New battery generates electricity from sewage

A novel new battery that uses "wired microbes" to generate electricity from dissolved organic matter found in wastewater and sewage has been developed.
Engineers at 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.
Study Co-authors Yi Cui, Craig Criddle and Xing Xie, hope their microbial battery will 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 fertiliser 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.
What is new about the microbial battery is a simple yet efficient design that puts these exoelectrogenic bacteria to work.
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, 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 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 absorbed a full load of electrons and has largely been converted into silver.

At that point it is removed from the battery and re-oxidised back to silver oxide, releasing the stored electrons.
The Stanford engineers estimate that the microbial battery can extract about 30 per cent of the potential energy locked in wastewater. That is roughly the same efficiency at which the best commercially available solar cells convert sunlight into electricity.