Assessment of organic removal in series- and parallel-connected microbial fuel cell stacks

Microbial fuel cells (MFCs) degrade organic contaminants in wastewater while simultaneously producing electricity, but must be stacked to yield adequate voltage and current. This study examined the evolution of the chemical oxygen demand (COD) removal rate and efficiency in two identical individual MFCs (i-MFCs) in series- and parallel-connected stacks (sc- and pc-MFCs, respectively) under batch and continuous operation. The stack voltage and current increased in the respective series and parallel connections of the two i-MFCs (MFC unit 1 and MFC unit 2). Voltage reversal was observed in the sc- MFC below an external load of 100 Ω. Regardless of occurrence of the voltage reversal, organic reduction between i-MFCs and sc-MFCs showed no significant difference (gap of < 9% and < 6% in COD removal rate and efficiency, respectively); additionally, organic removals between the two individual MFCs in series indicated differences less than 9% of COD removal rate and 5% of COD removal efficiency in batch mode. Continuous operation also yielded similar organic removals as the MFCs in individual and series connection (voltage reversal occurred) mode, even over 8 days operation. Parallel connection yielded identical organic removals and currents in the two individual MFCs of the pc-MFC, even though the two separate i-MFCs showed different organic removal rates and current productions. This study provides the guide for the application of stacked MFCs for power source and efficient organic pollutant removal in wastewater treatment process.