In Situ Biofilm Monitoring Using a Heat Transfer Sensor: The Impact of Flow Velocity in a Pipe and Planar System

Industrially applied bioelectrochemical systems require long-term stable oper-
ation, 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 indus-
trial 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
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pipes, as well as improved sensitivity at higher flow velocities.