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

concentrations of these rise. Methanol and other simple alcohols, carbon dioxide
and hydrogen are also produced during acidogenesis, the exact proportions of the
different by products 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 sub-divide them into a cidogenesis
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 break-
down process, since by converting VFAs into methane, they effectively 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 effec-
tively overcome. This is particularly important for methanogens themselves, since
they thrive in a relatively narrow pH threshold of 6.6-7.0, becoming progres-
sively more impaired as the pH falls below 6.4. In this event, the persistence of
unmodified VFAs can have potentially serious implications for the final use or
disposal of the material derived.
There are four main groups of bacteria involved in AD, as shown below, with
some typical examples of each:

• Hydrolytic fermentative bacteria: Clostridium and Peptococcus .
• Acetogenic bacteria: Syntrophobacter and Syntrophomonas .
• Acetoclastic methanogens: Methanosarcina and Methanothrix .
• Hydrogenotrophic methanogens: Methanobacterium and Methanobrevibac-
terium .

In reality, these are not the only species present in a digester and, though the
stages previously described represent the main desired biochemical reactions, a