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AWWA WQTC64082
- Effect of Coagulant Selection on Lead Leaching: Importance of the Chloride to Sulfate Mass Ratio
- Conference Proceeding by American Water Works Association, 11/01/2006
- Publisher: AWWA
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Switching from alum (aluminum sulfate) to polyaluminum chloride(PACl) as coagulant has triggered problems with lead at the tap, for several utilities thatwere previously in compliance with the US Environmental Protection Agency lead action level. This seeminglyinnocuous change in coagulant type results in a higher chloride to sulfate ratio of finishedwater, since PACl adds chloride to water whereas alum adds sulfate. Bench scale testingrevealed that PACl-treated water (high Cl<sup>-</sup>:SO<sub>4</sub><sup>-2</sup>), typically increases lead leaching fromleaded brass by a factor of 1.2-2.7 times, compared to alum-treated water (low Cl<sup>-</sup>:SO<sub>4</sub><sup>-2</sup>ratio). In the case of 50:50 lead:tin solder connected to copper pipe, and when aphosphate inhibitor was added to the water, a much more dramatic difference of 40 timeswas noted after two weeks. The mechanism driving galvanic corrosion is the lowering ofpH near the solder surface in a water of low buffering capacity, which in turn increaseslead solubility. Local pH on the surface of galvanic solder dropped from the neutralrange to as low as 3.4 after exposure to PACl-treated water, and to as low as 4.4 afterexposure to alum-treated water. Includes 13 references, table, figures.
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