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This paper describes a multi-phase systematic process in which several direct and indirectparameters and methods were used to optimize the selection of phosphate treatment under highpH conditions to mitigate calcium precipitation while simultaneously evaluating the impact ofsuch mitigation on lead solubility. The primary study objective was to find a solution that both protected the public from harmful leadlevels and prevented calcium carbonate from precipitating. Two possible methods to control CaCOsub3/subprecipitation were evaluated: lowering the pH; and, maintaining an elevated pH (9.0) with theaddition of a phosphate-based inhibitor. CaCOsub3/sub deposition is inhibited by the presence ofphosphates, particularly polyphosphates, which act in essence as a crystal growth "poison."A series of desktop and bench-scale studies were structured to answer the following questions:what is the pH level required to eliminate (or minimize) calcium carbonate precipitation;what effect, if any, would such a pH change have on lead solubility in the distribution system;how effective are various sequestering agents in controlling calcium deposition; and,does the addition of a sequestering agent affect lead solubility? The success of the recommended treatment, including post-implementationmonitoring results demonstrating reduction in both 90th percentile leadconcentration and customer complaints are also covered. Includes 8 references.

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Edition: Vol. - No. Published: 11/01/2005 Number of Pages: 5File Size: 1 file , 150 KB