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

Fouling behavior of nanofiltration membrane during the refining treatment of morphlines-dominant reverse osmosis concentrate

Nanofiltration (NF) has been proven to be with great potential for the separation of morpholines with molecular weight less than 200 Da in refining reverse osmosis concentrate (ROC), but its application is significantly restricted by the membrane fouling, which can reduce the rejection and service time. To enable the long-term operation stability of nanofiltration, this work focuses on the fouling behavior of each substance in the hydrosaline organic solution on nanofiltration membrane, aiming to give insight into the fouling mechanism. To this end, in this work, the effects of salts (i.e NaCl and Na2SO4), organic substances (including N-(2-hydroxypropyl)morpholine(NMH) and 4-morpholineacetate(MHA)) and representative divalent ions (Ca2+ and Mg2+) on the performance and physicochemical properties of DK membrane were systematically investigated. The results show that both salts and organics can induce DK membrane swelling, leading to an increase of the mean effective pore size. After the filtration of Na2SO4-NaCl-H2O, the mean pore size increased by 0.002 nm, resulting in the decrease of the removal ratio of NMH and MHA for 3.82% and 13.10%, respectively. With static adsorption of NMH and MHA, the mean pore size of DK membrane increased by 0.005 and 0.003 nm. The swelling slowed the entrance of more organic molecules into membrane pores. Among them, MHA led to the terrible irreversible pore blocking. As the concentration of Ca2+ increased, gypsum scaling was formed on the membrane surface. During this process, NMH and MHA played different roles, i.e. NMH accelerated the CaSO4 crystallization while MHA inhibited. As a conclusion, the fouling behavior of substances in the high saline organic wastewater on DK membrane were systematically revealed with the fouling mechanisms proposed, which could provide an insightful guidance for membrane fouling control and cleaning in the treatment of high salinity and organic wastewater.

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.

Non-destructive monitoring and prediction of fouling by organic matters and residual anionic coagulant during membrane process

Physical fouling characteristics on silicon carbide (SiC) membranes induced by various organic matter compounds vary depending on the presence of calcium ions (Ca2+). Both destructive techniques (morphological surface analysis) and non-destructive techniques (fouling properties monitoring) were used to determine the fouling mechanisms and behavior during the membrane filtration systems. Destructive analysis and a modified Hermia model were employed to assess the fouling mechanisms. Fouling behavior was also analyzed through non-destructive monitoring techniques including optical coherence tomography (OCT) and three-dimensional laser scanning confocal microscopy (3D-LSM). At concentrations of 10, 30, and 100 mg/L without Ca2+, the flux decreased by 57-95% for humic acid (HA) and anionic polyacrylamide (APAM). APAM exhibited a notable removal rate of up to 56% without Ca2+. At concentration of 10, 30, and 100 mg/L in the absence of Ca2+, the flux decreased by 6-8% for sodium alginate (SA). However, the addition of Ca2+ led to a reduction in the flux for SA by up to 91% and resulted in a removal rate of 40%. Furthermore, addition of Ca2+ led to an alteration of the fouling characteristics of HA and SA. In the case of HA, higher concentrations resulted in elevated thickness and roughness with correlation coefficients of 0.991 and 0.992, respectively. For SA, increased SA concentration led to a thicker (correlation coefficient of 0.999) but smoother surfaces (correlation coefficients of 0.502). Monitoring of these physical characteristics of the fouling layer through non-destructive analysis is crucial for effective fouling management, optimization of the system performance and extending the lifespan of the membrane. By continuously assessing the fouling layer thickness and surface roughness, we expect to be able to provide insights on the fouling behavior, identify trends, that can help scientists and engineers to make informed decisions regarding fouling control strategies in future.

Using organic fertilizer to mitigate organic-inorganic fouling in agricultural saline wastewater irrigation systems

Recycling saline wastewater for agricultural irrigation offer a promising solution to address both water scarcity and anthropogenic pollution. However, organic-inorganic fouling in saline wastewater irrigation systems (SWIS) poses significant technical and economic challenges. Traditional chemical biocides are currently insufficient for controlling composite organic-inorganic fouling and may pose environmental hazards. This study proposed a greener approach using organic acid (OA) fertilizers to alleviate organic-inorganic fouling in agricultural SWIS. The treatment performances were assessed employing four types of OA fertilizers (i.e., humic acid, alginic acid, nucleotide, and ammonia acid) and a negative control. Results showed that three types of OA, i.e., alginic acid, nucleotide, and ammonia acid, effectively reduced the total SWIS fouling content by 11.2%-57.4%, whereas humic acid exacerbated fouling by 11.2%-57.4%. Specifically, all types of OA significantly mitigated the content of inorganic fouling (precipitates and silicates) by 10.7%-42.3% by forming loosed and sparser structures. However, OA exhibited minimum effects on controlling silica fouling. Meanwhile, except the humic acid, other types of OA decreased the total content of organic fouling by 17.2%-39.5% by reducing the content of humic substances and building block fractions. In addition, the significant binary interactions of organic-inorganic fouling indicated the active role of calcium silica and biomineralization fouling. These findings provide insight into the development of appropriate and eco-friendly antifouling strategies for SWIS, with implications for recycling and reusing saline wastewater.

The intelligent prediction of membrane fouling during membrane filtration by mathematical models and artificial intelligence models

Recently, membrane separation technology has been widely utilized in filtration process intensification due to its efficient performance and unique advantages, but membrane fouling limits its development and application. Therefore, the research on membrane fouling prediction and control technology is crucial to effectively reduce membrane fouling and improve separation performance. This review first introduces the main factors (operating condition, material characteristics, and membrane structure properties) and the corresponding principles that affect membrane fouling. In addition, mathematical models (Hermia model and Tandem resistance model), artificial intelligence (AI) models (Artificial neural networks model and fuzzy control model), and AI optimization methods (genetic algorithm and particle swarm algorithm), which are widely used for the prediction of membrane fouling, are summarized and analyzed for comparison. The AI models are usually significantly better than the mathematical models in terms of prediction accuracy and applicability of membrane fouling and can monitor membrane fouling in real-time by working in concert with image processing technology, which is crucial for membrane fouling prediction and mechanism studies. Meanwhile, AI models for membrane fouling prediction in the separation process have shown good potential and are expected to be further applied in large-scale industrial applications for separation and filtration process intensification. This review will help researchers understand the challenges and future research directions in membrane fouling prediction, which is expected to provide an effective method to reduce or even solve the bottleneck problem of membrane fouling, and to promote the further application of AI modeling in environmental and food fields.

A review on polyester and polyester-amide thin film composite nanofiltration membranes: Synthesis, characteristics and applications

Nanofiltration (NF) membranes have been widely used in various fields including water treatment and other separation processes, while conventional thin film composite (TFC) membranes with polyamide (PA) selective layers suffer the problems of fouling and chlorine intolerance. Due to the abundant hydrophilic hydroxyl groups and ester bonds free from chlorine attack, the TFC membranes composed of polyester (PE) or polyester-amide (PEA) selective layers have been proven to possess enhanced anti-fouling properties and superior chlorine resistance. In this review, the research progress of PE and PEA nanofiltration membranes is systematically summarized according to the variety of hydroxyl-containing monomers for membrane fabrication by the interfacial polymerization (IP) reaction. The synthesis strategies as well as the mechanisms for tailoring properties and performance of PE and PEA membranes are analyzed, and the membrane application advantages are demonstrated. Moreover, current challenges and future perspectives of the development of PE and PEA nanofiltration membranes are proposed. This review can offer guidance for designing high-performance PE and PEA membranes, thereby further promoting the efficacy of nanofiltration.

Evaluation of Nanofiltration Membranes for the Purification of Monosaccharides: Influence of pH, Temperature, and Sulfates on the Solute Retention and Fouling

Furfural, acetic acid, and sulfates are found in the hemicellulose (HMC) fraction of lignocellulosic biomass. Separation of furfural, acetic acid, and sulfates from monosaccharides by four nanofiltration (NF) membranes was evaluated with a model solution of glucose, xylose, furfural, acetic acid, and sulfates. Results showed that Alfa Laval NF99HF is the most promising membrane to purify monosaccharides, with the retentions of xylose (85%), glucose (95%), and with the minimum sulfate retention. pH has the highest impact on the retention of all solutes and there is no significant effect of temperature on the retentions of sulphates and acetic acid. Lower pH and temperature are favored to maximize the monosaccharide retention and to remove acetic acid while retaining more furfural with the monosaccharides. Moreover, fouling tendency is maximized at lower pH and higher temperatures. According to the statistical analysis, the retentions of glucose, xylose, furfural, sulfates, and acetic acid are 95%, 90%, 20%, 88%, and 0%, respectively at pH 3 and 25 °C. The presence of sulfates favors the separation of acetic acid and furfural from monosaccharides.

Periodic fouling control strategies in gravity-driven membrane bioreactors (GD-MBRs): Impact on treatment performance and membrane fouling properties

This study aims to assess the effects of periodic membrane fouling control strategies in Gravity-Driven Membrane Bioreactor (GD-MBR) treating primary wastewater. The impact of each control strategy on the reactor performance (permeate flux and water quality), biomass morphology, and fouling composition were evaluated. The application of air scouring coupled with intermittent filtration resulted in the highest permeate flux (4 LMH) compared to only intermittent filtration (i.e., relaxation) (1 LMH) and air scouring under continuous filtration (2.5 LMH). Air scouring coupled with relaxation led to a thin (~50 μm) but with more porous fouling layer and low hydraulic resistance, presenting the lowest concentration of extracellular polymeric substance (EPS) in the biomass. Air scouring under continuous filtration led to a thin (~50 μm), dense, compact, and less porous fouling layer with the highest specific hydraulic resistance. The employment of only relaxation led to the highest fouling formation (~280 μm) on the membrane surface. The highest TN removal (~62%) was achieved in the reactor with only relaxation (no aeration) due to the anoxic condition in the filtration tank, while the highest COD removal (~ 60%) was achieved with air scouring under continuous filtration due to the longer aeration time and the denser fouling layer. The results highlighted the importance of performing in-depth fouling characterization to link the membrane fouling properties to the hydraulic resistance and membrane bioreactor performances (i.e., water quality and water production). Moreover, this work proven the versatility of the GD-MBR, where the choice of the appropriate operation and fouling control strategy relies on the eventual discharge or reuse of the treated effluent.

Fouling characteristics and cleaning approach of ultrafiltration membrane during xylose reductase separation

Many operating parameters of ultrafiltration (UF) are playing a crucial role when using a polyethersulfone membrane to separate xylose reductase (XR) enzyme from reaction mixtures during xylitol synthesis. The present study focuses on the separation of XR enzyme using a cross-flow ultrafiltration (UF) membrane. The filtration process was analyzed using the three effective variables such as filtration time, cross-flow velocity (CFV), and the transmembrane pressure (TMP), which were ranging from 0 to 100 min, 0.52 to 1.2 cm/s and 1-1.6 bar, respectively. Then, using the resistance in series model, the hydraulic resistance for alkali chemical cleaning during XR separation was estimated. During separation, increased TMP showed a positive-flux effect as a driving force, however, fouling and polarized layer were more prominent under higher TMP. Increased CFV, on the other hand, was found more efficient in fouling control. In terms of the membrane cleaning techniques, an alkaline solution containing 0.1 M sodium hydroxide was shown to be the most effective substance in removing foulants from the membrane surface in this investigation. Cleaning with an alkaline solution resulted in a maximum flux recovery of 93% for xylose reductase separation. This work may serve as a useful guide to better understand the optimization parameters during XR separation and alleviating UF membrane fouling induced during XR separation.

New insights into effect of alkaline cleaning on fouling behavior of polyamide nanofiltration membrane for wastewater treatment

Membrane fouling is an intractable issue in wastewater treatment by nanofiltration (NF) membrane, and alkaline cleaning is the most effective approach to remove organic fouling on NF membrane. However, it was found that pore swelling of NF membrane induced by alkaline cleaning might reduce cleaning efficiency, and it is never quantified and its effect on membrane fouling behavior is still mysterious. In this work, membrane pore swelling effect (~9.7%, increment of effective pore size) induced by alkaline cleaning (pH 11) is confirmed and its effect on fouling behavior of the polyamide NF membrane is investigated based on experimental and modelling results. It is found that the alkali-induced pore swelling phenomenon would disappear after water filtration at neutral pH for 30 min, and if such cleaned membrane is faced by the small foulants during this pore shrinkage period, the concentration polarization and membrane fouling would be severer, and the subsequent alkaline cleaning is less effective because more foulants enter the enlarged pores and are tightly embedded in the membrane. Thus, the irreversible fouling of the NF membrane increases from 20% to 40% while its permeability recovery declines from 100% to 67% after six fouling/cleaning cycles. When an anionic surfactant sodium dodecyl sulfate (SDS, 10 mM) is added in the alkaline cleaning solution, the adsorption of SDS in/on the membrane can not only improve its hydrophilicity and negative charge, but also quickly eliminate the alkali-induced pore swelling effect and avoid the accumulation of foulants in the pores, thereby enhancing the antifouling performance of the NF membrane. Using the alkaline SDS cleaning, the irreversible fouling of the NF membrane maintains below 10% while its permeability recovery keeps above 100% in six continuous fouling/cleaning cycles.

Evaluating an on-line cleaning agent for mitigating organic fouling in a reverse osmosis membrane

Cleaning-in-place (CIP) is a representative fouling management process from which the filtration performances of fouled membranes can be recovered. However, CIP can cause significant inefficiency in water production because frequent system restabilization is necessary for cleaning processes. This study applied a newly developed on-line cleaning agent (OCA, a feed water additive for fouling mitigation), to reduce the number of CIP by enhancing water productivity. Reverse osmosis filtration was performed to evaluate the effect of on-line cleaning on the mitigation of organic fouling originating from humic acid (HA) and bovine serum albumin. OCA increased the permeate flux in proportion to OCA concentration. In particular, OCA effectively reduced the fouling layer thickness by 22% when fouling was influenced by HA-Ca²⁺ complexation, increasing water production by 5%. It also had a minor influence on bovine serum albumin fouling, producing a 1.4% increase in permeate flux. Furthermore, the pore blockage-cake filtration model was used to evaluate OCA cleaning performance through the reduction in fouling layer resistance and the growth parameter. The results demonstrated the advantages of OCA utilization for mitigating cake layer development. These findings imply that OCA can be an effective cleaning additive, especially in seawater and groundwater treatment processes with a high proportion of HA and calcium ions.

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.

Clarification of 1,3-Propanediol Fermentation Broths by Using a Ceramic Fine UF Membrane

This work examined the use of a ceramic fine ultrafiltration (UF) membrane for the pre-treatment of 1,3-propanodiol (1,3-PD) fermentation broths. It has been demonstrated that the membrane used provides obtaining a high-quality, sterile permeate, which can be sequentially separated by other processes such as nanofiltration (NF) and membrane distillation (MD). Special attention was paid to the impact of the operational parameters on the membrane performance. The series of UF experiments under transmembrane pressure (TMP) from 0.1 to 0.4 MPa and feed flow rate (Q) from 200 to 400 dm3/h were performed. Moreover, the impact of the feed pH, in the range from 5 to 10, on the flux was investigated. It has been demonstrated that for fine UF, increasing the TMP is beneficial, and TMP equal to 0.4 MPa and Q of 400 dm3/h ensure the highest flux and its long-term stability. It has been shown that in terms of process efficiency, the most favorable pH of the broths is equal to 9.4. An effective and simple method of membrane cleaning was presented. Finally, the resistance-in-series model was applied to describe resistances that cause flux decline. Results obtained in this study can assist in improving the cost-effectiveness of the UF process of 1,3-PD fermentation broths.

The role of ferric coagulant on gypsum scaling and ion interception efficiency in nanofiltration at different pH values: Performance and mechanism

Nanofiltration (NF) is extensively applied after coagulation, which is conducive to alleviate organic fouling on NF membranes and improve water purification performance. However, inorganic fouling, which remains the major obstacle to limit the wider application of NF, could be enhanced by even low dosage coagulant. Few researchers realize the existence of coagulant-enhanced scaling, much less control it. This study investigated the effects of pH values on ferric-coagulant-influenced membrane performance during the nanofiltration of brackish water. Both membrane flux behavior (initial membrane flux, normalized flux during filtration, scaling resistance and scaling composition) and ion interception (filtrate conductivity and ions removal) were considered. Solution properties (zeta potential and nanoparticle size) were measured, and coagulant speciation variation was stimulated by Visual MINTEQ software. Mechanisms of ferric-coagulant-influenced membrane performance were analyzed from two aspects on the basis of correlation analyses: interface interaction on membrane surface and salts crystallization process (bulk crystallization and surface crystallization). Results showed that both bulk crystallization in feed solution and surface crystallization on membrane surface were dramatically induced by coagulant. Coagulant-enhanced fouling layer resistance decreased after the initial increase when pH varied from 3.0 to 10.0. Fe(OH)₃, a kind of active ingredients in ferric coagulant, was highly responsible for the enhanced scaling layer resistance. Coagulant was found improving ionic removal under acidic conditions despite the fact that it could worsen removal under alkaline conditions. This study is of valuable reference to figure out the mechanisms of coagulant-influenced membrane performance and find a feasible approach to avoid membrane deterioration in coagulant-influenced NF process.

On the design of sustainable antifouling system for the crossflow filtration of oily water systems: A multicontinuum and CFD investigation of the periodic feed pressure technique

The problem of fouling is considered a major reason for deteriorating the performance of porous membranes. Even though the accumulations of materials at the membrane surface are inevitable, efforts are continuously spent to minimize their drawbacks. Several techniques have been tested to minimize the problem of fouling. Some of these methods, however, confront some technical difficulties that make their use unfeasible. For example, in polymeric-type membranes, back flushing may result in the loss of bonding between the active and the support layers resulting thereby to the disintegration of the membrane. Recently, an interestingly new approach has been proposed that minimizes the problem of fouling and maintains the integrity of the membrane. The so-called periodic feed pressure technique, PFPT, cleans the surface of the membrane by reducing the adherence of the droplets to the membrane giving the chance to the crossflow field to sweep off pinned droplets. In this work, some of the features of the PFPT technique are highlighted using results from CFD simulation. Then we further investigate the PFPT technique in the realm of the multicontinuum modeling approach in which both the emulsion and the membrane are treated as overlapping continua. The behavior of the membrane is studied considering different transmembrane pressure values to highlight the fates of the different oil continua upon interacting with membrane continua. From the CFD highlights, it is found that during the half cycle when the TMP is set to zero, oil droplets at the surface of the membrane becomes unstable and it becomes easier for the crossflow field to dislodge them. The multicontinuum study, on the other hand, provides macroscopic analysis on the effects of different TMP cycles on important macroscopic parameters that influence the design, including the rejection capacity of membranes.

Experimental equations of seawater salinity and desalination capacity to assess seawater irrigation

A central question in science and technology of desalting is, can we predict optimal coastal sites to implement seawater irrigation? Freshwater only makes up 2.5% of all water on Earth but crop irrigation is responsible for 70% of freshwater demand. First, we compared the growth rates and the dehydration rates of 5 alternative seawater irrigation experiments of wheatgrass over 3 weeks' periods. The average salt tolerance threshold of wheatgrass is 6 dS m^-1. When seawater salinity is increased >10.50 dS m^-1, the growth, drainage volumes, leaching, and drainage salinities of wheatgrass did not show significant variations. When seawater salinity is increased to 12.25 dS m^-1, grass leaves gradually turned light green, bent, and fell. Notably, pH in soil remained nearly constant in all experiments with mean pH of 6.05 ± 0.25 (mean ± SD). Next, we derived experimental equations to define a mechanistic link between salinity and desalination capacity in a Modified Saline Adjustable Desalination System (MSADS). A cost-benefit analysis for a MSADS in a coastal location of southern California indicated that this system is $0.84 m^-3 more expensive than using water from a natural reservoir, but $0.08 m^-3 less expensive than importing water. This study provides a general framework to assess the implementation of a desalination system in coastal locations.