"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. "