Understanding Factors that Affect Microbial Fuel Cell Performance: Inoculum Characteristics and Methanogenesis
Caitlyn S. Butler
Microbial fuel cells (MFCs) are a promising approach to wastewater treatment that use anode-respiring bacteria (ARB) to oxidize organic matter and generate electric current. Although these devices have great potential, MFCs are not yet commercialized primarily due to their low power output at pilot scale. Past studies have hypothesized power production may be largely limited by high internal resistances and competing microbial metabolisms (Logan et al., 2008).
The source of inoculum used to build MFC communities has been demonstrated to significantly influence cell resistance and microbial dynamics (Sun et al., 2008; Chae et al., 2008). Studies that have shown these effects have generally focused on anode acclimation using air cathode MFCs. Presently, the effects of inoculum source on power production or startup times have not been explored in MFC designs that incorporate cathode-oxidizing biofilms. An objective of this research was to observe if inocula source and initial biomass concentration could influence startup times and power production of MFCs with biocathodes.
The role of initial biomass concentration was investigated by seeding identical reactors sets with inoculum from the same source at different VSS concentrations. The results of these tests showed that the initial VSS concentration did not strongly influence MFC startup times or power production. Identical reactors inoculated with raw primary effluent (0.24 g VSS/L) and diluted primary effluent (0.08 g VSS/L) both obtained steady-state power values of 120 µW ± 40 µW and stable cell potentials of 27 mV ± 5.0 mV after approximately 10 days of operation.
A direct comparison of three sources of mixed culture inoculums, at similar initial VSS concentrations, was performed by seeding the anode and cathode compartments of triplicate Htype MFCs and monitoring their performance in a recycled batch-fed mode for extended periods. iv Inoculum sources included primary clarifier effluent from the Amherst WWTP, anaerobic digestate from Barstow Dairy Farms, and anode effluent from a pilot scale MFC. MFCs inoculated with anaerobic digestate or primary effluent achieved similar performance after 8-10 days of operation with steady-state power values of 150 µW ± 20 µW and stable cell potentials of 30 mV ± 5.0 mV. MFCs seeded with anode effluent obtained power values of 40 µW ± 5.0 µW and stable cell potentials of 10 mV ± 2.0 mV after 8-10 days of operation. The most efficient conversion of acetate to electricity was obtained by MFCs inoculated with anaerobic digestate that achieved efficiencies of 37 % ± 6% during periods of stable cell voltages. These efficiencies are low compared to other studies that commonly report values as high as 70% when using acetate as the sole electron donor in excess (Lee et al., 2008).
Many studies using mixed cultures have reported poor power efficiencies linked to competition between ARB and methanogens in the anode (Schaetzle et al., 2008). Past work has demonstrated nitrate dosing can effectively inhibit methanogenesis (Conrad et al., 1998). This inhibition approach is attractive for MFC wastewater treatment due to the potential availability of nitrate via nitrification. A separate objective of this research was to test the effectiveness of low dosing concentrations of nitrate (1 mg-N/L and 10 mg-N/L) on communities native to MFC anodes. Using anaerobic cultivation methods, bottle cultures were enriched for methanogens using inoculum from the anode of an operating MFC. After two batch cycles, test cultures were dosed with sodium nitrate at either 1 mg-N/L or 10 mg-N/L. In general, the 10 mg-N/L dosing suppressed methanogenesis longer than the 1 mg-N/L dosing. The 10 mg-N/L dosing scenario suppressed methane production for up to 7 days ± 2 days while the 1 mg-N/L dosing scenario inhibited samples for up to 3 days ± 1 day. Furthermore, cultures that contained graphite granules were generally inhibited for periods 1-2 days shorter than suspended growth cultures.