Understanding the Distinguishing Features of a Microbial Fuel Cell as a Biomass-Based Renewable Energy Technology - Emerging Environmental Technologies - page 20

membrane surface area (dm 2 ), J O2 the oxygen flux across the membrane (e - eq./
L 2 ), and 1/8 the conversion form of 8 g COD to 1 e - eq.
The following equations define the effluent terms for a biofilm-dominated
system:

Q COD liq ¼ A ð J biomass þ J H ; liq Þ ð 1e eq := 8 gCOD Þ (1 : 18)

Q gas COD gas ¼ A ð J H ; gas Þ ð 1e eq := 8 gCOD Þ

(1 : 19)

We define coulombic efficiency in two ways. The first definition, respiration
coulombic efficiency (RCE), is based on the conversion of the removed substrate
to electrical current by ARB respiration, which can be expressed as the ratio of
the current flux to the substrate flux [31]:

RCE ¼ J e
J s

(1 : 20)

The second coulombic efficiency, capture coulombic efficiency (CCE), is
defined based on the capture of influent COD into electrical current. The
coulombic efficiency is now the fraction of COD that was captured as current
over the COD fed in the influent:

CCE ¼ COD e A
QCOD in

J e A
QCOD in

¼ (1 : 21)

CCE is usually high (71-95%) when acetate is used [15, 21, 40, 48]. When a
fermentable substrate is used, the production of fermentation byproducts, such
as methane and hydrogen, can reduce the amount of recovered electrons.
However, the coulombic efficiency also depends on the reactor configuration
and conditions used that could lead to variances in the e - sinks. Table 1.3 shows
a wide variety of reported values of RCE and CCE.
CCE for an MFC can be low for three reasons. First, the volumetric COD
loading to an MFC is disproportionately high with respect to the surface
loading (which is proportional to current density). Second, a high-fraction of
the COD entering the reactor is not biologically available to ARB. Third,
alternate electron sinks compete with the anode electrode for COD. Two useful
parameters to understand the relative impact of these processes on CCE are
treatment efficiency (TE) and fraction of stabilization as current (FoSaC).
TE is a useful parameter for evaluating the COD removal (i.e., stabilization),
which is a major goal of an MFC as a wastewater treatment process. TE is the
percentage of COD removed from the influent by aerobic oxidation (J O2 )orby
conversion into electrical current (J e- ) and gaseous byproducts that includes
hydrogen and methane (J H,gas ). The definition of the TE is

TE ¼ A ð J e þ J H ; gas þ J O2 Þ= QCOD in ¼ð COD in COD liq Þ= COD in Þ (1 : 22)