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futurity_sci_tech 00903.txt

#Colonies of wired microbes turn sewage into electricity Stanford university rightoriginal Studyposted by Tom Abate-Stanford on September 19 2013a new way to generate electricity from sewage uses naturally occurring ired microbesas mini power plants to produce electricity as they digest plant and animal waste. Scientists hope the icrobial batterycan be used in places such as sewage treatment plants or to break down organic pollutants in the ead zonesof lakes and coastal waters where fertilizer runoff and other organic waste can deplete oxygen levels and suffocate marine life. At the moment however the laboratory prototype is about the size of A d-cell battery and looks like a chemistry experiment with two electrodes one positive the other negative plunged into a bottle of wastewater. Inside that murky vial attached to the negative electrode bacteria feast on particles of organic waste and produce electricity that is captured by the battery s positive electrode. e call it fishing for electronssays Craig Criddle a professor in the department of civil and environmental engineering at Stanford university. Scientists have known long of the existence of what they call exoelectrogenic microbesâ##organisms that evolved in airless environments and developed the ability to react with oxide minerals rather than breathe oxygen as we do to convert organic nutrients into biological fuel. Over the last dozen years or so several research groups have tried various ways to use these microbes as bio-generators but tapping this energy efficiently has proven challenging. What is new about the microbial battery is a simple yet efficient design that puts these exoelectrogenic bacteria to work. As reported in the Proceedings of the National Academy of Sciences at the battery s negative electrode colonies of wired microbes cling to carbon filaments that serve as efficient electrical conductors. Using a scanning electron microscope the Stanford team captured images of these microbes attaching milky tendrils to the carbon filaments. ou can see that the microbes make nanowires to dump off their excess electronscriddle says. To put the images into perspective about 100 of these microbes could fit side by side in the width of a human hair. As these microbes ingest organic matter and convert it into biological fuel their excess electrons flow into the carbon filaments and across to the positive electrode which is made of silver oxide a material that attracts electrons. The electrons flowing to the positive node gradually reduce the silver oxide to silver storing the spare electrons in the process. After a day or so the positive electrode has absorbed a full load of electrons and has largely been converted into silver says Xing Xie an interdisciplinary researcher. At that point it is removed from the battery and re-oxidized back to silver oxide releasing the stored electrons. Engineers estimate that the microbial battery can extract about 30 percent of the potential energy locked up in wastewater. That is roughly the same efficiency at which the best commercially available solar cells convert sunlight into electricity. Of course there is far less energy potential in wastewater. Even so the microbial battery is worth pursuing because it could offset some of the electricity now used to treat wastewater. That use currently accounts for about 3 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. Looking ahead the engineers say their biggest challenge will be finding a cheap but efficient material for the positive node. e demonstrated the principle using silver oxide but silver is too expensive for use at large scalesays Yi Cui an associate professor of materials science and engineering. hough the search is under way for a more practical material finding a substitute will take time. ource: Stanford Universit t

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