Clean water might not grow on trees, but those trees might do the next best thing—provide a cheap and easy way of filtering it.
Researchers from the Massachusetts Institute of Technology have been able to use wood from tree branches to filter water both in the laboratory and in the field. This could provide an inexpensive, biodegradable, low-tech alternative to costly methods of getting clean water in places that need it the most.
In a new study, researchers show that the natural structure of wood from non-flowering plants, such as pine and cedar, provides enough filtration to remove E. coli, rotavirus and other bacteria from water. This method was already proven to work in a lab setting back in 2014, but now the researchers have found, through interviews with potential users and tests of local water sources, that the tree-based purification system works just as well in the field in India and that locals are willing to use it.
The branches of gymnosperms, or non-flowering plants, are built up of thin, straw-like tubes called xylem in their sapwood, the layer of wood under the bark. The unique structure of the xylem tissue in the branches allows it to work as a water filter because these channels are connected to each other through porous membranes, says Krithika Ramchander, a graduate student at MIT and lead author of the new study.
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“The pores of these membranes range in size from a few nanometers to a few hundred nanometers, which is smaller than the size of water contaminants such as particulates, bacteria and protozoa,” she says. These membranes can “entrap” contaminants, similar to a water filter membrane made from synthetic materials you may find in a water filtration system for backpacking or camping.
Ramchander says a four centimeter wide, one centimeter thick disk of wood can be rigged to function similarly to a conventional gravity-based water filtration system. Water is poured into a vessel and runs through a tube hanging underneath. The bottom of the tube is fitted with the wood xylem filter, and as water passes through it drips slowly, now clean, into a receptacle at the bottom of the set-up at a rate of one liter per hour (which is still quite a bit slower than other options on the market). Ramchander says many different designs are possible, and replacing the filter disk is cheap and easy.
The researchers were even able to extend the shelf-life of dry wood filters to around two years by dipping them in a protective mix of hot water and ethanol. The wood xylem filters work so well, that by the World Health Organization classifying scheme they would be ranked in the two star “comprehensive protection” category, which is the second highest category for household water treatment.
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Diarrheal diseases are responsible for thousands of deaths per year around the world, particularly in children, and 88 percent of these deaths can be attributed to unsafe drinking water and sanitation issues—and in parts of India, these issues are especially rampant, inspiring the researchers to center their work there, Ramchander says.“India has the highest water-borne illness mortality rate in the world with more than 160 million people lacking access to safe and reliable water,” she says. In field tests, ease of use and accessibility are just as key as how effective the technology. Fortunately, India’s native chir pine wood worked brilliantly to safely, and simply, filter water out in the real world.
To make this potentially life-saving technology be reachable to those who need it, the construction of the filter is open source and instructions can be found on their website detailing how anyone can try to construct their own wood water filter. “We have received an incredible amount of interest from students around the world, as well as from non-profits and entrepreneurs,” says Rohit Karnik, a professor of mechanical engineering at MIT and study author.
He says that they have been contacted by entrepreneurs from developing countries who are interested in using a widely-available resource to make water filtration easier to access. “We hope that our work empowers such people to further develop and commercialize xylem water filters tailored to local needs to benefit communities around the world.”
It may take time to get xylem filters in large-scale use, says Karnik, but they are working on the challenges of logistics, quality control standards and production. “We are collaborating with a non-profit organization with the goal of setting up a pilot manufacturing plant, establishing a business model, and performing pilot studies of filter use.”
