Analyzing the Evolution of Membrane Fouling via a Novel Method Based on 3D Optical Coherence Tomography Imaging

Unveiling the spatiotemporal dynamics of membrane fouling: A focused review on dynamic fouling characterization techniques and future perspectives

Membrane technology has emerged as a crucial method for obtaining clean water from unconventional sources in the face of water scarcity. It finds wide applications in wastewater treatment, advanced treatment, and desalination of seawater and brackish water. However, membrane fouling poses a huge challenge that limits the development of membrane-based water treatment technologies. Characterizing the dynamics of membrane fouling is crucial for understanding its development, mechanisms, and effective mitigation. Instrumental techniques that enable in situ or real-time characterization of the dynamics of membrane fouling provide insights into the temporal and spatial evolution of fouling, which play a crucial role in understanding the fouling mechanism and the formulation of membrane control strategies. This review consolidates existing knowledge about the principal advanced instrumental analysis technologies employed to characterize the dynamics of membrane fouling, in terms of membrane structure, morphology, and intermolecular forces. Working principles, applications, and limitations of each technique are discussed, enabling researchers to select appropriate methods for their specific studies. Furthermore, prospects for the future development of dynamic characterization techniques for membrane fouling are discussed, underscoring the need for continued research and innovation in this field to overcome the challenges posed by membrane fouling.

Evaluation of Front-Face Fluorescence for Assessing Cyanobacteria Fouling in Ultrafiltration

Cyanobacteria fouling in ultrafiltration (UF) drinking water treatment poses a significant threat to the stability and sustainability of the process. Both phycocyanin found in cyanobacteria and the polymer membrane exhibit strong fluorescence, which could be readily detected using front-face excitation-emission matrix (FF-EEM) spectroscopy. In this study, FF-EEM was employed for the nondestructive and in situ characterization of algae fouling evolution in UF, while also analyzing fouling mechanisms and reversibility. The results indicated that phycocyanin fluorescence on the membrane surface showed a linear correlation with the specific algal cell count on the membrane surface before reaching saturation. As fouling progressed, membrane fluorescence decreased, which was associated with the extent of the surface coverage on the membrane. The plateau in membrane fluorescence indicated full coverage, coinciding with the cake filtration mechanism, cake compression, and deterioration of fouling reversibility. These findings highlight the promise of FF-EEM as a valuable tool for monitoring and evaluating fouling of cyanobacteria in UF systems.

Cake layer 3D structure regulation to optimize water channels during Al-based coagulation-ultrafiltration process

The variation in cake layer three-dimensional (3D) structures and related water channel characteristics induced by coagulation pretreatment remains unclear; however, gaining such knowledge will aid in improving ultrafiltration (UF) efficiency for water purification. Herein, the regulation of cake layer 3D structures (3D distribution of organic foulants within cake layers) by Al-based coagulation pretreatment was analyzed at the micro/nanoscale. The sandwich-like cake layer of humic acids and sodium alginate induced without coagulation was ruptured, and foulants were gradually uniformly distributed within the floc layer (toward an isotropic structure) with increasing coagulant dosage (a critical dosage was observed). Furthermore, the structure of the foulant-floc layer was more isotropic when coagulants with high Al₁₃ concentrations were used (either AlCl₃ at pH 6 or polyaluminum chloride, in comparison with AlCl₃ at pH 8 where small-molecular-weight humic acids were enriched near the membrane). These high Al₁₃ concentrations lead to a 48.4% higher specific membrane flux than that seen for UF without coagulation. Molecular dynamics simulations revealed that with increasing Al₁₃ concentration (Al₁₃: 6.2% to 22.6%), the water channels within the cake layer were enlarged and more connected, and the water transport coefficient was improved by up to 54.1%, indicating faster water transport. These findings demonstrate that facilitating an isotropic foulant-floc layer with highly connected water channels by coagulation pretreatment with high-Al₁₃-concentration coagulants (having a strong ability to complex organic foulants) is the key issue in optimizing the UF efficiency for water purification. The results should provide further understanding of the underlying mechanisms of coagulation-enhancing UF behavior and inspire precise design of coagulation pretreatment to achieve efficient UF.

Revisiting scaling of calcium sulfate in membrane distillation: Uncertainty of crystal-membrane interactions

Scaling of calcium sulfate (CaSO₄) is a stumbling block to the development of membrane distillation (MD), which holds promise for the treatment of saline water/wastewater. Despite increasing efforts made to understand the scaling behavior of CaSO₄ in a process of MD and thereby develop strategies for mitigating the negative effects, considerable uncertainty remains about occurrence of the wetting and structural damage that could result from the strong crystal-membrane interactions. This study combined experimental and theoretical approaches to corroborate that a higher degree of supersaturation could be achieved by concentrating the CaSO₄ in the feed at a faster rate; the elevated supersaturation would be in favor of exerting substantially high crystallization pressure on the membrane structures. In particular, the theoretical analysis established two dimensionless groups for measuring the relative importance of the concentration effect and quantifying the essential role played by the crystalline growth, respectively. In addition to alleviating the uncertainty, this study would be beneficial to the design of MD processes with improved scaling resistance.

Application of Optical Coherence Tomography (OCT) to Analyze Membrane Fouling under Intermittent Operation

There is increasing interest in membrane systems powered by renewable energy sources, including solar and wind, that are suitable for decentralized water supply in islands and remote regions. These membrane systems are often operated intermittently with extended shutdown periods to minimize the capacity of the energy storage devices. However, relatively little information is available on the effect of intermittent operation on membrane fouling. In this work, the fouling of pressurized membranes under intermittent operation was investigated using an approach based on optical coherence tomography (OCT), which allows non-destructive and non-invasive examination of membrane fouling. In reverse osmosis (RO), intermittently operated membranes were investigated by OCT-based characterization. Several model foulants such as NaCl and humic acids were used, as well as real seawater. The cross-sectional OCT images of the fouling were visualized as a three-dimensional volume using Image J. The OCT images were used to quantitatively measure the thickness of foulants on the membrane surfaces under different operating conditions. The results showed that intermittent operation retarded the flux decrease due to fouling compared to continuous operation. The OCT analysis showed that the foulant thickness was significantly reduced by the intermittent operation. The decrease in foulant layer thickness was found to occur when the RO process was restarted in intermittent operation.

In Situ Visualization of Membrane Fouling Evolution during Ultrafiltration Using Label-Free Hyperspectral Light Sheet Fluorescence Imaging

Profound understanding of fouling behaviors and underlying mechanisms is fundamentally important for fouling control in membrane-based environmental applications. Therefore, it entails novel noninvasive analytical approaches for in situ characterizing the formation and development of membrane fouling processes. This work presents a characterization approach based on hyperspectral light sheet fluorescence microscopy (HSPEC-LSFM), which is capable of discriminating various foulants and providing their 2-dimensional/3-dimensional spatial distributions on/in membranes in a label-free manner. A fast, highly sensitive and noninvasive imaging platform was established by developing a HSPEC-LSFM system and further extending it to incorporate a laboratory-scale pressure-driven membrane filtration system. Hyperspectral data sets with a spectral resolution of ∼1.1 nm and spatial resolution of ∼3 μm as well as the temporal resolution of ∼8 s/plane were obtained, and the fouling formation and development process of foulants onto membrane surfaces, within the pores and on the pore walls were clearly observed during the ultrafiltration of protein and humic substances solutions. Pore blocking/constriction at short times while cake growth/concentration polarization at longer times was found to have coupled effects for the flux decline in these filtration tests, and yet the contribution of each effect as well as the transition of the governing mechanisms was found distinct. These results demonstrate in situ label-free characterization of membrane fouling evolution with the recognition of foulant species during filtration and provide new insights into membrane fouling. This work offers a powerful tool to investigate dynamic processes for a wide range of membrane-based explorations.

Inorganic Scaling in Membrane Desalination: Models, Mechanisms, and Characterization Methods

Inorganic scaling caused by precipitation of sparingly soluble salts at supersaturation is a common but critical issue, limiting the efficiency of membrane-based desalination and brine management technologies as well as other engineered systems. A wide range of minerals including calcium carbonate, calcium sulfate, and silica precipitate during membrane-based desalination, limiting water recovery and reducing process efficiency. The economic impact of scaling on desalination processes requires understanding of its sources, causes, effects, and control methods. In this Critical Review, we first describe nucleation mechanisms and crystal growth theories, which are fundamental to understanding inorganic scale formation during membrane desalination. We, then, discuss the key mechanisms and factors that govern membrane scaling, including membrane properties, such as surface roughness, charge, and functionality, as well as feedwater characteristics, such as pH, temperature, and ionic strength. We follow with a critical review of current characterization techniques for both homogeneous and heterogeneous nucleation, focusing on the strengths and limitations of each technique to elucidate scale-inducing mechanisms, observe actual crystal growth, and analyze the outcome of scaling behaviors of desalination membranes. We conclude with an outlook on research needs and future research directions to provide guidelines for scale mitigation in water treatment and desalination.

Insights into the wetting phenomenon induced by scaling of calcium sulfate in membrane distillation

Development of water/wastewater treatment based on membrane distillation (MD) suffers from the drawback that the hydrophobic membrane could be wetted for various reasons. Despite significant efforts, there is uncertainty in addressing the wetting induced by scaling of calcium sulfate, which is ubiquitous and recalcitrant in MD processes. This study made the first attempt to analyze the interplay between the growing crystals and the porous structures in the framework of Stoney's equation. Optical coherence tomography (OCT) was exploited to measure the membrane shift, whereby the scaling-induced deformation was correlated with the variation in stress created in the crystal-containing layer. Along with the stress analysis, the OCT-based characterization was combined with conventional approaches to ascertain the dependence of the scaling-induced wetting on the rate of concentrating the crystallizing species when arriving at a high degree of supersaturation in the feed. This study would refine the physical picture for better understanding crystal-membrane interactions that result in not only the wetting phenomenon but also the irreversible damage of membrane structures, thereby lending itself to the development of strategies for MD-based applications with improved efficiency.

A Review on Ion-Exchange Membrane Fouling during the Electrodialysis Process in the Food Industry, Part 1: Types, Effects, Characterization Methods, Fouling Mechanisms and Interactions

Electrodialysis (ED) was first established for water desalination and is still highly recommended in this field for its high water recovery, long lifetime and acceptable electricity consumption. Today, thanks to technological progress in ED processes and the emergence of new ion-exchange membranes (IEMs), ED has been extended to many other applications in the food industry. This expansion of uses has also generated several problems such as IEMs' lifetime limitation due to different ageing phenomena (because of organic and/or mineral compounds). The current commercial IEMs show excellent performance in ED processes; however, organic foulants such as proteins, surfactants, polyphenols or other natural organic matters can adhere on their surface (especially when using anion-exchange membranes: AEMs) forming a colloid layer or can infiltrate the membrane matrix, which leads to the increase in electrical resistance, resulting in higher energy consumption, lower water recovery, loss of membrane permselectivity and current efficiency as well as lifetime limitation. If these aspects are not sufficiently controlled and mastered, the use and the efficiency of ED processes will be limited since, it will no longer be competitive or profitable compared to other separation methods. In this work we reviewed a significant amount of recent scientific publications, research and reviews studying the phenomena of IEM fouling during the ED process in food industry with a special focus on the last decade. We first classified the different types of fouling according to the most commonly used classifications. Then, the fouling effects, the characterization methods and techniques as well as the different fouling mechanisms and interactions as well as their influence on IEM matrix and fixed groups were presented, analyzed, discussed and illustrated.

In-situ 3D fouling visualization of membrane distillation treating industrial textile wastewater by optical coherence tomography imaging

Membrane fouling, which is caused by the deposition of particles on the membrane surface or pores, reduces system performance in membrane distillation (MD) applications, resulting in increased operational costs, poor recovery, and system failure. Optical Coherence Tomography enables in-situ foulant monitoring in both 2D and 3D, however, the 2D images can only determine fouling layer thickness in severe fouling. Therefore, in this study, an advanced 3D imaging analysis technique using intensity range filters was proposed to quantify fouling layer formation during MD through the use of a single 3D image. This approach not only reduces computational power requirements, but also successfully separated the fouling layer from the membrane at the microscale. Thus, the thickness, fouling index, and fouling layer coverage can be evaluated in real time. To test this approach, Polyvinylidene fluoride (C-PVDF) and polytetrafluoroethylene (C-PTFE) membranes were used to treat a feed consisting of industrial textile wastewater. Thin and disperse foulants was observed on the C-PTFE, with a 22 µm thick fouling layer which could not be observed using 2D images after 24 h. Moreover, the C-PTFE demonstrated better antifouling ability than the C-PVDF as demonstrated by its lower fouling index, which was also supported by surface energy characterization. This work demonstrates the significant potential of 3D imagery in the long-term monitoring of membrane fouling process to improve membrane antifouling performance in MD applications, which can lead to lowered operational costs and improved system stability.

Membrane Fouling Phenomena in Microfluidic Systems: From Technical Challenges to Scientific Opportunities

The almost ubiquitous, though undesired, deposition and accumulation of suspended/dissolved matter on solid surfaces, known as fouling, represents a crucial issue strongly affecting the efficiency and sustainability of micro-scale reactors. Fouling becomes even more detrimental for all the applications that require the use of membrane separation units. As a matter of fact, membrane technology is a key route towards process intensification, having the potential to replace conventional separation procedures, with significant energy savings and reduced environmental impact, in a broad range of applications, from water purification to food and pharmaceutical industries. Despite all the research efforts so far, fouling still represents an unsolved problem. The complex interplay of physical and chemical mechanisms governing its evolution is indeed yet to be fully unraveled and the role played by foulants' properties or operating conditions is an area of active research where microfluidics can play a fundamental role. The aim of this review is to explore fouling through microfluidic systems, assessing the fundamental interactions involved and how microfluidics enables the comprehension of the mechanisms characterizing the process. The main mathematical models describing the fouling stages will also be reviewed and their limitations discussed. Finally, the principal dynamic investigation techniques in which microfluidics represents a key tool will be discussed, analyzing their employment to study fouling.

Deep learning model for simulating influence of natural organic matter in nanofiltration

Controlling membrane fouling in a membrane filtration system is critical to ensure high filtration performance. A forecast of membrane fouling could enable preliminary actions to relieve the development of membrane fouling. Therefore, we established a long short-term memory (LSTM) model to investigate the variations in filtration performance and fouling growth. For data acquisition, we first conducted lab-scale membrane fouling experiments to identify the diverse fouling mechanisms of natural organic matter (NOM) in nanofiltration (NF) systems. Four types of NOMs were considered as model foulants: humic acid, bovine-serum-albumin, sodium alginate, and tannic acid. In addition, real-time 2D images were acquired via optical coherence tomography (OCT) to quantify the cake layer formed on the membrane. Subsequently, experimental data were used to train the LSTM model to predict permeate flux and fouling layer thickness as output variables. The model performance exhibited root mean square errors of <1 L/m²/h for permeate flux and <10 µm for fouling layer thickness in both the training and validation steps. In this study, we demonstrated that deep learning can be used to simulate the influence of NOMs on the NF system and also be applied to simulate other membrane processes.

A Comprehensive Review on Membrane Fouling: Mathematical Modelling, Prediction, Diagnosis, and Mitigation

Membrane-based separation has gained increased popularity over the past few decades, particularly reverse osmosis (RO). A major impediment to the improved performance of membrane separation processes, in general, is membrane fouling. Fouling has detrimental effects on the membrane’s performance and integrity, as the deposition and accumulation of foulants on its surface and/or within its pores leads to a decline in the permeate flux, deterioration of selectivity, and permeability, as well as a significantly reduced lifespan. Several factors influence the fouling-propensity of a membrane, such as surface morphology, roughness, hydrophobicity, and material of fabrication. Generally, fouling can be categorized into particulate, organic, inorganic, and biofouling. Efficient prediction techniques and diagnostics are integral for strategizing control, management, and mitigation interventions to minimize the damage of fouling occurrences in the membranes. To improve the antifouling characteristics of RO membranes, surface enhancements by different chemical and physical means have been extensively sought after. Moreover, research efforts have been directed towards synthesizing membranes using novel materials that would improve their antifouling performance. This paper presents a review of the different membrane fouling types, fouling-inducing factors, predictive methods, diagnostic techniques, and mitigation strategies, with a special focus on RO membrane fouling.

Three-Dimensional Analysis of the Natural-Organic-Matter Distribution in the Cake Layer to Precisely Reveal Ultrafiltration Fouling Mechanisms

Cake layer formation is the dominant ultrafiltration membrane fouling mechanism after long-term operation. However, precisely analyzing the cake-layer structure still remains a challenge due to its thinness (micro/nano scale). Herein, based on the excellent depth-resolution and foulant-discrimination of time-of-flight secondary ion mass spectrometry, a three-dimensional analysis of the cake-layer structure caused by natural organic matter was achieved at lower nanoscale for the first time. When humic substances or polysaccharides coexisted with proteins separately, a homogeneous cake layer was formed due to their interactions. Consequently, membrane fouling resistances induced by proteins were reduced by humic substances or polysaccharides, leading to a high flux. However, when humic substances and polysaccharides coexisted, a sandwich-like cake layer was formed owing to the asynchronous deposition based on molecular dynamics simulations. As a result, membrane fouling resistances were superimposed, and the flux was low. Furthermore, it is interesting that cake-layer structures were relatively stable under common UF operating conditions (i.e., concentration and stirring). These findings better elucidate membrane fouling mechanisms of different natural-organic-matter mixtures. Moreover, it is demonstrated that membrane fouling seems lower with a more homogeneous cake layer, and humic substances or polysaccharides play a critical role. Therefore, regulating the cake-layer structure by feed pretreatment scientifically based on proven mechanisms should be an efficient membrane-fouling-control strategy.

Analyzing scaling behavior of calcium sulfate in membrane distillation via optical coherence tomography

Deepening the understanding of scaling processes would facilitate the improvement of membrane distillation (MD) as a promising technique for sustainable development. This study investigated the scaling of calcium sulfate in MD via an approach based on optical coherence tomography (OCT). The OCT-based characterization enabled an analysis that correlated the flux decline with the morphological evolution of the scaling layer. It was revealed by this analysis that the reduction in the evaporation rate could be dominated by different mechanisms as the crystalline particles grew and deposited on the membrane surface; the striping phenomenon visualized by mapping the local growth rates provided evidence for the hydrodynamic instability induced by the coupled mass and heat transfer in MD. Moreover, the OCT-based characterization was exploited to unravel the interplay between the crystallization and the porous structure by quantifying the membrane deformation as a function of time; the varied precipitation kinetics in the boundary layer was confirmed by comparing the temporal variations in the OCT signals at different depths. All these results shed light on mechanisms underlying complex scaling processes, which are the basis for optimizing the design of MD.

Light sheet fluorescence microscopy applied for in situ membrane fouling characterization: The microscopic events of hydrophilic membrane in resisting DEX fouling

Membrane fouling restricts the wide applications of membrane technology and therefore, it is essential to develop novel analytical techniques to characterize membrane fouling and to further understand the mechanism behind it. In this work, we demonstrate a capability of high-resolution large-scale 3D visualization and quantification of the foulants on/in membranes during fouling process based on light sheet fluorescence microscopy as a noninvasive reproducible optical approach. The adsorption processes of dextran (DEX) on/in two polyvinylidene fluoride membranes with similar pore structure but distinct surface hydrophilicity were clearly observed. For a hydrophilic polyvinylidene fluoride (PVDF) membrane, the diffusion and adsorption of the DEX in membrane matrix were much slower compared to that for a hydrophobic membrane. A concentrated foulant layer was observed in the superficial potion of the hydrophilic membrane matrix while the foulants were observed quickly penetrating across the overall hydrophobic PVDF membrane during a short adsorption process. Both the inner concentrated fouling layer (in membrane superficial portion) and the foulant penetration (in membrane asymmetric structure) presented correlations with membrane fouling irreversibility, which could elucidate the microscopic events of hydrophilic membrane in resisting fouling. In addition, the imaging results could be correlated with the XDLVO analysis, suggesting how the membrane-foulant and foulant-foulant interfacial interactions resulted in a time-dependent membrane fouling process. This work provides a fast, highly-sensitive and noninvasive imaging platform for in situ characterization of membrane fouling evolution and should be useful for a wide range of membrane-based process explorations.

Recent advances in mitigating membrane biofouling using carbon-based materials

Biofouling is the Achilles Heel of membrane processes. The accumulation of organic foulants and growth of microorganisms on the membrane surface reduce the permeability, shorten the membrane life, and increase the energy consumption. Advancements in novel carbon-based materials (CBMs) present significant opportunities in mitigating biofouling of membrane processes. This article provides a comprehensive review of the recent progress in the application of CBMs in antibiofouling membrane. It starts with a detailed summary of the different antibiofouling mechanisms of CBM-containing membrane systems. Next, developments in membrane modification using CBMs, especially carbon nanotubes and graphene family materials, are critically reviewed. Further, the antibiofouling potential of next-generation carbon-based membranes is surveyed. Finally, the current problems and future opportunities of applying CBMs for antibiofouling membranes are discussed.

Membrane-based separation for oily wastewater: A practical perspective

The large volumes of oily wastewater generated by various industries, such as oil and gas, food and beverage, and metal processing, need to be de-oiled prior to being discharged into the environment. Compared to conventional technologies such as dissolved air flotation (DAF), coagulation or solvent extraction, membrane filtration can treat oily wastewater of a much broader compositional range and still ensure high oil removals. In the present review, various aspects related to the practical implementation of membranes for the treatment of oily wastewater are summarized. First, sources and composition of oily wastewater, regulations that stipulate the extent of treatment needed before discharge, and the conventional technologies that enable such treatment are appraised. Second, commercially available membranes, membrane modules, operation modes and hybrids are overviewed, and their economics are discussed. Third, challenges associated with membrane filtration are examined, along with means to quantify and mitigate membrane fouling. Finally, perspectives on state-of-the-art techniques to facilitate better monitoring and control of such systems are briefly discussed.

In Situ Visualization of Concentration Polarization during Membrane Ultrafiltration Using Microscopic Laser-Induced Fluorescence

A novel noninvasive technique-microscopic laser-induced fluorescence (micro-LIF)-has been applied to achieve in situ visualization of concentration polarization (CP) of nanoparticles during cross-flow ultrafiltration at high resolutions. The reversible, highly dynamic nature of CP and its sensitive response to the filtration conditions were investigated and validated by direct visualization of the CP layer and the well depicted concentration profile near the membrane surface. Using micro-LIF, the formation of a CP layer during filtration and its back-diffusion after the filtration ceased can be directly observed. The dynamic variation of the CP layer with the cross-flow velocity and transmembrane pressure (TMP) change has also been demonstrated. The results showed that CP reached the steady state approximately 1 min after the filtration condition change. A higher cross-flow velocity and/or a lower TMP decrease the CP concentration and thickness. Further quantitative analysis of the filtration test results using the film theory model helps to obtain the particle concentration at the membrane surface and the thickness of the CP layer (30-50 μm). Accordingly, the nature of CP dynamics was characterized and the deficiency of the traditional CP model was explored.

Microfluidic model systems used to emulate processes occurring during soft particle filtration

Cake layer formation in membrane processes is an inevitable phenomenon. For hard particles, especially cake porosity and thickness determine the membrane flux, but when the particles forming the cake are soft, the variables one has to take into account in the prediction of cake behavior increase considerably. In this work we investigate the behavior of soft polyacrylamide microgels in microfluidic model membranes through optical microscopy for in situ observation both under regular flow and under enhanced gravity conditions. Particles larger than the pore are able to pass through deformation and deswelling. We find that membrane clogging time and cake formation is not dependent on the applied pressure but rather on particle and membrane pore properties. Furthermore, we found that particle deposits subjected to low pressures and low g forces deform in a totally reversible fashion. Particle deposits subjected to higher pressures only deform reversibly if they can re-swell due to capillary forces, otherwise irreversible compression is observed. For membrane processes this implies that when using deformable particles, the pore size is not a good indicator for membrane performance, and cake formation can have much more severe consequences compared to hard particles due to the sometimes-irreversible nature of soft particle compression.

Characterization of fluorescence foulants on ultrafiltration membrane using front-face excitation-emission matrix (FF-EEM) spectroscopy: Fouling evolution and mechanism analysis

The understanding of fouling behavior and mechanism is critical for fouling control in membrane processes. This study adopted a novel fluorescence front-face excitation-emission matrix (FF-EEM) approach to characterize the fluorescence foulants deposited on membrane surface. Methods for quantifying protein and humic substances deposited on ultrafiltration (UF) membrane were established. Foulants deposited on the membrane surface during the UF of model foulants (bovine serum albumin (BSA) and humic acids (HA)) and wastewater effluent organic matter (EfOM) were quantified using the FF-EEM and liquid EEM coupled with mass balance calculation. The foulants mass data obtained by FF-EEM were further used to analyze fouling mechanism involved in UF. The FF-EEM based method was more accurate than the liquid EEM based method, as the problems associated with liquid EEM based method (such as the error propagation in the mass balance calculation and the ineffectiveness of inner filter correction) were avoided in FF-EEM based method. The fouling resistance did not correlate well with the amount of foulants, as the major fouling mechanism instead of the mass of foulants mainly determined the extent of fouling. This work demonstrated FF-EEM could be a powerful tool for investigating fouling evolution and fouling mechanism in UF process.

Characterization of the Evolution of Crystallization Fouling in Membranes

Liquid-to-air membrane energy exchangers (LAMEEs) are promising in heating, ventilating, and air-conditioning applications because they are able to use semipermeable membranes to transfer heat and moisture between air and liquid desiccant streams. However, the development of crystallization fouling in membranes may pose a great risk to the long-term performance of LAMEEs. The main aim of this paper is to characterize the evolution of crystallization fouling in membranes through the use of both noninvasive and invasive methods. Noninvasive methods are used to study the development of fouling in the LAMEE by monitoring the changes in moisture flux through the membrane and overall moisture-transfer resistance of the LAMEE. On the other hand, invasive methods are implemented to characterize fouled membranes by using optical microscopy and scanning electron microscopy (SEM) to depict the morphology of crystal deposits and energy-dispersive X-ray spectroscopy (EDX) to identify the composition of the deposits. Experiments are performed by using air to dehydrate MgCl₂(aq) at two operating conditions of low and high fouling rates. The results show that the moisture flux decreases and the moisture-transfer resistance increases more considerably during the test at the high fouling rate than in the test at the low fouling rate. SEM micrographs show that cake crystal deposits cover the membrane surface in the test at the high fouling rate, whereas only few crystal particles are observed on the membrane in the test at the low fouling rate. Furthermore, the crystal deposits undergo more structural changes in the tests at the high fouling rate than in the tests at the low fouling rate, possibly because of the higher moisture transfer rate through the membrane in the tests at the high fouling rate. Finally, the SEM-EDX analysis confirms that the crystal deposits primarily consist of Mg, Cl, and O elements.

Evaluation of fouling in nanofiltration for desalination using a resistance-in-series model and optical coherence tomography

Resistance-in-series models have been applied to investigate fouling behavior. However, it is difficult to model the influence of morphology on fouling behavior because resistance is indirectly calculated from the water flux and transmembrane pressure. In this study, optical coherence tomography (OCT) was applied to evaluate the resistance of the fouling layer based on fouling morphology. Sodium alginate, humic acid, and bovine serum albumin (BSA) with high salts concentrations (conductivity: 23 mS/cm) were used as model foulants. At the same total fouling resistance, BSA showed the highest cake layer thickness (BSA (114.5 μm) > humic acid (53.5 μm) > sodium alginate (20.0 μm)). However, a different order was found for the cake layer resistance (BSA > sodium alginate > humic acid). This indicates that fouling thickness is not correlated with cake layer resistance. According to the Carman-Kozeny equation, fouling layer porosity decreased in the following order: humic acid (0.30) > BSA (0.21) > sodium alginate (0.20). In addition, we provided a specific value that was calculated using the ratio between the fouling thickness and cake layer resistance. The results show that alginic acid induced a stronger cake layer resistance, despite its thin fouling layer, whereas BSA showed a relatively low potential for inducing cake layer resistance. The results obtained in this study could be used for estimating and predicting fouling behavior.