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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]

Critical and threshold flux concepts were recently developed to distinguish no fouling, slow fouling and rapid fouling regimes. Membrane fouling behavior is expected to vary with respect to the imposed flux relative to the critical and threshold flux values. However, crossflow fouling tests are often performed independent of critical and threshold flux determinations. In this study, constant flux fouling experiments were performed in connection with critical and threshold flux determination. Fouling behavior was examined in the context of critical and threshold flux. A poly(vinylidene fluoride) microfiltration membrane was challenged with various oil-in-water emulsions. The critical and threshold flux values were estimated using the flux-stepping technique. Constant flux crossflow fouling tests were performed at selected fluxes below and above the critical and threshold fluxes. Below the critical flux, mass transfer resistance remained constant at the clean membrane value. Above the critical flux but below the threshold flux, mass transfer resistance approached a steady state resistance, R B , which was determined from the linear regression of flux-stepping experiments. Above the threshold flux, a three-stage transmembrane pressure (TMP) was observed, consisting of: (1) an initial gradual increase, (2) a TMP jump stage, and (3) a pseudo-steady state. The pseudo-steady state TMP corresponded to the estimated critical pressure of the oil layer. [ABSTRACT FROM AUTHOR]

A fundamental study of microfiltration membrane fouling by emulsified oil was conducted using a combination of real-time visualization, force balance on a droplet, and permeate flux analysis. The model 0.1% v/v hexadecane-in-water emulsions contained sodium dodecyl sulfate (0.1 mM, 0.4 mM, or 0.8 mM) to regulate interfacial tension. Direct Observation Through the Membrane tests with Anopore ( d pore =0.2 µm) and track-etch ( d pore =5 µm) membranes revealed three characteristic stages of membrane fouling: (1) droplet attachment and clustering, (2) droplet deformation, and (3) droplet coalescence. In qualitative agreement with visualization results, the force balance predicted that droplets ≲ 36–40 µm would remain pinned at d pore =5 µm pores while larger droplets would be swept off the surface by the crossflow drag. In a separate set of constant pressure crossflow filtration tests with track-etch membranes, the average oil rejection was ≥98% while the permeate flux decreased to a pseudo-steady-state ~10% of the initial value. The results indicate that membrane fouling by emulsified oil is controlled by droplet coalescence and crossflow shear: the transport of oil to the membrane surface by the permeate flow is balanced by the shear-induced removal of the droplets that coalesce to exceed a critical size. [ABSTRACT FROM AUTHOR]

Patterned microfiltration membranes were fabricated via Phase Separation MicroFabrication (PSMF) using vapor-induced phase separation to avoid skin formation on the nonpatterned surface. A starting solution of 10% PES, 60% PEG400, 5% water and 25% NMP yielded symmetric membranes with similar pore size all throughout the cross section when 10 or 15 min of vapor exposure was used before further coagulation in water. On the other hand, 5 min of vapor exposure resulted in what may be called inverse asymmetric membranes, which had smaller pores on the patterned (mold) side and macrovoids throughout the cross-section. The fidelity in replicating the pattern was higher when the vapor exposure was shorter and surface area enhancements of 103% and 52% were obtained when vapor exposure times were 5 and 10 min, respectively. The phase inversion rates of the polymer solutions in liquid water, humid air and humid air followed by liquid water were examined by measuring the light transmission through the solutions in time. It was observed that water in the starting solution increased the overall phase inversion rate and decreased the delay time before phase separation, in consistence with the membrane morphologies obtained with the solutions with and without water under different coagulation conditions. The patterned and nonpatterned membranes were tested in constant flux crossflow filtration of yeast suspensions at different fluxes and it was seen that the fouling rate of nonpatterned membranes was higher than patterned membranes. However, the major contribution to the improved fouling behavior appears to be the increased surface area since when the fluxes were normalized by the patterned (actual) surface area for foulant deposition, the difference between the fouling rate of patterned and nonpatterned membranes was not significant. [ABSTRACT FROM AUTHOR]

Abstract: The behavior of an oil droplet pinned at the entrance of a micropore and subject to crossflow-induced shear is investigated numerically by solving the Navier–Stokes equation. We found that in the absence of crossflow, the critical transmembrane pressure required to force the droplet into the pore is in excellent agreement with a theoretical prediction based on the Young–Laplace equation. With increasing shear rate, the critical pressure of permeation increases, and at sufficiently high shear rates the oil droplet breaks up into two segments. The results of numerical simulations indicate that droplet breakup at the pore entrance is facilitated at lower values of the surface tension coefficient, higher oil-to-water viscosity ratio and larger droplet size but is insensitive to the value of the contact angle. Using simple force and torque balance arguments, an estimate for the increase in critical pressure due to crossflow and the breakup capillary number is obtained and validated for different viscosity ratios, surface tension coefficients, contact angles, and drop-to-pore size ratios. [Copyright &y& Elsevier]

In membrane filtration, the pore size of the membrane determines the size of ‘particles’ that should be rejected, leading to accumulation of particles on the membrane surface and changed particle retention in time. A process without accumulation and thereby constant retention as function of time would be well suited for fractionation of components close in size. In this research, emulsions consisting of small droplets (∼2.0µm) and large droplets (∼5.5µm), with total concentrations between 10% and 47%, were fractionated. The cross-flow module consisted of a closed channel to allow particles to migrate, followed by a membrane area with 20µm pores where emulsion fractions could be removed. Under appropriate process conditions, the permeate consisted of only small droplets, at concentrations higher than in the original emulsion, leading to very high selectivities. Especially at high concentrations, known to cause severe fouling in regular membrane filtration, these effects were occurring as a result of shear-induced diffusion of the droplets. If only small particles are targeted, the module can be operated at fluxes of 40L/(m2/h); if fractionation is targeted the fluxes can be considerably higher. These fluxes are comparable to current operational fluxes, but here cross-flow velocity and trans-membrane pressure are much lower (corresponding to fluxes of 1–4m3/(m2/h/bar)) with stable retention and flux as function of time. [ABSTRACT FROM AUTHOR]

Abstract: Cross-flow microfiltration of fermented mash containing native corn starch granules from a simultaneous saccharification and fermentation process, was investigated with respect to the process performance. Influence of transmembrane pressure and cross-flow velocity on normalized permeate flux rate decline and membrane fouling were examined. The microfiltration system was equipped with a tubular ceramic membrane with 0.45μm pore size. A combined pore-blockage–cake-filtration model was successfully used to determine the process performance and identify principal fouling mechanisms. Experimental results revealed that the fouling phenomenon taking place during microfiltration of the fermented mash follows a pattern of initial complete pore blockage and subsequent cake formation. As a result, initial rapid flux decline is followed by gradual flux decline. Fouled membranes were characterized using scanning electron microscopy (SEM). The membrane surface characteristics examined by SEM images evidenced formation of dense deposit on the upper membrane surface. Rinsing the fouled membrane with water followed by investigation of the permeate flux rate indicated occurrence of increased fouling, when microfiltration was carried out at high transmembrane pressure. The highest normalized flux decline was observed for microfiltration operated at the highest transmembrane pressure (1.4bar) and the lowest cross flow velocity (1ms−1). It is recommended to use high cross-flow velocity (4.55ms−1) and low transmembrane pressure (0.35bar) to minimize membrane fouling, triggered by components of the fermented mash. [Copyright &y& Elsevier]

Abstract: Intensification of microfiltration has been accomplished using motionless mixers consisting of a series of pairs of semielliptical blades as turbulence promoters. Blade mixers of two aspect ratios 2.5 and 1.3 were tested in the microfiltration of milk (0.1μm membrane). The permeate fluxes are substantially increased by application of blade mixers due to a reduction of both reversible and irreversible fouling. The highest flux improvements of 500–650% for the same cross-flow rate (relative to the conventional operation) were obtained by application of the blade mixer of aspect ratio 1.3.When compared for the same hydraulic dissipated power mixer of aspect ratio 2.5 proved to be slightly more efficient because it causes the lower pressure drop. Despite to the increased pressure drop, the energy savings obtained by application of blade mixers are considerable compared to the conventional operation and to the operation using some other mixers. In the membrane fitted with a blade mixer the flow field changes in a manner which afford the intensive disruption of boundary layer, scouring and removal of fouling forming material. The flow field is characterized by the high cross-flow velocities at membrane wall, alternation of stream line path and swirling motion. Therefore, the application of blade mixers affords an operation under the several times lower initial cross-flow velocity. [Copyright &y& Elsevier]

The aim of this study was to understand the limiting flux in cross-flow microfiltration of a colloidal suspension in terms of the balance of hydrodynamic forces on a particle. Numerical simulations were carried out to estimate the lift force resulting from the cross-flow, drag force resulting from the convective flux, and net force. Assuming that the limiting flux is the flux when the net force on a particle is zero, we analyzed the simulation data to determine the correlation of two particle Reynolds numbers: R e p _ J l i m i t , calculated from the limiting flux, and R e p _ u δ , calculated from the cross-flow rate. The results indicate that R e p _ J l i m i t is proportional to the square of R e p _ u δ when a particle is at a dimensionless distance (distance divided by diameter) of less than 6.7 from the cake layer, in spite of the experimental fact that R e p _ J l i m i t is proportional to the 1.5th power of R e p _ u δ . In contrast, R e p _ J l i m i t is proportional to the 1.5th power of R e p _ u δ when a particle is at a dimensionless distance of 48–60. This means that the force balance of a particle at such a position, not at the membrane surface, determines the limiting flux in cross-flow microfiltration of a colloidal suspension. Image 1 • Hydrodynamic force on a particle in cross-flow microfiltration is simulated. • We focus on drag force induced by flux and lift force induced by cross-flow. • The balance of forces on a particle in a viscous sublayer is discussed. • The relationship between the force balance and limiting flux is discussed. [ABSTRACT FROM AUTHOR]