In many parts of the world, the presence of harmful bacteria makes it vitally important that water from lakes or rivers be thoroughly filtered before being consumed. While materials such as silver nanoparticles and titanium dioxide will do the job, people in developing nations or rural settings typically need something a lot cheaper and easier to manufacture. As it turns out, wood from pine trees works great.
More specifically, it's pine sapwood that traps bacteria.
It's made up of a porous material known as xylem, that is used to draw sap up from the roots and into the rest of the tree. Xylem's internal structure consists of a series of microscopic vessels, which are linked together by pores in their walls. These pores, called pit membranes, allow sap to flow from vessel to vessel along the length of the tree. They're too small for air bubbles to pass through, though – that's a good thing, as bubbles in the "sapstream" can kill a tree.
As discovered by scientists at MIT, the pit membranes also let water pass through, yet are small enough that they block the passage of bacteria. In lab tests, pieces of the sapwood were glued into the inside of rubber tubes, which E. coli-contaminated water was then flowed through. When the wood was subsequently examined, approximately 99 percent of the bacteria that had been in the water was found to be trapped around the pit membranes in the first few millimeters of the filter.
According to the researchers' calculations, a single 1.5-inch-wide (38 mm) sapwood filter could be used to produce up to four liters (1 US gal) of drinking water per day. It couldn't be allowed to dry out when not in use, however.
Additionally, although the wood can catch most types of bacteria, it likely cannot filter out viruses, due to their smaller size. Sapwood from some other types of trees has smaller pores that could presumably trap smaller microbes, however, so the scientists are planning on conducting more research.
In fact, it's not just the wood from trees that can stop bacteria. A previous study indicated the seeds of the Moringa tree are also highly effective.
A paper on the MIT research was recently published in the journal PLoS ONE.