Axenic bacterial cultures
Some bacterial species in MFCs, of which metal-reducing
bacterial are the most important, have recently been reported to directly
transfer electrons to the anode. Metal-reducing bacteria are commonly found in
sediments, where they use insoluble electron acceptors such as Fe (III) and Mn
(IV). Specific cytochromes at the outside of the cell membrane make Shewanella putrefaciens electrochemically active
in case it is grown under anaerobic conditions. The same holds true for bacteria
of the family
Geobacteraceae, which have been
reported to form a biofilm on the anode surface in MFCs and to transfer the
electrons from acetate with high efficiency
Metal reducing bacteria applied in MFC
Shewanella putrefaciens
Geobacter sulfurreducens
Geobacter metallireducens
Desulfuromonas acetoxidans
Rhodoferax ferrireducens
Rhodoferax species isolated from an anoxic
sediment were able to efficiently transfer electrons to a graphite anode using glucose as a sole
carbon source. Remarkably, this bacterium is the first reported strain that can
completely mineralize glucose to CO2 while concomitantly generating electricity
at 90% efficiency. In terms of performance, current densities in the order of
0.2-0.6 mA and a total power density of 1-17 mW/m2 graphite surface have been
reported for Shewanella putrefaciens, Geobacter sulfurreducens and Rhodoferax
ferrireducens at conventional (woven) graphite electrodes. However,
in case woven graphite in the Rhodoferax study was replaced by highly porous graphite electrodes, the
current and power output was increased up to 74 mA/m2 and 33 mW/m2,
respectively. Although these bacteria generally show high electron transfer
efficiency, they have a slow growth rate, a high substrate specificity (mostly
acetate or lactate) and relatively low energy transfer efficiency compared to
mixed cultures. Furthermore, the use of a pure culture implies a continuous risk
of contamination of the MFCs with undesired bacteria.
Mixed bacterial cultures
MFCs that make use of mixed bacterial cultures have some
important advantages over MFCs driven by axenic cultures: higher
resistance against process disturbances, higher substrate consumption rates,
smaller substrate specificity and higher power output. Mostly, the electrochemically
active mixed cultures are enriched
either from sediment (bothmarine and lake sediment) oractivated sludge from
wastewater treatment plants. By means of molecular analysis, electrochemically
active species of Geobacteraceae, Desulfuromonas, Alcaligenes faecalis, Enterococcus faecium, Pseudomonas
aeruginosa, Proteobacteria, Clostridia, Bacteroides and Aeromonas species were detected. Most remarkably, the study also showed the presence of nitrogen fixing bacteria (e.g., Azoarcus and Azospirillum) amongst the
electrochemically active bacterial
populations. The study of Rabaey et al. (2004a) showed that by starting from methanogenic sludge and by continuously harvesting
the anodic populations over a
5-month period using glucose as carbon source, an electrochemically active consortium can be obtained that mainly
consists of facultative anaerobic
bacteria (e.g. Alcaligenes, Enterococcus and Pseudomonas species). In this particular study, very high
glucose-to-power efficiencies could be reached in the order of 80%. It should be remarked that in
order to maximize the power output, experiments with varying external resistance should be
performed.
To estimate the power per
unit surface to putative power output per unit reactor volume, one can take into account that at present some
100-500 m2 of anode surface can be installed per m3 anodic reactor volume.
Hence, the state of the art power supply ranges from approximately 1 to 1800 W
per m3 anode reactor volume installed.
To render the anode more susceptible for
receiving electrons from the bacteria, electrochemically active compounds can
be incorporated in the electrode material. In this way, the main disadvantages
of mediators in solution, namely toxicity and degradation, can thus be circumvented
since the mediator is not released from the electrode material and thus has a
longer life time. Moreover, bacteria are still able to form a biofilm on the
modified anode surface.
