Recycling urban wastewater and using it to grow food crops can help mitigate water scarcity problems and reduce water pollution, according to a new FAO report.

Use of reclaimed wastewater in agriculture has been reported in around 50 countries on what amounts to 10 percent of the world's irrigated land, according to a report published at the start of the World Water Week by the Rome-based FAO.

While on a global scale only a small proportion of treated wastewater is used for agriculture,?and in a few countries like Spain and Mexico, for example, use?a high proportion of reclaimed water in irrigation.

"The case studies in this report show that safely harnessing wastewater for food production can offer a way to mitigate competition between cities and agriculture for water in regions of growing water scarcity," said Pasquale Steduto, deputy director of FAO's Land and Water Division.

"In the right settings, it can also help to deal with urban wastewater effluent and downstream pollution," he said.

Farmers would also be able to avoid some of the costs of pumping groundwater, while the presence of nutrients in the wastewater would reduce their fertilizer expenses.

"Properly treated and safely recycled water can potentially offer a 'triple dividend' to urban users, farmers and the environment," said Steduto.

Benefits offset costs

While building treatment and recycling systems that are capable of adequately handling wastewater does involve both capital investment up front and ongoing operating costs, the major benefit of such schemes is likely to be the value of the fresh water freed up for high-value urban or industrial use — this would reduce the cost to municipal authorities of seeking additional supplies via more expensive means.

And costs could be further offset by harnessing biogas generated during intensive treatment as an energy source, or potentially through the sale of carbon credits.

"While re-using wastewater in agriculture is not the only way to tackle problems of scarcity and pollution, it is in many situations an extremely a cost-effective solution, as the growing number of reuse schemes that we look at in this report testify," Steduto said.

Location is everything

The feasibility of reusing water in agriculture depends on local circumstances and conditions, which will affect the balance of costs and benefits, FAO's report notes.

Economic appraisal of any proposed projects should be made from a regional basin viewpoint, and it will also be necessary to factor the needs of and benefits to various water users, it says.

"It is unlikely that such schemes could be economically justified with reference only to agriculture," the report says. "The benefits to urban and industrial users would be relatively sizeable, and in most cases would be the principal justification for the project."

FAO's report also stresses that raw or untreated wastewater is inappropriate for use in irrigation — adequate treatment and recycling is always required.