Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 236

Figure10.1 Thebiomassandbioenergycycle
the incarnations of bio-energy are viewed in the wider context of sustainability
has also gradually changed and policies and attitudes along with it. As the recent
inaugural report from the United Nations Environment Programme's International
Panel for Sustainable Resource Management pointed out, developing bio-energy
in a truly environmentally friendly way will call for a particularly sophisticated
approach (UNEP, 2009).
Derived Biofuels
Methane biogas
Biogas is a methane-rich gas resulting from the activities of anaerobic bacteria,
responsible for the breakdown of complex organic molecules. It is combustible,
with an energy value typically in the range of 21-28MJ/m 3 . The general pro-
cesses of anaerobic digestion and the biochemistry of methanogenesis have been
discussed in earlier sections of this topic, so they will not be restated here. As
mentioned previously, the main route for methane production involves acetic
acid/acetate and accounts for around 75% of gas produced. The remainder is
made up via methanol or carbon dioxide and hydrogen, as shown in Figure 10.2.
At various times a number of models have been put forward to aid the pre-
diction of biogas production, ranging from the simplistic to the sophisticated.
Many of these have been based more on landfill gas (LFG) generation than truly
representative anaerobic bioreactors, which does lead to some confusion at times.
However, it is generally accepted that the linked, interdependent curves for cel-
lulose decomposition and gas evolution can be broadly characterised as having
five principal stages, outlined below.
• Stage I: Peak biowaste cellulose loadings; dissolved oxygen levels fall to zero;
nitrogen, and carbon dioxide tend to atmospheric levels.