MRI investigations of filtration: Fouling and cleaning processes

Fouling and cleaning of hollow fiber membranes were investigated by in-situ magnetic resonance imaging. Two aqueous feed solutions of sodium alginate with and without Ca2+ were applied during dead-end filtration. Loose concentration polarization layers formed during filtration without Ca2+, whereas dense and compact fouling layers were observed in the presence of Ca2+. A major technical concern is cleaning, realized in the two-step process of backwashing and flushing. The concentration polarization layer formed during filtration without Ca2+ was easily removed from the membrane by backwashing. In contrast, the dense and compact layer of alginate with Ca2+ shows an irregular shape and only partly detaches from the membrane during cleaning. The residuals contribute to further membrane fouling in the following process steps. Backwashing and flushing were analyzed by dedicated image processing.