Anaerobic digestion is a method of producing biogas (60% CH4, 40% CO2) through the breakdown of organic matter by bacteria. One of the large advantages of anaerobic digestion is using methane for energy while being able to mitigate its effects on the environment by substituting it for less harmful carbon dioxide. A drawback to using food crops for anaerobic digestion is the potential to cause an increase in overall food prices. The process of anaerobic digestion is done in a low oxygen environment, and can be used with many types of plant matter, and manure (human and animal). The biogas produced is then captured by the digester, and has many uses that will be explained later on. Anaerobic digestion also produces inorganic matter that can be dried and sold to be used as a fertilizers and bedding .
In an anaerobic digester, there are multiple chemical reactions taking place. These reactions are characterized by three main types of bacteria: Hydrolytic and fermentative (HFB), acid-forming (AFB), and methane forming (MFB) . The organic feedstock of the fermenter is first broken down by the HFB to produce H2, CO2, soluble acids, alcohols, Acetate (C2H3O2‾), and Formate (HCO2). The resulting H2 and CO2 are either utilized by the AFB to produce more acetate, or used by the MFB to produce methane. The soluble acids, alcohols, and neutral compounds produced by the HFB are used by the AFB to produce H2 and acetate. All of the H2, Acetate, and Formate produced by the previous bacteria are used by the MFB to produce CH4, CO2, and trace gasses . These gasses are what make up the biogas that is utilized for energy. All of these reactions take place in different amounts and speeds depending on the type of feedstock used for the fermentation.
There are four main types of anaerobic digesters, covered lagoon, fixed film, plug flow, and complete mix. Each of these main designs is more effective with different climates, power production, and solid percentages. A comparative overview of each of these designs can be seen in figure 4.
Figure 1. Example of a fixed film digester
A covered lagoon digester is characterized by an in-ground tank covered by a flexible material that can expand and contract depending on the amount of biogas being produced. The main tank houses the slurry of material for a set amount of time allowing for maximum gas production. A second tank is housed near the first to hold the digester effluent being pushed out of the digester .
Figure 2. Flexible biogas storage material
A plug flow digester is normally built partially or fully underground, and is characterized by a long and narrow concrete tank with either a rigid or flexible cover. Plug flow is used mainly for dairy and swine operations that involve a slurry of manure. Plug flow digesters pump the slurry below the surface on one end, and the effluent exits the digester at the opposite end .
The third main type of digester, complete mix, is characterized by an above-ground cylindrical tank with either a flexible or rigid cover. Complete mix digesters have 2-4 hydraulic or mechanical mixing systems built into the tank that continually mix the slurry to prevent settling and keep an even temperature inside the tank. Complete mix digesters also have a heating system to keep the slurry at optimal temperatures for the anaerobic bacteria; approximately 40ºC.
Figure 3. view of complete mix digester
Figure 4. Table of digester design characteristics; modified from 
Biogas and Byproducts
The biogas produced from anaerobic digestion can be used in multiple ways. The methane produced can be combusted on site to produce electricity and heat, liquified to CNG for transportation fuels, or put through the Fischer-Tropsch process to produce other liquid fuels.
Combustion is a very simple process that can be done on-site to produce adequate amounts of electrical energy while also being able to utilize waste heat for added efficiency. A digester producing 7,100 cubic yards of biogas per day can power two 526 kW engines while producing another 540 kW of heat .
Figure 5. Engine used in combustion of biogas
Methane, once compressed (3,000-5,000 psi), can be stored in fuel tanks and used in vehicles ranging from small scooters to semi-trucks. The compressed natural gas (CNG) has a 25-50% lower cost than diesel or gas, and significantly reduces the greenhouse gas effects of methane . Methane, being much more harmful to the atmosphere than CO2 is combusted in engines to produce CO2 and H2O. A vehicle running off of CNG can travel over 3 times as far as a vehicle running off of ethanol (produced from equal amounts of corn) . This makes anaerobic digestion much more attractive than ethanol production in terms of transportation fuels.
Methane can also be put through the Fischer-Tropsch process to be used to synthesize other transportation fuels like diesel and gasoline. The methane first needs to be converted to syngas (CO + H2) so it can be used in normal Fischer-Tropsch techniques. To do this, the methane is put through four processes. The methane is first put through steam reforming, then partially oxidized, and finally autothermally reformed, or put through combined reforming for better syngas production . Once the methane is converted to syngas, it can be put through the normal Fischer-Tropsch process and then used for many kinds of transport fuels.
The process of anaerobic digestion also produces other byproducts that can be used to make an added profit. solid materials produced from digestion have many uses. Manure solids from manure slurries can be used as a safe bedding for the animals that produce it (cows, swine, horses) . The solids can also be used to create manure composite boards that can be sold . Inorganic material from the digestion process can also be used as a fertilizer for the fields used to grow the feedstock. This fertilizer can replace a large percentage of the soil nutrients lost during harvesting, but certain toxins may need to be filtered out beforehand .
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