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water pours into filtration baskets

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Dubai’s population boom long ago outgrew its groundwater supply. Today, it provides only 0.5 percent of the city’s demand. To meet the other 99.5 percent—a daily peak of 416 million gallons, on average—the Jebel Ali plant imbibes 2.8 billion gallons from the Persian Gulf every day, turning it into pristine drinking water. In the first step, the sea passes through filtration baskets, which isolate large materials ­including trash, seaweed, and occasionally marine life.
reverse-osmosis facility

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At the reverse-osmosis facility, motorized pumps force salt water through tubes filled with tightly wound membranes. The result? Highly concentrated salt water on one side, and something you’d actually want to drink on the other. Reverse osmosis used to be notoriously ­expensive and energy intensive. But over the past decade, major advances in membrane technology let plants like this one treat more water faster. Jebel Ali produces 30 million gallons per day via reverse osmosis alone.
man on a bicycle near the massive tubes that make up the plant

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The rest of the seawater—about 98.5 percent—goes through a multi-step heating and cooling process called flash distillation. Turbines send steam via pipelines to a series of evaporation chambers. There, steam heats the salt water, and the resulting vapor cools and condenses into a collector. Leftover salt water travels to the next chamber, which has lower pressure (and a lower boiling point). This repeats many times until pure H2O—and a dense brine—are all that remain.
water flows out of a spigot

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The water gets tested for pH, turbidity, and any chlorine dioxide left over from treatment. Then it’s transferred to reservoirs en route to the city. The Dubai Energy and Water Authority (DEWA), which ­operates the plant, services 666,430 customers with water as of December 2016, most of them residential. Currently, the total capacity of the plant is 564 million gallons a day, but demand is growing, so an expansion to accommodate an additional 48 million gallons is underway.
brine outflows in the the gulf

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Despite all that work, only about 9 percent of the total original intake ­becomes potable water (or goes to generate electricity). The rest—now with a higher salinity—gets pumped back into the Persian Gulf. In 2015, that translated to more than 1.3 trillion gallons of discharged brine. DEWA keeps a close watch to ensure all that warm salt water doesn’t harm the local ecosystem. At multiple distances from the plant, workers routinely test the sea for things like acidity levels and the health of marine life.

This article was originally published in the March/April 2017 issue of Popular Science, under the title “Where they tame the undrinkable ocean.”

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