5.7 Fluoridation Practice & Compliance

The California Regulatory Structure

Fluoridation in California is governed by 22 CCR Article 4.1 (sections 64433 through 64433.7) plus the definition at 64400.42, and by Health and Safety Code section 116410, which requires public water systems with 10,000 or more service connections to fluoridate when funding is made available from sources other than ratepayers. Systems below that threshold may fluoridate voluntarily, and many do.

The temperature dependence exists because people drink more water in hot climates. Table 64433.2-A assigns the optimal level and control range from the annual average of maximum daily air temperatures for the community. A system in the Imperial Valley targets the low end of the range; a coastal or mountain system targets the high end.

[!IMPORTANT] California maintains two different fluoride numbers that both matter. The 2.0 mg/L primary MCL is a health standard that applies to every system, fluoridating or not, and many California groundwater sources exceed it naturally and require defluoridation by activated alumina, reverse osmosis, or blending. The optimal level and control range apply only to systems that fluoridate, and they are a treatment performance standard, not an MCL. A question asking "what is the maximum allowable fluoride concentration in California drinking water" is asking about 2.0 mg/L.

---

The Three Additives

All three must meet NSF/ANSI/CAN Standard 60 for drinking water treatment chemicals - a requirement that applies to every direct additive, not just fluoride.

The Saturator

Sodium fluoride is unusual in that its solubility is nearly constant with temperature: about 4.0 percent by weight (roughly 18,000 mg/L fluoride ion) in a saturated solution. That makes the upflow saturator possible - a tank with a deep bed of NaF crystals through which softened water passes slowly upward, always leaving as a saturated 4 percent solution regardless of how much salt is in the bed. The operator only has to keep the bed covered and meter the solution. Softened make-up water is essential: hard water precipitates calcium fluoride and cements the bed.

---

Dose Calculation

The general pounds formula applies, adjusted for both purity and available fluoride ion, and reduced by the natural background fluoride already in the source:

$\text{lb/day} = \frac{\text{Flow (MGD)} \times (\text{Dose}_{\text{target}} - \text{Fluoride}_{\text{natural}}) \times 8.34}{\text{AFI} \times \text{Purity}}$

Worked example. A plant treats 3.5 MGD, the source contains 0.15 mg/L natural fluoride, the optimal level is 0.9 mg/L, and the additive is sodium fluorosilicate at 98.5 percent purity with an available fluoride ion of 0.607.

Fluoride to be added = 0.9 - 0.15 = 0.75 mg/L Pounds of fluoride ion needed = 3.5 x 0.75 x 8.34 = 21.9 lb/day Pounds of compound = 21.9 / (0.607 x 0.985) = 21.9 / 0.598 = 36.6 lb/day

Liquid example. The same requirement met with 23 percent fluorosilicic acid weighing 10.0 lb/gal: each gallon carries 10.0 x 0.23 x 0.792 = 1.82 lb of fluoride ion, so the feed rate is 21.9 / 1.82 = 12.0 gal/day.

---

Feed Equipment and Control

• Solution feeders (diaphragm metering pumps) for saturator solution and for fluorosilicic acid. Acid feed requires materials compatible with a strong, corrosive acid - PVC, CPVC, PVDF, or polyethylene, never glass, never ordinary steel, never concrete without a liner.
• Dry feeders (volumetric or gravimetric) with a dissolving chamber for sodium fluorosilicate. Gravimetric feeders are more accurate and are preferred where dose control is tight.
• Flow pacing off the plant flow signal, with secondary control from a continuous fluoride analyzer.
• Anti-siphon and injection-point placement downstream of filtration and after any lime or soda ash feed, because high pH and high calcium precipitate calcium fluoride and cause a loss of dose.
• Day tanks with scales or calibration cylinders so the operator can verify the actual feed against the calculated feed by drawdown, independent of the pump's dial setting.

---

Monitoring, Reporting, and Recordkeeping

Under 64433.3 and 64433.7, a fluoridating system must:

• Test fluoride daily at the point representing water entering the distribution system.
• Maintain records of daily results, chemical deliveries, and feed rates.
• Report monthly to the State Board, including the number of days sampled and the number of results outside the control range.
• Notify the State Board when fluoride equipment fails or when results fall outside the control range, and when the system starts or stops fluoridating.

The 20 percent rule is the compliance test that trips systems up: not every excursion is a violation, but if more than one day in five in a month falls outside the control range, the system is out of compliance with optimal levels. That is a treatment technique failure, distinct from exceeding the 2.0 mg/L primary MCL, which is an MCL violation requiring public notification.

---

Safety

Fluoride chemicals are among the more hazardous materials in a water plant, and safety practice is directly testable:

[!WARNING] Never feed fluoride into a line that is not flowing. Interlock every fluoride feeder to the plant flow signal so that a pump failure or a plant shutdown stops the feed instantly. Nearly every documented fluoride overfeed incident traces back to a feeder that kept running into a static line or into a line at greatly reduced flow.