Evaluating Orthophosphate-Silicate Blend as an Alternative to Blended Phosphates for Corrosion Control and Sequestration

The presence of iron and manganese in drinking water distribution systems can contribute to discoloration, taste and odor issues, scale buildup, deposition corrosion, and the adsorption and transport of lead. Sequestrants can minimize aesthetic concerns and scale buildup, but they are risky due to increased lead/copper solubility. Here, we used a bench-scale batch reactor to evaluate orthophosphate with sodium silicate or polyphosphate for simultaneous lead corrosion control in waters with and without iron/manganese. Consistent with previous work, ortho-trimetaphosphate increased the total lead (23%). Increased dissolved lead was also observed for both ortho-trimetaphosphate (50%) and ortho-silicate (30%) treatments. When iron/manganese was present, orthophosphate-silicate was associated with 5-12% less total lead relative to orthophosphate under the same conditions, and the effect of ortho-trimetaphosphate was pH dependent. The addition of silicate and trimetaphosphate also reduced water discoloration compared to orthophosphate, as measured by apparent color. Here, the orthophosphate-silicate blend did not significantly increase total lead in the absence of high iron and manganese but increases to the highly mobile, dissolved fraction should be noted. Utilities seeking to control lead for compliance purposes, while simultaneously managing iron and manganese for consumer confidence, should explore orthophosphate-silicate as a possible solution in their corrosion control assessments.