In situ detection of fouling in crossflow filtration using 3ω sensing

Membrane filtration is a widely applied technology for water and wastewater treatment and for separation and purification in e.g. food and pharmaceutical industry. However, the applicability is severely limited by fouling. Several methods have been proposed to monitor membrane fouling, yet none have proven effective for full-scale implementation. The 3ω sensing is introduced as a novel approach for monitoring membrane fouling and shows promising potential to scale for in situ fouling monitoring. Promising results have been obtained for measuring filter-cake build up and compression (fouling) in dead-end filtration. In the current study, 3ω sensing is investigated for monitoring fouling in crossflow filtration to simultaneously measure how heat convection from the surface of the membrane depends on crossflow and formation of organic and inorganic fouling. A 3ω sensor was integrated onto the surface of a microfiltration membrane, and crossflow filtrations of kaolin and E. coli suspensions were conducted. It was observed that application of crossflow leads to a reduction of 3ω signal as it enhances heat transfer from the sensor. Measurements of 3ω signals at stagnant conditions (no crossflow) showed lower signals for membranes with inorganic fouling (thermally conducting) compared to a clean membrane, while measurements of a membrane fouled with E. coli shows a signal similar to that of a clean membrane due to the similarity in thermal conductivity between the feed and the fouling layer. Hence, the E. coli fouling layer could not be sensed in stagnant conditions. However, measurements in crossflow mode showed increasing 3ω signals by the formation of both kaolin and E. coli fouling layers. This happens because the fouling layer acts as a protective barrier against heat convection from the 3ω sensor, initially increasing the 3ω signal, regardless of the thermal conductivity. This phenomenon is coined shielding and has the notable consequence of increasing resolution of 3ω sensing for a foulant with thermal properties similar to those of water. This makes 3ω sensing an effective technique for detecting membrane fouling, with the potential to characterize both the type and thickness of the fouling layer with high resolution in crossflow filtration. These findings pave the way for advanced fouling diagnostics, predictive maintenance, and optimized cleaning strategies, offering substantial benefits for full-scale membrane operations in water and wastewater treatment, food, and pharmaceutical industries.