Engineers at California’s Stanford University have devised a new way to generate electricity from sewage, using naturally occurring "wired microbes" as mini power plants, producing electricity as they digest plant and animal waste.
In a paper published in the Proceedings of the National Academy of Sciences, co-authors Yi Cui, a materials scientist, Craig Criddle, an environmental engineer, and Xing Xie, an interdisciplinary researcher, call their invention a microbial battery. The prototype is about the size of a D-cell battery and looks like a chemistry experiment, with two electrodes, one positive, the other negative, plunged into a bottle of wastewater.
Inside, attached to the negative electrode like barnacles to a ship's hull, an unusual type of bacteria feast on particles of organic waste and produce electricity that is captured by the battery's positive electrode. "We call it fishing for electrons," said Criddle, a professor in the department of civil and environmental engineering.
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. 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," Criddle said. To put the images into perspective, about 100 of these microbes could fit, side by side, in the width of a human hair.
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 electrons flowing to the positive node gradually reduce the silver oxide to silver, storing the spare electrons in the process. According to Xie, after a day or so the positive electrode has absorbed a full load of electrons and has largely been converted into silver.
At that point it is removed from the battery and re-oxidized back to silver oxide, releasing the stored electrons. The inventors say the microbial battery is worth pursuing because it could offset some of the electricity now used to treat wastewater. The team estimates the technology can harvest about 30% of the potential energy locked in wastewater.
Looking ahead, the Stanford engineers say their biggest challenge will be finding a 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."