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.     

Microfiltration of protein solutions at thin film composite membranes

An experimental study of the interaction of the enzyme yeast alcohol dehydrogenase (YADH) with polysulfone thin film composite microfiltration membranes (Dow-Danmark) has been carried out. It was found that the membranes adsorbed only 3/4 of a monolayer of the enzyme under the conditions studied. Even so, under filtration conditions, the membrane permeation rate decreased continuously with time. This decrease in permeation rate was due neither to concentration polarisation nor to protein adsorption alone. However, it could be quantified using the standard blocking filtration law, which describes a decrease in pore volume due to deposition of protein in the interior structure of the membrane. Reversal of the membrane, so that the supporting matrix faced the feed solution, gave more stable permeation rates. Implications for the microfiltration of industrial fermentation broths are discussed.     

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.     

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.     

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.     

Effect of backwash on the performance of submerged membrane filtration

Particles with a mean diameter of 5 μm were filtered by a ceramic tubular membrane to study the effects of backwash on the performance of submerged membrane filtration. A periodic backwash can completely remove the formed cake, diminishing a part of membrane internal fouling, and, therefore, recover the filtration flux. In a membrane-blocking/cake formation comparable filtration system, the filtration resistance due to membrane-internal fouling is over twice as high as that due to cake formation. The irreversible filtration resistance increases progressively during operation, and it can be regressed to a power-type empirical relationship. Filtration period data were analyzed using blocking models. Membrane blocking occurs in the early filtration periods and is followed by cake filtration. The filtration flux can be simulated by employing blocking models and empirical equations for filtration resistance. The backwash effectiveness was examined by comparing filtrate productivity and washing efficiency. The calculated results of productivity under various backwash durations agree well with experimental data. An increase in backwash flux or duration leads to higher productivity, when the duration is shorter than 2 min; however, the productivity may be decreased with an increase of backwash duration due to the back pumping of more filtrate. A longer filtration time in each cycle results in higher backwash efficiency since a formed cake may efficiently prevent further membrane pore clogging and is more easily removed by a backwash. The optimal backwash conditions can be determined appropriately by the proposed method, with respect to both backwash efficiency and filtrate productivity.     

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.     

Particle deposition and layer formation at the crossflow microfiltration

A microscopic model of the layer formation and the cake growth at the crossflow microfiltration will be introduced. The model considers the hydrodynamic, adhesive and friction forces acting on a single particle during the filtration process. It can be shown that mainly the balance between the lift force and the drag force of the filtrate flow determines the layer formation at the membrane. Particle attachment to the layer is mostly an irreversible process. This is due to the large influence of the adhesive forces. The irreversibility of particle attachment was proved by experiments with monodisperse particles. The introduced model allows the prediction of the instationary crossflow filtration processes. The filtration rate and structure of the formed layer can be calculated. In the case of a filtration at constant transmembrane pressure the model calculation shows a good correspondence to the experimental results.     

Hybride process,microfiltration-electroperoxidation,for water treatment

Cross-flow microfiltration through a 0.8 μm inorganic tubular membrane was enhanced by coupling with a two electrode electrolysis cell producing hydrogen peroxide at high rate, without adding any chemical, by use of carbon felt cathode and dimensional stabilised anode (titanium coated with RuO₂). Anodic oxygen and transfer from atmosphere supplied the required oxygen. The current should be maintained under a maximum value to avoid peroxide reduction. This electrochemical process, called electroperoxidation, upgraded the water quality by removing contaminants that limit mass transport through the membrane, i.e. turbidity, dissolved organic carbon (DOC) and microorganisms. Transient filtration was adjusted to an internal clogging model whose coefficient decreased at the same rate as DOC. The microfiltration steady state flux was multiplied by a factor proportional to the peroxide concentration introduced in the filtration loop. The induced resistance decreased simultaneously with chemical oxygen demand and 254 nm absorbance. Steady state fluxes 2.5 times higher than without treatment were experimentally obtained.     

Studies on secondary layer formation and its characterization during cross-flow filtration of microbial cells

The formation of membrane sublayers during cross-flow filtration was studied with a standardized E. coli suspension both in a tubular and a flat channel module with different membrane materials. The height of the layers was calculated for different experimental conditions. Transmembrane pressure, cross-flow velocity, compressibility of the suspended particles, properties of the suspension, particle size and concentration were all found to have a significant effect on the formation of membrane sublayers. A decrease of the layer thickness and corresponding filtration resistance with increasing channel length was observed due to the longitudal transmembrane pressure gradient. The filtration resistance of the layer is found to be the dominant factor determining the flux rate.     

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.     

Modeling of flux decline during crossflow ultrafiltration of colloidal suspensions

Mass transfer during crossflow ultrafiltration is mathematically expressed using the two-dimensional convective–diffusion equation. Numerical simulations showed that mass transfer in crossflow filtration quickly reaches a steady-state for constant boundary conditions. Hence, the unsteady nature of the permeate flux decline must be caused by changes in the hydraulic boundary condition at the membrane surface due to cake formation during filtration. A step-wise pseudo steady-state model was developed to predict the flux decline due to concentration polarization during crossflow ultrafiltration. An iterative algorithm was employed to predict the amount of flux decline for each finite time interval until the true steady-state permeate flux is established. For model verification, crossflow filtration of monodisperse polystyrene latex suspensions ranging from 0.064 to 2.16 μm in diameter was studied under constant transmembrane pressure mode. Besides the crossflow filtration tests, dead-end filtration tests were also carried out to independently determine a model parameter, the specific cake resistance. Another model parameter, the effective diffusion coefficient, is defined as the sum of molecular and shear-induced hydrodynamic diffusion coefficients. The step-wise pseudo steady-state model predictions are in good agreement with experimental results of flux decline during crossflow ultrafiltration of colloidal suspensions. Experimental variations in particle size, feed concentration, and crossflow velocity were also effectively modeled.     

Modeling of Fouling of Crossflow Microfiltration Membranes

Abstract

Steady-state and transient models are reviewed for predicting flux decline for crossflow microfiltration under conditions in which both external cake buildup and internal membrane fouling are contributing factors. Experimental work is not covered in the scope of this review, although reference is made to a few recent studies which have compared experimental measurements with theory. The steady-state cake thickness and permeate flux are governed by the concentration polarization layer adjacent to the cake of rejected particles which forms on the membrane surface. Depending on the characteristic particle size and the tangential shear rate, Brownian diffusion, shear-induced diffusion, or inertial lift is considered to be the dominant mechanism for particle back-transport in the polarization layer. For typical shear rates, Brownian diffusion is important for submicron particles, inertial lift is important for particles larger than approximately ten microns, and shear-induced diffusion is dominant for intermediate-sized particles. For short times, it is shown that the transient flux decline due to cake buildup is closely approximated by deadend batch filtration theory, independent of the tangential shear rate. For long times, however, the steady or quasi-steady flux increases with shear rate, because the tangential flow sweeps particles toward the filter exit and reduces cake buildup.     

A review on hollow fiber membrane module towards high separation efficiency: Process modeling in fouling perspective

Hollow fiber microfiltration (MF) and ultrafiltration (UF) membrane processes have been extensively used in water purification and biotechnology. However, complicated filtration hydrodynamics wield a negative influence on fouling mitigation and stability of hollow fiber MF/UF membrane processes. Thus, establishing a mathematical model to understand the membrane processes is essential to guide the optimization of module configurations and to alleviate membrane fouling. Here, we present a comprehensive overview of the hollow fiber MF/UF membrane filtration models developed from different theories. The existing models primarily focus on membrane fouling but rarely on the interactions between the membrane fouling and local filtration hydrodynamics. Therefore, more simplified conceptual models and integrated reduced models need to be built to represent the real filtration behaviors of hollow fiber membranes. Future analyses considering practical requirements including complicated local hydrodynamics and nonuniform membrane properties are suggested to meet the accurate prediction of membrane filtration performance in practical application. This review will inspire the development of high-efficiency hollow fiber membrane modules.     

Secondary membranes for flux optimization in membrane filtration of biologic suspensions

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

Membrane microfiltration of oily water

Separation of oil in water emulsion was carried out by crossflow microfiltration using 3 types of microporous glass tubular membrane with different pore size of 0.27, 0.75, and 1.47 μm. The effect of pore size on permeate flux and oil rejection was investigated and the filtration mechanisms were analyzed based on various types of filtration models.     

Local properties of cake in cross-flow microfiltration of submicron particles

The local properties of filter cakes, such as porosity and specific filtration resistance, in cross-flow microfiltration of submicron particles are studied based on an analysis of force. The packing of particles in a filter cake can be divided into two modes. When the solid compressive pressure is smaller than the critical value, there exists an equilibrium distance between neighbouring particles due to the electrostatic repulsive force, and the local cake porosity can be estimated by using the cell model proposed in this study. When the solid compressive pressure is greater than the critical value, the compressive force can overcome the repulsive barrier, the particles then come into contact with neighbours, and the power-type empirical relationship between cake porosity and solid compressive pressure can be employed to estimate the local cake porosity. It can be found that the half of the cake near the filter membrane has a compact structure, and a high filtration resistance within the operating conditions of this study. On the other hand, the portion of cake near the cake surface has a high porosity due to the separation of particles. By using this model, the effect of electrolyte concentration on cake properties can be analyzed, and the estimated values of average porosity and average specific filtration resistance under various electrolyte concentrations, cross-flow velocities, and filtration pressures agree fairly well with the experimental data.     

The effect of oscillatory flow on crossflow microfiltration of beer in a tubular mineral membrane system – Membrane fouling resistance decrease and energetic considerations

We have investigated the consequences due to the changes in hydrodynamics above the membrane surface brought about by an oscillatory flow in the crossflow microfiltration (CFMF) of beer on a tubular mineral membrane. Experimental results in oscillatory flow filtration were analysed in terms of membrane resistance to filtration and energy consumption and compared with steady flow filtration. Two types of beers were used: a clarified beer composed of colloids and macromolecular material and a rough beer containing in addition yeast cells. Oscillatory flow was found to decrease membrane fouling resistance (up to 100%) in rough beer filtration in the presence of a yeast cell cake layer on the membrane surface, whereas it has no effect in clarified beer filtration in the presence of membrane clogging. The detrimental effect of transmembrane pressure on membrane resistance (at ΔP>1 bar) has been emphasized in both oscillatory and steady flows. The time-average hydraulic power dissipated by friction in the filtration module, in relation with the absolute value of the time-average flow rate in oscillatory flow, was found to be systematically higher than for steady flow filtration. However, the hydraulic energy per unit volume of permeate in the microfiltration of rough beer under oscillatory flow was close to that in steady flow at a time-average tangential velocity of 3 m/s. By considering the specific energy (per m³ of permeate) related to the kinetic energy applied to fluid in oscillatory and steady flow modes, the system by gas compression in oscillatory flow led to a reduction of specific energy ranging from 15% to 40%. Finally the ratio of hydraulic power consumed in oscillatory and steady flow was compared with a theoretical calculation based on the assumption that the oscillating flow regime is quasi-steady.     

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.