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The Good News
2014 April
Pg 3 - The Sunshine Express
Nature¡¯s Low Tech Water Filter
the size of the pores in sapwood - which contains
xylem tissue evolved to transport sap up the length
of a tree - also allows water through while blocking
most types of bacteria.
Co-author Rohit Karnik, an associate professor of
mechanical engineering at MIT, says sapwood is
a promising, low-cost, and efficient material for
water filtration, particularly for rural communities
where more advanced filtration systems are not
readily accessible.
¡°Today¡¯s filtration membranes have nanoscale
pores that are not something you can manufacture
in a garage very easily,¡± Karnik says. ¡°The idea
here is that we don¡¯t need to fabricate a membrane,
because it¡¯s easily available. You can just take a
piece of wood and make a filter out of it.¡±
The paper¡¯s co-authors include Michael Boutilier
and Jongho Lee from MIT, Valerie Chambers from
Fletcher-Maynard Academy in Cambridge, Mass.,
and Varsha Venkatesh from Jericho High School in
Jericho, N.Y.
Tapping the flow of sap
There are a number of water-purification technolo-
gies on the market today, although many come
with drawbacks: Systems that rely on chlorine
treatment work well at large scales, but are expen-
sive. Boiling water to remove contaminants re-
quires a great deal of fuel to heat the water. Mem-
brane-based filters, while able to remove microbes,
are expensive, require a pump, and can become
easily clogged.
Sapwood may offer a low-cost, small-scale alter-
native. The wood is comprised of xylem, porous
tissue that conducts sap from a tree¡¯s roots to its
crown through a system of vessels and pores. Each
vessel wall is pockmarked with tiny pores called
pit membranes, through which sap can essentially
hopscotch, flowing from one vessel to another as
it feeds structures along a tree¡¯s length. The pores
also limit cavitation, a process by which air bubbles
can grow and spread in xylem, eventually killing
a tree. The xylem¡¯s tiny pores can trap bubbles, pre-
venting them from spreading in the wood.
¡°Plants have had to figure out how to filter out
bubbles but allow easy flow of sap,¡± Karnik ob-
serves. ¡°It¡¯s the same problem with water filtration
where we want to filter out microbes but maintain
a high flow rate. So it¡¯s a nice coincidence that the
problems are similar.¡±
Seeing red
To study sapwood¡¯s water-filtering potential, the
researchers collected branches of white pine and
stripped off the outer bark. They cut small sections
of sapwood measuring about an inch long and half
an inch wide, and mounted each in plastic tubing,
sealed with epoxy and secured with clamps.
Before experimenting with contaminated water,
the group used water mixed with red ink particles
ranging from 70 to 500 nanometers in size. After all
the liquid passed through, the researchers sliced
the sapwood in half lengthwise, and observed that
much of the red dye was contained within the very
top layers of the wood, while the filtrate, or filtered
water, was clear. This experiment showed that sap-
wood is naturally able to filter out particles bigger
than about 70 nanometers.
However, in another experiment, the team found
MIT group shows xylem tissue in sapwood
can filter bacteria from contaminated water
Jennifer Chu, MIT News Office, Feb 26, 2014: If
you¡¯ve run out of drinking water during a lakeside
camping trip, there¡¯s a simple solution: Break off
a branch from the nearest pine tree, peel away the
bark, and slowly pour lake water through the stick.
The improvised filter should trap any bacteria,
producing fresh, uncontaminated water.
In fact, an MIT team has discovered that this low-
tech filtration system can produce up to four liters
of drinking water a day; enough to quench the
thirst of a typical person.
In a paper published this week in the journal PLoS
ONE, the researchers demonstrate that a small
piece of sapwood can filter out more than 99 per-
cent of the bacteria E. coli from water. They say
Need a water
filter? Peel a
tree branch.
Image cour-
tesy of the
A Balanced Plan
that sapwood was unable to separate out 20-nano-
meter particles from water, suggesting that there is
a limit to the size of particles coniferous sapwood
can filter.
Picking the right plant
Finally, the team flowed inactivated, E. coli-con-
taminated water through the wood filter. When
they examined the xylem under a fluorescent mi-
croscope, they saw that bacteria had accumulated
around pit membranes in the first few millimeters
of the wood. Counting the bacterial cells in the
filtered water, the researchers found that the sap-
wood was able to filter out more than 99 percent
of E. coli from water.
Karnik says sapwood likely can filter most types
of bacteria, the smallest of which measure about
200 nanometers. However, the filter probably can-
not trap most viruses, which are much smaller in
Karnik says his group now plans to evaluate the
filtering potential of other types of sapwood. In
general, flowering trees have smaller pores than
coniferous trees, suggesting that they may be
able to filter out even smaller particles. However,
vessels in flowering trees tend to be much longer,
which may be less practical for designing a com-
pact water filter.
Designers interested in using sapwood as a filter-
ing material will also have to find ways to keep
the wood damp, or to dry it while retaining the
xylem function. In other experiments with dried
sapwood, Karnik found that water either did not
flow through well, or flowed through cracks, but
did not filter out contaminants.
¡°There¡¯s huge variation between plants,¡± Karnik
says. ¡°There could be much better plants out there
that are suitable for this process. Ideally, a filter
would be a thin slice of wood you could use for
a few days, then throw it away and replace at
almost no cost. It¡¯s orders of magnitude cheaper
than the high-end membranes on the market
While the pores in sapwood are too big to filter
out salts, Saurya Prakash, an assistant professor of
mechanical engineering at Ohio State University,
says the design could be useful in parts of the
world where people collect surface water, which
can be polluted with fine dust and particles of
decaying plant and animal matter. Most of this
detritus, Prakash says, could easily be filtered out
by the group¡¯s design.
¡°The xylem tissue acts as a natural filter, similar to
a manmade membrane,¡± says Prakash, who was
not involved in the research. ¡°The study by the
Karnik group shows that use of abundant, natu-
rally occurring materials could pave the way for a
new generation of water filters that are potentially
low-cost enough to be disposable.¡±
This research was supported by the James H. Ferry
Jr. Fund for Innovation in Research Education.
(Reprinted with permission of MIT News, newsof-
BLM releases Colorado River Valley
Resource Management Plan
SILT: The Bureau of Land Management, on March
24, released the Proposed Resource Management
Plan and Final Environmental Impact Statement
for the Colorado River Valley Field Office in north-
western Colorado, that balances continued oil
and gas development and other resource use with
protection of sensitive resources.
The plan will provide a framework to guide sub-
sequent management decisions on 505,200 surface
acres and 707,200 acres of subsurface mineral
estate administered by the BLM¡¯s Colorado River
Valley Field Office in Eagle, Garfield, Mesa, Pitkin,
Rio Blanco and Routt counties for approximately
the next two decades.
¡°The extensive involvement from the public as
well as our local, state and federal cooperating
agencies for the past six years has been critical to
developing this Resource Management Plan,¡± said
Steve Bennett, Field Manager for the Colorado
River Valley Field Office. ¡°We have developed a
plan that balances protection of sensitive resources
with resource use.¡±
The Proposed RMP is based largely on the Pre-
ferred Alternative presented in the Draft EIS re-
leased for public comment in September 2011, but
it includes elements of other alternatives analyzed
in that Draft.
Key decisions in the Proposed RMP include
designating approximately 675 miles of routes for
motorized use and 587 miles for non-motorized
use,managing five areas totaling 34,400 acres to
protect existing wilderness characteristics, includ-
ing Thompson Creek and Deep Creek; and recom-
mending Wild and Scenic River designation for
the two BLM segments of Deep Creek.
The majority of the 147,500 acres with high po-
tential for oil and natural gas production within
the Colorado River Valley Field Office are already
leased and, under the Proposed RMP, will con-
tinue producing. The 98,100 acres closed to future
oil and gas leasing in the Proposed RMP include
State Wildlife Areas, areas managed for wilder-
ness characteristics, municipal boundaries, suit-
able Deep Creek Wild and Scenic River segments
and other designated recreation areas or areas of
critical environmental concern.
The Proposed RMP is now available for a 30-day
public protest period from the date the Notice of
Availability is published in the Federal Register,
which ends April 27, 2014. Information about the
protest period is available at:
gsfo.html or by calling the Colorado River Valley
Field Office at 970.876.9000. The BLM has also sent
the Proposed RMP to the Governor¡¯s Office for a
60-day consistency review.
The BLM will issue a Record of Decision detailing
the final decisions in the RMP after the consistency
review and resolution of any protests.
The BLM manages more than 245 million acres
of public land primarily located in 12 Western
states, including Alaska. The BLM also adminis-
ters 700 million acres of sub-surface mineral estate
throughout the nation