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.     

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.     

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.     

Critical flux concept for microfiltration fouling

Several constant-flux filtration experiments for yeast cell suspensions, yeast cell debris, and dodecane-water emulsion were performed at various operating conditions in both flat-sheet and tubular-membrane systems. The aim of the paper is two-fold. Firstly the relationship between constant-flux behaviour and membrane fouling is discussed. In some cases constant-flux filtration was realized at a constant transmembrane pressure which was below a critical value. In general constant-flux filtration was obtained with moderately increasing transmembrane pressure, and this approach is shown to have some advantages over normal constant-pressure filtration because it clearly provides for the possibility of avoiding over-fouling and so reduces the severity of fouling. Secondly, the concept of critical flux is introduced. Whilst it has long been recognised that low-pressure microfiltration is much more effective than high-pressure microfiltration, the emphasis in this work is upon the possible existence of a critical flux and the desirability of starting filtration operations at a low flux. The critical-flux hypothesis is that on start-up there exists a flux below which a decline of flux with time does not occur. Equations which may enable identification of the appropriate flux level are included.     

Control of concentration polarization and fouling of membranes in medical,food and biotechnical applications

Concentration polarization and fouling pose serious problems in ultrafiltration and microfiltration of protein-containing liquids. An empirical approach is reported to reduce fouling through surface modification by rf-cathode sputtering and plasma deposition. Different effects on ultrafiltration and microfiltration membranes have been found. In a second approach, by application of pulsatile flow the transmembrane flux of ultrafiltration membranes could be improved up to 50%.     

Spatial distribution of foulants on membranes during ultrafiltration of protein mixtures and the influence of spacers in the feed channel

Using matrix assisted laser desorption ionisation mass spectrum (MALDI-MS), this study reports the observations of the fouling distribution and composition along the membrane channel after the membranes were subjected to ultrafiltration of protein mixture solution in a crossflow module with and without spacer. In the fouling layer on a fully retentive membrane, the protein components with high molecular weight has higher presentation after 2 h of filtration and the presentation reduced to be lower than the smaller components after 6 h of filtration due to protein exchange and displacement phenomena in deposition layer caused by the differences in structure and diffusivity of different components. The protein exchange and replacement in the deposition layer was also observed on partial retentive membrane using a sequential fouling procedure. Fouling distribution along the membrane channel with spacer inserted in the module was more uniform and the flux was higher than that without spacer despite higher protein deposition observed in some cases.     

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.     

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.     

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.     

Fouling of reverse osmosis membranes by biopolymers in wastewater secondary effluent: Role of membrane surface properties and initial permeate flux

Reverse osmosis (RO) is being increasingly used in treatment of domestic wastewater secondary effluent for potable and non-potable reuse. Among other solutes, dissolved biopolymers, i.e., proteins and polysaccharides, can lead to severe fouling of RO membranes. In this study, the roles of RO membrane surface properties in membrane fouling by two model biopolymers, bovine serum albumin (BSA) and sodium alginate, were investigated. Three commercial RO membranes with different surface properties were tested in a laboratory-scale cross-flow RO system. Membrane surface properties considered include surface roughness, zeta potential, and hydrophobicity. Experimental results revealed that membrane surface roughness had the greatest effect on fouling by the biopolymers tested. Accordingly, modified membranes with smoother surfaces showed significantly lower fouling rates. When Ca²⁺ was present, alginate fouled RO membranes much faster than BSA. Considerable synergistic effect was observed when both BSA and alginate were present. The larger foulant particle sizes measured in the co-existence of BSA and alginate indicate formation of BSA-alginate aggregates, which resulted in greater fouling rates. Faster initial flux decline was observed at higher initial permeate flux even when the flux was measured against accumulative permeate volume, indicating a negative impact of higher operating pressure.     

Fouling of nanofiltration membranes used for separation of fermented glycerol solutions

In this study, the glycerol solutions were fermented using Lactobacillus casei bacteria. The broths were pre-treated by microfiltration, followed by a further separation with nanofiltration. The latter process was carried out in two stages, using the NF270 and NF90 membranes, respectively. The concentrates thus obtained were enriched with citric acid (first stage) and then with lactic acid and glycerol (second stage). By means of SEM and AFM microscopy, as well as ATR-FTIR analysis, the intensity of membrane-fouling was studied. The colloidal fouling and bio-fouling caused a more than two-fold decrease in the permeate flux during microfiltration of the broth. This pre-treatment stage was effective, and a permeate turbidity of less than 0.2 NTU was obtained. However, the nanofiltration membranes exhibited a 30 % flux decline over the course of the process, mainly due to the organic fouling.     

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.     

Removal of natural organic matter by ultrafiltration with TiO2-coated membrane under UV irradiation

This study investigates the performance of ultrafiltration (UF) by membranes coated with titanium dioxide (TiO2) photocatalyst under ultraviolet (UV) illumination in removing natural organic matter (NOM) and possibly in reducing membrane fouling. Experiments were carried out using heat-resistant ceramic disc UF membranes and humic acids as model substances representing naturally occurring organic matter. Membrane sizes of 1, 15, and 50 kDa were used to examine the effects of coating under ultraviolet irradiation. A commercial humic solution was subjected to UF fractionation (batch process); gel filtration chromatography was applied to study the effects of molecular weight distribution of NOM on UF membrane fouling. When compared to naked membranes, UV254 (ultraviolet light of lambda=254 nm) illumination of TiO2-coated membranes exhibits more flux decline with similar effluent quality. Although the UF membrane is able to remove a significant amount of humic materials, the incorporated photocatalysis results in poor performance in terms of permeate flux. The TiO2-coated membrane under UV254 irradiation alters the molecular weight (MW) distribution of humic materials, reducing them to <1 kDa, which is smaller than the smallest (1-kDa) membrane in this study. Thus, TiO2-coated membranes under UV254 irradiation do not perform any better in removing natural organic matter and reducing membrane fouling.     

Effect of the surface modification of polymer membranes on their microbiological fouling

Composite polymer membranes with chemically different surfaces are prepared by the photochemical modification of Millipore microfiltration poly(vinylidene fluoride) and polysulfone membranes using 2-acrylamido-2-methyl-1-propanesulfonic acid, 2-hydroxyethyl methacrylate, and 2-(dimethylamino)ethyl methacrylate quaternized with methyl chloride. It is shown that, during the filtration of an E. coli suspension, the membrane flux substantially decreases with time owing to the fouling of the membrane surface by bacterial cells. The membranes with the hydrophilic surface are less susceptible to fouling than hydrophobic membranes, while the ability to recover the performance upon washing is higher for the membranes with a chemically neutral surface than for charged membranes. It is shown that the susceptibility of membranes to microbiological fouling reduces with a decrease in the roughness of the membrane surface. It is established that the membranes modified with the quaternized 2-(dimethylamino)ethyl methacrylate possess antibacterial properties. These membranes proved to be the most efficient in the filtration of natural surface water in a noncontinuous regime, a result that is explained by the ability of membranes to prevent the formation of a fouling biofilm on their surfaces.     

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.     

Probing Membrane Fouling via Infrared Attenuated Total Reflection Mapping Coupled with Multivariate Curve Resolution

Understanding membrane fouling induced by dissolved organic matter (DOM) is of primary importance for developing effective fouling control and prevention strategies. In this work, we combine multivariate curve resolution–alternating least squares analysis with infrared attenuated total reflection mapping to explore the fouling process of microfiltration and ultrafiltration membranes caused by two typical DOMs, humic acid (HA) and bovine serum albumin (BSA). The spectral contributions of different foulants and the membrane substrate were successfully discriminated, thereby enabling the diagnosis of fouling origins. Membrane fouling caused by HA is more severe than that by BSA. Three periods, the initial adsorption stage, the equilibrium stage, and the accumulation stage, were observed for the HA‐induced fouling process. The integrated approach presented herein elegantly demonstrates the spatial and temporal characterization of membrane fouling processes, along with relative concentrations of the involved species, and suggests a promising perspective for understanding the interaction mechanisms between foulant species and membranes at the molecular level.     

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.     

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

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

Performance of partially permeable microfiltration membranes under low fouling conditions

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