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Using bacteria to create a water filter that kills bacteria
21 January 2019
More than one in 10 people in the world lack
basic drinking water access, and by 2025, half of the world's population
will be living in water-stressed areas, which is why access to clean
water is one of the National Academy of Engineering's Grand Challenges.
Engineers at Washington University in St. Louis have designed a novel
membrane technology that purifies water while preventing biofouling, or
buildup of bacteria and other harmful microorganisms that reduce the
flow of water.
And they used bacteria to build such filtering membranes.
Srikanth Singamaneni, professor of mechanical engineering &
materials science, and Young-Shin Jun, professor of energy,
environmental & chemical engineering, and their teams blended their
expertise to develop an ultrafiltration membrane using graphene oxide
and bacterial nanocellulose that they found to be highly efficient,
long-lasting and environmentally friendly. If their technique were to be
scaled up to a large size, it could benefit many developing countries
where clean water is scarce.
The results of their work were published as the cover story in the 2 January issue of Environmental Science & Technology.
Biofouling accounts for nearly half of all membrane fouling and is
highly challenging to eradicate completely. Singamaneni and Jun have
been tackling this challenge together for nearly five years. They
previously developed other membranes using gold nanostars, but wanted to
design one that used less expensive materials.
Their new membrane begins with feeding Gluconacetobacter hansenii
bacteria a sugary substance so that they form cellulose nanofibers when
in water. The team then incorporated graphene oxide (GO) flakes into the
bacterial nanocellulose while it was growing, essentially trapping GO
in the membrane to make it stable and durable.
After GO is incorporated, the membrane is treated with base
solution to kill Gluconacetobacter. During this process, the oxygen
groups of GO are eliminated, making it reduced GO. When the team shone
sunlight onto the membrane, the reduced GO flakes immediately generated
heat, which is dissipated into the surrounding water and bacteria
nanocellulose.
Ironically, the membrane created from bacteria also can kill bacteria.
"If you want to purify water with microorganisms in it, the reduced
graphene oxide in the membrane can absorb the sunlight, heat the
membrane and kill the bacteria," Singamaneni said.
Singamaneni and Jun and their team exposed the membrane to E. coli
bacteria, then shone light on the membrane's surface. After being
irradiated with light for just 3 minutes, the E. coli bacteria died. The
team determined that the membrane quickly heated to above the 70
degrees Celsius required to deteriorate the cell walls of E. coli
bacteria.
While the bacteria are killed, the researchers had a pristine
membrane with a high quality of nanocellulose fibres that was able to
filter water twice as fast as commercially available ultrafiltration
membranes under a high operating pressure.
When they did the same experiment on a membrane made from bacterial
nanocellulose without the reduced GO, the E. coli bacteria stayed
alive.
"This is like 3-D printing with microorganisms," Jun says. "We can
add whatever we like to the bacteria nanocellulose during its growth. We
looked at it under different pH conditions similar to what we encounter
in the environment, and these membranes are much more stable compared
to membranes prepared by vacuum filtration or spin-coating of graphene
oxide."
While Singamaneni and Jun acknowledge that implementing this
process in conventional reverse osmosis systems is taxing, they propose a
spiral-wound module system, similar to a roll of towels. It could be
equipped with LEDs or a type of nanogenerator that harnesses mechanical
energy from the fluid flow to produce light and heat, which would reduce
the overall cost.
Microbial Removals by a Novel Biofilter Water Treatment System
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4385770/
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