1.3.3 Microbial health-based targets

The health outcome target of 1 µDisability Adjusted Life Years (1 µDALY) per person per year (pppy) is used to determine the level of treatment, expressed as log reduction values (LRVs), required to remove enteric pathogens that may be present in source waters. The rationale is that meeting identified treatment performance targets will ensure that the health outcome of 1 µDALY pppy is not exceeded. LRVs are calculated for reference pathogens selected as conservative models for the three groups of enteric microorganisms (protozoa, viruses and bacteria). In the absence of system-specific pathogen data LRVs are typically calculated on the basis of source water vulnerabilities and E. coli results from raw water monitoring. Where sufficient system-specific pathogen data is available to enable calculation of LRVs, as shown in Figure 1.3, it is still important to undertake the default approach to support confidence in the identification of LRVs (see Box 5.7). Significant differences in the two sets of LRVs should lead to further investigations.

While the aim for all drinking water supplies should be to meet the target of 1 µDALY pppy, the Guidelines incorporate a level of flexibility in meeting the target in the form of a water safety continuum. The continuum provides a basis for designing and implementing water quality improvement programs. It is essential that the continuum should only be applied in consultation with the relevant health authority or drinking water regulator.

Exceeding the target of 1 µDALY pppy or a modified target based on application of the water continuum should be a signal to investigate the cause and, if appropriate, to take remedial action. See Section 5.4.3 for more information on microbial safety and the water safety continuum.