In pursuing their mandate of providing good and safe drinking water, water system operators and asset owners work continuously towards developing and maintaining the trust and confidence of their customers. The goal of developing and maintaining customer trust has led to an increased emphasis in the water sector to explicitly assess and manage risks. In this paper, we introduce Layers of Protection Analysis (LOPA) as an approach to assessing technical and organizational risk in the water industry. LOPA offers a robust, rational and defensible approach for assessing the adequacy of independent protection layers (IPLs), such as treatment units, standard operating procedures and incident response procedures, used to mitigate environmental and process risks in water supply systems. The strength of LOPA is not only its ability to facilitate a review of technical systems' reliability, but also its versatility to be used for the review of organizational resilience under trying conditions and the opportunity to learn from failure. This is demonstrated in this paper with a review of more than 400 incidents affecting a large water utility serving some six million customers between 1997 and 2006.

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