**Table 4.1 A framework for the management of microbial risks from wastewater use in agriculture**

|**Step objective**|**Step activities and comments**|
|---|---|
|**Step 1**<br>Define tolerable maximum<br>additional burden of disease|The metric for disease burden is the ‘disability-adjusted life year’ (DALY) (see<br>Box 4.1). The 2006 WHO Guidelines used a default value of ≤10<sup>−6</sup>DALY loss<br>per person per year (pppy) as consistent with accepted risks from drinking fully<br>treated drinking water. A more appropriate value, at least initially in low- and<br>middle-income countries, is ≤10<sup>−4</sup>DALY loss pppy (see section A.1 of Annex<br>A).|
|**Step 2**<br>Derive tolerable disease and<br>infection risks|The tolerable risk of a disease per person per year is obtained from the<br>equation: “tolerable DALY loss pppy ÷ DALY loss per case of the disease”,<br>and the tolerable risk of infection pppy from the equation: “tolerable disease<br>risk ÷ disease/infection ratio” (see section 4.2.1).|
|**Step 3**|QMRA-Monte Carlo simulations (Box 4.2) are used to estimate the required<br>|
|Conduct quantitative<br>microbial risk analyses to<br>determine required<br>minimum total pathogen<br>reductions|minimum total pathogen reductions—for example, from raw wastewater to<br>ingestion—such that the tolerable risk of infection determined in Step 2, and<br>hence the tolerable additional burden of disease selected in Step 1, are not<br>exceeded. The exposure scenario of involuntary ingestion of wastewater-<br>contaminated soil particles is used for restricted irrigation, and that of the<br>consumption of wastewater-irrigated foods eaten uncooked (salad crops, some<br>vegetables) for unrestricted irrigation. The ranges of parameter values used in<br>the QMRA-Monte Carlo simulations must be chosen to reflect local<br>circumstances (see Table A.7 in Annex A).|
|**Step 4**<br>Determine how the required<br>pathogen reductions are to<br>be achieved|The pathogen reduction to be achieved by wastewater treatment is that required<br>to protect the health of those who work in wastewater-irrigated fields (i.e.,<br>restricted irrigation)—but see section 4.3.2 for non-treatment options. If<br>unrestricted irrigation is practiced additional post-treatment health-protection<br>control measures must be selected to protect the health of those who consume<br>wastewater-irrigated foods (Table 4.2); alternatively, but entailing additional<br>costs, the degree of wastewater treatment may be increased.|
|**Step 5**<br>Verification monitoring|Verification monitoring must be carried out to confirm that the required<br>pathogen reduction is being achieved by wastewater treatment (details are given<br>in Annex B).<br>Unrestricted irrigation: a Hazard Analysis Critical Control Point (HACCP)<br>system should be put in place to monitor the efficacy of the health protection<br>control measures listed in Table 4.2 (details are given in section B.2 of Annex<br>B).|

_Sources:_ Adapted from WHO, (2006; and Godfree and Godfrey, 2008.

This approach includes both wastewater treatment and post-treatment health-protection control measures (Table 4.2 and section 4.3.1) and non-treatment options to be used in circumstances where, for one reason or another, wastewater treatment does not exist and is unlikely to exist for the immediate future (Table 4.2 and section 4.3.2).

QMRA determines numerical values for the health risks that result from particular sets of conditions relating to local practices in using wastewater in agriculture. As illustrated in Annex A, it is relatively straightforward to use the results from QMRA to select the most cost-effective set of conditions—i.e., 34