Investigating the effects of fluidic connection between microbial fuel cells

Microbial fuel cells (MFCs) can ‘treat’ wastewater but individually are thermodynamically restricted. Scale-up might, therefore, require a plurality of units operating in a stack which could introduce losses simply through fluidic connections. Experiments were performed on two hydraulically joined MFCs (20 cm apart) where feedstock flowed first through the upstream unit (MFCup) and into the downstream unit (MFCdown) to explore the interactive effect of electrical load connection, influent make-up and flow-rate on electrical outputs. This set-up was also used to investigate how calculating total internal resistance based on a dynamic open circuit voltage (OCV) might differ from using the starting OCV. When fed a highly conductive feedstock (~4,800 μS) MFCdown dropped approximately 180 mV as progressively heavier loads were applied to MFCup (independent of flow-rate) due to electron leakages through the medium. The conductivities of plain acetate solutions (5 and 20 mM) were insufficient to induce losses in MFCdown even when MFCup was operating at high current densities. However, at the highest flow-rate (240 mL/h) MFCdown dropped by approximately 100 mV when using 5 and 220 mV using 20 mM acetate. When the distance between MFCs was reduced by 5 cm, voltage drops were apparent even at lower flow-rates, (30 mL/h decreased the voltage by 115 mV when using 20 mM acetate). Shear flow-rates can introduce dissolved oxygen and turbulence all capable of affecting the anodic biofilm and redox conditions. Calculating total internal resistance using a dynamic OCV produced a more stable curve over time compared to that based on the starting constant OCV.