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A Low-Tech Water Filter

Xylem tissue in sapwood can take bacteria out of contaminated water.
April 23, 2014

If you run out of drinking water during a lakeside camping trip, there’s a simple solution: break off a pine branch, peel away the bark, and pour lake water through the stick. The improvised filter should trap any bacteria, producing fresh, uncontaminated water.

false-color electron
A false-color electron microscope image shows E. coli (green) trapped over sapwood xylem pit membranes (red and blue).

An MIT team has discovered that this low-tech filtration system can produce up to four liters of drinking water a day—enough to quench the thirst of a typical person.

In a paper published in PLOS One, the researchers demonstrate that a small piece of sapwood can filter out more than 99 percent of the E. coli bacteria present in water. They say the size of the pores in sapwood allows water through while blocking most types of bacteria.

Rohit Karnik, an associate professor of mechanical engineering and lead author of the paper, says sapwood is promising as a low-cost and efficient water filtration material, particularly for rural communities that lack access to advanced filtration systems.

Sapwood is composed of xylem, porous tissue that conducts sap from a tree’s roots to its crown through a system of vessels and pores. Each vessel wall is pockmarked with tiny pores called pit membranes, through which sap can essentially hopscotch, flowing from one vessel to another as it feeds structures along a tree’s length. The pores also limit cavitation, a process by which air bubbles can grow and spread in xylem, eventually killing a tree. The xylem’s tiny pores can trap the bubbles, preventing them from spreading.

To study sapwood’s water-filtering potential, the researchers collected branches of white pine and stripped off the outer bark. They cut small sections of sapwood and mounted and sealed each in plastic tubing.

The researchers mixed water with red ink particles ranging from 70 to 500 nanometers in size. When they poured the mixture through the tubes, the sapwood filtered out particles bigger than 70 nanometers.

The team then flowed water contaminated with inactivated E. coli through the tubes. When they counted the bacterial cells in the filtered water, they found that the sapwood filtered out more than 99 percent of the E. coli.

Karnik says sapwood can probably filter most types of bacteria, the smallest of which measure about 200 nanometers.

“There’s huge variation between plants,” he says. “There could be much better plants out there that are suitable for this process. Ideally, a filter would be a thin slice of wood you could use for a few days, then throw it away and replace at almost no cost.”

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