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Anaerobic Digestion: Why Not Everywhere?

Bagley, David M.1
1 Civil and Architectural Engineering, University of Wyoming, Laramie, WY.

Anaerobic digestion is the process of converting waste materials into methane-containing biogas using microorganisms in the absence of oxygen. This process has long been used at municipal wastewater treatment plants to treat sludges, at industrial facilities to treat wastewaters containing high concentrations of biodegradable material, and at livestock production facilities to treat manures. Additionally, anaerobic digestion occurs naturally in landfills and has recently been incorporated into landfill design to optimize biogas production. In an era of highly volatile energy prices, national security challenges, and concern for greenhouse gas emissions, the process of anaerobic digestion is conceptually attractive: Methane-containing biogas is produced from renewable sources using low temperature and low pressure microbial processes. Given this attractiveness, why isn't anaerobic digestion utilized everywhere, on every possible waste organic stream?

The objective of this paper is to identify answers to that question. The underlying principles of anaerobic digestion will be briefly examined including the thermodynamic and kinetic limitations of the process. Technologies currently used to treat municipal sludges and the organic fraction of municipal solid waste will be reviewed. Processes used to convert biogas into electricity and thermal energy will also be reviewed. In addition to examining the technical issues related to anaerobic digestion, this paper will also examine key economic factors. Greenhouse gas accounting will be briefly reviewed. Finally, the challenges of applying anaerobic digestion to convert in situ materials such as landfill material and low-rank coals into biogas will be identified.

Anaerobic digestion can be applied more widely for waste material treatment than it currently is. Although there are key technical and economic challenges to be addressed, another challenge is philosophical. Anaerobic digestion has historically been used to manage waste materials and has been considered a necessary but undesirable cost. Anaerobic digestion should instead be considered a profit engine, capable of converting raw materials into energy with the added benefits of minimizing the environmental impacts due to waste disposal while decreasing the greenhouse gas emissions due to energy production. This philosophical change will facilitate the developments needed to improve the technical performance and decrease the cost of anaerobic digestion.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009