Membrane fouling during filtration of milk––a microstructural study

The surface morphology and internal microstructure of a membrane are the result of membrane manufacturing processes and subsequent use during fluid processing in industry. Both these structural factors have a great effect on fouling and filtration performance.

In this study, scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy were used to comprehensively characterise the surface of unused microfiltration and ultrafiltration membranes, the fouled layer on the surface of membranes used for milk filtration and the internal fouling within the used membranes.

A simple and effective sample preparation method was developed to study the internal membrane structure using high-resolution field emission SEM with low-accelerating voltage. Various methods of structural characterisation were compared and the results showed that for flat sheet membranes AFM is an appropriate and convenient technique for examining the surface topography of membranes. In contrast SEM is a very appropriate technique for examining the cross-sectional and internal structure of a membrane, either unused or fouled.

Using these complimentary techniques it has been shown that internal fouling, during filtration of skim milk, proceeds by protein–polymer and protein–protein interactions. A gel layer forms on the surface of the membrane and has been imaged using SEM. This layer is slightly compressible and densifies as it grows. Fouling initiation commences after a very short filtration time.