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Posted on 01/12/2008 9:17:50 AM PST by Uncledave
More Insights into Using Bacteria to Generate Electricity from Waste Tempe, Arizona [RenewableEnergyAccess.com] Researchers at the Biodesign Institute are using the tiniest organisms on the planet -- bacteria -- as a viable option to make electricity. In a new study featured in the journal Biotechnology and Bioengineering, lead author Andrew Kato Marcus and colleagues Cesar Torres and Bruce Rittmann have gained critical insights that may lead to commercialization of a promising microbial fuel cell (MFC) technology.
"We can use any kind of waste, such as sewage or pig manure, and the microbial fuel cell will generate electrical energy," said Marcus, a Civil and Environmental Engineering graduate student and a member of the institute's Center for Environmental Biotechnology. Unlike conventional fuel cells that rely on hydrogen gas as a fuel source, the microbial fuel cell can handle a variety of water-based organic fuels.
"There is a lot of biomass out there that we look at simply as energy stored in the wrong place," said Bruce Rittmann, director of the center. "We can take this waste, keeping it in its normal liquid form, but allowing the bacteria to convert the energy value to our society's most useful form, electricity. They get food while we get electricity."
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. "We like to work with bacteria, because bacteria provide a cheap source of electricity," said Marcus.
There are many types of MFC reactors and research teams throughout the world. However, all reactors share the same operating principles. All MFCs have a pair of battery-like terminals: an anode and cathode electrode. The electrodes are connected by an external circuit and an electrolyte solution to help conduct electricity. The difference in voltage between the anode and cathode, along with the electron flow in the circuit, generate electrical power.
In the first step of the MFC, 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.
What is the matrix?
"We knew that the MFC process is relatively stable, but one of the biggest questions is: How do the bacteria get the electrons to the anode?" said Marcus.
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.
"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.
The treatment of the biofilm matrix as a conductor allowed the team to describe the transport of electrons driven by the gradient in the electrical potential. The relationship between the biofilm matrix and the anode could now be described by a standard equation for an electrical circuit, Ohm's law.
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."
Life at the Jolt
The concept of the ‘biofilm anode' allowed the team to describe the transport of electrons from bacteria to the electrode and the electrical potential gradient. The importance of electrical potential is well known in a traditional fuel cell, but its relevance to bacterial metabolism has been less clear. The next important concept the team had to develop was to understand the response of bacteria to the electrical potential within the biofilm matrix.
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.
Previous studies have shown that the rate of bacterial metabolism at the anode increases when the electrical potential of the anode increases. The researchers could now think of the electrical potential as fulfilling the same role as a bacterial nutrient broth. 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.
Promise meeting 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.
"Modeling the potential in the biofilm anode, we now have a handle on how the MFC is working and why. We can predict how much voltage we get and how to maximize the power output by tweaking the various factors," said Marcus. For example, the team has shown that the biofilm produces more current when the biofilm thickness is at a happy medium, not too thick or thin.
"If the biofilm is too thick," said Marcus, "the electrons have to travel too far to get to the anode. On the other hand, if the biofilm is too thin, it has too few bacteria to extract the electrons rapidly from the fuel."
To harvest the benefits of MFCs, the research team is using its innovative model to optimize performance and power output. The project, which has been funded by NASA and industrial partners OpenCEL and NZLegacy, lays out the framework for MFC research and development to pursue commercialization of the technology.
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We have a sewage treatment plant in our area. Sure would be nice if this could result in energy. At the present time, the sewage is treated and fed into Lake Ontario by a pipeline. I forget the length of the pipeline...maybe 5 miles into the Lake.
Yawn. Yet another "breakthrough" based on a math model. Call me when they have an actual working cell.
Hmmm. Is MFC better than a straight methane-power conversion?
Something to think about. I admit it has intriguing possibilities, especially for pig farmers - use the pig’s waste to provide the power needs of the farm. Surplus electricity can be fed into the grid.
Lastly, I wonder what the waste is from a MFC system? Is it something that can be used beneficially?
Never thought I’d live to see the day that (your favorite expletive for it here) would become valuable.
This will never be allowed. I'm sure Pope Algore would rather be flooded with pig manure than be subjected to more CO2 pollution.
Now we can power the Thunderdome!
I’ve watched this for years. Power companies don’t want the surplus fed into the the system because they have to pay YOU. POWER COMPANIES LIKE THE MONOPOLY THEY HAVE.
Or you could just...
no, that would be gross.
Ummm, yes and no, I suppose. The problem is power companies need to pay SOMEONE for the energy they transmit. This could be the coal or gas companies. This could be the debt they incurred for building the hydroelectric or nuclear power plants. Bottom line, someone is getting paid for the power transmitted.
As long as the power producer maximizes his profit, any additional source of power from clients that are fed back into the grid, would be welcomed since they pay a purchase price that is much lower than their selling price.
Here’s an example. Let’s say people pay 6 cents/kWh in a particular region where the power producer pays 2 cents/kWh to produce and transmit the power. The power provider still makes 4 cents/kWh off the electricity fed back into the grid since someone will inevitably pay for it.
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