Out Of The Labs
Water WizardryJonathan Fahey, 08.26.09, 6:00 AM ET
Seventeen billion gallons of sweet, fresh water are produced from salty water every day, enough to slake the thirst of 350 million people. Yet scientists don't really know how it is done.
Good thing they at least know how to make the process, which takes lots of energy, better.
Many desalination plants remove salt by forcing seawater against a membrane that allows fresh water through, but not salt ions. This is called
reverse osmosis, and anyone with a set of taste buds can tell that it works. But scientists still haven't been able to model exactly what is going on.
"I can make you a membrane that does what you want," says Eric Hoek, a professor at UCLA's Henry Samueli School of Engineering and Applied Science. "But I can't give you an equation that describes it."
Hey, whatever works. Hoek developed a membrane now in the process of being commercialized by a start-up company called NanoH20 that the company says could double the amount of fresh water produced per day compared with conventional membranes.
Desalination is booming worldwide, both because there are ever more people who need ever more scarce, fresh water and because desalination has been getting cheaper. Part of this is because desalination plant designers have incorporated clever energy recovery devices to reduce the amount of power needed to run the plants. (See "Making Sweet Water From (Almost) Perpetual Motion.") And part is due to big improvements in membrane technology.
Nikolay Voutchkov of Water Globe Consulting says membranes have gotten 2.5 to 3 times more efficient in the last decade, helping to drive the cost of desalinated water down from $6 to $7 per 1,000 gallons of fresh water to between $2.50 and $3.20.
But Jeff Green, chief executive of NanoH20, says that while costs came down through about 2003, they started to level off and even creep up a little because improvement of current membrane technology stalled.
In order to squeeze the salt out of water, seawater has to be pushed against the reverse osmosis membrane at very high pressures. Engineers have made the membranes, which are polymers very similar to Kevlar, stronger, more uniform and better at rejecting salt. But whenever they try to increase production by increasing their permeability, too much salt gets through.
"Polymer chemistry has been around for decades," says Green. "These membranes have been optimized."
Another issue: These membranes, constantly wet, are wonderful places for bacteria to flourish. The membranes get fouled and have to be treated with chemicals or replaced.
Hoek, a member of UCLA's
Water Technology Research Center, knew that one way to both get water through faster and to make things less hospitable to bacteria was to incorporate so-called hydrophilic, or water-loving materials.
Current membrane polymers are hydrophobic; water beads on them like on a recently waxed car. That increases the pressure needed to force water through, and it creates comfy microscopic dry patches for bacteria to grab onto.
Hoek decided to try some well-known, porous, clay-like materials called zeolites, made of alumina and silicates. He knew particles of 100 nanometers could be made with pore sizes as small as just 0.2 nanometers, about the same size as a water molecule, but smaller than the 0.8 nanometer size of a salt ion. "We wanted to make a pore that water wanted to go into," he says.
People have tried (and are still trying) to make pure-zeolite films, but have failed in part because they are too difficult to control and too expensive to manufacture.
Hoek decided to make the zeolite nanoparticles first, then bake them into conventional polymers. The nanocomposite result wasn't quite as hydrophilic as pure zeolite, but also not as hydrophobic as plain polymers.
Also, he was able to add tiny traces of silver onto the nanoparticles, which act as an antimicrobial and make them even more resistant to bacteria. (See: Pure Bioscience Looks for a Silver Lining.)
He put his new nanoparticle-spiked polymers through the ringer, exposing them to high-pressure water and thriving bacteria. The results were good enough that NanoH20 was able to raise $15 million from venture capitalist firms Khosla Ventures and Oak Investment Partners to try to commercialize it.
NanoH20's Green says the company has modified Hoek's work substantially to improve and perfect the nanoparticle membrane, but he won't say how. He says the company is targeting nearly 100% improvement in water production, from 6,000 to 7,500 gallons per day per eight-inch area of membrane to 12,000 gallons per day. The membrane will be the same size and shape as current membranes, so plants won't have to be retrofitted. The company is building enough capacity to produce "tens of thousands" of membranes--a big plant incorporates 10,000 to 20,000. The first membranes will go on sale early next year.
Hoek, though, remains steadfastly humble about his discovery. "I threw one material that was already known into this membrane that was already known," he shrugs.
If they work, these membranes will be an impressive step in reducing the cost and energy required to deliver fresh water. If we still don't understand the physics of what's going on? The water will taste just as sweet.
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