Dana Kirk, manager, Anaerobic Digestion Research and Education Center at MSU (kirkdana@msu.edu)
Steve Safferman, associate professor of biosystems and agricultural engineering (safferma@msu.edu)

Anaerobic digestion
Anaerobic digestion is the biological conversion of organic matter (biomass) in an atmosphere without oxygen to biogas and a stabilized slurry. Figure 1 depicts the basic process flow of an anaerobic digestion system.

Feedstocks for anaerobic digestion
The feedstock into the anaerobic digester determines the type of anaerobic digestion technology to deploy and quantities and characteristics of the products. Potential feedstocks include the following:  

• Animal manure (dairy, swine, beef and poultry).
• Energy crops (silages and grass clippings).
• Food service residuals.
• Food processing byproducts and off-spec products.
• Municipal wastewater biosolids.
• Industrial processing byproducts, including those from the renewable biofuels industry.

Manure is generally one of the lower energy-dense feedstocks; fats, oils and grease are energy-rich.

( Figure 1 Anaerobic digester system process flow diagram (MDA, 2007).)

Benefits of anaerobic digestion
In addition to the production of renewable energy, anaerobic digestion offers many manure management benefits, including the following:

• Odor control.       
• Reduced greenhouse gas emissions by producing renewable energy and carbon credits.
• Pathogen reduction.
• Reduced organic load, resulting in improved water quality.
• Improved treatability of manure.

Selecting technology
Several types of anaerobic digesters are currently being operated in the United States, including covered lagoon, plug flow, complete mix and fixed film. The selection depends on the feedstock characteristics and the operational and management preferences of the facility. Livestock producers considering anaerobic digestion and other renewable energy projects should use a stepwise approach to evaluate technologies. Adapting anaerobic digestion technologies can affect both up- and downstream management practices.  Important steps in the evaluation of renewable energy projects are:

• Define overall facility and management goals relating to anaerobic digestion.
• Estimate the energy potential from the planned feedstock and determine if that production meets objectives.
• Conduct simple biogas assays to determine if estimates are realistic and to identify potential complications.
• Understand potential variability of byproduct formation and assess tolerance for risks.
• Initiate detailed studies that allow for designing an integrated manure management system containing an anaerobic digester.

These steps must be taken in partnership with all who have a stake in farm ownership, management and operation with guidance from experienced professionals. Following a defined evaluation path will provide the basic information needed for a cost-benefit analysis so that risks can be understood. Figure 2 provides a high-level evaluation of manure characteristics and appropriate anaerobic digester technologies.

(Figure 2. Manure characteristics and anaerobic digestion options (U.S.EPA, 2002).)

Options for biogas utilization
The biogas produced during the digestion process consists of methane, CH4 (about 60 percent by volume) and carbon dioxide, CO2 (40 percent). Biogas also contains trace gases, moisture and other impurities. Hydrogen sulfide is an impurity that receives significant attention because it poses human health hazards and mechanical concerns. 
Biogas has a heating value of approximately 600 Btu/cubic foot (ft3). In comparison, natural gas has a heating value of roughly 1,000 Btu/ft3. The least complicated use of biogas is to fire a boiler for heat and/or steam. Adding a generator set makes it possible to produce electricity. Biogas can also be upgraded and compressed for inclusion into natural gas lines. Still somewhat experimental is use of digestion to produce methanol and hydrogen.  

Methane emissions and abatement
Methane is one of the most abundant gases in the Earth’s atmosphere and a potent greenhouse gas. Its formation during natural degradation of organic matter accounts for 45 percent of atmospheric methane. According to the U.S. Emissions Inventory (2005), the other 55 percent of atmospheric methane is attributed to human-related activities including:

• Landfills - 24%
• Natural gas system - 23%
• Enteric fermentation - 21%
• Manure management - 7%

Open anaerobic lagoons are the highest contributors to atmospheric methane of all manure practices. Capturing the biogas converted in a closed anaerobic system results in the destruction of methane, thus a reduction in a potent global warming gas.

(Figure 3. Energy production by anaerobic digesters (U.S. EPA, 2007).)

State of the technology in industry
Michigan currently has six operating anaerobic digesters on livestock farms. Five are complete mixed design and one is plug flow. The U.S. EPA AgStar estimates that 121 systems were operating in the United States in 2008. As shown in Figure 3, the deployment of anaerobic digester technology and subsequent energy production have increased rapidly since 2003. According to the European Biomass Industry Association (EUBIA), more than 2,000 anaerobic digester systems have been installed to treat manure and produce renewable energy.

According to the Energy Information Administration 2001 statistics, the average household in the Midwest uses approximately 9,000 kWh annually. According to the USDA (2008), there are approximately 300,000 lactating cows in Michigan. If the manure from one quarter of the cows was anaerobically digested, the resulting biogas could provide the electrical energy for 20,000 to 25,000 Michigan households. There are approximately 3.8 million households in Michigan (HUD).

Anaerobic digester technology on livestock farms can contribute to the overall renewable energy needs of Michigan, but the largest benefit will be realized by offsetting on-farm needs and developing economic development opportunities. Included is the beneficial use of all byproducts, including the fiber, nutrients and carbon dioxide. Further, odor control and nutrient management benefits must be counted in considering a cost/benefit.

This article is from the 2009 Manure $esnse Guide. To download the entire guide, click here.