Pulsed-electric-field crossflow ultrafiltration of bovine serum albumin

A conventional crossflow ultrafiltration (CUF) apparatus was modified by the inclusion of electrodes which permitted a pulsed electric field to be produced across the ultrafiltration membrane (PEF-UF process). Using this apparatus, a discontinuous electrophoretic velocity was imposed upon the proteins being concentrated, opposing their convective movement toward the CUF membrane. This resulted in a lower concentration of rejected solute protein in the fluid boundary layer adjacent to the high-pressure side of the membrane and, hence, in a lower solute-related filtration resistance than in the case of conventional ultrafiltration (zero electric field). Studies of the PEF-UF process with bovine serum albumin (BSA) in the range of 0.5–5% w/v demonstrated a 25–40% decrease in the solute-related resistance to the permeate flux compared to the case of a zero electric field. Accordingly, higher permeate fluxes and, therefore, higher rates of concentration of the protein solution were obtained than for conventional crossflow ultrafiltration. When the electric field was reimposed following a period of operation under conventional CUF conditions, the permeate flux could be restored to nearly the same higher value observed initially for the PEF-UF process.     

Fouling reduction in poly(acrylonitrile-co-acrylamide) ultrafiltration membranes

Ultrafiltration membranes with similar pore sizes were prepared from acrylonitrile homopolymer and copolymers with increasing acrylamide content. The membranes containing acrylamide were more hydrophilic, had a smaller dispersion force component of the surface energy, and a smaller negative zeta potential than those prepared from the homopolymer. The effect of the differing surface chemistry of these membranes with similar pore sizes was examined by studying the ultrafiltration of bovine serum albumin (BSA) as a function of feed pH. The hydrophilic membranes showed higher permeate fluxes and flux recoveries than the hydrophobic membrane, in spite of their reduced repulsive electrostatic interaction. With increasing pH, protein transmission increased markedly for the acrylamide containing membranes whereas the transmission through the hydrophobic membrane remained low. These rejection data are explained by the combined effects of the increased hydrophilicity, decreased dispersive surface energy and reduced electrostatic repulsion of the acrylamide containing membranes.     

Effect of ionic strength and protein concentration on the transport of proteins through chitosan/polystyrene sulfonate multilayer membrane

The influence of ionic strength and protein concentration on the transport of bovine serum albumin (BSA), ovalbumin and lysozyme through chitosan (CHI)/polystyrenesulfonate (PSS) multilayers on polyether sulfone supports are investigated under ultrafiltration conditions. The percentage transmission and flux of BSA, ovalbumin and lysozyme were found to increase with increase in salt concentration in the protein. The percentage transmission of BSA through 9 bilayer membrane was found to increase from 5.3 to 115.6 when the salt concentration was varied from 0 to 1 M. It was observed that 0.1 M NaCl in BSA solution is capable of permeating all the BSA. When the salt concentration in BSA was further increased, a negative solute rejection (solute enrichment in permeate) was found to take place. With 9 bilayer membrane, the percentage transmission of ovalbumin was found to increase from 23.3 to 125.8 when the salt concentration in protein was increased from 0 to 0.05 M. The effect of protein concentration on protein transport is studied taking BSA as a model protein. BSA was rejected by the multilayer membrane at all the studied concentrations (0.25, 0.5, 1 and 2 mg/ml). With increase in feed concentration, maximum rejection of protein occurred at higher number of CHI/PSS bilayers. BSA solution flux was found to decrease with an increase in BSA concentration. This study indicates that it is possible to fine tune the transport properties of proteins through multilayer membranes by varying the concentration and ionic strength of protein solutions.     

Fractionation of BSA and lysozyme using ultrafiltration: effect of pH and membrane pretreatment

Selective transmission of a solute through membranes proves to be a challenge in ultrafiltration processes. This is because the transport of a solute through an ultrafiltration membrane does not depend on size alone, but on several other factors such as solute-solute and solute-membrane interactions. By manipulating physicochemical parameters and process variables (eg. pH, ionic strength, concentration of solute, etc.) and by membrane modification, it is possible to enhance the transmission of a particular solute and thus enhance fractionation of solutes. In this paper, the effect of pH on fractionation of BSA and lysozyme by ultrafiltration through 50 kDa MWCO (molecular weight cut off) polysulfone membrane has been examined. It was found that the selectivity of solute separation for dilute mixtures of BSA and lysozyme was very much pH dependent and varied from 3.3 at pH 5.2 to 220.0 at pH 8.8. However, at a higher feed concentration, the transmission of lysozyme through polysulfone membrane decreases quite dramatically resulting in lower throughput of product. An attempt has been made to enhance the transmission of lysozyme through the polysulfone ultrafiltration membrane by pretreating the surface of the membrane by adsorption of another protein, myoglobin. An increase in lysozyme transmission of up to 63% with respect to native membrane was observed. The stability of this pretreatment and its effect on permeate flux have been examined. The pretreated membrane was used to fractionate BSA/lysozyme mixtures. Even at higher feed concentration, enhanced fractionation with respect to native membrane was observed due to highly enhanced transmission of lysozyme through the pretreated membrane.     

Prediction of dynamic permeate flux during cross-flow ultrafiltration of polyethylene glycol using concentration polarization-gel layer model

A tubular ultrafiltration model which couples the formation of a cake layer on the membrane surface and the presence of a polarized layer above the cake has been developed, which contains a single constant and the cake layer resistance to be evaluated from experiments. In the model, the tangential flow of feed material is assumed to induce a shearing effect on the cake layer resulting in the re-entrainment the particles into the bulk stream. The validity of the model over a range of cross-flow velocity, transmembrane pressure (TMP) and solute concentration was confirmed using experimental permeate fluxes obtained from the ultrafiltration of polyethylene glycol. Excellent prediction is observed for solute concentrations above some critical value at which a well developed cake layer is believed to have been formed. For concentrations below this value, the model under predicted the steady-state permeate fluxes. By ignoring the presence of the polarized layer, the model always over predict the dynamic fluxes.     

Lactic acid recovery from cheese whey fermentation broth using combined ultrafiltration and nanofiltration membranes

The separation of lactic acid from lactose in the ultrafiltration permeate of cheese whey broth was studied using a cross-flow nanofiltration membrane unit. Experiments to test lactic acid recovery were conducted at three levels of pressure (1.4, 2.1, and 2.8 MPa), two levels of initial lactic acid concentration (18.6 and 27 g/L), and two types of nanofiltration membranes (DS-5DK and DS-5HL). Higher pressure caused significantly higher permeate flux and higher lactose and lactic acid retention (p<0.0001). Higher initial lactic acid concentrations also caused significantly higher permeate flux, but significantly lower lactose and lactic acid retention (p<0.0001). The two tested membranes demonstrated significant differences on the permeate flux and lactose and lactic acid retention. Membrane DS-5DK was found to retain 100% of lactose at an initial lactic acid concentration of 18.6 g/L for all the tested pressures, and had a retention level of 99.5% of lactose at initial lactic acid concentration of 27 g/L when the pressure reached 2.8 MPa. For all the test when lactose retention reached 99–100%, as much as 64% of the lactic acid could be recovered in the permeate.     

Effect of surfactant agents and lipids on optical resolution of amino acid by ultrafiltration membranes containing bovine serum albumin

Ultrafiltration experiments for the optical resolution of racemic phenylalanine were performed in a solution system containing bovine serum albumin (BSA) and surfactant agents (Triton X-100, Tween 20, sodium dodecyl sulfate), lipid (phosphaticylcholine) and fatty acid (palmitic acid sodium salt). It was found that -phenylalanine preferentially existed in the permeate at pH 7.0 due to the binding of BSA to -phenylalanine in the feed and that the separation factors (=concentration ratio of -isomer to -isomer in the permeate) increased with a decrease in the BSA solution containing no additives and in the BSA solution containing Triton X-100 or Tween 20. The unusual tendency that the separation factors were less than unity was observed and the separation factors decreased with a decrease in the feed concentration of phenylalanine during the ultrafiltration containing the palmitic acid sodium salt or the phosphatidylcholine. This is caused by the fact that the binding constants of -phenylalanine to BSA are higher than those of -phenylalanine in the BSA solution containing the palmitic acid sodium salt or phosphatidylcholine. Since there were found conformational changes of BSA in the presence of palmitic acid sodium salt based on circular dichroism measurements of BSA solution, the conformational changes of BSA were attributed to the higher affinity of -phenylalanine to BSA than that of -phenylalanine in the BSA solution containing the palmitic acid sodium salt or phosphatidylcholine.     

Microfiltration of protein mixtures and the effects of yeast on membrane fouling

The effects of yeast cells on membrane fouling by a protein mixture were studied in dead-end filtration. A 0.2 μm cellulose acetate membrane was used with a 1 g/l protein mixture consisting of equal amounts of bovine serum albumin, lysozyme, and ovalbumin. Yeast cells were used either in suspension or as preformed yeast cakes on top of the membrane. A small concentration of 0.022 g/l yeast cells in suspension enhanced the permeate flux and maintained protein transmission at nearly 100%, compared with a 60% reduction in the protein concentration in the permeate obtained after 3 h for the protein mixture filtered alone. Higher suspended yeast concentrations of 0.043 and 0.18 g/l resulted in lower fluxes and intermediate values for the protein transmission. For the three different thicknesses of preformed yeast cakes studied (0.025, 0.05, and 0.10 cm), the cake with intermediate thickness resulted in protein transmission of nearly 100% and the highest permeate flux. The thinner yeast cake resulted in a lower permeate flux, but it maintained protein transmission at nearly 100%, whereas the thicker cake resulted in a reduction in both permeate flux and protein transmission. The mechanism proposed to explain the results is based on the formation of a secondary membrane by the yeast cells on top of the original membrane. This secondary membrane entraps protein aggregates, which would otherwise cause membrane fouling and reductions in permeate flux and protein transmission.     

Conversion of sodium lactate to lactic acid with water-splitting electrodialysis

The conversion of sodium lactate to lactic acid with water-splitting electrodialysis was investigated. One way of reducing the power consumption is to add a conductive layer to the acid compartment. Doing this reduced the power consumption by almost 50% in a two-compartment cell, whereas the electric current efficiency was not affected at all. Three different solutions were treated in the electrodialysis unit: a model solution with 70 g/L of sodium lactate and a fermentation broth that had been prefiltered two different ways. The fermentation broth was either filtered in an open ultrafiltration membrane (cut-off of 100,000 Dalton) in order to remove the microorganisms or first filtered in the open ultrafiltration membrane and then in an ultrafiltration membrane with a cut-off of 2000 Dalton to remove most of the proteins. The concentration of sodium lactate in the fermentation broth was 70 g/L, as well. Organic molecules present in the broth (peptides and similar organic material) fouled the membranes and, therefore, increased power consumption. Power consumption increased more when permeate from the more open ultrafiltration membrane was treated in the electrodialysis unit than when permeate from the membrane with the lower cut-off was treated, since there was a higher amount of foulants in the former permeate. However, the electrodialysis membranes could be cleaned efficiently with a 0.1 M sodium hydroxide solution.     

Prediction of permeate flux during osmotic pressure-controlled electric field-enhanced cross-flow ultrafiltration

Electric field-enhanced cross-flow ultrafiltration has been carried out to separate protein, bovine serum albumin, from aqueous solution using a 30,000 molecular weight cutoff membrane. A theoretical model is developed to predict permeate flux under a laminar flow regime including the effects of external d.c. electric field and suction through the membrane for osmotic pressure-controlled ultrafiltration. The governing equations of the concentration profile in the developing mass transfer boundary layer in a rectangular channel are solved using a similarity solution method. The effect of d.c. electric field on the variation of membrane surface concentration and permeate flux along the length of the channel is quantified using this model. The expression of Sherwood number relation for estimation of mass transfer coefficient is derived. The analysis revealed that there is a significant effect of electric field on the mass transfer coefficient. A detailed parametric study has been carried out to observe the effect of feed concentration, electric field, cross-flow velocity, and pressure on the permeate flux. For 1 kg/m3 BSA solution, by applying a d.c. electric field of 1000 V/m, the permeate flux increases from 42 to 98 L/m2 h compared to that with zero electric field. The experimental results are successfully compared with the model predicted results.     

Micellar Enhanced Ultrafiltration of Reactive Black 5 from Aqueous Solution by Cationic Surfactants

Reactive black 5 (RB-5) dye was removed from a water stream using two cationic surfactants, cetyltrimethylammonium bromide (CTAB) and cetylpyridinium chloride (CPC), via micellar enhanced ultrafiltration. Three membranes with different pore size were used for the determination of rejection coefficient and permeate flux of the solution at 1.5 bar trans-membrane pressure (TMP). The two surfactants (CPC and CTAB) played an almost negligible role in rejection efficiency with 5000 and 10,000 molecular weight cut-off membrane (MWCO), respectively. In this case, high rejection and low permeate flux was the result of a larger molecular size of RB-5 DYE being retained by comparatively smaller sized pores of membrane via ultrafiltration. However, CPC and CTAB surfactants showed 83% and 98% rejection coefficient, respectively, at a concentration greater than their CMC values against 30,000 MWCO. Permeate flux remained low and constant in presence of 5000 and 10,000 MWCO with a small variation against 30,000 MWCO for the two surfactants, thereby no appreciable effect on both surfactant concentrations on concentration polarization was estimated. Thus, RB-5 dye alone was determined to be responsible for membrane plugging or concentration polarization and ultimately for low permeate flux. The effect of trans-membrane pressure was also investigated during this study.     

Secondary membranes for flux optimization in membrane filtration of biologic suspensions

We employ in situ deposited secondary membranes of yeast (SMYs) to optimize permeate flux during microfiltration and ultrafiltration of protein solutions. The deposited secondary membrane was periodically removed by backflushing, and a new cake layer was deposited at the start of the next cycle. The effects of backflushing time, backflushing strength, wall shear rate, and amount of secondary membrane deposited on the permeate flux were examined. Secondary membranes were found to increase the permeate fluxin microfiltration by severalfold. Protein transmission was also enhanced owing to the presence of the secondary membrane, and the amount of protein recovered was more than twice that obtained during filtration of protein-only solutions under othewise identical conditions. In ultrafiltration, the flux enhancement owing to the secondary membrane was only 50% or less. In addition, the flux for ultrafiltration was relatively insensitive to changes in the concentration of yeast used during deposition of SMY and to the backflushing strength used to periodically remove the secondary membrane.     

A constant flux based mathematical model for predicting permeate flux decline in constant pressure protein ultrafiltration

This paper discusses a novel approach for predicting permeate flux decline in constant pressure ultrafiltration of protein solutions. A constant pressure process is assumed to be made up of a large number of small, sequential, constant flux ultrafiltration steps: the flux decreasing due to fouling and other related factors at the end of each step. The advantage of this approach is that constant flux ultrafiltration is easier to study, characterize, and model than constant pressure ultrafiltration. Consequently model parameters can be obtained in reliable and reproducible manner. Constant pressure ultrafiltration is dynamic in nature since both the magnitude of osmotic back-pressure and the extent of membrane fouling decrease as the permeate flux decreases with time. The proposed model takes into consideration the interplay between permeate flux, concentration polarization, and membrane fouling. The model demonstrates that the initial rapid flux decline is due to a combination of concentration polarization and membrane fouling while during the remaining part of the process, the effect of concentration polarization becomes negligible. The model also shows that concentration polarization affects the initial flux decline only at higher transmembrane pressures. This model which was validated using experimental data is conceptually simpler than other available models and easy to use. In addition to its value as a predictive tool it would particularly be useful for deciding appropriate start-up conditions in ultrafiltration processes.     

Ultrafiltration of BSA in pulsating conditions: an artificial neural networks approach

The aim of the present paper is to analyze membrane systems behavior, operating in pulsating conditions, by means of artificial neural networks (ANNs). Different ANNs have been developed, by means of Matlab^® Neural Network Toolbox, to model the ultrafiltration process of aqueous BSA solutions through poly-ethersulfone membranes. A specific neural network architecture, constituted by one input layer, two hidden layers and one output layer, has been finally identified by a trial-and-error procedure. The network has been trained through a selected set of experimental data obtained for a lab-scale flat sheet membrane module, equipped with a device capable of producing periodic pulses of the applied trans-membrane pressure (TMP) and feed flow rate. It has been found that the developed neural network is capable of offering very accurate predictions of actual system behavior either when it is tested within the range used for training or when the inputs combination has been never exploited during learning phase. The observed reliability of neural networks predictions of membrane performances has suggested to use them for searching an optimal pulsation frequency profile able to maximize permeate flux. The utilization of such a pulse frequency profile allows obtaining, on the basis of theoretical evaluations only, significant improvements of membrane performances with respect to UF experiments performed at fixed and constant pulsation frequencies.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

Influence of permeate flux on fouling during the microfiltration of β-lactoglobulin solutions under cross-flow conditions

Experiments to investigate the microfiltration fouling behaviour of a β-lactoglobulin solution were performed on a constant-flux, computer-controlled, cross-flow membrane rig equipped with zirconium oxide membranes. Fouling was dependent upon the permeate flux, being light at low flux (50 l/m² h) and severe at high flux (200 l/m² h). The fouling increased in severity as the flux was increased from 50 to 200 l/m² h. At 50 l/m² h, protein transmissions of>90% were observed. At higher fluxes, the protein transmission decreased with increasing fouling resistance. The relationship between fouling resistance and protein transmission was similar for 50 and 100 nm membranes and was independent of the starting permeate flux. Standard poreplugging and pore-narrowing models did not describe the observed behaviour. Development of a model to predict protein transmission from the fouling resistance indicated that fouling occurred only in a small part of the membrane pore, most likely at the pore entrance. It is proposed that the microfiltration pore acts in a way similar to a pressure-relief valve where shear-induced protein denaturation has been observed. Shear forces on the protein perhaps lead to protein denaturation and aggregation, and narrowing of the pore in the immediate vicinity of the pore entrance.     

Performance improvement of polysulfone ultrafiltration membrane by blending with polyaniline nanofibers

Polyaniline (PANI) nanofibers were used to improve hydrophilic property and permeability of polysulfone (PS) membrane. PS membrane and PS/PANI nanofibers blended membranes with different PANI–PS mass ratios (1, 5, 10, and 15 wt.%) were prepared by phase inversion process. The blended membranes showed similar bovine serum albumin (BSA) and albumin egg (AE) rejections to PS membrane. The blended membranes had larger porosity and better hydrophilic property than PS membrane, which caused the improvement of their permeability. Pure water fluxes of the blended membranes with PANI–PS mass ratios of 1 and 15 wt.% were 1.6 and 2.4 times that of PS membrane, respectively. During the filtration of BSA solution, the blended membranes had slower flux decline rate than PS membrane. Moreover, stable permeate fluxes of the blended membranes with PANI–PS mass ratios of 1 and 15 wt.% were 2.0 and 2.5 times that of PS membrane, respectively. Compared with PS membrane, mechanical property and thermal stability of the blended membranes with less PANI–PS mass ratio, e.g. 1 wt.%, had no obvious change. For the blended membrane with PANI–PS mass ratio of 15 wt.%, breaking strength increased 28% and elongation at break decreased 30.6%.     

Reversibility of fouling formed in activated sludge filtration

This paper investigates the reversibility of membrane fouling by activated sludge in a membrane bioreactor equipped with a 0.1 μm pore ceramic membrane. The membrane was submitted to a series of tests in which the permeate flux, the transmembrane pressure (TMP) or the circulation velocity were successively varied in cycles by step increments or decreases. When the permeate flux is set below the critical flux, the TMP remains stable and fouling is reversible. On the contrary, when the critical flux is exceeded, the TMP increases and does not stabilize, as in dead-end filtration. The fouling formed is partly irreversible when the flux is lowered again. When the TMP is first increased up to 400 kPa and then decreased back at constant velocity, no hysteresis is found on the flux–TMP graph, showing that fouling is reversible in this case. Velocity cycles were performed by first lowering the velocity from 5 to 1 m/s and raising it again to 5 m/s. In this case again, the fouling induced by reducing the velocity was found to be reversible. However, when the same pressure and velocity cycle tests were performed with activated sludge collected in the aeration tank of a classical wastewater treatment plant, fouling was found to be partly irreversible, showing that the cake formed in the absence of shearing is much more cohesive. In the final part of the paper, we tested a hydrodynamic method of fouling control consisting in alternating short periods of filtration (1–4 s) and short periods of washing (1 or 2 s) at low TMP and high velocity. This method yielded to a 20% permeate flux increase with a 10% reduction in hydraulic energy consumption for classical plant activated sludge.     

Membrane fouling in the ultrafiltration of polyelectrolyte solutions: Polyacrylic acid and bovine serum albumin

Batch ultrafiltration was used to study the reduction in membrane permeability due to fouling by polyelectrolyte solutes. The flexibility of the polymer chain was found to be an important factor in determining the degree of fouling in the ultrafiltration of poly(acrylic acid) (PAA) solutions, as the effects observed were dependent on the pH and the cation concentration. These parameters were found to have a less pronounced effect in the ultrafiltration of bovine serum albumin (BSA) solutions. An observation of the effects of temperature and applied pressure led to the conclusion that membrane fouling by PAA was due to a pore-plugging effect while that caused by BSA was attributed to hydrophobic adsorption.     

Influence of protein–protein interactions on bulk mass transport during ultrafiltration

Bulk mass transfer limitations can have a significant effect on the flux and selectivity during membrane ultrafiltration. Most previous studies of these phenomena have employed the simple stagnant film analysis, but this model is unable to account for the effects of solute–solute interactions on mass transport. We have developed a generalized framework for multicomponent mass transfer that includes both thermodynamic and hydrodynamic (frictional) interactions. Thermodynamic (virial) coefficients were evaluated from osmotic pressure data for albumin (BSA) and immunoglobulins (IgG), while hydrodynamic interaction parameters were determined from filtrate flux data obtained in a stirred cell using fully retentive membranes. The protein concentration profiles in the bulk solution were evaluated by numerical solution of the governing continuity equations incorporating the multicomponent diffusive flux. This model was used to analyze flux and protein transmission data obtained for the filtration of BSA and IgG mixtures through partially permeable membranes. The model accurately predicted the large reduction in flux and BSA transmission upon addition of IgG. These effects were due to the coupling between BSA and IgG mass transfer caused by protein–protein interactions.