* Big rise in use of water from sewers, drains seen -report
* N.American "Niagara" of treated wastewater flows into sea
By Environment Correspondent Alister Doyle
OSLO, Sept 5 (Reuters) - The world is set to use far more treated wastewater to help irrigate crops and feed a rising population as fresh water supplies dry up, a team of U.N.-backed experts said on Thursday.
A study led by Japan's Tottori University and U.N. University's Canadian-based Institute for Water, Environment and Health (UNU-INWEH) forecast "a rapid increase in the use of treated wastewater for farming and other purposes worldwide".
It did not forecast volumes, saying that many nations lack data on sewer and drain water. Of 181 nations studied, only 55 had information on wastewater generation, treatment and re-use.
Many governments and companies have so far overlooked the economic potential of vast amounts of wastewater, UNU-INWEH director, Zafar Adeel, said.
North America generates about 85 cubic km (20 cubic miles) of wastewater every year, of which about 61 cubic km is treated, roughly the amount flowing over Niagara Falls, and only four percent of that is re-used.
Wastewater also often contained nutrients such as potash, nitrogen and phosphorus which saved fertiliser costs, the study published in the journal Agricultural Water Management said.
"Properly treated, wastewater is a huge economic resource," Adeel told Reuters.
However, many developing nations cannot afford the equipment to treat wastewater even though recycling it can be cheaper in the long term than pumping water from deep aquifers, the report said, and, in Pakistan, like many other emerging economies, large areas are irrigated with mostly untreated wastewater.
Per-Arne Malmqvist, an associate of the Stockholm International Water Institute, said treatment costs were coming down. Orange County in California found it cheaper to recycle wastewater into drinking water than alternatives such as pumping it from the distant Colorado River, he said.
"It costs a lot of energy to treat the water with membranes but the technology is getting cheaper," he said.
Manzoor Qadir, an author of the study at UNU-INWEH, said costs of treatment could be kept down according to purity - for drinking water, for food crops or for crops such as biofuels.