A three mechanism model to describe fouling of microfiltration membranes

Mathematical modeling of flux decline during filtration plays an important role in both sizing membrane systems and in the understanding of membrane fouling. Protein fouling is traditionally modeled using one of three classical fouling mechanisms: pore blockage, pore constriction or cake filtration. Here, we have developed a mathematical model to describe flux decline behavior during microfiltration accounting for all three classical fouling mechanisms. Pore constriction was assumed to first reduce the size of internal pores. Pore blockage then occurs at the top of the membrane, preventing further fouling to the interior structure. Finally the foulants at the top of the membrane form a cake, which controls the late stages of the filtration. The model prediction shows excellent agreement with experimental data for 0.25 μm polystyrene microspheres filtered through 0.22 μm Isopore membranes (where pore constriction is expected to be minimal) as well as non-aggregated bovine serum albumin solution through hydrophobic Durapore membranes (where pore constriction is expected to dominate). The effects of different fouling mechanisms on the flux decline were characterized by the ratio of characteristic fouling times of the different mechanisms. In this way the model can provide additional insights into the relative importance of different fouling mechanisms as compared to an analysis by a single mechanism model or by derivative plots, and it can be used to provide important insights into the flux decline characteristics.     

A model for determining the steady state flux of inorganic microfiltration membranes

The present paper provides a model based on dimensional analysis that gives the basis for design of the cross-flow microfiltration processes. This gives the permeate flux f in terms of the pressure drop across the filtration membrane ΔP and the velocity V of cross-flow of the feed fluid in the membrane tubes. The model is compared with an extensive series of experimental results with magnesium hydroxide slurries. The model has certain similarities with previous ones and can be used for unit optimization.     

Regularities of latex filtration through microfiltration membranes

Filtration of suspensions of emulsifier-free monodisperse polystyrene latexes with particle sizes of 0.25, 0.3, and 0.4 μm through acetylcellulose microfilters is studied as depending on the composition of liquid phases, the rate and time of filtration, and the particle-to-pore size ratios. The effect of particle-membrane interactions, which are governed by the electrostatic repulsion and molecular attraction forces, on particle rejection by membranes is considered. It is shown that, when analyzing the mechanism for the rejection, it is necessary to take into account the electrophoretic motion of particles in the field of the streaming potential arising in the course of suspension filtration.     

Layer-by-layer-assembled microfiltration membranes for biomolecule immobilization and enzymatic catalysis

Multilayer assemblies of polyelectrolytes, for protein immobilization, have been created within the membrane pore domain. This approach was taken for two reasons: (1) the high internal membrane area can potentially increase the amount of immobilized protein, and (2) the use of convective flow allows uniform assembly of layers and eliminates diffusional limitations after immobilization. To build a stable assembly, the first polyelectrolyte layer was covalently attached to the membrane surface and inside the pore walls. Either poly(L-glutamic acid) (PLGA) or poly(L-lysine) (PLL) was used in this step. Subsequent deposition occurs by multiple electrostatic interactions between the adsorbing polyelectrolyte [poly(allylamine) hydrochloride (PAH) or poly(styrenesulfonate) (PSS)] and the oppositely charged layer. Three-layer membranes were created: PLL-PSS-PAH or PLGA-PAH-PSS, for an overall positive or negative charge, respectively. The overall charge on both the protein and membrane plays a substantial role in immobilization. When the protein and the membrane are oppositely charged, the amount immobilized and the stability within the polyelectrolyte assembly are significantly higher than for the case when both have similar charges. After protein incorporation in the multilayer assembly, the active site accessibility was comparable to that obtained in the homogeneous phase. This was tested by affinity interaction (avidin-biotin) and by carrying out two reactions (catalyzed by glucose oxidase and alkaline phosphatase). Besides simplicity and versatility, the ease of enzyme regeneration constitutes an additional benefit of this approach.     

Studies on the electrochemical and permeation characteristics of asymmetric charged porous membranes

Asymmetric charged porous membranes were prepared by imbedding 10% (W/W) ion-exchange resin in cellulose acetate binder. Membrane potential and conductance measurements have been carried out in sodium chloride solutions at different concentrations to investigate the relationship between concentration of fixed charges and electrochemical properties of developed nonselective cation- and anion-exchange membranes. Counterion transport number and permselectivity of these membranes were found to vary due to the presence of ion-exchange resin. The hydrodynamic and electroosmotic permeability of sodium chloride solutions has been studied in order to compute equivalent pore radius. For cation- and anion-exchange membranes good agreement was observed between pore radius values estimated from hydrodynamic and electroosmotic permeability coefficient separately, while for nonselective membranes no correlation was found. Membrane conductance data, along with values of concentration of fixed charges, were used for the estimation of the tortuosity factor, salt permeability coefficient, and frictional coefficient between solute and membrane matrix employing an interpretation by nonequilibrium thermodynamic principles based on frictional forces. Moreover, surface morphological studies of these membranes also have been carried out and the membranes were found to be reasonably homogeneous.     

Novel compaction resistant and ductile nanocomposite nanofibrous microfiltration membranes

Despite promising filtration abilities, low mechanical properties of extraordinary porous electrospun nanofibrous membranes could be a major challenge in their industrial development. In addition, such kind of membranes are usually hydrophobic and non-wettable. To reinforce an electrospun nanofibrous membrane made of polyethersulfone (PES) mechanically and chemically (to improve wettability), zirconia nanoparticles as a novel nanofiller in membrane technology were added to the nanofibers. The compressive and tensile results obtained through nanoindentation and tensile tests, respectively, implied an optimum mechanical properties after incorporation of zirconia nanoparticles. Especially compaction resistance of the electrospun nanofibrous membranes improved significantly as long as no agglomeration of the nanoparticles occurred and the electrospun nanocomposite membranes showed a higher tensile properties without any brittleness i.e. a high ductility. Noteworthy, for the first time the compaction level was quantified through a nanoindentation test. In addition to obtaining a desired mechanical performance, the hydrophobicity declined. Combination of promising properties of optimum mechanical and surface chemical properties led to a considerably high water permeability also retention efficiency of the nanocomposite PES nanofibrous membranes. Such finding implies a longer life span and lower energy consumption for a water filtration process.     

Convective model of cross-flow microfiltration

In membrane microfiltration of colloidal and cellular suspensions tangential feed ensures continuous operation. Neglecting diffusion (Pe ⪢ 1) the purely convective approach predicts well observed dependences between technological parameters and solves the so called microfiltration paradox.     

Colloidochemical properties of ultra-and microfiltration membranes in electrolyte solutions

Dependences of the structural, electrokinetic, and adsorption characteristics on solution pH and background electrolyte (NaCl) concentration are extensively studied for Sartorius and Vladisart cellulose acetate microfiltration membranes with pore sizes of 0.45 and 0.2 μm and a Vladisart ultrafiltration membrane with the rejection of 20 kD. It is revealed that effective hydrodynamic pore radii and maximum pore radii of the microfiltration membranes are 1.5-to 2-and 2.5-to 4-fold, respectively, larger than those presented in the catalog, which is related to the membrane calibration relative to the sizes of rejected particles. For the ultrafiltration membrane, it is shown that, when the pressure increased from 0.5 to 8.0 atm, filtration factor of a liquid and streaming potential substantially decrease owing to the contraction of the polymer network. Measurements of membrane conductivity by the difference and contact methods suggest that a structural anisotropy is virtually absent in the microfiltration membranes and that the ultrafiltration membrane has a nonuniform structure. Negative electrokinetic potentials, whose absolute values increase with the pH and dilution of a background electrolyte solution, are observed for all studied membranes. Isoelectric points of the ultrafiltration and microfiltration membranes are observed at pH ≤ 3 and 2.1 ± 0.2, respectively.     

Effects of sintering on properties of alumina microfiltration membranes

The transformation of membrane channels during the sintering process conforms to the Rhines' topological decay model of intermediate sintering stage. The pore size of the membranes enlarges with the increase of sintering temperature. The pore size increment caused by the increase of sintering temperature is more obvious for thin membranes than for thick membranes. With the increase of sintering temperature, the water permeance of membranes increases at first and then decreases after a turning point of sintering temperature.     

Development of a coating technique for the internal structure of polypropylene microfiltration membranes

A novel method of coating hydrophobic polyolefinic microfiltration (MF) membranes to produce a more hydrophilic membrane has been developed. A modified interfacial polymerization technique was used to coat the internal surface of a polypropylene (PP) membrane (about: 1.1 μm pore size, 84% void volume, 84 μm thick). 1,8-octanediamine (selected from several possible diamines) is dried onto the membrane internal surface from methanol and then reacted with a disulfonyl chloride (plus trisulfonyl chloride crosslinking agent) from a mixed solvent system of CHCl₃ and CCl₄, forming a polysulfonamide coating. Key polymerization parameters were identified as time and temperature of polymerization, concentrations of the diamine and the sulfonyl chlorides, and the ratio of CHCl₃ to CCl₄. The coating was uniform and stable. Permeation measurements were performed with various size polystyrene latex spheres and carboxylic modified polystyrene latex spheres in aqueous solution. Coating significantly increased hydrophillicity, and hence flux, and reduced membrane fouling for latex sphere solutions.     

High-flux,porous and homogeneous PVDF/cellulose microfiltration membranes

Low hydrophilicity of membranes is probably the biggest concern in membrane filtration since it increases the costs for water treatment. Conversely, application of hydrophilic biopolymers (such as cellulose) is limited because of its complex and crystalline structure. Enabling the wide use of the most common biopolymer in nature is crucial to improve the performance of water treatment, especially in terms of membrane sustainability. Here, we study the effect of cellulose dissolution in the synthesis of homogeneous PVDF/cellulose membranes. Although only partial dissolution was achieved for studied samples, adding cellulose to the membranes greatly improved their water flux. Besides, the porous structure obtained after partial solvent removal indicates the water flux (and consequently the pore size) may be tailored according to the membrane production method. Therefore, the homogeneous cellulose microfiltration membranes studied here may have potential for water treatment considering their high-water flux and low complexity to produce.

     

Dimensional analysis of steady state flux for microfiltration and ultrafiltration membranes

Dimensional analysis of the mass, length and time shows that the steady state flux observed for microfiltration or ultrafiltration through inorganic composite membrane can be expressed using two dimensionless numbers. The shear stress number N_S 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 drived 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 hydrocarbon emulsions and microfiltration of methanogenic bacteria suspensions and secondary treated wastewater were recalculated in terms of these dimensionless groups. Straight lines were plotted whose slope depends solely on the suspension and the membrane and not on the solute concentration. A negative slope and a positive intersection with the N_S axis means that a cake layer or a polarization layer can be completely eliminated at a critical cross-flow velocity; this was the case for an inorganic particles suspension and for the methanogenic suspension. A straight line of negative slope followed by a plateau means that an irreversible fouling is superimposed to the reversible phenomenon; this was observed for a secondary treated wastewater. A positive slope means that fouling predominates; this was observed with hydrocarbon emulsions.     

Use of gas-liquid porometry measurements for selection of microfiltration membranes

The process of crossflow microfiltration is hindered by the significant problem of fouling due to a pore size which favours penetration of the solutes. This leads to an internal fouling (adsorption and pore obstruction) which reduces permeate flux and makes any regeneration difficult. This study outlines a method of choosing an appropriate microfiltration membrane. Choice of membrane nature and pore size has been made in accordance with rapid dead-end filtration tests and the use of liquid-gas permporometry. Measuring pore size by porometry allows a choice of material which is non-adsorbent with regard to specific solutions to be microfiltered. Moreover, the internal fouling can be detected quickly by backflush washing after several minutes of dead-end filtration, and by measuring pore size distribution of the fouled membrane. Thus, choice of pore size will tend towards a membrane which bears slight internal fouling. The methodology described in this paper has allowed an appropriate choice of microfiltration membrane for use in recycling alkaline cleaning solutions in the dairy products industry.     

Yeast cell microfiltration: optimization of backwashing for delicate membranes

Crossflow microfiltration of yeast solutions was performed with crossflushing (done by stopping the permeate flow while maintaining the flow across the membrane) at intervals of 0.5 to 6 min and lasting for 5 to 90 s. A quantitative method was determined to compare the effectiveness of the various crossflushing times, independent of the initial cleanliness of the membrane. If crossflushing is performed every 0.5 to 1 min, 10–15 s is required to affect the steady-state flux. More time is required if crossflushing is performed less frequently. The total crossflushing time is also important. The more time spent crossflushing, the more slowly flux declines. However, since no product is being purified during crossflushing, economics will be an important part of determining the optimal crossflushing time.     

Performance of partially permeable microfiltration membranes under low fouling conditions

Controlling the onset of fouling and concentration polarization is critical in many membrane operations, particularly in the bioseparation area. By using stepping and constant flux experiments, the fouling threshold or `incipient fouling' region was studied for various microfiltration membranes, pH's, and bulk concentrations using bovine serum albumin. Experiments were conducted to try to decouple effects such are porosity and pore size on incipient fouling by using a combination of tracked etched and polyvinylidene difluoride membranes. Changes in protein transmission and wall concentrations near the fouling threshold were also compared across these membranes. While porosity determined the fouling rate after the exceeding the fouling threshold, pore size appear to be an dominant factor in determining level of the fouling threshold itself. The effect of pH also supports the hypothesis that the rejections are initially dominated by membrane–solute interactions but are subsequently modified by protein adsorption to the surface as the wall concentration increases. Repulsive forces between membrane and solute allow greater rejection (greater wall concentration) to be maintained without fouling but did not increase the critical flux substantially. Attractive electrostatic forces allow greater passage of solute (lower wall concentration), but the protein adsorption soon dominated and the onset of fouling occurred much more quickly. Using a conventional concentration polarization model, analysis of the results indicates that the onset of fouling is occurring at a relatively low wall concentrations.     

Chemical cleaning of oil contaminated polyethylene hollow fiber microfiltration membranes

The primary aim of this paper was to develop a more effective and economical procedure for cleaning polyethylene hollow fiber microfiltration membranes that have been used for removing oil from contaminated seawater. Alkaline cleaning showed higher recovery of operating cycle time but lower permeate flux recovery than acid cleaning. The combination of both alkaline and acid cleaning agents gave the best operating cycle time and flux recoveries (e.g. 96% and 94%, respectively). As the cleaning agent soaking time was reduced, the actual operating cycle time was reduced. However, the ratio of operating time/chemical cleaning time increased as the soaking time was reduced. The soaking time was recommended to be as short as possible (8–10 h) in the design of small capacity plants and 30 h or higher in case of large capacity plants. SEM analysis showed that in case of alkaline cleaning, most of the pores remained covered with a foulant layer, resulting in low flux recovery. The SEM results of acid cleaned membranes showed more complete removal of the foulant layer from the pores resulting in better flux recovery. Surface analysis of membranes cleaned with combined acid/base agents showed the best results. A membrane surface similar to the original one was obtained. The long-term objective is to increase the understanding of membrane fouling phenomena, preventive means and membrane cleaning processes as it applies to the clean-up and desalination of oil contaminated seawater.     

Colloid chemical characteristics of nanoporous glasses with different compositions in solutions of simple and organic electrolytes. 3. Calculation of electrochemical characteristics of membranes in terms of a homogeneous model

The electrosurface characteristics of nanoporous glass membranes–ion concentrations in pores with taking into account the specificity of counterions, electrokinetically mobile charge, the convective component of pore solution electrical conductivity, electroosmotic mobility of a liquid in the field of streaming potential and ion mobilities in pore space–were calculated within the homogeneous model. The effects of the type of counterion (sodium, potassium, ammonium, tetramethylammonium, and tetraethylammonium ions), solution concentration, glass composition, and pore size on the equilibrium and transport characteristics of membrane systems have been analyzed. A method for the determining of electrolyte activity coefficients in the membranes has been proposed.     

Thin layer molecularly imprinted microfiltration membranes by photofunctionalization using a coated alpha-cleavage photoinitiator.

A novel approach towards thin-layer molecularly imprinted polymer (MIP) composite membranes was developed based on using benzoin ethyl ether (BEE), a very efficient alpha-scission photoinitiator. The triazine herbicide desmetryn was used as the template, and a mixture of the functional monomer 2-acrylamido-2-methyl-1-propane sulfonic acid (AMPS) and the cross-linker N,N'-methylene-bis-acrylamide (MBAA) in methanol was copolymerised via photoinitiation followed by deposition on the surface of either hydrophobic or hydrophilically precoated polyvinylidene fluoride (PVDF) microfiltration membranes. Blanks were prepared under identical conditions, but without the template. Especially, the degree of functionalization (DF) of the PVDF membranes with poly(AMPS-co-MBAA), the membrane permeabilities and non-specific vs. MIP-specific template binding from aqueous solutions during fast filtration were studied in detail to evaluate the effects of the preparation conditions, in particular the coating of the membrane surface with the photoinitiator prior to UV irradiation and the influence of the precoated hydrophilic layer on PVDF. Significant template specificities of the MIP membranes compared with the blanks were only achieved for the preparations including coating the two types of PVDF membranes with BEE. In contrast, a homogeneous photoinitiation of the copolymerisation in the membrane pore volume yielded functional layers with similar DF but with only non-specific desmetryn binding. All data clearly indicate the significant contribution of MIP stabilization by the support material in layers of optimum thickness to the MIP specificity. Main advantages of the novel approach are the potential to synthesize MIP composite membranes by controlled deposition onto any kind of polymer support, and the very fast MIP preparations due to a very efficient photoinitiator and small MIP layer thickness. Due to the mechanical and chemical stability in combination with high permeabilities, thin-layer MIP composite membranes have a large application potential, e.g., in solid phase extraction.     

The effect of uni-axial stretching on the roughness of microfiltration membranes

Atomic force microscopy (AFM) was used to characterize the surface morphology of uni-axially stretched and non-stretched microporous microfiltration (MF) membranes. The effect of stretching on the pore structure and bulk properties of MF membranes has been previously reported [J.A. Morehouse, L.S. Worrel, D.L. Taylor, D.R. Lloyd, B.D. Freeman, D.F. Lawler, The effect of uni-axial orientation on macroporous membrane structure, J. Porous Mater. 13 (2006) 63–75.]; this paper focuses solely on the use of AFM to characterize the surface of stretched and non-stretched MF membranes. A new way of representing surface roughness that may prove useful in relating roughness to performance in cross-flow applications is presented.     

Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect

The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The zeta-potential of Membralox^® ceramic microfiltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zeta-potential is zero.The iso-electric points of new α-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the α-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point.