Effect of inoculum on bioelectricity yield and the use of factorial experiments for assessing microbial fuel cells

Abstract

The study aim was to understand the effect of inoculum on bioelectricity production and the interactions that occur between organic load, external resistance and fuel type during the operation of a Microbial Fuel Cell (MFC). The first experiment explored the effect of four different environmental inocula (freshwater sediment, two types of return activated sludge (RAS) and anaerobic sludge) on microbial fuel cell performance. The number of bacteria in each of the inocula were standardised prior to experiments to achieve an inoculum density of 1.29 x 107cells ml-1 so that the comparison between treatments could be carried out fairly. For almost every metric (voltage, current and coulombic efficiency) the RAS inoculum outperformed freshwater sediment and anaerobic sludge inoculum. The treatment efficiency was high in all instances (>79%) with the exception of anaerobic sludge (33%). Microbial community analysis showed that anodes from MFCs exhibited a more complex microbial community profile than anodes from MFCs inoculated with anaerobic sludge. Two experiments were performed to investigate the relationship between fuel type, organic load and external resistance and their effects on MFC performance using an iterative Design of Experiments (DoE) approach. In the first experiment, a half factorial design was used as a screening study to investigate the main effects of fuel type (glucose vs acetate), organic load and external resistance. The study found that acetate performed poorly compared with glucose and that the experimental settings for external resistance should be modified for future experiments. The second experiment used a full factorial design and showed that only organic load exerted a statistically significant effect on cell potential, current and coulombic efficiency and that a statistically significant interaction effect between organic load and external resistance is exerted on cell potential and coulombic efficiency. The dominant effect of organic load was also apparent in DGGE community fingerprint profiles, which clustered according to organic load, of the anode community samples taken from MFCS in this study. In conclusion, the experiments yielded useful insights into inoculum effects and the interactions between basic operational parameters in an MFC that will be useful for selecting the operational parameters of MFCs depending on the field conditions and process requirements. The novelty of the techniques deployed in this study – standardisation the inoculum and exploring MFCs within a Design of Experiments framework – are noted along with the advances to our understanding of MFCs and the fact they have provided new tools with which to study MFCs systems. The wider implications of the performance characteristics of the MFCs used in this study and the findings presented within are discussed.