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.     

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.     

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.     

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.     

Nanofiltration membrane fouling by oppositely charged macromolecules: investigation on flux behavior, foulant mass deposition, and solute rejection

Nanofiltration membrane fouling by oppositely charged polysaccharide (alginate) and protein (lysozyme) was systematically studied. It was found that membrane flux decline in the presence of both lysozyme and alginate was much more severe compared to that when there was only lysozyme or alginate in the feed solution. The flux performance for the mixed foulants was only weakly affected by solution pH and calcium concentration. These effects were likely due to the strong electrostatic attraction between the two oppositely charged foulants. Higher initial flux caused increased foulant deposition, more compact foulant layer, and more severe flux decline. The deposited foulant cake layer had a strong tendency to maintain a constant foulant composition that was independent of the membrane initial flux and only weakly dependent on the relative foulant concentration in feed solution. In contrast, solution chemistry (pH and [Ca2?]) had marked effect on the foulant layer composition, likely due to the resulting changes in the foulant-foulant interaction. The mixed alginate-lysozyme fouling could result in an initial enhancement in salt rejection. However, such initial enhancement was not observed when there was 1 mM calcium present in the feedwater, which may be attributed to the charge neutralization of the foulant layer.     

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.     

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...     

不同基团对丙烯酸酯乳液涂饰剂性能影响的研究

介绍了通过乳液共聚分别把－CN，－COOH，－CONH2，－C6H5，－OH等基团引入丙烯酸酯大分子链中，研究这些基团分别对丙烯酸酯乳液涂饰性能的影响。实验结果表明：这些极性基团的引入使涂饰剂性能有不同程度的提高，其中以－CONH2提高幅度最大，其次为－COOH，－OH。提高幅度最小的是－C6H5和－CN。     

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.     

THE REMOVAL OF β-LACTOGLOBULIN FROM STAINLESS STEEL SURFACES AT HIGH AND LOW TEMPERATURE AS INFLUENCED BY THE TYPE AND CONCENTRATION OF CLEANING AGENT

In the food industry, severe fouling of equipment surfaces is common and frequent cleaning is necessary. The aim of this work was to contribute to an understanding of the cleaning process. The adsorption of β-lactoglobulin (β-LG) onto stainless steel surfaces and its removal by detergent were followed at 24 and 82C using in situ ellipsometry. Soil removal using different concentrations of sodium hydroxide was studied. Furthermore, NaOH was combined with the anionic surfactant sodium dodecyl sulphate (SDS). Increasing the NaOH concentration resulted in an enhanced cleaning rate and better cleanability. When the concentration was increased to 0.8 mg/mL, the cleaning process could no longer be monitored by ellipsometry. Combining SDS with NaOH resulted in improved cleaning compared to if using the individual components. Adsorption and removal of β-LG at high temperature led in all cases to decreased cleanability compared to the results obtained at room temperature.     

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.     

Cadmium adsorption on aluminum oxide in the presence of polyacrylic acid

Adsorption of metals from aqueous solution onto oxide and other surfaces is known to affect trace metal transport in many natural and engineered systems. It is therefore important to understand whether dissolved metal inputs will be easily bound to particles or will be strongly complexed in solution and transported with the water phase. The effect of poly(acrylic acid) (PAA), representing a model compound for natural organic matter, on the adsorption of Cd(II) onto gamma-Al2O3 was determined using batch adsorption experiments over a pH range from 4 to 10. Initially, interactions among the individual components were evaluated. Cadmium adsorption onto alumina showed a typical S-shaped metal adsorption curve. PAA adsorption onto gamma-Al2O3 decreased with increase in pH. The affinity of PAA for Cd2+ increased strongly with pH. In ternary systems, the presence of PAA resulted in an enhancement of Cd(II) adsorption below pH 6, apparently due to ternary surface complex formation. Above pH 6, a decrease in cadmium adsorption onto gamma-Al2O3 was observed resulting from an increase in the concentration of soluble Cd-PAA complexes. Overall, results indicate that the presence of natural organic matter could have a significant impact on the distribution and mobility of cadmium in the environment. Simple surface complexation modeling was insufficient to describe behavior in the ternary systems due to the complexity of the PAA polymer.     

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.     

Three-component competitive adsorption model for flow-through PAC systems. 2. Model application to a PAC/membrane system

A three-component competitive adsorption kinetic model, developed and validated in part 1 of this study, was applied to a continuous-flow PAC/membrane system to study the effects of various system and operating parameters on organic removal. The model quantitatively describes the two competitive adsorption mechanisms that occur during adsorption of trace organic compounds by powdered activated carbon (PAC) in flow-through systems where the PAC is retained in the system: pore blockage and direct competition for adsorption sites. Model simulations were conducted to investigate the effects of influent water composition, membrane cleaning water quality, PAC pore size distribution, and system operation conditions such as hydraulic retention time, membrane cleaning interval, and PAC dosing method on treatment efficiency. Effects of these factors on adsorption capacity as well as surface diffusion rate and consequent removal of the trace organic compound were discussed. It was found that optimal operating conditions for maximum trace organic compound removal must be determined on the basis of the adsorption properties and concentrations of the competing compounds in the influent. For the conditions investigated in this study, the small strongly competing compound, p-DCB, had greater impact on atrazine removal than the large pore-blocking compound, PSS-1.8k. Various process design and operating parameters had complex and interrelated effects on the impact of competitive adsorption and corresponding trace contaminant removal efficiency in hybrid PAC/membrane systems.     

Microbiological and chemical analyses of stainless steel and ceramics subjected to repeated soiling and cleaning treatments

Stainless steel and ceramic surfaces were subjected to repeated soiling and cleaning procedures, using a milk powder soil inoculated with Pseudomonas aeruginosa and Staphylococcus aureus, followed by spraying with water with or without 0.1% (vol/vol) nonionic detergent. Test surfaces were removed after 1, 5, 10, and 20 soiling-cleaning cycles and were analyzed for attached microorganisms (total viable count of bacteria removed by swabbing and percentage of coverage of cells plus soil stained with acridine orange). Surfaces were additionally examined using a range of analytical techniques: X-ray photoelectron spectroscopy (XPS), dynamic and imaging secondary ion mass spectroscopy (SIMS), and time-of-flight SIMS. Both microbiological and chemical analyses revealed an accumulation of material over the cycles to a maximum level. Surfaces were conditioned rapidly to saturation with organic material within one cycle (XPS), whereas fouling by microorganisms was less rapid (five cycles). Cleaning with detergent tended to retard the cumulative fouling process when compared with spray cleaning using water alone. The methods described provide a more realistic scenario for testing the cleanability of surfaces routinely found in food processing locations and for screening novel cleaning regimens and/or surface materials. The analytical techniques provide additional information on the kinetics of soiling, which complement the more familiar microbiological methods.     

Facilitation of cleaning of alumina surfaces fouled with heat-treated bovine serum albumin by ozone treatment

Facilitation of cleaning of alumina (A12O3) particles fouled with heat-treated bovine serum albumin (BSA), which contains sulfhydryl groups on the molecule, by gaseous ozone was studied. With increasing temperature of heat treatment, the amount of adsorbed BSA onto A12O3 surfaces increased, whereas the rate of BSA desorption during alkali cleaning decreased markedly, resulting in the larger amounts of BSA remaining on 12O3 surfaces. No significant amounts of BSA were removed from A12O3 surfaces by alkali cleaning alone when treated at temperatures above 120 degrees C. Before alkali cleaning, the heat-treated, BSA-fouled AI2O3 at 150 degrees C were treated with 0.05 to 0.30% (vol/vol) gaseous ozone at room temperature. Ozone pretreatment markedly accelerated the rate of BSA desorption during subsequent alkali cleaning. The effect of ozone pretreatment on BSA removal depended on the concentration of ozone and treatment time and hence on the total amount of ozone supplied. The molecular weight (MW) of desorbed BSA during alkali cleaning without ozone pretreatment coincided with the MW of the native BSA, whereas the MW of desorbed BSA during the combined ozone-alkali cleaning was lower than the MW of the native BSA. This indicated that the heat-treated BSA molecules adsorbed on A12O3 were partially decomposed into some fragments by ozone pretreatment, resulting in the facilitation of the removal of BSA during alkali cleaning.     

NOM fractionation and fouling of low-pressure membranes in microgranular adsorptive filtration

Membrane fouling by natural organic matter (NOM) was investigated in microgranular adsorptive filtration (μGAF) systems, in which a thin layer of adsorbent is predeposited on low-pressure membranes. The adsorbents tested included heated aluminum oxide particles (HAOPs), ion exchange (IX) resin, and powdered activated carbon (PAC). Size exclusion chromatography (SEC) separated the NOM into four apparent MW fractions with significant UV???. HAOPs and the IX resin performed almost identically with respect to removal of these fractions, and differently from PAC. However, while HAOPs and PAC reduced fouling substantially, IX resin did not, indicating that fouling could not be attributed to the NOM fractions detected by SEC. Rather, the key foulants appear to comprise a very small fraction of the NOM with almost no UV??? absorbance. Alginate, a strongly fouling surrogate for natural polysaccharides, is adsorbed effectively by HAOPs, but not by IX resin or PAC, suggesting that polysaccharides sometimes play a key role in membrane fouling by NOM.     

Mechanism involved in the evolution of physically irreversible fouling in microfiltration and ultrafiltration membranes used for drinking water treatment

Control of membrane fouling is important for more efficient use of membranes in water treatment. Control of physically irreversible fouling, which is defined as fouling that requires chemical cleaning to be cancelled, is particularly important for reduction of operation cost in a membrane process. In this study, a long-term filtration experiment using three different types of MF and UF membranes was carried out at an existing water purification plant, and the evolution of physically irreversible fouling was investigated. The experimental results demonstrated that the extent of physically irreversible fouling differed significantly depending on the membrane type. Cleaning of the fouled membranes with various chemical reagents demonstrated that organic matter was mainly responsible for physically irreversible fouling. Organic matter that had caused physically irreversible fouling in the long-term operation was desorbed from the fouled membranes and was subjected to Fourier transform infrared and 13C nuclear magnetic resonance analyses. These analyses revealed that carbohydrates were dominant in the membrane foulant regardless of the type of membrane. Based on measurements of molecular weight distribution of organic matter in the feedwater and the permeates from the membranes, a two-step fouling mechanism is proposed to explain the dominance of carbohydrates in the foulant: hydrophobic (humic-like) components with small molecular weight are first adsorbed on the membrane and, consequently, narrow the size of micro-pores of membranes, and then hydrophilic (carbohydrate-like) compounds with larger molecular weight plug the narrowed pores or the hydrophilic compounds are adsorbed onto the membrane surface conditioned by the hydrophobic components.     

Adsorption of nonionic aromatic compounds to single-walled carbon nanotubes: effects of aqueous solution chemistry

We systematically studied effects of pH, ionic strength, and presence of Cu2+ (50 mg/L) or a dissolved humic acid (HA, Fluka) (50 mg/L) on adsorption of three nonionic aromatic compounds, naphthalene, 1,3-dinitrobenzene, and 1,3,5-trinitrobenzene to single-walled carbon nanotubes. Presence of Cu2+ or variance in the ionic strength between 0.02 and 0.1 M (NaNO3) only slightly affected adsorption affinities. Presence of HA reduced adsorption of the three compounds by 29-57% for CNTs, as measured by change in distribution coefficient(Kd), and by 80-95% for graphite. In contrast to nonporous graphite, whose surface area was completely accessible in adsorption, CNTs formed aggregates with microporous interstices in aqueous solution, which blocked large HA molecules from competing with the surface area. Changing the pH from 2 to 11 did not affect adsorption of naphthalene, while it increased adsorption of 1,3-dinitrobenzene and 1,3,5-trinitrobenzene by 2-3 times. Increasing pH apparently facilitated deprotonation of the acidic functional groups (-COOH, -OH) of CNTs, which promoted the pi-electron-donor ability of the graphene surface, therefore enhancing pi-pi electron-donor-acceptor (EDA) interactions of the two nitroaromatics (pi-electron acceptors).     

Interactions of hydrophobic fractions of dissolved organic matter with Fe(3+) - and Cu(2+)-montmorillonite

Interactions of dissolved organic matter (DOM) with clays can significantly affect a variety of soil processes. We studied adsorption and fractionation of hydrophobic acid (HoA) and hydrophobic neutral (HoN) fractions of DOM on Cu(2+)- and Fe(3+)-montmorillonite. Adsorption of both samples was higher on Fe(3+)-montmorillonite than on Cu(2+)-montmorillonite. A pH increase of about one unit was recorded followed by HoA adsorption by Fe(3+)-montmorillonite. This suggested exchange of negatively charged DOM groups on surface hydroxyl groups of Fe(3+)-montmorillonite surfaces. Adsorption of HoA on Cu(2+)-montmorillonite and HoN on Fe(3+)- and Cu(2+)-montmorillonites was governed mainly by van der Waals interactions. Spectroscopic analyses showed a distinct HoA fractionation by molecular size and aromaticity only by Fe(3+)-montmorillonite. On the basis of the pH measurements (increase in pH following adsorption of acid components) and enhanced DOM fractionation by molecular size and aromaticity we suggest that DOM reacted with Fe(3+)-montmorillonite similar to goethite.