Many health-protection control measures can be applied on-farm, at local produce markets, in streetfood/fast-food outlets and restaurants, as well as domestic kitchens or food preparation areas (Table 4.2).<sup>24</sup> Simple on-farm filtration practices suitable for use in urban agriculture include sand filters made from oil drums or buckets filled with fine-grained sand, soil filtration by digging an infiltration well adjacent to a wastewater canal, and covering the outlet of watering cans with mosquito netting to reduce pathogen-containing debris being applied to the crop. Debris can also be avoided by training urban farmers to collect their irrigation wastewater from well above the sludge layer in shallow sedimentation ponds.

Urban farmers often use watering cans to apply wastewater to their crops. This can be highly contaminating, but the farmers can be trained to use their watering cans in a less contaminating way by slightly changing their customary practice: capping the can outlet with a perforated shower rose, and irrigating from a height of less than 0.5 m, showed a combined average reduction of 2.5 log units of faecal coliforms and more than 2 helminth eggs per 100 g lettuce compared to the practice of uncapped spouts lifted more than 1 m high for irrigating (Drechsel _et al_ . 2008). The pathogen reductions achieved were ascribed to reduced wastewater splashing which minimizes crop contamination by soil-based pathogens which are present from manure application and previous applications of wastewater.

# **4.4 Applying quantitative microbial risk analysis**

# _4.4.1 Recommendations for current practice_

QMRA combined with Monte Carlo risk simulations (QMRA-MC) is a powerful tool to use when assessing the safety of local practices for wastewater use in both large-farm agriculture and smallscale urban agriculture. It is also very easy and fast to use (Mara and Sleigh, 2010d).

If governments wish to regulate wastewater use in agriculture, it would be sensible for them to undertake QMRA risk simulations so that they have an estimate of the health risks associated with the agricultural and food-preparation practices in their locality. Unless they undertake rigorous and expensive epidemiological studies, it is only through QMRA-MC that policymakers can make rational decisions about how these practices can be best improved to reduce to an acceptable level the health risks to those who work in wastewater-irrigated fields and to those who consume wastewaterirrigated foods. Box 4.3 presents two examples of the usefulness of QMRA in making socioeconomic decisions regarding wastewater use for unrestricted irrigation.

The examples in Box 4.3 raise two questions: (1) whether the risks from consuming wastewaterirrigated foods should be so much lower than those from drinking fully-treated drinking water, and (2) whether very large expenditures on wastewater treatment to achieve such very low risks are justified. If the answer to the first question is ‘No’ (and there does not appear to be any valid reason why the answer should not be ‘No’), then it follows that the answer to the second question is also ‘No’. This demonstrates the need to base decisions on _actual_ risks, rather than on merely _potential_ risks (Box 3.2). It also amply demonstrates the power of QMRA to assist in cost-effective decision making.

> 24 Further details are given in Ilic et al. (2010) and Keraita et al. (2010).

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