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.

Industrially applied bioelectrochemical systems require long-term stable operation, and hence the control of biofilm accumulation on the electrodes. An optimized application of biofilm control mechanisms presupposes on-line, in-situ monitoring of the accumulated biofilm. Heat transfer sensors have successfully been integrated into industrial systems for on-line, non-invasive monitoring of biofilms. In this study, a mathematical model for the description of the sensitivity of a heat transfer biofilm sensor was developed, incorporating the hydrodynamic conditions of the fluid and the geometrical properties of the substratum. This model was experimentally validated at different flow velocities by integrating biofilm sensors into cylindrical pipes and planar mesofluidic flow cells with a carbonaceous substratum. Dimensionless sensor readings were correlated with the mean biovolume measured gravimetrically, and optical coherence tomography was used to determine the sensors’ sensitivity. The biofilm sensors applied in the planar flow cells revealed an increase in sensitivity by a factor of 6 compared to standard stainless steel pipes, as well as improved sensitivity at higher flow velocities.

Industrially applied bioelectrochemical systems require for long-term stable operation and hence a control of the biofilm accumulation on the electrodes. An optimized application of biofilm control mechanisms presupposes an on-line, in-situ monitoring of the accumulated biofilm. Heat-transfer sensors have successfully been integrated into industrial systems for the on-line non-invasive monitoring of biofilms. In this study, a mathematical model for the description of the sensitivity of a heat-transfer biofilm sensor has been developed, incorporating the hydrodynamic conditions of the fluid and the geometrical properties of the substratum. This model was experimentally validated at different flow velocities by integrating biofilm sensors into cylindrical pipes and planar mesofluidic flow cells with a carbonaceous substratum. Dimensionless sensor readings were correlated with the mean biovolume measured gravimetrically and using optical coherence tomography to determine the sensor sensitivity. The biofilms sensors applied in the planar flow cells revealed an increase in sensitivity by a factor of 6 compared to standard stainless-steel pipes, as well as an improved sensitivity at higher flow velocities.