These Special Sewage Bacteria Can Turn Dirty Water into Energy

Some bacteria can already do it—generate electric current, that is—and those microbes are called "electrogenic." Now, thanks to the work of a research group from the University of California, Santa Barbara, we know how to easily turn non-electrogenic bacteria into electricity producers.

One important key to their success was the bacteria they chose—Shewanella oneidensis MR-1. This organism uses current-conducting proteins in their cell membranes to breathe in metal minerals instead of air.

Unlike previous efforts that involved genetically altering the bacteria, this new method simply involved feeding the bacteria a synthetic form of iron and providing an environment rich in organic fuel. The bacteria did the rest.

To turn the Shewanella oneidensis into electrogenic bacteria, the researchers built a compound called DFSO+ that has an iron core, according to their recent published paper in the journal Chem.

Test tubes of DSFO+ iron compound used in the study. Image via Zachary Rengert

After dissolving the DFSO+ powder in water and adding the solution to bacteria, the synthetic DFSO+ moved through the bacteria's cell membranes and began conducting current through its iron core. The bacteria readily takes in iron as part of its metabolic processes, and the DFSO+ molecule was transported across the cell's membrane the same way other iron-containing protein are usually taken in by the bacteria.

By a biochemical process called reduction, the bacteria reduced the iron, releasing electrons that were shuttled from the inside of the bacterial cell to the outside. This flow of electrons was a form of electrical current.

The Bacterial Powerhouses—No Oxygen Required

Bacteria use organic material as food and wastewater is rich in organic fuel sources, including pathogenic microbes, metals, salts, ammonia, pesticides, pharmaceuticals, and chemical compounds.

Shewanella is an organism that lives at the bottom of lakes where there is no oxygen, and it has naturally evolved to use minerals to breathe. But it doesn't eat them—it uses them to collect negatively charged particles called electrons that are produced when the bacteria digests organic material.

Shewanella oneidensis up close and personal. Image by Liza Gross/Public Library of Science

Co-first author Zachary Rengert, a chemistry graduate student at the University of California, Santa Barbara, told Invisiverse:

Minerals are solid materials and they have properties very similar to an electrode. So in this case, the bacteria are cleaning the water (by using the organic contaminates as food) and the electrode (the iron compound) is collecting the waste electrons (from the digestion of organic contaminates) and producing electricity as well.

— Zachary Rengert

Unlike Shewanella, the bacteria in a wastewater treatment tank usually require oxygen to breathe as well as degrade organic material. That means, the water must be oxygenated to keep the process of digestion going of organic material, and therefore the cleaning of wastewater—and that is an expensive process.

The researchers measured the electricity produced by the bacteria, but did not capture it. "You can think of the fuel cell as a special type of battery. In our study, we just measured it, because we wanted to know how much electricity it was producing, but the electricity is definitely usable!" Rengert told us.

Right now, the amount of electricity produced is small, but can offset the usual cost associated with oxygenating the wastewater.

To have a bacteria that can digest the material without added oxygen and—after simply feeding some iron—produce some electrons that can be captured as electrical current, is an efficient process that could lead to more efficient biological electricity-generating technology.

Cover image via USDA/Wikimedia Commons

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