Anaerobic digestion is a process in which waste is broken down by microorganisms in an environment without oxygen to produce biogas. This biogas is approximately 50-70% methane, 30-50% CO2, and the rest composed of other trace gases; the exact ratios depend on the feed and management of the system (1).
Anaerobic Digestion Flowchart
Anaerobic digestion occurs in three basic stages. The first group of microorganisms converts waste to a form that a second group of microorganisms can use to create organic acids. A third type of bacteria (the Methanogens), common in the gut of humans and ruminant animals, then breaks down these acids to produce methane. The digester must be kept at a constant temperature, as to not upset this sensitive process. The optimum temperature in the thermophilic range is about 130°F. At this temperature, the decomposition process occurs at a higher rate, but is more sensitive to disturbances. In the mesophilic range, the optimum temperature is around 98°F. This system is more stable, but requires a larger digester to accommodate for the longer residence time at a slower rate (2).
Energy is produced when the methane accumulated by the system is burned. This combustion of methane gives off carbon dioxide. Methane is over 20 times more effective than carbon dioxide at trapping heat in the atmosphere, thus, capturing and combusting it before releasing the gas greatly slows the global warming process (3). Germany is home to more than 6,100 anaerobic digesters, of which, over 70% are located on farms (4). In 2011, over 60 plants were directly feeding biomethane into the natural gas grid. Biomethane is projected to produce around 10% of Germany’s energy demand by the year 2030, increasing their energy independence and reducing the amount of natural gas needed to be imported (5).
Anaerobic Digesters at ADREC
Anaerobic digestion has many benefits. Primarily, the system works to reduce greenhouse gas emissions by capturing methane. Digesters actually reduce odor, as manure is not directly applied to the soil. After the gas collection is complete, the remaining digestate can be separated and used to create additional products, such as fertilizer. By creating energy from what was once waste, this renewable process demonstrates a system that is both profitable and beneficial for the environment. With the utilization of combined heat and power to maximize efficiency, as Ashley describes, anaerobic digestion brings us one step closer to the notion of a carbon-neutral energy supply (6).
Mixer in a digestion tank
Anaerobic digestion has great potential in the United States. AgStar estimates that digestion is feasible on over 8,000 American farms (1). Yet, there are currently only 176 on-farm systems in place (7). The economic barrier to starting a biogas project is the main inhibiting factor. One must consider the cost of the investment and the payback period when designing and implementing an anaerobic digester.
Feedstock loading equipment
There are currently only two full-scale sites in North America that process municipal solid waste, both located in Canada (8). Our visit to the Montabaur anaerobic digestion facility showed us the usefulness that comes with a plant of this type. They use only compostable waste, sorted by the people, with no added manure or energy crops. This eliminates the food vs. fuel debate that so many are worried about. Frank, our tour guide at Montabaur, was highly opposed to energy crops and stated that the future of anaerobic digestion is the role it plays in the waste disposal system.
Biogas reservoir at the Munich zoo
In 2010, Americans generated around 250 million tons of trash. Of this, only 85 million tons was recycled or composted. On average, an individual generates about 4.43 pounds of waste each day and recycles only 1.51 pounds of that (9). At less than a 35% recycling rate, America must wake up and realize the impact this has on the environment. We must be conscious of our actions and take action to reduce this unnecessary waste. Municipal solid waste digesters implemented on a large scale are one solution to this problem. With potential to take our trash out of the environment, use it to generate energy and heat, and produce fertilizer for American crops as a byproduct, there is no reason to continue land-filling our waste in the 21st century.
Digester under construction on the Bauer Farm
References
1. Anaerobic Digestion. 21 October 2011. United States Environmental Protection Agency. Retrieved 17 May 2012 from http://www.epa.gov/agstar/anaerobic/index.html
2. How Anaerobic Digestion (Methane Recovery) Works. 9 February 2011. United States Department of Energy. Retrieved 17 May 2012 from http://www.energysavers.gov/your_workplace/farms_ranches/index.cfm/mytopic=30003
3. Methane. 1 April 2011. United States Environmental Protection Agency. Retrieved 17 May 2012 from http://www.epa.gov/methane/
4. Biogas in Germany: a model to follow or avoid? 11 July 2011. National Non-Foods Crop Center. Retrieved 17 May 2012 from http://www.nnfcc.co.uk/news/biogas-in-germany-a-model-to-follow-or-avoid
5. Biomethane – The Smart Solution for the Future. August 2011. German Energy Agency (dena).
6. Recovering Value from Waste. December 2011. United States Environmental Protection Agency.
7. Kirk, D. On-Farm Anaerobic Digestion in the United States. March 2012. Michigan State University.
8. Where and how Anaerobic Digestion Technology is currently used in the US. Department of Ecology: State of Washington. Retrieved 4 June 2012 from http://www.ecy.wa.gov/programs/swfa/ad/us.html
9. Municipal Solid Waste. 3 April 2012. Environmental Protection Agency. Retrieved 4 June 2012 from http://www.epa.gov/osw/nonhaz/municipal/index.htm
Renewable Bioenergy Systems 
In addition to Alex’s overview of anaerobic digestion after being exposed to the anaerobic digesters in Germany, it is interesting seeing the different feedstocks each individual digester consumed. Energy crops, which do make up a large percentage of the anaerobic digesters, appeared to not be as successful as other materials we saw. For instance, the Ropa digester that used sugar beets (which is a very high sugar content energy crop) did not seem nearly as efficient and beneficial as say the municipal waste digester at Montabaur AD plant that took in everyday waste and was converted into biogas and fertile compost.
The amount of heat/electricity and by products does seem to be directly associated with not just the feedstock the companies end up using but the design of their system that best suits their need. With the Ropa case, they had an upflow downflow anaerobic digestion system installed. However, this design and/or its features did not accommodate to the large near by feedstock that they could be utilizing which is reflected on their system not being able to sustain their company. In comparison, the anaerobic digester are the Montabaur AD plant used a gasification process which was able to supply the buildings and grid with electricity. Given the fact that different design processes may be better suited than others, what, in your opinion, is a more beneficial anaerobic digestion system for energy crops?