3 Stages of Anaerobic Digestion | Microbiology

Anaerobic digestion is accomplished in three stages, solubilisation, acidogenesis and methanogenesis.

Stage # 1. Solubilisation:

It is the initial stage when feed stock is solubilized by water and enzyme. The feed stock is dissolved in water to make slurry. The complex polymers are hydrolysed into organic acids alcohols by hydrolytic fermentative methanogenic bacteria which are mostly anaerobes (Fig. 33.3A).

Stage # 2. Acidogenesis:

In this stage, the second group of bacteria i.e. facultative anaerobic and H₂ producing acidogenic bacteria convert the simple organic material via oxidation/reduction reactions into acetate, H₂ and CO₂.

These substances serve as food for the final stage. Fatty acid is converted into acetate, H₂ and CO₂ via acetogenic dehydrogenation by obligate H₂ producing acetogenic bacteria. There is another group of acetogenic bacteria which produce acetate and other acids from H₂ and CO₂ via acetogenic hydrogenation (Fig. 33.3B).

Stage # 3. Methanogenesis:

This is the last stage of anaerobic digestion where acetate and H₂ plus CO₂ are converted by methane producing bacteria into methane, carbon dioxide, water and other products (Fig. 33.3C).

Different species of methanogens are involved in breakdown of complex organic matter into acetate or other organic acids. Acetate is one of the substrates of methanogens. Hydrogen with CO₂ in general acts as substrate for methanogenesis.

All the bacteria require H₂ and formate (except M. bryantii, M. thermoautotrophicum and M. arboriphilus) for growth and methane production, whereas M. barkerii requires (besides H₂) methanol, methyl amine and acetate for their growth. Thus, the methanogens are either autotrophs, or utilize simple organic compounds as formate, acetate, and methyl amine and occupy the terminal position in anaerobic food chain.