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The fouling of ultrafiltration membranes by natural organic matter (NOM), isolated from a potable surface water source, was studied with an emphasis on elucidating fouling modes and the role of aggregates. NOM size was related to membrane pore sizes using parallel membrane fractionation and size exclusion chromatography, such analyses confirmed the predominance of low MW species and identified the presence of aggregates in concentrated NOM solutions. Cake formation was the dominant mode of fouling by the unfiltered feed, which contained aggregates. This was identified by a constant rate of increase in membrane resistance with permeate throughput and was independent of pore size over a 10-1000 kDa molecular weight cutoff (MWCO) range. Prefiltration (to remove aggregates) and dilution (to reduce aggregate concentration) reduced the rate of increase in membrane resistance for the low MWCO membranes but did not change the fouling mode. In contrast, such pretreatment prevented cake formation on the larger MWCO membranes and shifted the mode of fouling to pore blockage. The date lend support for the idea that an initial fouling layer of large aggregates can catalyze the fouling by lower MW species. The fouling layer could be removed from the large MWCO membranes by backwashing, but the lower MWCO membranes exhibited some irreversible fouling, suggesting that low MW species penetrated into the pore structure. A combined pore blockage-cake formation model described the data well and provided insight into how fouling modes evolve during filtration.

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Contact angle measurements have been widely used to estimate the surface energy of various materials. Such measurements are severely limited with substrate surfaces that exhibit surface restructuring, are contaminated, and/or are porous. Although the captive bubble/drop method addresses the capillarity problem, surface undulations have not previously been accounted for in a quantitative way. We do so here with a series of 8 different pore size synthetic polymer membranes, all fabricated from poly(ether sulfone), as model rough porous surfaces of the same surface chemistry. Also, 3 of the 8 different pore size membranes were rendered hydrophilic through photoinduced graft polymerization producing 17 different modified membranes that are similarly tested. By incorporation of roughness parameters obtained from AFM measurements, corrections to the captive bubble/drop constant angle measurements were successfully made using a simple model of the surface. The predicted average value for the sessile drop contact angle of poly(ether sulfone) accounting for undulations (44.5 ± 1.3°) was, within error, equal to that value estimated for a smooth relatively nonporous PES nanofiltration membrane (42.9 ± 2.5°).