Environmental Engineering Reference
In-Depth Information
food industry wastewaters is high. However, realization of the complete potential
requires further work with insoluble substrates. There are several technical issues
that need to be considered for application of MFCs to wastewater treatment. These
can be classified into two main categories: MFC design or engineering parame-
ters and process or operational parameters. The MFC design parameters include
electrode spacing, type of membrane, ratio of anode to cathode surface area, poros-
ity of electrodes and chemical vs. biological cathode. The MFC design parameters
have been reviewed in several recent reviews [7, 21, 22] and the issues related to
cathode systems have been discussed in two recent reports [51, 52]. The operating
parameters include ionic strength, buffering capacity, flow rate of liquid through
the anode and/or cathode chamber, oxygen content of the wastewaters and organic
loading.
The ionic strength of the wastewater has been shown to affect the power den-
sity of MFCs [53, 54]. The buffering capacity is a related parameter, which also
affects stable power generation as observed in the Mayfield wastewater case study.
Continuous power generation at high rates is possible by amending the wastewater
with buffering salts to maintain the ionic strength and buffering capacity. However,
this may not be a practical option due to the cost of the buffer salts, especially
because the salts cannot be easily recovered from the treated wastewater. Use of
membrane-free MFCs alleviates the problem of buffering capacity to some degree,
since it minimizes pH polarization [55]. However, the buffering capacity will still
be an issue in larger scale systems. In absence of membranes, diffusion of oxygen
towards anode and carbon source towards cathode become significant resulting in
reduced coulombic efficiency.
The anode flow rate controls the carbon and nutrient supply to the microbes typ-
ically present as biofilms on the electrodes. Presence of pH gradients, which occurs
in biofilms and non-porous electrodes can also be minimized by designing systems
with flow-through capability. Use of porous, three-dimensional anodes incorporat-
ing these principles has been reported to result in higher power densities [11, 12].
The flow rate is also know to affect biofilm formation and its subsequent impact
on power densities [56]. In a pilot-scale study with brewery wastewater as the feed,
several operating parameters were investigated [57]. One of the parameters was the
dissolved oxygen present in the incoming stream. The study reported formation of
thick biofilms which was a problem for stable, continuous operation of the MFC.
The presence of dissolved oxygen can promote aerobic growth of biofilms which is
undesirable for optimumMFC performance. The study also found a number of other
process and design parameters including proton transfer, liquid flow management,
electrode conductivity, electrical contacts, issues related to scale-up and handling of
operational upsets, which need to be carefully considered and optimized in order to
achieve power densities similar to those observed in the laboratory.
5.3.5.3 Potential for Water Reuse and Recycle
One of the selling points of the MFC technology has been its ability to treat wastew-
ater while generating energy. While removal of organic carbon from wastewaters
