Before the age of 13, there was a family reunion and one of my uncles ran salt mines for Cargill MacMillan in upstate New York. In the brief moment he was no longer swarmed by family and friends, I told him, “You should consider convincing the company to move from mining to desalination and pipe the water to the Midwest.” I’m not sure if the reason his jaw dropped speechless was because I was under the age of 13, or if he simply thought I was trying to kill his career.
As you will see, Dubai took my idea and improved, by adding power generation to the process.
One of the fastest-growing cities in the world is also among the driest. Situated between the Persian Gulf and the Arabian Desert, the DEWA Jebel Ali Power Plant & Desalination Complex makes the metropolis of Dubai possible.
Dubai’s population boom long ago outgrew its groundwater supply. Today, it provides only 0.5 per cent of the city’s demand. To meet the other 99.5 percent a daily peak of 1.57 billion litres, on average, the Jebel Ali plant imbibes 10 billion litres 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 junk, seaweed, and occasionally marine life.
At the reverse-osmosis facility, motorised 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 113 million litres per day via reverse osmosis alone.
The rest of the seawater (about 98.5 percent) goes through a multistep 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 vapour 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.
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 early 2017, most of them residential. Currently, the total capacity of the plant is 2.1 billion litres a day, but demand is growing, so an expansion to accommodate an additional 180 million litres is underway.
Despite all that work, only about 9 per cent 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 nearly 5 trillion litres 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.
Popular Science, Australia, March 2017