Online monitoring of biofouling using coaxial stub resonator technique

Here we demonstrate the proof-of-principle that a coaxial stub resonator can be used to detect early stages of biofilm formation. After promising field tests using a stub resonator with a stainless steel inner conductor as sensitive element, the sensitivity of the system was improved by using a resonator of shorter physical length, implying higher resonance frequencies (and by that a higher frequency range of operation) and improved sensitivity towards dispersion. In addition, the space between inner and outer conductor was filled up with glass beads, thereby exploiting the larger surface area available for biofilm formation.Analysis of the biofilm and the stub resonator signal, both as function of time, indicates that the sensor allows detection of early stages of biofilm formation. In addition, the sensor signal clearly discriminates between the first stages of biofilm formation (characterized by separated, individual spots of bacterial growth on the glass beads) and the presence of a nearly homogeneous biofilm later on in time. Model simulations based on the transmission line theory predict a shift of the sensor response in the same direction and order of magnitude as observed in the biofouling experiments, thereby confirming the operating principle of the sensor.