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2.2.2 Acidogenesis

The second step of the anaerobic digestion process is acidogenesis or acidification; the
products from hydrolysis step were converted into simple molecules with a low molecular
weight such as volatile fatty acids (acetic, propionic, butyric, and lactic acids), alcohol (ethanol,
and butanol), aldehydes, and gases (hydrogen, carbon dioxide, and traces of methane and
hydrogen sulfide) by fermentative bacteria. Angelidaki et al. (2002) reported approximately 51%
of acetate, 19% of hydrogen, and the rest are more products were comprised alcohols or
aldehydes as the products from the acidogenesis step.

2.2.3 Acetogenesis

The products from acidogenesis such as fatty acids with longer than two carbon atoms,
longer than one carbon atom of alcohols, and branched-chain and aromatic fatty acids, which
cannot be converted to methane by methanogenic bacteria directly, were converted to
methanogenic substrates (acetate, hydrogen, and carbon dioxide) in the acetogenesis stage.
However, there are several factors affecting to hydrogen producing acetogens and
homoacetogens comprehensive physical and chemical conditions (pH, temperature, acetate
accumulation, and partial hydrogen pressure).

2.2.4 Methanogenesis

The final step of the anaerobic digestion process, the products achieved from the first
three step of the anaerobic digestion mostly acetic acid, hydrogen, and carbon dioxide was
further converted to methane and carbon dioxide by aceticlastic and hydrogenotrophic
methanogens. However, 70% of methane was produced via the aceticlastic pathway by
aceticlastic methanogens and the rest of 30% was produced via the hydrogenotrophic pathway by
hydrogenotrophic methanogens. However, some of acetate-consuming methanogens are the
slow-growing microbes with a generation time of 1 to 12 days; in contrast, the hydrogen-
utilizing methanogens are the fastest-growing microbes with a generation time of 6 h. In
addition, hydrogen-consuming methanogens are more tolerant to environmental changes than
aceticlastic methanogens. Nevertheless, methanogenesis is affected by environmental changes
and reactor operating conditions include pH, temperature, hydraulic loading rate, organic loading
rate, substrate composition (Boe, 2006).