The feasibility of methane as a feed-source for a microbial fuel cell.

Microbial fuel cells (MFCs) directly use the energy contained in organic matter to generate electricity. This work investigated the potential of using methane-oxidizing bacteria as biocatalysts in a microbial fuel cell with mediators to facilitate charge transfer. Mediators enhance the power output of MFCs by acting as intermediate electron shuttles between the bacterial cell and the anode. The ability of methanotrophic bacteria to use mediators as the terminal electron acceptor in place of oxygen during methane oxidation has been speculated. However, the existing literature is inconclusive. An initial attempt to isolate the pure methanotroph which previously indicated an ability to reduce the mediator potassium ferricyanide was unsuccessful. A mixed methane-oxidizing culture was subsequently enriched from soil in the Canterbury region and cultivated in a mineral medium using a fed-batch bioreactor operated at 30 oC and pH 6.8. Through online monitoring of carbon dioxide production and sodium hydroxide addition, it was confirmed that the mixed culture reduced potassium ferricyanide with methane as the input energy source. In an anoxic environment with 5% methane and 95% nitrogen and an initial ferricyanide concentration of 2 mM, the maximum specific reduction rate was 0.4 mmol/gh using cells in the exponential phase (DCW 0.2 g/L). Reduction of ferricyanide also occurred in the absence of methane (100% nitrogen) at a rate of 1.3 mmol/gh. Several scenarios were identified to explain this behaviour, all of which could be applied to a methane-fed MFC: . A novel oxidation pathway was operational where the methanotrophs directly reduced ferricyanide during methane utilisation. This was unlikely as ferricyanide could not be used as a growth substrate in place of oxygen. However, the ferricyanide also had an inhibitory effect on the cells which may have hindered growth. . A microbial consortium existed which used intermediate compounds produced by the methanotrophs as a carbon source and ferricyanide as the terminal electron acceptor. This was identified as the most likely scenario, with either ethanol or acetate produced during aerobic methane oxidation as the reducing species. . The methanotrophs reduced ferricyanide by oxidising internal storage polymers such as glycogen or PHA. This was less likely, as the mixed culture reduced ferricyanide during the exponential growth phase before these polymers tend to accumulate. Methylene blue, thionine acetate, resorufin and neutral red were also tested as external electron acceptors. In serum vials, methylene blue was reduced only in the presence of methane. However, this result was inconclusive, as reduction did not occur in the bioreactor. Thionine acetate was reduced in serum vials in the presence and absence of methane at 0.6 mmol/g.h. Resorufin and neutral red were not reduced by the mixed culture. At the current mediator reduction rates, the estimated power density of a methane-fed MFC was 7 W/ m³ (anode volume) using ferricyanide and 200 W/m³ with thionine acetate. This is similar to traditional, liquid-fed MFCs and would be limited to low power consuming devices such as remote biosensors. If future work demonstrates an ability to generate meaningful power densities (> 1kW/m³), methane-fed MFCs could be an option for small agricultural farms producing biogas via anaerobic digestion. This would compete with gas combustion engines which are limited by high operating costs and low energy efficiencies