Filters made from branches used to decontaminate drinking water

Nonflowering trees such as gingko and pine contain sapwood, which is lined with straw-like conduits known as xylem. This is a type of transport tissue which moves water and nutrients from the roots to stems and leaves of plans. Xylem conduits are interconnected via thin membranes which act like sieves, filtering bubbles from water and other liquids.

The MIT team had previously created low-tech filters from the peeled cross-sections of sapwood branches, demonstrating that the design is an effective filter for bacteria. Now, they have advanced the natural filter and shown that it works well in real-world scenarios, filtering pathogens such as E. coli and rotavirus from contaminated spring, tap and groundwater.

“Because the raw materials are widely available and the fabrication processes are simple, one could imagine involving communities in procuring, fabricating and distributing xylem filters,” said Professor Rohit Karnik, a mechanical engineer at MIT involved with the study. “For places where the only option has been to drink unfiltered water, we expect xylem filters would improve health and make water drinkable.”

In previous studies, Karnik and his colleagues found that there were limitations to the natural filtering ability of sapwood: as the wood dried, its membranes began to stick to the walls, reducing its permeance. The filters also appeared to 'self-block' over time as they became clogged with woody water.

Two simple treatments were found to overcome these limitations. By soaking small cross-sections of sapwood in hot water for an hour, dropping them in ethanol and leaving them to dry, the material can retain its permeance. Its filtering can also be improved by tailoring a filter’s thickness according to the type of tree it is sourced from.

They sliced and treated cross-sections of white pine to create filtration systems and found that they could maintain a permeable comparable to commercial filters, even after being stored in dry form for up to two years. They also found that they could remove more than 99 per cent of E. coli and rotavirus (the most common cause of diarrhoeal disease); this could meet the WHO’s “two-star comprehensive protection” category of water treatment quality.

The researchers fabricated new xylem filters from native Indian pine and tested them with people from mountain and urban regions. Based on their feedback and findings, they created a prototype filtration system which effectively removes bacteria found in local groundwater. It takes the form of a metre-long tube containing a xylem filter, with a receptable at the top to fill with water and a spout at the bottom. The xylem filter can be swapped out either daily or weekly. The system can purify water at a rate of one litre per hour.

They are now exploring options for making this system available at large scale - such as by providing replacement filters in affordable packets - particularly in areas where contaminated drinking water remains a serious cause of disease and death. In the meantime, they have launched a website with guidelines for creating xylem filters from various types of tree.

“Xylem filters are made from inexpensive and abundantly available materials, which could be made available at local shops where people can buy what they need without requiring an upfront investment, as is typical for other water filter cartridges,” said Karnik. “For now, we’ve shown that xylem filters provide performance that’s realistic.”