Millions eat food watered with wastewater
Millions of hectares of agricultural land around the world are being watered with untreated or partially treated wastewater.
Revealing the results of the study at World Water Week, in Stockholm, Sweden, the IWMI said it is used more commonly for growing vegetables and cereals, especially rice.
The findings have raised fears about health risks for consumers, particularly from vegetables that are consumed uncooked.
"Irrigating with wastewater isn't a rare practice limited to a few of the poorest countries," said IWMI researcher and lead author of the report Liqa Raschid-Sally.
"It's a widespread phenomenon, occurring on 20m hectares across the developing world, especially in Asian countries, like China, India and Vietnam, but also around nearly every city of sub-Saharan Africa and in many Latin American cities as well."
However, the IWMI study found wastewater agriculture makes an important contribution to urban food supplies and helps provide a living for the urban poor, especially women.
She added: "It isn't just affluent consumers of exotic vegetables whose welfare is at stake. Poor consumers of inexpensive street food also depend on urban agriculture."
According to the report, in Ghana's capital Accra, an estimated 200,000 of the city's two million inhabitants regularly buy vegetables produced on just 100 hectares of land irrigated with wastewater.
Consumers across the 53 cities said they would prefer to avoid wastewater produce, but the IWMI said most of the time these shoppers have no way of knowing the origin of the products they buy.
Farmers are also aware of the health risks for themselves and their consumers, but have little choice as safe groundwater is rarely an accessible alternative, the study found.
The IWMI said there are few official guidelines and those that exist are rarely enforced.
It recommended reducing health risks through low-cost methods such as the use of drip irrigation, correct washing of fresh produce, and storing wastewater in ponds to allow suspended solids to settle out.
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