WASHINGTON, Aug. 18 (Xinhua) -- A new 53-city survey conducted by the International Water Management Institute (IWMI) indicates that most cities are using untreated or partially treated wastewater for agriculture.

Wastewater use is critical to farmers' incomes and urban food security but raises health concerns, IWMI said in a statement released on Monday.

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.

"Irrigating with wastewater isn't a rare practice limited to a few of the poorest countries," said IWMI researcher Liqa Raschid-Sally and lead author of a report on survey results. "It's a widespreadphenomenon, occurring on 20 million hectares across the developing world, especially in Asian countries, but also around nearly every city of sub-Saharan Africa and in many Latin American cities as well."

Wastewater is most commonly used to produce vegetables and cereals, according to the report, raising concerns about health risks for consumers, particularly of vegetables that are consumed uncooked.

But 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.

"The negative and positive implications of wastewater agriculture have only recently received attention," noted Colin Chartres, director general of IWMI. "This study offers the first comprehensive, cross-country analysis of the conditions that account for the practice and the difficult tradeoffs that arise from it."

Survey results on the forces driving wastewater use in urban agriculture suggest that it is not only widespread but practically inevitable. As long as developing countries lack suitable transport for delivering large quantities of perishable produce to urban areas, vegetable production in urban agriculture will remain important. And in the face of water scarcity generally and a lack of access to clean water, urban farmers will have no alternative except to use diluted or untreated wastewater or polluted river water.

Under those conditions, the report asserts, extreme measures, like banning the use of polluted water, or even stricter water quality guidelines are of no avail. In fact, they could adversely affect urban consumers, farmers and others who depend on urban agriculture.

The report praises new guidelines established by the World Health Organization, which replace often unachievable water quality thresholds with more realistic health-based targets. As a result, countries lacking the means to treat wastewater adequately can still reduce health risks through low-cost interventions, such as the use of drip irrigation and correct washing of fresh produce.

Another option is to build on a wide range of innovative indigenous practices that can greatly reduce the health risks from wastewater agriculture. According to the report, in Indonesia, Nepal, Ghana and Vietnam, farmers store wastewater in ponds to allow suspended solids to settle out. This practice also permits worm eggs to settle out, possibly reducing bacteria in the water. 　
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