Sapwood is a promising low-cost and efficient material for water filtration that could be used to provide filtered water in rural communities.

Massachusetts Institute of Technology (MIT) researchers have found that a small piece of sapwood from pine trees can filter more than 99% of E.coli from water. The wood’s pores allow water through while blocking most types of bacteria.

Rohit Karnik, an associate professor of mechanical engineering at MIT, says, “Today’s filtration membranes have nanoscale pores that are not something you can manufacture in a garage very easily. The idea here is that we don’t need to fabricate a membrane, because it’s easily available. You can just take a piece of wood and make a filter out of it.”

Karnik says sapwood could be used to filter water for rural communities where more advanced filtration systems are not readily accessible. It may offer a low-cost, small-scale alternative to the numerous water purification technologies on the market, many of which come with drawbacks. For example, systems that rely on chlorine treatment work well at large scales, but are expensive while boiling water to remove contaminants requires a great deal of fuel. Membrane-based filters are also expensive and can become easily clogged.

Pine sapwood can filter most types of bacteria, the smallest of which measure about 200 nanometers, but probably cannot trap most viruses, which are much smaller in size. The researchers plan to evaluate the filtering potential of other types of sapwood, including those with smaller pores.

The research

The researchers collected branches of white pine and stripped off the outer bark, cutting small sections of sapwood measuring about an inch long and half an inch wide.

Before experimenting with contaminated water, researchers used water mixed with red ink particles ranging from 70 to 500 nanometers in size. They observed that much of the red dye was contained within the top layers of the wood, while the filtered water was clear.

The experiment showed that sapwood is naturally able to filter out particles bigger than about 70 nanometers. However, there is a limit to the size of particles coniferous sapwood can filter. Another experiment found that sapwood was unable to separate out 20-nanometer particles from water.

When the researchers put inactivated, E. coli-contaminated water through the wood filter they found that the bacteria had accumulated around pit membranes in the first few millimetres of the wood. The sapwood was able to filter out more than 99% of E. coli from water.