September 17, 2013
Engineers Devise Method To Turn Sewage Into Electricity
April Flowers for redOrbit.com - Your Universe Online
Using naturally-occurring "wired microbes" as mini power plants, a team of engineers at Stanford University devised a method to generate electricity from sewage. The findings, published in Proceedings of the National Academy of Sciences (PNAS), reveal that these mini power plants produce electricity as they digest plant and animal waste.
The team, comprised of Yi Cui, a materials scientist, Craig Criddle, an environmental engineer, and Xing Xie, an interdisciplinary fellow, calls their new device a microbial battery and hope that it will be used in places such as sewage treatment plants.
The device could also be useful 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.
Their laboratory prototype at the moment, however, is the size of a D-cell battery. It looks like a chemistry experiment with two electrodes - negative and positive - plunged into a bottle of wastewater filled with an unusual type of bacteria. The bacteria attach to the negative electrode, and their feasting on organic waste particles creates electricity that is captured by the positive electrode.
"We call it fishing for electrons," said Criddle, a professor in the department of civil and environmental engineering.
Scientists call these types of bacteria 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. They have known of the existence of such microbes for a long time. During the last dozen years or so, several research groups have tried various ways to tap into this energy. So far, however, this has proved challenging.
The new aspect of this microbial battery is the design: simple, yet efficient at putting the exoelectrogenic microbes to work.
The negative filament has carbon filaments that the colonies of wired microbes cling to. These filaments work as electrical conductors. The researchers used a scanning electron microscope to capture 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," Criddle said. Criddle says that to put this into perspective, imagine that 100 microbes can fit, side by side, in the width of a human hair.
The microbes ingest organic matter and convert it into biological fuel. The excess electrons flow into the carbon filaments and cross to the positive electrode. This electrode is made of silver oxide, which attracts electrons.
As the electrons flow to the positive electrode, they reduce the silver oxide to silver, storing the spare electrons in the process. After a day or so, the positive electrode absorbs a full load of electrons and has mostly been converted to silver, according to Xie. The researchers remove the positive electrode from the battery and re-oxidize it back to silver oxide. This releases the stored electrons.
The microbial battery can extract about 30 percent of the potential energy locked in wastewater, according to the team's estimates, equaling roughly the same efficiency at which the best commercially available solar cells convert sunlight into electricity.
There is far less potential energy in wastewater, however, than in sunlight. The research team says the microbial battery is still worth pursuing because it could offset some of the electricity now used to treat wastewater. Treating wastewater currently accounts for approximately three percent of the total electrical load in developed nations -- mostly to pump air into the wastewater at conventional treatment plants where ordinary bacteria use oxygen in the course of digestion, just like humans and other animals.
The research team says the biggest future challenge will be finding cheap but efficient material for the positive node.
"We demonstrated the principle using silver oxide, but silver is too expensive for use at large scale," said 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."