7.2 Fluoridation: Chemicals, Dosage & Safety

7.2 Fluoridation: Chemicals, Dosage & Safety

Fluoridation is one of the few treatment processes added purely for public health benefit rather than to remove something. The Need-to-Know Criteria list fluoride among the chemicals operators must "ensure the proper use of" and among the analyses operators must perform and interpret. It is also a process where a feed error has immediate consequences, so dose control and safety are heavily emphasized.

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The Numbers That Matter

[!NOTE] In 2015 the Public Health Service replaced the older recommended range of 0.7 to 1.2 mg/L, which varied by average regional air temperature, with a single value of 0.7 mg/L. The rationale was that people now receive fluoride from many sources, principally toothpaste, and that air conditioning and modern beverage habits have weakened the link between climate and water consumption. Study guides and training materials still circulate the old temperature-based table; the current value is a flat 0.7 mg/L.

Note the direction of the two standards: the secondary standard, at 2.0 mg/L, is lower than the primary standard at 4.0 mg/L, because cosmetic mottling occurs at lower concentrations than skeletal effects. Exceeding the secondary standard requires public notice but is not an enforceable violation; exceeding 4.0 mg/L is.

Natural fluoride above 4.0 mg/L occurs in parts of Arizona's groundwater, in which case the operator's job is removal — by activated alumina, reverse osmosis, or blending with a low-fluoride source — not addition.

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The Three Additives

Fluorosilicic acid is the most widely used at medium and large systems because it is fed as a liquid with no dust and no dissolving step. Sodium fluoride, with the lowest fluoride content, is common at very small systems using saturator feed.

The Saturator

A sodium fluoride saturator holds an excess of NaF crystals in contact with water; because NaF has nearly constant solubility, the saturated solution is reliably about 18,000 mg/L (18 g/L) of fluoride ion regardless of exact conditions. That constancy is the point — a small system can feed a known solution strength without titrating it. Use softened water in a saturator: hardness precipitates calcium fluoride, which plugs the bed and reduces solution strength.

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Dosage Calculation

The base relationship is the pounds formula:

$\text{lb/day} = \text{Dose (mg/L)} \times \text{Flow (MGD)} \times 8.34$

Two adjustments are specific to fluoride:

1. Subtract the natural background fluoride already present in the raw water.
2. Divide by the purity (available fluoride ion) and by the commercial solution strength where applicable.

$\text{lb/day of chemical} = \frac{(\text{Target} - \text{Natural}) \times \text{Flow (MGD)} \times 8.34}{\text{Purity} \times \text{Solution strength}}$

Worked example — sodium fluorosilicate. A plant treats 2.4 MGD. Raw water contains 0.2 mg/L natural fluoride, and the target is 0.7 mg/L. The additive is sodium fluorosilicate at 98% commercial purity with 60.6% available fluoride ion.

Dose to add: 0.7 − 0.2 = 0.5 mg/L

$\text{Fluoride ion needed} = 0.5 \times 2.4 \times 8.34 = 10.0\text{ lb/day}$

$\text{Chemical needed} = \frac{10.0}{0.606 \times 0.98} = \frac{10.0}{0.594} = 16.8\text{ lb/day}$

Worked example — fluorosilicic acid. Same plant and target, using 23% FSA with 79% fluoride ion in the pure acid:

$\text{Chemical solution} = \frac{10.0}{0.79 \times 0.23} = \frac{10.0}{0.1817} = 55.0\text{ lb/day of solution}$

[!WARNING] Forgetting to subtract natural fluoride is the single most common fluoride calculation error. If the raw water already contains 0.3 mg/L and you dose for the full 0.7 mg/L, you deliver 1.0 mg/L — above target, and a reportable overfeed at many systems.

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Feed Equipment, Monitoring, and Control

• Metering pumps (diaphragm or peristaltic) for liquid FSA; volumetric or gravimetric dry feeders with a dissolving chamber for dry chemicals; saturators for small systems.
• Inject at a point of good mixing, and downstream of lime or filtration where practical, since high pH and coagulation can precipitate fluoride and reduce the delivered dose.
• Anti-siphon devices and no cross-connections between the fluoride feed and the potable line. A siphon that drains a day tank into the main is a classic and dangerous overfeed scenario.
• Daily monitoring of finished water fluoride, plus periodic split samples sent to a certified laboratory for verification.
• Analytical methods: the SPADNS colorimetric method and the ion-selective electrode are the standard field and laboratory techniques.
• Maintain records of the amount of chemical fed, the water volume treated, and the measured concentration; the calculated and measured values should agree closely, and a persistent gap means a feed or metering problem.

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Safety

Fluoride chemicals are among the more hazardous materials at a small plant.

• Fluorosilicic acid is highly corrosive. It attacks skin, eyes, and the respiratory tract, and it will etch glass and attack concrete and many metals. Store in acid-resistant, vented containment sized to hold the largest container, on a spill-containment pad.
• Required protection: chemical splash goggles and a face shield, acid-resistant gloves and apron, and boots. An emergency eyewash and safety shower must be immediately accessible.
• Dry sodium fluoride and sodium fluorosilicate generate dust. Use local exhaust ventilation and appropriate respiratory protection when handling and filling hoppers.
• Never mix fluoride chemicals with acids or with other treatment chemicals; contact between fluorosilicates and strong acid can liberate hydrogen fluoride gas.
• Follow the safety data sheet, label all containers and feed lines, and keep the fluoride room separate and secured.