Understanding the Distinguishing Features of a Microbial Fuel Cell as a Biomass-Based Renewable Energy Technology - Emerging Environmental Technologies - page 13

where J s,deep is the substrate flux in a deep biofilm. Thus, a substrate-limited
MFC must produce an electrical current that is proportional to J s, deep .
In order to convert J s into current density, j, we must take into consideration the
fact that a fraction of the electrons contained in the e - donor will be used for biomass
synthesis by ARB and another fraction of electrons will end up in byproducts (such
as methane, fermentation products, and soluble microbial products). The current
density contains the electrons not going to those sinks, but to respiration:

j ¼ s J s ð 1 f s H Þ (1 : 10)

where s is a conversion factor from mass to coulombs for the substrate ([nF]/
MW), f s 0 is the fraction of electrons used for cell synthesis, and H is the fraction
of electrons converted into byproducts [33]. In addition to the substrate flux,
biomass decay contributes to the total current, but this current has been
reported to be about 1% of the maximum current [15] and will not be consid-
ered in our discussion. We note that the calculation of j deep depends on para-
meters that are specific to the microbial community and donor substrate:
q max , K, H,andf s 0 . Assuming our community is similar to a pure culture of
G. sulfurreducens, a known ARB, we can estimate how j deep depends on S s and
X f . Table 1.1 shows the parameters for G. sulfurreducens. We assumed H=0 for
simplicity. These parameters are similar to those of slow-growing anaerobic
cultures, such as sulfate reducers or methanogens [32].
Figure 1.6 shows the maximum current density obtained by an ARB com-
munity that is substrate-limited, utilizing the parameters in Table 1.1. At a
typical X f of 50 mg VS/cm 3 , j deep increases as S s increases, with values between 0

Table 1.1 Electron-donor (ED) kinetic and stoichiometric parameters for G. sulfurreducens,
based on the literature
Symbol Description Value Units Reference

1.9 10 -3 mg BOD/
cm 3

[39] a K Half-maximum-rate acetate

concentration

q max Maximum specific rate of ED

utilization

8.4 mg BOD/mg

VS day

Calculated based on
Bond and Lovley
[40] b

Fraction of e - from electron
donor utilized in byproduct
formation

H 0 - Assumed

f s 0 Fraction of e - from the e - donor

utilized for biomass synthesis

0.05 - Based on Bond and
Lovley [40]
a We averaged values for K Sd for G. sulfurreducens utilizing acetate as the ED and two different
electron acceptors: fumarate and iron citrate as electron acceptors [39].
b For G. sulfurreducens catalyzing acetate oxidation at an anode, the maximum rate of acetate
utilization observed was 1.2 mmole mg /protein min (Bond and Lovley [40]). We made unit
conversion assuming a protein content of about 55% (Whitman et al. [41]), C 5 H 7 O 2 N as the
formula for cells grown with ammonium as the nitrogen source (Rittmann andMcCarty [32]),
and cells are 90% organic matter.