Wired microbes generate electricity from sewage

ENGINEERS at Stanford University have devised
a new way to generate electricity from sewage
using naturally occurring “wired microbes” as
mini power plants, producing electricity as
they digest plant and animal waste.
In a paper published Tuesday in the
Proceedings of the National Academy of
Sciences, co-authors Yi Cui, a materials
scientist, Craig Criddle, an environmental
engineer, and Xing Xie, an interdisciplinary
fellow, call their invention a microbial battery.
One day they hoped it would be used in
places such as sewage treatment plants, or to
break down organic pollutants in the “dead
zones” of lakes and coastal waters where
fertiliser runoff and other organic waste can
deplete oxygen levels and suffocate marine life.
At the moment, however, their laboratory
prototype is about the size of a D-cell battery
and looks like a chemistry experiment, with
two electrodes, one positive, the other
negative, plunged into a bottle of wastewater.
Inside that murky vial, attached to the
negative electrode like barnacles to a ship’s
hull, an unusual type of bacteria feast on
particles of organic waste and produce
electricity that is captured by the battery’s
positive electrode.
“We call it fishing for electrons,” said Criddle,
a professor in the Department of Civil and
Environmental Engineering and a senior fellow
at the Stanford Woods Institute for the
Environment.
Scientists have long known of the existence of
what they call exoelectrogenic microbes —
organisms that evolved in airless environments
and developed the ability to react with oxide
minerals rather than breathe oxygen as we do
to convert organic nutrients into biological
fuel.
During the past dozen years or so, several
research groups have tried various ways to use
these microbes as bio-generators, but tapping
this energy efficiently has proven challenging.
What is new about the microbial battery is a
simple yet efficient design that puts these
exoelectrogenic bacteria to work.  At the
battery’s negative electrode, colonies of wired
microbes cling to carbon filaments that serve
as efficient electrical conductors. Using a
scanning electron microscope, the Stanford
team captured images of these microbes
attaching milky tendrils to the carbon
filaments.
“You can see that the microbes make
nanowires to dump off their excess electrons,”
Criddle said. To put the images into
perspective, about 100 of these microbes could
fit, side by side, in the width of a human hair.
As these microbes ingest organic matter and
convert it into biological fuel, their excess
electrons flow into the carbon filaments and
across to the positive electrode, which is made
of silver oxide, a material that attracts
electrons.
The electrons flowing to the positive node
gradually reduce the silver oxide to silver,
storing the spare electrons in the process.
According to Xie, after a day or so the positive
electrode has absorbed a full load of electrons
and has largely been converted into silver.
At that point it is removed from the battery
and re-oxidised back to silver oxide, releasing
the stored electrons.
The Stanford engineers estimated that the
microbial battery could extract about 30 per
cent of the potential energy locked in
wastewater. That is roughly the same efficiency
at which the best commercially available solar
cells convert sunlight into electricity.
Of course, there is far less energy potential in
wastewater. Even so, the inventors say the
microbial battery is worth pursuing because it
could offset some of the electricity now use to
treat wastewater. That use currently accounts
for about three per cent of the total electrical
load in developed nations.
Most of this electricity goes toward pumping
air into wastewater at conventional treatment
plants where ordinary bacteria use oxygen in
the course of digestion, just like humans and
other animals.
Looking ahead, the Stanford engineers say
their biggest challenge will be finding a cheap
but efficient material for the positive node.
“We demonstrated the principle using silver
oxide, but silver is too expensive for use at
large scale,” said Cui, an associate professor of
materials science and engineering, who is also
affiliated with the SLAC National Accelerator
Laboratory. “Though the search is underway
for a more practical material, finding a
substitute will take time.”