The future may not smell too rosy – it may lie in sewage. As cities and industries suck up ever more of the world’s scarce water resources, agriculture is destined to rely increasingly on recycling the contents of urban sewers, according to a new international study of “wastewater agriculture”.

The good news – for farmers at least – is that the irrigation water from sewers comes with free fertiliser in the form of the nitrates and phosphates bound up in human faeces. The bad news is that this coprological cornucopia is filling vegetables sold in city markets with heavy metals, pathogenic bacteria and worms.

An estimated one fifth of the world’s food is growing in urban areas, with perishables like vegetables to the fore. But a 50-city study by the International Water Management Institute (IWMI) – a World Bank-backed research agency based in Sri Lanka – finds that often the only source of the essential irrigation water to grow many of those crops is city sewage.

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Half of urban fields are irrigated with sewage, suggesting that a tenth of the world’s food is already grown this way. IWMI’s director Colin Chartres warned this week: “This figure is bound to increase as growing cities coincide with escalating food shortages to create a squeeze on agricultural water supply.”


Theoretically, irrigating food crops with untreated wastewater is banned in many countries, one reason why there is virtually no research on the practice. But “while it may be theoretically forbidden, it is unofficially tolerated”, says the report’s authors, who found that city authorities in Faisalabad in Pakistan auction untreated sewage to farmers during droughts.

Some countries, including Israel, Mexico and Tunisia, treat sewage before delivering it to farmers, which removes bacteria and lumps, at least. But this is rare. In the Ghanaian capital Accra, 200,000 people buy vegetables grown on urban fields irrigated with tanker loads of wastewater that is untreated because the city’s sewage treatment works long since ceased to function.

Toxic build-up

“I am worried about the toxins, especially heavy metals, accumulating in foods like root crops,” says Chartres. “But often there is simply no other water. In many ways it is a great use of the waste and the nutrients it contains.” He says the best answer is not to ban the practice, but to improve it.

“Even without expensive infrastructure, common sense measures can make wastewater irrigation safer.” Storing the wastewater in ponds allows solids to settle out, including the eggs of intestinal worms. And farmers should wash vegetables in clean water before selling them to markets.

The bottom line is that increasing numbers of people will starve, and many more will lose their livelihoods, without the benefits of recycled sewage.