## **Box  4.3 Examples of the usefulness of QMRA**

Shuval _et al_ . (1997) used QMRA to estimate that the risk of hepatitis A from eating 100 g of lettuce irrigated with treated wastewater containing 1000 fecal coliforms per 100 mL (the 1989 WHO Guideline) on alternate days was ~10<sup>−7</sup> per person per year (pppy). They also determined that the extra expenditure required to treat wastewater from ≤1000 fecal coliforms per 100 mL to the 1992 USEPA/USAID guideline value of zero fecal coliforms per 100 mL, which would reduce the risk to ~10<sup>−10</sup> pppy, was US$35 million per case of hepatitis A averted. This is a clearly unjustifiable expenditure—a 300-bed hospital would cost about the same as 3−5 cases of hepatitis A averted.

Tanaka _et al_ . (1998) used a dataset of viral concentrations in the effluents of advanced wastewater treatment plants in California, designed to achieve the Californian standard of ≤2.2 total coliforms per 100 mL for unrestricted irrigation to show by QMRA that the infection risks to consumers of wastewater-irrigated salad crops irrigated with ‘fully’ treated wastewater (‘fully’ here means a 5.2-log unit virus reduction after primary, secondary and tertiary treatment and chlorination) were in the range 10<sup>−8</sup> −10<sup>−10</sup> pppy—i.e., 4−6 orders of magnitude lower than the value of 10<sup>−4</sup> pppy accepted by US EPA as the tolerable waterborne disease risk from drinking fully-treated drinking water (Macler and Regli, 1992).

QMRA-MC risk simulations can also be used to investigate “what if” scenarios: a question such as “What would the health risks be if people started to eat more lettuce?” can be answered simply by using the appropriate ranges of lettuce consumption in the QMRA-MC computer programs to reflect the anticipated increases in lettuce consumption.<sup>25</sup> Moreover Table 4.2 can be used to answer questions such as “If a campaign to improve the foodhygiene practices of street-food vendors were successful, what effect would this have on the required level of wastewater treatment?” Box 4.4 details the successful QMRA investigations in urban Ghana where there was concern about the adverse health effects resulting from the consumption of wastewater-irrigated lettuce.

# _4.4.2 Current challenges_

QMRA combined with Monte Carlo risk simulations (QMRA-MC) is a powerful tool for assessing the safety of local practices for wastewater use in agriculture. However, there are several challenges that will need to be met if QMRA is to be adopted in countries where wastewater is used in agriculture. These challenges will be particularly acute in low- and middle-income countries.

1. Regional training courses in QMRA need to be established so that local sanitary/environmental and agricultural engineers and water-resource planners become competent in its use and application to local conditions.

2. Local regulators need to be trained in QMRA so that they can establish locally appropriate regulations for wastewater use in agriculture that reflect the local epidemiology of excretarelated diseases and that are both cost-effective and effective in protecting the health of those who work in wastewater-irrigated fields and consume wastewater-irrigated foods.

3. Government officials and extension workers need to understand the rationale of QMRAbased approaches to controlling the health risks posed by wastewater use in agriculture, in

> 25 Lettuce consumption might well increase if the Ministry of Health were to encourage people to eat more fresh fruit and vegetables (see the ‘Five a Day’ campaign by the UK government at <u>http://www.5aday.nhs.uk).</u>

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