Scientists seek cheaper strategies for producing freshwater.
The world is on the verge of a water crisis.
Rainfall shifts caused by climate change plus the escalating water demands of a growing world population threaten society’s ability to meet its mounting needs. By 2025, the United Nations predicts, 2.4 billion people will live in regions of intense water scarcity, which may force as many as 700 million people from their homes in search of water by 2030.
Those water woes have people thirstily eyeing the more than one sextillion liters of water in Earth’s oceans and some underground aquifers with high salt content. For drinking or irrigation, the salt must come out of all those liters. And while desalination has been implemented in some areas — such as Israel and drought-stricken California — for much of the world, salt-removal is a prohibitively expensive energy drain.
Scientists and engineers, however, aren’t giving up on the quest for desalination solutions. The technology underlying modern desalination has been around for decades, “but we have not driven it in such a way as to be ubiquitous,” says UCLA chemical engineer Yoram Cohen. “That’s what we need to figure out: how to make desalination better, cheaper and more accessible.”
Recent innovations could bring costs down and make the technology more accessible. A new wonder material may make desalination plants more efficient. Solar-powered disks could also serve up freshwater with no need for electricity. Once freshwater is on tap, coastal floating farms could supply food to Earth’s most parched places, one scientist proposes.
Taking the salt out of water is hardly a new idea. In the fourth century B.C., Aristotle noted that Greek sailors would evaporate impure water, leaving the salt behind, and then condense the vapor to make drinkable water. In the 1800s, the advent of steam-powered travel and the subsequent need for water without corrosive salt for boilers prompted the first desalination patent, in England.
Most modern desalination plants use a technique that differs from these earlier efforts. Instead of evaporating water, pumps force pressurized saltwater from the ocean or salty underground aquifers through special sheets. These membranes contain molecule-sized holes that act like club bouncers, allowing water to pass through while blocking salt and other contaminants.
The membranes are rolled like rugs and stuffed into meter-long tubes with additional layers that direct water flow and provide structural support. A large desalination plant uses tens of thousands of membranes that fill a warehouse. This process is known as reverse osmosis and the result is salt-free water plus a salty brine waste product that is typically pumped underground or diluted with seawater and released back into the ocean. It takes about 2.5 liters of seawater to make 1 liter of freshwater.
In 2015, more than 18,000 desalination plants worldwide had the annual capacity to produce 31.6 trillion liters of freshwater across 150 countries. While still less than 1 percent of worldwide freshwater usage, desalination production is two-thirds higher than it was in 2008. Driving the boom is a decades-long drop in energy requirements thanks to innovations such as energy-efficient water pumps, improved membranes and plant configurations that use outbound water to help pressurize incoming water. Seawater desalination in the 1970s consumed as much as 20 kilowatt-hours of energy per cubic meter of produced fresh-water; modern plants typically require just over
Water, water, everywhere
Desalination plants supply water to more than 300 million people worldwide and experts expect that number to grow. Blue dots in this map represent the more than 500 large desalination plants currently in operation. Each plant produces more than 20 million liters of freshwater daily from seawater and salty groundwater. The number of smaller plants, such as those that provide freshwater on ships or for personal use, is unclear.
There’s a limit, however, to the energy savings. Theoretically, separating a cubic meter of freshwater from two cubic meters of seawater requires a minimum of about 1.06 kilowatt-hours of energy. Desalination is typically only viable when it’s cheaper than the next alternative water source, says Brent Haddad, a water management expert at the University of California, Santa Cruz. Alternatives, such as reducing usage or piping freshwater in from afar, can help, but these methods don’t create more H2O. While other hurdles remain for desalination, such as environmentally friendly wastewater disposal, cost is the main obstacle.
The upfront cost of each desalination membrane is minimal. For decades, most membranes have been made from polyamide, a synthetic polymer prized for its low manufacturing cost — around $1 per square foot. “That’s very, very cheap,” says MIT materials scientist Shreya Dave. “You can’t even buy decent flooring at Home Depot for a dollar a square foot.”
But polyamide comes with additional costs. It degrades quickly when exposed to chlorine, so when the source water contains chlorine, plant workers have to add two steps: remove chlorine before desalination, then add it back later, since drinking water requires chlorine as a disinfectant. To make matters worse, in the absence of chlorine, the membranes are susceptible to growing biological matter that can clog up the works.
With these problems in mind, researchers are turning to other membrane materials. One alternative, graphene oxide, may knock polyamide out of the water.
Since its discovery in 2004, graphene has been touted as a supermaterial, with proposed applications ranging from superconductors to preventing blood clots (SN: 10/3/15, p. 7; SN Online: 2/11/14). Each graphene sheet is a single-atom-thick layer of carbon atoms arranged in a honeycomb grid. As a hypothetical desalination membrane, graphene would be sturdy and put up little resistance to passing water, reducing energy demands, says MIT materials scientist Jeff Grossman.
Pure graphene is astronomically expensive and difficult to make in large sheets. So Grossman, Dave and colleagues turned to a cheaper alternative, graphene oxide. The carbon atoms in graphene oxide are bordered by oxygen and hydrogen atoms.
Those extra atoms make graphene oxide “messy,” eliminating many of the material’s unique electromagnetic properties. “But for a membrane, we don’t care,” Grossman says. “We’re not trying to run an electric current through it, we’re not trying to use its optical properties — we’re just trying to make a thin piece of material we can poke holes into.”
The researchers start with graphene flakes peeled from hunks of graphite, the form of carbon found in pencil lead. Researchers suspend the graphene oxide flakes, which are easy and cheap to make, in liquid. As a vacuum sucks the liquid out of the container, the flakes form a sheet. The researchers bind the flakes together by adding chains of carbon and oxygen atoms. Those chains latch on to and connect the graphene oxide flakes, forming a maze of interconnected layers. The length of these chains is fine-tuned so that the gaps between flakes are just wide enough for water molecules, but not larger salt molecules, to pass through.
The team can fashion paperlike graphene oxide sheets a couple of centimeters across, though the technique should easily scale up to the roughly 40-square-meter size currently packed into each desalination tube, Dave says. Furthermore, the sheets hold up under pressure. “We are not the only research group using vacuum filtration to assemble membranes from graphene oxide,” she says, “but our membranes don’t fall apart when exposed to water, which is a pretty important thing for water filtration.”
The slimness of the graphene oxide membranes makes it much easier for water molecules to pass through compared with the bulkier poly-amide, reducing the energy needed to pump water through them. Grossman, Dave and colleagues estimated the cost savings of such highly permeable membranes in 2014 in a paper in Energy & Environmental Science. Desalination of ground-water would require 46 percent less energy; processing of saltier seawater would use 15 percent less, though the energy demands of the new proto-types haven’t yet been tested.
So far, the new membranes are especially durable, Grossman says. “Unlike polyamide, graphene oxide membranes are resilient to important cleaning chemicals like chlorine, and they hold up in harsh chemical environments and at high temperatures.” With lower energy requirements and no need to remove and replace chlorine from source water, the new membranes could be one solution to many desalination challenges.
In large quantities, the graphene oxide membranes may be economically viable, Dave predicts. At scale, she estimates that manufacturing graphene oxide membranes will cost around $4 to $5 per square foot — not drastically more expensive than polyamide, considering its other benefits. Existing plants could swap in graphene oxide membranes when older polyamide membranes need replacing, spreading out the cost of the upgrade over about 10 years, Dave says. The team is currently patenting its membrane—making methodology, though the researchers think it will take a few more years before the technology is commercially viable.
“We are at a point where we need a quantum leap, and that can be achieved by new membrane structures,” says Nikolay Voutchkov, executive director of Water Globe Consulting, a company that advises industries and municipalities on desalination projects. The work on graphene oxide “is one way to do it.”
Other materials are also vying to be poly-amide’s successor. Researchers are testing carbon nanotubes, tiny cylindrical carbon structures, as a desalination membrane. Which material wins “will come down to cost,” Voutchkov says. Even if graphene oxide or other membranes save money in the long run, high upfront costs would make them less appealing.
Plus, those new membranes won’t solve the problems of desalination in less-developed areas. The costs of building a large plant and pumping freshwater over long distances make desalination a hard sell in rural Africa and other water-starved places. For hard-to-reach locales, scientists are thinking small.
A portable approach
The researchers have produced 2.5-centimeter-wide disks of the new material so far, which are light enough to float. The black disks absorb more than 96 percent of incoming sunlight and about 90 percent of the absorbed energy is used in evaporating water, the researchers reported in the June Nature Photonics.
The evaporated water condenses and collects in a transparent box containing stainless steel. In laboratory tests, the researchers successfully desalinated water from China’s Bohai Sea to levels low enough to meet drinking water standards. The researchers reckon that they can produce around five liters of fresh-water per hour for every square meter of material under intense light. In early tests, the disks held up after multiple uses without dropping in performance.
Aluminum is cheap and the material’s fabrication process can easily scale, Zhu says. While the disks can’t produce as much drinkable water as quickly as big desalination plants, the new method may serve a different need, since it’s more affordable and more portable, he says. “We are developing a personalized water solution without big infrastructure, without extra energy consumption and with a minimum carbon footprint.” The researchers hope that their new desalination technique will find use in developing countries and remote areas where conventional desalination plants aren’t feasible.
The disks are worth pursuing, says Haddad at UC Santa Cruz. “I say let’s try it out. Let’s work with some villages and see how well the tech works and get their feedback. That to me is a good next step to take.”
Desalinating water by evaporation has a downside, though, Voutchkov says. Unlike most methods for removing salt, evaporation produces pure distilled water without any important dissolved minerals such as calcium and magnesium. Drinking water without those minerals can cause health issues over time, he warns. “It’s OK for a few weeks, but you can’t drink it forever.” Minerals would need to be added back in to the water, which is hard to do in remote places, he says.
Freshwater isn’t just for filling water bottles, though. With a nearly endless supply of salt-free water at hand, desalination could bring agriculture to new places.
When Khaled Moustafa looks at a beach, he doesn’t just see a place for sunning and surfing. The biologist at the National Conservatory of Arts and Crafts in Paris sees the future of farming.
In the April issue of Trends in Biotechnology, Moustafa proposed that desalination could supply irrigation water to colossal floating farms. Self-sufficient floating farms could bring agriculture to arid coastal regions previously inhospitable to crops. The idea, while radical, isn’t too farfetched, given recent technological advancements, Moustafa says.
Floating farms would lay anchor along coastlines and suck up seawater, he proposes. A solar panel–powered water desalination system would provide freshwater to rows of cucumbers, tomatoes or strawberries stacked like a big city high-rise inside a “blue house” (that is, a floating greenhouse).
In remote Africa, electricity is hard to come by. Materials scientist Jia Zhu of Nanjing University in China and colleagues are hoping to bring drinkable water to unpowered, parched places by turning to an old-school desalination technique: evaporating and condensing water.
Their system runs on sunshine, something that is both free and abundant in Earth’s hotter regions. Using the sun’s rays to desalinate water is hardly new, but most existing systems are inefficient. Only about 30 to 45 percent of incoming sunlight typically goes into evaporating water, which means a big footprint is needed to create sizable amounts of freshwater. Zhu and colleagues hope to boost efficiency with a more light-absorbing material.
The material’s fabrication starts with a base sheet made of aluminum oxide speckled with 300-nanometer-wide holes. The researchers then coat this sheet with a thin layer of aluminum particles.
When light hits aluminum particles inside one of the holes, the added energy makes electrons in the aluminum start to oscillate and ripple. These electrons can transfer some of that energy to their surroundings, heating and evaporating nearby water without the need for boiling (SN Online: 4/8/16).
Each floating farm would stretch 300 meters long by 100 meters wide, providing about 3 square kilometers of cultivable surface over only three-tenths of a square kilometer of ocean, Moustafa says. The farms could even be mobile, cruising around the ocean to transport crops and escape bad weather.
Such a portable and self-contained farming solution would be most appealing in dry coastal regions that get plenty of sunshine, such as the Arabian Gulf, North Africa and Australia.
“I wouldn’t say it’s a silly idea,” Voutchkov says. “But it’s an idea that can’t get a practical implementation in the short term. In the long term, I do believe it’s a visionary idea.”
Floating farms may come with a large price tag, Moustafa admits. Still, expanding agriculture should “be more of a priority than building costly football stadiums or indoor ski parks in the desert,” he argues.
Whether or not farming will ever take to the seas, new desalination technologies will transform the way society quenches its thirst. More than 300 million people rely on desalination for at least some of their daily water, and that number will only grow as needs rise and new materials and techniques improve the process.
“Desalination can sometimes get a rap for being energy intensive,” Dave says. “But the immediate benefits of having access to water that would not otherwise be there are so large that desalination is a technology that we will be seeing for a long time into the future.”
This article appears in the August 20, 2016, Science News with the headline, "Quenching society’s thirst: Desalination may soon turn a corner, from rare to routine."