3.5 Corrosion Control, Lead & Copper Rule Revisions (LCRR) & Fluoridation Practices

Corrosion Control, Lead & Copper Rule Revisions (LCRR) & Fluoridation Practices

Water leaving a treatment plant must be chemically stable. Corrosive water degrades distribution infrastructure, dissolves toxic heavy metals from customer plumbing, and creates aesthetic red/black water complaints. Concurrently, public water systems adjust finished water chemistry to support dental public health through community water fluoridation.

1. Lead and Copper Rule Framework (LCR, LCRR & LCRI)

Under EPA regulations and CDPHE Regulation 11, lead and copper are regulated via treatment technique Action Levels (AL) rather than standard Maximum Contaminant Levels (MCLs), because these contaminants originate predominantly in customer service lines and indoor plumbing rather than raw water sources.

Core Regulatory Thresholds

• Lead Action Level: $\mathbf{0.015\text{ mg/L (15 }\mu\text{g/L or 15 ppb)}}$.
• Lead Trigger Level (LCRR): $\mathbf{0.010\text{ mg/L (10 }\mu\text{g/L or 10 ppb)}}$ — Mandates progressive corrosion control planning and goal-based lead service line replacement.
• Copper Action Level: $\mathbf{1.3\text{ mg/L (1,300 }\mu\text{g/L or 1.3 ppm)}}$.

90th Percentile Compliance Determination

Compliance is evaluated based on the 90th percentile concentration of 1-liter, first-draw tap water samples collected from high-risk customer homes (Tier 1 sites with confirmed Lead Service Lines [LSL], lead pipes, or copper pipes with lead solder installed between 1983 and 1986) after a minimum 6-hour stagnation period.

$\text{90th Percentile Rank} = \text{Total Number of Valid Samples} \times 0.90$

To determine compliance:

1. Sort all sample results in ascending order (from lowest to highest concentration).
2. Calculate the rank position ($N \times 0.90$).
3. If the concentration at the 90th percentile rank exceeds $0.015\text{ mg/L}$ for lead or $1.3\text{ mg/L}$ for copper, an Action Level Exceedance (ALE) occurs.

Example (20 Samples): Rank = 20 x 0.90 = 18th sample value.
If the 18th sorted sample is 0.018 mg/L Lead -> System exceeds 0.015 mg/L Action Level.

Mandatory Actions Following an Action Level Exceedance:

• Monitor water quality parameters (WQPs) including pH, alkalinity, calcium, and inhibitor residual.
• Submit an Optimal Corrosion Control Treatment (OCCT) recommendation to CDPHE.
• Deliver mandatory public education materials to all consumers within 60 days (and public notice within 24 hours for lead exceedances under updated rules).
• Accelerate mandatory Lead Service Line (LSL) replacement programs.

2. Optimal Corrosion Control Treatment (OCCT) Mechanisms

Corrosion in water distribution systems is an electrochemical process where metals give up electrons to water (anodic reaction), dissolving metal ions into solution. Utilities control internal pipe corrosion through two primary strategies:

Strategy A: Calcium Carbonate ($\text{CaCO}_3$) Precipitation / Saturation Control

Utilities adjust water pH, calcium hardness, and alkalinity to deposit a thin, protective eggshell layer of calcium carbonate scale along pipe interiors.

• Langelier Saturation Index (LSI): $\text{LSI} = \text{pH} - \text{pH}_s$ Where $\text{pH}$ is measured water pH and $\text{pH}_s$ is the theoretical saturation pH where water is in thermodynamic equilibrium with $\text{CaCO}_3$.
  • $\text{LSI} = 0.0$: Water is in perfect chemical equilibrium; neither scale-forming nor corrosive.
  • $\text{LSI} > 0.0$ (Positive): Water is supersaturated with $\text{CaCO}_3$; tends to precipitate protective scale.
  • $\text{LSI} < 0.0$ (Negative): Water is undersaturated (aggressive); tends to dissolve calcium scale and corrode plumbing metals.
• Ryznar Stability Index (RSI): $\text{RSI} = 2(\text{pH}_s) - \text{pH}$. Values $< 6.5$ indicate scale-forming water; values $> 7.0$ indicate corrosive water.

Strategy B: Chemical Passivation via Phosphate Inhibitors

In soft or low-alkalinity waters where calcium carbonate precipitation is impractical, utilities add orthophosphate ($\text{PO}_4^{3-}$) (fed as phosphoric acid $\text{H}_3\text{PO}_4$, monosodium phosphate, or zinc orthophosphate). Orthophosphate chemically reacts with dissolved lead and copper ions to form a micro-crystalline, insoluble mineral passivation layer (e.g., lead hydroxyapatite $\text{Pb}_5(\text{PO}_4)_3\text{OH}$ and pyromorphite) that shields the pipe wall from the flowing water. Target residual concentrations typically range from 1.0 to 3.0 mg/L as $\text{PO}_4$.

Common Chemicals for pH and Alkalinity Adjustment

3. Community Water Fluoridation Practices

Community water fluoridation is the controlled addition of a fluoride compound to public drinking water to achieve an optimal concentration that prevents dental caries (tooth decay).

Public Health Benchmarks & Regulatory Limits

• CDC / U.S. Public Health Service Optimal Concentration: $\mathbf{0.7\text{ mg/L}}$ (standardized nationwide to balance cavity prevention against enamel fluorosis).
• EPA Primary Maximum Contaminant Level (MCL): $\mathbf{4.0\text{ mg/L}}$ (enforceable health standard to prevent crippling skeletal fluorosis).
• EPA Secondary Maximum Contaminant Level (SMCL): $\mathbf{2.0\text{ mg/L}}$ (non-enforceable aesthetic/cosmetic guideline to protect children from dental fluorosis / enamel mottling).

Approved Fluoride Chemicals

1. Fluorosilicic Acid (Hydrofluorosilicic Acid, $\text{H}_2\text{SiF}_6$):

   • Straw-yellow, fuming, highly corrosive liquid supplied at 23% to 25% active strength.
   • Specific gravity is approximately 1.21 (weighs $\approx 10.2\text{ lb/gal}$).
   • Delivers 79.2% fluoride ion within the pure active molecule.
   • Most common chemical in medium and large water treatment plants; fed directly neat using peristaltic or diaphragm metering pumps from day tanks on load cells/scales.

2. Sodium Fluoride ($\text{NaF}$):

   • White crystalline powder or coarse granules (98% pure, containing 45.2% available fluoride ion).
   • Predominantly fed using downflow saturators: water passes through a bed of granular $\text{NaF}$ to produce a constant, self-limiting 4.0% saturated solution (18,000 mg/L F⁻) at room temperature ($77^\circ\text{F}$).

3. Sodium Fluorosilicate ($\text{Na}_2\text{SiF}_6$):

   • White, odorless powder (98.5% pure, containing 60.7% available fluoride ion).
   • Fed via gravimetric or volumetric dry chemical feeders with mechanical dissolving tanks.

Safety, Interlocks, and Operational Monitoring

• Feed Interlocks: Fluoride chemical feed pumps must be electrically interlocked with primary raw/finished water flow meters so the pump cannot operate if water flow ceases, preventing dangerous chemical overfeeds.
• Daily Testing: Operators must test finished water fluoride concentrations daily using the SPADNS colorimetric method or an Ion-Selective Electrode (ISE) probe.