The interiors of nonflowering trees such as pine and ginkgo contain sapwood lined with
straw-like conduits known as xylem, which 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.
MIT engineers have been investigating sapwood’s natural filtering ability, and have previously
fabricated simple filters from peeled cross-sections of sapwood branches, demonstrating that the
low-tech design effectively filters bacteria.
Now, the same team has advanced the technology and shown that it works in real-world situations.
They have fabricated new xylem filters that can filter out pathogens such as E. coli and
rotavirus in lab tests, and have shown that the filter can remove bacteria from contaminated
spring, tap, and groundwater.
They 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.
Their results, published in the journal Nature Communications, 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 has launched an open-source website, with guidelines for designing and fabricating
xylem filters from various tree types.
The website is intended to support entrepreneurs, organizations, and leaders to introduce the
technology to broader communities, and inspire students to perform their own science experiments
with xylem filters.
The researcher says that 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.
For places where the only option has been to drink unfiltered water, the researchers
expect xylem filters would improve health, and make water drinkable.
In their prior studies of xylem, the researchers found that the woody material’s natural
filtering ability also came with 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.
Surprisingly, 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 researchers found that 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 sliced and treated small cross-sections of white pine from branches around the MIT
campus and showed that the resulting filters maintained a permeance comparable to commercial
filters, even after being stored for up to two years, significantly extending the filters’
shelf life.
The researchers also 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 water treatment level
that meets the “two-star comprehensive protection” category set by the World Health Organization.
But there are chemical contaminants like arsenic and fluoride where the researchers don’t
know the effect yet.
Encouraged by their results in the lab, the researchers moved to field-test their designs
in India, a country that has experienced the highest mortality rate due to water-borne
disease in the world, and where safe and reliable drinking water is inaccessible to more than
160 million people.
The researchers fabricated filters from native pine trees and tested them, along with filters
made from ginkgo trees in the U.S., with local drinking water sources.
These tests confirmed that the filters effectively removed bacteria found in the local water.
The researchers also held interviews, focus groups, and design workshops to understand
local communities’ current water practices, and challenges and preferences for water treatment
solutions.
They also gathered feedback on the design.
One of the things that scored very high with people was the fact that this filter is a
natural material that everyone recognizes.
The researchers also found that people in low-income households prefer to pay a smaller
amount on a daily basis, versus a larger amount less frequently.
That was a barrier to using existing filters, because replacement costs were too much.
With information from more than 1,000 potential users across India, they 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-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.
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 practice water purification as part
of their daily lives — for instance, by providing replacement filters in affordable,
pay-as-you-go packets.
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.