Increasingly, problems of rising energy demands, dwindling resources,
and pollution concerns are being mitigated by turning
waste into usable products. Now some researchers are eyeing organic wastes
from homes, food processing, and other sources as
an energy feedstock--bacteria including Rhodoferax and Geobacter are
being harnessed in devices called microbial fuel cells (MFCs) to break down
organic
waste products, converting the energy of their chemical
bonds into electricity and hydrogen.
Significant Energy Resource
In the United States, 46 trillion liters of household wastewater
are treated annually, according to an article by Bruce Logan, director
of the Hydrogen Energy Center at The Pennsylvania State University,
in the 1 May 2004 issue of Environmental Science & Technology.
This costs $25 billion, and the electricity required--mostly for
aeration--constitutes 1.5% of the electricity used in the nation,
says Lars Angenent, an assistant professor in the Department of
Chemical Engineering at Washington University in St. Louis. According
to Angenent, most of that energy could be saved by treating wastewater
using MFCs. He says one of these devices could produce enough extra
energy to power 900 homes by treating the wastes from a single
large food processing plant. According to Logan, MFCs would cut
the cost of aerating activated sludge in wastewater by as much
as 50% of the electricity usage, and should generate 50-90% less
solids to be disposed of.
Logan put this potential in context in his 1 May 2004 article
when he wrote that the United States consumed 97 quads (short for “quadrillion
British thermal units”) of total energy in 2002; of this,
13 quads were generated electricity. Should hydrogen become the
transportation fuel of choice, as many believe it will--with most
hydrogen produced ultimately from fossil fuels--another 12 quads
would be required to make hydrogen from water, he wrote.
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Skimming the surface. Bruce Logan and colleagues
at Penn State have begun demonstrating that MFCs
can produce electricity directly from wastewater,
potentially cutting both power costs and solid
wastes.
image: Greg Grieco/The Pennsylvania
State University |
According to Logan, all the U.S. household wastewater produced
in one year contains 0.11 quad organic matter, livestock
production wastewater contains 0.3 quad, and food processing wastewater
possibly
0.1 quad. Though small, these amounts are potentially
significant, says Scott Sklar, the former executive director of
the Solar Energy
Industries Association and current president of The
Stella Group, an energy generation marketing and policy analysis
firm. There
will be no one-size-fits-all solution to the nation’s energy
problems, he says. Instead, energy will come from
many sources, many of them small sources, and power will be created
through a
patchwork of technologies tailored to local circumstances
and needs.
MFCs could also become important energy sources in the lesser
developed parts of the world, says Logan. These fuel cells used
locally produced fuel, and their power output can be managed locally. “Microbial
fuel cells [appear] destined, at least at this moment, to utilize
some energy resources that are not otherwise available on an industrial
scale, like sea bottom sediments, or some biomass from waste,” says
Plamen Atanassov, an assistant professor of chemical engineering
at the University of New Mexico. One candidate bacterium for MFCs, Rhodoferax
ferrireducens, was first isolated from sediments collected
in Oyster Bay, Virginia; Geobacter metallireducens was first
isolated from sediments from the Potomac River.
Breakthroughs Boost Prospects
MFCs go back to the early 1900s, says Angenent. It was at a 1996
American Chemical Society meeting titled “Emerging Technologies
in Hazardous Waste Management” that Korean scientists Byung
Hong Kim and Doo-Hong Park first described the use of a “mediator-less
biofuel cell” to treat wastewater. Breakthroughs in the last
five years have suggested fresh promise for this technology.
One breakthrough was the discovery, reported in the 18 January
2002 issue of Science by Derek Lovley, a professor in the
Department of Microbiology at the University of Massachusetts Amherst,
that Geobacter produces electricity. That followed the discovery
by German and Australian researchers, published in Bacteriology in
July 1998 (issue 14), that in certain iron-reducing bacteria, the
cytochromes--specialized enzymes known to transfer electrons to
other proteins--span the outer cell membrane, enabling direct transfer
of electrons to external metals and the creation of a circuit.
This is the ultimate source of electricity in MFCs. These discoveries
opened up the possibility of engineering both the bacteria and
the electrodes in the MFC to improve electron transfer.
In the 23 June 2005 issue of Nature, Lovley announced
the discovery of “nanowires,” literally tiny wires
produced by Geobacter, which the bacterium presumably uses
to transfer electrons. This discovery opened up further possibilities
for electron transfer. He also published a study in the Octobe
2003 issue of Nature Biotechnology showing that Rhodoferax provides
a constant flow of electrons while oxidizing glucose at 80% electron
efficiency--a boon for drawing power from carbohydrates.
Still another breakthrough was the discovery, published by Park
and University of Michigan molecular biologist J. Greg Zeikus in
the June 2002 issue of Applied Microbiology and Biotechnology,
that one could increase power output in MFCs by about sixfold by
using mixed microbial communities rather than pure cultures. This
is a big advantage for harvesting energy from wastewater, which
is microbially diverse, says Angenent. The question of exactly
why this is so is an area Angenent plans to address in future research.
The technology has also seen the benefit of engineering advances.
A year ago, in unpublished research, Angenent combined the “upflow” system
used in methane digesters with the MFC technology to eliminate
the need for mechanical pumping and mixing. In the upflow system,
wastewater is piped from above the fuel cell, down, around, and
then upwards into the bottom of the anode powered by gravity--the
opposite of a syphon. Thus, pumping and mixing become unnecessary.
The first microbial fuel cells produced between 1 and 40 milliwatts
per square meter (mW/m2) of anode electrode surface
area, says Logan. In just the past year, he says, his laboratory
has generated power in the range of up to 500 mW/m2 using
domestic wastewater and 1,500 mW/m2 with glucose and
air. He adds that researchers in Belgium recently achieved 3,600
mW/m2 using glucose, although they needed a nonrenewable
chemical instead of air for their process.
Electric versus Hydrogen
MFCs generate electricity, but can be modified to produce hydrogen
instead. In both systems, the source of electricity is the chemical
energy contained in the bonds of organic compounds. Bacteria, living
in biofilms on the anode, break down the organics, separating electrons
from protons. These electrons and protons then travel to the cathode,
the former via an external wire, the latter by diffusing through
the electrolyte, a substance that does not conduct electricity.
In the electricity-generating MFCs, the protons and electrons
combine at the cathode with oxygen to form water. This “uses
up” the electrons, allowing more to keep flowing from the
anode to the cathode.
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image: Chris Reuther/EHP |
In the MFC modified to produce hydrogen, the cathode is kept
free of oxygen. But in order to make hydrogen, a thermodynamic
barrier must be breached. To overcome this barrier,
Logan uses a power source to add voltage into the circuit.
The hydrogen MFC appears to be twice as efficient as the electricity-producing
cells, says Logan, because in the latter some oxygen leaks back
into the anode. However, adding the voltage in the hydrogen-producing
system requires about one-sixth of the energy that is produced
as hydrogen. Further losses occur if the hydrogen is converted
into other forms of energy. Bottom line: in terms of efficiency
for electricity as a final product, neither electricity nor hydrogen
production possesses a clear advantage.
The main benefit of hydrogen-producing MFCs is that they would
provide additional options to fit production to energy needs, says
Logan. For example, hydrogen could be stored to make off-peak electricity
or for use as a transportation fuel. “But if you just want
to use electricity locally, you are probably better off making
electricity to start with,” he says.
Many Technological Challenges
MFC technology is still strictly at the laboratory scale. “[It]
doesn’t have its own design principals, and borrows from
neighboring technologies,” says Atanassov. “It is absolutely
premature to even address [questions of design].”
The cathode oxygen in electricity-producing devices creates a
big challenge for MFCs. A “proton exchange membrane” separates
anode from cathode, allowing protons to pass, but blocking the
larger oxygen molecules from diffusing to the anode. However, some
oxygen manages to cross the proton exchange membrane into the anode,
where it takes electrons that would otherwise flow in the circuit,
reducing the power, says Lovley.
The low power density of MFCs is also a major problem. Researchers
working on MFCs measure power density in W/m2, while
those working on conventional fuel cells measure power density
in W/cm2, a highly illustrative disparity, says Atanassov.
That low power density of MFCs means electrodes--which aren’t
cheap--must be exceptionally bulky.
Power density is a function of the interface between the microbes
and the electrodes, says Harold Bright, a program manager in the
Office of Naval Research, which is funding studies on MFCs. “We
have fairly slow electron transfer from the bacteria into the electrode.”
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Big plans for small microbes. Jason
He (left) and Lars Angenent inspect their MFC.
In Derek Lovley’s lab (right), a model SUV is
powered by marine geobatteries.
image: Derek Lovley; David
Kipler/Washington University in St. Louis |
Scale-up for commercial uses adds to the challenges. The current
laboratory-scale prototypes use materials that aren’t sturdy
enough to be used in a commercial system, such as
carbon paper and carbon cloth electrodes. Further, experimental
MFCs, now smaller
than a beer mug, would need to be as big as a mansion
(in large part to compensate for the low power density), undoubtedly
greatly
increasing the distance between anode and cathode.
That, in turn, would slow diffusion of hydrogen from the former
to the latter,
damping efficiency.
To be competitive with methane digester technology, MFCs’ practical
predecessor, the power density must more than double the maximum
achieved so far, to 8,500 mW/m2, says Angenent. And
for this, he says, “another breakthrough is required.”
Advances in microbiology and electrode technology leading to
higher rates of electron transfer could improve power density;
bacteria could be engineered for better electron transfer. Lovley
has been systematically deleting genes for outer membrane cytochromes
in order to discern which cytochrome was essential for electricity
production. “Now we can determine if engineering Geobacter to
produce more of this cytochrome and/or modifying the electrode
to better interact with the cytochrome will result in more power
production,” he says.
There is ample room for improvement. “If Geobacter could
transfer electrons to electrodes as fast as it can to its natural
electron acceptor, ferric iron, the rate of electron flow--that
is, the current--could possibly be ten thousand times higher,” says
Lovley.
The use of wastes as cost-free substrates will further improve
economics, says Logan. Wastes are ideal since their disposal, he
says, “is already an economic burden.”
Currently, there is virtually no government funding for MFCs
except for use in applications such as remote sensors,
which are funded by the Navy, the Department of Energy, and the
Defense Advanced
Research Projects Agency. “The current laboratory systems
that we build cost way too much money for the amount
of electricity we get back,” Logan admits. “[But] the same was
true of solar energy fifty years ago.” Now solar has become an
important--if still small--contributor to the nation’s energy
supply, and Logan predicts that MFCs will follow suit.
David C. Holzman
Suggested Reading
Holmes DE, Nicoll JS, Bond DR, Lovley DR. 2004. Potential role of a novel
psychrotolerant member of the family Geobacteraceae, Geopsychrobacter
electrodiphilus gen. nov., sp. nov., in electricity production
by a marine sediment fuel cell. Appl Environ Microbiol 70:6023-6030.
Liu H, Grot S, Logan BE. 2005. Electrochemically assisted microbial
production of hydrogen from acetate. Environ Sci Technol 39:4317-4320.
Logan BE. 2004. Extracting hydrogen and electricity from renewable
resources [review]. Environ Sci Technol 38: 160A-167A.
Reguera G, McCarthy KD, Mehta T, Nicoll JS, Tuominen MT, Lovley DR.
2005. Extracellular electron transfer via microbial nanowires. Nature
435:1098-1101.