Healthy soil and sustainable growing

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Sewage waters a tenth of world's irrigated crops
  12:01 18 August 04   NewScientist.com news service
http://www.newscientist.com/news/news.jsp?id=ns99996297

A tenth of the world’s irrigated crops - everything from lettuce and tomatoes 
to mangoes and coconuts - are watered by sewage. And much of that sewage is 
raw and untreated, gushing direct from sewer pipes into fields at the fringes 
of the developing world’s great megacities, reveals the first global survey 
of the hidden practice of waste-water irrigation.

And, however much consumers may squirm, farmers like it that way. Because the 
stinking, lumpy and pathogen-rich sewage is rich in nitrates and phosphates 
that fertilise crops free of charge, suggests the survey presented at the 
Stockholm Water Symposium on Tuesday.

“Wastewater irrigation is in an institutional no-man’s land,” said Chris 
Scott of the Sri Lanka-based International Water Management Institute, 
co-editor of the study. “Water, health and agriculture ministries in many 
countries outlaw the practice, but refuse to recognise that it is widespread.”

He estimates that 20 million hectares of the world’s farms are irrigated with 
sewage. A quarter of Pakistan’s vegetables, including salad crops, are grown 
in sewage effluent, the study found.

And business is booming. One farmer in the heart of an un-named West African 
city grows 12 crops of lettuce a year from his sewage farm. In many 
fast-growing megacities, clean water is in desperately short supply, where 
sewage is plentiful. And the sewage pipes keep flowing even in the dry 
season, when irrigation canals often dry up.


Toxic waste

Farms hooked up to sewage pipes make big profits. The study found that in 
parts of Pakistan the price of fields watered by sewage pipes is twice that 
of neighbouring fields irrigated with clean water.

In Mexico, Jordan, Israel and Tunisia, sewage is specially treated to remove 
pathogens and make it safe for irrigation. But in India, China and Pakistan, 
the study found that treatment is rare. The sewage is added to fields 
complete with disease-causing pathogens and toxic waste from industry.

Sewage is probably the biggest source of water for urban farming, which 
provides an estimated one fifth of the world’s food, said Scott. In 
Hyderabad, the Indian city where he works, “pretty much a 100 per cent of the 
crops grown around the city rely on sewage,” he said. “There is no other 
water available.”

Many consumers would not buy produce at markets if they knew it had been 
grown in sewage, he agrees. “Often farmers take the produce to distant 
markets, where customers don’t know how it is produced.” And farmers 
themselves run the greatest risks of disease, he points out.

But the study concludes that banning the practice is not usually practicable. 
“We need to recognise that sewage is a valuable resource that grows huge 
amounts of food. So instead we should help the millions of farmers involved 
to do it better,” said Scott.