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August 20, 2008

Much of world agriculture depends on waste water

Vegetables, rice and other cereals in at least 53 cities in Asia, Africa and Latin America may someday come with warning labels that read "this is a byproduct of raw sewage."

Against the backdrop of rising food prices and a shortage of drinking water worldwide, urban farmers are being forced to use either untreated wastewater or polluted river water both for their agricultural needs and for their economic survival.

A 53-city survey finds the practice most common in some of the world's poorer nations where wastewater use is critical both to farmer's incomes and urban food security while simultaneously raising critical health risks.

The study conducted by the Sri Lanka-based International Water Management Institute (IWMI) -- and released to coincide with World Water Week in the Swedish capital of Stockholm -- indicates that about 80 percent of the cities surveyed are using untreated or partially treated wastewater for agriculture.

In over 70 percent of the cities studied, more than half of urban agricultural land is irrigated with wastewater that is either raw or diluted in streams. The use of waste water, which usually includes sludge, industrial effluent, kitchen and bathroom waste, is a widespread phenomenon, occurring on 20 million hectares across the developing world, according to the survey.

This is particularly prevalent in Asian countries, including China, India and Vietnam, but also around nearly every city of sub-Saharan Africa, as well as in many Latin American cities, including Lima, Santiago, La Paz, Bogota and Sao Paulo.

The study says that wastewater is most commonly used to produce vegetables and cereals, especially rice, raising concerns about health risks for consumers, particularly in vegetables consumed uncooked. At the same time, wastewater agriculture contributes importantly to urban food supplies and helps provide a livelihood for the urban poor, especially women, and recent migrants from the countryside.

As a notable example, the study points out that Accra, Ghana's capital city with an urban population of nearly 2.0 million people, illustrates those tradeoffs particularly well. An estimated 200,000 of the city's inhabitants make daily purchases of vegetables produced on just 100 hectares of urban agricultural land irrigated with wastewater.

Consumers across the 53 cities said they would prefer to avoid wastewater produce. But most of the time, they have no way of knowing the origin of the products they buy. Farmers too are aware that irrigating with wastewater may pose health risks both for themselves and the consumers of their produce, but they simply have little choice, since safe groundwater is seldom an accessible alternative, according to the IWMI report.

Liqa Raschid-Sally of IWMI-West Africa and lead author of the study, told IPS there are no comprehensive studies on benefits versus risks. "But it is abundantly clear that if you put an immediate stop to this, you will certainly cut off supplies of some types of vegetables to cities, as almost 75 percent of the cities source at least some of their vegetables from urban and peri-urban agriculture which uses wastewater." Simple economics, she argued, will indicate that this would cause a rise in vegetable prices in cities.

The health risks can be managed because there are various interventions on farms, markets and households which can effectively reduce the health risks. In Indonesia, Nepal, Ghana and Vietnam, for example, farmers store wastewater in ponds to allow suspended solids to be eliminated. And inadvertently, this practice also permits worm eggs to settle out, possibly reducing bacteria in the water.

In Ouagadougou, Burkina Faso, farmers using water from a brewery build storage basins for wastewater and fill them only when they judge the quality of the wastewater to be acceptable (that is, not acidic), based on its appearance, odor and even taste.

So the message to the poorest nations is that they can apply these methods to reduce the risks and it just needs to be incorporated within agriculture extension services, Raschid-Sally said. "This is not an encouragement to use wastewater but a push to improve practices," she added.

To maintain the urban supply with these vegetables, the WHO is recommending multiple barriers which can easily reduce the pathogen level on the crop.

An officiall said there is a significant water quality dimension which is interlinked: "On the one hand, where wastewater is not or only partially treated it is polluting and further diminishing our clean water resources. On the other hand, where we are able to make an asset out of wastewater, we are gaining a valuable resource back."


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