The overall objective of our work is to identify the organic fraction responsible for fouling of lowpressure (microfiltration and ultrafiltration) membranes, and to understand fouling mechanisms. Several natural organic matter (NOM) fractions isolated from Ribou Reservoir have been ultrafiltered with two different 100 KD membranes (regenerated cellulose and polyethersulfone) using a non-stirred cell unit. Results have shown that the organic colloidal fraction (bacterial peptidoglycan residue) shows the most significant fouling. The dissolved fraction (<0.45 μm) of NOM, which contains solutes larger and smaller than the pore size of 100 KD membranes, contributes to fouling through pore blockage and/or adsorption mechanisms. Atomic force microscopy (AFM) examination reveals morphological changes during membrane fouling. The polyethersulfone and regenerated cellulose membranes are relatively smooth. However, based on AFM, some of the fouled membrane surfaces appear rougher than the corresponding clean membrane surface. These results demonstrate the role of surface coverage in ultrafiltration membranes. It appears that membrane roughness is a key physical parameter in membrane fouling. More analysis is being undertaken with scanning electron microscopy images to determine pore size distribution for the clean and the fouled membranes, providing more information in terms of fouling mechanisms.

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