What is Anaerobic Digestion?
Also known as a biogas recovery system, an anaerobic digester uses bacteria to break down waste (such as manure or food waste) in an oxygen-free environment. The significant end product of this process is biogas. Biogas is a mixture of different gases such as methane (CH4) and carbon dioxide (CO2) among other gases. When combusted, biogas releases energy that can be used for different applications such as cooking and heating or further converted to electricity.
A mass balance can be performed around the digestion process:
There are three main sub-processes (with specific bacteria for each) that go into the anaerobic digestion process (as shown above):
1. Hydrolysis: The first step, solid waste is liquified. The large macromolecules are broken into simpler elements such as glucose from cellulose.
2. Acidogenesis and Acetogenesis: Next, Acidogenesis involves transforming the simpler elements from the hydrolysis step into acids such as lactic acid. Also produced are alcohols such as bicarbonate ethanol. Acetogenesis involves bacteria that then produce hydrogen sulphide.
3. Methanogenesis: The final step in the anaerobic digester. Two kinds of bacteria reduce methane acetate, CH4, bicarbonate and methane bicarbonate to achieve a final biogas product.
Here is a diagram of a simple anaerobic digester: this simplified model shows the inputs and outputs of the anaerobic digestion process.
Biodigester at Fabio:
The anaerobic biodigester at fabio uses a combination of food waste and animal waste (and sometimes plant matter). The carbon and nitrogen ratios have been optimized in order to achieve a large output of methane (CH4) gas. The biodigester is unique in that it is a continuous stir tank reactor. This requires an energy input in order to drive the stirring. Energy from solar panels adjacent to the tanks (object 4 in diagram below) provides energy for the operation.
The first step of the process is entering the waste into the system. The second step is grinding the waste, this allows the waste to breakdown and mix so that the bacteria is exposed to more surface area. Next, the waste is mixed with water. This mixture then enters the biodigester (item 3).
After 20 days, the anaerobic digestion cycle is complete. Solids and liquids are filtered out of the products to give biogas, a combination of methane and other biogases. This gas is trapped in a large bag and then used to generate power.
As mentioned above, the main outputs of the anaerobic digestion process are biogas and solid/liquid. Large solids (greater than 6mm) in diameter are filtered out first, but smaller solids remain in the slush. Our design project was to develop a filter to remove smaller solids before they enter a wetland. Part of the research at Fabio is related to treating water using an eco-friendly wetland. The wetland takes the slush (mostly water) and uses plants (through root uptake) to treat the water.
Biodigester at the University of Georgia:
Another biodigester that we were able to see was the one at the University of Georgia. Not only did we get to see the biodigester, but we were able to eat dinner that was cooked from biogas from the digester. The biodigester at UGA is similar to the one at Fabio, but is much larger and features a plug flow reactor (PFR) design, rather than a continuous stir tank reactor (CSTR). The benefits of a PFR design is that no energy is needed to power an impeller used to mix the solution, making this very low maintenance and cost-friendly. Another difference between this anaerobic digester is the influent. At UGA, human waste from the residential campus is used to feed the system.
When the biogas bag is filled, it can provide enough methane gas to cook one dining hall meal per day, sometimes that is over 100 people! At UGA the idea of a biodigester is accepted and looked at with positivity. The idea of a biodigester being used to generate biogas (that can cook meals) is still not widely accepted in the United States. However, in the future with more public education about the subject, this could be a very real and sustainable alternative to waste management.
Currently the American Biogas Council, the United States Department of Agriculture and the U.S. Environmental Protection agency are working together to educate the public about biodigester processes. This diagram from the American Biogas Council gives an overview of the entire process from organic material influent to what can be done with biogas and the solid/liquid effluent.
To sum things up:
Transforming plant, food, animal and human waste into fuel and fertilizer are what biodigesters can do. They use special kinds of bacteria to produce methane and further breakdown waste into smaller components. Biodigesters are generally two different kinds of reactors: a continuous stir tank (CSTR) or plug flow (PFR), with each having its own benefits. Sometimes the reaction is done under mesophilic conditions or thermophilic conditions, where energy from the biogas can be used to heat the system. The biogas generated is a good alternative to natural gas and can be used in a series of applications such as fuel and heat.
For more information on anaerobic digestion, visit:
Check out how anaerobic digestion is being used at MSU: http://msutoday.msu.edu/news/2013/new-msu-anaerobic-digester-to-supply-power-for-south-campus-buildings/