A cheap and effective water filter has been made from the sapwood of pine tree branches. MIT researchers found that xylem – the porous transport tissue in plants – filters microbes from water, making it safer to drink.
Lead researcher, Professor Rohit Karnik, explains that xylem tissue allows sap to flow through it while preventing the spread of vapour bubbles. ‘Xylem’s structure comprises closed-end conduits (tubes) arranged in parallel, with membranes connecting adjacent conduits. The porosity of the membranes that trap these bubbles is in the same range required to filter microbes from water – essentially making the xylem tissue a filter material.’
Having studied the xylem structures of several species, the team settled on pine conifers because the xylem conduits are short and constitute most of the sapwood. Karnik says, ‘This makes it easy to isolate the xylem and enables the construction of thinner filters compared to that from other types of plants.’
The process of creating these filters is straightforward. Simply take a branch of pine, peel off the bark, clamp it onto a tube and flow water through the sapwood using pressure. ‘This is not recommended as a replacement for certified filters,’ Karnik says, ‘but it does provide a method to make water safer to drink.’
The aim, however, is not to provide rustic filters for parched scouts. The technology performs too well for that. So far, the pine sapwood has been used to filter out about 99.99% of E. coli bacteria from water. The intention is to create low-cost, replaceable xylem filters for portable devices.
Before this can come to pass, the researchers have several problems to overcome, the main one being that the filter only works when the wood is wet. Karnik notes, ‘[At the moment], conifer xylem has flexible membranes that plug to prevent bubble flow, and drying has the same effect of plugging the pores.’
The team is investigating how to use dry sapwood filters and is also looking to improve filtration performance. Karnik says, ‘It may take two to three years to develop the first prototypes for large-scale use. If these prototype filters tackle the spread of waterborne diseases, we will strive to make them available to as many people as possible.’