by Emil Morhardt
Bacteria are good at getting energy out of sewage; that’s what wastewater treatment plants are mostly about…converting the organic carbon that we didn’t extract from the food during it’s passage through our guts into something that won’t pollute the water bodies we dump the treated wastewater into. In the closed anaerobic digester tanks you can see at any wastewater treatment plant the microorganisms are busy converting it into methane. Sometimes this methane gets used onsite to generate power, or is further processed and piped off as “biogas” for some other use, maybe even to power city buses. More often than not it is just released into the atmosphere where, although it can no longer pollute any water, it is a powerful greenhouse gas. What if we could skip the methane production step and just generate electricity directly from the sewage by sticking electrodes in it? Sounds impossible, but there is new science that is making it happen, at least at laboratory scale. Xie et al. at Stanford University have constructed what they call a microbial battery that makes just as much electricity out of a given amount of wastewater as you can get from first using the microorganisms to produce methane, then burning it…without the intervening gas handling and power plant, not to mention the likely leaks of methane to the atmosphere in the process. The secret is a solid-state cathode which makes the system act like a rechargeable battery, with exoelectrogens—microorganisms that oxidize the electron-donating chemicals in the sewage and transfer the electrons to the anode. The electrons then pass through an external circuit as an electrical current, on their way to the cathode. Voila! Electricity that can be used for anything you like.
Xie, X., Ye, M., Hsu, P.-C., Liu, N., Criddle, C.S., Cui, Y., 2013. Microbial battery for efficient energy recovery. Proceedings of the National Academy of Sciences 110, 15925-15930. http://bit.ly/1siA7Ji