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Desalination pours more toxic brine into the ocean than previously thought

https://www.sciencenews.org/article/desalination-pours-more-toxic-brine-ocean-previously-thought  The supersalty water is a byproduct in producing potable water, BY JEREMY REHM , JANUARY 14, 2019

Technology meant to help solve the world’s growing water shortage is producing a salty environmental dilemma.

Desalination facilities, which extract drinkable water from the ocean, discharge around 142 billion liters of extremely salty water called brine back into the environment every day, a study finds. That waste product of the desalination process can kill marine life and detrimentally alter the planet’s oceans, researchers report January 14 in Science of the Total Environment.

“On the one hand, we are trying to provide populations — particularly in dry areas — with the needed amount of good quality water. But at the same time, we are also adding an environmental concern to the process,” says study coauthor Manzoor Qadir, an environmental scientist at the United Nations University Institute for Water, Environment and Health in Hamilton, Canada.

Between human population growth and climate change, water is becoming increasingly scarce (SN: 8/18/18, p. 14). Desalination technology has become a viable solution to this problem and has grown exponentially in popularity since the 1980s. Almost 16,000 plants now operate worldwide.

Desalination relies on evaporation or specialized membranes to either chemically or electrically separate pure water from a stream of saltwater. But two streams always flow out of the system: one that becomes water that people can use, and another with the leftover, extra-salty brine, which is released back into the environment.

Previous evaluations didn’t assess how much brine these facilities produced, Qadir says. Scientists assumed that desalination facilities on average equally produced brine and pure water — one liter of brine for every liter of pure water. That turned out to be wrong.

Using data on the water sources and technology used at desalination facilities around the globe, Qadir and his colleagues estimated for the first time how much brine is discharged daily. For every liter of pure water made, they found that on average 1.5 liters of highly concentrated brine is released back into the environment. Per day, that value translates to more than half the daily volume of water pouring over Niagara Falls, with 70 percent of it originating from desalination plants in arid North Africa and the Middle East.

As brine re-enters the ocean, “it creates a kind of local environment,” Qadir says. The highly concentrated discharge, which can also contain metals and antifouling chemicals, is denser than seawater, so it flows as a salty plume to the seafloor and can poison marine organisms living nearby. Some brine can also still be hot from evaporative processes during desalination. Because hot water doesn’t hold oxygen as well as cold water, ocean areas where brine enters can become depleted of oxygen.

An international standard requiring wastewater treatment and the use of environmentally friendly chemicals in desalination discharge does exist, says Yoram Cohen, a chemical engineer at UCLA. “But whether all people follow it, I don’t know.”

Save for some scientific studies, not much is being done to resolve the issue, Qadir says. “At the government level, I don’t see that there is a serious attempt that has been made.”

Suggestions have been proposed for repurposing the brine, including for watering salt-tolerant agricultural fields, extracting metals such as magnesium or uranium, or harvesting salt versus mining for it. In terms of technology, you can take the brine “and evaporate it to recover the salt,” Cohen says. “But the price is huge.”

Depending on location and type of technology, desalination alone can cost between $0.50 and over $2 to produce 1,000 liters of drinkable water — about what two people in the United States use in a day. Further evaporating the brine waste only increases the cost.

Modern desalination technologies, such as graphene oxides, are becoming more cost effective and releasing less brine discharge (SN: 8/20/16, p. 22). But they are not universally distributed and are uncommon in the Middle East where desalination is most used. “We need to make sure that with our efforts, we are able to use more of those types of technology which produce more desalinated water than brine,” Qadir says.

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January 15, 2019 - Posted by | 2 WORLD, oceans, water

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