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.





Make a Free Website with Yola.