3.4 Fluoridation, Corrosion Control & Disinfection Byproducts

Fluoridation, Corrosion Control & Disinfection Byproducts

Water leaving the treatment plant enters an extensive distribution system of buried transmission mains, storage tanks, and consumer plumbing. Operators must manage three critical post-clarification priorities: mandatory fluoridation, internal distribution corrosion control, and the minimization of carcinogenic disinfection byproducts (DBPs).

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1. Illinois Mandatory Community Water Fluoridation

Illinois has mandated community water fluoridation since 1967. The statutory duty now sits in the Public Water Supply Regulation Act at 415 ILCS 40/7a, which requires owners and official custodians of public water supplies to comply with the optimal community fluoridation level recommended by the U.S. Department of Health and Human Services and the CDC, together with the rules adopted by the Illinois EPA and the Pollution Control Board. The Illinois Department of Public Health tracks compliance through the Illinois Fluoridation Reporting System.

• Target Concentration: 35 Ill. Adm. Code 604.1150(a) requires fluoride feed equipment to have the capacity to maintain finished water fluoride at 0.7 mg/L. Illinois adopted 0.7 mg/L in 2016, replacing the older 0.9-1.2 mg/L optimal range, after HHS revised its recommendation to account for fluoride from toothpaste and rinses. Under 604.1150(e), samples go to a certified laboratory monthly to determine compliance with 35 Ill. Adm. Code 611.125.
• Regulatory Limits: Primary Maximum Contaminant Level (MCL) of 4.0 mg/L (protects against skeletal fluorosis); Secondary MCL of 2.0 mg/L (prevents dental fluorosis / tooth enamel mottling in children).

Illinois Fluoride Feed Design Rules (604.1150)

Four Illinois design requirements are frequently tested:

1. A free chlorine residual of 10 mg/L must be maintained in solutions prepared from dry chemicals — and that residual does not count toward the distribution chlorination requirement of Section 604.725.
2. Chlorine must not be added to hydrofluosilicic or fluorosilicic acid solutions.
3. Fluoride compound must not be added ahead of the filters at plants that lime soften or coagulate for turbidity removal, and must not be added ahead of ion exchange softeners.
4. Two diaphragm-operated anti-siphon devices are required on saturator and fluorosilicic acid feed systems: one on the discharge side of the feed pump and a second at the point of application, unless a suitable air gap is provided. Water used to dissolve sodium fluoride must be softened when hardness exceeds 75 mg/L as CaCO3 — the rule behind the saturator softening requirement.

Fluoride Chemical Feed Systems

• Hydrofluorosilicic Acid (H2SiF6): Corrosive, straw-colored liquid (23% to 25% active; ~19% available F-, specific gravity ~1.23). Attacks glass and releases toxic hydrogen fluoride vapor. Fed neat using diaphragm or peristaltic metering pumps from day tanks on load cells.
• Sodium Fluorosilicate (Na2SiF6): Dry white crystalline powder (98% to 99% pure). Fed using volumetric or gravimetric feeders into dissolving tanks with mechanical mixers.
• Sodium Fluoride (NaF): White crystal or powder used in saturator tanks. Crucially, NaF maintains a constant 4.0% solubility (40,000 mg/L NaF, or 18,000 mg/L available F-) across standard water temperatures (0°C to 30°C).

Saturator Operation & Safety Interlocks

In downflow NaF saturators, makeup water percolates through a bed of crystalline NaF. Softened water is mandatory; unsoftened water contains calcium that precipitates insoluble calcium fluoride scale (CaF2↓), cementing the bed and clogging distributor tubes.

To prevent catastrophic fluoride overdoses:

• Electrical Interlock: Chemical feed pumps must be electrically interlocked with well or service pumps so they de-energize instantly when water flow stops.
• Anti-Siphon Protection: Injection quills must have spring-loaded anti-siphon check valves, and day tanks must be placed lower than injection points.
• PPE: Handling fluorosilicic acid requires an acid suit, rubber boots, neoprene gloves, face shield, and an acid-gas vapor respirator.

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2. Internal Corrosion Chemistry & Galvanic Cells

Internal pipe corrosion is an electrochemical reaction where refined metals oxidize into dissolved ions:

1. Anode: Reactive site where metal atoms dissolve: Fe → Fe2+ + 2e- or Pb → Pb2+ + 2e-.
2. Cathode: Non-reactive site where electrons are consumed by dissolved oxygen: O2 + 2 H2O + 4e- → 4 OH-.
3. Electrolyte: Conductive water transporting dissolved ions.
4. Metallic Path: Pipe wall conducting electrons from anode to cathode.

Water Quality Drivers

• pH & Alkalinity: Low pH (< 7.0) accelerates cathodic reduction and dissolves protective scales. Low alkalinity lacks carbonate buffering.
• Dissolved Oxygen: Primary cathodic electron acceptor driving rapid oxidation.
• Chloride-to-Sulfate Mass Ratio (CSMR): $\text{CSMR} = \frac{[\text{Cl}^-, \text{mg/L}]}{[\text{SO}_4^{2-}, \text{mg/L}]}$ A CSMR > 0.58 drastically accelerates galvanic corrosion at lead-tin solder joints connected to copper pipe. Switching coagulants from alum to ferric chloride raises chloride, spiking the CSMR and inducing lead leaching.

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3. Lead and Copper Rule (LCR / LCRR) Standards

The Lead and Copper Rule protects consumers from heavy metals leaching from service lines and plumbing fixtures.

• Lead Action Level: 0.015 mg/L (15 ppb) (LCRR establishes a 10 ppb trigger level).
• Copper Action Level: 1.3 mg/L (1,300 ppb).

Compliance is based on the 90th percentile of first-draw 1-liter cold water tap samples collected at targeted Tier 1 homes (lead service lines, lead pipes, or copper with lead solder installed before 1986). Samples must be collected after a minimum of 6 hours of stagnation.

Exceeding an action level triggers mandatory Corrosion Control Treatment (CCT) optimization, Water Quality Parameter (WQP) monitoring, public education within 60 days, and lead line replacement under the Illinois Lead Service Line Replacement and Notification Act (Public Act 102-0613).

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4. Corrosion Mitigation Strategies & LSI

• Orthophosphate Passivation: Phosphoric acid or zinc orthophosphate (PO4 3-) reacts with lead and copper to deposit an insoluble passivating film on pipe walls: $5 \text{ Pb}^{2+} + 3 \text{ PO}_4^{3-} + \text{H}_2\text{O} \rightleftharpoons \text{Pb}_5(\text{PO}_4)_3\text{OH}\downarrow \quad \text{(Hydroxypyromorphite)}$ Plants maintain 1.0 to 3.0 mg/L as PO4 residual at pH 7.2 to 7.8.
• Polyphosphates vs. Orthophosphate: Polyphosphates sequester iron and manganese but do not passivate lead. Polyphosphates can disperse lead into water and are prohibited as sole lead corrosion inhibitors.
• pH and Alkalinity Adjustment: Dosing caustic soda (NaOH), soda ash (Na2CO3), or lime raises pH (7.8 to 8.5) to reduce metal carbonate solubility.
• Langelier Saturation Index (LSI): Evaluates calcium carbonate scaling tendency (LSI = pH_actual - pH_s):
  • $\text{LSI} > 0$: Supersaturated; scale-forming tendency forms protective film.
  • $\text{LSI} = 0$: Stable, in saturation equilibrium.
  • $\text{LSI} < 0$: Undersaturated; aggressive water dissolves protective scales and attacks metal.

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5. Disinfection Byproducts & Stage 2 DBPR Compliance

DBPs form when free chlorine reacts with precursor Natural Organic Matter (NOM) (measured as TOC) and bromide.

Regulated DBPs & Formation Factors

• Total Trihalomethanes (TTHM): Chloroform, bromodichloromethane, dibromochloromethane, and bromoform. Primary MCL = 0.080 mg/L (80 ppb).
• Haloacetic Acids (HAA5): Monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acids. Primary MCL = 0.060 mg/L (60 ppb).
• Key Drivers: High TOC precursors, high chlorine dose, long contact time, elevated temperature, and pH:
  • Higher pH (≥ 8.0): Accelerates haloform reactions, increasing TTHMs.
  • Lower pH (≤ 7.0): Stabilizes haloacetic acid intermediates, increasing HAA5.

Stage 2 DBPR Compliance Rules

• Locational Running Annual Average (LRAA): Compliance is assessed quarterly at each individual sampling location by averaging the current and previous three quarters. A violation at any single monitoring point puts the entire utility out of compliance: $\text{LRAA} = \frac{Q_1 + Q_2 + Q_3 + Q_4}{4}$
• Operational Evaluation Levels (OEL): Early warning calculation identifying sites at risk of future exceedance: $\text{OEL} = \frac{Q_1 + Q_2 + (2 \times Q_3)}{4}$ Exceeding 80 ppb TTHM or 60 ppb HAA5 requires an operational evaluation report submitted to IEPA within 90 days.

Mitigation Strategies

• Enhanced Coagulation: Lowering coagulation pH (5.5–6.5) and increasing coagulant dose to maximize TOC precursor removal before chlorination.
• Secondary Disinfection: Transitioning from free chlorine to chloramines (NH2Cl) for distribution residual maintenance.
• Distribution Management: Operating tanks at lower levels in winter, tank aeration to strip volatile TTHMs, and unidirectional flushing to reduce water age.