Environmental Engineering Reference
In-Depth Information
Current
Density,
j
Bulk Liquid
Biofilm
J
s
(1-
f
s
0
- H
)
Diffusive Substrate
Flux, J
s
(e
-
donor)
J
s
f
s
0
J
s
H
Microbial by-products
(soluble and/or gases)
Fig. 1.9 Schematic electron flow from the e
-
donor in the bulk liquid to the anode. Cell
respiration and cell decay are the two contributors to current density. Cell decay is not
considered in our discussion for simplicity and because it is often small
Q
gas
∗
COD
gas
1/8AJ
e
-
Q
∗
COD
in
C
a
t
h
o
d
e
A
n
o
d
e
COD
O2
Q
∗
COD
liq
Fig. 1.10 COD balance in a continuous MFC reactor
where J
e-
is the fraction of substrate utilized for respiration and is equal to j/g
s
(e
-
eq./L
2
), J
H
is the fraction of substrate used for byproduct formation and is
equal to J
s
*H (e
-
eq./L
2
), and J
biomass
is the fraction of substrate used for
biomass synthesis and is equal to J
s
*f
s
0
(e
-
eq./L
2
). Note that Eq. (1.16) is
derived from Eq. (1.10).
We can perform a similar mass balance for COD at the reactor level. In this
analysis, we include the flux of O
2
(J
O2
) across the ion-exchange membrane
separating the anode compartment from the cathode. Reduction of O
2
in the
anode compartment is an electron sink. Thus, the COD of the influent substrate
is divided into liquid effluent COD, gas effluent COD, O
2
reduction, and
current generation:
Q
COD
in
¼
Q
COD
liq
þ
Q
gas
COD
gas
þ
1
=
8AJ
e
þ
1
=
8
m
J
O2
(1
:
17)
where Q is the influent liquid flow rate (L/day), Q
gas
the effluent gas flow rate
(L/day), COD
in
the influent COD concentration (g COD/L), COD
liq
the efflu-
ent COD concentration (g COD/L), COD
gas
the COD concentration in the gas
effluent (g COD/L of gas), A the anode surface area (dm
2
), A
m
the ion-exchange