Internal Membrane Fouling Research Articles

Comparative evaluation of adsorptive and repulsive separation using cellulose nanofiber/calcium alginate composite hydrogel filtration membranes with improved degradability

Among several approaches to address hard-to-manage dye wastewater, adsorption is one of the widely used methods, but the usage of these adsorbents can be constrained by their high material and process costs depending on the adsorbent materials as well as handling a large amount of adsorbents in a bulk solution. To overcome the problems, hydrogel membranes have recently attracted much attention because they can provide several strengths such as adsorption removal, confined adsorption sites, and filtration separation. However, to the best of our knowledge, it is still quite rare to find an in-depth comparative evaluation of adsorptive removal and repulsive separation in hydrogel membrane filtration. Herein, this study aims to bridge the gap between existing knowledge and undisclosed phenomena. Through a thorough comparative evaluation of adsorptive removal and repulsive separation in hydrogel membrane filtration, we found that repulsive separation was more suitable and sustainable than adsorptive removal because it was free from internal membrane fouling, adsorption sites’ saturation, and the cake layer formation on the surface. As a result, the hydrogel membranes could maintain excellent real-time separation performance while mitigating severe flux and rejection decline. Furthermore, we significantly improved the degradability of the spent hydrogel membrane under high saline conditions by 6.3 times compared to the control membrane by fine-tuning the structural properties via dual crosslinking, while maintaining the mechanical properties during filtration. It was possible due to the extended distance between polymer chains by dual crosslinking, reducing the burden imposed by plastic waste generated from the spent membrane.

Biogenic silver nanoparticles-modified forward osmosis membranes with mitigated internal concentration polarization and enhanced antibacterial properties

Thin-film composite forward osmosis (TFC-FO) membranes show the potential to be applied in water treatment applications. However, the TFC-FO membrane performance was limited by internal concentration polarization (ICP) and membrane fouling. In this study, a well-functioning TFC-FO membrane was produced by embedding biogenic silver nanoparticles (BioAg) into the polysulfone (PSf) substrate of a FO membrane. Effects of BioAg on membrane structure, filtration performance, antifouling, and antibacterial properties of the as-prepared FO membranes were examined. Results indicated that BioAg-modified FO membranes achieved improved surface hydrophilicity, higher porosity, and mitigated ICP, resulting in 2.5–4.4 times higher water flux than the pristine FO membranes. The BioAg-modified FO membrane also showed significantly improved antifouling and antibacterial ability. Moreover, the release velocity of silver nanoparticles in FO membranes was significantly slowed down by the construction of the polyamide layer. After 30 days of immersion in an aqueous solution, 96.5% of silver was retained in the optimized BioAg-modified FO membrane. The study offers an effective approach to simultaneously mitigate ICP and enhance the antifouling property of TFC-FO membranes through substrate modification.

Preparation and characterization of double-skinned FO membranes: Comparative performance between nanofiber and phase conversion membranes as supporting layers

To address the internal concentration polarization (ICP) effect and membrane fouling problems of forward osmosis technology in pressure retarded osmosis (PRO) mode, another highly selective polyamide (PA) layer was introduced on the other side of the single-skinned forward osmosis (FO) membrane. Recently, a new symmetrical nanofiber support layer has shown excellent performance. For better proof, the nanofiber support layer and the phase inversion support layer were prepared separately in this study, respectively, and two double-skinned FO membranes were obtained via interfacial polymerization. By comparing various surface physicochemical properties of the nanofiber support layer and the phase inversion support layer, it can be seen that the double-skinned PA layer formed on the surface of the nanofiber support layer had better performance. The nanofiber double-skinned FO membrane showed water flux of 19.99 LMH when using 2 M NaCl and DI water as the draw and feed solution, much higher than that of the phase inversion double-skinned FO membrane. Furthermore, the anti-fouling performance of the nanofiber double-skinned FO membrane was also better than that of the phase inversion double-skinned FO membrane. This study pointed out a new direction for a double-skinned FO membrane with high permeability selectivity and provided a promising method for anti-membrane fouling.

Fouling of microfiltration membranes by bidisperse particle solutions

Fouling of filtration membranes is a key process that degrades performance and reduces membrane lifetime. Nevertheless, obtaining direct information about fouling mechanisms is still a significant challenge, especially fouling that occurs within the internal pore spaces. Here, we combined highly multiplexed single-particle tracking methods with alternating laser excitation, which allowed the direct visualization of the transport of nanoparticles within microfiltration membranes, where the advective motion and retention of 200 nm and 40 nm diameter particles was simultaneously imaged within filtration membranes with 650 nm nominal pore size, as membranes became increasingly fouled as a function of total particle throughput. We found that internal membrane fouling consisted of three stages, fouling site formation, fouling site growth, and fouling site coalescence. Larger particles had a greater chance to be retained initially, nucleating the fouling sites. These sites tended to capture other particles passing through the membrane, causing the fouling sites to grow in size. Eventually, isolated fouling sites grew large enough to coalescence with neighboring sites, blocking a large region of membrane pores, leading to significant fouling. Importantly, the presence of small numbers of large particles significantly increased the retention of small particles and also accelerated the rate of particle retention. This mechanistic information about internal membrane fouling will assist in the design and optimization of filtration processes to reduce membrane fouling and expand the fundamental understanding of complex mass transport of polydisperse particle distributions.

Internal membrane fouling by proteins during microfiltration

The current study aimed to understand both external and internal membrane fouling by three proteins with different net charges, namely, negatively charged pepsin and bovine serum albumin (BSA), as well as positively charged lysozyme. Polycarbonate track-etched (PCTE) membranes were used. Per electrostatic attraction, the flux decline was the worst for lysozyme, which is attributed by the fouling model to the greatest pore blockage (α) and pore constriction (β), and by field-emission scanning electron microscope (FESEM) and optical coherence tomography (OCT) to the most extensive external fouling. Between pepsin and BSA, BSA gave worse flux decline despite its more negative net charge. The fouling model indicates that BSA gave greater pore blockage (α) and denser internal cake (Rc/Rm), while the quartz crystal microbalance with dissipation (QCM-D) indicates a rigid cake structure. Notably, despite monotonic flux decline with filtration, the OCT fouling voxel trends show significant fluctuations, which has not been reported before and thus signify the unique behavior of protein foulants in straight-through pores. Specifically, the trends below and above the −4.5 μm layer (i.e., 4.5 μm below the feed-membrane interface) are perfectly opposite, indicating the non-uniform protein deposits slipping downwards in the membrane pores as filtration progressed. The dynamic movements of the protein cakes unveiled here warrant more understanding in future studies.

Effective membrane backwash with carbon dioxide under severe fouling and operation conditions

Membrane fouling is an intrinsic deficiency common for all membrane processes. Fouling mitigation is therefore required to achieve sustainable membrane performance. Our study suggests a novel backwash concept which utilizes network of ultrafiltration interconnected membrane pores as a substrate for CO2 bubbles nucleation from a saturated CO2 solution which enters membrane pores from the support side. As a result, enhanced fouling removal was achieved due to additional hydrodynamic forces caused by expanded and lifted CO2 bubbles. An investigation of CO2 nucleation kinetics using a high speed camera revealed that initial CO2 nucleation rate is strongly determined by the module height and feed water type. A saturated CO2 solution backwash effectively removed bovine serum albumin (BSA) which caused both internal and external membrane fouling. A fouling reduction was also observed in BSA/seawater matrix opposite to cake layer buildup observed experiments with Milli-Q backwash. CO2 nucleation allowed to remove hydraulically irreversible fouling which was caused by transparent exopolymer particles (TEP) at pH 4 and 8. This is a promising result as TEP is biofouling precursor which tends to adsorb to a membrane surface making conventional cleaning practices inefficient. Complete transmembrane pressure recovery was achieved with a feed water containing sodium alginate and SiO2 nanoparticles with sizes compatible with membrane pores. The observed results emphasized the importance of the specific interactions in membrane/foulant/CO2 bubble triangle for a successful membrane recovery.

Internal fouling during microfiltration with foulants of different surface charges

In view of the inevitability of membrane fouling, it is critically important to understand the underlying mechanisms to alleviate fouling and thereby improve the feasibility of membrane-filtration for more applications. This study was targeted at understanding the internal membrane fouling mechanisms by three types of polystyrene (PS) particles (sized at ~ 0.45 μm; different surface charges) during microfiltration with the same polycarbonate track-etched (PCTE) membrane (nominal pore size of 2 μm). Optical coherence tomography (OCT) technique, which was employed to monitor membrane fouling, indicated greater extents of particle deposition in the case of the positive and less negative PS particles, which is consistent with the more severe flux declines relative to the more negative PS particles. The new fouling model developed that additionally accounts for internal cake filtration, on top of the well-reported pore constriction and pore blockage, revealed the fouling mechanisms at play. For the positive PS particles, the attractive particle-membrane interaction led to a more homogeneous layer-by-layer cake growth, which resulted in denser packing in the pore and higher internal cake resistance, and thereby the worst flux decline. In contrast, for the negative PS particles, the repulsive particle-particle and particle-membrane interactions resulted in more random deposition of the particles inside the pores, resulting in more extensive pore constriction and blockage, as well as lower resistance of the internal cake. This study underscored the importance of surface charge in internal fouling and provided insights on the lesser reported internal fouling mechanisms.

Forward osmosis membranes modified with laminar MoS2 nanosheet to improve desalination performance and antifouling properties

Internal concentration polarization (ICP) and membrane fouling are factors limiting the performance of forward osmosis (FO) membranes. Aiming at alleviating these problems, this study demonstrated the fabrication of FO membrane incorporated with MoS2 by layer-by-layer assembly with the synergistic effects of surface hydrophilicity, porosity and fouling-release properties. The grafting of MoS2 on the membrane surface was confirmed through the SEM, EDS and AFM analysis. The results demonstrated that the MoS2-coated-FO membrane could achieve transmembrane water flux of 27.15 LMH which was 35.34% higher than that obtained from a commercial polyethersulfone (PES) FO membrane. The reverse flux of NaCl ions was 16.42 gMH for the MoS2-coated-FO, 35.91% less than blank FO membrane. Meanwhile, coating MoS2 nanosheets endowed the resultant composites with higher water flux and lower reverse solute diffusion. In addition, the results of dynamic operation in a cross-flow test cell demonstrated that MoS2-coated-FO membrane exhibited good antifouling and fouling-release properties. This study indicates that use of MoS2 nanosheet is an attractive and feasible approach for the development of a dual-functional FO membrane by simultaneously improving the antifouling performance and reducing the ICP issues.

Optimization of coagulation pre-treatment for alleviating ultrafiltration membrane fouling: The role of floc properties on Al species

This study investigated membrane fouling in a coagulation/ultrafiltration (C-UF) process by comparing the floc properties and humic acid (HA) removal efficiency of three hydrous Al(III) species (Ala, Alb, and Alc). The results indicated that the coagulation and membrane mechanisms were different for all three Al species because of the differences in floc properties. The HA removal efficiency increased with increasing Al dosage until an equilibrium was reached at the optimal dosage of 6 mg L−1. In addition, membrane fouling gradually decreased as the Al dosages increased. Regardless of coagulant type, the OH and COOH functional groups of HA reacted with the Al species. Both external and internal membrane fouling were strongly dependent on the porosity of the cake layer and on the size distribution of the floc particulates, respectively. The pore area of the cake layer formed by the Ala-coagulated effluent was large because of the strong charge neutralization. Moreover, Ala generated large and loose flocs with a porous cake layer that mitigated external fouling. However, the internal fouling with the Alc coagulant was significant because the concentration of residual aggregates in the membrane pores was high.

Analyzing external and internal membrane fouling by oil emulsions via 3D optical coherence tomography

Membrane-based filtration is an emerging technique for processing oily wastewater. Improving the efficiency of oil-water separation via membranes entails novel techniques for studying the complex interactions between the oil droplets and membrane underlying fouling. This paper presents the first study that applies optical coherence tomography (OCT) to the characterization of membrane fouling by oil emulsions. A series of dead-end filtration experiments was performed to characterize the rejection of oil droplets (~ 10µm; hexadecane) by the membrane structure (0.45µm PVDF) via three-dimensional (3D) OCT scanning in real time. The experimental results were compared with the control experiments with ~ 10µm glass beads to identify the optical artifacts. Both the external and internal fouling by the oil droplets were successfully revealed by analyzing the variation in OCT intensity at various layers that were mathematically defined in terms of the coordinate surfaces parallel to, and above and below, the feed-membrane interface. The evolution of membrane fouling was quantified by evaluating the fraction of fouling voxels as a function of time at varied depths. This study demonstrates that the OCT-based characterization has the potential to shed light on the complex interactions occurring in oil-water separations via membrane filtration, particularly by providing real-time non-invasive monitoring of internal fouling, the understanding of which is valuable for both fundamental research and practical applications.

Pressure-retarded osmosis with wastewater concentrate feed: Fouling process considerations

Pressure-retarded osmosis (PRO) has attracted worldwide attention for its potential applications in renewable osmotic energy harvesting, low energy seawater desalination, and industrial waste brine disposal. However, membrane fouling is one of the major issues limiting PRO performance with the use of a high-salinity draw solution and a low-value impaired feedwater. This study systematically investigated membrane fouling in the PRO process using a real wastewater retentate (a waste byproduct from wastewater reclamation plants) as the PRO feed. Organic fouling (e.g., by humic substances) and inorganic fouling (e.g., by calcium phosphate) were identified to be the major types of fouling, where the latter dominated the overall water-flux decline. The internal PRO membrane fouling was exacerbated by internal concentration polarization (ICP) and intermolecular interactions between the organic macromolecules and the scaling precursor ions (e.g., Ca2+). A limiting flux that was independent of the initial flux (i.e., applied pressure) was observed during the PRO fouling and was explained by a novel PRO limiting flux model. A membrane with a smaller structural parameter could achieve a higher limiting flux in PRO. A simple and effective pressure-assisted osmotic backwashing protocol was developed to clean the internally fouled membrane that could restore over 92% of the water flux within a short cleaning period (15mins). The PRO performance could also be significantly improved by decreasing the feedwater pH that mitigated alkaline scaling. Finally, performing the PRO membrane fouling test in the active-layer-facing-feed-solution (AL-FS) orientation revealed that the integral cellulose triacetate (CTA) membrane exhibited strong mechanical stability and anti-fouling tendency whereas the thin-film composite (TFC) membrane was not mechanically stable in this orientation. This study for the first time identified that an integral membrane instead of a TFC membrane is an excellent candidate for PRO operation in the AL-FS mode for fouling control.

Mechanistic modeling of the loss of protein sieving due to internal and external fouling of microfilters.

Fed-batch and perfusion cell culture processes used to produce therapeutic proteins can use microfilters for product harvest. In this study, new explicit mathematical models of sieving loss due to internal membrane fouling, external membrane fouling, or a combination of the two were generated. The models accounted for membrane and cake structures and hindered solute transport. Internal membrane fouling was assumed to occur due to the accumulation of foulant on either membrane pore walls (pore-retention model) or membrane fibers (fiber-retention model). External cake fouling was assumed to occur either by the growth of a single incompressible cake layer (cake-growth) or by the accumulation of a number of independent cake layers (cake-series). The pore-retention model was combined with either the cake-series or cake-growth models to obtain models that describe internal and external fouling occurring either simultaneously or sequentially. The models were tested using well-documented sieving decline data available in the literature. The sequential pore-retention followed by cake-growth model provided a good fit of sieving decline data during beer microfiltration. The cake-series and cake-growth models provided good fits of sieving decline data during the microfiltration of a perfusion cell culture. The new models provide insights into the mechanisms of fouling that result in the loss of product sieving. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:1323-1333, 2017.

Fabrication of mesh-embedded double-skinned substrate membrane and enhancement of its surface hydrophilicity to improve anti-fouling performance of resultant thin-film composite forward osmosis membrane

Forward osmosis (FO) has been postulated as a potential viable alternative to conventional pressure-driven membrane treatment processes for water treatment, wastewater reclamation and desalination. Nevertheless, FO suffers from inherent drawbacks such as internal concentration polarization (ICP) and membrane fouling. Its practicality is further limited by the operational dilemma of either experiencing a more severe dilutive ICP in active-layer-facing-feed-solution (AL-FS) orientation or suffering a higher fouling propensity in active-layer-facing-draw solution (AL-DS) orientation, especially with the conventional asymmetric single-skinned FO membranes. Aiming at alleviating the problem of internal fouling while still exploiting the lower ICP and higher water flux of the AL-DS orientation, this study has demonstrated the fabrication of a flat-sheet mesh-incorporated double-skinned substrate membrane for thin-film composite (TFC) FO membrane. Through a progressive substrate membrane surface modification via polydopamine (PDA) deposition, this study explicitly correlates the increment of substrate membrane surface hydrophilicity to the improved fouling resistance and fouling reversibility of resultant TFC-FO membrane in the AL-DS orientation. The effects of foulant types (i.e., alginate and silica), cross-flow rates (i.e., high flow and low flow) and membrane orientations (i.e., AL-DS and AL-FS) have also been systematically studied to better understand FO operation characteristics and enhance its practical applications. The PDA modified double-skinned membrane developed in this study, e.g., TFC-Psf-6, has demonstrated more than 93.0% of water flux recovery in the AL-DS orientation after cleaning as well as significantly surpassing the performance over unmodified double-skinned TFC-Psf-0 (81.7%) and single-skinned TFC-Psf-S (61.7%) under the same testing condition.

Comparison of epichlorohydrin–dimethylamine with other cationic organic polymers as coagulation aids of polyferric chloride in coagulation–ultrafiltration process

Epichlorohydrin–dimethylamine (DAM–ECH) copolymer was acquired by polycondensation of hazardous reagents: epichlorohydrin (analytical reagent, A.R.) and dimethylamine (A.R.) with ethanediamine (A.R.) as cross-linker. Its coagulation and membrane performance as coagulation aid of polyferric chloride (PFC) was evaluated by comparing with other two cationic coagulation aids: poly dimethyl diallyl ammonium chloride (PDMDAAC) and polyacrylamide (PAM) in humic acid–kaolin (HA–Kaolin) simulated water treatment. Firstly, optimum dosages of PFC&DAM–ECH, PFC&PDMDAAC and PFC&PAM were identified according to their coagulation performance. Then their impacts (under optimum dosages) on membrane fouling of regenerated cellulose (RC) ultra-membrane disc in coagulation–ultrafiltration (C–UF) process were reviewed. Results revealed that small addition of DAM–ECH was the effective on turbidity and DOC removal polymer. Furthermore, in the following ultra-filtration process, external membrane fouling resistance was demonstrated to be the dominant portion of the total membrane fouling resistance under all circumstances. Meanwhile, the internal membrane fouling resistance was determined by residual of micro-particles11Micro-particles: Humic acid molecules, Kaolin and micro-floc formed in coagulation process that cannot be intercepted by cake layer or ultrafiltration membrane. that cannot be intercepted by cake layer or ultrafiltration membrane.

Evaluating the viability of double-skin thin film composite membranes in forward osmosis processes

Internal concentration polarization (ICP) and membrane fouling are two major factors impairing the performance of forward osmosis membranes. Their relative severity differs in the two operating modes of forward osmosis. The PRO mode (where the skin layer is facing the draw solution) is less susceptible to ICP, and as a result, produces a higher water flux than the FO mode (where the skin layer is facing the feed solution). Membrane fouling is severe in the PRO mode, resulting in the decline of membrane performance with time. Membrane cleaning is also more difficult in the PRO mode. There have been suggestions to add another skin layer to alleviate membrane fouling. The effects of the sandwiched membrane design on normal operations other than fouling abatement have yet to be systematically examined. In this study, the forward osmosis performance of double-skin membranes was evaluated and compared with the single-skin membranes by both theoretical calculations and experimental measurements. The results from a series of membranes with different transport properties suggested that the double-skinned membranes are not superior to the single-skinned membranes in any aspect other than their low fouling properties. Even for forward osmosis applications with serious fouling, the single-skinned membranes operating in the FO mode can still be a better alternative.

Insights into the operational characteristics of a multi-habitat membrane bioreactor: Internal variation and membrane fouling

This work investigated the operational characteristics of a multi-habitat membrane bioreactor to reveal its internal variation and membrane fouling. During the 100d operating period, a combined anaerobic and aerobic zone gradually formed in a single membrane bioreactor. In the anaerobic zone, DO decreased to nearly zero, while in the aerobic zone, DO value still remained at above 3.1mg/L, which created beneficial conditions for the removal of nitrogen (TN removal=87.1%) and phosphorus (TP removal=90.2%), although the phosphorus removal efficiency decreased in the later stage of operation (TP removal=44.6%). The increase of torque in the bioreactor accorded with the Boltzmann model (r2=0.97891), and the growth trend between torque and apparent viscosity of the fluid was highly consistent, which implied that an on-line torque approach may play an important role in the timely detection of rheological information about the mixed liquor suspended solids. As operational time increased, the size of sludge particles in the aeration zone tended to decrease, and the permanent membrane resistance gradually increased. These relationships revealed the reason for accelerated membrane fouling. Scanning electron microscopy revealed that the membrane fouling was mainly caused by a compact fouling layer that formed on the surface of membrane module.

Forward osmosis as a pre-treatment for treating coal seam gas associated water: Flux and fouling behaviour

In this study, a bench scale forward osmosis (FO) process was operated using two commonly available FO membranes in different orientations in order to examine the removal of foulants in the coal seam gas (CSG) associated water, the water flux and fouling behaviours of the process were also investigated. After 48h of fouling simulation experiment, the water flux declined by approximately 55 and 35% of its initial level in the TFC-PRO and CTA-PRO modes (support layer facing the feed), respectively, while the flux decline in the TFC-FO and CTA-FO modes (active layer facing the feed) was insignificant. The flux decline in PRO modes was caused by the compounding effects of internal concentration polarisation and membrane fouling. However, the declined flux was completely recovered to its initial level following the hydraulic cleaning using deionised water. Dissolved organic carbon (DOC), adenosine tri-phosphate (ATP) and major inorganic scalants (Ca, Mg and silica) in the CSG feed were effectively removed by using the FO process. The results of this study suggest that the FO process shows promising potential to be employed as an effective pre-treatment for membrane purification of CSG associated water.

Effect of pH, transmembrane pressure and whey proteins on the properties of casein micelle deposit layers

Deposit layers arising during milk microfiltration have not yet been well described with regard to their structural build-up. Besides casein micelles forming the bulk of the deposit, whey proteins are integrated into the milk protein deposit. Both the deposited protein layer and the membrane are involved in the retention of both protein fractions. Therefore, this study focusses on the impact of whey proteins on casein micelle deposit compressibility and reversibility. Compressibility and specific fouling resistance of casein micelle deposits were reduced in the presence of whey proteins. In other words, the deposit layer becomes more open or more porous due to whey protein attachment to the casein micelle surface. Furthermore, diffusive removability of the deposit layers was affected by pH and whether or not whey proteins were present. Convective removal by fluid forces was found to be almost entirely independent of pH or the presence of whey proteins. This shows that deposit layers arising during milk microfiltration consist of three phases, a diffusively removable micro-porous deposit, a gel-like convectively removable layer and internal membrane fouling.

Pressure retarded osmosis for osmotic power production: influence of internal concentration polarization and membrane fouling

Pressure retarded osmosis for osmotic power production: influence of internal concentration polarization and membrane fouling

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 6m/s, and transmembrane pressure of 159kPa, for 90min. 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.