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 UN-backed study said on January 14.

According to a study by the United Nations University Institute for Water Environment and Health (UNU-INWEH), desalination plants pump out 142 million cubic metres of salty brine every day, 50 per cent more than previous estimates, to produce 95 million cubic metres of fresh water.

The study found that 55 per cent of global brine is produced in just four countries: Saudi Arabia (22 per cent), UAE (20.2 per cent), Kuwait (6.6 per cent), and Qatar (5.8 per cent). West Asian plants, which largely operate using seawater and thermal desalination technologies, typically produce four times as much brine per cubic metre of clean water as plants where river water membrane processes dominate, such as in the US.

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

Brine - water comprising about 5 per cent salt - often includes toxins such as chlorine and copper used in desalination, it said. By contrast, global seawater is about 3.5 per cent 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. UNU-INWEH director Vladimir Smakhtin said the study was part of research into how best to secure fresh water for a rising population without harming the environment. “There are all sorts of under-appreciated sources of water,” he said, ranging from fog harvesting to aquifers below the seabed.

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.

Meanwhile, the paper also highlights economic opportunities to use brine in aquaculture, to irrigate salt-tolerant species, to generate electricity, and by recovering the salt and metals contained in brine. With better technology, a large number of metals and salts in desalination plant effluent could be mined.  These include sodium, magnesium, calcium, potassium, bromine, boron, strontium, lithium, rubidium and uranium - all used by industry, in products, and in agriculture.

The needed technologies are immature, however; recovery of these resources is economically uncompetitive today.