The Newest Development in Killing Waterborne Bacteria Is Plain Old Wood

Imagine the entire city of Los Angeles dying for lack of clean water. Then, imagine it dying again every year with some brand new population. In fact, 3.4 million people (pretty close to LA’s population), mostly children, do die every year from causes related to dirty drinking water. Chlorine treatments, boiling, and membrane-based filters are good ways to provide clean water, but they’re expensive and many lack access. It’s a vexing problem being approached from many different angles. The latest: a team at MIT recently discovered that xylem tissue in sapwood can remove many dangerous bacteria from water sources, including 99 percent of E. coli, and it can do it all with just one filter.

Xylem tissue is what the wood is in trees. It’s funny stuff, enabling the transport of water and minerals from the organism’s roots to its very highest branches, all while providing the trees with an amazingly resilient skeleton. Part of the transport process in xylem tissue involves very, very tiny pores that allow water to move between otherwise rigid cell walls. What the MIT team found is that these pores are also useful in trapping bacteria. “The idea is that these filters should be inexpensive enough to dispose of, rather than [the user] attempting to clean them,” Rohit Karnik, an associate professor of mechanical engineering at MIT and one of the coauthors of a new paper describing the method, told me.

It’s still a brand new concept, but it could drastically change the landscape of water filtration in places of dire need, like Africa. “A filter could take various forms, but one that is especially appealing is a compact device that simply attaches to a faucet, with a replaceable xylem filter inside it,” Karnik said. “It may take two to three years to develop the first prototypes for larger scale use, but I believe that there will be many developments to follow.”

The key to using the filters properly is keeping them damp when not in use. Karnik explains that these pores “are flexible structures that are designed to cut off flow in the presence of bubbles.” So users need water to flow freely, with few bubbles, which can be accomplished by the material being pre-wet. With dry sapwood the xylem often clogs or doesn’t allow a consistent trickle.

Unfortunately, the pores in the filter are too large to filter salt, so it wouldn’t be useful for desalination. Though in places like Africa much of the water available comes in the form of surface water anyway (water found on and collected from the ground). And with 345 million people without access to clean water in Africa alone, this is a much-needed development.