Effect of Phosphate Inhibitors on Lead Release From Pipes

Hexametaphosphate tends to increase release of both particulate and soluble lead to drinking water. For every mg/L of residual hexametaphosphate, soluble lead increased by 1.6 – 2.5 mg/L after 72 hour stagnation in pure lead pipes compared to the same condition dosed with orthophosphate. Utilities should consider these adverse effects whenever polyphosphate is used to prevent scaling or iron precipitation; in fact, polyphosphates cannot be recommended for lead corrosion control. Though soluble lead release was in reasonable agreement with predictions of solubility models, particulate lead release was dominant in these experiments. In some cases, a peak in lead release was observed with increasing stagnation time in pipes— analysis of that data indicated that particulate lead can re-attach to the pipe surface under at least some circumstances. INTRODUCTION The beneficial use of phosphates in preventing calcium scale build-up and iron precipitation in water distribution systems has been established for well over 50 years. It is also known that, under at least some circumstances, phosphates can also prevent certain aspects of iron pipe corrosion including red water, weight loss and tubercle build-up. Consequently, when the USEPA Lead and Copper Rule effectively expanded the meaning of the term “corrosion control” at utilities to explicitly consider lead and copper concentrations at the consumers tap, it was hoped that phosphate dosing would provide a low cost approach to meeting multiple water quality objectives. Phosphates can be dosed as either phosphoric acid, combinations of orthophosphoric acid and zinc orthophosphate, polyphosphates, or blends of orthophosphoric acid and polyphosphate. More than one half of U.S. utilities reported that control of lead corrosion by-products was an important reason why they used phosphate based corrosion inhibitors in a 1992 survey. A