Microbial Fuel Cells for Wastewater Treatment - Biotechnologies and Biomimetics for Civil Engineering - page 418

a synergistic biofilm consortium, it is likely that a nonelectrogenic microbe may
secrete mediators that may help the electrogenic microbe to perform better electron
transfer.
18.7 Microbial Community of Electroactive Biofilms
Biofilms more than ten micrometers in thickness are typically formed on the anode
surfaces (Bond and Lovley 2003 ). They contain a complex microbial population
(Kim et al. 2004 ; Rabaey et al. 2004 ), apart from the known electrogenic bacteria
(Geobacter, Shewanella). Identifying members of the microbial community will
be a valuable aid in terms of improving the performance of MFCs and a more
comprehensive understanding of the key microbes required for exoelectrogenesis.
Up to now, there are many publications associated with microbial communities in
MFCs by means of PCR-amplified 16S rRNA gene fragments and sequencing such
as denaturing gradient gel electrophoresis (DGGE) (Table 18.2 ). Analysis of the
populations inhabiting such systems demonstrates that microbial communities are
phylogenetically diverse in most MFCs. Microbial populations are affected by
numerous factors, such as the substrate, cultivation mode, system architectures,
anaerobiosis degree, as well as the conditions within the cathode chamber (Logan
and Regan 2006a ).
The composition of substrates has a close relationship with the microbial
populations within the anode biofilms and MFC performance, as they serve as the
carbon (nutrient) and energy source for the microbiological process. Commonly,
the carbon sources contain pure compounds (acetate, glucose, lactic acid, etc.)
(Chaudhuri and Lovley 2003 ; Liu et al. 2005b ) and a variety of wastewaters
(brewery, chocolate, meat packing and paper recycling wastewaters, etc.) (Feng
et al. 2008 ; Huang and Logan 2008 ). The pure substrate inoculated systems are
found to produce more power than those fed with wastewater perhaps as the result
of different solution conductivity and buffer capacity (Pant et al. 2010 ). Based on
16S rRNA gene sequences, the dominant community members in the MFCs with
pure substrate are more known exoelectrogens (Geobacter sp., Desulfuromonas
sp., Rhodopseudomonas sp., etc.) and other bacteria with special function, such as
Clostridium sp., which is useful for lignocellulose degradation in cellulose-fed
MFCs (Cheng et al. 2011 ) (Table 18.2 ).
The highest power density of 4.31 W/m 2 was achieved using a mixed culture in a
fed-batch MFC and glucose as the substrate in the reactor with a Coulombic effi-
ciency (defined as the fraction of electrons recovered as current versus the maximum
possible recovery) of 81 %. The analysis of the population using DGGE showed
great phylogenetic diversity, with a complex mixture of bacteria (Firmicutes, c-, b-,
and a-Proteobacteria). Facultative anaerobic bacteria capable of hydrogen produc-
tion (Alcaligenes faecalis, Enterococcus gallinarum) were predominant (Rabaey
et al. 2004 ), probably owning to using a fermentable substrate with a mixed culture
inocula (Debabov 2008 ). It was deduced that mediator production accounted for the