Adsorption behavior of BSA in microfiltration with porous glass membrane

The fouling mechanism during dead-end microfiltration of bovine serum albumin (BSA) with porous glass membrane was investigated from the point of BSA adsorption onto the pore surface of membrane under the condition of pH 5.0 and ionic strength 0.01. The location of BSA retention was confirmed by comparing the filtration performance between dead-end mode and cross-flow mode. During the dead-end microfiltration BSA was retained only by the adsorption on the pore surface. The adsorption was irreversible and of multilayer type, which consists of the adsorption on clean pore surface, i.e. the primary adsorption, and that on preadsorbed pore surface, i.e. the secondary one. The adsorption isotherm was high affinity type. The adsorption rate was proportional to the feed rate of BSA, and the proportional coefficient was dependent on the adsorption process. The flux decline was correlated quantitatively with the amount of adsorbed BSA from the pore radius narrowing model by adsorption.     

Flux decline behaviors in dead-end microfiltration of activated sludge and its supernatant

The properties of dead-end microfiltration were explored under constant pressure using two types of activated sludge controlled under the condition of different air flow rates. The activated sludge cultured at the air flow rate of 0.15 L min⁻¹ (the anaerobic condition) exhibited a significant flux decline compared with the case of the air flow rate of 2.33 L min⁻¹ (the aerobic condition). It was found from the results of microfiltration of the supernatant separated by centrifugation that the constituents in the supernatant caused a major cake resistance in microfiltration of the activated sludge. The average specific filtration resistance for filtration of the activated sludge was closely consistent with that for filtration of the supernatant at low pressure (49 kPa). However, the cake resistance of the microbial floc in microfiltration of the activated sludge became substantial with increasing filtration pressure because of high compressibility of the microbial floc. Moreover, the foulant and the fouling mechanism in microfiltration of the supernatant were evaluated from both microfiltration test of the supernatant and microfiltration test of the filtrate collected thereby. As a result, the effects of the pore size and material of the microfiltration membrane on the flux decline behaviors in dead-end microfiltration were reasonably elucidated.     

A Combined Pore Blockage and Cake Filtration Model for Protein Fouling during Microfiltration

Previous studies of protein fouling during microfiltration have shown significant discrepancies between filtrate flux data and predictions of the classical pore blockage, pore constriction, and cake filtration models. A new mathematical model was developed for the filtrate flux which accounts for initial fouling due to pore blockage and subsequent fouling due to the growth of a protein cake or deposit over these initially blocked regions. The model explicitly accounts for the inhomogeneity in the cake layer thickness over different regions of the membrane arising from the time-dependent blockage of the pore surface. The model was shown to be in excellent agreement with experimental data obtained during the stirred cell filtration of bovine serum albumin solutions through polycarbonate track-etched microfiltration membranes over the entire course of the filtration. The model provides a smooth transition from the pore blockage to cake filtration regimes, eliminating the need to use different mathematical formulations to describe these two phenomena. In addition, the model provides the first quantitative explanation for some of the unusual observations reported previously in investigations of protein microfiltration. The results provide important insights into the underlying mechanisms of protein fouling during microfiltration. Copyright 2000 Academic Press.     

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.     

Protein fouling in microfiltration: deposition mechanism as a function of pressure for different pH

The influence of applied pressure on the fouling mechanism during bovine serum albumin (BSA) dead-end microfiltration (MF) has been investigated for a polyethersulfone acidic negatively charged membrane (ICE-450) from Pall Co. BSA solutions at pH values of 4, 5 (almost equal to the protein isoelectric point, IEP), and 6 were microfiltered through the membrane at different applied transmembrane pressures. Results have been analyzed in terms of the usual blocking filtration laws and a substantial change in the fouling mechanism was observed as the pressure was increased, this change can be related to the specific membrane-protein and protein-protein interactions.     

Humic acid fouling during microfiltration

A major factor limiting the use of microfiltration for surface water treatment is membrane fouling by natural organic matter. The extent and mechanisms of humic acid fouling during microfiltration have been examined using stirred cell filtration experiments and scanning electron microscopy. The extent of fouling was strongly dependent on both the source and preparation of the humic acid solutions. The large flux decline observed during constant pressure microfiltration was caused by the formation of a humic acid deposit located on the upper surface of the membrane. Prefiltration of the humic acid solutions dramatically reduced the rate of fouling through the removal of large humic acid aggregates. The initial fouling in this system was determined almost entirely by the convective deposition of these large particles/aggregates on the membrane surface. This initial deposit accelerated the subsequent rate of humic acid fouling, possibly serving as a nucleation site for deposition of macromolecular humic acids.     

Sieve mechanism of microfiltration separation

A theoretical model of dead-end microfiltration (MF) of dilute suspensions is proposed. The model is based on a sieve mechanism of MF and takes into account the probability of membrane pore blocking during MF of dilute colloidal suspensions. An integro-differential equation (IDE) that includes both the membrane pore size and the particle size distributions is deduced. According to the suggested model a similarity property is applicable, which allows one to predict the flux through the membrane as a function of time for any pressure, and dilute concentration, based on one experiment at a single pressure and concentration. The suggested model includes only one fitting parameter, β>1, which takes into account the range of the hydrodynamic influence of a single pore. For a narrow pore size distribution in which one pore diameter predominates (track-etched membranes), the IDE is solved analytically and the derived equation is in good agreement with the measurements on different track-etched membranes. A simple approximate solution of the IDE is derived and that approximate solution, as well as the similarity principal of MF processes, is in good agreement with measurements using a commercial Teflon microfiltration membrane. The theory was further developed to take into account the presence of multiple pores (double, triple and so on pores) on a track-etched membrane surface.

A series of new dead-end filtration experiments are compared with the proposed initial and modified pore blocking models. The challenge suspension used was nearly monodispersed suspension of latex particles of 0.45 μm filtered on a track-etched membrane with similar sized pores 0.4 μm. The filtered suspension concentration ranged from 0.00006 to 0.01% (w/w) and the cross-membrane pressures varied from 1000 to 20,000 Pa. Three stages of microfiltration have been observed. The initial stage is well described by the proposed pore blocking model. The model required only a single parameter that was found to fit all the data under different experimental operational conditions. The second stage corresponds to the transition from the blocking mechanism to the third stage, which is cake filtration. The latter stage occurred after approximately 10–12 particle layers were deposited (mass = 0.006 g) on the surface of the microfiltration membrane.     

Analysis of humic acid fouling during microfiltration using a pore blockage–cake filtration model

Fouling by natural organic matter, such as humic substances, is a major factor limiting the use of microfiltration for water purification. The objective of this study was to develop a fundamental understanding of the underlying mechanisms governing humic acid fouling during microfiltration using a combined pore blockage–cake filtration model. Data were obtained over a range of humic acid concentrations, transmembrane pressures, and stirring speeds. The initial flux decline was due to pore blockage caused by the deposition of large humic acid aggregates on the membrane surface, with a humic acid deposit developing over those regions of the membrane that have first been blocked by an aggregate. The rate of cake growth approaches zero at a finite filtrate flux, similar to the critical flux concept developed for colloidal filtration. The data were in good agreement with model calculations, with the parameter values providing important insights into the mechanisms governing humic acid fouling during microfiltration. In addition, the basic approach provides a framework that can be used to analyze humic acid fouling under different conditions.     

Characterization and theoretical analysis of protein fouling of cellulose acetate membrane during constant flux dead-end microfiltration

A rapid characterization method was used to study protein fouling of cellulose acetate membrane during dead-end, in-line, constant flux microfiltration. Based on pressure-permeate volume profiles, two fouling phases could be identified and compared at different permeate fluxes. Using protein staining dyes, the model foulant (bovine serum albumin) was found to deposit on the upstream side of the membrane as a loose cake at its isoelectric point. The effects of solution pH on both the nature and extent of membrane fouling, and membrane cleaning were examined. To further understand and quantitatively analyze the fouling behavior, a combined mathematical model which took into account pore blocking, cake formation and pore constriction was developed based on existing fouling models. The data obtained by modeling was in good agreement with experimental fouling data. Theoretical analysis of data clearly indicated that cake formation was the main fouling mechanism. Using methods such as dynamic light scattering, the significant role of large protein aggregates in membrane fouling was confirmed. The dimer composition of protein did not change significantly during the fouling experiments, clearly indicating that smaller aggregates played less important role in membrane fouling.     

Effect of membrane morphology on the initial rate of protein fouling during microfiltration

Protein fouling remains a major problem in the use of microfiltration for many bioprocessing applications. Experiments were performed to evaluate the effect of membrane morphology and pore structure on protein fouling using different track-etched, isotropic, and asymmetric microfiltration membranes. Fouling of membranes with straight-through pores occurred by pore blockage caused by deposition of large protein aggregates on the membrane surface. However, the rate of blockage was a function of the membrane porosity due to the possibility of multiple pore blockage by a single protein aggregate on high porosity membranes. Membranes with interconnected pores fouled more slowly since the fluid could flow around the blocked pores through the interconnected pore structure. This behavior was quantified using model membrane systems with well-defined pore morphology constructed from track-etch and isotropic membranes in a layered series combination. These results provide important insights into the effects of membrane pore structure and morphology on protein fouling.     

Experimental study of ultrafiltration membrane fouling by sodium alginate and flux recovery by backwashing

Membrane fouling is the major limitation for a broader application of membrane technology. One of the main causes of membrane fouling in advanced wastewater reclamation and in membrane bioreactors (MBR) are the extracellular polymeric substances (EPS). Among the main constituents in EPS, polysaccharides are the most ubiquitous. This study aims at a better understanding of the fouling mechanisms of EPS and the efficiency of backwashing technique, which is applied in practice to restore membrane flux. For that purpose, the evolution of fouling by sodium alginate, a microbial polysaccharide, is studied in ultrafiltration. Fouling experiments are carried out in a single fiber apparatus, aiming at identifying the significance of distinct fouling mechanisms and their degree of reversibility by backwashing. An important parameter considered in the study is the concentration of calcium ions, which promote sodium alginate aggregation and influence the rate of flux decline, the reversibility of fouling and rejection. A rapid irreversible fouling takes place due to internal pore constriction, at the beginning of filtration, followed by cake development on the membrane surface. With increased calcium addition, cake development becomes the dominant mechanism throughout the filtration step. Furthermore, fouling reversibility is increased with the increase of calcium concentration. A unique behavior of sodium alginate solution in the absence of calcium is also noted, i.e. the formation of a labile layer on the membrane surface, which is affected by the small cross-flow that exists inside hollow fibers, even in the nominally dead-end mode of operation.     

Modeling of the permeate flux decline during MF and UF cross-flow filtration of soy sauce lees

Cross-flow ultrafiltration and microfiltration have been used to recover refined soy sauce from soy sauce lees for over 25 years. The precise mechanism which dominated the permeate flux during batch cross-flow filtration has not been clarified. In the present study, we proposed a modified analytical method incorporated with the concept of deadend filtration to determine the initial flux of cross-flow filtration and carried out the permeate recycle and batch cross-flow filtration experiments using soy sauce lees. We used UF and MF flat membrane (0.006 m² polysulfone) module under different transmembrane pressures (TMP) and cross-flow velocities. The modified analysis provided an accurate prediction of permeate flux during the filtration of soy sauce lees, because this model can consider the change in J₀ at initial stage of filtration which was caused by the pore constriction and plugging inside membrane, and these changes may not proceed when the cake was formed on the membrane surface. Mean specific resistance of the cake increased with TMP due to the compaction of the cake and decreased with cross-flow velocity due to the change of deposited particle size, but less depended on the membrane in the present study. These results indicate that the value of J₀ determined by modified method was relevant to exclude the effects of the initial membrane fouling by pore constriction due to protein adsorption and plugging with small particles. The modified analytical method for the cake filtration developed in the present study was considered to be capable of selecting an appropriate operating conditions for many cross-flow filtration systems with UF, MF membranes.     

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.     

Microfiltration of protein-cell mixtures with crossflushing or backflushing

Crossflow microfiltration experiments were performed with and without crossflushing or backflushing using yeast suspensions, bovine serum albumin (BSA) solutions, and mixtures of yeast and BSA. Scanning electron microscope (SEM) photographs of the membrane surface were taken before and after crossflushing and backflushing. Backflushing is highly effective, while crossflushing is partially effective, in removing the external cake formed during filtration of yeast suspensions. Crossflushing is completely ineffective and backflushing is only partially effective for removal of internal foulants during filtration of BSA solutions. During backflushing or crossflushing of yeast–BSA mixtures, complete or partial removal of the yeast cake reduces the hydraulic resistance to permeate flow. However, this removal also exposes the primary membrane to internal fouling by BSA, against which neither crossflushing nor backflushing is very effective.     

Fouling during constant flux crossflow microfiltration of pretreated whey. Influence of transmembrane pressure gradient

Pretreatment of whey by microfiltration (MF) has emerged as a necessary step in producing high purity whey protein concentrates. In the MF of pretreated whey using a Carbosep M14 membrane (pore diameter 0.14 μm), proteins and calcium phosphate aggregates were responsible for fouling, which increased according to the “complete blocking” filtration law and accounted for a progressive decrease of the active filtering area. An operating mode with dynamic counter pressure (recirculation of the permeate co-current to the retentate), as opposed to static counter pressure, allowed lower overall fouling, a longer time of operation and better protein recovery because of more evenly distributed fouling along the membrane tube. At shorter times of operation, fouling was greater under higher transmembrane pressure (TP), so that the less fouled areas under lower TP were forced to filter larger volumes and consequently became fouled more rapidly. This involved a movement of the effective filtering area along the membrane tube, as evidenced by the systematic evolution of fouling heterogeneity as measured by infra-red spectroscopy.     

Microfiltration of post-fermentation broth with backflushing membrane cleaning

Separation of microorganism cells from broth is a very important stage in the recovery of fermentation products. The microfiltration of fermented glycerol solutions was studied. During this process, the filter cake building up on the membrane surface caused an increase of filtration resistances, resulting in the decrease of the permeate flux. In this work, short time reverse flow of permeate was used to remove the fouling layer after each cycle of the filtration. The applied periodical membrane cleaning led to minimization of the observed fouling effects.     

Cut-off of dilute O/W emulsions through a microfiltration membrane

In order to obtain a monodispersed emulsion, we have used a cut-off process through a microfiltration membrane. Generally in the microfiltration process, a self-rejecting cake-layer formed at the initial stage of filtration would retain droplets, regardless of their size. It was therefore believed that separation based on relative size of pores and droplets through a microfiltration membrane was an impossible process. In the present study, it is assumed that removal of the self-rejecting cake-layer might enable cut-off to be realized through a microfiltration membrane. Based on this idea, both dead-end and cross-flow filtrations with stirred cell under conditions that avoid cake-layer formation were carried out. It is clear from the present experimental results that the cut-off process through microfiltration can be used to control droplet size under the special condition of no cake-layer formation, and the yield of this process can be predicted by values of the cut-off curve. A sieving mechanism should be the process responsible for the cut-off in the present experimental system.     

Effects of nanoparticles on the ultrafiltration of surface water

The effects of nanoparticles on the fouling behavior of UF membranes were investigated by filtering river water containing natural organic matter (NOM). Self-dispersible carbon black (70–200 nm) was employed to model nanoparticles in natural water. The presence of nanoparticles transformed the mode of initial fouling from internal pore adsorption of NOM to intermediate pore blocking, which caused a significant flux reduction. The use of powdered activated carbon to adsorb organic micromolecules reduced internal pore fouling, but this effect on initial fouling mode did not much mitigate the overall flux decline. As filtration proceeded, cake filtration became the dominant fouling mode. The resistance-in-series model revealed that boundary-layer resistance contributed significantly to increased filtration resistance in the filtration of river water. The nanoparticles nullified boundary-layer resistance plausibly by removing organic macromolecules from river water, but aggravated cake resistance, which required chemical cleaning. Addition of calcium significantly increased the aggregate size of nanoparticles from 0.18–0.35 μm to 3.4 μm, and thus reduced pore blocking and total cake resistance.     

Beer clarification by microfiltration — product quality control and fractionation of particles and macromolecules

Beer clarification by microfiltration demands a finely balanced retention of colloidal particulates (yeast cells, chill haze flocs, etc.) and transmission of soluble macromolecules including carbohydrates, proteins, flavour, and colour compounds which give the “whole some” quality of a beer. The required porous transmission of these macromolecular species led to an unavoidable, complex and dynamic in-pore membrane fouling in terms of fouling constituents, formation, structure and kinetics, which are the main obstacles in obtaining an economically viable flux and consistency in permeate quality.This experimental study was carried out with the aims of understanding the dynamic inter-relation between flux, fouling and system selectivity during a cross-flow beer microfiltration process so that an effective operating strategy for flux optimisation could be formulated in conjunction with the parallel objective of good product (permeate) quality control. Tubular ceramic membranes (Ceramem) with nominal pore diameters of 0.2, 0.5, and 1.3 μm were used. Simultaneous measurement of flux and permeate qualities, such as specific gravity and chill haze level enabled identification of the effect of anti-fouling techniques, such as backflushing on transmission of essential beer components and on the filtered beer quality. The experimental evidence lead to an understanding that the drastic flux enhancement achieved by employing backflushing at reversed membrane morphology was associated with enhanced solute transmission which could, without careful control, upset a balanced transmission of essential beer components and the retention of unwanted “chill haze” components. Further operating parameters and varying system configurations were investigated over their effect on both flux performance and system selectivity. These include membrane pore size, filtration temperature, and the addition of an amorphous silica particles as coagulation agent for hydrophilic proteins.