Colloidal interactions and fouling of NF and RO membranes: a review

Colloids are fine particles whose characteristic size falls within the rough size range of 1-1000 nm. In pressure-driven membrane systems, these fine particles have a strong tendency to foul the membranes, causing a significant loss in water permeability and often a deteriorated product water quality. There have been a large number of systematic studies on colloidal fouling of reverse osmosis (RO) and nanofiltration (NF) membranes in the last three decades, and the understanding of colloidal fouling has been significantly advanced. The current paper reviews the mechanisms and factors controlling colloidal fouling of both RO and NF membranes. Major colloidal foulants (including both rigid inorganic colloids and organic macromolecules) and their properties are summarized. The deposition of such colloidal particles on an RO or NF membrane forms a cake layer, which can adversely affect the membrane flux due to 1) the cake layer hydraulic resistance and/or 2) the cake-enhanced osmotic pressure. The effects of feedwater compositions, membrane properties, and hydrodynamic conditions are discussed in detail for inorganic colloids, natural organic matter, polysaccharides, and proteins. In general, these effects can be readily explained by considering the mass transfer near the membrane surface and the colloid-membrane (or colloid-colloid) interaction. The critical flux and limiting flux concepts, originally developed for colloidal fouling of porous membranes, are also applicable to RO and NF membranes. For small colloids (diameter?100 nm), the limiting flux can result from two different mechanisms: 1) the diffusion-solubility (gel formation) controlled mechanism and 2) the surface interaction controlled mechanism. The former mechanism probably dominates for concentrated solutions, while the latter mechanism may be more important for dilute solutions. Future research needs on RO and NF colloidal fouling are also identified in the current paper.     

Influence of membrane structure on fouling layer morphology during apple juice clarification

The flux behavior of 0.2 μm nylon, polysulfone (PS), polyvinylidene fluoride (PVDF) and polyethersulfone (PES) membranes was examined during dead-end microfiltration of commercial apple juice. On nylon membranes, a 0.1 μm thick surface fouling layer rapidly formed that acted as a secondary membrane. The colloidal particles retained by this surface layer aggregated to form a thick loose gel structure, producing an anisotropic fouling structure. In contrast, the 4 μm thick surface fouling layer of PES was slower to form and had a more open structure with a lower flux resistance per unit thickness. The morphology of the PES surface layer also did not differ dramatically from the loose gel structure that subsequently formed on top of this secondary membrane. The PS surface fouling layer was similar in structure to nylon whereas the PVDF layer more closely resembled that found with PES. The density of the surface fouling layer did not directly correlate to membrane surface hydrophobicity or pure water flux. Atomic force microscopy (AFM) indicated that surface roughness strongly influenced surface fouling layer morphology. The membrane surface appears to act as a template for the fouling process; therefore, smooth membranes (nylon and PS) produce a dense surface fouling layer whereas this same layer on rough membranes (PES and PVDF) is much more open. Consequently, the fluxes of PES and PVDF membranes are less affected by fouling formation.     

Pore blocking mechanisms during early stages of membrane fouling by colloids

A method based on a simple linear regression fitting was proposed and used to determine the type, the chronological sequence, and the relative importance of individual fouling mechanisms in experiments on the dead-end filtration of colloidal suspensions with membranes ranging from loose ultrafiltration (UF) to nanofiltration (NF) to non-porous reverse osmosis (RO). For all membranes, flux decline was consistent with one or more pore blocking mechanisms during the earlier stages and with the cake filtration mechanism during the later stages of filtration. For ultrafiltration membranes, pore blocking was identified as the largest contributor to the observed flux decline. The chronological sequence of blocking mechanisms was interpreted to depend on the size distribution and surface density of membrane pores. For salt-rejecting membranes, the flux decline during the earlier stages of filtration was attributed to either intermediate blocking of relatively more permeable areas of the membrane skin, or to the cake filtration in its early transient stages, or a combination of these two mechanisms. The findings emphasize the practical importance of the clear identification of, and differentiation between mechanisms of pore blocking and cake formation as determining the potential for the irreversible fouling of membranes and the efficiency of membrane cleaning.     

Influence of membrane fouling by (pretreated) surface water on rejection of pharmaceutically active compounds (PhACs) by nanofiltration membranes

The effects of surface water pretreatment on membrane fouling and the influence of these different fouling types on the rejection of 21 neutral, positively and negatively charged pharmaceuticals were investigated for two nanofiltration membranes. Untreated surface water was compared with surface water, pretreated with a fluidized anionic ion exchange and surface water, pretreated with ultrafiltration. Fouling the nanofiltration membranes with anionic ion exchange resin effluent, resulted in the deposition of a mainly colloidal fouling layer, with a rough morphology. Fouling the nanofiltration membranes with ultrafiltration permeate, resulted in the deposition of a smooth fouling layer, containing mainly natural organic matter. The fouling layer on the nanofiltration membranes, caused by the filtration of untreated surface water, was a combination of both colloids and natural organic matter.Rejection of pharmaceuticals varied the most for the membranes, fouled with the anionic ion exchange effluent, and variations in rejection were caused by a combination of cake-enhanced concentration polarisation and electrostatic (charge) effects. For the membranes, fouled with the other two water types, variations in rejection were smaller and were caused by a combination of steric and electrostatic effects.Changes in membrane surface hydrophobicity due to fouling, changed the extent of partitioning and thus the rejection of hydrophobic, as well as hydrophilic pharmaceuticals.     

Polysilicato-iron for improved NOM removal and membrane performance

The natural organic matter (NOM) removal efficiency of polysilicato-iron (PSI) coagulants and the fouling potential of PSI pretreated waters have been studied using two microfiltration (MF) membrane types: polyvinylidene fluoride (PVDF-2) and polypropylene (PP). The results showed that PSI coagulant with a Si/Fe ratio of 1 (PSI-1) was the most effective, compared to conventional coagulants, in removing dissolved organic carbon (DOC) and in improving the fouling potential. A relative flux of unity through PVDF-2 membrane was achieved for both water sources pretreated with PSI-1.

Aluminium-based coagulants, particularly aluminium chlorohydrate (ACH), worked best at lower coagulant dose. Increasing the coagulant dose to improve DOC removal led to increased membrane fouling, possibly due to increased level of unsettleable flocs and pore blocking. For PSI with larger floc size, the advantage of increased DOC removal was not overridden by the adverse effect of pore blocking. In addition, the residual neutral fraction in the waters and/or the presence of a filter cake on the membrane surfaces seemed to have a limiting effect on the fouling rates through both PP and PVDF-2 membranes to the extent that similar rates were obtained, despite substantial differences in DOC removal.

In contrast, these limiting factors did not influence the fouling potential of PSI-1 treated waters through the PVDF-2 membrane, as suggested by the relative flux of unity for both water sources. It is suggested that the oxide deposits on the PVDF-2 membrane may act as a ‘screening layer’, acting as pre-filtration by the filter cake. This layer may be effectively removed by backwashing, together with deposited NOM, throughout the experiment to maintain the flux at unity. The hydrophobic nature of the PP membrane may discourage the deposition of the oxides, thus minimising the positive effects of the oxides in the system. The high removal of hydrophobic fractions by PSI-1 may also lead to less association between residual NOM and less binding to the membranes, particularly on the PVDF-2 membrane.     

Formation of polysulfone colloids for adsorption of natural organic foulants

An ever-present problem in the use of commercial membranes for treatment of drinking water is fouling of the membranes by natural organic matter (NOM). This work describes a new approach to elimination or minimization of membrane fouling by NOM. When a 2% solution of polysulfone in NMP and propionic acid is slowly injected into water, approximately 50 nm polysulfone particles are spontaneously formed, and these hydrophobic particles quickly coagulate into approximately 12-microm diameter aggregates; the formed material has a surface area of approximately 100 m(2)/g and an equivalent "pore" size of 25 nm. When 50 mg/L of the new material is equilibrated with a local drinking water supply, virtually all adsorptive fouling of a 20-kDa molecular weight cutoff ultrafiltration membrane is eliminated. Interestingly, although only a very small percentage of the NOM is removed by adsorption on the polysulfone aggregates, it appears that exactly this small NOM component is responsible for nearly all of the membrane fouling. This paper describes the fabrication and characterization of the new polysulfone adsorbent and offers an hypothesis for the formation of the product via spontaneous emulsification and spinodal decomposition.     

Enhancing the antifouling property of polyethersulfone ultrafiltration membranes through surface adsorption-crosslinking of poly(vinyl alcohol)

An adsorption-crosslinking process of poly(vinyl alcohol) (PVA) was introduced to modify the surface of polyethersulfone (PES) ultrafiltration membranes for enhancement of their antifouling property. XPS and water contact angle measurement confirmed the obvious enhancement of surface hydrophilicity. Ultrafiltration results showed that the spreading of PVA chains over the hydrophobic membrane surface caused substantial but acceptable decrease on membrane flux. The fouling type analysis indicated that PVA adsorption effectively improved the antifouling property of PES membranes. With a PVA concentration of 0.5 wt% and three cycles of alternative adsorption-crosslinking, the total and irreversible fouling ratio of modified membranes were 0.38 and 0.22, respectively, much lower than those of control PES membrane (0.61 and 0.47), and the flux recovery ratio was increased accordingly. The long-term ultrafiltration experiment demonstrated the improvement of recycling property and the reliability of adsorption-crosslinking process.     

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.     

Chemical and physical aspects of organic fouling of forward osmosis membranes

The growing attention to forward osmosis (FO) membrane processes from various disciplines raises the demand for systematic research on FO membrane fouling. This study investigates the role of various physical and chemical interactions, such as intermolecular adhesion forces, calcium binding, initial permeate flux, and membrane orientation, in organic fouling of forward osmosis membranes. Alginate, bovine serum albumin (BSA), and Aldrich humic acid (AHA) were chosen as model organic foulants. Atomic force microscopy (AFM) was used to quantify the intermolecular adhesion forces between the foulant and the clean or fouled membrane in order to better understand the fouling mechanisms. A strong correlation between organic fouling and intermolecular adhesion was observed, indicating that foulant–foulant interaction plays an important role in determining the rate and extent of organic fouling. The fouling data showed that FO fouling is governed by the coupled influence of chemical and hydrodynamic interactions. Calcium binding, permeation drag, and hydrodynamic shear force are the major factors governing the development of a fouling layer on the membrane surface. However, the dominating factors controlling membrane fouling vary from foulant to foulant. With stronger intermolecular adhesion forces, hydrodynamic conditions for favorable foulant deposition leading to cake formation are more readily attained. Before a compact cake layer is formed, the fouling rate is affected by both the intermolecular adhesion forces and hydrodynamic conditions. However, once the cake layer forms, all three foulants have very similar flux decline rates, and further changes in hydrodynamic conditions do not influence fouling behavior.     

Fouling mechanism of separator membranes for the iron/chromium redox battery

The NASA chromium/iron redox battery being developed for photovoltaic and load-levelling storage applications uses an anionic permselective membrane to keep the reactants separate while providing electrical continuity. The membrane resistance increases as a function of time when exposed to a ferric chloride solution. The resistance increase termed “fouling” is related to the ability of the ferric ion to form ferric chloride complexes which are not electrically repelled by the anionic membrane. Electron microprobe analyses show that fouling is associated with an uptake of iron that approaches a uniform distribution across the membrane thickness. Since the iron does not cause fouling by covering the surface of the membrane, alternative explanations were considered. The amount of water in the membrane was found to decrease when the membrane became fouled. Percolation theory was used to relate the resistance of fouled membranes to the volume fraction of water. The fit was excellent and the fitting parameters were physically reasonable. The water loss is caused by an osmotic effect from placing the membrane in a ionic solution and a displacement effect when the ferric chloride complex enters the membrane.     

Novel ultrafiltration membranes with the least fouling properties for the treatment of veterinary antibiotics in the pharmaceutical wastewater

Dendrimers have received more attention in all fields of research these days. In the present study, polyamidoamine (PAMAM) dendrimers were synthesized on the acrylic ultrafiltration membranes to minimize fouling as an important deficiency in the separation process. The antifouling activity of these dendrimers with different generations (G0‐3) was tested to restrict three macrolides (tylvalosin, tylosin, and tulathromycin) and two pleuromutilins (tiamulin and valnemulin) as veterinary antibiotic drugs with amine groups and positive charges at pH = 7 of the membrane surface. These compounds are risky for human consumption. Due to having several amine functional groups and branches, PAMAM dendrimers can be a great coating agent for antifouling. G3 PAMAM dendrimer‐coated membranes had the best performance (water flux: 130.7 L/m²·h, rejection of tulathromycin: 91.4%, flux recovery ratio: 86.3%). The function of this ultrafiltration process depended on pore size and also charge surface. A significant reduction for irreversible and reversible fouling was observed for this new ultrafiltration membrane (F_ir: 14.5%, F_re: 21.9%). This observation was confirmed by the power law model. Three 5‐hour cycle ultrafiltration processes were carried out for veterinary antibiotic wastewater that showed 3.18% loss of initial water flux (for the third cycle), final cleaning efficiency of 96.82%, and tylvalosin rejection of 94.1%.     

Blocking of capillaries as fouling mechanism for dead-end ultrafiltration

In this paper plugging of capillaries in the potting is investigated. A lot of research has been done on fouling of the membrane surface (pore blocking, cake filtration) but research on other types of membrane fouling like plugging of capillaries is not so common. Experiments were performed with a lab-scale test installation under constant flux conditions with synthetic feed water containing ferric hydroxide flocks as a fouling component. The experiments showed that during operation capillaries became blocked by fouling plugs. The presence of blockages, especially in the potting at the concentrate side of the capillaries, could not be detected by measuring the clean water resistance. However such blockages did result in an increased forward flush pressure. A combination of the clean water resistance and the forward flush pressure is suitable for determining the fouling of a membrane and the effectiveness of a cleaning procedure. The part of the capillaries in the potting is not backwashed and therefore the hydraulic as well as the chemical cleaning is not efficient at this place.     

Gel layer formation and hollow fiber membrane filterability of polysaccharide dispersions

Gel layer formation on the membrane surface during filtration plays a significant role in membrane fouling that, in many instances, controls water production and energy consumption in the treatment of waters and wastewaters. In this study, alginate is selected as a model of the polysaccharides prevalent in wastewaters which, on membrane filtration, may form a gel on the membrane surface which subsequently limits filtrate throughput. We show that over the range of the applied pressures of 11.7–135 kPa considered here, constant pressure ultrafiltration of alginate follows the behavior of cake filtration. The material properties of the alginate are determined by the employment of the previously developed steady-state filtration approach. The consolidation of the gel layer is found to be controlled by the hydraulic flow resistance rather than the rearrangement of particles. Under these conditions, it is valid to apply the derived material properties for the quantification of both constant pressure and constant flux filtration. The gel layer formed from alginate is very compressible and far from uniform over its depth. Within the range of the applied pressures, the gel layer is very porous with a water content of more than 96% but very low Darcy permeability of less than 1 × 10⁻¹⁷ m². During hollow fiber membrane filtration, the local flux is neither uniform nor constant along the fiber length, resulting in non-uniformity of the growth rate, the average porosity and the thickness of the gel layer. The non-uniformity is most apparent at the start of filtration and then gradually diminishes as the gel layer builds up with ongoing filtration.     

Adsorptive fouling of modified and unmodified commercial polymeric ultrafiltration membranes

The fouling tendency, due to adsorption on the pore walls, of two pairs of modified and unmodified ultrafiltration membranes, with similar observed retentions determined by dextran and gel permeation chromatography, was studied. The membranes investigated were made of modified and unmodified polyaramide (PA) and modified and unmodified polyvinylidene fluoride (PVDF). The PVDF membrane was surface-modified and the PA membrane was made from a modified polymer solution. Membrane modification was used to reduce fouling by adsorption. Octanoic acid was used as the fouling substance, representing a large number of small, hydrophobic compounds. It is demonstrated in this investigation that membrane modification is not always successful. It was determined that at lower concentrations of octanoic acid, the modified PA membrane exhibits a smaller fouling tendency than the unmodified PA membrane, while the result is reversed for concentrations above 60% of the saturation concentration. The fouling tendency of the unmodified PVDF membrane is much lower than that of the modified PVDF membrane at all concentrations. The cross-sections of the membranes were visually examined with scanning electron microscopy, but no difference could be observed between the modified and unmodified membranes. The membranes were also examined with Fourier transform infrared spectroscopy. The spectra of the two PA membranes were different, while no difference was observed for the unmodified and surface-modified PVDF membranes. Remains of octanoic acid were found in the membranes, although they had been thoroughly rinsed with deionized water and the initial pure water flux was recovered.     

Modeling of concentration polarization and depolarization with high-frequency backpulsing

Rapid backpulsing to reduce membrane fouling during crossflow microfiltration and ultrafiltration is studied by solving the convection-diffusion equation for concentration polarization and depolarization during cyclic operation with transmembrane pressure reversal. For a fixed duration of reverse filtration, there is a critical duration of forward filtration which must not be exceeded if the formation of a cake or gel layer on the membrane surface is to be avoided. The theory also predicts an optimum duration of forward filtration which maximizes the net flux, since backpulsing at too high of frequency does not allow for adequate permeate collection during forward filtration relative to that lost during reverse filtration, whereas backpulsing at too low of frequency results in significant flux decline due to cake or gel buildup during each period of forward filtration. In general, short backpulse durations, low feed concentrations, high shear rates, and high forward transmembrane pressures give the highest net fluxes, whereas the magnitude of the reverse transmembrane pressure has a relatively small effect.Rapid backpulsing experiments with yeast suspended in deionized water performed with a flat-sheet crossflow microfiltration module and cellulose acetate membranes with 0.07 μm average pore diameter. The optimum forward filtration times were found to be 1.5, 3, and 5 s, respectively, for backpulse durations of 0.1, 0.2, and 0.3 s. Both theory and experiment gave net fluxes with backpulsing of about 85% of the clean membrane flux (0.022 cm/s = 790 l/m² h), whereas the long-term flux in the absence of backpulsing is an order-of-magnitude lower (0.0026 cm/s = 94 l/m² h).     

Analysis of constant permeate flow filtration using dead-end hollow fiber membranes

Dead-end filtration of colloids using hollow fibers has been analysed theoretically and experimentally. A mathematical model for constant flux filtration using dead-end hollow fiber membranes has been developed by combining the Hagen–Poiseuille equation, the (standard) filtration equation, and cake filtration theory of Petsev et al. [D.N. Petsev, V.M. Starov, I.B. Ivanov, Concentrated dispersions of charged colloidal particles: sedimentation, ultrafiltration and diffusion, Colloid Surf. A: Physicochem. Eng. Aspects, 81 (1993) 65–81.] to describe the time dependence of the filtration behavior of hollow fiber membranes experiencing particle deposition on their surface. Instead of using traditional constitutive equations, the resistance of the cake layer formed by the deposited colloids has been directly correlated to the cake structure. This structure is determined by application of a force balance on a particle in the cake layer combined with the assumption that an electrostatically stable cake layer of mono-sized particles would be ordered in a regular packing geometry of minimum energy. The developed model has been used to identify the relationship between the filtration behavior of the hollow fiber membrane and the particle properties, fiber size, and imposed average flux. Filtration experiments using polystyrene latex particles of relatively narrow size distribution with a single dead-end hollow fiber membrane demonstrate good consistency between experimental results and model prediction. The developed model has been used to simulate the distribution of the cake resistance, transmembrane pressure, and flux along the hollow fiber membrane and used to assess the effect of fiber size, particle size, zeta potential, and the average imposed flux on the suction pressure-time profiles, flux, and cake resistance distributions. These results provide new insights into the filtration behavior of the hollow fiber membrane under constant flux conditions.     

Coagulation and ultrafiltration: Understanding of the key parameters of the hybrid process

A hybrid coagulation–ultrafiltration process has been investigated to understand membrane performance. Coagulation prior to ultrafiltration is suspected to reduce fouling by decreasing cake resistance, limiting pore blockage and increasing backwash efficiency. Coagulation followed by tangential ultrafiltration should gather the beneficial effects of particle growth and cross-flow velocity. Our study aims at determining the key parameters to improve membrane performance, by describing floc behaviour during the hollow fibre ultrafiltration process. Flocs encounter a wide range of shear stresses that are reproduced through the utilization of different coagulation reactors. Performing a Jar-test enables the formation of flocs under soft conditions, whereas Taylor-Couette reactors can create the same shear stresses occurring in the hollow fibres or in the pump. Synthetic raw water was made by adding bentonite into tap water. Five organic coagulants (cationic polyelectrolytes) and ferric chloride were selected. Floc growth was thoroughly monitored in the different reactors by laser granulometry. Coagulation–ultrafiltration experiments revealed different process performance. The effect on the permeate flux depended on the coagulant used: some coagulants have no influence on permeate flux, another enables a 20% increase in permeate flux whereas another coagulant leads to a decrease of 50%. Flocs formed with ferric chloride do not resist shear stress and consequently have no influence on permeate flux. These results show the necessity to create large flocs, but the size is not sufficient to explain membrane performance. Even if flocs show a good resistance to shear stress, a high compactness (D_f = 3) will lead to a dramatic decrease of permeate flux by increasing the mass transfer resistance of the cake. On the contrary, flocs less resistant to shear stress, then smaller and also more open have no effect on permeate flux. An optimum was quantified for large flocs, resistant enough to shear stress facilitating flow between aggregates.     

Fabrication of blend hydrophilic polyamide imide (Torlon®)-sulfonated poly (ether ether ketone) hollow fiber membranes for oily wastewater treatment

Blend hydrophilic polyamide imide (PAI)-sulfonated poly (ether ether keton) (SPEEK) hollow fiber membranes were fabricated for oil-water emulsion separation. The structure and performance of the membranes were examined by FESEM analysis, N₂ permeation, overall porosity, collapsing pressure, water contact angle, pure water flux, molecular weight cutoff (MWCO), and oil rejection tests. By studying ternary phase diagrams of polymer/solvent-additive/water system, the higher phase-inversion rate was confirmed for the solutions prepared at higher PAI/SPEEK ratio. A more open structure with larger finger-likes was observed by increasing PAI/SPEEK ratio. Mean pore size of 81 nm, overall porosity of 79% and water contact angle of 58° were obtained for the improved membrane prepared by PAI/SPEEK ratio of 85/15. Increasing SPEEK ratio resulted in lower mechanical stability in terms of collapsing pressure. Pure water flux of about 2.5 times of the plain PAI membrane was found for the improved membrane. MWCO of 460 kDa was found for the improved blend membrane. From oil rejection test, all the membranes demonstrated an oil rejection of over 95%. The improved membrane showed a lower rate of permeate flux reduction compared to the plain membrane which was related to the smaller fouling possibility. Less fouling resistance of the improved membrane was related to the higher flux recovery ratio (about 92%). For all the membranes, the dominant fouling mechanism was found to be the cake filtration. The improved PAI-SPEEK hollow fiber membranes was found to be practical for ultrafiltration of oily wastewaters.     

Development of an ultrasonic technique for in situ investigating the properties of deposited protein during crossflow ultrafiltration

Although an amount of research has reported that a flux minimum occurs at the isoionic/isoelectric points (pH 4.6-5.0) in the absence of salts in the ultrafiltration of bovine serum albumin (BSA), the real mechanism remains incompletely understood due to the lack of additional techniques in real time to detect the properties of deposited BSA (gel) layers formed during ultrafiltration (UF). An ultrasonic technique was developed as an analytical noninvasive tool to in situ investigate the properties of deposited BSA layers at pH 4.9 (isoionic or isoelectric point, IEP) and 6.9 during crossflow ultrafiltration. The membrane was a polysulfone (PSf) UF membrane with molecular weight cut-off (MWCO) 35 kDa. The feed used was 0.5 g/l BSA solution. Results show good correspondence between the ultrasonic signal responses and the development of BSA gel layers on the membranes. The deposit is thicker at pH 6.9 than at pH 4.9. However, the deposited gel layers are more compressible at pH 4.9 than at pH 6.9. The flux decline is mainly controlled by the density (packing) of the deposit layer. At pH 6.9, protein mainly deposits on the membrane surface. Around the isoelectric point, protein absorbs within and on the membranes. A functional relationship between acoustic signals and fouling resistance exists. The fouling resistance is mainly attributed to pore blocking or pore constriction.     

Free-solvent model shows osmotic pressure is the dominant factor in limiting flux during protein ultrafiltration

In protein ultrafiltration (UF), the limiting flux phenomenon has been generally considered a consequence of the presence of membrane fouling or the perceived formation of a cake/gel layer that develops at high operating pressures. Subsequently, numerous theoretical models on gel/cake physics have been made to address how these factors can result in limiting flux. In a paradigm shift, the present article reestablishes the significance of osmotic pressure by examining its contribution to limiting flux in the framework of the recently developed free solvent osmotic pressure model. The resulting free-solvent-based flux model (FSB) uses the Kedem–Katchalsky model, film theory and the free solvent representation for osmotic pressure in its development. Single protein tangential-flow diafiltration experiments (30 kDa MWCO CRC membranes) were also conducted using ovalbumin (OVA, 45 kDa), bovine serum albumin (BSA, 69 kDa), and immuno-gamma globulin (IgG, 155 kDa) in moderate NaCl buffered solutions at pH 4.5, 5.4, 7 and 7.4. The membrane was preconditioned to minimize membrane fouling development during the experimental procedure. The pressure was randomly selected and flux and sieving were determined. The experimental results clearly demonstrated that the limiting flux phenomenon is not dominated by membrane fouling and the FSB model theoretically illustrates that osmotic pressure is the primary factor in limiting flux during UF. The FSB model provides excellent agreement with the experimental results while producing realistic protein wall concentrations. In addition, the pH dependence of the limiting flux is shown to correlate to the pH dependency of the specific protein diffusion coefficient.