Fouling of ultrafiltration (UF) and microfiltration (MF) membranes by proteins is a major challenge in the bioprocessing and dairy industries, as well as in surface and wastewater treatment applications. This review attempts at presenting a comprehensive state-of-the-art understanding on protein fouling of membranes. Effects of operating conditions, along with properties of proteins and membranes, are discussed. Various tools and techniques used to characterize and monitor fouling are described. Different mitigation techniques and cleaning methods used are also presented. Two main factors have been identified as playing important roles in governing protein fouling, namely, ratio of protein size to membrane pore size and interfacial interactions (i.e., protein-protein and protein-membrane). Some directions for future research are suggested: (1) explore a wider range of proteins and their mixtures with respect to their fouling tendencies; and (2) create a comprehensive dataset that can be used to develop machine-learning models to enhance both predictive capabilities and mechanistic understanding.

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Membrane-filtration is promising for treating the voluminous oily wastewater, especially when the oil emulsions are smaller than 20 μm. However, studies on the inevitable membrane fouling phenomenon by oil are rather scarce. In particular, a question that remained to be addressed was whether the DLVO or XDLVO model provides better predictions of the oil-membrane interfacial interactions and thereby the extent of fouling. Accordingly, this study investigated four oil types (namely, hexadecane, soybean oil, fish oil and crude oil) that were stabilized by the same non-ionic surfactant (namely, Tween 20) and had similar mean droplet diameters. The direct observation through the membrane (DOTM) technique was used to quantify the critical flux of the different emulsions, and both the XDLVO and DLVO models were used to quantify the foulant-membrane and foulant-foulant interactions. DOTM results indicated that the critical flux values were similar for all oils under the conditions tested, except crude oil. Although the XDLVO model appears to be more comprehensive than the DLVO in terms of accounting for the additional Lewis acid-base polar (AB) interaction that is acknowledged to be important in membrane-filtration, results indicate that the dominance of the AB component drowns out the other interactions like that of electrostatics (EL) in this case, which impedes accurate prediction of the different fouling tendencies by the different oil types.@en

The striping phenomenon in membrane filtration, whereby the foulants deposit as regular streaks rather than a more uniform layer, was first observed more than 30 years ago (Jonsson, 1987), but the understanding has remained limited to a few subsequent studies (Henriksen and Hassager, 1993; Larsen, 1991; Li et al., 2016; Tanudjaja et al., 2017; Tarabara et al., 2002). In view of the potential practical implications of the stripes in terms of membrane fouling and fouling mitigation, this study was targeted at an in-depth characterization of the stripes. The direct observation through the membrane (DOTM) technique was employed to observe the conditions when the stripes formed by oil emulsions stabilized by three Tween surfactants at a lower cross-flow velocity, and higher oil concentrations and permeate fluxes. The results indicate that (i) other than hydrodynamic factors (e.g., permeate drag, tangential shear), foulant-membrane and foulant-foulant interactions played a role in stripe formation or disappearance, as evident in the formation of stripes only by the oil emulsion stabilized by the Tween surfactants and the effect of pH; (ii) stripes made up of oil droplets appeared to be easier to remove than the more uniform layer of oil droplets, as evident in the shorter time taken for the former to detach; and (iii) among the three Tween surfactants, the striping characteristics investigated were largely similar, except for the time taken for the deposits to detach. These are expected to have implications for fouling control and mitigation.

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Membrane-based filtration is promising for the treatment of oily wastewater with stable micron-sized oil droplets, but is unfortunately limited by the inevitable membrane fouling phenomenon. To advance the understanding on membrane fouling by oil emulsion, this study made use of the direct observation through the membrane (DOTM) technique to experimentally visualize the evolution of fouling and determine critical flux, and also molecular dynamics simulations to unveil the interfacial interactions and behaviors underlying the different fouling behaviors. Three oil emulsion types with similar mean droplet sizes were studied, namely, one without surfactant, one stabilized by sodium dodecyl sulfate (SDS; a negatively charged surfactant) and one stabilized by dodecyltrimethylammonium bromide (DTAB; a positively charged surfactant). DOTM results indicate that the critical flux was the highest in the absence of surfactant and lowest for the DTAB-stabilized ones, while simulation results indicate that the interaction energies are clearly different among the different oil emulsion types. Both hence affirm that the presence of surfactant and different surfactant type distinctly changes the fouling tendencies. The key conclusions on the different fouling tendencies among the three oil emulsion types are summarized as follows. Firstly, the highest critical flux in the absence of any surfactant is linked to greatest oil-water interaction and least oil-membrane interaction, both of which cause the oil molecule to prefer being in the bulk aqueous feed. The radial distribution function (RDF) further substantiates this. Secondly, the higher critical flux exhibited by the SDS-stabilized oil emulsion compared to the DTAB-stabilized ones could be attributed to the negative charge of the former. Simulations affirm the comparatively greater oil-membrane repulsion through the RDF profile, lower oil-membrane interaction and higher oil-water interaction of the SDS-stabilized oil emulsion. Thirdly, despite surfactants generally have a stabilizing effect on oil emulsions, DOTM images show that coalescence was most extensive for the DTAB-stabilized oil emulsion, because of the greatest oil-membrane attraction and least oil-oil repulsion. Coupling both experiments and simulations, this study enhanced the mechanistic understanding on the effect of surfactant on membrane fouling by emulsions of the same oil.

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