3.2. Batch mode operation
Polarization and power density curves obtained after 25 fedbatch
cycles show that the reactor operated under mesophilic conditions
produced a slightly higher maximum power density than
reactors operated at ambient temperatures (Fig. 2A). The mesophilic
reactor (S30) produced 334 mW/m2 (10.2 W/m3, 0.977 A/
m2), or about 12% more power than the other two reactors (298
or 302 mW/m2, 9.07 or 9.18 W/m3, for P23 and S23, respectively).
CEs varied substantially for all reactors, with the highest CEs of 47–
57% obtained at the lowest current densities (Fig. 2B).
Energy recovery was the greatest for MFCs operated under
ambient temperatures, with a maximum of 0.69 Wh/g COD removal
at the lowest current density, and 0.03 Wh/g COD removal
at the highest current density (Fig. 2B). Energy recovery was
slightly lower for mesophilic reactors with a maximum of
0.52 Wh/g COD removal. In all cases the current density (and
therefore the circuit load) affected energy recovery more than
the inoculum or reactor conditions.
COD removal was greater than 88% in all reactors at all current
densities, except at the highest current density (2 A/m2) at the
lowest resistance tested (50 X) (Fig. 2C). Similarly, high particulate
COD removals were achieved (>89%) except at this highest current
density.
The ORP rapidly decreased in all reactors at the beginning of the
cycle 400 to 500 mV (Fig. 3A and B). Acetate was the main component
of the VFAs. Other intermediate byproducts, including propionate,
butyrate and ethanol were below the detection limit
(Fig. 3C and D). COD degradation rates (kt for total COD, and kp
for particulate COD) were higher under mesophilic conditions
(69.3 mg tCOD/L-h, and 22.0 mg pCOD/L-h) than under ambient
conditions (48.5 mg tCOD/L-h, and 12.5 mg pCOD/L-h). There were
relatively little differences in the degradation rates (kt2) of the
slowly biodegradable organic fraction in the wastewater under
the two temperature conditions (9 mg tCOD/L-h for mesophilic,
and 11 mg tCOD/L-h for ambient conditions) (Fig. 3C and D).
Total nitrogen removal in all cases of fed-batch MFCs was greater
than 63%. Nitrogen was more effectively removed in mesophilic