of teeth.
• Primary Maximum Contaminant Level (MCL = 4.0 mg/L): A federally enforceable health standard established under the Safe Drinking Water Act. Chronic consumption of drinking water exceeding 4.0 mg/L leads to skeletal fluorosis, a painful, debilitating bone disorder characterized by dense bone calcification, ligament ossification, and increased fracture susceptibility.

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The Three Common Fluoridation Chemicals

Three commercial chemicals are certified under ANSI/NSF Standard 60 for drinking water fluoridation:

1. Sodium Fluoride (NaF)

Sodium fluoride was the first chemical employed in water fluoridation. Its defining operational characteristic is its fixed solubility constant of 4.0 grams per 100 mL of water (a 4.0% solution) across all normal operating temperatures (0°C to 100°C). One gallon of water dissolves approximately 0.33 pounds of NaF to yield a saturated solution containing approximately 18,000 mg/L of fluoride ion ($F^-$). It is shipped in 50-lb bags or drums.

2. Sodium Fluorosilicate ($Na_2SiF_6$)

Sodium fluorosilicate (also called sodium silicofluoride) is a white crystalline dry powder produced as a byproduct of phosphate fertilizer manufacturing. It has a low and temperature-dependent solubility (requiring large dissolution tanks with mechanical jet mixers and warm dilution water). It is fed via gravimetric (loss-in-weight) or volumetric dry chemical screw feeders equipped with extension hoppers and dust collection bags.

3. Fluorosilicic Acid ($H_2SiF_6$)

Fluorosilicic acid (hydrofluorosilicic acid, FSA) is the most widely utilized fluoridation chemical in the United States. Shipped as a 23% to 25% aqueous solution, it is a dense (10.2 to 10.3 lb/gal), highly corrosive, transparent liquid that fumes in moist air and emits a pungent, suffocating odor. It freezes at approximately 4°F (-15.5°C) at 23% strength and attacks glass, concrete, and stoneware. It must be stored in specialized cross-linked high-density polyethylene (XLPE), polypropylene, or fiber-reinforced polymer (FRP) tanks with viton/PTFE seals and vented to the outdoors.

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Feed Equipment, Saturators, and Anti-Siphon Safety

Sodium Fluoride Saturator Tanks

Saturator tanks generate a continuous 4.0% sodium fluoride solution without complex weighing or mechanical mixing.

                    [ Softened Water Inlet (<50 mg/L Hardness) ]
                                     |
                                     v (Float Valve)
     +-------------------------------------------------------------+
     |                                                             |
     |    [ Liquid Zone: Constant 4.0% Saturated NaF Solution ]    | ---> Floating Suction
     |    (~18,000 mg/L Fluoride Ion)                              |      to Metering Pump
     |                                                             |
     |-------------------------------------------------------------|
     |    [ Bed Zone: Coarse Granular Crystal NaF (min 6-12 in) ]  |
     |-------------------------------------------------------------|
     |    [ Gravel Underdrain Support: 3/8" x 1/8" graded rock ]    |
     +-------------------------------------------------------------+

1. Upflow vs. Downflow Saturators: In an upflow saturator, softened makeup water enters a manifold at the bottom of the tank, percolates upward through a layer of graded gravel and a bed of sodium fluoride crystals, and achieves 4.0% saturation before being drawn from the top by a floating suction intake. In a downflow saturator, water sprays over the top of the bed and drains through the gravel underdrain.
2. Chemical Form Mandate: Operators must load granular crystal NaF into saturators. Powdered NaF must never be used in saturators because fine powder forms an impermeable, insoluble paste that plugs the gravel bed and prevents water flow.
3. Water Softening Requirement: Saturator makeup water hardness must be less than 50 to 75 mg/L as $CaCO_3$. In hard water, calcium ions react immediately with dissolved fluoride to precipitate insoluble calcium fluoride scale ($CaF_2$), which petrifies the bed, clogs suction lines, and binds metering pump valves. Utilities treating hard water must route saturator feed water through an auxiliary sodium-cycle ion exchange water softener.

Liquid Feed Engineering and Anti-Siphon Controls

Fluorosilicic acid is fed undiluted directly from shipping drums or bulk day tanks using positive displacement diaphragm metering pumps. Diluting fluorosilicic acid in ordinary water is prohibited because it hydrolyzes and precipitates gelatinous silica ($SiO_2$) that permanently binds pump check valves.

• Anti-Siphon Protection: If the injection point has a negative pressure (such as an intake suction line) or is located lower than the chemical day tank, chemical can be siphoned uncontrollably into the water main. Mandatory anti-siphon defenses include:
  1. Installing day tanks at an elevation lower than the pump discharge and pipeline injection point.
  2. Spring-loaded diaphragm anti-siphon check valves installed at the pipe injection quill.
  3. Atmospheric vacuum breakers on all chemical and dilution lines.
• Electrical Interlocks: The fluoride chemical pump power circuit must be interlocked in series with the plant raw water influent flow switch and finished water pump starters. If plant water flow stops for any reason, electrical power to the chemical metering pump is immediately cut, preventing chemical injection into standing water.

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Fluoride Feed Calculations and Gravimetric Tracking

Operators must perform daily dosage calculations to verify compliance and calibrate feed equipment.

Universal Chemical Feed Formula

The required chemical feed rate in pounds per day depends on plant flow, desired dosage increase, available fluoride ion (AFI) fraction, and chemical purity:

$\text{Feed Rate (lb/day)} = \frac{\text{Plant Flow (MGD)} \times \text{Dosage (mg/L)} \times 8.34 \text{ lb/gal}}{\text{AFI} \times \text{Purity}}$

$\text{Dosage (mg/L)} = \text{Target Concentration (0.7 mg/L)} - \text{Natural Background Fluoride (mg/L)}$

NaF Saturator Feed Formula

Because a 4% NaF saturator always delivers 18,000 mg/L of active fluoride ion, the required saturator pump flow rate in gallons per minute (gpm) or gallons per day (gpd) is calculated directly:

$\text{Saturator Feed (gpm)} = \frac{\text{Plant Flow (gpm)} \times \text{Dosage (mg/L)}}{18,000 \text{ mg/L}}$

$\text{Saturator Feed (gpd)} = \frac{\text{Plant Flow (MGD)} \times \text{Dosage (mg/L)} \times 1,000,000}{18,000 \text{ mg/L}}$

Class II Operational Diagnostic: Operators verify liquid feed using platform scales under the day tank. By comparing the calculated theoretical pounds of chemical used over 24 hours against the actual measured scale loss, operators detect pump diaphragm degradation, air binding, or check valve clogging.

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Overfeed Response and Emergency Protocols

Fluoride overfeeds represent acute chemical poisoning hazards requiring immediate intervention.