WASHINGTON, Sept. 16 (Xinhua) -- U.S. researchers said Monday they have created a "microbial battery" that harnesses a special type of microbe to produce electricity by digesting the plant and animal waste dissolved in sewage.
The invention, published in the journal Proceedings of the National Academy of Sciences, could potentially be used in places such as sewage treatment plants, or to break down organic pollutants in the "dead zones" of lakes and coastal waters where fertilizer runoff and other organic waste can deplete oxygen levels and suffocate marine life.
Scientists have long known of the existence of what they call exoelectrogenic microbes -- organisms that evolved in airless environments and developed the ability to react with oxide minerals rather than breathe oxygen as we do to convert organic nutrients into biological fuel.
During the past dozen years or so, several research groups have tried various ways to use these microbes as bio-generators, but tapping this energy efficiently has proven challenging.
In the new study, researchers from the Stanford University devised "a simple yet efficient" way that puts naturally-occurring "wired microbes" to work as mini power plants.
The researchers plunged two electrodes, one positive, the other negative, into a bottle of wastewater to create a laboratory prototype about the size of a D-cell battery.
At the battery's negative electrode, colonies of wired microbes cling to carbon filaments that serve as efficient electrical conductors. Using a scanning electron microscope, the Stanford team captured images of these microbes attaching milky tendrils to the carbon filaments.
"You can see that the microbes make nanowires to dump off their excess electrons," said co-author Craig Criddle, a professor in the department of civil and environmental engineering.
About 100 of these microbes could fit, side by side, in the width of a human hair, the researchers said.
As these microbes ingest organic matter and convert it into biological fuel, their excess electrons flow into the carbon filaments and across to the positive electrode, which is made of silver oxide, a material that attracts electrons, the researchers said.
The electrons flowing to the positive node gradually reduce the silver oxide to silver, storing the spare electrons in the process. According to the researchers, after a day or so the positive electrode has absorbed a full load of electrons and has largely been converted into silver.
When the positive electrode is spent, it can be removed from the battery and re-oxidized back to silver oxide, releasing the stored electrons. Simply reinstalling the electrode resumes power, they said.
The researchers estimated that the microbial battery can extract about 30 percent of the potential energy locked in wastewater, "roughly the same efficiency at which the best commercially available solar cells convert sunlight into electricity."
Although there is far less energy potential in wastewater, the microbial battery is worth pursuing because it could offset some of the electricity now use to treat wastewater, which currently accounts for about three percent of the total electrical load in the developed countries, said the researchers.
The Stanford team, however, cautioned that finding a cheap but efficient material for the positive node will be their biggest challenge for developing devices for large-scale power production.
"We demonstrated the principle using silver oxide, but silver is too expensive for use at large scale," said co-author Yi Cui, an associate professor of materials science and engineering. " Though the search is underway for a more practical material, finding a substitute will take time."