Microbial health-based targets for drinking water: current state and Australian case study

Through the avoidance of a substantial health burden globally, access to safe drinking water is an important foundation of public health¹. An emerging development in this regard is the use of public health metrics, such as disability-adjusted life years, to inform water safety planning². This paper examines the hypothesis that confidence in the protection of public health, on the part of water suppliers, health regulators, and ultimately consumers is strengthened through the implementation of a health outcome target for the microbial safety of drinking water. A case study demonstrating the implementation of the target is presented.

The recent contamination of Campylobacter spp. in drinking water supplies in Havelock North, New Zealand highlights that access to safe drinking water remains an area of importance to public health even in developed countries. It is estimated that, of the 14 000 inhabitants, 5500 (39%) contracted campylobacteriosis³. This was not an isolated issue: waterborne outbreaks in developed countries continue to be attributable to deficiencies in municipal drinking water services⁴. Even seemingly unrelated disturbances to municipal drinking water supply can be implicated in microbial disease outbreaks. Secondary impacts of the water supply lead contamination in Flint, Michigan, USA are thought to include the increased incidence of shigellosis and Legionnaires’ disease^5,6. Further, health impacts of waterborne microbial contaminants are not limited to infectious disease; elevated levels of microcystins (a group of toxic cyanobacterial metabolites) triggered the issuance of a precautionary ‘do not drink’ notice for the 400 000 inhabitants of Toledo, Ohio, USA, lasting several days in 2014⁷. Thus, drinking water supplies represent an environmental exposure route of far-reaching and wide-ranging public health consequence. Comprehensive planning for safe water supplies is therefore a prudent investment in public health.

The WHO promotes the use of a preventive risk management system (the water safety plan) as an international norm for the assurance of drinking water safety⁸. Water safety plans facilitate the establishment of a comprehensive set of preventive management actions for the quality assurance of drinking water safety, similar to the hazard analysis and critical control point (HACCP) principles and ISO 22000 standard for food safety management^9,10. Its use has been demonstrated to be an effective public health intervention^11–13. Water safety planning further considers the need to meet the unique challenges associated with drinking water supply. Challenges include variation in quality of source water, the need for continuity of supply, and the consideration of aesthetic and other physical characteristics¹⁰.

In Australia, water safety planning is authoritatively guided by the Australian Drinking Water Guidelines¹⁴. Since 2004, the Australian Drinking Water Guidelines has recommended preventive risk management of water supplies through the Framework for Management of Drinking Water Quality. In effect, implementation of water safety planning (in a manner consistent with the Australian Drinking Water Guidelines or an analogous regime) is required by the health regulators of all Australian states, the Northern Territory, and the Australian Capital Territory. The most recent development in Australian water safety planning is the expected incorporation of a health outcome target for drinking water treatment into the Australian Drinking Water Guidelines^2,15. The health-based target approach is currently under development by the National Health and Medical Research Council to be included in the revised Guidelines. The target, 10^–6 disability-adjusted life years per person-year, can be met by having drinking water treatment performance limits set commensurate to the microbial challenge expressed by the source water^16,17.

The Water Services Association of Australia method for the derivation of water treatment process criteria for the achievement of the health outcome target¹⁶ was followed for a large water treatment plant in New South Wales. A graphical representation of the method and the resultant set of process criteria are shown in Figure 1. The plant’s historical drinking water treatment process data, as recorded through the supervisory control and data acquisition (SCADA) system, were then compared to these criteria. They were found to be consistent with meeting the targeted health outcome. The target’s associated water treatment process criteria were then codified as critical limits—demarcating acceptable water treatment performance—under the water safety plan and SCADA system. This was to allow the water treatment plant operators an ongoing, instantaneous indication of the target’s achievement. It also resulted in later infrastructure planning necessarily considering the water treatment plant’s continued ability to meet the targeted health outcome over time.

Figure 1. Derivation of water treatment process criteria for the achievement of the health outcome target for the microbial safety of drinking water, based on the Water Services Association of Australia method, showing results for a large water treatment plant in New South Wales, Australia. QMRA, quantitative microbial risk assessment; HBT, health-based target of 10^–6 disability-adjusted life years per person-year; IFE, individual filter effluent; CFE, combined filter effluent; Ct, primary disinfectant contact-time; NTU, nephelometric turbidity units.

The introduction of the health outcome target signifies a challenge and opportunity. For water suppliers and regulators, its implementation is a non-trivial investment of effort yet offers a substantial opportunity to gain increased confidence in the adequacy of drinking water treatment. It can thus provide valuable evidence for improvement to operations and infrastructure for the achievement of safe drinking water supplies.