Replacing polyphosphate with silicate to solve lead, copper, and source water iron problems

Abstract

In 1990, the town of Hopkinton, Mass., instituted a polyphosphate sequestrant feed in two of its five wells to address colored water complaints caused by source water iron and manganese. Failure to meet the Lead and Copper Rule (LCR) action levels and continuing intermittent red water problems prompted the water utility to seek an alternative treatment strategy. After analyses of the water quality of the different wells, the scales on some lead service lines, the operational patterns, the location of lead service lines, and the distribution system configuration, a combination of sodium silicate (with chlorination) and sodium hydroxide addition at different wells was investigated. At study monitoring sites, an initial silicate dose of 25–30 mg/L elevated the pH from 6.3 to 7.1 and immediately resulted in a 55% reduction in lead levels and an 87% reduction in copper levels. An increase to a silicate dose of 45–55 mg/L elevated the pH to 7.5 and produced greater reductions in lead and copper. The treatment change reduced 90th percentile lead and copper levels by at least 95%, enabling compliance with the LCR. The aesthetic quality of the drinking water after treatment was equal or superior to the quality before treatment. The study showed that for many small and medium water systems with multiple wells and entry points, simultaneous overall corrosion control and sequestration of iron and manganese is possible, and it avoids the adverse drinking water/wastewater consequences associated with polyphosphate addition.