Fouling of nanofiltration, reverse osmosis, and ultrafiltration membranes by protein mixtures: the role of inter-foulant-species interaction

Protein fouling of nanofiltration (NF), reverse osmosis (RO), and ultrafiltration (UF) membranes by bovine serum albumin (BSA), lysozyme (LYS), and their mixture was investigated under cross-flow conditions. The effect of solution chemistry, membrane properties, and permeate flux level was systematically studied. When the solution pH was within the isoelectric points (IEPs) of the two proteins (i.e., pH 4.7-10.4), the mixed protein system experienced more severe flux decline compared to the respective single protein systems, which may be attributed to the electrostatic attraction between the negatively charged BSA and positively charged LYS molecules. Unlike a typical single protein system, membrane fouling by BSA-LYS mixture was only weakly dependent on solution pH within this pH range, and increased ionic strength was found to enhance the membrane flux as a result of the suppressed BSA-LYS electrostatic attraction. Membrane fouling was likely controlled by foulant-fouled-membrane interaction under severe fouling conditions (elevated flux level and unfavorable solution chemistry that promotes fouling), whereas it was likely dominated by foulant-clean-membrane interaction under mild fouling conditions. Compared to nonporous NF and RO membranes, the porous UF membrane was more susceptible to dramatic flux decline due to the increased risk of membrane pore plugging. This study reveals that membrane fouling by mixed macromolecules may behave very differently from that by typical single foulant system, especially when the inter-foulant-species interaction dominates over the intra-species interaction in the mixed foulant system.     

A physicochemical investigation of membrane fouling in cold microfiltration of skim milk

The main challenge in microfiltration (MF) is membrane fouling, which leads to a significant decline in permeate flux and a change in membrane selectivity over time. This work aims to elucidate the mechanisms of membrane fouling in cold MF of skim milk by identifying and quantifying the proteins and minerals involved in external and internal membrane fouling. Microfiltration was conducted using a 1.4-μm ceramic membrane, at a temperature of 6 ± 1°C, cross-flow velocity of 6 m/s, and transmembrane pressure of 159 kPa, for 90 min. Internal and external foulants were extracted from a ceramic membrane both after a brief contact between the membrane and skim milk, to evaluate instantaneous adsorption of foulants, and after MF. Four foulant streams were collected: weakly attached external foulants, weakly attached internal foulants, strongly attached external foulants, and strongly attached internal foulants. Liquid chromatography coupled with tandem mass spectrometry analysis showed that all major milk proteins were present in all foulant streams. Proteins did appear to be the major cause of membrane fouling. Proteomics analysis of the foulants indicated elevated levels of serum proteins as compared with milk in the foulant fractions collected from the adsorption study. Caseins were preferentially introduced into the fouling layer during MF, when transmembrane pressure was applied, as confirmed both by proteomics and mineral analyses. The knowledge generated in this study advances the understanding of fouling mechanisms in cold MF of skim milk and can be used to identify solutions for minimizing membrane fouling and increasing the efficiency of milk MF.     

Relating organic fouling of reverse osmosis membranes to intermolecular adhesion forces

Organic fouling of reverse osmosis (RO) membranes and its relation to foulant--foulant intermolecular adhesion forces has been investigated. Alginate and Suwannee River natural organic matter were used as model organic foulants. Atomic force microscopy was utilized to determine the adhesion force between bulk organic foulants and foulants deposited on the membrane surface under various solution chemistries. The measured adhesion force was related to the RO fouling rate determined from fouling experiments under solution chemistries similar to those used in the AFM measurements. A remarkable correlation was obtained between the measured adhesion force and the fouling rate under the solution chemistries investigated. Fouling was more severe at solution chemistries that resulted in larger adhesion forces, namely, lower pH, higher ionic strength, presence of calcium ions (but not magnesium ions), and higher mass ratio of alginate to Suwannee River natural organic matter. The significant adhesion force measured with alginate in the presence of calcium ions indicated the formation of a crossed-linked alginate gel layer during fouling through intermolecular bridging among alginate molecules.     

Characterization of Proteinaceous Membrane Foulants and Flux Decline During the Early Stages of Whole Milk Ultrafiltration

Pasteurized whole milk was fractionated with a pilot-scale, plate and frame, ultrafiltration system to study membrane fouling and flux decline. Concentration factor was set at approximately 1.4× to simulate the first stage of a multistage UF system. Proteinaceous membrane foulant was characterized by SDS-PAGE. Distribution of proteins in the foulant was very different from distribution of proteins in milk. Whey proteins, α-lactalbumin and β-lactoglobulin, accounted for 95% of the proteinaceous membrane foulants. Very little casein was identified as membrane foulant.The approximate amount of protein in the membrane foulant was estimated to be .6 g/m² of membrane area. Permeate flux studies indicated that flux decline is severe in the early stages of milk ultrafiltration and is associated with irreversible adsorption of protein on the membrane surface. A threefold difference between the water flux of clean membranes and fouled membranes was attributed to the adsorbed foulant. Identification and characterization of membrane foulants and the mechanism of their interaction with membrane surfaces should lead to the design of more efficient ultrafiltration systems for the dairy industry.     

Organic fouling and chemical cleaning of nanofiltration membranes: measurements and mechanisms

Fouling and subsequent chemical cleaning of nanofiltration (NF) membranes used in water quality control applications are often inevitable. To unravel the mechanisms of organic fouling and chemical cleaning, it is critical to understand the foulant-membrane, foulant-foulant, and foulant-cleaning agent interactions at the molecular level. In this study, the adhesion forces between the foulant and the membrane surface and between the bulk foulant and the fouling layer were determined by atomic force microscopy (AFM). A carboxylate modified AFM colloid probe was used as a surrogate for humic acid, the major organic foulant in natural waters. The interfacial force data were combined with the NF membrane water flux measurements to elucidate the mechanisms of organic fouling and chemical cleaning. A remarkable correlation was obtained between the measured adhesion forces and the fouling and cleaning behavior of the membrane under various solution chemistries. The AFM measurements further confirmed that divalent calcium ions greatly enhance natural organic matter fouling by complexation and subsequent formation of intermolecular bridges among organic foulant molecules. Efficient chemical cleaning was achieved only when the calcium ion bridging was eliminated as a result of the interaction between the chemical cleaning agent and the fouling layer. The cleaning efficiency was highly dependent on solution pH and the concentration of the chemical cleaning agent.     

Membrane independent limiting flux for RO and NF membranes fouled by humic acid

The flux decline of reverse osmosis and nanofiltration membranes was investigated under constant pressure conditions during humic acid fouling tests. For a given membrane type under a given feedwater composition, increasing pressure resulted in increased flux reduction and foulant accumulation. A limiting flux seems to exist beyond which the membrane flux cannot be sustained. Membranes with initial fluxes greater than the limiting flux experienced severe fouling and their pseudo stable fluxes approached the limiting flux. Flux reduction was much milder when the initial flux was lower than the limiting flux. Furthermore, the limiting flux seems to be independent of membrane properties, probably due to the dominance of foulant--deposited-foulant interaction upon complete foulant coverage over membrane surfaces. On the other hand, strong dependence of the limiting flux on the feedwater composition was observed. The limiting flux was reduced at higher proton, calcium, and/or background electrolytes concentrations, likely due to reduced electrostatic repulsion under these conditions.     

Studying the role of common membrane surface functionalities on adsorption and cleaning of organic foulants using QCM-D

Adsorption of organic foulants on nanofiltration (NF) and reverse osmosis (RO) membrane surfaces strongly affects subsequent fouling behavior by modifying the membrane surface. In this study, impact on organic foulant adsorption of specific chemistries including those in commercial thin-film composite membranes was investigated using self-assembled monolayers with seven different ending chemical functionalities (-CH(3), -O-phenyl, -NH(2), ethylene-glycol, -COOH, -CONH(2), and -OH). Adsorption and cleaning of protein (bovine serum albumin) and polysaccharide (sodium alginate) model foulants in two solution conditions were measured using quartz crystal microbalance with dissipation monitoring, and were found to strongly depend on surface functionality. Alginate adsorption correlated with surface hydrophobicity as measured by water contact angle in air; however, adsorption of BSA on hydrophilic -COOH, -NH(2), and -CONH(2) surfaces was high and dominated by hydrogen bond formation and electrostatic attraction. Adsorption of both BSA and alginate was the fastest on -COOH, and adsorption on -NH(2) and -CONH(2) was difficult to remove by surfactant cleaning. BSA adsorption kinetics was shown to be markedly faster than that of alginate, suggesting its importance in the formation of the conditioning layer. Surface modification to render -OH or ethylene-glycol functionalities are expected to reduce membrane fouling.     

Membrane fouling in pilot-scale membrane bioreactors (MBRs) treating municipal wastewater

The main obstacle for wider use of membrane bioreactors (MBRs) for wastewater treatment is membrane fouling (i.e., deterioration of membrane permeability),which increases operating costs. For more efficient control of membrane fouling in MBRs, an understanding of the mechanisms of membrane fouling is important. However, there is a lack of information on membrane fouling in MBRs, especially information on features of components that are responsible for the fouling. We conducted a pilot-scale experiment using real municipal wastewater with three identical MBRs under different operating conditions. The results obtained in this study suggested that the food-microorganisms ratio (F/M) and membrane filtration flux were the important operating parameters that significantly influenced membrane fouling in MBRs. Neither concentrations of dissolved organic matter in the reactors nor viscosity of mixed liquor, which have been thought to have influences on fouling in MBRs, showed clear relationships with membrane fouling in this study. Organic substances that had caused the membrane fouling were desorbed from fouled membranes of the MBRs at the termination of the operation and were subjected to Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (NMR) analyses. These analyses revealed that the nature of the membrane foulant changes depending on F/M. It was shown that high F/M would make the foulant more proteinaceous. Carbohydrates were dominant in membrane foulants in this study, while features of humic substances were not apparent.     

Chicory juice clarification by membrane filtration using rotating disk module

Clarification is the first step of inulin production from chicory juice, and membrane filtration as an alternative can greatly simplify this process, increase juice yield, improve product quality, and reduce the cost and waste volume. In this study, a rotating disk module (RDM) was used to investigate the clarification of chicory juice by four micro- and ultrafiltration membranes. Compared with dead end filtration, the RDM had a much higher permeate flux and product quality. High rotating speeds produced high permeate fluxes and reduced flux decline, because of the strong back transport of foulant from fouling layer to feed solution. At high rotating speeds of 1500–2000 rpm, the permeate flux increased with membrane pore size and transmembrane pressure (TMP), while at low rotating speeds (<1000 rpm), permeate flux was independent of membrane type and TMP due to a thick deposited fouling layer as a dominant filtration resistance, while carbohydrate transmission decreased at higher TMP because of denser cake layer as an additional selective membrane. The highest carbohydrate transmission (∼98%) and desirable permeate turbidity (2.4 NTU) was obtained at a TMP of 75 kPa and a rotating speed of 2000 rpm for FSM0.45PP membrane. With the RDM, the Volume Reduction Ratio (VRR) could reach 10 with a high permeate flux (106 L m⁻² h⁻¹) in the concentration test, and permeate was still rich in carbohydrate and well clarified. Chemical cleaning with 0.5% P3-ultrasil 10 detergent solution was able to recover 90% water flux of fouled membrane.     

Fouling of microfiltration and ultrafiltration membranes by natural waters

Membrane filtration (microfiltration and ultrafiltration) has become an accepted process for drinking water treatment, but membrane fouling remains a significant problem. The objective of this study was to systematically investigate the mechanisms and components in natural waters that contribute to fouling. Natural waters from five sources were filtered in a benchtop filtration system. A sequential filtration process was used in most experiments. The first filtration steps removed specific components from the water, and the latter filtration steps investigated membrane fouling by the remaining components. Particulate matter (larger than 0.45 microm) was relatively unimportant in fouling as compared to dissolved matter. Very small colloids, ranging from about 3-20 nm in diameter, appeared to be important membrane foulants based on this experimental protocol. The colloidal foulants included both inorganic and organic matter, but the greatest fraction of material was organic. When the colloidal fraction of material was removed, the remaining dissolved organic matter (DOM), which was smaller than about 3 nm and included about 85-90% of the total DOM, caused very little fouling. Thus, although other studies have identified DOM as a major foulant during filtration of natural waters, this work shows that a small fraction of DOM may be responsible for fouling. Adsorption was demonstrated to be an important mechanism for fouling by colloids.     

Ultrafiltration fouling of amylose solution: Behavior, characterization and mechanism

Applications of ultrafiltration membrane often deal with feed streams containing amylose starch. This paper describes a detailed investigation of amylose fouling during ultrafiltration. Commercial membranes made of polysulfone and fluoro polymer were used. Both adsorptive and ultrafiltration fouling were investigated. Experiments using different membrane characteristics, feed concentrations and trans-membrane pressures were carried out. The resulting fouling was characterized by water flux and contact angle measurements and was visualized by scanning electron microscopy (SEM). The results suggest that solute adsorption has occurred as noticed by significant water flux reductions as well as changes in membrane characteristics. Further, both reversible and irreversible fouling have occurred during ultrafiltration with irreversible fouling was more dominant. Apparently, cake layer formation initiated by either adsorption due to hydrophobic–hydrophobic interactions or pore blocking is the dominant fouling mechanism. However, pore narrowing instead of pore blocking was also observed for the membrane having large and relative uniform pore structure or for the ultrafiltration using low trans-membrane pressure or low solute concentration. Membrane autopsy using SEM confirmed the formation of solute layer on the membrane surface.     

Polyelectrolyte and silver nanoparticle modification of microfiltration membranes to mitigate organic and bacterial fouling

Membrane fouling remains one of the most problematic issues surrounding membrane use in water and wastewater treatment applications. Organic and biological fouling contribute to irreversible fouling and flux decline in these processes. The aim of this study was to reduce both organic and biological fouling by modifying the surface of commercially available poly(ether sulfone) (PES) membranes using the polyelectrolyte multilayer modification method with poly(styrenesulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), and silver nanoparticles (nanoAg) integrated onto the surface as stable, thin (15 nm) films. PSS increases the hydrophilicity of the membrane and increases the negative surface charge, while integration of nanoAg into the top PSS layer imparts biocidal characteristics to the modified surface. Fouling was simulated by filtering aqueous solutions of humic acid (5 and 20 mg L(-1)), a suspension of Escherichia coli (10(6) colony-forming units (CFU) mL(-1)), and a mixture of both foulants through unmodified and modified PES membranes under batch conditions. Filtration and cleaning studies confirmed that the modification significantly reduced organic and biological fouling.     

正渗透膜污染的影响因素及清洗效果研究

为研究正渗透(FO)浓缩过程中的膜通量衰减规律,本文以牛血清白蛋白(BSA)为特征污染物,研究了正渗透过程中原料液的离子强度及BSA浓度、膜方位等参数不同时FO膜的污染规律,以提高膜通量和截留率为目标,对驱动液的种类、浓度,料液流速进行了优化,并优化了适宜的膜清洗方案.结果表明:原料液中离子强度越大,FO膜的初始通量越...     

Fouling Analysis and Performance of Tubular Ultrafiltration on Pretreated Olive Mill Waste Water

This study deals with the performance of a tubular ultrafiltration system on sieved and centrifuged olive mill waste water. A generalized statistical model was developed describing the impact and the relative importance of major experimental parameters (membrane pore size, transmembrane pressure, feed flow rate, and feed temperature) on permeate flux. According to this model, process pressure appeared to have the largest impact on permeate flux, followed by process temperature. As membrane treatment of such a difficult material largely depends on fouling, a systematic analysis of prevailing fouling mechanisms was also run. Despite sieving and centrifugation of the original waste, membrane fouling caused a flux decline of 60–65% within 15–20 min. Internal fouling, pore blocking, and cake layer formation were all responsible for membrane fouling during the first 40 min of operation. After that period, cake formation appeared to play a predominant role. Based on the proposed generalized model, the relative importance of process parameters can be evaluated and process performance can be improved by proper interventions. Independent of membrane size, fouling is a serious problem to be resolved. The qualitative performance of this process, including chemical oxygen demand distribution, polyphenol profile, and antioxidant capacity, is discussed in a separate paper.     

THE FOULING AND CLEANING OF ULTRAFILTRATION MEMBRANES DURING THE FILTRATION OF MODEL TEA COMPONENT SOLUTIONS

Proteins and polyphenols are the principal fouling constituents in the ultrafiltration (UF) of black tea liquor. The aim of this study was to determine the relative importance of individual components in the fouling process, to investigate any synergetic interactions that were occurring and to compare the cleaning characteristics of different fouled membranes. A 30‐kD molecular weight cutoff polysulfone UF membrane in dead‐end mode was challenged with model solutions of tea components. Model solutions consisted of tea proteins, theaflavins (TFs), thearubigins and caffeine. Sodium hydroxide was used as a cleaning reagent. Permeate flux decline curves were presented for single components and mixtures. Individual component transfer fluxes and rejections were also presented. An unexpected finding was that protein in a mixture with TFs could permeate the membrane to a degree, while a protein solution in the absence of the polyphenol was completely rejected. The inspection of membranes fouled by different solutions revealed different foulant morphologies. Membrane cleaning with 0.2 wt % sodium hydroxide was generally found to be effective.     

Recovery of Citrus Cloud from Aqueous Peel Extract by Microfiltration

Microfiltration of reconsituted aqueous peel extract resulted in linear flux decline with time. Feed stream of reconstituted peel extract does not foul the membrane and flux decline is due solely to increased retentate viscosity. Cloud was concentrated up to 6.3% of washed dry matter, compared with 1.5% achieved by vacuum heat concentration. Permeate was at constant low viscosity during filtration experiment, free of insolubles, and crystal clear. Soluble sugars do not contribute to fouling gel layer; their permeation flux is constant, and is only a function of viscosity. Soluble high-molecular-weight polymers, such as pectins, cause membrane fouling and flux depression. Fluxes for soluble constituents, sucrose and pectin, were higher than those of reconstituted peel extract with similar viscosity.     

Characterization of humic acid fouled reverse osmosis and nanofiltration membranes by transmission electron microscopy and streaming potential measurements

Reverse osmosis and nanofiltration membranes fouled by humic acid were systematically characterized by transmission electron microscopy. All fouled membranes, except those with very low initial flux, were completely covered by a layer of humic acid whose thickness and density were greatly affected by the feedwater composition ([H+] and [Ca2+]) and initial flux. A low-density humic layer (about 0.1 g of purified Aldrich humic acid (PAHA)/cm3) was formed at low initial flux (2 m/day or less) at pH 7 without calcium. It was several times denser at a higher initial flux, pH 4.5, or 1 mM Ca2+. Corresponding to the denser foulant layers under these conditions, PAHA accumulation was greatly increased. The denser foulant layers together with the greater PAHA accumulations were responsible for the severe flux reductions. Both virgin and fouled membranes were characterized by streaming potential measurements. While considerable differences existed for virgin membranes, humic acid fouled membranes exhibited identical surface charge properties. The zeta potential of the fouled membranes was controlled by the humic acid layer due to its complete coverage of the membrane surfaces.     

An innovative lab-scale strategy for the evaluation of Grape Processing Residues (GPR) filterability: Application to GPR valorization by ultrafiltration

The objective of this work was to characterize the foulant propensity of three Grape Processing Residues (GPR) in relation with their physicochemical properties. Therefore, a physicochemical characterization of GPR was firstly carried-out, with a focus on the separation/characterization of relevant size-classes of particles. Then, lab-scale experiments were performed using a dead-end filtration module with different membranes (three average pore diameters) in order to identify for each GPR the main foulant size-classes of particles and the related fouling mechanisms. Results showed that this study allowed getting more insight into how different fractions of GPR are involved in membrane fouling according to their size. Moreover, size-cartographies of GPR foulant compounds were built to provide a better understanding and control of membrane fouling. The obtained results showed that it was possible to use this simple and cost-effective tool to choose the appropriate filtration conditions.Industrial relevanceThe valorization of Grape Processing Residues (GPR) is a major economic and ecological challenge. GPR contain a high amount of phenolic compounds, well known for their several biological properties. In the light of this, membrane technologies have been widely used for the extraction and the purification of these bioactive compounds from GPR. However, the performance of these processes is highly challenged by membrane fouling, which is partially dependent on the feed suspension characteristics. To date, different operating strategies have been used industrially to prevent membrane fouling. However, the use of costly and time-consuming strategies has not often been validated by characterization of the feed suspension prior to filtration. Thus, it is mandatory to analyze GPR intrinsic characteristics in relation with their fouling in order to identify the appropriate operation conditions for large-scale filtration. For this purpose, an innovative strategy was developed in this work. This study was useful to develop a simple and cost-effective tool providing reliable guidelines for the development of efficient pilot-scale processes for the extraction of phenolic compounds from GPR.     

Evaluation of membrane fouling for in-line filtration of oil sands process-affected water: the effects of pretreatment conditions

Membrane filtration is an effective reclamation option for oil sands process-affected water (OSPW). However, fresh OSPWs contain suspended solids and inorganic constituents in suspended and dissolved forms that can severely foul membranes. Pretreatment of OSPW with coagulation-flocculation (CF) was investigated to determine the effects of different coagulant aids (anionic, cationic, and nonionic polymers) on membrane surface properties and fouling. Our results showed that CF pretreatment effectively enhanced nanofiltration (NF) and reverse osmosis (RO) membrane permeate flux and salt rejection ratio through reducing membrane fouling. It was shown that coagulants and coagulant aids applied to OSPW feedwater can affect membrane physicochemical properties (surface hydrophilicity, zeta potential, and morphology), membrane performance, and the fouling indexes. Membrane rejection of ionic species increased significantly with the inclusion of an anionic coagulant aid and slightly with a cationic coagulant aid. Among three coagulant aids tested, anionic coagulant aids led to the most enhanced membrane performance through increasing membrane surface negativity and decreasing the formation of a fouling layer. Conversely, although cationic coagulant aids were the most effective in reducing OSPW turbidity, the application of cationic coagulant aids promoted the adsorption of foulants on membrane surfaces.     

Fundamental Understanding of Fouling Mechanisms During Microfiltration of Bitter Gourd (<Emphasis Type="Italic">Momordica charantia</Emphasis>) Extract and Their Dependence on Operating Conditions

Microfiltration of bitter gourd (Momordica charantia) extract using hollow fiber membrane module was carried out in the present study. To identify the dominant fouling mechanism, flux decline behavior was examined using Field model. At lower transmembrane pressure, pore blocking mechanism was found to be more important, while cake filtration was dominant at higher pressure. Higher cross flow rate reduced filtration constant indicating slower rate of membrane fouling. Additionally, surface and particle size analyses were undertaken to validate the findings of modeling. Scanning electron microscope analysis clearly showed prevalence of pore blocking mechanism at lower transmembrane pressure drop, whereas cake filtration was dominant fouling mechanism at higher pressure. Fourier transform infrared spectroscopy analysis supported the role of cake layer as a secondary membrane retaining some amount of polyphenols. Analysis of flux decline ratio also confirmed that for transmembrane pressure of 104 kPa and beyond, cake layer became compact, and hence, increase in cross flow rate was unable to influence the improvement of permeate flux. The current study provides an insight into the fouling mechanism involved in scaling up of clarification of bitter gourd extract for successful processing of this medicinal herb.