AD is a synergistic process of a consortium of microbes which
can be classified along with a series of metabolic pathways. The
major reactions of the AD process are shown in Fig. 1. The whole
process is divided into following 4 steps [11–31].
3.1. Enzymatic hydrolysis
This is the first step of the AD process; in this step substrate
undergoes hydrolysis reaction that reduces the complex organic
polymers to simple soluble molecules with the help of extracellular
enzymes. During this reaction protein, lipids and carbohydrate
polymers are hydrolyzed to amino acids, long-chain fatty
acids and sugars, respectively.
3.2. Fermentation
The reduced compounds are then converted to a mixture of
short chain volatile fatty acids (VFAs) and other minor products
such as carbon dioxide, hydrogen and acetic acid with the help of
fermentative bacteria.
3.3. Acetogenesis
Acetogenic bacteria further convert the organic acids to acetate,
carbon dioxide, and/or hydrogen which are used as direct substrates
for methane production.
3.4. Methanogenesis
The final step of AD is methanogenesis, where a variety of
methanogenic bacteria consume acetate, carbon dioxide and
hydrogen to produce methane.