5.1 Introduction

This chapter discusses the microbial characteristics of drinking water and provides quantitative health-based targets for assessing microbial risk. It describes the health risks from disease causing microorganisms (pathogens) and toxic cyanobacteria that may be found in drinking water.

The important concepts relating to assessing microbial risk are set out in Section 5.3. The principles and approach for assessing the contamination of source waters and management of those risks are set out in Sections 5.4, 5.5 and 5.6. Adoption of these concepts should be integrated through the 12 Element Framework for Managing Drinking Water Quality outlined in Chapter 3 (see Box 5.1).

Box 5.1

Integrating health-based targets into the Framework for Managing Drinking Water Quality (Chapter 3)

Health-based targets provide a quantitative measure of the microbial safety of drinking water. The Australian Drinking Water Guidelines (the Guidelines) promote preventive risk-based management of drinking water quality from source to consumer with the Framework for Managing Drinking Water Quality (Chapter 3). Health-based targets provide an assessment of enteric pathogen risks in the source water and inform appropriate risk management measures (barriers). These assessment and preventive measures support Elements 2 and 3 of the Framework.

It is expected that implementing health-based targets into drinking water management, particularly for small water suppliers, will take time. Health-based targets are not a pass/fail metric, instead they provide the basis for assessing the level of treatment required to manage source water microbial risks. Shortfalls in achieving the required treatment targets (expressed as $\text{log}_{10}$ reduction values or LRVs) to manage source water pathogen risks should be used to prioritise improvements (see Section 5.4.3 Management of Risk from Enteric Pathogens: Microbial Safety and the Water Safety Continuum).

Focus should be maintained on selecting the best quality source water, catchment protection, multiple barriers and management of critical control points. The introduction of health-based targets should not take focus away from the guiding principles of the Guidelines (see Section 1.1) and implementing all 12 Elements of the Framework for Management of Drinking Water Quality. The introduction of health-based targets must not be used as a licence to degrade source water quality.

This chapter addresses the overarching scientific principles and concepts. A detailed guide for practical implementation is beyond the scope of the Guidelines. Further practical details can be found in the following suggested industry implementation documents, noting that there might be some costs associated with accessing these:

• Deere and Mosse (2016) Water Industry Operators Association of Australia: Practical guide to the operation and optimisation of distribution systems, 3rd ed.

• Water Research Australia (WaterRA 2015) Good practice guide to the operation of drinking water supply systems for the management of microbial risk.

• Water Research Australia (WaterRA 2021), Good practice guide to sanitary surveys and operational monitoring to support the assessment and management of drinking water catchments.

• Water Services Association Australia (WSAA 2015) Manual for the application of health-based targets for drinking water safety.

• World Health Organization (WHO) (WHO 2016) Protecting surface water for health. Identifying, assessing and managing drinking-water quality risks in surface-water catchments.

“Nuisance organisms” that may affect the taste, odour or appearance of water, but do not cause disease (non-pathogenic), are discussed in Section 5.7. Quantitative targets are not included for nuisance microorganisms, but the fact sheet on Taste and Odour in Part V provides guidance on their detection and treatment.

The supply of safe drinking water involves the use of multiple barriers to prevent the entry and transmission of pathogens (see Section 1.1 Guiding principles). The effectiveness of these barriers should be monitored by a program based on operational characteristics (critical control points) and testing for microbial indicators (see Sections 3.4 and 3.5). Advice on monitoring to confirm effectiveness of the preventive measures and barriers to microbial contamination is outlined in Part III (Chapters 9 and 10).