MIT

For people in too many developing

countries, clean water is often a luxury. Chlorine treatments are too expensive for small villages, boiling requires a hefty investment in fuel, and UV radiation demands regular high-tech maintenance. But now, scientists say that a simple, inexpensive water filter might be only a tree branch away from reality.

As reported in a paper published yesterday on PLOS One, MIT researchers ran contaminated water through a sapwood branch and found that the plant tissue successfully filtered experimental dye and actual bacteria out of the mix. The filter required only a fresh branch of white pine and some cheap plastic tubing—simplicity that could be ideal in remote villages or emergency situations.

"It's too early to compare, but with further development of xylem-based filters, I think it would be fair to compare it to other filtering methods," says Rohit Karnik, mechanical engineer at MIT and coauthor of the study.

Karnik's sapwood filters harness the hardware already built into plant tissue. In transit from plant's roots to its leaves, water bounces through a meshwork xylem, a series of conduits and membranes with tiny pores that blocks bubbles from clogging the system. Karnik first heard about these plant pores at a NATO-sponsored conference in 2012. "I listened to this talk about how water flows through plants, and it occurred to me that the flow of water has to pass through these membranes in xylem," he says.

After further investigation, Karnik discovered that most xylem pores are about the same size as the average pathogenic bacterium. Intrigued by the possibility of using plant tissue to filter bacteria out of water, he recruited Varsha Venkatesh, a high school student from New Jersey and coauthor of the paper, to run preliminary experiments. By the end of last summer, the lab began to see positive results. "We tried freshly cut sapwood and found that it actually works," Karnik says. "We were able to show that it can filter bacteria out of water."

MIT

Inexpensive, disposable filters could bring clean water to millions of people in developing nations. But Karnik believes that the filter might also help stranded hikers distill clean water in emergency situations. "White pine is so common the Northeast that just cutting off a branch and using it as a filter would be fairly straightforward," he says.

Internationally, sapwood filters could provide a much needed low-cost and low-tech solution to water purification, says Rick Andrew, global business development director of water systems at the National Sanitation Foundation (NSF) International. "It's always interesting to see these novel techniques being investigated," he says. "Regardless of whether or not it has real world potential—and it might—it gives us insight into ideas and other technology that could be developed based on the study."

But Andrew has a few concerns about the prototype, and wonders whether heavily polluted water would clog up the sapwood branch and reduce flow rate. Water flow rates are an especially important part of developing a filter for drinking water, and Andrew notes that the study was performed in a laboratory using only pure, deionized water. "Water in an impacted area may have high turbidity and high organic content. One of the next steps would be to look at a more real-world type of water supply," he says.

Karnik is quick to note that his sapwood filter is still in its early testing stages, and that further research will undoubtedly be necessary. "This is the first demonstration that it is possible to use xylem for filtration," he says. "But it's only the tip of the iceberg."

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