Who can forget the historical sci-fi moment when the Dr Emmet Brown of “Back to the Future” fame shoved some garbage into his time car to make it run? It was such a memorable moment in sci-fi history because it had a truthful ring to it—why can’t we power the world with garbage instead of expensive, polluting oil? We can, according to new research. In fact, a garbage-fueled society is a smart alternative. It’s like killing two birds with one stone. You can take common sources of organic waste products such as human sewage, animal waste, or agricultural runoff that isn’t being used for anything and convert it into cheap, renewable electricity.
“Performing double-duty of energy generation and pollution prevention
is a huge advantage of an microbial fuel cell,” researcher Andrew Kato
Marcus told The Daily Galaxy. He and his colleagues recently published
a study featured in the journal Biotechnology and Bioengineering, which
offers some key insights into the process. According to Marcus, in may
ways “garbage power” is the ideal solution. And it’s already being done
notes, “last year by a group in Harbin Institute, China actually used
landfill leachate as the fuel for an MFC [microbial fuel cell].”
Bruce Rittmann, director of the Center for Environmental Biotechnology at the Biodesign Institute explained to The
Daily Galaxy that the impact could be huge. "If all the residual
(waste) biomass from agriculture, the food-processing industry, and a
number of industries could be collected and converted to electricity,
we could displace up to 25% of the world’s energy demand today. Of
course, we really cannot collect and convert all of it, but we can see
that biomass conversion via MFCs or some other microbial systems can
have a big impact on displacing fossil fuels.”
Bacteria have such a rich diversity that researchers can find a
bacterium that can handle almost any waste compound in their daily
diet. By linking bacterial metabolism directly with electricity
production, the MFC eliminates the extra steps necessary in other fuel
cell technologies.
How does it work? An anode respiring bacterium breaks down the organic
waste to carbon dioxide and transfers the electrons released to the
anode. Next, the electrons travel from the anode, through an external
circuit to generate electrical energy. Finally, the electrons complete
the circuit by traveling to the cathode, where they are taken up by
oxygen and hydrogen ions to form water.
The bacteria depend on the anode for life. The bacteria at the anode
breathe the anode, much like people breathe air, by transferring
electrons to the anode. Because bacteria use the anode in their
metabolism, they strategically position themselves on the anode surface
to form a bacterial community called a biofilm.
Bacteria in the biofilm produce a matrix of material so that they stick
to the anode. The biofilm matrix is rich with material that can
potentially transport electrons. The sticky biofilm matrix is made up
of a complex of extracellular proteins, sugars, and bacterial cells.
The matrix also has been shown to contain tiny conductive nanowires
that may help facilitate electron conduction.
Bacteria have evolved to utilize almost any chemical as a food source.
"Our numerical model develops and supports the idea that the bacterial
matrix is conductive," said Marcus. In electronics, conductors are most
commonly made of materials like copper that make it easier for a
current to flow through. "In a conductive matrix, the movement of
electrons is driven by the change in the electrical potential." Like a
waterfall, the resulting voltage drop in the electrical potential
pushes the flow of electrons.
Within the MFC is a complex ecosystem where bacteria are living within
a self-generated matrix that conducts the electrons. "The whole biofilm
is acting like the anode itself, a living electrode," said Marcus.
"This is why we call it the 'biofilm anode.'"
Bacteria will grow as long as there is an abundant supply of nutrients.
Jacques Monod, one of the founding fathers of molecular biology,
developed an equation to describe this relationship. While the team
recognized the importance of the Monod equation for bacteria bathed in
a rich nutrient broth, the challenge was to apply the Monod equation to
the anode, a solid. The team recognized that the electrical potential
is equivalent to the concentration of electrons; and the electrons are
precisely what the bacteria transfer to the anode.
Equipped with this key insight, the team developed a new model, the
Nernst-Monod equation, to describe the rate of bacterial metabolism in
response to the "concentration of electrons" or the electrical
potential.
In their model, the team identified three crucial variables to
controlling an MFC: the amount of waste material (fuel), the
accumulation of biomass on the anode, and the electrical potential in
the biofilm anode. The third factor is a totally novel concept in MFC
research.
But how practical is this technology? Could this replace conventional
forms of generating electricity in most parts of the world?
“I imagine MFCs becoming a competitive, renewable energy not too far in
the future,” Markus told The Daily Galaxy. “The field is gaining
momentum and some of our colleagues in Australia are building pilot
plants for energy generation to answer that very question. Challenges
are lowering the capital cost and improving process efficiencies. For
the latter, our model will be useful for making improvements.”
Posted by Rebecca Sato
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Links:
http://microbialfuelcell.org
http://www.eurekalert.org/pub_releases/2008-01/asu-lat010308.php