The British company Geneco has begun implementing prominent buses with a cartoon graphic detailing its power source: human waste. The concept of using waste to produce fuel is not novel, and is termed biomethane or renewable natural gas (RNG). It is most commonly used to power vehicles. The biomethane is collected from sewage treatment plants that process human waste while producing methane and carbon dioxide as byproducts. Typically, the resulting gases are simply released into the atmosphere, with plants in Oslo, Norway producing and releasing approximately 17,000 tons of carbon dioxide a year (Demerjian 2009). Continue reading →
Disposal of waste animal carcasses is expensive, a nuisance, and more trouble than its worth, at least some of the time in China. Consider that in 2013 over 16,000 dead pigs were dumped into one of Shanghai’s primary drinking water sources (Zhang and Ji, 2024). If this waste product were more valuable, than nothing of the sort would happen. According to these authors the carcasses are, in fact, worth $56/tonne, when converted to biofuel. To prove their point, they used pig carcasses to make biodiesel and biogas. They cooked them in water in an autoclave (basically a pressure cooker) for six hours then extracted the pig fat from the water and converted it to biodiesel. The remaining water was inoculated with anaerobic bacteria from a pig farm digester, and allowed to form biogas, in this case 63% methanol. Continue reading →
The paper by Xie et al. (discussed in my August 10 post) didn’t say much about the electrogenic bacteria needed to make their microbial battery work. Just a few days ago, however, researchers in Beijing and Singapore published a paper focussed on such bacteria (Klebsiella sp. in this case) isolated out of a microbial fuel cell, that can do the job in wastewater heavily contaminated with cyanide, almost completely degrading the cyanide in the process. Even without the electricity generation this is interesting, because these bacteria do a better job of removing cyanide than the much more expensive chemical oxidation methods more commonly used by industry. Microbial fuel cells get electricity out of microbes differently than the “microbial battery” of Xie et al.; they consist of two wastewater-filled chambers seperated by a proton exchange membrane. The bacteria in the anode chamber strip protons (hydrogen ions) off the feedstock—a cyanide/glucose mixture in this experiment—and the protons migrate through the membrane to the cathode. The electrons flow as an electrical current from the anode to the cathode in a wire, where they can be used as electricity. Interestingly, the bacteria continued to generate electricity from cyanide alone when they ran out of glucose.
Wang, W., Feng, Y., Tang, X., Li, H., Du, Z., Yang, Z., Du, Y., 2014. Isolation and Characterization of an Electrochemically Active and Cyanide-degrading Bacterium Isolated from a Microbial Fuel Cell. RSC Advances, DOI: 10.1039/C1034RA04090B. Abstract at: http://rsc.li/XbkskA
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