Probabilistic microbial risk assessment and management implications for urban water supply systems

Urban drinking-water supplies are still implicated as pathways for the transmission of waterborne disease. A move toward risk informed, proactive water system management has occurred over the past decade and is advocated in current international drinking water guidelines. Quantitative Microbial Risk Assessment (QMRA) is a tool with potential for aiding health risk management; however the refinement of scientific based practical methods to support that philosophy still requires development. This thesis focused on the water utility, its responsibility to manage microbial water safety, and how probabilistic QMRA may aid in developing management strategies. A framework for waterborne disease risk assessment from urban supply systems was derived and tested on an Australian case study. The main premise was that, in order for risk assessment outcomes to inform the management process, the steps should incorporate the concepts of risk variability, the explicit event conditions that can drive it, and that examination of QMRA sensitivity to various risk scenarios/model uncertainties is undertaken. The identified management uses were: (i) prioritising for attention issues hampering the system's ability to meet or the assessor's ability to interpret against (e.g. knowledge gaps about the system), a water quality health target; and (ii) identifying potential strategies or control points for addressing those issues. Additionally, rarely occurring, high impact, adverse fluctuations in treated water quality (and consumer infection risks), especially from source water contaminant "peaks", are highly, nearly totally, influential over the extent of risks averaged over longer, say annual, periods. As such, a case is made calling for widespread adoption of health targets that refer to tolerable consumer risks per exposure, rather than or as well as the current common practice of expressing targets in terms of risks from exposure over a year or lifetime. Doing so may provide incentive and