Iron and Copper Release in Drinking-Water Distribution Systems
Shi, Baoyou, Taylor, James S., Journal of Environmental Health
Unlined metal pipes are widely used in drinking-water distribution systems. In the United States, more than 60 percent of distribution pipes are composed of cast iron, ductile iron, and steel (McNeill & Edwards, 2001). Copper pipe is a primary material for home plumbing systems. Corrosion of all unlined metal pipes is ubiquitous and can result in a contaminant release that adversely affects health and the aesthetic quality of the water.
Corrosion in drinking-water distribution systems results from chemical reactions between pipe metals (or plumbing fixtures) and finished water; it results in formation of a solid corrosion product: scale. Disruption of the solid scale is undesirable, as more contaminants are typically released until new scale is formed. If the composition of the water frequently varies, the equilibrium between the corrosion scale and the water is disrupted, and contaminant release occurs. Many studies have been done on the formation and properties of corrosion scales to gain insight into corrosion mechanisms as affected by water quality in distribution systems (Blengino, Keddam, Labbe, & Robbiola, 1995; Palit & Pehkonen, 2000; Refait, Abdelmoula, & Genin, 1998; Sarin, Snoeyink, Bebee, Kriven, & Clement, 2001). These scales can be porous or impervious. Impervious scales can stop corrosion, but porous scales can accelerate corrosion and increase iron and copper release.
Many water quality parameters, including pH, alkalinity, sulfate, chloride, phosphate, silicates, natural organic matter, dissolved oxygen, disinfectantresiduals, and temperature, can affect iron and copper release under different specific conditions (Boulay & Edwards, 2001; Broo, Berghult, & Hedberg, 1997; Edwards, Schock, & Meyer, 1996; Larson & Skold, 1958; Sander, Berghult, Broo, Johansson, & Hedberg, 1996). The mechanisms involving metal pipe corrosion and corrosion product release, however, are still unclear, and contradictory results have been reported by different researchers.
Because of a lack of well-accepted guidelines for corrosion and metal release control, a pilot study was needed to reveal water quality problems that might occur in a distribution system because of changes in water source and treatment process. Findings from such a study could help utility operators work out effective measures to ensure high-quality drinking water for consumers.
With the joint support of the American Water Works Association Research Foundation (AWWARF) and Tampa Bay Water (TBW) in Florida, a large-scale pilot study was carried out at the University of Central Florida. The purpose was to determine what effect blending finished waters from ground, surface, and saline sources would have on distribution system water quality Treatment and distribution systems were built for this pilot study.
This paper focuses on seven finished waters produced from different treatment systems. The quality of these waters is characterized, and their influences on iron and copper release are interpreted in terms of primary water quality parameters and changing operation conditions.
Water Treatment Systems
Finished waters were produced by seven pilot treatment systems (Table 1). The selection of the treatment processes was based on research interests and the needs of TBW. The integrated membrane system (S2) was selected to provide a point of high water quality. The nanofiltration system (G4) was selected to provide data for a TBW member government that was considering nanofiltration of all treated waters. The softening system (G3) was selected to provide data for a second member government that was considering softening all waters in an existing softening facility. The remaining four systems (G1, G2, S1, and RO) were existing water treatment systems. Source groundwater was drawn from the project site, the Cypress Creek well field in Pasco County, Florida. …