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
 
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