Biomedical Engineering Reference
In-Depth Information
diffusion, Li et al. [ 12 ] proposed a innovative overflow-type wetted-wall reactor by
integrating a tubular anode chamber reactor and a baffle-chamber reactor. During
operation, the anodic medium, after reaction, continuously overflows into the
cathode chamber through a baffle and forms a falling-water film on the cathode
surface. Nevertheless, this design cannot avoid crossover of substrate and thus still
has low overall coulombic efficiency (CE).
2.1.2 Forced-Flow MFCs
Fluid mixing is an important factor of MFC operation, which may affect biofilm
formation on the electrode and mass/proton transfer within the reactor chamber.
This is especially true in larger-scale systems where mechanical agitation is
usually needed to increase the electrochemical reactions. Rabaey et al. [ 13 ] found
that adding baffles into the granular-bed anode chamber caused a forced flow and
increased voltage. It has also been reported that a thick biocathode biofilm was
beneficial for restricting oxygen transport. Based on this recognition, Hu [ 14 ]
designed a baffle-chamber membrane-less MFC by adding a baffle into the reactor
chamber to reduce mixing in the vicinity of the cathode and thus facilitate the
formation of a thick biofilm. This slight modification of the reactor configuration
led to improved CE. The baffle structure was also applied to a single-chamber
MFC by setting baffles at the input and output of the reactor, which enabled
adequate mixing during continuous operation [ 15 ]. Moreover, this configuration
decreases the electrode space and offeres more convenience for electrode stacking.
Recently, a novel baffle-chamber continuous-flow MFC with tubular air-cathode
was designed by Feng et al. [ 16 ]. This baffle structure was effective in promoting
fluid mixing and mass transfer in the graphite-granule-packed anode chamber and
accelerating electrode reactions while maintaining an anaerobic environment. The
maximum power density of this system reached 15.2 W/m 3 with an overall internal
resistance of only 13.7 X, and the average chemical oxygen demand (COD)
removal was 88.0% even at a high influent COD load.
Another approach to enhance fluid mixing by forced flow is to adopt upflow
configuration. An upflow MFC incorporates the operating mode of upflow
anaerobic sludge blanket (UASB) into MFC design [ 17 ]. Generally, an upflow was
created by feeding influent from the bottom of the anode chamber and discharging
the effluent from the top, with partial effluent feeding back to the bottom to form a
recirculation. In such a way, adequate anolyte mixing can be achieved without the
need for mechanical agitation. A first attempt was made by He and his colleagues
[ 18 ], who constructed a two-chamber upflow MFC with the cathode chamber
placed on top of the anode chamber. This group further improved the configuration
by putting a U-shaped cathode chamber inside the anode chamber [ 19 ]. Both
chambers were filled with granular activated carbon to increase the electrode area
while a PEM was assembled as the separator. This setup significantly lowered the
internal resistance to 17 X, likely due to a reduced ohmic resistance of the MFC at
decreased electrode spacing and increased PEM surface area compared with the
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