THIRSTY

Sunlight Might Be the Key to Turning Our Oceans Into Drinkable Water

The Department of Energy is betting big on a new technology to turn saltwater into freshwater.

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Two-thirds of the world is water, but only a fraction of it serves any real purpose for life on Earth. Already, one in nine people lack access to clean water—and the World Health Organization expects half the world’s population to live in water-stressed environments by 2025. Desalination—the removal of salts and other minerals from water — could be a key innovation in resolving this upcoming mess, but only if it’s turned into a sustainable, affordable technology. A group of scientists think they’re closer to achieving that goal than anyone before them, and might finally be able to turn the world’s vast salt water bodies into a never-ending quarry for potable water, thanks to the power of the sun.

According to Akshay Deshmukh, a researcher at Yale University working on desalination technologies, the gold standard desalination technology is powered by reverse osmosis, in which a pressure gradient is induced to move water through a semipermeable membrane that’s capable of trapping salts and turning saltwater into freshwater. It’s extremely effective, but the pumps used to induce this pressure are extraordinarily expensive to build, ship, and operate—especially when it comes to treating water with higher degrees of salinity.

“Because reverse osmosis uses so much electricity,” says Deshmukh, “you need quality infrastructure to keep these pumps going.” Off-grid and poor communities in the developing world simply don’t have access to the type of power needed to run reverse-osmosis desalination. For these communities, you want a technology that can run off external energy. Waste energy might work, but if you’re looking to run desalination on a practical level—basically 24/7—you want to turn to renewable power to drive the process.

So you have to get away from reverse osmosis. Deshmukh’s research group at Yale is just one of several parties heavily involved with Nanotechnology Enabled Water Treatment (NEWT) Systems: a consortium of industry, academic, and government partners launched by the National Science Foundation to develop inexpensive, sustainable water-treatment systems that could be used off-grid and potentially give millions of people around the world access to clean water. NEWT is a pioneer of a desalination method called membrane distillation. Unlike reverse osmosis, membrane distillation is driven by heat, not pressure—saltwater is injected with heat from various sources (maybe waste heat from another industrial process), and as the water comes in contact with a membrane, it evaporates. The water vapor diffuses through the membrane and condenses onto the cold side, while leaving salts behind on the other, hot side.

“The materials are broadly similar,” says Deshmukh, “to, like, breathable waterproof coats, where your sweat and water vapor can go through the coat because it’s hotter, but rain can’t go through because it’s colder.”

The type of membrane distillation technology NEWT is pursuing adds a green twist to the equation: the side of the membrane that faces the salty water is coated with carbon black nanoparticles that absorb light and convert this energy into thermal energy that heats the water and drives desalination. Thus the whole process, called nanophotonics-enabled solar membrane distillation (NESMD), is essentially driven by solar power. It can be used anywhere the sun shines, making it a potentially breakthrough water treatment system for poor and rural communities around the world.

The Department of Energy just awarded a $1.7 million grant to NEWT to run a new round of testing on an NESMD prototype. Previous tests indicated the membrane was capable of desalinating about 6 liters of water per square meter of membrane area, every hour, using a simple lens that concentrated sunlight to around 17.5 kilowatts. The upcoming trials should strike for something closer to 10 liters of water every hour. That means a square meter of membrane could provide as much as 240 liters of water per day.

Admittedly, that’s less than what many of us are used to: the average American uses between 300 and 380 liters of water every day. But Deshmukh emphasizes that the prototype is a modular system. You can easily add more membranes to one another and have stacks of them running in parallel at the same time, depending on one’s needs. The new testing is working under the idea that a large capacity desalination plant using NESMD technology would produce 10,000 liters of freshwater per day, while a small capacity plant would be aimed at producing 2,000 liters per day—more than enough for a single household (even an egregious American one). And of course, the idea would be to scale this up to even larger targets once the testing demonstrates what materials and processes could be further optimized.

NEWT is already making strides to figure out how this technology could be deployed by manufactured and deployed through commercial partners, like Montrose Environmental Group (whose company vice president, Joon Min, is a co-investigator of the DOE-funded study). If the trial goes well, we might quickly see NESMD structures being unrolled sooner than we think, granting those who are struggling with water a reprieve they’ve been waiting a long time for.