AWWA WQTC52971

Hexametaphosphate tends to increase release of both particulate and soluble leadto drinking water. For every mg/L of residual hexametaphosphate, soluble leadincreases by about 1.6-2.5 mg/L after 72 hours of stagnation in pure lead pipescompared to the same condition dosed with orthophosphate. Utilities shouldconsider these adverse effects whenever polyphosphate is used to prevent scalingor iron precipitation; in fact, polyphosphates cannot be recommended for leadcorrosion control. Though soluble lead release was in reasonable agreement withpredictions of solubility models, particulate lead release was dominant in theseexperiments. In some cases, a peak in lead release was observed with increasingstagnation time in pipes--analysis of that data indicated that particulate leadcan re-attach to the pipe surface under at least some circumstances. Includes 22 references, tables, figures.