Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 193

down into amino acids, fats into long-chain fatty acids and carbohydrates into
simple sugars, while the liquefaction of complex biological polymers, especially
cellulose, to simple, soluble substances is often the rate-limiting step in digestion.
The rate of hydrolysis is governed by the nature and availability of the substrate,
bacterial population, temperature and pH.

Acidogenesis
The monomers released by hydrolysis, together with volatile fatty acids (VFAs)
derived from the protein, fat and carbohydrate components of the material being
treated are converted to acetic, lactic and propionic acids, the pH falling as the
concentrations of these rise. Methanol and other simple alcohols, carbon dioxide
and hydrogen are also produced during acidogenesis, the exact proportions of the
different byproducts being dependent on bacterial species and the environmental
conditions within the reactor. Though we have considered these events as a single
stage in the process, some authorities prefer to subdivide them into acidogenesis
and acetogenesis to highlight the importance of acetic acid, which accounts for
around 75% of the methane produced by the next step.

Methanogenesis
Relying on obligate anaerobes whose overall growth rate is slower than those
responsible for the preceding stages, this final phase involves the production of
methane from the raw materials previously generated. Of these, acetic acid and
the closely related acetate are the most important, for the reason mentioned above.
There are other potential substrates for methanogenic bacteria, but we will limit
the scope of this discussion to the most usual examples, as represented in the
following equations:

Acetic acid

CH 3 COOH −−−→ CH 4 + CO 2

Methanol

CH 3 OH + H 2 −−−→ CH 4 + H 2 O
Carbon dioxide and hydrogen
CO 2 + 4H 2 −−−→ CH 4 + 2H 2 O

Methanogenic bacteria also play an important part in the wider overall breakdown
process, since by converting volatile fatty acids (VFAs) into methane, they effec-
tively act to limit pH decrease within the digester. With the acid/base equilibrium
naturally regulated in this way, any potential bacterial inhibition by acidification is
effectively overcome. This is particularly important for methanogens themselves,