There are four key biological and chemical stages of anaerobic digestion:^[40][41]

1. Hydrolysis
2. Acidogenesis
3. Acetogenesis
4. Methanogenesis

In most cases biomass is made up of large organic polymers. In order for the bacteria in anaerobic digesters to access the energy potential of the material, these chains must first be broken down into their smaller constituent parts. These constituent parts or monomers such as sugars are readily available by other bacteria. The process of breaking these chains and dissolving the smaller molecules into solution is called hydrolysis. Therefore hydrolysis of these high molecular weight polymeric components is the necessary first step in anaerobic digestion.^[42] Through hydrolysis the complex organic molecules are broken down into simple sugars, amino acids, and fatty acids.

Acetate and hydrogen produced in the first stages can be used directly by methanogens. Other molecules such as volatile fatty acids (VFA’s) with a chain length that is greater than acetate must first be catabolised into compounds that can be directly utilised by methanogens.^[43]

The biological process of acidogenesis is where there is further breakdown of the remaining components by acidogenic (fermentative) bacteria. Here VFAs are created along with ammonia, carbon dioxide and hydrogen sulfide as well as other by-products.^[44] The process of acidogenesis is similar to the way that milk sours.

The third stage anaerobic digestion is acetogenesis. Here simple molecules created through the acidogenesis phase are further digested by acetogens to produce largely acetic acid as well as carbon dioxide and hydrogen.^[45]

The terminal stage of anaerobic digestion is the biological process of methanogenesis. Here methanogens utilise the intermediate products of the preceding stages and convert them into methane, carbon dioxide and water. It is these components that makes up the majority of the biogas emitted from the system. Methanogenesis is sensitive to both high and low pHs and occurs between pH 6.5 and pH 8.^[46] The remaining, non-digestable material which the microbes cannot feed upon, along with any dead bacterial remains constitutes the digestate.

A simplified generic chemical equation for the overall processes outlined above is as follows:

C₆H₁₂O₆ → 3CO₂ + 3CH₄

References:

1. Anaerobic digestion, www.waste.nl, retrieved 19.08.07 Ciborowski, P (2004)Anaerobic Digestion in the Dairy Industry, Minnesota Pollution Control Agency Air Innovations Conference, www.epa.gov, retrieved 19.08.07
2. Sleat, R. & Mah, R. (2006) Hydrolytic Bacteria in Anaerobic digestion of biomass, p15 Boone, D. & Mah, R. (2006) Transitional bacteria in anaerobic digestion of biomass, p35
   
3. What is anaerobic digestion, www-sop.inria.fr, retrieved 24.10.07
4. Anaerobic digestion, www.biotank.co.uk, retrieved 24.10.07
5. Martin, A.D. (2007) Understanding Anaerobic Digestion, Presentation to the Environmental Services Association, 16.10.07, www.esauk.org, retrieved 22.10.07

Anaerobic digestion is a series of processes in which microorganisms break down biodegradable material in the absence of oxygen. It is widely used to treat wastewater sludges and organic waste because it provides volume and mass reduction of the input material.[1] As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digestion is a renewable energy source because the process produces a methane and carbon dioxide rich biogas suitable for energy production helping replace fossil fuels. Also, the nutrient-rich solids left after digestion can be used as fertiliser.

The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria. Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Methanogens, finally are able to convert these products to methane and carbon dioxide.[2]

Previously, the technical expertise required to maintain anaerobic digesters coupled with high capital costs and low process efficiencies had limited the level of its industrial application as a waste treatment technology.[3] Anaerobic digestion facilities have, however, been recognised by the United Nations Development Programme as one of the most useful decentralised sources of energy supply, as they are less capital intensive than large power plants.[4]

References:

1. Anaerobic digestion http://www.monsal.com, retrieved 18.09.07
2. waste.nl Anaerobic digestion reference sheet, www.waste.nl, retrieved 25.10.07
3. waste.nl Anaerobic digestion reference sheet, www.waste.nl, retrieved 26.10.07
4. Biogas Bonanza for Third World Development, www.i-sis.org.uk, retrieved 4.11.07, cites United Nations Development Programme (UNDP) 1997 Report, Energy After Rio: Prospects and Challenges