Membrane fouling monitoring by 3ω sensing

Abstract

A platinum wire with a diameter of 20 µm and length of 14 mm was attached to the surface of a ceramic membrane for in situ fouling monitoring. By measuring the voltage across the wire while applying an AC current, the amplitude of the third harmonic wave, the so-called 3ω signal, which is inversely proportional to the thermal conductivity around the wire, was obtained. It was demonstrated that the 3ω signal is influenced by changing the environment around the wire as a result of differences in thermal conductivity and heat convection. Measurements of the 3ω signals on the membrane surface covered in varying amounts of acrylic varnish showed an increase in 3ω signal amplitude as the thermally insulating varnish layer increases. At higher AC frequencies, the effect becomes more pronounced as the penetration depth of the thermal wave is decreased. Thus, the frequency of the AC current can be varied to measure thermal conductivity inside and beyond the fouling layer. The method was applied in semi-dead-end filtration of polymeric core-shell particles and crossflow filtration of diluted milk. Results showed increasing 3ω signals over time and correlates directly to the hydraulic resistance of the formed fouling layer, as the fouling layers have low thermal conductivity and because the fouling layers reduced heat convection by crossflow. After membrane cleaning, the magnitude of the 3ω signal is restored to its initial level as before filtration, showing that the 3ω method can be used to monitor and control cleaning. Hence, the 3ω method is a novel and highly promising method to characterize and quantify fouling during filtration and cleaning, and to assess flow conditions locally on membranes.