Iron release in drinking water distribution systems by feeding desalinated seawater: characteristics and control

A study was conducted to study the effects of feeding desalinated seawater (DSW) to an existing distribution system and subsequent effects on red water formation due to iron release. Iron release from existing pipe scales was investigated using pipe section reactors filled with the DSW containing different chloride concentrations. Iron release was linearly correlated to desalted seawater chloride concentration. When chloride concentration increased from 50 to 240mg/L, the total iron concentration after 8h stagnation increased from 1.50 to 6.50mg/L, respectively. A mechanistic explanation for iron release based on chloride was proposed, wherein it was concluded that chloride could penetrate and disrupt stable tubercles existing on the pipe wall. "Red water" was shown to be controlled by adjusting the pH, alkalinity, and hardness of the desalted permeate; enhancements were seen when phosphate inhibitors were added to the desalinated permeate. It is suggested that water quality targets for the distribution of DSW into an existing cast iron pipeline include maintaining the pH above 7.7, alkalinity over 80mg/L, hardness more than 80mg/L, and providing 0.25-0.50mg/L blended phosphate inhibitor.