News and analysis about energy in California with an eye toward renewables.

Study: Dirty Water Can Create Clean Power

This microbial generator is 1/300th the width of a human hair. | Photo: Xing Xie, Stanford Engineering
Scientists at Stanford University may have unlocked a way to convert some of the energy stored in one of our society's most prevalent waste products into electricity. With the help of some clever engineering, the Stanford engineers have harnessed tiny microorganisms that give off small amounts of electrons as they digest organic matter in sewage and other wastewater, hooking them into a circuit that charges a battery.

In a paper published this month, materials scientist Yi Cui, environmental engineer Craig Criddle, and their team describe their breakthrough in coaxing anaerobic organisms that can use and give off electrical energy into sharing some of the electrical power they generate with human engineers.

Referred to as exoelectrogenic microbes, these microscopic generators have been the subject of engineering attention for some time. Scientists have known about the microbes' generating capacity, but the matter of getting the electrons from the microbes into an electrical circuit has vexed researchers. Cui and colleagues may have demonstrated the feasibility of one approach.

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The paper, published in the Proceedings of the National Academy of Sciences, describes how Cui and Criddle's team connected the bacteria to their battery: by offering the microbes a compatible dock. Attaching conductive carbon filaments to their solid state storage battery's anode, researchers found that the microbes were able to grow thin tendrils to attach themselves to the carbon. These tendrils, called pili, conducted the microbial electric charge to the circuit, charging a battery.

"We call it fishing for electrons," Criddle said in a Stanford press office article.

The team reports rather astonishing energy conversion efficiency for their prototype wastewater battery. Once you account for inefficiencies in the battery design, the prototype recovered 30 percent of the energy contained in the wastewater's organic matter contaminants.

The team hopes that, with refinement, the process might help clean up sewage and other polluted water, while paying back some of the significant amount of energy we use every day to treat sewage and other effluent -- as much as three percent of electrical power demand in developed nations.

But don't expect to be able to charge your phone in your bathroom anytime soon. A number of details still need to be worked out to make these microbial generators a commonplace part of our energy infrastructure, not least of which was the Stanford team's use of silver oxide as a solid state battery material. When charged, the silver atoms have more and more of their available orbital spaces filled with electrons, displacing the oxygen atoms and creating metallic silver. When the charged battery is removed and its stored power tapped, that silver re-oxidizes back into silver oxide. It's a nifty process, but that precious metal feedstock is notoriously pricey.

"Silver is too expensive for use at large scale," Cui says. "Though the search is underway for a more practical material, finding a substitute will take time."


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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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