By Mark Kinver
Science and nature reporter, BBC News
The survey found that 1.1m urban farmers used wastewater for irrigation
Urgent action is needed to remove pollutants from urban wastewater, which is often used in cities to grow food, an international study has warned.
Data collected by the International Water Management Institute (IWMI) found that 85% of cities discharged the water without any appropriate treatment.
With many developing nations swiftly urbanising, the authors said people were at increasing risk of disease.
The findings are being presented at an international water summit in Sweden.
"As the world flips over from a predominantly rural to a predominantly urban population base, cities are going to take more and more water for agriculture," explained IWMI director general Colin Chartres.
"However, most of the water going into urban areas comes out the other end in the sewers," he told BBC News.
"We know that there is an informal sector within many cities that is using [wastewater] to grow vegetables, but there has been no data on how much of this water was being used or what the risks were."
Waste not, want not
The study, based on case studies from 53 cities in developing nations, examined where wastewater was being generated, how much was being used in urban agriculture, and to what degree the water was being treated.
Food irrigated with wastewater can ease the pressure on supplies
With increasing food prices and growing concerns about water scarcity, the authors of the report highlighted a number of benefits of using wastewater to irrigate crops.
They said that it allowed food production in places where there was a lack of water, or where no alternative clean water sources were available.
It also recycled nutrients, meaning that farmers did not have to buy expensive fertilisers.
And irrigating farmland with wastewater also has environmental benefits, explained Dr Chartres.
"It is a pretty useful way of treating water in the sense that if the water just went straight into a river, it would cause a lot more eutrophication problems further downstream.
"So in a way it is performing an ecological service by cleaning up some of the water and recycling the nutrients."
Samples of wastewater taken at regular intervals from a river in India, showing how the water gets cleaner as it flows through natural wetlands
However, Dr Chartres warned that using wastewater for irrigation was not risk free, especially as the world became more urbanised.
"If this practice is going to be increasingly commonplace and more and more people are going to be eating food produced this way, then there needs to be a bit more concern about the heavy metals and other contaminants in there.
WHAT IS WASTEWATER?
Urban wastewater is usually a combinations of one or more of the following:
domestic effluent, including "blackwater" (excreta, urine) and "greywater" (water from kitchen sinks etc)
water from commercial buildings and institutions, eg hospitals
storm and other urban run-off
"Ideally, the end product should be treating the water to a standard that means there is no risk, but most developing nations cannot afford to do this.
"Apparently, what happens now in areas with very polluted water is that the farmers do a smell test or a taste test," he added.
"If the water tastes too foul or smells too bad, then they won't use it to irrigate their crops - but that's a pretty dangerous way to go about things."
The IWMI report found that wastewater was being used in 80% of the cities surveyed.
This equated to 5.6 million farmers and family members dependent upon the wastewater in order to earn a living.
It added that very often industrial and domestic waste streams were mixed together. Ensuring the two were kept apart would reduce the risks from chemical contamination.
"It is really just about minimising the risks from field to fork with a series of simple measures," Dr Chartres explained.
"[These include] letting the water settle in a pond, so a lot of the eggs from worms drop out of the water, and irrigating around the crops rather than on top of them.
"When the crop is harvested, it also needs to be washed with fresh, clean water in the market, and that water needs to be constantly changed so everything else is not contaminated."
The authors said that the international community needed to develop policies and practices to reduce the health and environmental risks, while maintaining the financial and food production benefits.
The findings are being presented at the World Water Week summit in Stockholm, Sweden - a week-long conference attended by water and sanitation experts from more than 140 countries.