Scientists and engineers have known for some time about "exoelectrogenic microbes." These tiny lifeforms live in environments without oxygen, and when they consume organic matter as fuel, they generate extra electrons which they must excrete into their environment. Yes, that means these are power-pooping microbes!
That sounds like a perfect source of alternative energy, except for one small problem (who can know if that pun is intended?): How can one collect electrons from microscopic creatures?
Stanford University engineers have found the solution. In a paper published in PNAS, Xing Xie and colleagues describe how carbon filaments at the negative anode encourage the microbes to grow vine-like tendrils. Materials scientist and research collaborator Craig Criddle describes what we are seeing in the image above (100 of the microbes pictured would fit in the width of a single human hair):
You can see that the microbes make nanowires to dump off their excess electrons.
The carbon filaments pick up the electrons being excreted by the microbes and efficiently conducts them to a positive silver oxide anode. The silver oxide converts to silver, effectively storing the electrical energy output by the sewage munching microbes.
The team estimates that the process can convert about 30% of the energy stored in sewage to usable energy, a rate comparable with solar panel efficiencies or the energy gained in heat and power from methane fermentation. The microbes convert 44-49% of the organic mass (depending on measurement method) but the recovery of the energy from the solid silver state drops the overall efficiency of the process.
There may be another benefit as well. The Stanford press release notes that the electricity used to treat wastewater "currently accounts for about three percent 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." Since these microbes process their food anaerobically, wastewater treatment systems relying on their digestive capabilities could save electricity on pumping air and generate electricity to boot!
The next steps will include scaling up and finding cheaper materials to make the process more cost effective.