Detecting Excess Biofilm Thickness in Microbial Electrolysis Cells by Real‐Time In‐Situ Biofilm Monitoring

Long-term stable operation of bioelectrochemical systems (BES) presupposes the avoidance of mass transfer limitations of the electroactive biofilm. Excessive pH-gradients from bulk to electrode interface or substrate limitations of the electroactive biofilm are known to diminish the electrical performance of BES. In this study the impact of the morphology of a mixed-species electroactive biofilm cultivated on the electrical performance of a microbial electrolysis cell (MEC) was investigated to identify the optimal biofilm for real-life applications in wastewater treatment. Noninvasive monitoring by means of optical coherence tomography and an industrial biofilm sensor allowed for a real-time evaluation of the morphology of the biofilm. The maximum current density of approximately 3.5 A/m² was found for a mean biofilm thickness in the range of 100–150 µm, beyond which thicker biofilms caused mass transfer limitations. Along with local biofilm detachment a continuous decline in efficiency demonstrates the need for active biofilm control to adjust the biofilm thickness.