Biotechnology and Waste - Environmental Biotechnology: Theory and Application - page 192

The key to effective practical applications of AD technology lies in regulating
and optimising the internal environment of an enclosed bioreactor vessel such
that the ideal conditions for the process are produced and maintained. Under
these circumstances, in the absence of free oxygen, anaerobic bacteria convert
the large organic molecules mainly into methane CH 4 and carbon dioxide CO 2 .
The actual progression of this breakdown is chemically very complex, poten-
tially involving hundreds of intermediary reactions and compounds, many of
which have their own additional requirements in terms of catalysts, enzymes or
synergistic chemicals. Unlike composting, AD occurs at one of three distinct
temperature ranges, namely:
• Cryophilic ( < 20 ◦ C).
• Mesophilic (20-45 ◦ C).
• Thermophilic ( > 45 ◦ C).
Since AD is very much less exothermic than composting, within a landfill or in
bogs and swamps, it proceeds under cryophilic conditions. This largely accounts
for the relatively protracted timescale and the irregular progress of breakdown
typically encountered in these examples. In order to overcome these drawbacks,
engineered anaerobic bioreactors are usually run at one or other of the higher
ranges, with additional heat supplied by external means to elevate the temperature
to the required level. A variety of technology vendors have developed commercial
systems based around either thermophilic or mesophilic digestion, which have
their own particular characteristics. Without entering into a lengthy discussion of
the relative merits of these approaches, it is important to note that the internal
conditions favour different bacterial complements and that certain aspects of
the reaction details also differ. Consequently, for any given application, one or
other may be particularly suited, dependent on the specifics of the material to be
processed and the overall requirements for treatment.
The digestion process
Hydrolysis
Carbohydrates, cellulose, proteins and fats are broken down and liquefied by the
extracellular enzymes produced by hydrolytic bacteria. The proteins are broken
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 proprionic acids, the pH falling as the