Constructing spherical-beads-on-string structure of electrospun membrane to achieve high vapor flux in membrane distillation

Hydrophobic membranes with a reentrant-like structure have shown high hydrophobicity and high anti-wetting properties in membrane distillation (MD). Here, PVDF spherical-beads-on-string (SBS) fibers were electrospun on nonwoven fabric and used in the MD process. Such a reentrant-like structure was featured with fine fibers, a low ratio of bead length to bead diameter, and high bead frequency. It was revealed that the SBS-structured membranes exhibited an exceptional capability for vapor flux, due to the formation of a network of more interconnected macropores than that of fibers and fusiform-beads-on-string structures, ensuring unimpeded vapor diffusion. In the desalination of formulated seawater (3.5 wt.% NaCl solution), a vapor flux of 61 ± 3 kg m-2 h-1 with a salt rejection of >99.98 % was achieved at a feed temperature of 60 °C. Furthermore, this SBS structured membrane showed satisfactory seawater desalination performance with a stable flux of 40 kg m-2 h-1 over a 27 h MD process. These findings suggest a viable approach for fabricating SBS-structured membranes that significantly enhance vapor flux in MD for desalination applications. Besides, the hydrophobic membranes with SBS structure can be prepared by single-step electrospinning, and it is facile to scale-up manufacture. This strategy holds promise for advancing the development of high-performance MD membranes tailored for efficient seawater desalination processes.

Crosslinking and fluorination reinforced PTFE nanofibrous membrane with excellent amphiphobic performance for low-scaling membrane distillations

Membrane distillation (MD) has emerged as a promising technology for desalination and concentration of hypersaline brine. However, the efficient preparation of a structurally stable and salinity-resistant membrane remains a significant challenge. In this study, an amphiphobic polytetrafluoroethylene nanofibrous membrane (PTFE NFM) with exceptional resistance to scaling has been developed, using an energy-efficient method. This innovative approach avoids the high-temperature sintering treatment, only involving electrospinning with PTFE/PVA emulsion and subsequent low-temperature crosslinking and fluorination. The impact of the PVA and PTFE contents, as well as the crosslinking and subsequent fluorination on the morphology and MD performance of the NFM, were systematically investigated. The optimized PTFE NFM displayed robust amphiphobicity, boasting a water contact angle of 155.2º and an oil contact angle of 132.7º. Moreover, the PTFE NFM exhibited stable steam flux of 52.1 L·m-2·h-1 and 26.7 L·m-2·h-1 when fed with 3.5 wt % and 25.0 wt % NaCl solutions, respectively, and an excellent salt rejection performance (99.99 %, ΔT = 60 °C) in a continuous operation for 24 h, showing exceptional anti-scaling performance. It also exhibited stable anti-wetting and anti-fouling properties against surfactants (sodium dodecyl sulfate) and hydrophobic contaminants (diesel oil). These results underscore the significant potential of the PTFE nanofibrous membrane for practical applications in desalination, especially in hypersaline or polluted aqueous environments.

Progress in membrane distillation processes for dye wastewater treatment: A review

Textile and cosmetic industries generate large amounts of dye effluents requiring treatment before discharge. This wastewater contains high levels of reactive dyes, low to none-biodegradable materials and chemical residues. Technically, dye wastewater is characterised by high chemical and biological oxygen demand. Biological, physical and pressure-driven membrane processes have been extensively used in textile wastewater treatment plants. However, these technologies are characterised by process complexity and are often costly. Also, process efficiency is not achieved in cost-effective biochemical and physical treatment processes. Membrane distillation (MD) emerged as a promising technology harnessing challenges faced by pressure-driven membrane processes. To ensure high cost-effectiveness, the MD can be operated by solar energy or low-grade waste heat. Herein, the MD purification of dye wastewater is comprehensively and yet concisely discussed. This involved research advancement in MD processes towards removal of dyes from industrial effluents. Also, challenges faced by this process with a specific focus on fouling are reviewed. Current literature mainly tested MD setups in the laboratory scale suggesting a deep need of further optimization of membrane and module designs in near future, especially for textile wastewater treatment. There is a need to deliver customized high-porosity hydrophobic membrane design with the appropriate thickness and module configuration to reduce concentration and temperature polarization (CP and TP). Also, energy loss should be minimized while increasing dye rejection and permeate flux. Although laboratory experiments remain pivotal in optimizing the MD process for treating dye wastewater, the nature of their time intensity poses a challenge. Given the multitude of parameters involved in MD process optimization, artificial intelligence (AI) methodologies present a promising avenue for assistance. Thus, AI-driven algorithms have the potential to enhance overall process efficiency, cutting down on time, fine-tuning parameters, and driving cost reductions. However, achieving an optimal balance between efficiency enhancements and financial outlays is a complex process. Finally, this paper suggests a research direction for the development of effective synthetic and natural dye removal from industrially discharged wastewater.

Scaling behavior in membrane distillation: Effect of Biopolymers and Antiscalants

Fouling and scaling are inherent characteristics of membrane-based separation. They lead to a reduced membrane throughput. In the case of membrane distillation (MD), they can possibly result in pore wetting and irreversible failure to sustain the mass transfer interface. Most prior research on understanding fouling and scaling uses indirect measurements (flux) or ex-situ analyses methods (such as SEM and EDX), which limit the outcomes to indirect qualitative conclusions. Particularly, studying scaling tends to be more challenging due to the complexity of the experiments and the method of investigation; it is imperative to distinguish the contributions from the bulk phase and heterogeneous nucleation. In this work, we established a non-invasive, in-situ, real-time imaging experimental apparatus to study the scaling mechanism. Our experimental setup assisted us in distinguishing distinct phases of scaling during the filtration tests. We studied the scaling mechanism of various single-component systems (sodium chloride, strontium sulfate, calcium sulfate, and calcium carbonate) in vacuum MD filtration. The effect of natural organic matter and antiscalants on gypsum scaling were systematically investigated. Overall, organic fouling on the membrane surface expedited heterogeneous crystallization while decelerating crystal growth in the bulk phase. For instance, deposited humic acid (HA) on the membrane surface promoted gypsum heterogeneous nucleation on the membrane surface due to the interactions between HA carboxylic functional groups and calcium ions. The adsorption of HA on the salt crystal also decelerated crystal growth in the bulk phase. Antiscalants delayed and decelerated both crystal nucleation and crystal growth. PAA, a polycarboxylate antiscalant at 5 ppm, was found to effectively delay the onset of nucleation and crystal growth in the bulk phase, while phosphorous antiscalants at 5 ppm only delayed the onset of nucleation in the bulk phase with a negligible influence on crystal growth. Real-time, in-situ, and non-invasive monitoring shed light on the scaling mechanism and can further be used to identify mitigation strategies.

Molecular insights into the adsorption and penetration of oil droplets on hydrophobic membrane in membrane distillation

Membrane fouling induced by oily substances significantly constrains membrane distillation performance in treating hypersaline oily wastewater. Overcoming this challenge necessitates a heightened fundamental understanding of the oil fouling phenomenon. Herein, the adsorption and penetration mechanism of oil droplets on hydrophobic membranes in membrane distillation process was investigated at the molecular level. Our results demonstrated that the adsorption and penetration of oil droplets were divided into four stages, including the free stage, contact stage, spreading stage, and equilibrium stage. Due to the extensive non-polar surface distribution of the polytetrafluoroethylene (PTFE) membrane (comprising 95.41 %), the interaction between oil molecules and PTFE was primarily governed by van der Waals interaction. Continuous oil droplet membrane fouling model revealed that the new oil droplet molecules preferred to penetrate into membrane pores where oil droplets already existed. The penetration of resin (a component of medium-quality oil droplets) onto PTFE membrane pores required the "pre-paving" of light crude oil. Finally, the ΔE quantitative structure-activity relationships (QSAR) models were developed to evaluate the penetration mechanism of pollutant molecules on the PTFE membrane. This research provides new insights for improving sustainable membrane distillation technologies in treating saline oily wastewater.

A novel electro-Fenton hybrid system for enhancing the interception of volatile organic compounds in membrane distillation desalination

Membrane distillation (MD) is a promising alternative desalination technology, but the hydrophobic membrane cannot intercept volatile organic compounds (VOCs), resulting in aggravation in the quality of permeate. In term of this, electro-Fenton (EF) was coupled with sweeping gas membrane distillation (SGMD) in a more efficient way to construct an advanced oxidation barrier at the gas-liquid interface, so that the VOCs could be trapped in this layer to guarantee the water quality of the distillate. During the so-called EF-MD process, an interfacial interception barrier containing hydroxyl radical formed on the hydrophobic membrane surface. It contributed to the high phenol rejection of 90.2% with the permeate phenol concentration lower than 1.50 mg/L. Effective interceptions can be achieved in a wide temperature range, even though the permeate flux of phenol was also intensified. The EF-MD system was robust to high salinity and could electrochemically regenerate ferrous ions, which endowed the long-term stability of the system. This novel EF-MD configuration proposed a valuable strategy to intercept VOCs in MD and will broaden the application of MD in hypersaline wastewater treatment.

Design of a multistage hybrid desalination process for brine management and maximum water recovery

Hypersaline brine production from desalination plants causes huge environmental stress due to the untenable conventional discharge strategies. Particularly, brine production is expected to drastically increase in the coming few decades due to the increasing desalination capacity in attempts of forestalling water scarcity. Thereby, zero liquid discharge (ZLD) is a worth-considering solution for strategic brine management. ZLD or minimal liquid discharge (MLD) systems provide maximum water recovery with least or zero liquid waste generation and valuable salt production. In this work, a theoretical design of ZLD/MLD system is proposed for reverse osmosis (RO) brine management. Different scenarios are investigated utilizing multistage freeze desalination (FD) and its hybridization with multistage direct contact membrane distillation (DCMD), and eutectic freeze crystallization (EFC) technologies. The design is based on the experimental assessment of the indirect FD process at different feed salinities, i.e., 2 g/L to 155 g/L. FD experiments showed that ice quality is reduced at greater crystallinity levels and initial concentration. Moreover, a computational fluid dynamics (CFD) model is utilized to assess the performance of DCMD. A single DCMD module could produce 53 kg/(m2.h) of pure water operating with 69% thermal efficiency. Eventually, water recovery, water quality, as well as specific energy consumption (SEC) are evaluated for the whole system. Based on different configurations of the hybrid ZLD system, the proposed design can achieve water recovery between 40 and 93% with SEC range of 28-114 kWh/m3. Results also showed that the produced water quality exceeds drinkable water standards ( ≪ 500 mg/L). This work has provided great evidence in the practicality of ZLD/MLD systems for sustainable brine management.

Novel coiled hollow fiber module for high-performance membrane distillation

Membrane distillation (MD) scale-up is challenged by ineffective heat recovery and the temperature polarization effect. Direct contact membrane distillation (DCMD) modules suffer high thermal conduction losses due to feed flow direction along the length of the membrane, resulting in low thermal efficiency. We propose a novel module design named coiled hollow fiber (CHF) to decouple the flow direction from the membrane surface in hollow fiber (HF) DCMD. Experimental and computational analyses were employed to compare the performance of CHF and the conventional design. The CHF module design successfully mitigates the TP effect in HF DCMD, increasing the flux by 148 % and 163 % in cross-flow and localized heating (LH) modes, respectively. Moreover, CHF operated in LH mode exhibits the lowest energy consumption of all configurations (81 % decrease) compared to the conventional design. This novel module design represents a new pathway for efficient and highly performing DCMD module.

Engineered eco-friendly composite membranes with superhydrophobic/hydrophilic dual-layer for DCMD system

Considerable advancements have been made in the development of hydrophobic membranes for membrane distillation (MD). Nonetheless, the environmentally responsible disposal of these membranes poses a critical concern due to their synthetic composition. Herein, an eco-friendly dual-layered biopolymer-based membrane was fabricated for water desalination. The membrane was electrospun from two bio-polymeric layers. The top hydrophobic layer comprises polycaprolactone (PCL) and the bottom hydrophilic layer from cellulose acetate (CA). Additionally, silica nanoparticles (SiO2 NPs) were electrosprayed onto the top layer of the dual-layered PCL/CA membrane to enhance the hydrophobicity. The desalination performance of the modified PCL-SiO2/CA membrane was compared with the unmodified PCL/CA membrane using a direct contact membrane distillation (DCMD) unit. Results revealed that silica remarkably improves membrane hydrophobicity. The modified PCL-SiO2/CA membrane demonstrated a significant increase in water contact angle of 152.4° compared to 119° for the unmodified membrane. In addition, PCL-SiO2/CA membrane has a smaller average pore size of 0.23 ± 0.16 μm and an exceptional liquid entry pressure of water (LEPw), which is 3.8 times higher than that of PCL/CA membrane. Moreover, PCL-SiO2/CA membrane achieved a durable permeate flux of 15.6 kg/m2.h, while PCL/CA membrane showed unstable permeate flux decreasing approximately from 25 to 12 kg/m2.h over the DCMD test time. Furthermore, the modified PCL-SiO2/CA membrane achieved a high salt rejection value of 99.97% compared to a low value of 86.2% for the PCL/CA membrane after 24 h continuous DCMD operation. In conclusion, the proposed modified PCL-SiO2/CA dual-layer biopolymeric-based membrane has considerable potential to be used as an environmentally friendly membrane for the MD process.

Numerical evaluation of sweeping gas membrane distillation for desalination of water towards water sustainability and environmental protection

Sweeping gas membrane distillation (SGMD) is considered a membrane distillation configuration. It uses an air stream to collect the water vapour. A 2D mathematical model is prepared in the current study to predict the effect of various operating parameters on the SGMD performance. Also, the temperature distribution in the SGMD was obtained. The effect of air inlet temperature, salt concentration, feed and air flowrate on air and salted solution outlet temperature and vapour flux through the membrane is investigated. There was good agreement between experimental data and modelling outputs. It was found that increase in air inlet temperature from 40 to 72 °C was increased the outlet temperature of air stream and cold solution from 37 to 63 °C and 38 to 65 °C respectively. Furthermore, increase in air inlet temperature led to the enhancement of vapour flux in the membrane distillation. Also, the salt concentration and feed flow rate did not have meaningful influence on the outlet temperatures, however, the flux was increased by increasing feed flowrate.

Enhanced landfill leachate treatment performance by adsorption-assisted membrane distillation

Membrane fouling and high-strength membrane concentrate production are two limitations of membrane distillation (MD) for landfill leachate treatment. In this study, activated carbon- and biochar-based adsorption processes were integrated into a conventional MD system to overcome these limitations. The organic matter fractionations of the leachate were thoroughly investigated during the treatment. Membrane-reversible and irreversible foulants differed remarkably from the inlet leachate in the non-assisted MD system. Specifically, reversible foulants were characterized by a high abundance of humic-like fluorescent components, high-molecular-weight humic-size constituents, peptides, and unsaturated compounds. In contrast, irreversible foulants were enriched with fulvic-like fluorescent components, low-molecular-weight neutrals, unsaturated compounds, and polyphenols. The adsorption-based pre-treatment effectively removed foulant precursors from landfill leachate, with a relatively higher (20%) adsorption performance for specific biochar used in this study than for activated carbon. Compared with the non-assisted MD system, the biochar-assisted MD system showed improved performance, achieving 40% overall membrane flux recovery, 42% higher filtration fluxes, and 53% lower concentrate production. In addition, a 15% higher removal of irreversible foulants was observed as compared to the reversible foulants, which can potentially increase the membrane lifespan. This study demonstrates the effectiveness of an adsorption-assisted MD system supported by increased filtration, membrane fouling alleviation, and low-strength leachate concentrate generation.

Making waves: Magneto-responsive membranes with special and switchable wettability: new opportunities for membrane distillation

Membrane distillation (MD) has promising potential in the water purification and wastewater treatment industries; however, fouling and wetting are the main obstacles to its commercialization, and higher fluxes and energy efficiencies are essential. Magneto-responsive membranes (MagMem) with integrated magnetic nanoparticles (MNPs) enable in situ fouling mitigation and switchable separation by nano-mixing or nano-heating, triggered by external magnetic fields, in a range of membrane processes, but not yet been demonstrated in MD. This perspective discussed the potential paths of MagMem utilization in MD based on the research status and dilemmas of MD. It can be envisioned that MagMem will lead to a paradigm shift in MD, especially by in situ fouling/wetting mitigation and enhancing energy efficiency via in-place actuation and localized heating by MNPs. Moreover, remotely controllable pore tuning and specific or switchable wettability can also be anticipated. Overall, MagMem provides attractive opportunities for advanced robust and efficient MD.

Synergetic effect on fouling alleviating of membrane distillation in urine resource recovery by thermally activated peroxydisulfate pretreatment

Given that the spontaneous precipitation of minerals caused by urea hydrolysis and abundant organic compounds, membrane fouling became a major obstacle for urine recovery by membrane distillation (MD). Herein, this study developed a combined system (TAP-MD) by integrating thermally activated peroxydisulfate (TAP) and MD process to inhibit membrane fouling and improve separation efficiency. Based on the TAP-MD system, the separation performance was improved significantly, improving nutrient recovery efficiency and quality of reclaimed water. More than 80% of water could be recovered from urine, and about 94.13% of total ammonia nitrogen (TAN), 99.02% of total nitrogen (TN), 100% of total phosphate (TP), and 100% of K+ were rejected. The mechanism for alleviating urine-induced fouling was systematically and intensively studied. With TAP pretreatment, the TAN concentration of pretreated urine was kept at a low level steadily and the pH was at neutral or weakly acidic. Hence, inorganic scaling represented by carbonate and phosphate precipitates were significantly inhibited by creating unfavorable solvent environment for crystallization with TAP pretreatment. Additionally, aromatic proteins were found as the main organic foulants. According to the secondary structure of protein, the proteins were degraded by the cleavage of peptide bonds by TAP pretreatment. Meanwhile, the hydrophilicity of protein increased, which reduced the hydrophobic interaction of protein and membrane surface and thus alleviated protein-induced membrane fouling. This study revealed the inorganic and organic foulants in urine that caused membrane fouling and demonstrated the mechanism of membrane fouling alleviation by TAP-MD system. The experimental results will be instrumental in better understanding the mechanisms of membrane fouling induced by urine and optimize MD process for resource recovery from urine.

Antiscalants for mitigating silica scaling in membrane desalination: Effects of molecular structure and membrane process

Silica scaling is a major type of mineral scaling that significantly constrains the performance and efficiency of membrane desalination. While antiscalants have been commonly used to control mineral scaling formed via crystallization, there is a lack of antiscalants for silica scaling due to its unique formation mechanism of polymerization. In this study, we performed a systematic study that investigated and compared antiscalants with different functional groups and molecular weights for mitigating silica scaling in membrane distillation (MD) and reverse osmosis (RO). The efficiencies of these antiscalants were tested in both static experiments (for hindering silicic acid polymerization) as well as crossflow, dynamic MD and RO experiments (for reducing water flux decline). Our results show that antiscalants enriched with strong H-accepters and H-donors were both able to hinder silicic acid polymerization efficiently in static experiments, with their antiscaling performance being a function of both molecular functionality and weight. Although poly(ethylene glycol) (PEG) with abundant H-accepters exhibited high antiscaling efficiencies during static experiments, it displayed limited performance of mitigating silica scaling during MD and RO. Poly (ethylene glycol) diamine (PEGD), which has a PEG backbone but is terminated by two amino groups, was efficient to both hinder silicic acid polymerization and reduce water flux decline in MD and RO. Antiscalants enriched with H-donors, such as poly(ethylenimine) (PEI) and poly(amidoamine) (PAMAM), were effective of extending the water recovery of MD but conversely facilitated water flux decline of RO in the presence of supersaturated silica. Further analyses of silica scales formed on the membrane surfaces confirmed that the antiscalants interacted with silica via hydrogen bonding and showed that the presence of antiscalants governed the silica morphology. Our work indicates that discrepancy in antiscalant efficiency exists between static experiments and dynamic membrane filtration as well as between different membrane processes associated with silica scaling, providing valuable insights on the design principle and mechanisms of antiscalants tailored to silica scaling.

Fenton pretreatment to mitigate membrane distillation fouling during treatment of landfill leachate membrane concentrate: Performance and mechanism

Membrane distillation (MD) is regarded as a promising technology for treatment of landfill leachate membrane concentrate (LLMC) due to its merits of low cost and high rejection of non-volatile components. However, the high concentration of pollutants in the wastewater will cause severe membrane fouling, resulting in costly cleaning and maintenance. In this study, Fenton pretreatment was applied to alleviate membrane fouling during MD treatment of LLMC. Compared to rapid flux decline of 88.2% at concentration factor (CF) of 3 for raw LLMC, MD flux only decreased by 17.4% at CF = 6 for treating acidic Fenton effluent without subsequent pH adjustment (Fe2+ and H2O2 concentration were 600 mg/L and 1457 mg/L, respectively). The pH neutralization of Fenton effluent or merely acidification of LLMC could not achieve such excellent fouling mitigation. It was concluded that both oxidation and acidification were critical and the collaboration mechanism was revealed to explain low membrane fouling. Firstly, Fenton oxidation removed organic contaminants, reduced the hydrophobicity of organic substances and increased the percentage of carboxylic group within LLMC. Thus, hydrophobic (HP) attraction was weakened but multivalent cation bridging became dominant fouling mechanism for neutral Fenton effluent. Then, acidification weakened multivalent cation bridging by inhibiting the deprotonation of carboxylic group, further mitigating membrane fouling. However, acidification of LLMC caused more severe organic fouling due to decrease in electrostatic (EL) repulsion. In addition to low membrane fouling, satisfactory total organic carbon (TOC) rejection rate of 96.23% was achieved during combined Fenton-MD process. This study demonstrated that Fenton pretreatment without pH neutralization could effectively alleviate MD fouling and elucidated the synergistic mechanism between oxidation and acidification for fouling mitigation.

Thin continuous membrane coating with high surface energy for comprehensive antifouling seawater distillation

Membrane distillation (MD) has prominent advantages such as treating high-salinity wastewater with a low-grade thermal energy, high salt rejection, and zero discharge. However, organic fouling and mineral scaling are two major challenges for hydrophobic MD membranes when used for practical applications. Commonly, improving organic fouling- and mineral scaling-resistance require oppositely enhanced wetting properties of membrane, thus is difficult to simultaneously realize dual resistance with one membrane. Here, we proposed to use underwater thermodynamically stable high-surface-energy coating to modify the hydrophobic membrane with Janus structures comprising different surface energy. The underlayered structure meets the hydrophobicity requirements of the MD membrane, while the coating layer realizes dual resistance to organic and inorganic foulants. Theoretical analysis and experimental proof reveal that the membrane with the high-surface-energy coating layer outperforms the pristine one with approximately 10 times of longevity. This strategy provides a new way for the use of high-surface-energy materials in versatilely fouling-resistant MD process.

Anisotropic gypsum scaling of corrugated polyvinylidene fluoride hydrophobic membrane in direct contact membrane distillation

Membrane distillation (MD) technology has gained a lot of attention for treatment of geothermal brine, high salinity waste streams. However, mineral scaling remains a major challenge when treating complex high-salt brines. The development of surface-patterned superhydrophobic membranes is one of the core strategies to solve this problem. We prepared flat sheet membranes (F-PVDF) and hydrophobic membranes with micron-scale corrugated pattern (C-PVDF) using a phase separation method. Their scaling behavior was systematically evaluated using calcium sulfate solutions and the impact of the feed flow was innovatively investigated. Although C-PVDF shows higher contact angle and lower sliding angle than F-PVDF, the scaling resistance of C-PVDF in the perpendicular flow direction has worst scaling resistance. Although the nucleation barrier of the corrugated membrane is the same at both parallel and perpendicular flow directions based on the traditional thermodynamic nucleation theory, experimental observations show that the C-PVDF has the best scaling resistance in the parallel flow direction. A 3D computational fluid dynamics (CFD) model was used and the hydrodynamic state of the pattern membranes was assessed as a determinant of the scaling resistance. The corrugated membrane with parallel flow mode (flow direction in parallel to the corrugation ridge) induces higher fluid velocity within the channel, which mitigated the deposition of crystals. While in the perpendicular flow mode (flow direction in perpendicular to the corrugation ridge), the solutions confined in the corrugated grooves due to vortex shielding, which aggravates the scaling. These results shed light on the mechanism of scaling resistance of corrugated membranes from a hydrodynamic perspective and reveal the mechanism of anisotropy exhibited by corrugated membranes in MD.

Challenges and advancements in membrane distillation crystallization for industrial applications

Membrane distillation crystallization (MDC) is an emerging hybrid thermal membrane technology that synergizes membrane distillation (MD) and crystallization, which can achieve both freshwater and minerals recovery from high concentrated solutions. Due to the outstanding hydrophobic nature of the membranes, MDC has been widely used in numerous fields such as seawater desalination, valuable minerals recovery, industrial wastewater treatment and pharmaceutical applications, where the separation of dissolved solids is required. Despite the fact that MDC has shown great promise in producing both high-purity crystals and freshwater, most studies on MDC remain limited to laboratory scale, and industrializing MDC processes is currently impractical. This paper summarizes the current state of MDC research, focusing on the mechanisms of MDC, the controls for membrane distillation (MD), and the controls for crystallization. Additionally, this paper categorizes the obstacles hindering the industrialization of MDC into various aspects, including energy consumption, membrane wetting, flux reduction, crystal yield and purity, and crystallizer design. Furthermore, this study also indicates the direction for future development of the industrialization of MDC.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

Thermal Effect on Algae, Biofilm and Their Composition Towards Membrane Distillation Unit: A Mini-review

Membrane distillation (MD) has lower operating temperature and potential to recycle waste heat for desalination which catches much attention of the researchers in the recent years. However, the biofouling is still a challenging hurdle to be overcome for such applications. The microbial growth rate, secretion and biofilm formation are sensitive to heat. Membrane distillation is a thermally driven separation, so the increase of temperature in the seawater feed could influence the extent of biofouling on the unit parts. In this review, we present the effect of temperature on algal growth, the range of temperature the microbes, marine algae and planktons able to survive and the changes to those planktons once exceed the critical temperature. Thermal effect on the biofilm, its composition and properties are discussed as well, with association of the biofilm secreting microbes, but the study related to membrane distillation unit seems to be lacking and MD biofouling factors are not fully understood. Characterization of the algae, biofilm and EPS that govern biofouling are discussed. This information not only will help in designing future studies to fill up the knowledge gaps in biofouling of membrane distillation, but also to some extent, assist in pointing out possible fouling factors and predicting the degree of biofouling in the membrane distillation unit.

Quasi-critical condition to balance the scaling and membrane lifespan tradeoff in hypersaline water concentration

Mineral scaling is an inconvenient obstacle for membrane distillation in hypersaline wastewater concentration applications, compromising membrane lifespan to maintain high water recovery. Although various measures are devoted to alleviating mineral scaling, the uncertainty and complexity of scale characteristics make it difficult to accurately identify and effectively prevent. Herein, we systematically elucidate a practically applicable principle to balance the trade-off between mineral scaling and membrane lifespan. Through experimental demonstration and mechanism analysis, we find a consistent concentration phenomenon of hypersaline concentration in different situations. Based on the characteristics of the binding force between the primary scale crystal and the membrane, the quasi-critical concentration condition is sought to prevent the accumulation and intrusion of mineral scale. The quasi-critical condition achieves the maximum water flux on the premise of guaranteeing the membrane tolerance, and the membrane performance can be restored by undamaged physical cleaning. This report opens up an informative horizon for circumventing the inexplicable scaling explorations and develops a universal evaluation strategy to provide technical support for membrane desalination.

Patterned dense Janus membranes with simultaneously robust fouling, wetting and scaling resistance for membrane distillation

Membrane fouling, wetting and scaling are three prominent challenges that severely hinder the practical applications of membrane distillation (MD). Herein, polyamide/polyvinylidene fluoride (PA/PVDF) Janus membrane comprising a hydrophobic PVDF substrate and a patterned dense PA layer by reverse interfacial polymerization (R-IP) was developed. Direct contact MD experiments demonstrated that PA/PVDF Janus membrane could exhibit simultaneously superior resistance towards surfactant-induced wetting, oil-induced fouling and gypsum-induced scaling without compromising flux. Importantly, the size-sieving effect, rather than the breakthrough pressure of the membrane, was revealed as the critical factor that probably endowed its resistance to wetting. Furthermore, a unique possible anti-scaling mechanism was unveiled. The superhydrophilic patterned dense PA layer with strong salt rejection capability not only prevented scale-precursor ions from intruding the substrate but also resulted in the high surface interfacial energy that inhibited the adhesion and growth of gypsum on the membrane surface, while its relatively low surface -COOH density benefited from R-IP process further ensured the membrane with a low scaling propensity. This study shall provide new insights and novel strategies in designing high-performance MD membranes and enable robust applications of MD facing the challenges of membrane fouling, wetting and scaling.

Optimization and prediction of lead removal from aqueous solution using FO-MD hybrid process: Statistical and artificial intelligence analysis

Heavy metals (HMs) has become one of the most serious pollutants that are harmful to the environment and ecology. This paper focused on the removal of lead contaminant from wastewater by forward osmosis-membrane distillation (FO-MD) hybrid process using seawater as draw solution. Modeling, optimization, and prediction of FO performance are developed using complementary approach based on response surface methodology (RSM) and an artificial neural network (ANN). FO process optimization using RSM revealed that under initial lead concentration of 60 mg/L, feed velocity of 11.57 cm/s and draw velocity of 7.66 cm/s, FO process achieved highest water flux of 6.75 LMH, lowest reverse salt flux of 2.78 gMH and highest lead removal efficiency of 87.07%. Fitness of all models was evaluated based on determination coefficient (R²) and mean square error (MSE). Results showed highest R² value up to 0.9906 and lowest RMSE value up to 0.0102. ANN modeling generates the highest prediction accuracy for water flux and reverse salt flux, while RSM produces the highest prediction accuracy for lead removal efficiency. Subsequently, FO optimal conditions are applied on FO-MD hybrid process using seawater as draw solution and evaluate their performance to simultaneously remove lead contaminant and desalination of seawater. Results displays that FO-MD process shows a highly efficient solution to produce fresh water with almost free heavy metals and very low conductivity.

Control the hydrophilic layer thickness of Janus membranes by manipulating membrane wetting in membrane distillation

Janus membranes with asymmetric wettability have attracted wide attentions for their robust anti-oil-wetting/fouling abilities in membrane distillation (MD). Compared to traditional surface modification approaches, in this study, we provided a new approach which manipulated surfactant-induced wetting to fabricate Janus membrane with a controllable thickness of the hydrophilic layer. The membranes with 10, 20, and 40 μm of wetted layers were obtained by stopping the wetting induced by 40 mg L^-1 Triton X-100 (J = 25 L m^-2 h^-1) at about 15, 40, and 120 s, respectively. Then, the wetted layers were coated using polydopamine (PDA) to fabricate the Janus membranes. The resulting Janus membranes showed no significant change in porosities or pore size distributions compared with the virgin PVDF membrane. These Janus membranes exhibited low in-air water contact angles (< 50°), high underwater oil contact angles (> 145°), and low adhesion with oil droplets. Therefore, they all showed excellent oil-water separation performance with ∼100% rejection and stable flux. The Janus membranes showed no significant decline in flux, but a trade-off existed between the hydrophilic layer thicknesses and the vapor flux. Utilizing membranes with tunable hydrophilic layer thickness, we elucidated the underlying mechanism of such trade-off in mass transfer. Furthermore, the successful modification of membranes with different coatings and in-situ immobilization of silver nanoparticles indicated that this facile modification method is universal and can be further expanded for multifunctional membrane fabrication.

Early monitoring of pore wetting in membrane distillation using ultrasonic time-domain reflectometry (UTDR)

Pore wetting induced by surfactants and salt scaling is a major obstacle to the industrial application of membrane distillation (MD). Identifying the transition of wetting stages and achieving early monitoring of pore wetting are crucial for wetting control. Herein, we made a pioneering attempt to use ultrasonic time-domain reflectometry (UTDR) technique to non-invasively detect the pore wetting in a direct contact MD, and explain the UTDR waveform with the help of optical coherence tomography (OCT) imaging. The results showed that the water-vapor interface had a strong reflection to ultrasound (reflection coefficient = 0.9995), while the water-membrane and water-scaling layer interfaces showed relatively weak reflection. Therefore, UTDR could effectively detect the movement of water-vapor interface with the low interference from the signals generated by the membrane and scaling layer. For the surfactant-induced wetting, the occurrence of wetting could be successfully detected by the right-shift in phase and the reduction in amplitude of the UTDR waveform. Moreover, the wetting depth could be accurately calculated by the time of flight (ToF) and ultrasonic velocity. For scaling-induced wetting, the waveform slightly shifted to the left at the beginning due to the growth of scaling layer, then to the right because the left-shift was surpassed by the right-shift of the waveform caused by pore wetting. Both for the surfactant- and scaling-induced wetting, the variation of the UTDR waveform was sensitive to wetting dynamics, and the right-shift of phase and the reduction in amplitude of the waveform could act as early monitoring signals to the occurrence of wetting.

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.

Mixed scaling deconstruction in vacuum membrane distillation for desulfurization wastewater treatment by a cascade strategy

Mineral scaling is one key obstacle to membrane distillation in hypersaline wastewater desalination, but the scaling or fouling mechanism is poorly understood. Addressing this challenge required revealing the foulants layer formation process. In this work, the scaling process was deconstructed with a cascade strategy by stepwise changing the composition of the synthetic desulfurization wastewater. The flux decline curves presented a 3-stage mode in vacuum membrane distillation (VMD). Heterogeneous nucleation of CaMg(CO₃)₂, CaF₂, and CaCO₃ was the main incipient scaling mechanism. Mg-Si complex was the leading foulant in 2nd-stage, during which the scaling mechanism shifted from surface to bulk crystallization. The flux decreased sharply for the formation of a thick and compacted scaling layer by the bricklaying of CaSO₄ and Mg-Si-BSA complexes in the 3rd-stage. Bulk crystallization was identified as the key scaling mechanism in VMD for the high salinity and concentration multiple. The organic matter had an anti-scaling effect by changing the bulk crystallization. Humic acids (HA) and colloidal silica also contributed to incipient scaling for the high affinity to membrane, bovine serum albumin (BSA) acting as the cement of Mg-Si complexes. Mg altered the Si scaling from polymerization to Mg-Si complex formation, which significantly influence the mixed scaling mechanism. This work deconstructed the mixed scaling process and illuminated the role of main foulants, filling in the knowledge gap on the mixed scaling mechanism in VMD for hypersaline wastewater treatment and recovery.

Virus rejection and removal in pilot-scale air-gap membrane distillation

Membrane distillation (MD) is a thermally-driven process that can treat high concentration streams and provide a dual barrier for rejection and reduction of pathogens. Thus, MD has potential applications in treating concentrated wastewater brines for enhancing water recovery and potable water reuse. In bench-scale studies, it was demonstrated that MD can provide high rejection of MS2 and PhiX174 bacteriophage viruses, and when operating at temperatures greater than 55 °C, can reduce virus levels in the concentrate. However, bench-scale MD results cannot directly be used to predict pilot-scale contaminant rejection and removal of viruses because of the lower water flux and higher transmembrane hydraulic pressure difference in pilot-scale systems. Thus far, virus rejection and removal have not been quantified in pilot-scale MD systems. In this work, the rejection of MS2 and PhiX174 at low (40 °C) and high (70 °C) inlet temperatures is quantified in a pilot-scale air-gap MD system using tertiary treated wastewater. Both viruses were detected in the distillate which suggests the presence of pore flow; the virus rejection at a hot inlet temperature of 40 °C for MS2 and PhiX174 were 1.6-log₁₀ and 3.1-log₁₀, respectively. At 70 °C, virus concentrations in the brine decreased and were below the detection limit (1 PFU per 100 mL) after 4.5 h, however, viruses were also detected in the distillate in that duration. Results demonstrate that virus rejection is lower in pilot-scale experiments because of increased pore flow that is not captured in bench-scale experiments.

Treatment to surfactant containing wastewater with membrane distillation membrane with novel sandwich structure

Surfactant containing wastewater widely exists in textile industry, which hardly to be treated by membrane technology due to its high in salinity and wetting potential. In this study, PVDF membrane was modified by constructing a PDMS-SiO₂-PDMS "sandwich" structure on top of its surface via coating to achieve resistance to surfactant induced wetting. The "sandwich" layer was optimized based on the membrane performance during membrane distillation. Compared to the pristine PVDF membrane with contact angle of 92°, the water contact angle of the membrane with a "sandwich" layer of 0.44 μm increased to 153°. For the feed contained 0.5 wt% NaCl and 0.25 wt% surfactant, there was no membrane wetting occurred during the experiment period using the membrane with a "sandwich" structure, in comparison to the pristine PVDF membrane being wetted from beginning. For a challenge experiment to the developed membrane lasting for 100 h using a surfactant containing feed, there is no wetting sign observed and the stable flux is 20 kg·m^-2·h^-1.

Nutrient recovery of the hydrothermal carbonization aqueous product from dairy manure using membrane distillation

Phosphorus is a crucial resource for the agricultural industry, but its limited supply requires recovery from waste materials before it is lost and leads to eutrophication. Dairy manure is rich with phosphorus, and the growth and consolidation within the dairy industry has led to dairy manure management becoming a significant concern. Hydrothermal carbonization (HTC) and membrane distillation (MD) were investigated as an alternative to treat dairy manure and recover nutrients, specifically phosphorus and nitrogen. HTC is a thermal treatment process that converts organic matter into a hydrochar analogous to a low-grade coal, and MD is a thermally-driven separation process that can utilize low-grade waste heat from HTC, thus the two processes are synergetic. A byproduct of the HTC process is the aqueous product (HAP) that contains the water-soluble nutrients and organic components of dairy manure. In this work, the efficacy of MD to concentrate the nutrients in the presence of dissolved organic carbon was assessed. Samples included synthetic nutrient-rich streams as well as HAP produced at HTC temperatures ranging from 200 °C to 260 °C. In each case, the nutrients were successfully concentrated in the feed loop with rejections >99%. Dissolved carbon was found to foul the MD membrane at levels proportional to its hydrophobicity, with little fouling observed for glucose and substantial fouling observed for HAP solutions created at higher temperatures.

Engineered multi-scale roughness of carbon nanofiller-embedded 3D printed spacers for membrane distillation

Membrane distillation (MD) transfers heat and mass simultaneously through a hydrophobic membrane. Hence, it is sensitive to both concentration and temperature polarisation (CP and TP) effects. In this study, we fabricated feed spacers to improve MD efficiency by alleviating the polarisation effects. First, a 3D printed spacer design was optimised to show superior performance amongst the others tested. Then, to further enhance spacer performance, we incorporated highly thermally stable carbon nanofillers, including carbon nanotubes (CNT) and graphene, in the fabrication of filaments for 3D printing. All the fabricated spacers had a degree of engineered multi-scale roughness, which was relatively high compared to that of the polylactic acid (PLA) spacer (control). The use of nanomaterial-incorporated spacers increased the mean permeate flux significantly compared to the PLA spacer (27.1 L/m²h (LMH)): a 43% and 75% increase when using the 1% graphene-incorporated spacer (38.9 LMH) and 2% CNT incorporated spacer (47.5 LMH), respectively. This could be attributed to the locally enhanced turbulence owing to the multi-scale roughness formed on the spacer, which further increased the vaporisation rate through the membrane. Interestingly, only the CNT-embedded spacer markedly reduced the ion permeation through the membrane, which may be due to the effective reduction of CP. This further decreased with increasing CNT concentration, confirming that the CNT spacer can simultaneously reduce the CP and TP effects in the MD process. Finally, we successfully proved that the multi-scale roughness of the spacer surface induces micromixing near the membrane walls, which can improve the MD performance via computational fluid dynamics.

Electrospun nanofiber composite membranes for geothermal brine treatment with lithium enrichment via membrane distillation

In this study, a composite electrospun nanofiber membrane was fabricated and used to treat a geothermal brine source with lithium enrichment. An in-situ growth technique was applied to incorporate silica nanoparticles on the surface of nanofibers with (3-Aminopropyl) triethoxysilane as the nucleation site. The fabricated composite nanofiber membrane was heat pressed to enhance the integration of the membrane and its mechanical stability. The fabricated membranes were tested to evaluate their performance in feedwater containing different concentrations of NaCl in the range of 0-100 g/L, and the wetting resistivity of the membranes was examined. Finally, the optimal membrane was applied to treat the simulated geothermal brine. The experimental results revealed that the in-situ growth of nanoparticles and coating of flourosilane agent dramatically improved the separation performance of the membrane with high salt rejection, and adequate flux was achieved. The heat-pressed membrane obtained >99% salt rejection and flux of 14-19 L/m²h at varying feedwater salinity (0-100 g/L), and the concentration of the Li during the 24 h test reached >1100 ppm from the initial 360 ppm. Evaluation of the energy efficiency of the membranes showed that the heat-pressed membrane obtained the optimum energy efficiency in the high concentration of salts. Additionally, the economic analysis indicated that MD could achieve a levelized cost of 2.9 USD/m³ of lithium brine concentration as the heat source is within the feed. Overall, this technology would represent a viable alternative to the solar pond to concentrate Li brine, enabling a compact, efficient, and continuous operating system.

Integrated membrane electrochemical reactor-membrane distillation process for enhanced landfill leachate treatment

Treatment of recalcitrant landfill leachate (LFL) induces huge energy consumption and carbon emissions due to its complex composition. Although membrane distillation (MD) exhibits good potential in LFL treatment with waste heat utilization, membrane fouling and ammonia rejection are still the major problems encountered that hinder its application. Herein, membrane electrochemical reactor (MER) was coupled with MD for simultaneous membrane fouling control and resource recovery. LFL pretreatment with membrane-less electrochemical reactor (EO) and without pretreatment were also purified by MD for comparison. Results showed that the MER-MD system rejected almost all COD_Cr, total phosphorus, metal salts, and ammonia nitrogen (increased by 33.5%-43.5% without chemical addition), and recovered 31% of ammonia nitrogen and 48% of humic acid in the raw LFL. Owing to the effective removal of hardness (61%) and organics (77%) using MER, the MER-MD system showed higher resistance to the membrane wetting and fouling, with about 61% and 14% higher final vapor flux than those of the MD and EO-MD systems, respectively, and the pure water flux could be fully recovered by alkaline solution cleaning. Moreover, SEM-EDS, ATR-FTIR and XRD characterization further demonstrated the superiority of the MD membrane fouling reversibility of the MER-MD system. Energy consumption and carbon emissions analysis showed that the MER-MD system reduced the total energy consumption/carbon emissions by ∼20% and ∼8% compared to the MD and EO-MD systems, respectively, and the ammonia nitrogen recovered by MER could offset 8.25 kg carbon dioxide equivalent. Therefore, the introduction of MER pretreatment in MD process would be an option to decrease energy consumption and reduce carbon emissions for MD treatment of LFL.

Comprehensive experimental and theoretical investigations on the effect of microbubble two-phase flow on the performance of direct-contact membrane distillation

This study provides a comprehensive and systematic overview of the application of gas-liquid two-phase flow with microbubbles in the feed stream to improve heat and mass transfer in direct-contact membrane distillation (DCMD) processes for seawater desalination. A swirl-flow-type microbubble generator (MBG) was installed at the feed-side inlet of the DCMD module to investigate its effect on transmembrane flux. The maximum improvement in the MBG-assisted DCMD permeation flux was found to be approximately 18% at a lower feed temperature (40 °C) and optimal air flow rate (50 cc/min), and an optimal MBG geometry comprising a swirler, a nozzle tip of diameter 2 mm, and a diffuser at an angle of 30°. The results were observed to be related to the number density of microbubbles less than 100 µm in size, which plays an important role in improving heat and mass transfer in two-phase flow. In addition, the simulation results based on conventional heat transfer correlations of bubbly flow underestimated the experimental results. Therefore, this study also aims to propose and verify a new two-phase flow heat transfer correlation. The proposed correlation considers the effects of bubble size distribution to accurately predict the performance of MBG-assisted DCMD processes.

Understanding the membrane fouling control process at molecular level in the heated persulfate activation- membrane distillation hybrid system

Sulfate radical (SO₄^●-) based advanced oxidation is considered as a promising pretreatment strategy to degrade organic pollutants and thereby mitigate the membrane fouling in the membrane process. In this study, heat-activated persulfate (PS) activation was integrated with the membrane distillation (MD) process for the alleviation of membrane fouling in treatment of wastewater treatment plant (WWTP) secondary effluent and surface water. In-depth understanding of the molecular fate during membrane fouling control process was performed by using a non-targeted screening method of two-dimensional gas chromatography-time-of-flight mass spectrometry (GC × GC-TOF-MS) coupling with multiple characterizations. It was found that the heat-activated PS activation pretreatment could effectively degrade the dissolved organic matter (DOM) and change its molecular conformation, wherein the relative abundance of oxygen-containing substances was remarkably increased through oxygenation reactions. Moreover, the refractory organics with higher molecular weight (MW) and unsaturation degree were more inclined to be destroyed, following by partial mineralization during pretreatment process. It was also identified that oxygen-deficient compounds and the molecular formulas featuring higher double bond equivalent (DBE) values and lower MW tended to be deposited on the membrane surface to cause the membrane fouling. In particular, the aliphatic substances were the predominant components irrespective of membrane foulant samples from secondary effluent or surface water. Meanwhile, the complexation between organic compounds and high valence cations as well as the precipitation of inorganics were restrained owing to the reduction of DOM concentration and the transformation of molecular structure, consequently leading to reduced membrane fouling. This study is believed to further provide new insight into the membrane fouling control mechanism at molecular level.

Desalination technologies, membrane distillation, and electrospinning, an overview

Water is a critical component for humans to survive, especially in arid lands or areas where fresh water is scarce. Hence, desalination is an excellent way to effectuate the increasing water demand. Membrane distillation (MD) technology entails a membrane-based non-isothermal prominent process used in various applications, for instance, water treatment and desalination. It is operable at low temperature and pressure, from which the heat demand for the process can be sustainably sourced from renewable solar energy and waste heat. In MD, the water vapors are gone through the membrane's pores and condense at permeate side, rejecting dissolved salts and non-volatile substances. However, the efficacy of water and biofouling are the main challenges for MD due to the lack of appropriate and versatile membrane. Numerous researchers have explored different membrane composites to overcome the above-said issue, and attempt to develop efficient, elegant, and biofouling-resistant novel membranes for MD. This review article addresses the 21st-century water crises, desalination technologies, principles of MD, the different properties of membrane composites alongside compositions and modules of membranes. The desired membrane characteristics, MD configurations, role of electrospinning in MD, characteristics and modifications of membranes used for MD are also highlighted in this review.

Evaluation of membrane fouling at elevated temperature impacted by algal organic matter

Membrane distillation (MD) is a thermally driven technology applied in desalination and water reuse with utilisation of sustainable energy. However, algal organic matter (AOM) could foul membrane critically and plague MD's long-term operational stability. In this study, the soluble extracellular polymeric substance (sEPS) and intracellular organic matter with bound extracellular polymeric substance (IOM + bEPS) of two algal species (Amphora coffeaeformis and Navicula incerta) were exposed to 60 °C, 70 °C and 80 °C for 8 h with polypropylene hydrophobic membrane, simulating heated AOMs contacted with membrane inside MD unit, to study the temperature effect on membrane fouling. The dissolved carbohydrate and protein in the sEPS and IOM + bEPS samples generally increased after being heated. Heating caused cell lysis and the release and dissolution of carbohydrate and protein from sEPS, IOM and bEPS into water. As heating temperature increased, the carbohydrate release from the AOM usually increased. The contact angle of membrane contacted with sEPS and IOM + bEPS reduced significantly after heat treatment. The reduction in IOM + bEPS was larger than sEPS, in line with SEM analysis, indicating membrane surfaces and pores with IOM + bEPS fouled more severely than sEPS. It is due to higher hydrophobicity in IOM + bEPS causing adherence to membrane and presence of amphiphiles. High protein, lipid, and saturated fats proportions also cause severe fouling. SEM-EDX analysis indicated presence of O, Na, Cl and Mg elements, pointing to carbohydrate and lipids, and salt trapped in foulants. AOM heating and composition had direct effect to the membrane integrity, dictating severity of fouling in MD operations.

Structural design of the electrospun nanofibrous membrane for membrane distillation application: a review

Although membrane distillation (MD) is a promising technology for water desalination and industrial wastewater treatment, the MD process is not widely applied in the global water industry due to the lack of a suitable membrane for the MD process. The design and appropriate manufacture are the most important factors for MD membrane optimization. The well-designed porous structure, superhydrophobic surface, and pore-wetting prevention of the membrane are vital properties of the MD membrane. Nowadays, electrospinning that is capable of manufacturing membranes with superhydrophobic or omni phobic properties is considered a promising technology. Electrospun nanofibrous membranes (ENMs) possess the characteristics of cylindrical morphology, re-entrant structure, and easy-shaping for a specific purpose, benefiting the membrane design and modification. Based on that, this review investigates the current state and future progress of the superhydrophobic, multi-layer, and omniphobic ENMs manufactured with various structural designs for seawater desalination and wastewater purification. We expect that this paper will provide some recommendations and guidance for further fabrication research and the configuration design of ENMs in the MD process for seawater desalination and wastewater purification.

Feasibility assessment of bioethanol production from humic acid-assisted alkaline pretreated Kentucky bluegrass (Poa pratensis L.) followed by downstream enrichment using direct contact membrane distillation

The effective fractionation of structural components of abundantly available lignocellulosic biomass is essential to unlock its full biorefinery potential. In this study, the feasibility of humic acid on the pretreatment of Kentucky bluegrass biomass in alkaline condition was assessed to separate 70.1% lignin and hydrolyzable biocomponents. The humic acid-assisted delignification followed by enzymatic saccharification yielded 0.55 g/g of reducing sugars from 7.5% (w/v) pretreated biomass loading and 16 FPU/g of cellulase. Yeast fermentation of the biomass hydrolysate produced 76.6% (w/w) ethanol, which was subsequently separated and concentrated using direct contact membrane distillation. The hydrophobic microporous flat-sheet membrane housed in a rectangular-shaped crossflow module and counter-current mode of flow of the feed (hot) and distillate (cold) streams yielded a flux of 11.6 kg EtOH/m²/24 h. A modular, compact, flexible, and eco-friendly membrane-integrated hybrid approach is used for the first time to effectively valorize Kentucky bluegrass biomass for sustainable production of biofuel.

Elucidating biofouling over thermal and spatial gradients in seawater membrane distillation in hot climatic conditions

Biofouling is a hurdle of seawater desalination that increases water costs and energy consumption. In membrane distillation (MD), biofouling development is complicated due to the temperature effect that adversely affects microbial growth. Given the high relevance of MD to regions with abundant warm seawater, it is essential to explore the biofouling propensity of microbial communities with higher tolerance to elevated temperature conditions. This study presents a comprehensive analysis of the spatial and temporal biofilm distribution and associated membrane fouling during direct contact MD (DCMD) of the Red Sea water. We found that structure and composition of the biofilm layer played a significant role in the extent of permeate flux decline, and biofilms that built up at 45°C had lower bacterial concentration but higher extracellular polymeric substances (EPS) content as compared to biofilms that formed at 55 °C and 65°C. Pore wetting and bacterial passage to the permeate side were initially observed but slowed down as operating time increased. Intact cells in biofilms dominated over the damaged cells at any tested condition emphasizing the high adaptivity of the Red Sea microbial communities to elevated feed temperatures. A comparison of microbial abundance revealed a difference in bacterial distribution between the feed and biofilm samples. A shift in the biofilm microbial community and colonization of the membrane surface with thermophilic bacteria with the feed temperature increase was observed. The results of this study improve our understanding of biofouling propensity in MD that utilizes temperature-resilient feed waters.

Thermally assisted efficient electrochemical lithium extraction from simulated seawater

Extracting lithium electrochemically from seawater has the potential to resolve any future lithium shortage. However, electrochemical extraction only functions efficiently in high lithium concentration solutions. Herein, we discovered that lithium extraction is temperature and concentration dependent. Lithium extraction capacity (i.e., the mass of lithium extracted from the source solutions) and speed (i.e., the lithium extraction rate) in electrochemical extraction can be increased significantly in heated source solutions, especially at low lithium concentrations (e.g., < 3 mM) and high Na⁺/Li⁺ molar ratios (e.g., >1000). Comprehensive material characterization and mechanistic analyses revealed that the improved lithium extraction originates from boosted kinetics rather than thermodynamic equilibrium shifts. A higher temperature (i.e., 60 ^oC) mitigates the activation polarization of lithium intercalation, decreases charge transfer resistances, and improves lithium diffusion. Based on these understandings, we demonstrated that a thermally assisted electrochemical lithium extraction process could achieve rapid (36.8 mg g^-1 day^-1) and selective (51.79% purity) lithium extraction from simulated seawater with an ultrahigh Na⁺/Li⁺ molar ratio of 20,000. The integrated thermally regenerative electrochemical cycle can harvest thermal energy in heated source solutions, enabling a low electrical energy consumption (11.3-16.0 Wh mol^-1 lithium). Furthermore, the coupled thermal-driven membrane process in the system can also produce freshwater (13.2 kg m^-2 h^-1) as a byproduct. Given abundant low-grade thermal energy availability, the thermally assisted electrochemical lithium extraction process has excellent potential to realize mining lithium from seawater.

Desalinating a real hyper-saline pre-treated produced water via direct-heat vacuum membrane distillation

Membrane distillation (MD) is an emerging thermal desalination technology capable of desalinating waters of any salinity. During typical MD processes, the saline feedwater is heated and acts as the thermal energy carrier; however, temperature polarization (as well as thermal energy loss) contributes to low distillate fluxes, low single-pass water recovery and poor thermal efficiency. An alternative approach is to integrate an extra thermal energy carrier as part of the membrane and/or module assembly, which can channel externally provided heat directly to the membrane-feedwater interface and/or along the feed channel length. This direct-heat delivery has been demonstrated to increase single-pass water recovery and enhance the overall thermal efficiency. We developed a bench-scale direct-heated vacuum MD (DHVMD) process to desalinate pre-treated oil and gas "produced water" with an initial total dissolved solids of 115,500 ppm at a feed temperature ranging between 24 and 32 °C. We evaluated both water flux and specific energy consumption (SEC) as a function of water recovery. The system achieved a 50% water recovery without significant scaling, with an average flux >6 kg m^-2 hr^-1 and a SEC as low as 2,530 kJ kg^-1. The major species of mineral scales (i.e., NaCl, CaSO₄, and SrSO₄) that limited the water recovery to 68% were modeled in terms of thermodynamics and identified by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In addition, we further developed and employed a physics-based process model to estimate temperature, salinity, water transport and energy flows for full-scale vacuum MD and DHVMD modules. Model results show that a direct-heat input rate of 3,600 W can increase single-pass water recovery from 2.1% to 3.1% while lowering the thermal SEC from 7,800 kJ kg^-1 to 6,517 kJ kg^-1 in an unoptimized module. Finally, the scaling up potential of DHVMD process is briefly discussed.

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.

Recent technological advancements in membrane distillation and solar stills: preheating techniques, heat storage materials, and nanomaterials - a detailed review

Freshwater and energy are critical components for the growth and progress of societies. The scarcity of freshwater and rapid population growth, especially in remote countries, has led to an urgent need to develop desalination technologies in order to raise its productivity and reduce its energy consumption rates. Membrane distillation is one of the effective methods characterized by its high productivity, but its disadvantage by higher electricity consumption. Also, solar stills are one of the sustainable and economical technologies, but the disadvantage by lower productivity. Accordingly, this manuscript dealt with a comprehensive review and detailed comparison of the most important modifications and innovations that were made to the design of the membrane distillation units, which aim to reduce electricity consumption rates, as well as the design of solar stills, which aims to maximize the productivity and efficiency. This was done by providing a detailed comparison of the most important three axes of modifications and innovations that were addressed by recent previous studies on the design of membrane distillation units and solar stills, and their statement as follows: preheating technology, use of the thermal storage materials, and nanomaterials technology. Finally, based on this review, the authors make some recommendations for future work in the field of solar and membrane desalination.

Synergy of feed-side aeration and super slippery interface in membrane distillation for enhanced water flux and scaling mitigation

Membrane distillation (MD) is an acknowledged promising technology for desalinating hypersaline brine, and as such can be a suitable candidate to further concentrate the seawater discharged from reverse osmosis process. Mineral scaling represents a major constraint against the application of MD for further desalination of concentrated seawater, especially when considering CaSO₄ (gypsum) and NaCl. Up until now, it has been difficult to rely solely on membrane modification to mitigate CaSO₄ scaling. Permeate-side aeration can lessen CaSO₄ scaling, but does not permit to increase the water flux. Herein, we proposed the synergy of feed-side aeration and super slippery interface to perform concentrated seawater desalination via direct contact membrane distillation. The results of this study show that this synergistic effect could significantly increase the water flux, which was approximately 1.5 times higher in comparison to the membrane without aeration. Moreover, the synergistic effect effectively alleviates the complex scaling of concentrated seawater, achieving 90 wt% water recovery rate. Based on the observed results, we elucidated the mechanisms governing the enhanced water flux and scaling mitigation driven by the synergistic effect. In addition, we studied the optimal working condition for this system, unveiling that low-intensity large bubbles are more suitable as they lead to a better equilibrium between the economics and functionality of the process.

Water reclamation and microbial community investigation: Treatment of tetramethylammonium hydroxide wastewater through an anaerobic osmotic membrane bioreactor hybrid system

Tetramethylammonium hydroxide (TMAH) is a toxic photoresist developer used in the photolithography process in thin-film transistor liquid crystal display (TFT-LCD) production, and it can be removed through anaerobic treatment. TMAH cannot be released into the environment because of its higher toxicity. A tight membrane, such as a forward osmosis (FO) membrane, together with an anaerobic biological process can ensure that no TMAH is released into the environment. Thus, for the first time, an anaerobic osmotic membrane bioreactor (AnOMBR) hybrid system was developed in this study to treat a low-strength TMAH wastewater and to simultaneously investigate its microbial community. Microfiltration extraction was used to mitigate the salinity accumulation, and a periodically physical water cleaning was utilized to mitigate the FO membrane fouling. The diluted draw solute (MgSO₄) was reconcentrated and reused by a membrane distillation (MD) process in the AnOMBR to achieve 99.99% TMAH removal in this AnOMBR-MD hybrid system, thereby ensuring that no TMAH is released into the natural environment. Moreover, the membrane fouling in the feed and draw sides were analyzed through the fluorescence excitation-emission matrix (FEEM) spectrophotometry to confirm that the humic acid-like materials were the primary membrane fouling components in this AnOMBR. Additionally, 16S rRNA metagenomics analysis indicated that Methanosaeta was the predominant contributor to methanogenesis and proliferated during the long-term operation. The methane yield was increased from 0.2 to 0.26 L CH₄/g COD when the methanogen species acclimatized to the saline system.

Insights into the enhanced flux of graphene oxide composite membrane in direct contact membrane distillation: The different role at evaporation and condensation interfaces

Graphene oxide (GO) coating has recently been reported as a novel approach to increase membrane flux of membrane distillation (MD), yet the phenomena underlying the process are still not fully understood. In this study, a mathematical model based on capillary-film assumption was developed and validated with the results (R²>0.99) from a series of MD experiments. According to the model, when GO layer was placed at the evaporation interface, the temperature difference across the membrane surface increases significantly (44.2%∼92.0%) and the temperature polarization coefficient is increased greatly from 0.29∼0.38 to around 0.55. This leads to a big increase of driving force for higher heat flow and subsequently mass flux (17.8∼45.5%). However, the vapor pressure on membrane surface was decreased due to Kelvin effect of GO capillary pores, which has a negative influence on the driving force, accounting for about 26.9% to 52.6% drop in the achieved flux. In comparison, when GO layer was placed at the condensation interface, the temperature difference across the membrane surface decreases slightly (7.2∼12.2%), but the reduced vapor pressure on GO capillary pores due to Kelvin effect become the dominant factor affecting membrane flux, resulting in an increase mass flux of 12.4∼16.4%. The model developed in this study provides a theoretical foundation for understanding the role of GO coating on flux improvement, and can be used for further development of high flux membranes.

Seasonal shift of water quality in China Yangtze River and its impacts on membrane fouling development during the drinking water supply by membrane distillation system

Membrane distillation (MD) technique is increasingly regarded as a promising process for drinking water supply and wastewater treatment owing to its great water purification and usage of renewable energy. Like other membrane separation processes, the membrane fouling issue is widely considered as the main obstacle for real applications of large-scale MD systems. Feedwater characteristics, as the predominant factors for membrane fouling layer formation, mostly determined the membrane fouling trend of MD. Thus the impacts of seasonal shifts of initial feedwater quality on the MD membrane fouling were detailedly researched in this study, and the biofilm development mechanism was especially explored. The bacterial community structure of membrane biofilms was clearly clarified in MD runs of Yangtze River waters that collected in four seasons. The results revealed that the winter run posed a quite sharp flux drop, while a relatively milder flux decline behaviour was seen for other groups despite of the higher bacteria concentration of initial feedwaters. The poorer water quality in winter induced the establishment of a rather thick biofilm on the MD membrane, in which the biofilm-forming bacteria (Gammaproteobacteria and Alphaproteobacteria) and organic matters (EPS) were remarkably observed. Comparatively, a relatively thin biofilm containing abundant live cells and fewer organics finally formed in summer and autumn runs, causing a mitigated flux decline trend. Hence, it can be inferred that the membrane flux decline of MD was likely to be more sensitive to the organic attachment on the membrane in comparison with the bacteria adhesion. Finally, a three-phase pretreatment method was suggested for MD fouling control, including heating course, sterilization course, and filtration course.

Nanobubble-assisted scaling inhibition in membrane distillation for the treatment of high-salinity brine

In this study, we report the use of nanobubbles (NBs) as a simple and facile approach to effectively delay scaling in membrane distillation (MD) during the treatment of highly saline feed (100 g L-1). Unlike conventional gas bubbling in MD for improving the hydrodynamic flow conditions in the feed channel, here we generated air NBs with an average size of 128.81 nm in the feed stream and examined their impact on membrane scaling inhibition during MD operation. Due to their small size, neutral buoyancy, and negative surface charge, NBs remain in suspension for a longer time (14 days), providing homogenous mixing throughout the entire feed water. The MD performance results revealed that severe membrane scaling happened during the DCMD treatment of high salinity brine in the absence of nanobubbles, which dramatically reduced the distillate flux to zero after 13 h. A one-time addition of air NBs in the saline feed significantly reduced salt precipitation and crystal deposition on the PVDF membrane surface, delayed the occurrence of flux decline, prevented membrane wetting, thereby prolonging the effective MD operating time. With similar feed concentration and operating conditions, only 63% flux decline after 98 h operation was recorded in nanobubble-assisted MD. Two key explanations were suggested for the delayed membrane scaling upon addition of air NBs in the MD feed: (1) NB-induced turbulent flow in the feed channel that increases the surface shear forces at the membrane surface, alleviating both temperature and concentration polarization effect, (2) electrostatic attractions of the counterions to the negatively charged NBs, which reduces the availability of these ions in the bulk feed for scale formation.

Superhydrophobic modification of electrospun nanofibrous Si@PVDF membranes for desalination application in vacuum membrane distillation

Superhydrophobic nanofibers have received prominent attention owing to their exceptional properties and researchers are focused on developing high-performing MD membranes. Herein, we fabricate superhydrophobic electrospun nanofibrous membranes using polyvinylidene fluoride (PVDF) solutions with silica nanoparticles (0 wt% to 6 wt%) to create multiscale (or hierarchical) surface roughness. For superhydrophobicity, the composite membranes (Si@PVDF) were subjected to a two-step modification that included acid pre-treatment and silanization with fluoroalkylsilane (FAS) compound of low surface energy. The acid pre-treatment enhances the hydroxyl group of SiO₂ nanoparticles and create active sites in abundance for silanization. The modified membranes (FAS-Si@PVDF-A) having 6 wt% SiO₂ showed excellent wetting resistance with water contact angle (WCA) up to 154.6 ± 2.2°, smaller average pore size of 0.27 ± 0.3 μm, and high liquid entry pressure (LEP) of 143 ± 4 kPa. It was observed, increasing silica content decreased the fiber diameter and average pore size and increased WCA and LEP of modified membranes. The modified superhydrophobic membranes gave stable permeate flux, exhibited strong wetting resistance and excellent salt rejection in vacuum membrane distillation (VMD) test. The optimal FAS-Si@PVDF-A membrane (6 wt% SiO₂) of thickness 98 ± 5 μm produced a stable permeate flux of more than 11.5 kg m^-2 h^-1 and salt rejection as high as 99.9% after 22 h of continuous operation using NaCl solution (3.5 wt%) as feed. Therefore, this modification provided superhydrophobic membranes possessing robust anti-wetting properties with significant permeability and has encouraging application in membrane distillation for desalination.