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.