A scanning electron microscope image of the exoelectrogenic microbes with wire-like tendrils attached to a carbon filament (Credit:�Xing Xie)

'Wired microbes' generate electricity from sewage

Engineers have created a “microbial battery” driven by bacteria evolved to produce electricity as they digest organic material.

An interdisciplinary team at Stanford University have built a laboratory prototype about the size of a D-cell battery, which looks like a chemistry experiment, with two electrodes, one positive, the other negative, plunged into a bottle of wastewater.

But inside the vial an unusual type of naturally occurring bacteria, attached to the negative electrode like barnacles to a ship’s hull, produces electricity as they digest plant and animal waste that is then captured by the battery’s positive electrode.

"We call it fishing for electrons," said co-author Craig Criddle, a professor in the department of civil and environmental engineering and a senior fellow at the Stanford Woods Institute for the Environment.

The 'battery' harnesses electricity produced when the microbes digest plant and animal waste dissolved in sewage (Credit: Xing Xie) 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. What is new about the microbial battery is a simple yet efficient design that puts these exoelectrogenic bacteria to work.

In a paper published yesterday in the Proceedings of the National Academy of Sciences Criddle, along with co-authors Yi Cui, a materials scientist and Xing Xie, an interdisciplinary fellow, calls the invention a microbial battery.

One day they hope it 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 fertilizer runoff and other organic waste can deplete oxygen levels and suffocate marine life.

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.

More than 100 of the microbes can fit side by side in the width of a human hair (Credit: Xing Xie) 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 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.

While there is far less energy potential in wastewater, the inventors say the microbial battery is worth pursuing because it could offset some of the electricity now used to treat wastewater – currently about three per cent of the total electrical load in developed nations.

Most of this electricity goes toward pumping air into wastewater at conventional treatment plants where ordinary bacteria use oxygen in the course of digestion, just like humans and other animals.

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, who is also affiliated with the SLAC National Accelerator Laboratory. "Though the search is underway for a more practical material, finding a substitute will take time."

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