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Granular Activated Carbon Filtration as a Lead Control Strategy

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Overview

This bench-scale study tested granular activated carbon filtration as a treatment-train strategy for reducing lead corrosion from lead solder at a surface water utility in Atlantic Canada. The study compared alum versus polyaluminum chloride coagulation, with and without GAC filtration, across a range of zinc-orthophosphate doses (0, 1, 2 mg PO4/L). Polyaluminum chloride increased the chloride-to-sulfate mass ratio (CSMR) of treated water, driving galvanic corrosion of lead solder. GAC reduced natural organic matter (NOM) in treated water, which improved the performance of orthophosphate as a lead corrosion inhibitor by removing NOM that would otherwise complex with lead and inhibit protective scale formation.

Key numbers

Bench-scale corrosion cell baseline (conditioning phase prior to treatment comparisons):

  • Median dissolved lead: 23.5 µg/L (interquartile range 10.1 to 86.5 µg/L)
  • Median total lead: 98.8 µg/L (interquartile range 49.1 to 205.2 µg/L)

Treatment effects relative to orthophosphate-only control (percent change in geometric mean):

  • Trimetaphosphate (polyphosphate): total lead +22 percent (95% CI: 11 to 35 percent); dissolved lead +54 percent (95% CI: 25 to 91 percent) — net increase
  • Sodium silicate-orthophosphate blend: total lead +8 percent (not statistically significant); dissolved lead +30 percent (95% CI: 5 to 61 percent)
  • pH 9.5 versus pH 7.5: total lead -24 percent; dissolved lead -34 percent (95% CI: -15 to -48 percent) — protective
  • Iron and manganese co-present (versus absent): total lead +41 percent; dissolved lead -44 percent (adsorption to Fe/Mn colloids reduces dissolved fraction while increasing total)

Utility context: Health Canada maximum acceptable concentration for lead lowered to 5 µg/L in 2019.

Methods (brief)

Bench-scale glass jar system with 52 lead coupons (2.7 by 1.5 by 0.2 cm, 99.9 percent lead). Treated water from Halifax Water’s J.D. Kline plant at approximately pH 7.5, alkalinity 18 to 21 mg/L as CaCO3. Five water changes per week; 24-hour and 72-hour stagnation cycles. ICP-MS (Standard Methods 3125 and 3030). Dissolved samples filtered at 0.2 µm. Mixed effects models for paired treatment comparisons.

Evidence fitness

Supports: identifying the mechanism by which polyphosphate sequestrants increase dissolved lead solubility; demonstrating that NOM removal via GAC can improve orthophosphate efficacy for lead corrosion control; establishing that polyaluminum chloride coagulation elevates CSMR, increasing galvanic corrosion risk.

Does not support: direct extrapolation to field-scale treatment plants (bench-scale static conditions differ from distribution system hydraulics); estimating absolute lead concentrations at residential taps; applying these findings to plumbing materials other than 50:50 lead (Pb)/tin (Sn) solder.

Limitations

Bench-scale static systems do not replicate shear forces, flow dynamics, or pipe scale development in distribution systems. Coupon geometry and surface area differ from actual soldered joints. The study used a single source water chemistry; utilities with different alkalinity, hardness, and NOM profiles may see different results. Polyphosphate effects are measured at water treatment scale; some utilities use polyphosphates specifically for iron and manganese sequestration, and the trade-off documented here (sequestration versus lead solubility increase) requires utility-specific evaluation.

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