Environmental Engineering Reference
In-Depth Information
converted into electrical energy (E
out
). These analyses indicate that understand-
ing the flow of electrons in an MFC is important to understand the interaction
of energy generation and water treatment. In this regard, careful studies that
track electrons based on mass balance provide valuable information for under-
standing the overall process dynamics for MFCs and other energy-to-biomass
processes [21, 48].
Another important concept to determine the efficiency of a biomass-to-
energy process is the PEE:
PEE
¼
E
out
E
input
E
out
(1
:
25)
in which E
input
is the amount of energy required to run the energy-to-biomass
process. Therefore, PEE evaluates the net energy produced in the process, while
ignoring the input energy as substrate (E
substrate
). Combining PEE and ECE
gives an overall efficiency (OE) from the point of view of E
substrate
:
OE
¼
PEE
ECE
¼
E
out
E
input
E
substrate
(1
:
26)
1.3 Perspectives on Different Energy-to-Biomass Technologies
ECE and PEE are valuable tools for comparing different energy-to-biomass
technologies. PEE estimates can only be obtained once the process has been
tested in pilot or full scale. Therefore, it is premature to place a PEE value for
MFCs, although we can predict it will be high, since MFCs do not require much
energy input. ECE values for MFCs vary depending on the substrate used,
experimental conditions, and acclimation of the ARB community. Under the
best conditions, assuming a VE = 0.5 and CCE = 0.85 (typical of value for
acetate consumption), ECE = 42.5%. For comparison, ethanol fermentation
and methanogenesis are anaerobic metabolisms that synthesize biomass with a
small yield. Thus, both processes produce only a small amount of microbial
biomass and capture 80-90% of the electrons (CCE) as CH
4
or ethanol. How
do they compare with electricity generation by an MFC?
1.3.1 Methanogenesis
Methanogenesis is a long-standing technology for stabilizing sludges and other
organic wastes [9], as it has natural advantages. First, a complex community
develops so that the electrons and energy in sewage, industrial, and agricultural
sources can be routed to H
2
, acetate, and then CH
4
. Second, the anaerobic
metabolism synthesizes biomass with a small yield. Thus, a methanogenesis
