| Turning Trash Into Power -- Inside Science
Reported October 2006
BACKGROUND: Environmental engineers at the University of California, Davis are building a full-scale anaerobic digester that can convert any type of solid organic waste into electricity -- even leftovers from restaurants. The system is part of the $100,000 Sacramento Municipal Utility District (SMUD) pilot project, but engineers are putting into place an even larger digester system in San Francisco.
HOW IT WORKS: In the process, engineers feed food waste collected from restaurants and institutions to bacteria that thrive in low-oxygen environments. It's called anaerobic digestion, a naturally occurring process of decomposition.
One type of bacteria turns carbohydrates into simple sugars, amino acids and fatty acids. A second group of bacteria eats those compounds and turns them into hydrogen gas, carbon dioxide, and acetic acid -- the primary component of vinegar. Then a third group of bacteria takes those broken-down compounds and turns them into methane and carbon dioxide. Between 60 percent and 80 percent becomes methane. The methane can be used as fuel for an internal combustion engine that provides electricity.
TYPES OF DIGESTION: Anaerobic digestion is not the same thing as human digestion, since the type of bacteria that produce methane don't live in the human digestive tract. Industrial anaerobic digesters can also harness this natural process to treat waste, provide heat, and increase nutrients in soil. They are most commonly used for sewage treatment and for managing animal waste.
BENEFITS: The goal of SMUD is to obtain 20 percent of its electricity from renewable sources such as wind, solar, and biodegradable matter by 2011. Currently, SMUD derives 10 percent of its electricity from renewable sources, of which biomass accounts for 2.5 percent. The UC-Davis digester would keep food and other biodegradable waste out of landfills; food leftovers account for 18 percent of a landfill's contents. One tone of leftover food can produce enough fuel to power 18 homes for one day.
The Institute of Electrical and Electronics Engineers, Inc., and the American Society for Microbiology contributed to the information contained in the TV portion of this report.
If you would like more information, please contact:
Ruihong Zhang, Ph.D.
Professor, Biological and Agricultural Engineering Department
University of California, Davis
(530) 754-9530
rhzhang@ucdavis.edu
American Society for Microbiology
Washington, D.C. 20036-2904
(202) 737-3600
http://www.asm.org
Institute of Electrical and Electronics Engineers, Inc.
Washington, D.C. 20036-5104
(202) 785-0017
ieeeusa@ieee.org
http://www.ieee.org
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