AD explained
The generic AD process
Anaerobic Digestion (AD) is a natural biological process using naturally occurring micro-organisms to break down organic matter into a valuable fertiliser whilst producing biogas. The anaerobic bacteria utilised in the process are commonly found in soils and deep waters.
The science of AD can be split into three main stages:
Hydrolysis:
Hydrolysis is the splitting (lysis) of a compound with water (hydro). Carbohydrates, fats and proteins undergo hydrolysis to break them up into smaller more soluble molecules. Fermentative bacteria convert insoluble complex organic matter, such as cellulose into soluble molecules such as fatty acids, amino acids and sugars. The liquefaction of complex compounds has the greatest effect on the rate of the digestion process.Acidogenesis:
Acetogenic bacteria convert the hydrolysis products into simple organic acids (acetic, butyric and proprionic acids), ethanol and volatile fatty acids. The pH falls, with carbon dioxide and hydrogen being produced.Methanogenesis:
Methane is finally produced by methanogenic bacteria breaking down the acetic acid and VFA molecules. The methanogenic bacteria produce a biogas, a mixture of methane (60%) and carbon dioxide (40%) and other trace elements.
A combined heat and power (CHP) engine is used to convert the biogas into electricity and heat.
