URBAN agriculture is destined to rely increasingly on water and nutrients from sewage, even though irrigation with untreated waste water is currently banned in many countries. Half the urban fields in developing countries are already irrigated with raw sewage.
The practice is illegal in many parts of the world because of possible health dangers. But as the world faces escalating shortages of both water and food, developing countries will have to devise ways to make it safer and lift the bans.
These predictions come in a report by the International Water Management Institute (IWMI) based in Sri Lanka, which is backed by the World Bank. The most detailed of its type, it is based on a survey of 53 cities in Africa, Asia, the Middle East and Latin America.
Advertisement
Charles Chartres, director of the IWMI, warned this week that the proportion of food grown using waste water is bound to increase 鈥渁s growing cities coincide with escalating food shortages to create a squeeze on agricultural water supply鈥�.
Growing 1 kilocalorie of food typically requires 1 litre of water, Chartres says. 鈥淲ith 2.5 billion extra mouths to feed by 2050, that will require at least 2000 cubic kilometres more water annually.鈥� That鈥檚 more than twice the volume of irrigation water now used worldwide.
An estimated 20 per cent of the world鈥檚 food is grown in urban areas. Irrigation water from sewers comes with free fertiliser in the form of the nitrates and phosphates bound up in human faeces. As the oil crisis sends fertiliser prices skyrocketing, this is a resource poor urban farmers can鈥檛 afford to ignore.
Some countries, including Mexico and Tunisia, treat sewage before delivering it to farmers, but for many others this is too expensive. While cultivating crops with sewage is theoretically forbidden, it is unofficially tolerated, say the report鈥檚 authors. They found that in Faisalabad, Pakistan, city authorities auction untreated sewage to farmers during droughts. In Accra, Ghana, 200,000 people have to rely on vegetables grown on urban fields irrigated with waste water that is untreated because the city鈥檚 sewage works have broken down.
Cholera outbreaks have been tied to vegetables irrigated with raw sewage. Waste that includes industrial effluent can also leave food contaminated with heavy metals such as cadmium, zinc and lead at levels well above those deemed safe by the UN. 鈥淥ften there is simply no other water,鈥� says Chartres.
The best answer, the report says, is not to ban waste-water irrigation, but to improve it. Yet partly because the practice is illegal, it has attracted virtually no research.
One simple way to treat waste water is to store it in ponds to allow solids to settle out, including the eggs of intestinal worms, the report suggests. Industrial and domestic waste should be separated to ensure that heavy metals do not get into food. And farmers should wash vegetables in clean water before selling them. 鈥淓ven without expensive infrastructure, common sense measures can make waste-water irrigation safer.鈥�
