Anaerobic Digestion itself occurs in four–stages, being

1. Hydrolysis: where the polymeric carbohydrate and protein compounds are broken into smaller chained monomeric compounds such as amino acids, sugars, and fatty acids
2. Acidogenesis: where the above compounds are oxidized to volatile fatty acids (C3–C6 acids), alcohols, aldehydes and carboxylic acids,
3. Acetogenesis: where the above are cleaved into two carbon acetic acid with production of carbon dioxide, ammonia, hydrogen and hydrogen sulfide and finally
4. Methanogenesis: where the acetic acids are further broken down into methane and carbon dioxide. The temperature at which the process is allowed to occur determines the rate or decomposition as well as the types of bacteria involved.

• Wet vs dry fermentation systems: The Ad process in categorised depending on the percentage total solids (TS). In wet fermentation systems the total solids is between 2%-10%, while the dry fermentation system handles >30. It is recommended that slurry be kept at 7- 9% for big reactors in order to facilitate pumping and mixing of the feedstock and digestate.
• Plug Flow hydraulics: The hydraulics of AD is similar to wastewater treatment. In plug flow system, the feedstock enters one end and pushes the digestate out the other end. The organic matter moves as a block through the system digesting as it goes along with most digested in front and less digested at the back. There are no mixing only agitators that work to push the waste forward.
• Continuous Stirred Reactor (CSTR): In CSTR, the feedstock is mixed thoroughly by mixers in the reactor so that at any time the rate of decomposition is similar everywhere in the tank. CSTR allows for faster degradation and biogas recovery but requires an input of energy.