Hold the Salt
An innovative desalination technique could go from the lab to the land next year–but skeptics abound.
We build cities in the desert, and then pump in water from mountains near and far. But as demand for water rises insatiably across the American West, the solutions are getting even tougher and more complex.
Cities such as Las Vegas and Phoenix now want to tap the vast saline aquifers that surround them. An offbeat desalination technique scheduled to arrive commercially next year could help to tackle the job–and to handle a host of water-purification duties for other customers.
Dubbed capacitive deionization (CD), the desalination technique was invented a decade ago at the Lawrence Livermore National Laboratory. It exploits carbon aerogel, an extremely porous material originally developed for aerospace applications such as “not toasting astronauts on re-entry,” says Dallas Talley, chairman and CEO of CDT Systems in Dallas.
After licensing the technology in 1997, CDT Systems spent some lean years dramatically lowering aerogel manufacturing costs and optimizing desalination performance. This month the company expects to finalize arrangements with the state of Texas to begin volume production of capacitive deionization modules by mid-2004, and to ink a joint venture agreement with Air Water of Osaka Japan, under which the giant environmental services company will assemble the modules for the Japanese market.
Composed almost entirely of air, aerogel looks like “frozen smoke,” in one common description. The capacitive deionization approach to desalination takes advantage of two of aerogel’s distinctive properties: its extremely high surface area and extremely low electrical resistance.
In operation, the salty water flows between paired sheets of aerogel. Electrodes embedded in the aerogel apply a small direct current; positively charged ions attach to the sheet with the negative electrodes, and negatively charged ions cling to the sheet with the positive electrodes. After a suitable number of hours or days, the current is reversed, rinsing the ions off into a concentrate stream.
A CDT Systems AquaCell module removes 1,000 parts per million (ppm) of suspended solids from about 3,800 liters of water daily. “This is all scaleable,” says Talley. “To raise volume, you put the modules in parallel. To raise purity, you put them in a series.” CDT Systems claims the design doesn’t require pre-treating the water, and that the modules take no more power than a 100-watt light bulb.
Most current and upcoming desalination plants in the United States work via a process called reverse osmosis, in which the salty water is pushed through a permeable membrane under high pressure. Reverse osmosis is thoroughly proven and can operate at energy efficiencies as high as 95 percent. But the energy requirements are still high and so are the costs.
Talley notes that capacitive deionization will not replace reverse osmosis equipment anytime soon for the big seawater desalination plants, which can pump out fresh water for as little as 60 cents per thousand liters. (This covers both capital and operational costs; expenses vary tremendously depending on such factors as a given plant’s feedwater characteristics, energy costs, and the method of disposing of the leftover water). Instead, CDT Systems initially will target applications treating water with solids concentrations of 8,000 ppm or less. (Seawater, by contrast, has salt levels of about 32,000 ppm.) These include brackish aquifers as well as the water produced by coalbed methane extraction or other petroleum operations.
One likely early use of the new technology will be in Texas, “which has a huge supply of brackish water,” Talley points out. Water at around 3,000 ppm can be treated at an operating cost of 9 cents per thousand liters, he estimates. That’s half or less of the cost of current reverse osmosis plants handling similar jobs, according to a study by the state of Texas.
Livermore is seeing a lot of excitement about the basic technology, says business development executive Annemarie Meike. The lab has licensed the technology to several other (unnamed) firms, and restarted its capacitive deionization research this year. While much of the interest has been in high-volume desalination, capacitive deionization may be more compelling for small-scale point-of-use jobs, Meike says. “That could be under a sink, in a camping vehicle, or anyplace else where you might need to process water at the source.” She also emphasizes the technology’s efficiency at removing heavy metals.
Capacitive deionization’s biggest issue remains the cost of making the aerogel. “The price of aerogel is unknown,” Meike says. CDT Systems expects to sell its modules in the range of $1,200 to $1,500–only about 2 percent of the manufacturing cost when the company acquired the technology from Livermore. “Our fingers are crossed” for CDT’s manufacturing efforts,” Meike says.
Show Me the Salt
Capacitive deionization has intrigued desalination experts since its original announcement, but they say they’re still waiting to see real-world results. “There’s a lot of hope for the process, but a long way to go,” says David Furukawa of Separation Consultants in Poway, CA, a former president of the International Desalination Association.
“I don’t know if you can go into large-scale production” without a better understanding of how the process works, adds Ron Linsky, executive director of the National Water Research Institution in Fountain Valley, CA.
“A lot of desalination technologies have come and gone, promising the world and delivering almost nothing,” adds Ian Watson of RosTek Associates in Tampa, one of the experts who contributed to the federal roadmap. When it first debuted, “this one looked good.” He’s very interested, but reserving judgment on capacitive deionization until the technology starts producing large volumes of fresh water in real-world applications. Watson says that he awaits the results of CDT’s work on brackish water with interest but the “proof is in the eating.” He also notes that the capital cost of the aerogel modules precludes high-volume seawater desalination.
The competition is still the tried-and-true reverse osmosis process, which has prospered while “crazy ideas” have come and gone, says Lisa Henthorne, vice president and membrane technology leader at environmental consultancy Metcalf & Eddy in Naples, FL. Reverse osmosis membranes have steadily improved; you get 27 times as much water for your capital investment as you got in 1980, she says.
“Hundreds of new desalination and water supply purification ideas are generated every year in the field and in the laboratory,” notes the Desalination and Water Purification Technology Roadmap, a report issued earlier this year by the U.S. Department of the Interior.
And whether based on capacitive deionization, reverse osmosis or any other desalination technology, inland water projects face another severe challenge–disposing of the leftover brine concentrate. “You have to keep it away from fresh water supplies,” points out Furukawa. Most states don’t allow it to be injected in deep wells, and evaporation ponds take considerable land. Researchers are looking at creative solutions to contain the concentrate, or turn it into useful products.
According to the federal roadmap, half of U.S. population growth is forecast to occur in California, Texas, and Florida–“regions already experiencing water shortages.” Moreover, the roadmap adds, “only with cost-effective and efficient revolutionary technologies will the nation be able to meet its future (25+ years out) demand for safe, sustainable, and affordable water.”
It’s too early to predict how much of a part capacitive deionization will play in desalination versus the well established rival technologies–not only reverse osmosis but also the thermal techniques popular in the Middle East, where energy is cheap, or the other approaches that crop up each year in labs worldwide. But the new technology’s commercial emergence highlights the advanced alternatives now poised to meet a basic human need.
Experts agree on one point: demand for water is soaring, and it will only be met by broad, deep, and continuously improving desalination efforts. “We’ve been blessed and spoiled, I suppose, by copious quantities of very low cost water,” says Furukawa. “That cost is going up.”
Become an MIT Technology Review Insider for in-depth analysis and unparalleled perspective.Subscribe today