If you live on the coast in an area prone to drought—say, Los Angeles or parts of India—there’s a good chance that as climate change progresses, your community may begin to rely more on desalination technology that can make ocean water drinkable. (One L.A.-area water agency is now considering a controversial plan to spend hundreds of millions on a new desalination plant; Chennai, India is currently building its third desalination plant as it faces severe water shortages.) But desal technology typically uses a lot of energy. New research has found an unlikely source to help change that: trees.
“We are trying to develop a new type of membrane material that is nature-based,” says Z. Jason Ren, an engineering professor at Princeton University and one of the coauthors of a new paper in Science Advances about that material, which is made from wood. It’s designed for use in a process called membrane distillation, which heats up saltwater and uses pressure to force the water vapor through a membrane, leaving the salt behind and creating pure water. The membranes are usually made from a type of plastic. Using “nanowood” membranes instead can both improve the energy efficiency of the process and avoid the environmental problems of plastic.
Wood is naturally good at transporting water through its capillaries. By treating the wood to remove lignin, the part of the plant that makes it rigid and “woody,” and hemicellulose, which links cells together, the researchers were left with a thin material called nanowood that still has tiny natural channels. They treated the material with silane, a coating that repels water, so that vapor could pass through but water, salt, and other impurities would stay on one side of the membrane. Because wood naturally provides insulation, the material also minimizes heat from leaking in the process, saving energy.
Unlike a plastic membrane made from fossil fuels—with emissions from digging up the fossil fuel and manufacturing and disposal issues—the wood-based membrane could potentially be carbon negative, capturing more CO2 than it emits when it’s made. “You grow a tree, where during the process, you’re actually not only not emitting greenhouse gas, you’re actually capturing CO2 from the atmosphere,” says Ren. Desalination using the nanowood is energy efficient enough that it could run completely on solar energy, solving a secondary problem: Because batteries are expensive, places with large amounts of renewable energy on the grid sometimes end up with excess solar power during the day that can’t be used. That extra energy could, in theory, power the creation of potable water.
The new material doesn’t address other environmental challenges from desalination, such as what happens to the toxic brine that’s left when the water is purified—right now, it’s often dumped back in the ocean, where it can harm marine ecosystems. But others are studying how brine can be turned into useful products. Ren and the others on the nanowood research team plan to continue working on improving the material, looking at issues like how it can be scaled up and how it can compete with the cheap cost of current plastic membranes. They also plan to experiment with different types of wood. “Different trees have different structures,” Ren says. “We need to optimize or find the best candidate for different pore structures and the strands of the wood we want.”