Pinewood stems as cheap water filter systems

CAN TREES like pine yield an efficient water filtration system that can reduce the disease burden associated with contaminated drinking water? A team at the Massachusetts Institute of Technology (MIT), which includes many India-born scientists, may have tapped the ability of stems of non-flowering trees like conifers to work as natural sieves. 

The interiors of these trees contain straw-like conduits known as xylem, which draw water up through a tree’s trunk and branches. These conduits are interconnected through thin membranes for filtering out bubble from water and sap that pass through. Exploiting this natural filtering ability of sapwood, engineers led by Rohit Karnik, professor of mechanical engineering at MIT, fabricated filters that can remove pathogens like E. coli and rotavirus from drinking water. Further, they devised simple techniques that can extend the filter’s shelf-life, enabling the woody disks to purify water after being stored in a dry form for at least two years. The scientists demonstrated that the filter can work in real-world situations by carrying out trials in India by purifying water from contaminated spring, tap and groundwater sources. 

The low-tech design was further perfected based on feedback received from users in India, leading to development of a prototype of a simple filtration system, fitted with replaceable xylem filters. This low-cost water purifying system, capable of giving potable water at a rate of one litre per hour, was reported in Nature Communications journal in late March. 

The scientists are now looking to make these filters available at large scale in areas where contaminated drinking water is a serious issue. They are also making the design of the filter freely available to anybody interested in making such filters, including entrepreneurs and civil society or community organisations working with people living in areas with poor quality of water. 
To make these filters work, the MIT researchers had to overcome some of the natural limitations faced by sapwood once it dries up. For instance, when the wood dies up, the sieve-like membranes begin to stick to the walls, reducing the filter’s ability to allow water to flow through. Besides, the build-up of woody matter inside can also clog the conduits. 

The researchers overcame these limitations through two simple treatments. They first soaked small cross-sections of sapwood in hot water for an hour and dipped them subsequently in ethanol. Once it dried, the material is capable of retaining its permeance as good as commercially available filters, they found. 

The researchers who worked on the xylem filers include Krithika Ramachander, Megha Hegde, Anish Antony, Luda Wang, Kendra Leith and Amy Smith. This research was supported, in part, by the Abdul Latif Jameel Water and Food Systems Lab at MIT and the MIT Tata Center for Technology and Design. 

Once satisfied with the performance of the filter in the lab, the researchers tested the filter – made using locally available wood – in communities in the Himalayan region as well as in a low-income urban settlement in New Delhi for two years. 

Based on feedback from over 1,000 potential users across India, the MIT engineers designed a prototype of a simple filtration system, fitted with a receptacle at the top that users can fill with water. The water flows down a 1-metre-long tube, through a xylem filter, and out through a valve-controlled spout. The xylem filter can be swapped out either daily or weekly, depending on a household’s needs. 

The team is exploring ways to produce xylem filters at larger scales, with locally available resources and in a way that would encourage people to practise water purification as part of their daily lives -- for instance, by providing replacement filters in affordable, pay-as-you-go packets.