Lead release from lead pipes into drinking water has been a critical concern as exemplified by high-profile events of high lead concentrations in Washington DC, Flint, and Newark. Orthophosphate addition is an established method of controlling lead concentrations. Many phosphate-based inhibitors currently used by utilities are blends of orthophosphate and polyphosphate. Orthophosphate can limit lead release from pipes by forming low-solubility lead phosphate solids on the pipe inner surfaces. In contrast, polyphosphate can accelerate lead release by forming soluble lead-phosphate complexes. Due to the potential conversion from polyphosphate to orthophosphate in the distribution system and to complex dynamics of water chemistry in lead service lines (LSLs), the net impact of a blended phosphate chemical on lead in tap water has been challenging to assess. To address this challenge, a laboratory-scale pipe loop study was performed with six harvested LSLs from a utility with a surface water source that currently adds a blended phosphate corrosion inhibitor. The pipes were conditioned for 6 months with artificial local water that received the same 70%/30% poly/ortho blended-phosphate chemical and dose as they had seen when in use. Three pipes were tested in two stages while three were controls that continuously received the baseline water used for conditioning. During Treatment Stage 1, three pipes were treated with additional orthophosphate (total of 1.5 mg/L as PO43-) while maintaining the presence of the blended phosphate. This treatment resulted in decreases in both total and dissolved lead that were greater than in the control pipes and that also resulted in more stable concentrations. During Treatment Stage 2, blended phosphate was no longer dosed in the water to the treated pipes, and they received only orthophosphate (1.5 mg/L as PO43-). The treated pipes continued to have more stable and lower total and lead concentrations than the controls, and the concentrations were lower at the end of this treatment stage than at the end of Treatment Stage 1. Scale analysis revealed that the calcium/lead phosphate solid phosphohedyphane had formed in the treated pipes and that its abundance increased in moving from Treatment Stage 1 to Treatment Stage 2. The results suggested that more lead-phosphate solid may form and further decreases in lead concentration will be possible if the polyphosphate is removed from blended-phosphate corrosion inhibitor.

Impacts of polyphosphate presence in blended-phosphate corrosion inhibitors on lead release from lead service lines

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.