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Anaerobic Digestion: Unaddressed Opportunity

While wind and solar generally grab the lion's share of headlines on renewable energy, there are less celebrated, untapped renewable energy resources that can add to the U.S.'s energy mix. One example is a form of waste-to-energy.

The most common waste-to-energy applications in the U.S. include the combustion of municipal solid waste (MSW), landfill gas-to-energy, and the digestion of farm waste or waste water. An often overlooked waste-to-energy resource, however, is mixed organic waste (for example, food and yard waste) anaerobic digestion (AD). AD technologies comes in a variety of shapes and sizes, so for now, we've used assumptions provided by a dry fermentation (that is a digester that accepts higher-solids waste) technology provider.

The U.S. Environmental Protection Agency estimates that in 2008, 250 million tons of municipal solid waste (including organic and non-organic) was generated in the U.S. While 22 million tons of organic waste was diverted for composting, an estimated 43 million tons of organic waste was sent to landfills. The total electric and thermal power (assuming a combined heat and power application) associated with this organic waste is approximately 1 GWe and 1.4 GWth, respectively. The total electric output is equivalent to serving close to 1 million homes.

While composting may appear to be a direct competitor to AD for organic waste, the two are mutually beneficial because remaining digestate from the AD process can be composted and sold. In fact, composting facilities such as Cedar Grove Composting in Everett, Wash., are a primary target for new AD projects.

European companies such as Germany's Viessmann (BIOFerm) and GICON Bioenergie GMBH and Austria's Entec Biogas GMBH have digester technology that is suitable to convert food waste, yard waste and other organic material into energy. Anaerobic digestion was first widely applied in Europe in the 1930 and 1940's and has a history of success due to beneficial waste management practices and energy policies. While the technology is fully commercial, the application in the U.S. has been limited to farm and wastewater treatment plant facilities. Given the greater land availability for landfills, the U.S. has enjoyed cheaper municipal waste disposal than densely populated Europe.

Take a large industrial or institutional facility, such as a naval base or university. The facility could consider converting its waste to energy to replace boiler fuel for steam generation or for combined heat and power (CHP). The facility can save on its cost of waste disposal while generating on-site, renewable energy. CHP analysis reveals a simple payback of 7 to 10 years, excluding incentives, which compares favorably to a waste combustion application, using the same analysis parameters, which has a payback of greater than15 years, even under the highest energy price scenario.

The lack of organic waste separation is the greatest logistical barrier for mixed organic waste AD project in the U.S. Similar to composting, AD conversion requires a specific organic waste composition and a sufficient supply, without which biogas output will be lower, and the project will be uneconomic.

The majority of the U.S. population still discards organic and non-organic waste into the same container. However, many U.S. universities, such as the University of Wisconsin Oshkosh, and local governments, such as the City of San Jose in California, have commissioned pilot studies or commercial projects for either composting and/or AD that require separation of organic waste from the regular waste stream.

Higher solids organic waste digester technology and its application are relatively new to the U.S., requiring education of new potential developers, policymakers and project-hosts in order for it to gain wider acceptance and adoption. Permitting can be a barrier if a state has no prior experience with this type of project, which is certainly the case in many states.

Successful implementation of AD faces its unique challenges in the U.S., but its outlook is positive. In fact, in mid-March, Harvest Power, one of the new dry fermentation AD project developers in the U.S., announced $51.7 million in funding, led by former Vice President Al Gore's investment firm. There will likely be higher demand for AD applications such as dry fermentation in the coming years due to the growing population, declining land availability for new landfills, a continued interest in renewable energy and pursuit of efficient resource use.

Given its greater land availability and more dispersed population, the U.S. has relied upon landfills for waste disposal in comparison to densely populated Europe, which focused on waste combustion and AD.

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