Location and quantification of biological foulants in a wet membrane structure by cross-sectional confocal laser scanning microscopy

Microfiltration of solutions containing proteins and polysaccharides suffers from permeate flux decline caused by membrane fouling, despite the small size of these species relative to the nominal membrane pore size. To develop fouling mitigation strategies for microfiltration of biological streams, it is important to understand the mechanisms that lead to fouling. In this contribution, we discuss the use of confocal laser scanning microscopy (CLSM) to determine where proteins and polysaccharides deposit inside polymeric microfiltration membranes when a fluid containing these materials is filtered. By using fluorescently labeled probes, CLSM permitted evaluation of the location and extent of fouling by individual components (protein: casein and polysaccharide: dextran) within wet, asymmetric polyethersulfone microfiltration membranes. By labeling the protein and polysaccharide with different fluorophores, we were able to locate each component separately and to visualize co-localization within the membrane. In addition, flux profiles and cross-sectional CLSM images were obtained for membranes that processed single-component solutions and mixtures to better understand the role of each on membrane fouling and to see how component interactions impact the fouling profiles. Analysis of the CLSM images at different levels of fouling for single-component solutions and mixtures provided concentration profiles versus depth for each individual component present in the feed solution. Using a new cross-sectional imaging protocol that we developed in a previous investigation, we were able to visualize fouling profiles throughout the entire thickness of the membrane, overcoming limitations of depth of penetration observed in previous CLSM work.