Compressibility characterization of organic and inorganic fouling by the 3ω method

• Fouling layer type is distinguishable based on its thermal conductivity. • 3ω signal increases with organic fouling and decreases with inorganic fouling. • Compactness of the fouling layer can be assessed by 3ω method. • 3ω signal of organic, compressible fouling layers increases with pressure. Membrane fouling is a major problem that reduces the sustainability and efficiency of separation processes. In this study, an online fouling monitoring technique using the 3ω method was investigated to distinguish between different compositions and compactness fouling layers based on their distinct thermal properties. Dead-end filtration of kaolin or humic acid suspensions were performed to represent both an inorganic and organic fouling while the 3ω signals were measured during the fouling layer build-up. The formation of an inorganic fouling layer reduced the 3ω signal due to its higher thermal conductivity compared to water. Limited compaction of the kaolin fouling layer was observed by elevating the pressure. In the organic fouling layer on the other hand, the 3ω signal showed higher values than for water, due to the insulating effect of organic materials. It was also possible to detect the compaction of the fouling layer, which was in good agreement with measurements of specific cake resistance and water content. The 3ω method can potentially characterize fouling layers in a non-destructive way at a distance ranging from micrometers to a few millimeters from the surface of the membrane, thus providing operators with insights for tailored fouling mitigation strategies.