Nothing to Shake a Stick At: MIT Introduces Tree-Based Water Filters


diy-survival-filter-dirty-water-with-stick.w654  The UN estimates that 85% of the world’s population lives in the driest half of the planet. 783 million people don’t have access to clean water, and roughly 2.5 billion people don’t have access to sanitation. Various systems have been developed using charcoal, sand or chlorination to filter impurities and contaminants. Some are costly, some are complicated, some require fuel or pumps, and need massively outweighs demand. But a group of engineers at MIT have discovered how to use trees to filter water for very little cost. The cost is so low that the system could potentially be disposable. The innovation is still in the process of development but if a prototype, expected in the next 2-3 years, proves to be a success, it may drastically increase access to water for people in the Global South and around the world.

Xylem is a tissue in plants that helps to bring the water and nutrients in the soil through the roots up to the leaves in the form of sap. Through evaporation, water is drawn up from the roots, through the stem to the leaves. During this process air bubbles form, which impede the ability for the sap to be drawn up into the leaves. The xylem in the plant has pores and membranes that prevent the bubbles from spreading, thereby allowing the sap to reach the leaves, and for the plant to receive the nourishment it needs. Rohit Karnik and his team at MIT is working to develop a water filter using this function.

rohit_karnikTo test the theory, Karnik and his engineers used the sapwood of a pine tree (the layer directly under the bark). They cut a piece only a few centimetres long and attached one end to a clear tube. They sent a solution with red dye through it first, to show that the wood was able to carry the solution through to the tube. The liquid that passed through the stick came out clear. They moved on to a solution containing a high concentration of E coli bacteria. The process was the same and the strip of wood was able to filter out 99.9% of the bacteria. Multiple tests determined that one 3cm-long strip will filter several litres of water a day, which is sufficient for a single person. The vision for marketing the filter involves attaching it to a water container that is placed above a tap or spigot. No pump is necessary. This set up could filter up to 4 litres of water a day, and each strip of wood can last a number of days, depending on how heavy the usage is. The cost of the wood strips would be so low that users, even in developing countries, could throw them away and replace them every few days.

xylem_water_filter_mitBut could a solution so seemingly simple and available in almost every wooded area across the globe be too good to be true? The concept has some specialists scratching their heads. Johnathan Ball, virologist at the University of Nottingham, noted that at present, the device is capable of filtering out bacteria and contaminants that are between 70 – 200 nanometres, but nothing smaller. This means that some contaminants, such as waterborne pathogens, may slip through.  Further, Daniele Lantagne, an Engineering professor at Tufts University, noted that the filter must be tested according to WHO guidelines for the evaluation of household water-treatment products. One guideline designates am amount of 0% as allowable e-coli in a 100 ml water sample. The 0.01% that the device leaves behind is problematic. Additionally, the wood must be kept damp for the xylem to do its work. This might be difficult in arid regions, such as parts of Africa, where the need is greatest.

Clearly the prototype will have to undergo further testing and revisions in order to become a viable alternative to currently available water filtration systems. The research team at MIT is aware of these limitations, and has plans to address them. In this study, the sapwood of a white pine was used. Plans to test other varieties of sapwood trees are underway, as are flowering trees. The rationale is that flowering trees have smaller pores than coniferous trees, thus they may be able to filter smaller particles. In its current state, Dr. Saurya Prakash at Ohio State University can see it being useful in areas where people collect surface water. Perhaps the water might not be filtered to the highest degree, but it would be enough to filter out dust and decaying plant and animal matter that often collects in rain or surface water pools. In terms of increasing access to water across the Global South, we may not be out of the woods yet. But with this device, we are moving in the right direction.

The study occurred at the Microfluids and Nanofluids Research Lab at MIT in Cambridge, Massachusetts, and was funded with a grant from the James H Ferry Jr. Fund for Innovation in Research Education.

Click here to read about the study in its entirety, and more about the project and it’s developments is here.

The innovation is featured during MIT’s first Water Summit. Find out more through a video here.


Sarah Anstett

Sarah Anstett

Sarah is a writer, researcher, and development practitioner currently based in Toronto, Canada.

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