The ability of microbial fuel cells (MFCs) to convert chemical or biochemical energy directly into electricity makes them well suited for treatment of wastewaters. Expanding the understanding of the processes and mechanisms that transpire in an MFC was the overall goal of this project with focus placed on the biological aspects of such systems. Biodegradation of model organic compounds of different structures commonly found in wastewaters, specifically lactate, acetate and phenol, and subsequently a representative wastewater was evaluated in H-type MFCs.
Biodegradation of evaluated compounds was achieved with concomitant generation of electricity. In MFCs with rod electrodes and suspended microbial cells, substrate biodegradation was accompanied by microbial growth and rise in open circuit potential (OCP). Preferential use of model compounds and effectiveness of biodegradation was observed with lactate being the most favourable substrate followed by acetate and phenol. To promote cell immobilization, granular electrodes were employed which resulted in higher biodegradation rates and electrochemical outputs than those with rod electrodes.
Biodegradation performance and associated power and current were improved in continuously operated MFCs. Higher biodegradation rates or removal efficiencies led to higher OCP, power and current indicating a correlation between biodegradation performance and electrochemical output. This correlation was evident in the biodegradation of organics in an internal process stream wastewater whereby the highest electrochemical output was attained when COD removal and coulombic efficiencies were at maximum.
Application of neutral red (NR) enhanced the biodegradation of phenol and eliminated inhibition effects in batch MFCs but showed no improvement during continuous operation. Phenol biodegradation was not enhanced when combined with lactate, while the presence of phenol at high concentrations (≥500 mg L-1) negatively impacted the biodegradation of lactate. Co-biodegradation of lactate was more effective than phenol, especially in the continuous systems, and as a result higher power and current output were obtained with lactate.
The developed biokinetic model was able to predict microbial growth and biodegradation kinetics in the MFC with high accuracy. The magnitude of biokinetics in MFCs were lower than those reported for conventional bioreactors indicating the need for development of suitable culture along with design of more sophisticated MFC bioreactors
