process: on any one day one tank (or pond) is filled with wastewater, the contents of another are settling, and the contents of the third are used for irrigation; this is a very reliable, almost foolproof, system. In small-scale urban agriculture, as opposed to large-farm agriculture, a single tank is generally sufficient (and more affordable): on any day in the morning the tank contents are used for crop watering, and the tank is then refilled and its contents allowed to settle until the following morning.

# _Helminth eggs_

If it is assumed that in areas where ascariasis is hyperendemic untreated wastewater contains 1000 _Ascaris_ eggs per liter (see Table A.8 in Annex A), a 3-log unit egg reduction is required to achieve ≤1 egg per liter. For root vegetables eaten raw, and assuming that a 2-log unit reduction occurs through produce peeling prior to consumption (Table 4.2), wastewater treatment is required to effect a reduction of only 1 log unit from 1000 to 100 eggs per liter. This reduction is also achieved by an anaerobic pond, a three-tank or three-pond system, or overnight settling.

# _4.3.2 Non-treatment options_

Most wastewater used for crop irrigation is untreated and this causes disease in both fieldworkers and consumers (Shuval _et al_ ., 1986; WHO, 2006; Scott _et al_ ., 2010). However, in many parts of the world it would be counter-productive, as well as unenforceable in practice, for governments to ban the use of untreated wastewater for irrigation, given the often informal character of this sector. However, the current situation in many low-income countries in which wastewater irrigation is unrecognized, or where the authorities choose in practice to ignore it, is not sustainable (Drechsel _et al_ . 2002, Ensink and van der Hoek 2009).

Clear policies are needed to balance the agricultural benefits and the associated risks to human health of wastewater use. Ensink and van der Hoek (2009) recommend a set of practical and easily enforceable measures to minimize the health risks associated with the use of untreated domestic wastewater:

- only produce that is cooked before being eaten can be grown with untreated wastewater

- root crops, such as potatoes, onions, and carrots, cannot be grown with untreated wastewater

- vegetables should be cultivated and irrigated using the ridge-and-furrow method

- biannual treatment of fieldworkers and their families with antihelminthic drugs

- general improvements in water supply and sanitation, including in local produce markets, in order to improve post-harvest practices and thus produce quality.

Improving hygiene in local markets can be very important: Ensink _et al_ . (2007) found that vegetable contamination due to poor hygiene in local markets in Pakistan was more important than the quality of irrigation water. Nguyen _et al_ . (2007) came to the same conclusion for fish grown in peri-urban wastewater-fed fishponds in Hanoi. However, in urban Ghana, where the irrigation water is more severely polluted, Amoah _et al_ . (2007a, p. 455) concluded the opposite: Despite poor sanitary conditions in markets, post-harvest handling and marketing did not further increase the farm-gate contamination levels. To reduce the health risk associated with the consumption of contaminated lettuce safer farming and irrigation practices are required, while the remaining risk could best be addressed where lettuce is prepared for consumption.

The variability found in these studies confirms the need to evaluate contamination levels and practices at the local level.

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