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Branching out for safer water

A new study taps a surprisingly simple, cheap and renewable resource for filtering water

9:00am, March 13, 2014

This electron microscope image (in false color) shows sapwood after being used for water filtration. The sapwood’s pit membranes (red and blue) successfully trapped E. coli bacteria (the tiny, leaf-green-colored dots and rod shapes). This prevented the germs from passing through to the drinking water. 

R. Karnik, et al. MIT

Can a tree branch save lives? Maybe, according to a new study. Certain types of wood, it finds, can filter harmful bacteria from drinking water.

Globally, illnesses caused by tainted water kill more than 2 million people each year. Most are children. The problem is especially grave in poor, developing countries where safe, clean water isn’t piped into homes as it is in the United States. Filtering and treatment systems can work well. But such systems can be expensive, complex or difficult to set up and maintain. And these can be especially hard to provide during emergencies, such as after a major storm or earthquake.

Rohit Karnik is a mechanical engineer at the Massachusetts Institute of Technology in Cambridge. There, he studies membranes, the materials used in filters. He focuses on ones that allow water to pass through while blocking the flow of contaminants. For a long time, Karnik hadn’t really thought about wood in connection to filters. Then he heard another scientist describe how sap moves through plants.

“Biologists have studied this process for a long time,” Karnik told Science News for Students. They just didn’t talk about this around people who study filtration. Maybe natural wood could filter bacteria from water, he realized. If so, it would be a cheap, renewable and readily available option for sanitizing water. His team’s new data now seem to confirm that.

How it works

The plant tissue that conducts water and sap is called xylem. It works a bit like a one-way straw. A network of conduits — tiny tubes with closed ends — makes up the xylem. Each tube contains a few microscopic holes, called pits. Those pits line up along the conduits like the holes in a recorder or flute. Water flows up through one conduit, then out and into the next conduit through these pits. To keep disruptive bubbles from making the same jump, a membrane covers each pit. In trees, this xylem network is called sapwood.

To test its ability to filter out bacteria, Karnik and his team first cut small plugs of sapwood from oak and balsa trees. They fitted the plugs into tubes containing germ-tainted water and applied some pressure. (They used “about the same pressure as if you had a tank on a roof and a hose running down into the house,” explains Karnik.) Then the researchers used a scanning electron microscope to see if the woody filters stopped the bacteria or if the microbes had simply flowed through.

The experiment did not go smoothly at first. In shaping the plugs, the researchers had unknowingly cut off the ends of the sapwood conduits. This allowed water — and its contaminants — to flow right through each conduit’s end, bypassing the pit membranes.

Then the team learned that in white-pine trees, conduits are no more than 10 millimeters (0.4 inch) long. That would mean that the conduits were short enough to remain intact as the test plugs were prepared. To confirm that, the team stripped the bark from freshly cut white-pine branches and tried again. This time, the water passed through the filter. And only the water did. The wood’s natural pit membranes stopped anything larger than about 70 nanometers (a nanometer is one-billionth of a meter). That’s good news, the researchers note, because most bacteria are larger than 200 nanometers.

High-school senior Varsha Venkatesh works in Karnik’s lab. She says it was thrilling to see the xylem membranes actually filter bacteria out of the water. “That was the first spark which kept me moving forward in my research.”

Venkatesh is originally from India. There, many people lack access to clean drinking water. If the sapwood system works as the researchers predict, each tiny filter could produce about four liters (3.5 quarts) of drinking water per day. That’s enough for one person.

Not yet ready for prime time

William Krantz is a materials scientist at Nanyang Technological University on the Pacific island nation of Singapore. He thinks the possibility of using sapwood to filter water is interesting. “But it has some problems,” he suspects. For instance, wood can crack. “Just one crack will let through unfiltered water containing bacteria that will increase exponentially,” he worries.

Karnik agrees that a wood filter system isn’t quite ready for real world use. “I think this is just the start,” he says. “There are a lot of questions yet to answer. For example, we’ve just investigated one type of plant. Maybe others are more suitable.” In addition, Karnik points out that many harmful viruses are smaller than bacteria. They might slip right through the sapwood’s pit membranes.

For now, Karnik emphasizes, this is just a prototype. So, he warns: “Don’t try this without adult supervision, and don’t drink the water.”

Power words

bacterium(plural bacteria)   A single-celled organism forming one of the three domains of life. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

exponentially   An adverb referring to something that increases dramatically — literally by 10-fold, 100-fold or more.

filter  A device that allows some materials to pass through but not others, based on their size or some other feature.

filtration  The process of blocking the passage or flow of some materials through a fabric, screen or some other type of matrix.

membrane A barrier which blocks some materials depending on their size or other features. Membranes are an integral part of filtration systems.

nanometer  One-billionth of a meter.

sanitize   The process of removing substances that can spread disease.

sapwood  A common name for the xylem in trees.

scanning electron microscope (SEM)   A scientific instrument in which the surface of a specimen is scanned by a beam of electrons that are reflected to form an image.

xylem  The part of a plant that conducts water, nutrients and sap. Also called sapwood.

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