3.2.3 Hazard identification and risk assessment
Summary of actions
Define the approach and methodology to be used for hazard identification and risk assessment.
Identify and document hazards, sources and hazardous events for each component of the water supply system.
Estimate the level of risk for each identified hazard or hazardous event.
Evaluate the major sources of uncertainty associated with each hazard and hazardous event and consider actions to reduce uncertainty.
Determine significant risks and document priorities for risk management.
Periodically review and update the hazard identification and risk assessment to incorporate any changes.
Effective risk management requires identification of all potential hazards, their sources and hazardous events, and an assessment of the level of risk presented by each. A structured approach is important to ensure that significant issues are not overlooked and that areas of greatest risk are identified.
In this context:
A hazard is a biological, chemical, physical or radiological agent that has the potential to cause harm.
A hazardous event is an incident or situation that can lead to the presence of a hazard (what can happen and how).
Risk is the likelihood of identified hazards causing harm in exposed populations in a specified timeframe, including the severity of the consequences.
The distinction between hazard and risk is important: attention and resources need to be directed to actions selected primarily on the basis of level of risk, rather than just the existence of a hazard.
To give an example, the protozoan parasite Cryptosporidium parvum is a hazard; failure at a water treatment plant leading to C. parvum passing into the distribution system is a hazardous event; and the likelihood of the organism being present in source water and passing through the treatment plant in sufficient numbers to cause illness is a risk.
Realistic expectations of hazard identification and risk assessment are important. Rarely will enough knowledge be available to complete a detailed quantitative risk assessment. Hazard identification and risk assessment are predictive activities that will often include subjective judgments, and will inevitably contain uncertainty. Given these inherent limitations, flexibility is vital, to ensure an effective response when the unexpected occurs. Staff should have a realistic understanding of the limitations of these predictions, and this should also be conveyed to the public.
A consistent methodology should be established for both hazard identification and risk assessment. The methodology needs to be transparent and fully understood by everyone involved in the process. Staff should be included and need to be aware of the outcomes of the risk assessment.
Hazard identification
A comprehensive list of potential hazardous agents in drinking water is provided in Part V. Hazardous agents include microbial, chemical, physical and radiological agents. All potential hazards, sources and events that can lead to the presence of these hazards (what can happen and how) should be identified and documented for each component of the water supply system, regardless of whether or not the component is under the direct control of the drinking water supplier. This includes point sources of pollution (e.g. human and industrial waste discharges) as well as diffuse sources (e.g. those arising from agricultural and animal husbandry activities). Continuous, intermittent or seasonal pollution patterns should also be considered, as well as extreme and infrequent events such as droughts or floods.
The hazard identification and risk assessment should be reviewed and updated periodically because changing conditions may introduce important new hazards or modify risks associated with identified hazards.
Risk assessment
Once potential hazards and their sources have been identified, the level of risk associated with each hazard or hazardous event should be estimated so that priorities for risk management can be established and documented. Although there are numerous contaminants that can compromise drinking water quality, not every potential hazard will require the same degree of attention.
The level of risk for each hazard or hazardous event can be estimated by identifying the likelihood of occurrence (e.g. certain, possible, rare) and evaluating the severity of consequences if the hazard were to occur (e.g. insignificant, major, catastrophic). The aim should be to distinguish between very high and low risks.
An example of an approach to estimating the level of risk is provided in Tables 3.1–3.3. These tables have been adapted from AS/NZS 4360:2004 (Risk Management), and can be modified to meet the needs of an organisation.
Chapter 5 introduces the use of quantitative microbial risk assessment (QMRA) which investigates the likelihood of disease along a risk pathway from the point at which pathogen concentration is quantified (e.g. in a water source) to the receptor (e.g. a consumer of drinking water). QMRA involves quantifying each component of the exposure pathway, together with the estimated health outcome for specific reference pathogens (currently representatives for human enteric viruses, bacteria and parasitic protozoa). The outcome of a QMRA is a quantitative assessment of risk and is most applicable for answering quantitative questions such as: “What is safe?” and “How much treatment is required to achieve safety?”. Figure A3.1 in Appendix 3 provides an overview of the QMRA process.
A likely outcome of risk assessment is the identification of specific areas where further information and research is required (see Box 3.7 in Section 3.9).
Risk prioritisation
Based on the assessment of risks, priorities for risk management and application of preventive measures can be established. Risk should be assessed at two levels:
maximum risk in the absence of preventive measures; and
residual risk after consideration of existing preventive measures.
Assessing maximum risk is useful for identifying high priority risks, determining where attention should be focused and preparing for emergencies. Residual risk provides an indication of the need for additional preventive measures.
Unforeseen and rare events
In well managed systems, problems should be rare, making them more challenging to anticipate and possibly to counter. This highlights the need to learn constructive lessons from the experiences of other Australian and international drinking water suppliers and water agencies. Many problems are triggered by short periods of sudden change, such as heavy rainfall or equipment failure. There are catalogues of waterborne disease outbreaks and the events that caused them. Some of these events should have been foreseeable while others have been attributable to more unusual or rare events. Maintaining awareness of such incidents can enable preventive measures to be implemented, to safeguard against similar occurrences (see Box 3.3 in Section 3.4).
Table 3.1 Qualitative measures of likelihood
A
Almost certain
Is expected to occur in most circumstances
B
Likely
Will probably occur in most circumstances
C
Possible
Might occur or should occur at some time
D
Unlikely
Could occur at some time
E
Rare
May occur only in exceptional circumstances
Table 3.2 Qualitative measures of consequence or impact
1
Insignificant
Insignificant impact, little disruption to normal operation, low increase in normal operation costs
2
Minor
Minor impact for small population, some manageable operation disruption, some increase in operating costs
3
Moderate
Minor impact for large population, significant modification to normal operation but manageable, operation costs increased, increased monitoring
4
Major
Major impact for small population, systems significantly compromised and abnormal operation if at all, high level of monitoring required
5
Catastrophic
Major impact for large population, complete failure of systems
Table 3.3 Qualitative risk analysis matrix: level of risk
A (almost certain)
Moderate
High
Very high
Very high
Very high
B (likely)
Moderate
High
High
Very high
Very high
C (possible)
Low
Moderate
High
Very high
Very high
D (unlikely)
Low
Low
Moderate
High
Very high
E (rare)
Low
Low
Moderate
High
High
Uncertainty
There will always be uncertainty associated with hazard identification and risk assessment. Uncertainty can be caused by a lack of knowledge or by variability in parameters. While variability can only be better understood (e.g. by improved characterisation of a hazard), uncertainty due to lack of knowledge can be reduced through better measurement and research. For example, uncertainty in our ability to identify the source, human infectivity or infectious dose of Cryptosporidium oocysts can be addressed through increased research.
Characterising the major sources and types of uncertainty can provide a better understanding of the limitations of the hazard identification and risk assessment and how these limitations can be reduced. Investigative studies and research monitoring can often provide further information for the risk assessment process and help to reduce uncertainty (see Section 3.9).
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