To Clean Drinking Water, All You Need Is A Stick
Removing all the dangerous bacteria from drinking water would have enormous health benefits for people around the world.
The technologies exist for doing that, but there's a problem: cost.
Now a scientist at the Massachusetts Institute of Technology thinks he's on to a much less expensive way to clean up water.
MIT's Rohit Karnik is a mechanical engineer who works on water technologies. He says it's relatively easy to make membranes that can filter the bacteria out of water. But making membranes cheaply, he says, is not so easy.
One day a few years ago, he was at a meeting on plants and water flow when a light bulb went off in his head. Why not, he thought, use the xylem tissue in plants for water filtration?
Now if you remember your high school biology, you'll know that xylem is the stuff in plants that transports water in the form of sap from the roots to the leaves.
"And the way the water is moved is by evaporation from the leaves," says Karnik.
It's somewhat like what happens when you put a straw into a glass of liquid. Evaporation from the leaves has the same effect as sucking on the straw.
Pulling water up to the leaves this way creates a problem for the plant, but also an opportunity for an inventor.
The plant's problem is something called cavitation, or the growth of air bubbles, which makes it harder for water to reach the leaves. But Karnik says xylem has a way of getting rid of these bubbles.
"The xylem has membranes with pores and other mechanisms by which bubbles are prevented from easily spreading and flowing in the xylem tissue," he says.
And it turns out these same pores that are so good at filtering out air bubbles are just the right size for filtering out nasty bacteria.
To prove it worked, he created a simple setup in his lab. He peeled the bark off a pine branch and took the sapwood underneath containing the xylem into a tube. He then sent a stream of water containing tiny particles through the tube and showed that the wood filter removed them.
"We also flowed in bacteria and showed we could filter out bacteria using the xylem," he says. Karnik estimates the xylem removed 99.9 percent of the bacteria.
The results were published Wednesday in the journal PLOS ONE.
Karnik says what makes wood such an attractive material for water filtration is that it's cheap. So he thinks it's worth trying to work out the technical hurdles to scaling up his system.
But Robert Jackson, an environmental expert at Stanford University, points out that at least as it stands now, the system doesn't do a good enough job at filtering out bacteria. He wrote in an email that filtering out almost all of the nasty bacteria is certainly helpful, "but when you can have hundreds of thousands, even millions, of them in a drop of water, you don't want to rely on something with 99 percent efficiency."
"In a survival or short-term situation this could work," he wrote. "As a longer-term or global solution to the billion people on Earth without access to clean water, call me skeptical."
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Transcript
MELISSA BLOCK, HOST:
Contrary to what you may have heard, George Washington's dentures were not made out of wood. But there are a lot surprising things that are made out of wood - a battery, for instance. NPR's Joe Palca reported on that last year as part of his series on scientists and inventors called Joe's Big Idea. Well, today, Joe tells us about an engineer at MIT who's trying to make a water purification system out of wood.
JOE PALCA, BYLINE: Even water that looks clean can have crud in it you don't want to drink: Vibrio cholera bacteria, Salmonella bacteria, nasty strains of E. coli bacteria - yuck.
MIT's Rohit Karnik is a mechanical engineer who works on water technologies. Now, it's relatively easy to make membranes that can filter this stuff out. But...
ROHIT KARNIK: Making membranes cheap is not so easy.
PALCA: One day a few years ago, he was at a meeting on plants and water flow when a light bulb went off in his head.
KARNIK: Why not use the xylem tissue that conducts sap in plants for water filtration?
PALCA: Karnik explained to me why using xylem tissue would be a good idea. First, he reminded me what xylem tissue was. It's the stuff in plants that transports water in the form of sap from the roots to the leaves.
KARNIK: And the way the water is moved is by evaporation from the leaves.
PALCA: It's somewhat like what happens when you put a straw in a glass of liquid. Evaporation from the leaves has the same effect as sucking on the straw. Pulling water up to the leaves this way makes a problem for the plant, but an opportunity for an inventor. The plant's problem is something called cavitation.
KARNIK: That is growth of bubbles.
PALCA: Air bubbles. In this case, bad air bubbles because air bubbles make it harder for the water to reach the leaves. But Karnik says xylem has a way of getting rid of the bubbles.
KARNIK: The xylem has membranes with pores and other mechanisms, by which bubbles are prevented from easily spreading a flowing in the xylem tissue.
PALCA: And it turns out these same pores that are so good at filtering out air bubbles are just the right size for filtering out nasty bacteria. To prove it, he created a simple set up in his lab. He peeled the bark off a pine branch and took the sapwood underneath containing the xylem and put it in a tube.
KARNIK: So the setup is really a piece of wood in a tube. That's it.
PALCA: Hmm. That is rather simple. To test his system, he sent a stream of water containing tiny particles through the tube, and showed that the wood filter filtered them out.
KARNIK: We also flowed in bacteria and showed that we can filter out bacteria using the xylem.
PALCA: Karnik says what makes wood such an attractive material for water filtration is it's cheap. So he thinks it's worth trying to work out the technical hurdles to scaling up his system, not that he expects it to happen fast.
KARNIK: Taking it from the idea to demonstration took more than a year.
PALCA: Going from this demonstration in a small tube to something that works in real-life applications will surely take even longer. More details about Karnik's work can be found in the journal PLOS ONE.
Joe Palca, NPR News. Transcript provided by NPR, Copyright NPR.
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