A3.3 QMRA framework for the calculation of log₁₀ reduction values (LRVs)

QMRA is a framework that can be applied to quantify the required level of drinking water treatment needed to achieve a defined health outcome target. The health outcome target is a quantitative measure of tolerable disease burden from drinking water that allows for quantitative treatment performance targets to be established. In this case, the health outcome target is defined as 1 x $10^{-6}$ Disability Adjusted Life Years (DALYs) per person per year (pppy) (see Box A3.1). This approach is intended to ensure that treatment is designed to be in proportion with the level of source water microbial contamination (i.e. that treatment is sufficient but not over engineered). The process of applying QMRA to quantify the required pathogen reduction is illustrated in Figure A3.2 (WHO 2016).

Figure A3.2 QMRA framework applied to quantify LRVs for drinking water treatment (adapted from WHO 2016)

The concentration of reference pathogens in source water is quantified as far as reasonably practicable. This is based on available microbial data and estimates of water consumption volume and health impact data (i.e. the potential DALY burden of disease outcomes are predicted: infectivity, morbidity, mortality and disability weightings). The critical pathogen reduction level (expressed as an LRV) is determined to meet the health-based outcome target. QMRA is a logical mathematical framework that relies on scientific data to inform the underlying model assumptions. The following sections summarise the selected model inputs.