Biogas is produced from anaerobic digestion, a multi-step process orchestrated by\u00a0a diverse community of microorganisms coexisting in an oxygen-free digester tank.\u00a0These microorganisms feast on organic matter loaded into the digester, typically some form of waste like livestock manure,\u00a0agricultural waste,\u00a0sewage, or food waste. Bacteria break down the organic matter into simpler molecules which are then fermented and broken down further by other bacteria. Finally, microorganisms known as methanogens use the resulting molecules to produce methane.\u00a0Methanogens aren’t bacteria though, they belong to the domain Archaea. We’re well-acquainted with these microorganisms since they are responsible for methane production in wetlands and in our own guts.<\/p>\n
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Digester Disparities<\/strong><\/p>\n Digesters in the U.S. are commonly found on farms and at wastewater treatment plants (WWTP) because these provide typical anaerobic digestion feedstocks (farm waste and sewage, respectively). In the U.S. there are about 200 farm digesters and 1500 WWTP digesters<\/a>. By contrast, Germany takes the lead in Europe with about 6800 digesters (2011 data<\/a>). That’s roughly 83 digesters per 1 million people in Germany versus\u00a05 digesters per 1 million people in the United States. A host of economic incentives (e.g., feed-in tariffs, see 2009 rates on pg 11<\/a>) and environmental legislation (e.g., the Landfill Directive<\/a>) supported the growth of anaerobic digestion systems in Europe.<\/p>\n <\/p>\n Food Waste as an Up-and-coming Feedstock<\/strong><\/p>\n Stronger efforts to reduce the amount of organic waste sent to landfills in the U.S. have led to more digesters that consume food waste.\u00a0WWTPs have increasingly been turning to co-digestion, in which the sewage from a WWTP is accompanied by other organic waste, like food waste, into anaerobic digesters. Other relatively new digesters have been consuming food waste alone as a feedstock. Food waste produces more biogas than sewage does, and digestion diverts food waste from landfills where it would emit greenhouse gases (but note that some landfills, about 500 in the U.S, collect landfill gas for fuel). The digested food waste can subsequently be sent to compost facilities or used as fertilizers.\u00a0The following examples are just a sample of the WWTP co-digestion facilities and food waste digesters that have been emerging in the U.S. within the past few years.<\/p>\n In 2004, the East Bay Municipal Utility District<\/a> in Oakland, CA was the first WWTP in the country to experiment with anaerobic digestion of collected food waste.\u00a0Just a few weeks ago, the Newtown Creek WWTP\u00a0announced it will begin co-digesting<\/a>\u00a0a portion of the municipal food waste collected by the\u00a0NYC Organics Collection<\/a>\u00a0program. With this pilot study, the city hopes to attract private firms to build more digesters around NYC.<\/p>\n UC Davis researchers have successfully\u00a0scaled-up commercial anaerobic digesters<\/a>\u00a0that can accept a variety of food wastes. They are\u00a0opening another digester this January<\/a>\u00a0to divert organic waste produced on campus and use it to generate electricity from biogas.\u00a0Madison, WI started a residential\u00a0organic waste collection program<\/a>\u00a0in 2011 to provide a feedstock for the University of Wisconsin Oshkosh anaerobic digester.\u00a0A new anaerobic digestion facility<\/a> broke ground in San Jose, CA this November with the aim of digesting residential food waste as well. The Kroger food store chain has also started digesting spoiled food from over 300 regional stores at a facility in Compton, CA (video<\/a>).<\/p>\n <\/p>\n A Class Experiment in Food Waste Digestion<\/strong><\/p>\n With food waste digestion catching on, researchers have been interested in creating stable microbial communities that are capable of producing as much methane as possible from a mixture of feedstocks. This fall in a Bioenergy course at Duke, we experimented with artificial sewage and food waste to compare the methane potential of different feedstocks. To make the food waste more palatable to microorganisms, we mashed up banana peels and squash rinds with a hand-held blender prior to the digestion.<\/p>\n We added the food waste alone, the artificial sewage alone, and a combination of the food waste and sewage into different glass bottles. We then added some substrate from an anaerobic digester in Marc Deshusses’s lab, which already contained an established community of microorganisms.<\/p>\n![\"\"](\"http:\/\/blogs.nicholas.duke.edu\/fuelforthought\/files\/2013\/12\/IMG_0053_21-1024x450.jpg\")
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