It’s a cruel irony of the blue planet: Most of the earth is awash in oceans, yet seawater is undrinkable. Large-scale efforts to remove salt from seawater -- the process known as desalination -- trace back to the 1950s, and today almost 20,000 facilities from China to Mexico are making salt water drinkable to sustain burgeoning populations. But this modern-day alchemy is under scrutiny as critics question whether the benefits of desalination justify its potential harm to marine environments and contribution to global warming.

1. How is desalination done?

Ancient Greeks understood that boiling seawater produces a drinkable vapor; 18th century sailors experimented with capturing this vapor in sponges to ease their thirst on long voyages. Modern desalination began similarly: boiling seawater (by burning oil or natural gas) and recovering droplets of fresh water in a process known as thermal desalination. Cleaner, more energy-efficient technologies such as reverse osmosis -- passing seawater through plastic membranes to remove salt and other impurities -- have become more prevalent. The big exception is the Middle East, where countries rely on fossil fuel-based thermal plants for two-thirds of their desalination needs. The region accounts for roughly 90 percent of thermal treatment of seawater worldwide, according to the International Energy Agency.

2. How much drinkable water is produced this way?

As of 2015, about 18,000 desalination plants had a combined production capacity of 86.55 million cubic meters a day, or 22,870 million gallons -- roughly 1 percent of the world’s need. Since then, about 2,000 additional plants have been built. Saudi Arabia produces the biggest share of desalinated water, about a fifth of the world total, followed by the U.S., the United Arab Emirates, China, Spain and Kuwait. (Because Persian Gulf countries use most of their limited groundwater for agriculture, they depend heavily on desalination to supply their populations and industries.) Governments and companies spend as much as $14 billion a year to make ocean and brackish water drinkable. Suez and Veolia Group of France and Doosan Heavy Industries & Construction Co. of South Korea are the biggest builders of installed capacity, according to the International Desalination Association.

3. How much does it cost?

Costs have tumbled by more than half over the past 30 years thanks to improvements in technology and energy efficiency. The cost of producing 1 cubic meter of treated water currently ranges from 50 cents to 90 cents, said Miguel Angel Sanz, president of the IDA. Saudi Arabia and the U.A.E. are among the cheapest places to desalinate water, given their comparatively low energy prices and the economies of scale at their large facilities. Increased use of solar and wind power to run the desalination process may make it even cheaper.

4. So what’s the problem?

Early criticism focused on the super-heated effluent that many thermal plants discharged back into the sea and how it could kill corals and other marine life. The IDA says desalination facilities now cool this bilge so that it no longer poses such a threat. Another environmental concern centers around the use of fossil fuels to power desalination, especially in the Middle East. A study published in January ignited fresh controversy about “toxic chemicals” in the brine that desalination plants pour into oceans.

5. What’s so bad about the brine?

This salty concentrate may contain harmful residues from anti-scaling and anti-fouling chemicals used in the plants, according to the study by the Hamilton, Canada-based UN Institute for Water, Environment and Health. Desalination creates enough of the pollutant each year to cover the state of Florida beneath 1 foot (30 centimeters) of brine, said Manzoor Qadir, one of the study’s authors. Brine can also deplete oxygen in surrounding waters, suffocating marine organisms and disrupting food chains. Saudi Arabia, the U.A.E., Kuwait and Qatar together generate 55 percent of global brine, the study said. Some facilities have tried to dilute brine by dispersing it over larger areas of the sea or processing it to extract valuable metals. But such methods are technically difficult and expensive.

6. Is the criticism valid?

Sanz of the IDA said the study failed to differentiate between thermal and membrane technologies and showed “a lack of real knowledge.” The study is “an opinion of some people working in a university” and not an official United Nations’ view of desalination, he said.

7. Can desalination be done without damaging the environment?

Some plants are located inland, away from the sea, and the brine they produce is put into evaporation ponds, injected deep underground or converted into a slurry that’s processed into dry salt for disposal. Wider adoption of membrane technologies, particularly in the Middle East, looks like the best way to reduce chemical-laden brine. Almost all new facilities planned in the region will use reverse osmosis, a trend likely to continue as budget-minded Gulf governments cut subsidies on fossil fuels. These cuts, by making thermal plants less competitive, are the main reason membrane technologies are finally taking off there.

8. Can desert countries do without desalination?

The imperative to make seawater drinkable shows no sign of easing. At current consumption rates, the U.A.E.’s largest emirate may run out of natural groundwater supplies “within a few decades,” the Abu Dhabi Environment Agency said in a 2017 report. Yet rising demand for limited water resources is spurring new ideas for food production as much as for desalination. The International Center for Biosaline Agriculture in Dubai recycles brine to irrigate salt-tolerant plants such as salicornia, which can be eaten or used for biofuel. The research institute also breeds food crops like quinoa that flourish in salty, desert soils.

To contact the reporter on this story: Bruce Stanley in Dubai at bstanley5@bloomberg.net

To contact the editors responsible for this story: Will Kennedy at wkennedy3@bloomberg.net, Laurence Arnold

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