Ultrafiltration Flux Research Articles

How unsteady filtration conditions can improve the process efficiency during cell cultures in membrane bioreactors

Abstract Among processes developed to increase biological performances, membrane bioreactors have provided the best results. The membrane bioreactor combines a continuous fermentor and a crossflow filtration module enabling separation of cells from liquid media. Very high biomass concentrations have thus been reached and important bioconversion yields obtained. However the potentiality of this process is mainly limited by the rapid decline in permeate flux due to membrane fouling. In our laboratory, various technological solutions, based on unsteady hydrodynamics inside the tubular filters to limit the external fouling, have been developed and applied during cell cultures in membrane bioreactors. The biological model was alcoholic fermentation. The first kind of flow unsteadiness was based on an air injection at the membrane inlet to create a gas/liquid slug flow. For the same energy consumption, this process enabled a mean twofold gain in ultrafiltration flux with a lower efficiency for microfiltration due to pore blocking by cell debris. The impact of an unsteady jet generated by a pneumatically controlled valve was also evaluated. Although the strong physico–chemical affinity between the membrane material and the culture medium, a flux enhancement of 1.3 was achieved at the end of fermentation. It was also pointed out that when the formation of a cell cake layer was expected to be the main mechanism for flux decline, flow unsteadiness failed to disrupt a previously built-up deposit and for a maximal efficiency it had to be started at the very beginning of the filtration operation. After these feasibility studies on a relatively simple and well-known biological model, further applications on environmental problems were carried out. The interest of a gas/liquid slug flow as a means to increase both the permeate flux and the oxygen transfer rate was demonstrated during continuous phenol degradation by Ralstonia eutropha . The active biomass could be doubled without encountering oxygen depletion while the permeate flux was 75% higher. This led to the complete degradation of a high phenol load higher than 70 kg m −3 day −1 . Finally, a new biological treatment process combining a gas/liquid contactor (‘aero-ejector’) and a membrane bioreactor was developed in order to ensure total microbial degradation of pollutants which were initially present in industrial gaseous effluents. The ‘aero-ejector’ technology allowed the solubilisation of gaseous compounds (here ethanol) in a liquid phase before their degradation in the bioreactor itself. During aerobic cultures of Candida utilis , almost all injected ethanol was transferred and degraded over 350 h of culture.

Tubular ultrafiltration flux prediction for oil-in-water emulsions: analysis of series resistances

Using the resistance-in-series (RIS) approach to permeate flux modeling, a general relationship between permeate flux, transmembrane pressure, cross-flow velocity, and feed kinematic viscosity was developed for the tubular ultrafiltration (UF) of synthetic oil-in-water emulsions. The fouling layer resistance, R f, was 63% of the total membrane resistance, R m′; however, concentration polarization was the predominant factor controlling resistance in the tubular UF system. An explicit form of the resistance index, Φ, was postulated based on the observed interactions between Φ, cross-flow velocity and feed kinematic viscosity and the RIS model was modified to further describe the interactions between permeate flux and operational parameters. The modified model adequately predicted flux–pressure data over the range of experimental variables examined in this study. Additionally, a set point operating pressure was determined as a function of cross-flow velocity and feed viscosity to achieve a balance between polarization and total membrane resistance.

The role of bentonite addition in UF flux enhancement mechanisms for oil/water emulsion

The main problem in treating oil/water emulsion from car wash waste-water by ultrafiltration (UF) is fouling caused by oil adsorption on the membrane surface and internal pore walls. This study demonstrates that the addition of bentonite clay can reduce the adsorption layer on cellulose acetate UF membrane, resulting in a reduction of total membrane resistance ( R t). Experiments were conducted to identify and describe three possible mechanisms: (i) bulk oil emulsion concentration reduction; (ii) particle aggregation and (iii) detachment of the adsorbed gel layer by shear force. Adsorption of oil emulsion by bentonite can lead to a significant reduction of bulk oil emulsion concentration, one of the major causes of flux enhancement. Results show that contact of oil emulsion with bentonite forms larger particles resulting in flux increment. An optimum particle size of 37 μm, corresponds with a bentonite concentration of 300 mg/l and provided the highest flux. Beyond this limiting concentration, flux improvement gradually declined, possibly due to the formation of packed cake of particles on the membrane surface. The presence of bentonite in the oil emulsion promotes high shear stress which acts against the gel layer. This high shear stress, caused by bentonite particles and cross-flow velocity, reverses the adsorbed gel layer to the bulk of the liquid phase.

Hollow-fibre membrane module design: comparison of different curved geometries with Dean vortices

The performance of several designs of curved membrane modules with Dean vortices was compared through experiments using a colloidal bentonite suspension and cellulose acetate hollow-fibre ultrafiltration (UF) membranes. The different module geometries were: straight, helically coiled, twisted and sinusoidal, or meander-shaped. The experiments show a remarkable increase in mass transfer in curved modules as compared to conventional straight ones. Comparisons were made for modules equipped with the same hollow fibres and the same Dean number (De) for a given Reynolds number (Re). At the same Dean number, all the curved geometries gave the same limiting permeate flux. A mass transfer correlation relating limiting UF flux with the mean wall shear stress has been obtained.

Ultrafiltration flux theory based on viscosity and osmotic effects: application to diafiltration optimisation

The optimisation of diafiltration is examined using the new theory of ultrafiltration based on osmotic pressure and the viscosity dependence of the mass transfer coefficient. It is shown that, if the permeate flux is maintained at the limiting value at each bulk macrosolute concentration, the diafiltration time always decreases with increasing concentration. However, the transmembrane pressure required to reach the limiting flux increases rapidly with concentration, thus putting a practical limit on the concentrations at which an ultrafiltration system can be operated under limiting flux conditions. In contrast, when the osmotic pressure is a polynomial function of the wall concentration and operation at a fixed transmembrane pressure is considered, a finite optimum concentration is found which minimises the diafiltration time. A methodology for computing this optimum concentration is developed. It is shown that the optimum concentration increases with increasing transmembrane pressure but is only weakly dependent on feed velocity and membrane resistance.

Pressure-Dependent Permeate Flux in Ultra- and Microfiltration

The effect of applied pressure on the permeate flux in cross-flow ultrafiltration (UF) and microfiltration (MF) was investigated both theoretically and experimentally. In UF and MF processes, the permeate fluxes are controlled by concentration polarization and cake formation over the membrane surface. As a better understanding of concentration polarization and cake formation becomes available, the permeate flux under any pressure can be theoretically predicted. Experiments were conducted in a ceramic tubular cross-flow filter with silica colloids of a narrow size distribution (model colloids). The pressure-dependent flux of the model colloidal suspension in cross-flow filtration was investigated under various experimental conditions. The experimental measurements were compared with the theoretical predictions, and the results showed that the pressure-dependent permeate flux in cross-flow filtration can be adequately predicted. Furthermore, theory and experiments demonstrated that the performance and opera...

Two-phase limiting flux in high-shear rotary ultrafiltration of oil-in-water emulsions

A study was conducted to assess the effect(s) of high oil concentration on the pressure-independent ‘limiting’ permeate flux and the ability of a thin film model to adequately predict the flux in the treatment of synthetic and waste metal working fluids using the high-shear rotary ultrafiltration (HSRUF) system. Two distinct limiting flux regions were ascertained, in contrast to a discrete flux decline with concentration predicted from the thin film model. Additionally, widely different gel layer oil concentrations were predicted based on data from each discrete region. Thus, it was not possible to predict the occurrence of the two-phase limiting flux or the closest packed arrangement of emulsified oil droplets using a thin film model. The net effect of the unique ‘wagon wheel’ turbulence promoters was to enhance hydraulic turbulence in the HSRUF system; thus, the transition from laminar to turbulent hydraulic flow occurred at lower Re r in the HSRUF system than reported previously for a solid plate turbulence promoter. Further, it was demonstrated that the occurrence of the two-phase limiting flux resulted in operating conditions under which high feed oil concentrations could be attained while maintaining acceptable limiting flux.

Dimensionless numbers for the steady-state flux of cross-flow microfiltration and ultrafiltration with gas sparging

The steady-state flux, observed for gas-sparged microfiltration or ultrafiltration through inorganic composite membranes, could be expressed using two dimensionless numbers. The generalised shear stress number N S ′, with introduction of an equivalent fluid density, compares the shear stress against the membrane wall to the driving pressure, while the resistance number N f compares the convective cross-flow transport to the driven transport through a layer, whose resistance is the sum of all the resistances induced by the different processes which limit the mass transport. Experimental data, obtained in ultrafiltration of dextran solutions and microfiltration of ferric hydroxide suspensions and secondary treated wastewater, were re-calculated in terms of these dimensionless groups. Straight lines were plotted; their slope is a decreasing function of the gas–liquid velocity ratio when particle deposition or polarisation limited the mass transport. A negative slope and a positive intersection with the N S ′-axis means that the induced resistance can completely be eliminated with gas sparging. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to a reversible phenomenon in the hydrodynamic conditions of the run. A positive slope means that the flux reaches a plateau when cross-flow is increased. Gas sparging allows then decreasing the slope and reaching negative values.

Hydrodynamics of slug flow applied to cross‐flow filtration in narrow tubes

AbstractThis article focuses on the characterization of slug‐flow hydrodynamics in two sizes of tubular membrane diameters (6 and 15 mm) to quantify the main mechanical phenomena involved in the limitation of particle fouling during cross‐flow filtration of suspensions. By using a conductance probe technique, the flow structure was accurately identified for two geometries, and noticeable differences were observed in terms of void fractions, velocities and lengths of Taylor bubbles, and liquid slugs. This characterization allowed some data to be theoretically estimated (wall shear stress, “falling” film velocity, film thickness) thanks to the application of a phenomenological model initially developed for oil pipes. The results obtained in a 15‐mm tube showed that the ultrafiltration flux improvement, experimentally achieved with bentonite and yeast suspensions, was partly due to the increase in the wall shear stress, induced by continuous gas sparging inside the tubular filtration module. Other hydrodynamic phenomena linked to the quasi‐periodic succession of Taylor bubbles and liquid slugs were also involved in the control of the particle entrainment: intermittency frequency, reversal of the wall shear stress, instantaneous pressure variations in the long bubble wake with a higher level of turbulence, and an enhanced local mixing.

Ultrafiltration of an olive oil emulsion stabilized by an anionic surfactant

In this paper we report results on limiting flux in ultrafiltration of an oil in water emulsion stabilized by an anionic surfactant (sodium dodecyl sulfate). Whereas most of previous investigations report variations of filtration flux as a function of pressure at different flow rates, here we also consider the influence of surfactant/oil ratio and emulsion’s ionic strength. The present study clearly suggests that filtration of stabilized emulsions exhibits paradoxical experimental similarities with filtration of macromolecules. However, classical film models used for the latter case did not yield proper values for mass transfer coefficients. When using flux models to account for shear enhanced diffusion and pseudo-plastic behavior of the concentrated oil in water emulsions a good agreement between calculated and experimental mass transfer coefficient is obtained.

Mass Transfer for Dialysis with Ultrafiltration Flux Declined in Cross-Flow Membrane Modules.

This study deals with the effect of an exponential-decay flux of ultrafiltration on the mass transfer rate of solute for dialysis in cross-flow membrane modules. The numerical method of orthogonal collocation was used to solve this problem. The calculated results of solute concentrations and separation efficiencies were analyzed under various operating conditions and decay coefficients of flux in ultrafiltration. The mass transfer rate is usually enhanced by the addition of the effect of ultrafiltration, but this effect decreases as the decay coefficient increases. A significant influence of the decay coefficient on the mass transfer rate appears when the solute transport is dominated by the effect of ultrafiltration, i.e. the conditions of lower dialysate flow rate, lower solute sieving coefficient, or lower overall mass transfer coefficient.

Preparation, structure, and transport properties of ultrafiltration membranes of poly(vinyl chloride) and poly(vinyl pyrrolidone) blends

Ultrafiltration (UF) membranes based on poly(vinyl chloride) and poly(vinyl pyrrolidone) blends were prepared by the phase inversion method, and the factors governing membrane properties were investigated. The membranes were characterized by scanning electron microscopy and atomic force microscopy. The fouling characteristics of the membranes were determined by UF of aqueous solutions of bovine serum albumin (BSA) over a pH range of 2-9 and varying salt concentrations. The maximum adsorption of the protein on the membrane surface occurred near the isoelectric point (pI 4.8) of BSA, and the presence of the salts increased the fouling of the membrane. The results can be explained in terms of the nature of the membrane polymer and the effect of different ionic environments on the permeability of the deposited protein layer. The net charge on the BSA molecules appears to be a dominant factor in determining the flux of water through the blend membranes. The UF flux is correlated by a model based on the membrane resistance, adsorbed protein resistance, and time dependent resistance of the concentration polarization layer near the membrane surface. The ζ potentials of the membranes were also determined before and after UF to characterize the surface potential of the membrane.

SIZING AGENT RECOVERY BY MEMBRANE ULTRAFILTRATION IN HOLLOW-FIBER MODULES

Ultrafiltration of an aqueous solution of polyvinyl alcohol was carried out in an Amicon model H1P30-20 hollow-fiber cartridge made of polysuifone. Fresh water was forced through the macroporous membrane, as the permeate, while polyvinyl alcohol was concentrated and re covered as the retentate. Correlation equations for calculating the permeate flux of membrane ultrafiltration of the polyvinyl alcohol were derived based on the resistance-in-series and exponential models. Correlation results were confirmed by the experimental data, especially for the results obtained from the exponential model. It was found that the permeate flux increases as transmembrane pressure or fluid velocity increases, but decreases when feed concentration increases. Because membrane ultrafiltration is a pressure-driven process, high cross-flow velocity enhances the mass transfer coefficient of the solute and high solution concentration in creases the thickness of the concentration polarization layer.

Enhancement of ultrafiltration flux by coupling static turbulence promoter and electric field

The purpose of this work was to determine the effect of the static metal deployed sheet on the performance of the ultrafiltration process. The sheet was first utilised to provoke turbulence near the membrane, and at the same time as an anode to create an electric field. It was shown that the electro-ultrafiltration module was more efficient for both low crossflow velocity and BSA initial concentration. The global hydraulic membrane resistance may be reduced by half in all cases. The polarisation layer and fouling membrane appeared reversible phenomena, which may be controlled by appropriate electric field strength.

Mass transfer in gas-sparged ultrafiltration: upward slug flow in tubular membranes

Gas sparging is recognized as an effective way to increase permeate flux in ultrafiltration processes. This paper discusses the mechanism of flux enhancement in ultrafiltration processes by gas sparging, in the special case of upward slug flow in tubular membrane module. An attempt is made to model gas sparged ultrafiltration with a view to predicting permeate flux in such processes. The model is based on dividing the region near the gas slug into three different zones depending on the nature of flow in the vicinity of the membrane. These different zones are (a) the film zone, where there is falling film flow, (b) wake zone, which is a region of free turbulence, and (c) liquid slug zone, where the flow could be either streamline or turbulent depending on the bulk fluid flow. The mass transfer coefficients for each of these zones can be determined for a particular feed solution, and from these, the averaged permeate flux for gas sparged ultrafiltration can be calculated. The model was tested for ultrafiltration of 167 kDa dextran. A reasonably good agreement between experimental and theoretically predicted data was obtained. The discrepancies observed are discussed. Simulations were run in order to examine the effects of operational parameters on permeate flux. The results suggest that gas sparging is more effective at higher transmembrane pressure, and increasing the liquid flow rate has opposite effects in single phase flow and gas-sparged ultrafiltration.

Rotation and Concentration Effects in High-Shear Ultrafiltration

A parametric waste-specific study was conducted to assess the effects of membrane rotational speed and feed oil concentration on the pressure independent “limiting” flux in a high-shear rotary ultrafiltration (HSRUF) system. The limiting flux data were adequately described by the thin-film model. The transition from pressure dependent to pressure independent behavior occurred at lower oil concentrations as membrane rotational speed was decreased and pressure was increased due to an increase in the thickness of the solute boundary layer at the membrane surface. A gel layer oil concentration, \IOC\N\dg\de\dl, of 39% was reported, and \IOC\N\dg\de\dl was determined to be constant with respect to average transmembrane pressure and membrane rotational speed. The solute mass transfer coefficient increased with membrane rotation and was greater then reported for conventional ultrafiltration systems. The greater mass transfer characteristics determined for the HSRUF system were attributed to the efficient delivery of “cleaning energy” to the membrane surface due to the effective decoupling of feed pressurization from recirculation/hydraulic turbulence.

Modified concentration polarization model for hollow-fiber membrane ultrafiltration

An improvement of a previous study for estimating the permeate flux of hollow-fiber membrane ultrafiltration has been made. The improvement was achieved by removing an unrealistic condition on the declination of permeate flux near the fiber outlet and the requirement of estimating the solute diffusivity at a condition corresponding to that existing at the membrane surface.

Permeate Flux in Crossflow Ultrafiltration under Intermediate Pressures

The intermediate pressure range stands for an important condition for ultrafiltration operations where permeate flux falls below that predicted from a linear flux-pressure relationship but does not reach the limiting flux. Optimal performance of ultrafiltration might be achieved on the upper end of the intermediate pressure range because the maximum permeate flux will be obtained at the lowest possible pressure. Unlike in the low pressure range or in the high pressure range, little has been known about the behavior of permeate flux in the intermediate pressure range. A general theory of permeate flux in the intermediate pressure range is developed in this study. The theory includes the linear-increasing and limiting fluxes as two special cases and, therefore, can be used to study the behavior of permeate flux in ultrafiltration in the whole pressure range.

Preparation, structure, and transport properties of ultrafiltration membranes of poly(vinyl chloride) (PVC), carboxylated poly(vinyl chloride) (CPVC), and PVC/CPVC blends

Ultrafiltration (UF) membranes were prepared from poly(vinyl chloride) (PVC), carboxylated poly(vinyl chloride) (CPVC), and PVC/CPVC blends by the phase-inversion method. The physical structure of the membranes was characterized by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). The fouling characteristics of all the three membranes and acrylamide (AA)-grafted PVC membranes were characterized by ultrafiltration of bovine serum albumin (BSA) solution over a range of pH and of salt concentrations. Maximum adsorption of the protein on the membrane occurred near the isoelectric point of BSA and in the presence of the salts. The charge on BSA appears to be a dominant factor in determining the fouling. The UF results are explained in terms of nature of the membrane polymer, and effect of different ionic environments on the conformational changes of the protein. The ultrafiltration fluxes are correlated by a model based on the membrane resistance and the time-dependent resistance of the concentration polarization layer of the protein. The values of a mass transfer coefficient and concentration polarization were determined. Zeta potential of the membranes were also determined before and after the UF. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1117–1130, 1999

Static protein adsorption, ultrafiltration behavior and cleanability of hydrophilized polysulfone membranes

A systematic study on the influence of incrementally increased hydrophilicity of UF membranes made from chemically modified polysulfone on static protein adsorption, protein ultrafiltration performance and cleanability was performed. Hydrophilicity was imparted to the polymeric backbone either by carboxylation or sulfonation. Membranes with 100% retention towards sulfhydryl modified bovine serum albumin (cys-BSA) were made by the phase inversion process from unmodified polysulfone, five carboxylated polysulfones (degrees of carboxylation ranging from 0.26 to 1.74) and two sulfonated polysulfones (degrees of sulfonation of 0.24 and 0.58). 1000 ppm aqueous solutions of cys-BSA at pH values 3, 4.8 and 9 were used in KCl medium at either low or high concentration. Static adsorption, ultrafiltration (UF) flux reduction and protein binding strength were lower at pH 3 than at pH 9 but reached an expected maximum at the protein's isoelectric point at pH 4.8. Hydrophilized polysulfone membranes show less static adsorption and lower UF flux reduction than hydrophobic unmodified membranes. In all experiments we found that the higher the degree of carboxylation or sulfonation, the greater this effect. High ionic strengths reduced these advantages at pH values above or below the IEP but had a positive effect at the IEP.