System assessment is the part of the Water Safety Plan that evaluates whether a water supply system is capable of producing drinking water that meets the health-based targets. System assessment can be done at increasing level of detail, requiring more site specific information as the level of detail increases. Four case studies are presented with increasing level of detail, showing the type of information that is required for each of these levels and how each level informs risk management. The first case study shows how a system assessment can be performed without other site specific information than the type of source water and the type of treatment processes. The required data for the system assessment are collected from the large body of literature available. The second case study uses site specific microbial indicator data. The third study uses pathogen data and the fourth case study combines data on pathogens, microbial indicators and process parameters. The case studies show that the level of detail required largely depends on the risk management question.

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