Membrane-based Water Treatment Processes Research Articles

A comprehensive review on the use of Ti3C2Tx MXene in membrane-based water treatment

Population growth, industrial activities, and the energy demands of water production intensify the global challenge of accessing clean water. Membrane-based water treatment processes have emerged as crucial solutions to address this crisis, these include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), forward osmosis (FO), reverse osmosis (RO), and membrane distillation (MD). This review summarizes the properties and use of Ti3C2Tx MXene, a material with notable chemical stability, specific surface area, hydrophilicity, thermal conductivity, and environmental compatibility. Ti3C2Tx fills in the class of two-dimensional materials, it is promising in various membrane processes by offering enhancement in hydrophilicity, permeability, and selectivity. In UF, Ti3C2Tx-based membranes have superior antifouling properties and high rejection rates of contaminants. Meanwhile, in NF, MXene nanosheets elevate the rejection of organics, dyes, and salts, and present valuable nanocomposite membranes. In desalination, incorporating Ti3C2Tx in RO membranes is found to increase water permeability while maintaining high salt rejection in desalination. Also, MXene-modified FO membranes had lower fouling and improved water flux compared with plain membranes, presenting a promising avenue for water purification. Additionally, Ti3C2Tx enhanced water vapor transport and resistance against wetting, which is favorable in MD.This review underscores the transformative potential of Ti3C2Tx MXene in revolutionizing membrane-based water treatment processes, providing innovative solutions to meet the escalating global demand for clean water. Future research directions should optimize MXene-based membrane designs and scale up their production for practical applications in water treatment, thereby contributing to a more sustainable water future.

Antibiofouling polyethersulfone ultrafiltration membranes functionalized with MoS2 nanosheets and self-cleaning with hydrogen peroxide

The development of biofouling mitigation and cleaning strategy is of paramount importance for ultrafiltration (UF) membrane biofouling. In this study, polyethersulfone (PES) UF membrane was functionalized, namely MoS2/PES composite membrane. The MoS2/PES composite membrane exhibited superior antifouling property as evidenced by antibacterial circle, static and dynamic biofouling experiments. The permeate fluxes of the pristine PES and MoS2/PES composite membrane respectively decreased by 76% and 31% at the end of 3-h filtration. After exposure of E. coli to MoS2, inhibited cell growth led to reduced membrane biofouling. The biofilm formation was retarded under the dynamic flow condition, which were further confirmed by Scanning Electron Microscopy (SEM) and Confocal Laser Scanning Microscopy (CLSM) observations. As for the enhanced biofouling removal by cleaning with H2O2, 62% of the flux recovery rate for the MoS2/PES composite membrane was much higher than the pristine PES membrane (24%). The peroxidase-like properties of MoS2 were responsible for the enhancement of the superior cleaning performance. The use of H2O2 to trigger the in-situ catalytic function of the MoS2 nano-enzyme generated more ROS and O2 for inducing bacterial cells death and facilitating detachment of biofilm. This work will be helpful to provide important insights into developing more effective strategies for biofouling control and cleaning using advanced nanomaterials in membrane-based water treatment processes.

Multi-layer structure toward simultaneous enhancement of forward osmosis membrane separation performance and anti-biofouling property

Biofouling is a critical issue in membrane-based water-treatment processes because the feed solution retains microorganisms despite rigorous pretreatment. The forward osmosis (FO) process has a drawback of severe biofouling tendency when the active layer faces the draw solution. Here, we fabricated a thin-film nanocomposite membrane with a multilayer structure consisting of an MXene/carbon nanotubes (MXene/CNT) interlayer and a CNT back layer (TFNi-CNT). The interlayer structure significantly enhanced the membrane separation performance whereas the CNT back layer did not significantly hamper the performance. The biofilm formed on the CNT back layer surface was reduced by approximately 90% compared to that on the pristine substrate, indicating that the CNT back layer has superior antibiofilm properties. The water flux of the TFNi-CNT membrane was well-maintained (approximately 46%) and reversibly recovered through facile physical flushing, even after four dynamic biofouling cycles, whereas that of the pristine membrane was reduced to approximately 10%. These results indicated that the TFNi-CNT membrane possesses excellent resistance to biofouling. The CNT layer acts as a barrier that effectively prevents bacteria from entering the inner porous substrate, thus alleviating the detrimental biofilm-enhanced internal concentration polarization. This study provides new insights into the rational design and fabrication of FO membranes to mitigate biofouling.

A review on UiO-66 applications in membrane-based water treatment processes

Metal-organic frameworks (MOFs) are a modern class of materials that got great academic attention over the last two decades. Recently, serious attempts were conscripted to scale up several types of them and involve industrial applications. UiO-66 is a zirconium-based MOF that was synthesized in 2008 and widely applied in many fields since then. As researchers investigated UiO-66 applications, this material continuously showed high performance and remarkable properties, which increased the interest for further exploitation. In water treatment, where many MOFs are utilized in various processes, UiO-66 is among the intensively applied ones mainly due to its high-water stability. However, the UiO-66's stability is not limited to water; it is stable in organics and acids, in addition to its high thermal stability. Moreover, UiO-66 has the general MOF properties of large surface area, proper pore diameter, large pore volume, functional surface, etc.In this review, we will focus on the use of UiO-66 in membrane-based water treatment processes. These processes include ultrafiltration, nanofiltration, forward osmosis, and reverse osmosis, which are used for dyes, heavy metals, proteins, and antibiotics removal, as well as desalination. Based on database information, UiO-66 use in water treatment is constantly increasing due to the promising opportunities this material raises. Here, it became necessary to explain the up-to-date progress in the membranes field and explore the available future aspects and aspirations of UiO-66.

Lamellar carbon nitride membrane for enhanced ion sieving and water desalination

Membrane-based water treatment processes offer possibility to alleviate the water scarcity dilemma in energy-efficient and sustainable ways, this has been exemplified in filtration membranes assembled from two-dimensional (2D) materials for water desalination purposes. Most representatives however tend to swell or disintegrate in a hydrated state, making precise ionic or molecular sieving a tough challenge. Here we report that the chemically robust 2D carbon nitride can be activated using aluminum polycations as pillars to modulate the interlayer spacing of the conjugated framework, the noncovalent interaction concomitantly affords a well-interlinked lamellar structure, to be carefully distinguished from random stacking patterns in conventional carbon nitride membranes. The conformally packed membrane is characterized by adaptive subnanochannel and structure integrity to allow excellent swelling resistance, and breaks permeability-selectivity trade-off limit in forward osmosis due to progressively regulated transport passage, achieving high salt rejection (>99.5%) and water flux (6 L m−2 h−1), along with tunable permeation behavior that enables water gating in acidic and alkaline environments. These findings position carbon nitride a rising building block to functionally expand the 2D membrane library for applications in water desalination and purification scenarios.

Rapid co-deposition of dopamine and polyethyleneimine triggered by CuSO4/H2O2 oxidation to fabricate nanofiltration membranes with high selectivity and antifouling ability

Membrane surface properties play important roles in the separation performance and fouling behavior during the membrane-based water treatment processes. Mussel-inspired polydopamine (PDA) layer with abundant functional groups can endow the membrane surface with increased hydrophilicity as well as extraordinary versatility and creative potential for further functionalization toward specific applications. However, the slow deposition rate still limits its application in membrane modification. In this study, rapid deposition of the PDA layer on the surface of the polysulfone membrane was realized by catalytic oxidation using CuSO4/H2O2. Meanwhile, polyethyleneimine (PEI) was introduced to co-deposit with PDA to obtain a hydrophilic, uniform, and electroneutral separation layer for the nanofiltration membrane. An optimized stoichiometry of modifiers and oxidants was obtained by UV–vis absorbance measurement. The membrane surface properties including surface morphology, roughness, chemical element compositions, wettability, and charge behavior before and after modification were characterized through various techniques. The optimized PDA/PEI modified membrane (a-Cu(15)-PDA/PEI) possessed higher MgCl2/NaCl and dye/salt selectivities as well as improved anti-fouling performance compared to the commercial NF-270 membrane. Furthermore, the Gaussian simulation combined with XPS analysis illustrated that the introduction of Cu2+/H2O2 accelerated the dopamine oxidation and facilitated the formation of CO quinoid structures, thus providing more reactive sites for the Schiff base reaction in the PDA/PEI co-deposition process. The results from this study provide theoretical foundations for improving nanofiltration membrane performances by PDA/PEI modification.

CO2 as an Alternative to Traditional Antiscalants in Pressure-Driven Membrane Processes: An Experimental Study of Lab-Scale Operation and Cleaning Strategies.

Scaling, or inorganic fouling, is a major factor limiting the performance of membrane-based water treatment processes in long-term operation. Over the past few decades, extensive studies have been conducted to control the scale growth found in membrane processes and to develop sustainable and greener processes. This study details the role of CO2 in scale inhibition in membrane processes. The core concept of CO2 utilization is to reduce the influent pH and to minimize the risk of scale formation from magnesium or calcium salts. Three reverse osmosis (RO) units were operated with a control (U1), CO2 (U2), and a commercial antiscalant, MDC-220 (U3). The performances of all the units were compared in terms of change in transmembrane pressure (TMP). The overall efficiency trend was found as U1 > U3 > U2. The membrane surfaces were analyzed using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) for the morphological and elemental compositions, respectively. The surface analysis signified a significant increase in surface smoothness after scale deposition. The noticeable reduction in surface roughness can be described as a result of ionic deposition in the valley region. A sludge-like scale layer was found on the surface of the control membrane (U1) which could not be removed, even after an hour of chemical cleaning. After 20–30 min of cleaning, the U2 membrane was successfully restored to its original state. In brief, this study highlights the sustainable membrane process developed via CO2 utilization for scale inhibition, and the appropriate cleaning approaches.

Ionic fluid as a novel cleaning agent for the control of irreversible fouling in reverse osmosis membrane processes

While a variety of chemical cleaning strategies has been studied to control fouling in membrane-based water treatment processes, the removal of irreversible foulants strongly bound on membrane surfaces has not been successful. In this study, we firstly investigated the diluted aqueous solutions of ionic fluid (IF, 1-ethyl-3-methylimidazolium acetate) as a cleaning agent for three model organic foulants (humic acid, HA; bovine serum albumin, BSA; sodium alginate, SA). The real-time monitoring of cleaning progress by optical coherence tomography (OCT) showed that fouling layer was dramatically swelled by introducing IF solution and removed by shear force exerted during cleaning. This phenomenon was induced due to the pre-existing interactions between organic foulants were weakened by the intrusion of IF into the fouling layer, which was analyzed by the measurement of adhesion forces using atomic force microscopy (AFM). In the experiments with model foulants and wastewater effluent, IF was added to alkaline cleaning agents (NaOH) to verify the applicability to be supplemented in commercial cleaning agents, and resulted in the significantly enhanced control of irreversible membrane fouling. Implication of utilizing recyclable IF with negligible volatility is that environmental effects of membrane cleaning solutions could be minimized by decreasing usage of cleaning chemicals, while increasing the cleaning efficiency.

Mechanistic insights into Ca-alginate gel-associated membrane fouling affected by ethylene diamine tetraacetic acid (EDTA)

While transparent exopolymer particles (TEP) is a major foulant, and ethylene diamine tetraacetic acid (EDTA) is a strong chelating agent frequently used for fouling mitigation in membrane-based water treatment processes, little has been known about TEP-associated membrane fouling affected by EDTA. This work was performed to investigate roles of EDTA addition in TEP (Ca-alginate gel was used as a TEP model) associated fouling. It was interestingly found that, TEP had rather high specific filtration resistance (SFR) of 2.49 × 1015 m−1·kg−1, and SFR of TEP solution firstly decreased and then increased rapidly with EDTA concentration increase (0–1 mM). A series of characterizations suggested that EDTA took roles in SFR of TEP solution by means of changing TEP microstructure. The rather high SFR of TEP layer can be attributed to the big chemical potential gap during filtration described by the extended Flory-Huggins lattice theory. Initial EDTA addition disintegrated TEP structure by EDTA chelating calcium in TEP, inducing reduced SFR. Continuous EDTA addition decreased solution pH, resulting into no effective chelating and accumulation of EDTA on membrane surface, increasing SFR. It was suggested that factors increasing homogeneity of TEP gel will increase SFR, and vice versa. This study revealed the thermodynamic mechanism of TEP fouling behaviors affected by EDTA, and also demonstrated the importance of EDTA dosage and pH adjustment for TEP-associated fouling control.

Development and performance of stable PANI/MWNT conductive membrane for contaminants degradation and anti-fouling behavior

• PANI/MWNT membranes were developed by incorporating PANI and MWNT through layer-by-layer assembly. • PANI/MWNT membranes exhibited good electrical conductivity and stability. • PANI/MWNT membranes showed an excellent degradation for the organic pollutants. • PANI/MWNT membranes can reduce fouling in electrochemical filtration systems. Electrically conductive membranes (ECMs) have the potential to conduct and distribute an externally applied potential across the membrane surface, which enable some phenomena at the membrane/water interface, including increase in electrostatic repulsive forces between foulants and the membrane surface, and electrochemical oxidation and reduction of toxic contaminants. The objective of this study was to develop highly stable and conductive polyaniline/multiwall carbon nanotubes (PANI/MWNT) membranes through vacuum filtration assisted layer-by-layer assembly technique coupled with heat treatment, and to demonstrate its application in electrochemical filtration systems to degrade contaminants and reduce membrane fouling. The PANI/MWNT conductive membrane was assembled for ten bilayers through the comprehensive evaluation of surface conductivity, pure water flux, pore size distribution and electrochemical properties. The surface conductivity, pure water flux and average pore size of the membrane were 732.0 S/m, 596.6 L/m 2 ·h·bar and 72.0 nm, respectively. The stability evaluation revealed that the conductive membrane exhibited excellent physical, electrochemical and chemical stability. The degradation rate of methylene blue, methyl orange and phenol can achieve 94.9%, 96.1% and 95.6% under the applied voltage of 2.5 V, much higher than the traditional electrochemical oxidation process. Anti-fouling test conducted with humic acid showed that the fouling was effectively alleviated and the flux recovery rate increased from 63.3% to 78.0% when a negative bias was applied. This study provides a new insight for the multifunctional application of ECMs and offers a promising enhancement to existing membrane-based water treatment processes.

A Review on Uio-66 Applications in Membrane-Based Water Treatment Processes

A Review on Uio-66 Applications in Membrane-Based Water Treatment Processes

Optimization of the Asymmetric Cellulose Acetate Membrane Synthesis Variables for Porosity and Pure Water Permeation Flux Using Response Surface Methodology: Microfiltration Application

Membranes play a vital role in membrane-based water treatment processes. Thus, improvement in the membrane fabrication stage can greatly affect the overall performance of these technologies. Accordingly, this study focuses on optimizing the fabrication process of a microfiltration cellulose acetate membrane prepared using a pore-templating method. The effects of synthesis variables and their interactions were investigated using face-centered composite design through response surface methodology (RSM). Calcium carbonate (CaCO3) and glycerin concentrations in the casting solution in addition to evaporation time and hydrochloric acid (HCl) bath time were selected as the independent variables, while the membrane porosity and pure water flux were considered as the response variables. Significant increase in both responses was observed by increasing CaCO3 and glycerin concentrations (P value < 0.05). However, the effect of evaporation time was found to be statistically insignificant on both responses. The effect of the HCl bath time was found to significantly affect the membrane porosity but not the pure water flux. RSM models for both responses were statistically evaluated in terms of the coefficient of determination (R2), and the estimated values (96.74% for the porosity model and 83.85% for the flux model) indicated good fit of the model to the experimental values. Multiple response optimization was employed, and the membrane produced at optimum conditions proved to have good agreement with the predicted values (16.2% error in flux and 3.66% error in porosity). The membrane was successfully applied for paint pigment rejection and oil emulsion rejection, and rejection efficiencies of 95.6% and 80% were achieved for color and oil, respectively, confirming the applicability of the membrane produced at the obtained optimum condition for wastewater treatment.

Alleviation of membrane fouling in a submerged membrane bioreactor with electrochemical oxidation mediated by in-situ free chlorine generation

The control of membrane fouling is still the biggest challenge that membrane bioreactor (MBR) for wastewater treatment faces with. In this report, we evince that an in-situ electrochemical free chlorine generation is effective for membrane fouling mitigation. An electrochemical oxidation (EO) apparatus with perforated Ti/IrO2 anodes and Ti/Pt cathodes was integrated into a conventional MBR with microfiltration module (EO-MBR). The membrane fouling characteristics of EO-MBR fed with synthetic wastewater were monitored for about 2 months in comparison to control MBRs. In the EO-MBR at a direct current density of 0.4 mA/cm2, the frequency of membrane fouling when the trans-membrane pressure (TMP) reached 30 kPa was effectively reduced by 40% under a physical membrane cleaning regime. The evolution patterns of TMP together with hydraulic resistance analysis based on resistance-in-series model indicated that the electrochemically generated active chlorine alleviated the physically irremovable membrane fouling. Further analysis on extracellular polymeric substances (EPS) of sludge cake layer (SCL) revealed significant reductions of protein contents in soluble EPS and fluorescence emission intensities from humic acids and other fluorophores in bound EPS, which in-turn would decrease the hydrophobic accumulation of organic foulants on membrane pores. The chlorine dosage from the EO apparatus was estimated to be 4.7 mg Cl2/g MLVSS/day and the overall physicochemical properties (bio-solids concentration, floc diameter, zeta-potential) as well as the microbial activity in terms of specific oxygen utilization rate and removal efficiency of dissolved organic carbon (>97%) were not affected significantly. A T-RFLP (terminal restriction fragment length polymorphism) analysis suggested noticeable shifts in microbial community both in mixed liquor and sludge cake layer. Consequently, our electrochemical chlorination would be an efficient fouling control strategy in membrane-based water treatment processes where additional electricity consumption and cathodic scale deposition are not of serious concerns.

Characterizing the scouring efficiency of Granular Activated Carbon (GAC) particles in membrane fouling mitigation via wavelet decomposition of accelerometer signals

Particle fluidization is a promising unsteady-state shear means of mitigating membrane fouling, thereby potentially lower the energy requirement in membrane-based water treatment processes. In particular, the particles facilitate back-transport in the polarization layer and also play the role of mechanical scouring agents, the latter of which is dominant for millimeter-sized particles. In this study, the scouring efficacy of three Granular Activated Carbon (GAC) particle diameters (namely, 1.20mm, 1.85mm, and 2.18mm) has been investigated using an accelerometer to reveal the fluid dynamics. Specifically, because both liquid and solid phases contribute to the accelerometer signal, wavelet decomposition was used to extract information in the higher-frequency detail signals that reflect the particle dynamics. The energy contained in the accelerometer signal indicative of the solid phase dynamics correlated well with the extent of fouling mitigation in the filtration tests; the liquid contribution was less significant when the GAC particles were fully fluidized. Results indicate that the smallest particle diameter of 1.20mm conferred the least scouring efficiency, while both the larger particle diameters of 1.85mm and 2.18mm provided similar scouring efficiency. Calculations of energy requirement indicate that the energy requirement of all three particle diameters (dp) were similar at lower scouring efficiency, whereas the energy requirement of the smallest dp of 1.20mm was the greatest at higher scouring efficiency. Optimization of the particle diameter hinges on the balance between particle inertia, and the difference between superficial liquid velocity and minimum fluidization velocity.

The significance of calcium ions on Pseudomonas fluorescens biofilms – a structural and mechanical study

The purpose of this study was to investigate the effects of calcium ions on the structural and mechanical properties of Pseudomonas fluorescens biofilms grown for 48 h. Advanced investigative techniques such as confocal laser scanning microscopy and atomic force spectroscopy were employed to characterize biofilm structure as well as biofilm mechanical properties following growth at different calcium concentrations. The presence of calcium during biofilm development led to higher surface coverage with distinct structural phenotypes in the form of a granular and heterogeneous surface, compared with the smoother and homogeneous biofilm surface in the absence of calcium. The presence of calcium also increased the adhesive nature of the biofilm, while reducing its elastic properties. These results suggest that calcium ions could have a functional role in biofilm development and have practical implications, for example, in analysis of biofouling in membrane-based water-treatment processes such as nanofiltration or reverse osmosis where elevated calcium concentrations may occur at the solid–liquid interface.

An analysis of the effects of osmotic backwashing on the seawater reverse osmosis process

Fouling control is an important consideration in the design and operation of membrane-based water treatment processes. It has been generally known that chemical cleaning is still the most common method to remove foultants and maintain the performance of reverse osmosis (RO) desalination. Regardless of the chemical membrane cleaning methods applied effectively, however, frequent chemical cleaning can shorten the membrane life. In addition, it also increases operating and maintenance costs due to the waste chemical disposal. As an alternative, osmotic backwashing can be applied to RO membranes by diluting the concentration polarization (CP) layer. In this study, the effects of osmotic backwashing were analysed under different total dissolved salts (TDSs) and backwashing conditions, and the parameters of the osmotic backwashing were evaluated. The results of the analysis based on the properties of the organic matters found in raw water showed that the cleaning efficiency in respect to the fouling by hydrophilic organic matters was the greatest. Osmotic backwashing was carried out by changing the TDS of the permeate. As a result, the backwashing volume decreased with time due to the CP of the permeate and the backwashing volume. The difference in the osmotic pressure between the raw water and the permeate (Δ π) also decreased as time passed. It was confirmed that when the temperature of the effluent was high, both the cleaning efficiency and the backwashing volume, which inpours at the same time, increased. When the circulation flow of the effluent was high, both the cleaning efficiency and the backwashing volume increased.

Thermally responsive membrane-based microbiological sensing component for early detection of membrane biofouling

Early detection of bacteria in water treatment is desired to mitigate system biofouling and potential human health impacts. This research involved the development of an antibody-based sensor, which was attached to a hydroxypropyl cellulose (HPC) modified cellulose acetate ultrafiltration membrane surface, to target bacteria. HPC was chosen because it collapses above and expands below a lower critical solution temperature (43 °C), thus allowing temperature modulation. The membrane had a high target recovery efficiency (10–18%) in both single bacterium and complex samples (simulated by adding organics and competitive organisms). Sensor recovery was decreased (to 5–7%) following temperature activation (above 50 °C) due to adverse impacts of temperature on the antibody. The membrane was able to be regenerated (with 0.1 M NaOH) and reused three consecutive times. The thermally responsive sensing membrane developed from this work was specifically developed to detect biofoulants in membrane-based water treatment processes. However, it could be adapted to address a range of environmental detection concerns, such as pathogen detection.

Management of Concentrated Waste Streams from High-Pressure Membrane Water Treatment Systems

The sustainable management of concentrated waste streams from high-pressure membrane-based water treatment processes are commonly the greatest limitations to the implementation of such processes. This applies to seawater desalination, brackish water desalination, groundwater softening, surface water treatment, and municipal water reclamation. This review provides an analysis of the potential environmental implications of concentrate disposal to marine, freshwater, and terrestrial environments. Although high-pressure membrane treatment plants generate a number of other waste streams of similar content and effect on the environment (such as spent pre-treatment filter backwash and spent cleaning chemicals used during periodic cleaning of reverse osmosis membranes), these waste streams are not discussed in detail herein. The focus is on Australian circumstances, but the issues raised are universal. Established management practices are critically reviewed, and a number of alternative practices canvassed. Given that large-scale high-pressure membrane water treatment plants are relatively recent developments in Australia, a significant amount of work has been undertaken. However, a considerable number of knowledge gaps are revealed, preventing a complete understanding of the risks associated with existing practices and the development of potentially more economically and environmentally sustainable concentrate management practices. As such, the review provides insight to current research needs and priorities.