Visualization of Protein Fouling inside a Hollow Fiber Ultrafiltration Membrane by Fluorescent Microscopy

Membrane fouling caused by macromolecular species such as proteins remains a major problem, especially in ultrafiltration processes. In this work, we developed a method of direct observation of protein fouling inside a hollow-fiber membrane using fluorescent microscopy. Fluorescent microscope images showed where proteins accumulated inside the membrane during filtration. Commercially available membranes with differing molecular weight cutoffs had differing geometries of internal protein deposition. Different types of proteins (BSA, casein, and lysozyme) also induced different internal membrane foulings. Poly(vinylidene fluoride) hollow-fiber membranes with and without a skin layer were prepared by phase separation methods, and the present observation method revealed that the structural differences between the membranes significantly affected internal membrane fouling. Finally, the protein-fouled membrane was backwashed with various cleaning agents. The fluorescent microscopic observation visualized the effects of backwashing with the cleaning agents. These results reveal fouling behaviors inside an ultrafiltration hollow-fiber membrane and provide important insights into membrane fouling.