Factors affecting lead release in sodium silicate-treated partial lead service line replacements

Abstract

Water quality parameters affecting sodium silicate performance in partial lead service line replacements were examined using a fractional factorial experimental design and static pipe systems. An external copper wire was used to create a galvanic connection between a former lead service line and a new copper pipe. The pipe systems were filled with lab prepared water made to mimic real water quality. Water was changed on a three times per week basis. A 2⁴⁻¹ fractional factorial design was used to evaluate the impact of alkalinity (15 mg L⁻¹ or 250 mg L⁻¹ as CaCO₃), nitrate (1 mg L⁻¹ or 7 mg L⁻¹ as N), natural organic matter (1 mg L⁻¹ or 7 mg L⁻¹ as dissolved organic carbon), and disinfectant type (1 mg L⁻¹ chlorine or 3 mg L⁻¹ monochloramine), resulting in eight treatment conditions. Fractional factorial analysis revealed that alkalinity, natural organic matter and monochloramine had a significant positive effect on galvanic current. Natural organic matter and monochloramine also had a significant positive effect with respect to both total and dissolved lead release. For the treatment conditions examined, 67–98% of the lead released through galvanic currents was stored as corrosion scales and predominantly comprised of particulate lead (96.1–99.9%) for all eight treatments. The use of monochloramine and the presence of natural organic matter (7 mg L⁻¹) were not favourable for corrosion control in sodium silicate-treated partial lead service line replacements, although further studies would be required to characterize optimal water quality parameters for specific water quality types. For utilities operating with sodium silicate as a corrosion inhibitor, this work offers further evidence regarding the consideration of chlorine as a secondary disinfectant instead of monochloramine, as well as the value of controlling natural organic matter in distributed water.

Keywords:

• Alkalinity
• chlorine
• galvanic corrosion
• monochloramine
• natural organic matter
• partial lead service line replacement
• sodium silicate

Acknowledgments

The authors wish to thank Jim Wang at the Drinking Water Research Group (University of Toronto) for technical support during the setup and operation of these trials as well as The City of London for providing the lead service lines.

Funding

The authors also gratefully acknowledge the financial support for this project from the Canadian Water Network and the Natural Sciences and Engineering Research Council of Canada Industrial Chair program.