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Graphical abstract Highlights • Evolution of oil-emulsion cake layer on HF membranes observed in real time. • At the same permeate flux, TMP of oil emulsion was higher than latex particle. • Oil gave a thinner cake layer that was denser and moving with the cross flow. • Drastic fluctuations of cake thickness and porosity versus permeate volume for oil. • Greater oil-oil and oil-membrane interaction energies underlie the fouling observed. Abstract In view of membrane-filtration being a promising means for treating the large volumes of oily wastewater, much efforts have been dedicated to understanding membrane fouling by oil emulsions. However, the understanding on cake fouling, which is the governing form of fouling throughout filtration, by oil emulsions remain poorly understood. To bridge this gap, this study focused on characterizing the evolution of the oil-emulsion cake layer characteristics on hollow fiber (HF) membranes in real time using the direct observation through the membrane (DOTM) technique. The more well-studied latex particles were studied alongside to provide a benchmark for comparison. Despite the same feed concentration and crossflow velocity used, the transmembrane pressure (TMP) and cake layer resistance magnitudes were greater for oil emulsion than for latex particles at the same permeate flux. DOTM images revealed that this was linked to the following behaviors: (i) oil tended to deposit as a moving cake layer, whereas latex as scattered deposits; and (ii) oil gave a thinner yet denser cake layer. Moreover, it was observed that the cake thickness and porosity fluctuated significantly with respect to permeate volume for the oil emulsions, because the moving cake layer gave greater variations of cake thicknesses along the membrane. Finally, the interfacial interaction energy predicted by the XDLVO model gave higher oil-membrane and oil-oil affinities compared to that of latex, which underlie respectively the more extensive fouling and denser cake exhibited by oil. The unique characteristics of the cake layer of oil emulsion vis-à-vis the well-studied particulate foulant (i.e., latex) shown here provides new perspectives on the monitoring and control of oil fouling. [ABSTRACT FROM AUTHOR]

The large volumes of oily wastewater generated by various industries, such as oil and gas, food and beverage, and metal processing, need to be de-oiled prior to being discharged into the environment. Compared to conventional technologies such as dissolved air flotation (DAF), coagulation or solvent extraction, membrane filtration can treat oily wastewater of a much broader compositional range and still ensure high oil removals. In the present review, various aspects related to the practical implementation of membranes for the treatment of oily wastewater are summarized. First, sources and composition of oily wastewater, regulations that stipulate the extent of treatment needed before discharge, and the conventional technologies that enable such treatment are appraised. Second, commercially available membranes, membrane modules, operation modes and hybrids are overviewed, and their economics are discussed. Third, challenges associated with membrane filtration are examined, along with means to quantify and mitigate membrane fouling. Finally, perspectives on state-of-the-art techniques to facilitate better monitoring and control of such systems are briefly discussed. • Large volumes of oily wastewater need to be de-oiled prior to discharge. • Summarize practical implementation of membranes for treating oily wastewater. • Present sources and composition of oily wastewater, and regulatory requirements. • Review commercial membranes, membrane modules, operation modes and hybrids. • Discuss challenges associated with membrane filtration and means to mitigate. [ABSTRACT FROM AUTHOR]

Highlights • Mixtures of casein, lactose and soybean oil emulsion used to mimic skim milk. • DOTM used to quantify critical flux of the mixtures under different conditions. • Deposits at the feed-membrane interface clearly slowed down with viscosity. • Both casein and lactose increased viscosity, but lactose had lesser effect on fouling. • Good agreement between experiment and model at lower CFV but not at higher CFV. Abstract Membrane-based filtration is promising for applications in the food industry, for example, for processing dairy products, but membrane fouling restricts the efficiency of the process. The deposition of the main constituents in skim milk of casein, lactose and milk fat globules onto the membrane due to the permeate drag is inevitable, which necessitates a mechanistic study of the fouling phenomena. The Direct Observation through the Membrane (DOTM) technique was used to understand the effects of viscosity and bidispersity on the microfiltration of oil emulsions. Deposits at the feed-membrane interface clearly slowed down as the feed viscosity increased. The higher drag to the crossflow as a result of the higher viscosity caused more oil droplets to foul the membrane. However, although both casein and lactose increased the feed and permeate viscosity, the presence of lactose did not alter the fouling phenomena of a moving cake layer of oil droplets much, but the presence of casein caused the oil droplets to deposit instead as clusters. For casein, other than the viscosity effect, it also resulted in a bidisperse suspension, with the smaller particulates promoting membrane fouling. Agreement between the experimental data and the shear-induced diffusion model was good at the lower cross-flow velocities (CFVs) but poorer at higher CFVs, which indicate that the effect of viscosity appeared negligible at the lower CFVs but more dominant at the higher CFVs. The DLVO model agreed with the J crit and TMP results. This study sheds light on the membrane fouling phenomena for viscous feeds, and are expected to be useful particularly for food applications. [ABSTRACT FROM AUTHOR]

Highlights • Studied the striping phenomenon whereby the foulants deposited as regular streaks. • The direct-observation DOTM technique employed to characterize the stripes. • Stripes formed by oil drops stabilized by Tween surfactants but not other surfactants. • Stripes easier to remove from membrane than the more uniform layer of oil droplets. • Among the three Tween surfactants, the striping characteristics were largely similar. Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Microfiltration is an attractive means for treating oily wastewater, especially when the size of the oil droplets are micrometer-sized since the conventional techniques become deficient. A systematic study on the critical flux of oil-in-water emulsion, which behaves differently from other colloidal foulants with regards to deformation, coalescence and splitting, has not been carried out to date. This was the goal of the current study, which employed the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of oil-in-water emulsions of various concentrations, and at various cross-flow velocities (CFV) and salt concentrations. Five observations can be highlighted here. Firstly, the oil droplets with a mean droplet diameter of approximately 5 µm exhibited critical fluxes equal to or greater than latex particles of 10 µm. This is likely due to the twin effects of membrane oleophobicity promoting back-transport of the oil foulant from the membrane and the presence of a droplet size distribution with larger drops that can enhance the shear-induced diffusion of the average drops. Secondly, the critical flux values did not agree with the model that is valid for the size range the mean droplet diameter falls in, but instead agreed with the model adapted for smaller particulate foulants. Thirdly, the increase in the critical flux with CFV was more significant for the lower oil concentration. Fourthly, a striping phenomenon was observed at higher oil concentrations and lower CFV values. Striping was not observed for latex particles. Fifthly, the critical flux decreased with salt concentration. These findings highlight the unique fouling behavior of oil-in-water emulsions in microfiltration. [ABSTRACT FROM AUTHOR]

Membrane fouling by particulates is largely driven by permeate drag that causes the foulants to move towards the membrane. Cross-flow is used to induce shear at the membrane surface to mitigate the fouling. This study describes the flow-field mitigation of membrane fouling (FMMF) technique that reduces the fouling based on a judicious manipulation of the flow-field by inclining the channel walls to counter the permeate drag experienced by the foulants. The key mechanism is the additional transverse (i.e., orthogonal to the fluid flow direction) fluid vector caused by a small inclination of the channel walls (on the order of one to a few degrees). Both simulation and experiments confirm the efficacy of FMMF in mitigating fouling at a reduced energy requirement compared with the conventional channels with parallel walls. Simulations over a range of permeate fluxes indicated that a slight inclination angle of 1.15° can give a deposition factor less than that in the conventional channel with parallel walls even at twice the permeate flux. Direct observation through the membrane (DOTM) experiments for fouling using two particle diameters over a range of power inputs showed that the critical fluxes were significantly increased in the FMMF module compared with the conventional module. This study provides a proof-of-concept of the principle underlying FMMF and underscores the potential benefits. The direction for further development and scale-up is suggested. [ABSTRACT FROM AUTHOR]

Crossflow microfiltration is a popular application spanning various industries. Although the impacts on fouling of feed bidispersity, crossflow velocity (CFV) and spacer, all of which are present in practical operations, are known separately, the understanding of the interplay of these three factors on fouling is lacking. Accordingly, this study used the Direct Observation Through the Membrane (DOTM) technique to characterize the critical flux of monodisperse and bidisperse polystyrene particles in both the absence and presence of a spacer over a range of CFV values. The results indicate that (i) the combined effects of both bidispersity ( d p =3 µm and 10 µm) and spacer gave the highest J crit values for the smaller particles throughout the CFV range investigated; (ii) bidispersity was more effective in enhancing J crit at a lower CFV, while the presence of a spacer was more effective at a higher CFV; (iii) a higher CFV diminished the enhancement induced by bidispersity more than that by the spacer; and (iv) comparisons between models and experimental data reveal that shear-induced diffusion models based on monodisperse particles are deficient for bidisperse mixtures, because they cannot allow for particle size segregation effects that occur in flowing mixed systems. [ABSTRACT FROM AUTHOR]

Membrane fouling in organic solvent environments remains poorly explored despite its significance in chemical and pharmaceutical industries. This study uses molecular dynamics (MD) simulations and experiments to explore lysozyme fouling in water, as well as four organic solvent environments, namely, 30 % v/v and 50 % v/v isopropyl alcohol (IPA), and 30 % v/v and 50 % v/v dimethyl sulfoxide (DMSO). Experimentally, flux declines were least with IPA and worst with DMSO. Biased simulations indicate the worst fouling in DMSO is tied to the most attractive lysozyme-membrane energy in the presence of DMSO. However, the relative attractive energies for IPA and water do not agree with the relative flux declines, indicating other factors are more influential when the interaction energies are similar. To understand the gentler flux decline for IPA despite the more attractive lysozyme-membrane energy, radial distribution functions (RDFs) were obtained from unbiased simulations. Analyss of the water and solvent films around both the membrane and the lysozyme molecule reveal that the denser water film around both entities induced by the presence of IPA serves as a barrier for fouling and thus leads to less flux decline. The results underscore the complexity of fouling in organic solvent systems, cautioning against direct use of the understanding based on aqueous systems. [Display omitted] • Membrane fouling by lysozyme in water, IPA and DMSO studied. • Flux decline worst in DMSO and least in IPA. • Worst fouling in DMSO tied to most significant lysozyme-membrane attraction. • Least fouling in IPA tied to denser water film around membrane and lysozyme. • Both biased and unbiased molecular dynamics simulations needed for full explanation. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Three oil emulsion types in the absence and presence of surfactant studied. • DOTM used to quantify critical flux and visualize evolution of fouling. • Presence of either surfactant reduced the critical flux of the oil emulsion. • Molecular dynamics simulation revealed different interaction energies. • Coalescence worst for DTAB-stabilized emulsion due to the least oil-oil repulsion. [ABSTRACT FROM AUTHOR]

• Overview of mechanisms of membrane fouling by oil. • Focus on physicochemical bases, detection, diagnosis, and visualization. • Review of the recent work on membrane materials for fouling mitigation. • Survey of invasive and non-invasive methods of fouling visualization. • Discussion of knowledge gaps and future research outlook. Separation of liquid–liquid emulsions is a challenging problem that has gained importance with the development of unconventional oil resources. Large volumes of water contaminated by emulsified oil need to be treated for safe disposal or efficient reuse. Among technologies capable of removing oil dispersed into smaller size droplets (<15 µm), membrane processes occupy a unique niche where the required separation performance and throughput can be achieved at a relatively modest cost. As with most membrane-based processes, separation of emulsified oil entails membrane fouling that requires regular maintenance and imposes additional operational costs. Emulsions present unique challenges as their fouling behavior is affected by droplets' deformability, coalescence both in the bulk and on the membrane surface, membrane wetting by droplets and films, pore blockage and intrusion by oil. The purpose of this paper is to overview the literature on the separation of oil–water emulsions by pressure-driven membrane processes with an emphasis on how properties of emulsions and membranes affect separation performance. A particular focus is on membrane fouling by oil including physicochemical bases, detection, diagnosis, and visualization. The review spans studies with both industrial oily wastewater and synthetic model emulsions of various types of oil. The discussion of membrane materials is limited to surface modifications that render more fouling-resistant membranes. [ABSTRACT FROM AUTHOR]

Foulant-foulant and foulant-membrane interfacial interactions play an important role in dictating the extent of fouling. In order to understand the impact of the surface energy of particulate foulants on the fouling extent, the direct observation through the membrane (DOTM) technique was used to characterize critical flux and also assess the initial evolution of foulant deposition. Polystyrene and glass with diameters approximating 10 μm were used as foulants because (i) the similar particle diameter eliminated the differences due to particle back-transport, and (ii) the Gibbs free energies (Δ G ) of foulant-membrane and foulant-foulant interactions were both negative (i.e., attractive) for polystyrene and both positive (i.e., repulsive) for glass. Results indicate that: (1) because of the attractive and repulsive foulant-membrane interactions of the polystyrene and glass, respectively, (i) critical flux was lower for polystyrene than glass, (ii) the phenomenon of a flowing particle layer was observed at a lower cross-flow velocity (CFV) for glass than polystyrene, and (iii) relaxation was more effective for glass than polystyrene; (2) because of the attractive and repulsive foulant-foulant interactions of the polystyrene and glass, respectively, clustering was observed for the former but not latter; (3) out of the four particle deposition mechanisms (namely, convective, entrapment, dead zone and clustering) distinguished, the first three occurred for both attractive and repulsive interfacial interactions, but the forth (i.e., clustering) only occurred when foulant-foulant interactions were attractive. [ABSTRACT FROM AUTHOR]