Environmental Engineering Reference
In-Depth Information
implicated in the transfer of electrons from the biofilm bacteria to the anode
electrode, e.g., cytochromes, conductive nanowires, and adsorbed soluble-media-
tors such as pyocyanine [20, 44, 45]. Based on the presence of conductive materials
in the extracellular matrix of biofilm anodes, Kato Marcus et al. described the
extracellular matrix as a conductor that accepts electrons from the respiration
and conducts the electrons to the anode [31]. Because the biofilmmatrix functions
as part of the anode, the biofilm on the anode is termed a ''biofilm anode.''
Much like diffusion-limitation of a soluble acceptor in a biofilm, electron
conduction can create an electrical potential gradient along the depth of biofilm
anode, as illustrated in Fig. 1.5. Kato Marcus et al. used Ohm's law to describe
the gradient in the electrical potential in the biofilm anode [31]:
j
¼
bio
d
dz
(1
:
15)
where
bio
is the biofilm conductivity (mS/cm) and the local potential . Equa-
tions (1.14) and (1.15) combine to form an electron-balance that describes the
generation and conduction of electrons in the biofilm anode.
In addition to the biofilm anode mechanisms, an ARB community can
produce electron shuttles such as pyocyanine to facilitate the transfer of elec-
trons through the electron-shuttle mechanism [44, 46]. Interested readers may
refer to the original work that presents the model for the electron-shuttle
mechanism [47].
An important take-home message from this section is that ARB require
electrical potential to drive electrical current. From the perspective of an
MFC operator, this implies loss of useful voltage V
cell
. This analysis highlights
the need to investigate a potential trade-off between the current and potential.
In the next section, we introduce the concept of ECE, which is a useful para-
meter for assessing the tradeoff.
1.2.3 Coulombic Efficiency, Energy Capture Efficiency, and COD
Balance in an MFC
In order to complete the efficiency analysis of an MFC, we perform COD
balances to help understand the amount of current produced per substrate
feed and removed in the MFC anode. COD balances can be performed at the
biofilm level or at the reactor level. At the biofilm level (Fig. 1.9), a balance of
substrate flux in terms of electrons helps illustrate how the electrons flow into
the anode to produce an electrical current (Fig. 1.10). This is expressed math-
ematically by
J
s
¼
J
e
þ
J
H
þ
J
biomas
¼
j
=
s
þ
J
s
H
þ
J
s
f
s
(1
:
16)
