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).     

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.     

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.     

Role of backpulsing in fouling minimization in crossflow filtration with ceramic membranes

Effect of backpulsing on crossflow filtration of different process streams was studied. Laboratory scale experiments were conducted with synthetic electroplating wastewater containing Cr(OH)₃ suspension. Porous ceramic membranes of various pore sizes (0.05–5.0 μm) were evaluated. Filtration experiments with and without backpulsing show that backpulsing is effective in minimizing membrane fouling. Up to five-fold increase in steady-state permeate flux and 100% flux recovery were observed. Theoretical aspects are reviewed to develop a better understanding of the critical parameters associated with high-pressure backpulsing.Pilot and commercial scale operating results on several industrial applications, such as yeast filtration, process slurry filtration and oily wastewater filtration are presented. Data analysis shows the critical importance of backpulsing in reducing long-term membrane fouling while allowing the realization of high product recovery. Optimization of process parameters with backpulsing typically results in higher flux and reduces the total capital cost required to achieve the desired production rate.     

Experimental evaluation of a uniform transmembrane pressure crossflow microfiltration unit for the concentration of micellar casein from skim milk

A uniform transmembrane pressure (UTMP) crossflow microfiltration (CFMF) system maintains a low but uniform transmembrane pressure (ΔP_TM) with high crossflow velocity (CFV), which reduces fouling and cake build-up, and improves the utilization of available filtration area. A CFMF system, with a 0.2 μm nominal pore size ceramic filter, filtration area 0.184 m², was operated in both UTMP and non-UTMP modes. The two modes were compared for their effectiveness in maintaining a steady flux during the separation of casein micelles from skim milk up to a concentration factor (CF) 10 at 50°C. Experiments were performed at an average CFV of 7.2 m s⁻¹ and ΔP_TM from 89 to 380 kPa. Up to CF 4 the non-UTMP mode maintained a slightly better flux and process time than the UTMP mode, but reached the minimally acceptable flux (below 0.005 kg m⁻² s⁻¹) at CF 6. Depending upon the ΔP_TM maintained, the UTMP mode approached the minimal flux at CF 7 or 10 depending upon the combination of ΔP_TM and CFV used. Cake resistance (R_cm) was modified to include the effect of an increase in retentate viscosity with concentration. R_cm increased for the non-UTMP mode and decreased for the UTMP mode with a decrease in the ratio of permeation flux/wall shear stress (J_p/τ_w) (which occurred as the retentate gets concentrated). This indicated that the cake formed during the non-UTMP mode of operation was more compact and durable (harder to erode) than in the UTMP mode. A central composite rotatable design estimated the optimal operating region at a CFV of 7.1 m s⁻¹ and ΔP_TM of 241±10 kPa to achieve maximum flux and a high concentration.     

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.     

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.     

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.     

Evaluation of crossflow filtration models based on shear-induced diffusion and particle adhesion: Complications induced by feed suspension polydispersivity

Specific flux data were obtained during the transient period of flux decline in laminar crossflow filtration. Effects of hydrodynamics on cake parameters such as specific resistance, mass and particle size distribution were studied experimentally. An evaluation of crossflow filtration models suggests that a model based on shear-induced diffusion [1] is a better predictor of specific flux decline than a particle adhesion model [2]. Even for relatively narrowly distributed suspensions, polydispersivity complicates analyses in a manner that is not adequately addressed by these models. Changes in experimental specific cake resistances with module hydrodynamics coupled to the inadequacy of these models for accurately predicting time-dependent specific flux profiles, cake specific resistances, and mass suggests that cake morphology is a key variable that needs to be incorporated in future modeling efforts.     

A combined osmotic pressure and cake filtration model for crossflow nanofiltration of natural organic matter

A combined osmotic pressure and cake filtration model for crossflow nanofiltration of natural organic matter (NOM) was developed and successfully used to determine model parameters (i.e. permeability reduction factor (η) and specific cake resistance (α_cake)) for salt concentrations, NOM concentrations, and ionic strength of salt species (Na⁺ and Ca⁺⁺). In the absence of NOM, with increasing salt concentration from 0.004 to 0.1 M, permeability reduction factor (η)) decreased from 0.99 to 0.72 and 0.94 to 0.44 for monovalent cation (Na⁺) and divalent cation (Ca⁺⁺), respectively. This reduced membrane permeability was due to salt concentrations and salt species. In the presence of NOM, specific cake resistance tended to increase with increasing NOM concentration and ionic strength in the range of 0.85 × 10¹⁵–3.66 × 10¹⁵ m kg⁻¹. Solutions containing divalent cation exhibited higher normalized flux decline (J_v/J_vo = 0.685–0.632) and specific cake resistance (α_cake = 2.89 × 10¹⁵–6.24 × 10¹⁵ m kg⁻¹) than those containing monovalent cation, indicating a highly compacted NOM accumulation, thus increased permeate flow resistance during NF filtration experiments. After membrane cleaning, divalent cation exhibited lower water flux recovery than monovalent cation, suggesting higher non-recoverable (R_non-rec) resistance than monovalent cation.     

Effect of particle shape on crossflow filtration flux

In an effort to further increase the understanding of crossflow filtration, experiments were performed on the influence of particle shape on permeation flux. Five particles of similar density and size distribution but of different shapes were used to test the influence of particle shape, while varying experimental parameters such as crossflow velocity, filtration pressure, solids concentration, membrane morphology and pore size. Particle shape was found to influence the equilibrium flux by the structure of the cake layer formed. Irregularly shaped particles such as branched carbon particles provided higher fluxes due to the high voidage cakes. More regularly shaped particles such as glass spheres resulted in lower fluxes. Platelet aluminium particles had relatively high filtration rates due to the gaps between the plates. The effects of the other experimental parameters typically showed results consistent with previous publications. Using the measured cake mass, a theoretical model based on D'Arcy and Kozeny gave reliable filtration flux compared to the experimental results.     

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.     

A novel approach for modeling concentration polarization in crossflow membrane filtration based on the equivalence of osmotic pressure model and filtration theory

A theoretical model for prediction of permeate flux during crossflow membrane filtration of rigid hard spherical solute particles is developed. The model utilizes the equivalence of the hydrodynamic and thermodynamic principles governing the equilibrium in a concentration polarization layer. A combination of the two approaches yields an analytical expression for the permeate flux. The model predicts the local variation of permeate flux in a filtration channel, as well as provides a simple expression for the channel-averaged flux. A criterion for the formation of a filter cake is presented and is used to predict the downstream position in the filtration channel where cake layer build-up initiates. The predictions of permeate flux using the model compare remarkably well with a detailed numerical solution of the convective diffusion equation coupled with the osmotic pressure model. Based on the model, a novel graphical technique for prediction of the local permeate flux in a crossflow filtration channel has also been presented.     

Influence of inorganic scalants and natural organic matter on nanofiltration membrane fouling

The influence of inorganic scalants and NOM on nanofiltration (NF) membrane fouling was investigated by a crossflow bench-scale test cell. Mathematical fouling models were used to determine kinetics and fouling mechanisms of NF membrane. It was observed that, with natural organic matter (NOM) at a concentration of 10 mg L⁻¹, divalent cation, i.e. calcium (Ca²⁺), exhibited greater flux decline than monovalent cation, i.e. sodium (Na⁺), while solution flux curves dominated cake formation model, especially at high ionic strength. For inorganic scalants of polyanions, i.e. carbonate (CO₃²⁻), sulphate (SO₄²⁻), and phosphate (PO₄³⁻), solution flux curves were relatively fitted well with pore blocking model, possibly due to precipitated species formed and blocked on membrane surface and/or pores. For different divalent cations (i.e. calcium and magnesium (Mg²⁺)), calcium showed greater flux decline than magnesium, possibly due to higher concentration of precipitated calcium species than that of precipitated magnesium species based on the pC (−log concentration) and pH diagram.     

Synergetic cleaning procedure for a ceramic membrane fouled by beer microfiltration

Apart from considerations for hygienic operation, membrane cleaning is essential to maintain consistent permeability and selectivity of membrane systems for clarifying beer and beverages where balanced fractionation of particles/macromolecules is necessary. Experiments involved formulating and optimising chemical cleaning methods for a ceramic microfiltration membrane, which had been severely fouled during clarification of a commercial beer. The cleaning processes employed NaOH, HNO₃, H₂O₂, and Ultrasil 11 as the chemical cleaning agents. The cleaning ability and cleaning kinetics of the processes were evaluated in parallel with the study of the fouling mechanism, formation and strength so as to elucidate the synergetic relationship between fouling and cleaning. A three-step cleaning mechanism was postulated. This led to the development of a fast and effective combined simultaneous caustic cleaning and oxidation method (CSCCO), which was able to restore 87% of the original membrane's water permeability within 8 min. Analysis suggested the concept of a cleaning energy barrier E_c and a cleaning rate constant k_c0. This study confirmed the existence of a synergetic relationship between the prior fouling and optimum formulation of cleaner and optimal cleaning condition. The study varied beer filtration conditions. Transmembrane pressure (TMP) and crossflow velocity during fouling appeared to have a minimal effect on the membrane's subsequent cleanability, especially when the powerful CSCCO process was employed. The number of previous fouling/cleaning cycles was influential. A complete removal of the residual fouling, formed on the virgin membrane's surface proved beyond the means of the harsh chemical cleaning used under any conditions. The degree of residual fouling eventually reached a plateau and a level of 87% of the original water flux could be restored repeatedly.     

Self-forming dynamic membrane for ultrafiltration of pineapple juice

The formation of self-forming dynamic membrane on a porous ceramic support was studied. Pineapple juice of 12° Brix concentration was used in the experiments which were carried out at 25°C by circulating the pineapple juice at the applied pressure of 100, 200, and 300 kPa and at cross-flow velocities of 1.30–2.95 m s⁻¹ through the ceramic membrane module for 1 h. The experimental data of flux and rejections showed that the dynamic membrane was well-formed after 30 min of circulation under the applied pressure of 300 kPa and at a cross-flow velocity of 2.0 m s⁻¹ in which the steady values of flux and rejections of macromolecules and sugars obtained from the filtration mode were 6.0×10⁻³ m³/m² h, 84–87% and 6%, respectively. The corresponding values for ultrafiltration by alumina membrane of MW cut-off 50,000, using equivalent conditions, were 15.8×10⁻³ m³/m² h, 91% and 10.5%. Ultrafiltration was found to be more promising. The stability of the self-forming dynamic membrane was acceptable when subjected to change of filtration conditions. The permeation flux increased with cross-flow velocity and decreased when the applied pressure was reduced. The resistances for filtration by dynamic membrane and by ultrafiltration were calculated. For a porous support of large pore sizes, an in-pore blockage of solutes which were smaller than the membrane pores reduced the pore volume and induced fouling. Internal fouling resistance (R_f) was, therefore significant and responsible for the values of flux and rejection and was approximately 70% of total resistance. While in ultrafiltration, in which membrane with a smaller pore diameter was used, R_f was only 20% but R_p, the polarized layer resistance, was as high as 60% of total resistance.     

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.     

Optimisation of dead-end filtration conditions for an immersed anoxic membrane bioreactor

The optimisation of the energy demand in the application of dead-end filtration in an immersed membrane bioreactor applied to groundwater denitrification has been studied. Filtration cycle length was varied at a set flux to control the amount of foulant deposited at the membrane surface. Physical cleans comprising a simultaneous backflush and gas injection were subsequently instigated and the reversibility of the deposit determined by the residual resistance, R_res. Examination of R_res versus flux and cycle length variation indicated an operational envelope where limited fouling occurred. The transition from limited fouling to extensive fouling was indicated by a parameter based on the critical accumulated mass, indicating incipient deposit consolidation. The transition between regions became less severe when the solids retention time was increased from 10 to 25 and 40 days. This was apparently related to a shift in bulk physical characteristics. Nevertheless, low residual fouling was observed during long-term filtration when operating below the critical mass, resulting in a 20× reduction in energy demand over that of constant gas injection.     

Quantitative measurements of the concentration polarisation layer thickness in membrane filtration of oil-water emulsions using NMR micro-imaging

In this paper we report measurements of the thickness of the concentration polarisation layers formed during crossflow membrane filtration of an oil-water emulsion. The formation and development of the oil polarisation layers was visualised non-invasively using NMR chemical shift selective micro-imaging. A series of images was acquired during the transient state of the filtration, (i.e. while the polarisation layer was forming and the flux of filtrate was changing), prior to the establishment of steady state conditions. An estimate of the specific resistance of the concentration polarisation layers was then obtained by determining the average oil layer thicknesses and concentration at a given time from the resulting images and measuring the corresponding (length averaged) flux of filtrate gravimetrically. After the establishment of steady state conditions, the dependence of the steady state filtrate flux on crossflow Reynolds number was found to be consistent with Brownian diffusion being the main mechanism controlling the build-up of the oil polarisation layers, at least under our range of operating conditions.