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three areas related to distribution system integrity: physical, referring to the pipes as barriers; hydraulic, referring to the delivery of water at the desired quantity and pressure; and quality, referring to the maintenance of the water quality through its travel in the distribution system. All three areas of integrity must be addressed to ensure public health. High-priority areas for risk reduction identified by the panel included cross-connections, new and repaired water mains, and water storage (NRC, 2005).

Distribution systems represent the next frontier of research needs and technology challenges for the drinking water industry. In addition to the infrastructure replacement and management challenges, water-quality issues are highly scrutinized by the public and media. Tools are emerging to address these challenges but considerable work remains to fully address the issues.

DISTRIBUTION SYSTEM WATER QUALITY

One key to the protection of public health in the distribution system is the maintenance of a disinfectant residual, typically in the form of free chlorine or chloramine. As water travels through the distribution system the disinfectant oxidizes material in both the bulk water and at the pipe wall surface, thereby reducing the residual available to maintain disinfection (Figure 1). At the pipe wall the chlorine can react with corrosion products, sediments, and biofilms. Biofilms have been shown to grow on most common pipe materials, but the quantity of attached bacteria is several orders of magnitude higher on unlined cast iron pipes (Camper et al., 2003).

Drinking water regulations place limits on the minimum and maximum amounts of disinfectant allowable in the water. In addition, several classes of disinfection byproducts are regulated. These compounds, which form when chlorine

FIGURE 1 Schematic of chlorine decay reactions in a distribution system. Source: Adapted from Speight, 2003.



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