The development of potable water reuse systems (systems for the treatment of wastewater to quality adequate to augment other surface water supplies) requires careful attention to the consistent production of product water which has low levels of contaminants, such as infectious pathogens, capable of causing human health effects from acute exposure. Little consideration has been given to the formal specification of the degree of reliability of such systems. In this paper we present two ways to approach the problem. The first is a formal extension of the ‘multiple barrier’ concept often cited in the water treatment literature. The second is an application of probabilistic analysis. With either method, it is clear that much more information should be obtained with respect to the failure modes, and the frequency with which individual processes achieve a particular level of performance. However the conceptual framework presented here should enable a formal analysis of the problem to be conducted.

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