Environmental Engineering Reference
In-Depth Information
20
15
15
10
10
5
5
0
0
0
50 100 150 200 250
S
s
(mg BOD/l)
0
50
100
150
200
Biofilm Density (mg VS/cm
3
)
(a)
(b)
Fig. 1.6 Maximum current density for an ARB biofilmanode that is completely substrate-limited.
The ARB have parameters similar to those of G. sulfurreducens (Table 1.1). (a)Maximum
current density as a function of substrate concentration at the liquid-biofilm interface for a
biofilm with X
f
=50mgVS/cm
3
.(b) Maximum current density as a function of biofilm density
for an S
s
=50mgBOD/l
and 16 A/m
2
for S
s
= 0-250 mg BOD/l (Fig. 1.6a). At S
s
=50 mg BOD/l, j
deep
increases with an increase in X
f
, as shown in Fig. 1.6b, up to a maximum of 14
A/m
2
at 200 mg VS/cm
3
. Given a high S
s
and X
f
, for example, S
s
=500 mg
BOD/l and X
f
=200 mg VS/l, j
deep
=47 A/m
2
. These probably are not realistic
values in a wastewater treatment process that aims for low effluent BOD
concentrations, but they may be realistic for energy conversion from a high-
strength waste stream, such as pig manure.
The use of bacteria that have a higher q
max
can also increase the maximum
current density. However, an increase in q
max
usually implies an increase in
energy requirements for bacterial growth (leading to a lower VE), as well as a
higher f
s
0
[32, 34], both of which are not desirable in an MFC process. There-
fore, it would be valuable to find or engineer ARB that has a very high q
max
and
a very low f
s
0
.
Table 1.2 shows the maximum current densities obtained by various MFC
researchers. The maximum current densities reported (per anode surface area)
are usually around 5-6 A/m
2
[35, 36], although recently current densities
reported have increased up to 13 A/m
2
[26]. Reported current densities are as
much as an order of magnitude less than the highest current densities shown in
Fig. 1.6, but the computed values are for high substrate concentrations and the
assumption of only substrate limitation. This comparison suggests that sub-
strate limitation is not the only or main controlling factor for contemporary
MFCs. Overcoming the current limitations offers the potential to increase
current density significantly.
Recently, Torres et al. [25] showed that the transport of protons, produced
during substrate oxidation, from inside to outside the biofilm can cause a pH
inhibitory effect on ARB, thus limiting the total current produced. The
current density produced by the ARB biofilm decreased linearly as the buffer
