Oct. 09, 2008 -- The race is on to make commercial amounts of ethanol or other biofuels out of plant waste containing cellulose -- a fibrous component of plants that gives them structure. But some researchers are looking at a different option -- using cellulose to power microbial fuel cells, in which bacteria digest plant waste matter to create electricity directly.
These fuel cells could be used to charge batteries or power electrical devices.
Others are considering drawing power from microbes digesting human waste at wastewater treatment plants or manure from feedlot lagoons.
"Basically, we're converting cellulose into a different energy source than ethanol," said John Regan of Pennsylvania State University in University Park.
"It's not more efficient right now, but if you look at what's been done over the last decade, there has been about a five to six order-of-magnitude (100,000-1,000,000-fold) increase in power density."
Microorganisms generate electrons as they break down food sources for energy, but in most species the electrons are transferred to molecules inside the cell. Microbial fuel cells rely on the ability of certain bacteria to transport electrons to the outside of the cell. If provided with electrodes in the right arrangement, the bacteria can dump their exterior electrons through a circuit, providing power.
But these "exoelectrogenic" microbes, as Regan calls them, cannot digest cellulose. So the system relies on another type of bacteria to break the cellulose down into simple molecules that the electron dumpers can then use.
Regan found that wastewater, which contains a diverse community of microorganisms, could generate electricity from cellulose, too, though not as much. Adding extra cellulose-degraders to the wastewater sped up the process. He is presenting his results this week at a meeting of the Geological Society of America in Houston, Texas.
Regan envisions near-term applications that would not depend on cellulose, but rather would degrade the soup of compounds in wastewater.
"In waste treatment, the incoming product is free. It's waste material, so you could use that electricity to run pumps or aerators," he said. Even if the wastewater couldn't produce enough electricity to completely power the plant, it could at least reduce the plant's utility bill.
Others have found the necessary groups of microorganisms in different environments.
Ann Christy of Ohio State University in Columbus discovered that bacteria recovered from the guts of cows had both cellulose degraders and exoelectrogenic bacteria and could therefore be used to run a fuel cell.
"The fuel could be hay or any sort of leaf clippings or grass clippings. It could even be manure," she said.
Her lab has gathered enough power from these fuel cells to charge AA batteries. "At this point we can probably go to C's or D's," she said. It is already enough to charge an MP3 player.
Eventually, these chargers could be used in rural areas without electricity or following disasters where power is lost, Christy said.
Christy is also interested in the possibility of scaling up the process to harness power from manure waste lagoons or septic tanks in private homes.
Similar devices are already in place in the ocean, powering remote monitoring devices. In that case, the microbes living in ocean sediment convert sediment organic matter into electricity gathered by sediment fuel cells with an electrode stuck in the ocean floor.
"We have lots of questions. It's an exciting time to be studying this very innovative and not-so-well-known source of bioenergy," Christy said.
Source:
Discovery News
