Heavy metal, organic matter, and disinfection byproduct release from drinking water pipe scales under stagnant conditions

Abstract

Physico-chemical and microbiological processes occurring in water distribution systems (WDSs) can result in the formation of pipe scales which accumulate harmful metals. Irregular changes in either supply-water quality or physical disturbances can induce the mobilization and release of these metals. This study investigates morphological and physico-chemical characteristics of pipe scales from three different water supply zones in Croatia. Magnetite, siderite and goethite were the main constituents of pipe scales, with iron the most abundant metal across all three supply zones, followed by Al and Mn. Arsenic was present in only one pipe from the WDS supplied with elevated As-containing source water (30 to 50 μg L⁻¹). Stagnation release experiments were used to explore the potential metal and organic matter (OM) release from pipe scales using water from the three water supply zones. Different release patterns were observed for the monitored metals (Fe, Al, Mn, As, Pb) with a close relationship with OM components indicative of the important role of OM in metal mobilization. Significant correlations enabled the division of pipe scales into three groups; with group 1 exhibiting a close relationship between Fe, Pb and As, group 2 exhibiting a close relationship between Fe, Al and Zn, and group 3 exhibiting a close relationship between Mn and sulfate. These relationships imply that the co-existing metals in scales were co-released into the water. Experiments involving the addition of humic acid to the pipe exhibiting high As showed an elevated release of As and Pb, implying a greater risk of their release if concentrations of humic-like OM in source waters increase over time.