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.”