The pathways, through which water quality in the distribution network can be compromised, may be classified into five categories: intrusion of contaminants into the distribution system (e.g. through cross connection); regrowth of bacteria in pipes and distribution storage tanks; water treatment breakthrough; leaching of chemicals or corrosion products from system components (pipes, tanks, liners, etc.); and permeation of organic compounds through plastic pipe and pipe components in the system.

Quantification and characterization of the various risk factors in water distribution systems is a difficult task. Many kilometres of pipes of different ages and materials, uncertain operational and environmental conditions, unavailability of reliable data, and lack of understanding of some factors and processes affecting pipe performance make it extremely challenging. It is often difficult to identify or validate specific cause(s) for water contamination or waterborne disease outbreak because real-time data are rarely, if ever, available. For these reasons, high uncertainties are inherent in any risk measure that may be assigned to the distribution system. Further, the current inability to precisely quantify most of these risks may warrant the usage of a quantitative–qualitative framework.

In this paper, a framework is presented for the analysis of aggregative risk associated with water quality failure in the distribution system. Each risk item is defined by the product of the likelihood of a failure event and its consequence (peril). Both the likelihood and the consequences of a failure event are defined using fuzzy numbers to capture vagueness in the qualitative linguistic definitions. A multi-stage hierarchical model of aggregative risk for water quality failure is developed. An analytic hierarchy process is used for estimating the priority matrix (weights) for grouping risk attributes. The framework is applied on a simplified structure of risk hierarchies for a water distribution system.

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