Environmental Engineering Reference
In-Depth Information
than for the starting biomass. The fermentative bacteria retain a small fraction of
the carbon from the original compounds to synthesize new cells, but most of the
electrons and energy from the biomass are contained in the fermentation products.
Figure 1.2 shows different stages at which biofuels such as ethanol, hydrogen,
and methane are generated. Ethanol is generated directly from the fermentation
of sugars, such as glucose. The complete fermentation of one glucose molecule to
ethanol produces two moles of ethanol and two moles of carbon dioxide:
C
6
H
12
O
6
2CH
3
CH
2
OH
þ
2CO
2
This brings the average oxidation state of carbon to -2 for ethanol.
As shown in Fig. 1.2, methane is produced at the last stage of the biological
processes during methanogenesis. Anaerobic (methanogenic) digestion depends
on a sequence of steps that involves acidogenesis, acetogenesis, and methanogen-
esis. The overall goal of the anaerobic digestion technology is to convert carbons
in biomass to its most reduced oxidation state, -4 in CH
4
or methane.
Microorganisms that carry out methanogenesis are termed methanogens (i.e.,
methane generators), and they are in the domain Archaea. Two major types of
methanogens are hydrogenotrophic and acetoclastic methanogens. Hydrogeno-
trophs oxidize H
2
and reduce CO
2
to CH
4
in a form of anaerobic respiration:
4H
2
þ
CO
2
¼
CH
4
þ
2H
2
O
Acetoclastic methanogens ferment one molecule of acetate to produce one
molecule each of CH
4
and CO
2
:
CH
3
COO
þ
H
2
O
¼
CH
4
þ
HCO
3
As glucose and biomass have comparable average carbon oxidation states (i.e.,
0), many (e.g., Angenent et al. [9]) appropriately describe the overall reaction
for methanogenesis with glucose as
C
6
H
12
O
6
¼
3CO
2
þ
3CH
4
in which CH
4
with the carbon oxidation state of -4 captures all the electrons
from glucose. All the electrons must flow through H
2
and acetate, as shown in
Fig. 1.2 and the reactions just above. Approximately 70% of the methane
formed in conventional anaerobic treatment comes from acetate [9].
1.2 Microbial Fuel Cell
An MFC marries microbiological catalysis to the electrochemistry of a fuel cell.
Figure 1.3 describes the flow of electrons and ions in an MFC. The fuel for an
MFC is an electron donor for bacteria that live in a biofilm attached to the
