240 Environmental Biotechnology
- Stage V – Cellulose becomes fully decomposed, ultimately leading to zero
methane and carbon dioxide production; oxygen and nitrogen revert to atmo-
spheric levels.
Although beyond the scope of the present discussion to address fully, the posi-
tion of hydrogen as a regulator of methane production warrants a brief mention. In
the earlier examination of anaerobic digestion the obligate syntrophic relationship
between the hydrogen-producing acetogenic bacteria and the hydrogen-utilising
methanogens, was described. Essentially, higher fatty acids and alcohols are
converted to acetate, which requires an active population of hydrogenotrophic
methanogens to ensure a low hydrogen partial pressure, avoiding the preferential
production of butyric, lactic, proprionic and other acids instead of the desired
acetic. This has the potential to cause higher volatile fatty acids to accumulate
beyond the system’s ability to self-buffer, leading to a lowering of the pH. In turn,
as the increased acidity inhibits the methanogens themselves, methane production
ceases and ultimately the process will collapse.
A number of different applications have developed the idea of anaerobic
digestion for methane production, notably in the waste management, sewage
treatment, agricultural and food processing industries. The process has also been
successfully used at relatively small scale, commonly with animal manures as
its feedstock. Figure 10.3 shows an illustrative chart of methane generation for
many of the common biodegradable components of MSW.
Methane has an explosive range of 5–15% by volume and a density at 20◦Cof
0.72 kg/m^3 ; for hydrogen the same properties lie between 4–74% and 0.09 kg/m^3
at 20◦C, respectively. At 20◦C, carbon dioxide has a density of 1.97 kg/m^3 .The
calorific value of typical biogas, consisting of about 60% CH 4 , 40% CO 2 , lies
between 5.5–6.5 kWh/m^3 and it is this which makes its production attractive
as a means of generating renewable energy. As was mentioned in the earlier
section on anaerobic digestion, with a theoretical yield of 400 m^3 of biogas per
wet cellulosic tonne, the prospect of high energy returns simultaneous with waste
Figure 10.3 Methane generated from biowaste components