OSLO (Reuters) - Almost 16,000 desalination plants worldwide produce bigger-than-expected flows of highly salty waste water and toxic chemicals that are damaging the environment, a U.N.-backed study said on Monday.

FILE PHOTO: Workers walk outside a desalination plant, south of Riyadh, Saudi Arabia, May 2011. REUTERS/Fahad Shadeed

Desalination plants pump out 142 million cubic meters (5 billion cubic feet) of salty brine every day, 50 percent more than previous estimates, to produce 95 million cubic meters of fresh water, the study said.

About 55 percent of the brine is produced in desalination plants processing seawater in Saudi Arabia, the United Arab Emirates and Qatar, according to the study by the U.N. University’s Canadian-based Institute for Water, Environment and Health (UNU-INWEH).

The hyper-salty water is mostly pumped into the sea and, over a year, would be enough to cover the U.S. state of Florida with 30 cms (one foot) of brine, it said of the fast-growing and energy-intensive technology that benefits many arid regions.

Brine, water comprising about five percent salt, often includes toxins such as chlorine and copper used in desalination, it said. By contrast, global sea water is about 3.5 percent salt.

Waste chemicals “accumulate in the environment and can have toxic effects in fish”, said Edward Jones, the lead author at UNU-INWEH who also works at Wageningen University in the Netherlands.

Brine can cut levels of oxygen in seawater near desalination plants with “profound impacts” on shellfish, crabs and other creatures on the seabed, leading to “ecological effects observable throughout the food chain”, he said.

Vladimir Smakhtin, director of UNU-INWEH, said the study was part of research into how best to secure fresh water for a rising population without harming the environment.

LAKES, RIVERS

“There are all sorts of under-appreciated sources of water,” he said, ranging from fog harvesting to aquifers below the seabed. The study also involved the Gwangju Institute of Science and Technology in South Korea.

Brine from desalination plants that tap brackish lakes, aquifers or rivers far inland is harder to treat than brine from coastal plants that can be piped into the seas.

Gurpal Toor, a water quality expert at the University of Maryland who was not involved in the study, said brine “could be a serious issue in a small body of water” like an inland lake.

Brine can be used inland for irrigating salt-tolerant crops, for fish farming or to produce table salt, he said.

Alex Drak, an engineer at IDE Technologies in Israel, a leading water treatment firm, said properly processed brine from coastal plants dissolves quickly in the sea.

“We see a lot of marine life, different types of fish, in the vicinity to the discharge point,” said Drak, who was not involved in the UNU study.