A Cheap, Promising Way to Filter Water: Through A Twig

Plant's xylem tissue could be an efficient, low-cost way to filter water. 

By 

Douglas Main

 Posted 03.03.2014 at 6:00 pm 

How to make a xylem filter 

Boutilier et al / PLOS ONE

One way to avoid getting sick while traveling is to only eat fruit that you peel yourself, since plants can filter out bacteria and prevent it from traveling throughout their tissues. Well, why not apply this principle to filtering water directly? A team of scientists have done just that, testing how well pine wood filtered water with its xylem, the tube-like tissue that transports water from plant roots to leaves. The results, published last week in the journal PLOS ONE, were very promising. "Filtration using three different xylem filters showed nearly complete rejection of the bacteria," catching at least 99.9% of them, the authors wrote. 
The xylem filters used in the study appeared to catch nearly all particles larger than 100 nanometers in diameter, meaning they would exclude protozoa (like Giardia) as well. Smaller viruses would be expected to pass through, but research suggests that other types of wood with smaller pores could possibly be used to filter out these pathogens as well; pine was used in this experiment since a larger percentage of its cross-section is made up of xylem, making it a more feasible choice for a prototype. 
To make a filter, all you need to do is peel the bark off a pine twig and stick it into a tube, sealing holes between twig and tube with epoxy. Then the pressure must be optimized, which is a little bit trickier. But once that's worked out, each twig-filter processed 4 liters of water per day, enough for one person.
The particles are filtered out in the wood's pits, the sieve-like holes between adjacent tube-like conduits of xylem. The pits' nanoscale pores "perform the critical function of preventing bubbles from crossing over from one conduit to another," allowing water to flow from the ground to the leaves in live plants, but also filtering out bacteria for us humans. Fresh wood appears to filter material much better than dead wood.
"The simple construction of xylem filters, combined with their fabrication from an inexpensive, biodegradable, and disposable material suggests that further research and development of xylem filters could potentially lead to their widespread use and greatly reduce the incidence of waterborne infectious disease in the world," the authors concluded.