The interiors of nonflowering trees, such as pine and ginkgo, contain sapwood lined with straw-like conduits known as xylem that draw water up through a tree’s trunk and branches. Xylem conduits are interconnected via thin membranes that act as natural sieves, filtering out bubbles from water and sap.
Engineers have fabricated simple filters from peeled cross-sections of sapwood branches, demonstrating that the design effectively filters bacteria. New xylem filters were developed that can filter out pathogens such as E. coli and rotavirus in lab tests and can remove bacteria from contaminated spring, tap, and groundwater. The team also developed simple techniques to extend the filters’ shelf-life, enabling the woody disks to purify water after being stored in a dry form for at least two years.
The researchers took their techniques to India, where they made xylem filters from native trees and tested the filters with local users. Based on their feedback, the team developed a prototype of a simple filtration system, fitted with replaceable xylem filters that purified water at a rate of one liter per hour. The results show that xylem filters have potential for use in community settings to remove bacteria and viruses from contaminated drinking water.
The researchers are exploring options to make xylem filters available at large scale, particularly in areas where contaminated drinking water is a major cause of disease and death. The team launched an open-source website with guidelines for designing and fabricating xylem filters from various tree types.
The woody material’s natural filtering ability had some natural limitations. As the wood dried, the branches’ sieve-like membranes began to stick to the walls, reducing the filter’s permeance, or ability to allow water to flow through. The filters also appeared to “self-block” over time, building up woody matter that clogged the conduits.
Two simple treatments overcame both limitations. By soaking small cross-sections of sapwood in hot water for an hour, then dipping them in ethanol and letting them dry, the material retained its permeance, efficiently filtering water without clogging up. Its filtering could also be improved by tailoring a filter’s thickness according to its tree type.
The researchers tested the filters’ ability to remove contaminants such as E. coli and rotavirus — the most common cause of diarrheal disease. The treated filters removed more than 99 percent of both contaminants.
A prototype of a simple filtration system was fitted with a receptacle at the top that users can fill with water. The water flows down a 1-meter-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.