Integrated Environmental Biotechnology - Environmental Biotechnology: Theory and Application - page 238

Figure10.3 Methanegeneratedfrombiowastecomponents
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 diges-
tion for methane production, notably in the waste management, sewage treatment,
agricultural and food processing industries. The process has also been success-
fully used on a 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 ◦ C
of 0.72 kg/m 3 ; for hydrogen the same properties lie between 4 and 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 produc-
tion attractive as a means of generating renewable energy. As was mentioned
in the earlier section on anaerobic digestion, with a theoretical yield of 400m 3
of biogas per wet cellulosic tonne, the prospect of high energy returns simulta-
neous with waste treatment has clear appeal. However, as was also pointed out
in the same earlier segment, it is not feasible to optimise conditions such that
high levels of both waste reduction and gas generation are deliverable. More
commonly, in practice only around a quarter of the potential biogas yield is
actually achieved.
Using biogas
Although biogas from engineered AD processes share many similarities with
LFG, it is important to remember that it is of quite distinct quality, being much