PLUMBOSOLVENCY EFFECTS AND CONTROL IN HARD WATERS

A laboratory lead-pipe rig has been used to support Anglian Water's successful orthophosphate dosing programme to reduce plumbosolvency. The hard waters in the region generally fall into low or high plumbosolvency categories according to the types of crystalline deposit formed. To improve the cost-effectiveness of plumbosolvency control, the effects of temperature, phosphate doses, blending and alternation of these waters were investigated. Initial phosphate concentrations must be above 0.6 mg P/l (as phosphorus) to establish plumbosolvency control, Subsequently, phosphate doses may be reduced, provided that dosing is continuous and sufficient phosphate reaches the extremities of thc distribution system. When high and low plumbosolvency waters are blended before distribution, both (or the mixture) must be phosphate dosed. However, where waters alternate in distribution, laboratory studies have shown that low plumbosolvency deposits are more stable, resulting in low lead concentrations. The high plumbsolvency of some hard waters may be due to the presence of low concentrations of humic substances.