Water utilities commonly dose phosphates to potable water to (1) reduce aesthetic problems by sequestration, (2) inhibit calcium carbonate scale formation via threshold inhibition, and (3) reduce corrosion of lead or copper plumbing materials by forming protective pipe scales. Despite widespread use and increasing importance of phosphates, significant gaps in fundamental understanding still exist. This is partly due to the proprietary nature of some phosphate chemicals, causing experimental data and acquired knowledge for public water supplies to be considered trade secrets. Here, we summarize the current state of the science, provide operational guidance, and identify knowledge gaps regarding the use of ortho-and polyphosphates. The goal is to empower water scientists to improve phosphate chemical performance and to avoid unintended adverse consequences.

After a utility switched its source water from ground to surface water in 2017, first draw water lead levels spiked due to increased lead solder corrosion that could not be explained by existing knowledge. When lead release was not adequately reduced with a 90:10 orthophosphate/polyphosphate corrosion inhibitor blend or even high levels of 100% orthophosphate, an in-depth investigation of possible causes revealed a strong correlation between 90th percentile lead and seasonal fluctuations in surface water nitrate levels. Complementary bench-scale studies that tested new copper coupons with lead solder and harvested pipes from a worst case home verified a strong relationship between nitrate and elevated lead. Lead release in the presence of nitrate became increasingly erratic with time, resulting in the spalling of large lead solder particulates up to 7 mm in length into the water. Lead levels were occasionally >1000 ppb in homes and >100000 ppb in the bench experiments with harvested pipe. Orthophosphate was unable to sufficiently reduce lead levels below the action level during periods with high nitrate levels in the bench studies. Water utilities and regulators should proactively consider possible unintended consequences of higher nitrate levels on lead release when changing source waters or during seasonal runoff events.