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.     

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.     

Impact of ultrafiltration operating conditions on membrane irreversible fouling

The main limitation of the ultrafiltration (UF) process identified in drinking water treatment is membrane fouling. Although adsorption of natural organic matter (NOM) is known to cause irreversible fouling, operating conditions also impact the degree of irreversible fouling. This study examined the impact of several operating parameters on fouling including flux, concentrate velocity in hollow fibers, backwash frequency, and transmembrane pressure. A hydrophilic cellulose derivative membrane and a hydrophobic acrylic polymer membrane were used to conduct these tests. Pilot testing showed that when short-term reversible fouling was limited during a filtration cycle by increasing the concentrate velocity, reducing the flux, and increasing the backwash frequency, the evolution of the membrane toward irreversible fouling could be controlled. It appeared that operating parameters should be adjusted to maintain the increase of transmembrane pressure below a certain limit, determined to be approximately 0.85 to 1.0 bar for the tested UF membrane, in order to minimize the rate of irreversible fouling. This threshold for transmembrane pressure was confirmed empirically by compiling data from over 36 pilot studies. Other testing results demonstrated that hydraulic backwash effectiveness decreased as the transmembrane pressure applied in the previous filtration cycle increased. Backwash efficiency in terms of membrane flux recovery after hydraulic backwash was reduced by 50% when the transmembrane pressure was increased from 0.4 bar to 1.4 bar.     

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.     

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.     

Analysis of particle fouling during microfiltration by use of blocking models

Particle fouling mechanisms in “dead-end” microfiltration is analyzed using blocking models. The blocking index and resistance coefficient of the models during microfiltration are calculated under various conditions. The major factors affecting these model parameters, such as the filtration rate, the amount of particles simultaneously arriving at the membrane surface and particle accumulation, are discussed thoroughly. Instead of the four different blocking models previously proposed, a membrane blocking chart is established for relating the blocking index, filtration rate, and particle accumulation. Blocking index variation during microfiltration can be interpreted using this chart. Membrane blocking occurs during the initial filtration periods until the condition reaches a critical value; then, the blocking index suddenly drops to zero by following up the cake filtration model. Once the normalized resistance coefficient is regressed to an exponential function of the blocking index under a wide range of conditions, the blocking models can be used to quantitatively explain filtration flux attenuation by solving a unitary mathematical equation. Comparing the experimental filtration rates obtained under different conditions with the simulated results reveals a good agreement between them and demonstrates the reliability of this analysis method.     

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.     

Flux decline in crossflow microfiltration and ultrafiltration: experimental verification of fouling dynamics

The theory of fouling dynamics in crossflow membrane filtration is compared with ultrafiltration experiments with suspensions of 0.12 μm silica colloids. It has been experimentally verified that colloidal fouling in crossflow filtration is a dynamics process from non-equilibrium to equilibrium and that the steady state flux is the limiting flux. With the cake concentration c_g identified from an independent experiment and the specific cake resistance calculated by Carman–Kozeny equation, the time-dependent flux and the time to reach steady state in the experiments of this study are correctly predicted with the theory of fouling dynamics.     

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.     

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.     

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.     

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.     

A systematic insight into fouling propensity of soluble microbial products in membrane bioreactors based on hydrophobic interaction and size exclusion

Soluble microbial products (SMPs) contained in membrane bioreactor (MBR) supernatant have been proved to be main foulants. To obtain a comprehensive understanding of the fouling potential of SMPs on the basis of both hydrophilic/hydrophobic properties and molecular size, MBR supernatant of a pilot-scaled system treating municipal wastewater was partitioned into different hydrophilic/hydrophobic fractions by DAX-8 resins, with joint size partition of hydrophilic fraction also undertaken. A series of stirred dead-end filtration tests were conducted to investigate the flux decline. Hydrophilic fraction was found the dominant foulant responsible for flux deterioration, which was mainly attributed to the subclass of molecular weight above 100 kDa. The molecular weight distribution and atomic force microscopy images indicated that large molecules in hydrophilic fraction plugged the membrane pores. The backwash tests showed the flux decline caused by hydrophilic fraction was much less recoverable by hydraulic cleaning. It can be inferred that steric factor, i.e. size exclusion was the primary cause in the initial stage of fouling, while the role of hydrophobic interaction was of less significance. Additional modeling work indicates that the main fouling mechanism was complete blocking, further confirming the predominance of size exclusion contributing to membrane fouling by SMPs in MBR supernatant.     

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).     

Modelling of submerged membrane bioreactor: Conceptual study about link between activated slugde biokinetics,aeration and fouling process

A mathematical model was developed to simulate filtration process and aeration influence on submerged membrane bioreactor (SMBR) in aerobic conditions. The biological kinetics and the dynamic effect of the sludge attachment and detachment from the membrane, in relation to the filtration and a strong intermittent aeration, were included in the model. The model was established considering soluble microbial products (SMP) formation-degradation. The fouling components responsible of pore clogging, sludge cake growth, and temporal sludge film coverage were considered during calculation of the total membrane fouling resistance. The influence of SMP, transmembrane pressure, and mixed liquor suspended solids on specific filtration resistance of the sludge cake was also included. With this model, the membrane fouling under different SMBR operational conditions can be simulated. The influence of a larger number of very important process variables on fouling development can be well quantified. The model was developed for evaluating the influence on fouling control of an intermittent aeration of bubbles synchronized or not with the filtration cycles, taking into account the effects of shear intensity on sludge cake removal.     

Resistance analysis for enhanced wastewater membrane filtration

This study investigated enhancement techniques for synthetic wastewater filtration in a membrane bioreactor (MBR) at mixed liquor suspended solids concentrations (MLSS) of 12–18 g/L. Air sparging (AS), backflushing (BF) and a combined application of both (AS + BF) were applied to increase permeate flux compared to the conventional application (NON). Scanning electron microscope (SEM) measurements of cake thickness served for evaluating cleaning effectiveness and as input data for some of the model calculations. AS + BF showed the lowest overall resistance, and thus the highest permeate yield, for about 2 weeks of observation. The contribution of fouling resistance, cake resistance and membrane resistance to the overall resistance was evaluated, based on experimental data. Air sparging significantly lowered cake thickness and consequently cake resistance. The experimental cake resistance and the model resistances were compared. A model based on the measured cake thickness and literature values for the specific surface area proved most successful. Finally, a relationship between the backflush resistance and the permeate flow resistance was observed.     

Macroscopic and microscopic characterizations of a cellulosic ultrafiltration (UF) membrane fouled by a humic acid cake deposit: First step for intensification of reverse osmosis (RO) pre-treatments

One of the critical issues for the application of low-pressure membrane processes (microfiltration, MF or ultrafiltration, UF) as pre-treatment processes for freshwater preparation is membrane fouling due to natural organic matter (NOM). The aim of this preliminary study is to contribute to a better understanding of the fouling phenomena occurring on a regenerated cellulose UF membrane fouled with a humic acid cake deposit. The originality of this work is based on a double approach on surface analysis at both macroscopic and microscopic scales. It is presently reported that humic acid fouling is mainly governed by cake formation, which plays a major role in flux decline via the well-known model of resistances in series. We obtained that the adsorbed resistance is 2% of the total resistance while the cake resistance is 52% of the total resistance, which is higher than that of the virgin membrane. From field emission gun scanning electron microscopy (FESEM) it was found for the first time that the humic acid cake is well organized, and particularly in fractal forms. The fractal dimension (FD) of the cake is determined as 2.52, which is in good agreement with the theoretical fractal dimension of particle–cluster aggregation underlying diffusion-limited aggregation (FD = 2.51). This new microscopic fouling index decreases with the presence of cake and can be correlated with the decrease of the hydraulic permeability. The classical silt density index (SDI) and the new modified fouling index (denoted MFI-UF) were obtained and also proved the presence of the cake. To complete this approach transmembrane streaming potential (denoted SP) measurements were conducted with a new homemade apparatus developed in our lab and presented for the first time in the present article, helped us to observe also a penetration of low molecular fractions of humic acid inside the membrane. Indeed the displacement of the isoelectric point (iep) of the membrane from 2.3 to 1.5 for the virgin and fouled membranes, respectively, permitted to illustrate this penetration. This newly designed SP apparatus is a semi-automatic tool assisted by a software denoted as proFluid 1.2. Furthermore, preliminary experiments with seawater were realized in order to estimate the influence of seawater filtration on the hydraulic permeability and SP parameters for the RC 100-kDa membrane.     

Flux decline during nanofiltration of naturally-occurring dissolved organic matter: effects of osmotic pressure,membrane permeability,and cake formation

Nanofiltration of naturally-occurring dissolved organic matter (NOM) by an aromatic polyamide membrane was measured in a crossflow bench-scale test cell and modeled using a semi-empirical osmotic pressure/cake formation model. Our objective was to examine flux decline due to NOM fouling while explicitly accounting for flux decline due to osmotic effects and changes in membrane permeability. This approach allowed quantification of the effect of ionic composition on specific NOM cake resistance, and yielded insight into flux decline due to enhanced NaCl rejection by the NOM deposit. In the absence of NOM, increasing NaCl concentration reduced salt rejection and decreased membrane permeability. Flux decline was modeled by accounting for changes in osmotic pressure with time, and by employing an effective permeability. The addition of calcium significantly reduced rejection of sodium and feed conductivity, and thus mitigated flux decline. Increasing pH from 4 (near membrane pI) to 10 increased the effective permeability but also increased NaCl rejection, which resulted in greater flux decline. The presence of NOM caused greater flux decline resulting from a combination of NOM cake resistance and increased rejection of NaCl by negatively charged NOM functional groups. Increasing NaCl concentration had little effect on the mass of NOM deposited, but significantly increased the specific resistance of the NOM cake. The effect of ionic strength on specific resistance correlated with a reduction in NOM size, estimated by separate UF permeation experiments and size exclusion chromatography analysis of UF permeate. Therefore, increased specific cake resistance is consistent with a more compact, less porous cake. Flux decline by NOM solutions showed a maximum at pH 7, where salt rejection was also a maximum. Binding of calcium reduced the ability of NOM to enhance NaCl rejection, and likely increased NOM cake resistance. Flux decline caused by NOM fouling in the presence of calcium was only significantly different than that caused by NOM in a solution of NaCl at the same ionic strength when the calcium concentration corresponded to saturation of NOM binding sites.