Calcium Phosphate Precipitation as an Unintended Consequence of Phosphate Dosing to High-pH Water

Abstract

A calcium phosphate solid formed as an unintended consequence of a novel high-pH orthophosphate lead corrosion control strategy in Providence, RI, resulting in some consumer complaints and clogged plumbing. Investigations were carried out at pH levels of 8.9–10.4, at temperatures of 5–60°C, and with orthophosphate concentrations ranging from 0 to 4.0 mg/L as orthophosphate (PO₄) to examine this phenomenon. During field testing, the calcium phosphate initially precipitated at orthophosphate doses above about 2 mg/L as PO₄, and the extent of precipitation increased with water age and higher temperature. Laboratory-scale tests confirmed that doses above about 2 mg/L as PO₄ were required to form the precipitate in the absence of preexisting calcium phosphate solids and that the solid formed quickly at 60°C (upper range for hot water heaters) and tended to dissolve at lower pH. Solubility studies and solids characterization indicate that the particles are a mixture of noncrystalline compounds containing calcium and phosphate. For water systems currently practicing a high pH/low alkalinity corrosion control strategy, orthophosphate dosing can enhance plumbosolvency control without the risk of pH reduction, but formation of this solid can place an upper limit on the maximum orthophosphate residual level in the distribution system.