Microbes generate electricity from wastewater

Biological battery could provide energy to sewage treatment plants

Connected to the electrode via fine threads, these bacteria release electrons - thus generating electricity © Xing Xie / Stanford University
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Plenty of sewage, a few carbon fibers and silver oxide - these are the ingredients that a novel microbial battery needs. In it, bacteria generate electricity from wastewater - after all, with an efficiency of at least 30 percent. The electricity is generated when the microbes degrade organic material under exclusion of air. Corresponding procedures could cover part of the enormous power requirements of sewage treatment plants, the researchers report in the journal "Proceedings of the National Academy of Sciences".

Some microbes are loaded - you know that for some time. The so-called exoelektrogenen bacteria produce the degradation of organic matter under oxygen deficiency electrical potential. They do not need oxygen for their metabolism, like animals or most other bacteria, but use minerals as reactants in their anaerobic metabolism. Electrons are released during this process - researchers have been trying for years to capture them in order to use them for power generation.

Carbon fibers as a microbe lure

However, the "tapping" of the microbes has been found to be difficult and has not led to a satisfactory yield. Xing Xie of Stanford University and his colleagues have now developed a process in which the tiny biogenerators willingly network with the electrodes. Your microbial battery looks like a simple experimental setup from the chemistry class, in the wired vial with the murky broth but puts clever know-how.

Here is the fine thread with which the bacterium adheres to the carbon fiber. © Xing Xie / Stanford University

The electrobacteria sit on a special carbon fiber layer that covers the negative electrode (anode) of the battery. The carbon fibers are a good electrical conductor and the microbes apparently like them too: they form filamentous adhesive organs, which they use to bond to them, as electron micrographs show. About a hundred bacteria fit side by side on the width of a hair, the researchers explain.

Silver as an electron trap

The biological power plants are bathed in sewage, which serves as a source of food for them. When they metabolize the organic substance dissolved in them, electrons are liberated, which flow out via the carbon material of the anode. They migrate to the counterpart, the cathode, which consists of silver oxide and thus of a material that can take up electrons: the electrons reduce the silver oxide to silver a process that is reversible. display

Looks rather unspectacular: the microbial battery Xing Xie / Stanford University

The cathode has largely turned into silver after about a day. Now it can be removed from the battery and reoxidized by oxygen to resume electrons in the battery. According to the researchers, the bacteria can thus provide humans with about 30 percent of the energy that is bound in the wastewater. Above all, this energy could be used directly in wastewater treatment plants, because they have an enormous power consumption: it is largely generated by the systems that channel air into the wastewater, thus reducing the oxygen demand bacteria in the pond can do their cleansing work.

But for the system to be viable for electricity generation, there is one more barrier to overcome, as the researchers argue: silver as a cathode material is too expensive for commercial use. Xing Xie and his colleagues are now looking for cheaper alternatives for the recording medium of the electrons of their electro-bacteria. (Proceedings of the National Academy of Sciences, 2013; doi: 10.1073 / pnas.1307327110)

(PNAS / Stanford School of Engineering, 17.09.2013 - MVI)