Membrane Distillation Research Articles

Innovative hyper-thermophilic aerobic submerged membrane distillation bioreactor for wastewater reclamation

For the first time, a hyper-thermophilic aerobic (>60 °C) bioreactor has been integrated with direct submerged membrane distillation (MD), highlighting its potential as an advanced wastewater treatment solution. The hyper-thermophilic aerobic bioreactor, operating up to 65 °C, is tailored for high organic removal, while MD efficiently produces clean water. Throughout the study, high removal rates of 99.5% for organic matter, 96.4% for ammonia, and 100% for phosphorus underscored the impressive adaptability of microorganisms to challenging hyper-thermophilic conditions and a successful combination with the MD process. Despite the extreme temperatures and substantial salinity accumulation reaching up to 12,532 μS/cm, the biomass of microorganisms increased by 1.6 times over a 92-day period, representing their remarkable resilience. The distillation flux ranged from 6.15 LMH to 8.25 LMH, benefiting from the temperature gradient in the hyper-thermophilic setting and the design of the tubular submerged MD membrane module. The system also excels in pH control, utilizing fewer alkali and nutritional resources than conventional systems. Meiothermus, Firmicutes, and Bacteroidetes, the three dominant species, played a crucial role, showcasing their significance in adapting to high salinity and decomposing organic matter.

Maximizing the wetting resistance of fluorine-free omniphobic membranes for hypersaline wastewater desalination

Membrane distillation (MD) equipped with omniphobic (non-wetting) membranes has found a niche in water reclamation from hypersaline industrial wastewater. Here, we examined the efficacy of non-fluorinated materials as surface coating agents for omniphobic MD membrane fabrication, and identified necessary mechanisms to attain a maximized wetting resistance using fluorine-free materials. We first prepared MD membranes with different surface chemistries using a series of linear alkylsilanes and polydimethylsiloxane (PDMS) as representative fluorine-free, low surface energy materials. Membranes modified with a longer chain alkylsilane exhibited a lower surface energy and demonstrated a greater wetting resistance in direct contact MD experiments using feedwaters of various surface tensions. Despite the nearly identical surface energy measured for the longest alkylsilane and PDMS, PDMS-modified membrane exhibited an extended antiwetting performance as compared to the membrane treated with the longest alkylsilane. To elucidate the source of the distinctive wetting resistance, we examined the nucleation and condensation kinetics on the surfaces with the different surface chemistries via environmental scanning electron microscopy. Our analysis suggests that the membranes treated with long chain alkylsilanes contain surface defects (i.e., hydrophilic regions) whereas the high mobility of the PDMS effectively minimizes the defect exposure, slowing down the condensation and subsequent surface wetting.

Coupling of photovoltaic thermal with hybrid forward osmosis-membrane distillation: Energy and water production dynamic analysis

This study proposes a novel solar photovoltaic thermal (PVT)-driven forward osmosis (FO) and membrane distillation (MD) system for desalinating brackish water, addressing water scarcity issues by combining the advantages of FO's low operating cost with MD's ability to treat hypersaline water. A physics-based dynamic model was developed for the system, and the model equations were discretized and solved using an open-source algebraic modeling language. Model validation involved verifying the consistency of outputs between successive unit operations and conducting sensitivity analysis by varying one operating condition at a time. The analysis considered varying weather conditions while maintaining constant operating conditions. Increasing the flow rate of brackish water (cooling fluid) from 19.79 to 59.38 kg/h improved thermal efficiency by 28.7 %; however, FO and MD water fluxes decreased by 1.12 LMH and 7.77 kg/m2/h, respectively. Peak thermal efficiency (81.2 %) was achieved at the highest flow rate. Incorporating heat transfer analysis into FO models goes beyond the common focus on mass transfer, allowing for a more comprehensive understanding of energy dynamics within the system. Both FO and MD water fluxes improved with irradiance and ambient temperature but decreased with cooling fluid flow rate and wind velocity. Increasing feed concentration to 10,000 mg/l reduced water flux by 0.66 LMH, and for each 0.5 M increase in draw solution concentration, FO water flux increased by >10 LMH. Compared to MD water flux, FO water flux is more sensitive to changes in concentration than to changes in weather conditions.

“Two Birds One Stone” Strategy: PTFE Nanorods for Assembling Nanomaterials and Constructing Hydrophobic Porous Membranes for High-Performance Membrane Distillation

“Two Birds One Stone” Strategy: PTFE Nanorods for Assembling Nanomaterials and Constructing Hydrophobic Porous Membranes for High-Performance Membrane Distillation

Fabrication of Hydrophobic PET Track-Etched Membranes using 2,2,3,3,4,4,4-Heptafluorobutyl Methacrylate for Water Desalination by Membrane Distillation

The world is currently facing a drinking water problem. Human activity, climate change and pollution of existing water bodies are exacerbating the problem. The scientists around the world are currently trying to purify water using effective and inexpensive methods. One such method is membrane distillation. Membrane distillation is a versatile thermally driven membrane separation process. This method of water purification has the potential to remove salts and other non-volatile components. The work is concerned with the desalination of salt solutions by membrane distillation using ion-track membranes based on polyethylene terephthalate (PET). PET ion-track membranes (PET TMs) were modified by photoinitiated graft polymerization of 2,2,3,3,4,4,4-heptafluorobutyl methacrylate (HFBMA) to make them hydrophobic. Optimal polymerization conditions (monomer concentration, reaction time) were determined, which led to an increase of water CA from 51 to 105°. The obtained membranes were used to purify the solution from NaCl. The effect of salt concentration as well as membrane properties on performance and degree of purification was studied. The results show that large pore size PET TMs modified with HFBMA has the potential to desalinate water in an efficient manner.

Insights into the silica scaling behaviors in membrane distillation and anti-scaling mechanism of functional polymers

Silica scaling imposes a significant limitation on the efficacy of membrane distillation (MD) in the treatment of hypersaline wastewater. The complex dynamic behaviors of silica at the membrane-water-air interface and the poor understanding of molecular-level anti-scaling mechanism hampers the development of effective antiscalants for mitigating silica scaling in MD. Despite using functional polymers to prevent silica polymerization, the inhibition mechanisms are unclear. Here, the kinetic process of silica scaling during MD and the potential anti-scaling mechanism of poly-ethylenimine (PEI) were investigated at the molecular level via molecular dynamics simulations. The investigation reveals that silica scales were more likely to adhere to the water-PTFE interface with a free energy potential well of -40.0 kJ mol−1 than that of the water-air interface with a -11.4 kJ mol−1 potential well. Silica scales falling at the water-air interface also migrated on the water-air interface until captured by the PTFE membrane. In this work, a representative functional amino-rich polymer PEI was constructed as silica inhibitors and its scale inhibition mechanism was elucidated. Notably, the inclusion of PEI increased the free-energy barriers for the silica polymerization reaction from 72.0 kJ mol−1 to 86.1 kJ mol−1, compared to scenarios without the antiscalants. Moreover, quantitative structure-activity relationships (QSAR) model of ΔGwater−silica was developed to predict the anti-scaling efficiencies of typical antiscalants based on machine learning method. These findings provide valuable insights into enhancing the efficiency of silica scaling mitigation strategies.

Numerical investigation of multiple channels module for enhanced water production in membrane distillation

This paper introduces a novel numerical model for investigating the Multiple Channels Direct Contact Membrane Distillation (MCDCMD) process, which features multiple feed and permeate channels separated by membranes. We compare the performance of this new module with that of a conventional module under various operating conditions. The CFD model has been validated against experimental productivity data obtained from the conventional module and also compared with the MCDCMD module. Our findings reveal that the new MCDCMD module significantly improves productivity by 100 % across all tested conditions, as compared to the conventional DCMD laboratory setup. Additionally, employing a feed channel with a thickness of 0.5 mm leads to a 10 % improvement in productivity over the baseline module with a 1 mm thickness, owing to the substantial temperature variance between membrane surfaces. Turbulence intensity further boosts the productivity of pure water, with a 37 % increase observed at a lower velocity of 4.17 cm/s and a 27 % increase at a higher velocity of 68 cm/s. Overall, the proposed MCDCMD module exhibits a remarkable 137 % improvement in productivity relative to the conventional module. Finally, the experiments are performed with the new design of MCDCMD and verified with the CFD model. Our findings suggest that the proposed MCDCMD module could significantly advance the development of an industrial-scale, energy-efficient DCMD process, particularly for producing high-quality water with improved productivity. Future studies may propose various design modifications aimed at achieving an optimized configuration of commercial-scale DCMD technology.

Engineering multi-level structured electrospun nanofiber Janus membrane for direct contact membrane distillation

The membrane distillation (MD) has been considered as an attractive technique for the desalination and water treatments. However, the membrane with the superhydrophobic surface is still urgently needed for the wide application of MD process. Inspired by nature, the membranes with the highly porous structure have been fabricated by electrospinning to enhance the water vapor permeation in MD process. In this work, a hydrophobic layer with the hierarchical reentrant architecture was fabricated by electrostatically depositing silicon dioxide (SiO2) nanoparticles on the surface of polyvinylidenefluoride electrosprayed layer followed by the fluorination treatment. The properties of hydrophobic layers were evaluated by varying the electrospraying time and SiO2 loading amount. The optimal membrane had a water contact angle value of 145.7° and surface roughness of ∼275 nm, demonstrating a water flux of 21.2 L.m−2.h−1 and superior salt rejection of >99.9 % when 3.5 wt% NaCl solution was used as the feed solution and the temperature difference was set as 40 °C. The membrane showed an excellent operation stability and effective mitigation in scaling and fouling tendency. The prepared membrane could be applied as a good candidate for the MD-based water treatment processes.

Surface Modification of Polyethylene Terephthalate Track-Etched Membranes by 2,2,3,3,4,4,5,5,6,6,7,7-Dodecafluoroheptyl Acrylate for Application in Water Desalination by Direct Contact Membrane Distillation.

In this work, the surfaces of poly (ethylene terephthalate) track-etched membranes (PET TeMs) with pore sizes of 670-1310 nm were hydrophobized with 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate (DFHA) by photoinitiated graft polymerization. Attenuated total reflection FTIR spectroscopy (ATR-FTIR), scanning electron microscopy (SEM) coupled to an energy-dispersive X-ray spectrometer (EDX), and contact angle measurements were used to identify and characterize the TeMs. The optimal parameters for graft polymerization were determined as follows: polymerization time of 60 min, monomer concentration of 30%, and distance from the UV source of 7 cm. The water contact angle of the modified membranes reached 97°, which is 51° for pristine membranes. The modified membranes were tested for water desalination using direct contact membrane distillation (DCMD) method. The effects of membrane pore size, the degree of grafting, and salt concentration on the performance of membrane distillation process were investigated. According to the results obtained, it has been concluded that large pore size hydrophobic TeMs modified by using DFHA could be used for desalinating water.

Robust superhydrophobic Poly(vinylidene fluoride) membranes with spherulitic surface morphology against wetting and scaling in membrane distillation

Membrane distillation (MD) offers a theoretically complete interception of salts, presenting a promising desalination pathway for freshwater supply. However, conventional hydrophobic membranes fail to desalinate water containing surfactants or supersaturated gypsum over prolonged periods. To address wetting and scaling challenges, we developed a superhydrophobization methodology for poly (vinylidene fluoride) membranes by in situ controlling spherulitic surface morphology through thermally induced phase separation. The major advantages of this one-step strategy lie in its fluorination-free, template-free, and nanoparticle-free. The as-prepared membranes exhibit a rough morphology and superhydrophobic characters with high air-trapping pockets. They demonstrate stable vapor fluxes and high salt rejections (>99 %) when used in desalination processes for saline water containing 0.6 mM sodium dodecyl sulfate, 0.03 mM cetyltrimethylammonium bromide, or gypsum solution with a saturation index of 1.1. Also, the outstanding anti-wetting and anti-scaling properties were demonstrated using synthetic shale gas wastewater. The formation of an air layer on the integrated spherulitic surface highlights the local kinetic barrier and slippery surface, effectively preventing the attachment of amphiphilic surfactants and mineral ions. Importantly, due to the membrane's structural integrity, the spherulitic surface displays high robustness and minimal impact from abrasion cycles and ultrasonication. This work exemplifies a facile strategy to engineer micro-/nano-spherulitic surface morphology for robust superhydrophobic membranes used in MD desalination.

Preparation of polysulfone-based nanofiber Janus membrane for membrane distillation containing organic pollutants

Membrane distillation is an emerging wastewater treatment technology that harnesses low-grade heat as an energy source and exhibits potential for complete desalination. Nonetheless, two notable challenges hinder the practical application of this technology: membrane wetting and fouling. To counter these challenges, an innovative anti-fouling Janus membrane with asymmetric wettability was developed through electrospinning. The hydrophobic layer was formed using tetraethyl orthosilicate/polysulfone (PSF), and the superhydrophilic layer was created using polyvinylpyrrolidone (PVP)/PSF. A sensitive adhesion probe was used to assess the anti-fouling performance of the Janus membrane against oil. Molecular dynamics simulation suggested that PVP reduced the adsorption tendency of the membrane for humic acid (HA). Under experimental conditions involving saline water with HA and a saline oil–water emulsion, the non-Janus membrane suffered severe fouling, resulting in rapid water permeate flux decline. However, the Janus membrane demonstrated consistent permeate flux (26.84 LMH and 24.92 LMH) and an impressive salt rejection rate (> 99.99%). This study suggests that the Janus membrane, with its high permeate fluxes and remarkable resistance to fouling and wetting, could be an effective solution for wastewater treatment, with considerable potential for future application.

Investigation of Performance and Entropy Generation Rate of Direct Contact Membrane Distillation (DCMD) with Metal Foam: A CFD Study

Investigation of Performance and Entropy Generation Rate of Direct Contact Membrane Distillation (DCMD) with Metal Foam: A CFD Study

Construction of robust silica-titania polydopamine composite membrane with light-absorption property for photothermal vacuum membrane distillation

A robust silica-titania polydopamine composite membrane was fabricated adding a photothermal polydopamine coating to the surface of polypropylene membrane for photothermal vacuum membrane distillation. Detailly, the surface of an inert PP membrane was modified with atomically controllable titanium and silicon to enhance the hydroxyl groups on the membrane surface, then, a photothermal effect layer of polydopamine was generated on the composite membrane surface through the self-polymerization reaction of dopamine. The polydopamine coating has significant light absorption and heating properties, and it can act as a local heater when exposed to light sources. This local heating effect significantly raises the temperature at the membrane interface, creating a thermal gradient that drives the evaporation of water molecules. Specifically, we quantitatively analyzed the three reasons (surface energy, free radicals, and photothermal effect) for the increase in evaporation rate of the pristion PP membrane caused by the membrane composite membrane. Among these reasons, the photothermal effect caused by black polydopamine is the most important factor for improving evaporation efficiency. The dark-illumination alternating experiment revealed that the PP-TiO2-SiO2-PDA composite membrane delivered a noteworthy enhancement in vacuum membrane distillation performance (approximately 40%), while the improvement of the PP membrane was only 1.7%.

Thin film theory and boundary layer theory, an approach for analysing heat and mass transfer in spacer-filled Direct Contact Membrane Distillation

Thin film theory and boundary layer theory, an approach for analysing heat and mass transfer in spacer-filled Direct Contact Membrane Distillation

The role of mixing on the kinetics of nucleation and crystal growth in membrane distillation crystallisation

While mixing is considered critical in membrane distillation crystallisation, an explicit link between mixing and crystallisation mechanisms has yet to be made. This study therefore used in-line nucleation detection to determine induction time and metastable zone width, as a method to characterise the independent roles of interfacial boundary layer mixing and bulk crystalliser mixing on the kinetics of nucleation and crystal growth in membrane distillation crystallisation. Interfacial boundary layer mixing was investigated by changing Reynolds number (Re, 1300–2050), where an increase in Re also increased flux. This increased supersaturation within the boundary layer, shortened induction time, and was correlated to a higher nucleation rate. The high nucleation rates introduced smaller crystal sizes, which is an effect that can be correlated to classical nucleation theory. Mixing within the crystalliser did not modify the interfacial supersaturation at nucleation (ReN 1562–18741). However, a decrease in induction time was observed when transitioning from lower to higher mixing, which may arise from the improved distribution of the supersaturated feed within the crystalliser. The effect of crystalliser mixing was also evident on crystal growth, where larger crystals were produced at higher crystalliser stirrer speeds due to improved diffusion-controlled growth. This study therefore demonstrates that bulk and interfacial mixing can be collectively used to control crystallisation by decoupling conditions that determine nucleation and crystal growth. Morphological assessment was subsequently undertaken that evidenced how dendritic breeding and wider secondary nucleation mechanisms that dominate crystal growth at high levels of supersaturation can be mitigated through mixing. Membrane crystallisation offers a scalable geometry in which we demonstrate mixing to facilitate good control over crystal growth which is more difficult to realise in existing evaporative crystalliser design.

Cost-effective, highly efficient, and stable evaporator constructed of Chinese ink impregnation superhydrophobic Xuan paper for solar-driven interfacial organic solvent purification

The lack of cost-effective, highly efficient, and stable photothermal conversion materials in organic solvents greatly hinders the development of solar-driven interfacial evaporation for organic solvent purification. Herein, inspired by the traditional Chinese calligraphy culture, black ink impregnated superhydrophobic Xuan paper membrane (XPBI) is fabricated via a pressing-impregnation method for solar-driven interfacial organic solvent purification. The high photothermal conversion efficiency and long-term stability of XPBI are attributed to the inherent stability, superhydrophobic surface, and abundant channels of Xuan paper, as well as strong adhesion with carbon black particles. As a result, XPBI exhibited a high light absorption of 94.96 % within a broadband wavelength. The evaporation rates of DMF and NMP for XPBI under one sun were 3.63 and 1.44 kg·m−2·h−1, 49 and 127 times higher than that of natural evaporation. After natural sunlight irradiation for 8 h, XPBI evaporated 37.58 and 9.63 kg·m−2 IPA and DMF with organic dye rejection rates of 99.7 %. Furthermore, the simulation and optimization of cotton bud arrangement for organic solvent transport is proven to inhibit the membrane pollution of dye molecules for XPBI. This work demonstrates a low-cost, highly efficient, and stable photothermal membrane evaporator as an energy-saving and low-carbon technical prototype for organic solvent purification.

Investigating Permselectivity in PVDF Mixed Matrix Membranes Using Experimental Optimization, Machine Learning Segmentation, and Statistical Forecasting.

This research examines the correlation between interfacial characteristics and membrane distillation (MD) performance of copper oxide (Cu) nanoparticle-decorated electrospun carbon nanofibers (CNFs) polyvinylidene fluoride (PVDF) mixed matrix membranes. The membranes were fabricated by a bottom-up phase inversion method to incorporate a range of concentrations of CNF and Cu + CNF particles in the polymer matrix to tune the porosity, crystallinity, and wettability of the membranes. The resultant membranes were tested for their application in desalination by comparing the water vapor transport and salt rejection rates in the presence of Cu and CNF. Our results demonstrated a 64% increase in water vapor flux and a salt rejection rate of over 99.8% with just 1 wt % loading of Cu + CNF in the PVDF matrix. This was attributed to enhanced chemical heterogeneity, porosity, hydrophobicity, and crystallinity that was confirmed by electron microscopy, tensiometry, and scattering techniques. A machine learning segmentation model was trained on electron microscopy images to obtain the spatial distribution of pores in the membrane. An Autoregressive Integrated Moving Average with Explanatory Variable (ARIMAX) statistical time series model was trained on MD experimental data obtained for various membranes to forecast the membrane performance over an extended duration.

Selection of optimal draw solution recovery technology for forward osmosis desalination system using analytical hierarchy process

Water scarcity poses a significant threat to global food and water security, prompting a need for practical solutions. With 97% of Earth’s water situated in oceans, desalination emerges as a viable option. Among desalination technologies, forward osmosis (FO) using membrane-based technology stands out for its potential to reduce costs and energy requirements. The focus on energy consumption in FO has prompted an exploration of optimal technology selection through the Analytical Hierarchy Process (AHP), a multi-criteria decision-making method. Value judgments were collected through a questionnaire in consultation with two experts. Environmental aspects emerged as the most critical factor, weighted at 0.3963. The AHP analysis revealed nanofiltration (NF) as the optimal system, attaining a total weight of 0.2612. The NF scored highest in terms of environmental impact (C3), operating and maintenance costs (S6), and energy requirements (S4). Conversely, membrane distillation ranked as the least preferred alternative, with a total score of 0.1335, mainly due to lower maturity of technology (S3), higher capital costs (S5), and negative environmental impact (C3). Sensitivity analysis was conducted to investigate how changing weights for sub-criteria might affect the preferred technology. Notably, Reverse Osmosis became the most favored technology when efficiency (S1) and S3 weights were set at 0.3 and 0.2, respectively. Conversely, thermal separation gained preference when the weights for resistance to scaling and fouling (S2) and S5 were set at 0.3. Changes in S4, S6, and C3 have showed the most minor sensitivity.

A hybrid solar-driven membrane distillation-assisted liquid desiccant air conditioning system: Mathematical modeling and feasibility analysis

Membrane distillation (MD) is a promising method for liquid desiccant (LD) regeneration as it can mitigate LD carryover and produce freshwater as a by-product. Previous research on MD regeneration was mainly focused on performance evaluation and optimization of the regenerator itself. This paper proposed a hybrid solar-driven direct contact MD (DCMD) regeneration-assisted liquid desiccant air conditioning (LDAC) system for air dehumidification, cooling, and freshwater production. A mathematical model for the DCMD regenerator was first developed by considering both temperature and concentration polarizations. This model was validated against the experimental data in terms of the outlet channel temperature and LD solution concentration in the feed tank with relative deviations of ± 9.8 % and ± 0.5 %, respectively. The DCMD model was further integrated into an LDAC system with a batch-wise operation. To investigate the technical feasibility of the proposed system, a one-week simulation was conducted in a residential house during summer in a tropical climate area of Australia. The results showed that the latent heat load was effectively removed from the process air and the regenerated liquid desiccant solution can meet the dehumidification requirement with a concentration over 30.0 wt%. The dehumidification rate and regeneration rate were at 0.49–1.25 kg/h and 0.95–4.25 kg/h, respectively. Solar energy contributed to 52 %-78 % of the total system thermal energy consumption. Furthermore, this system can produce 11.0–12.8 kg of water daily.

Evaporative cooling and sensible heat recovery enable practical waste-heat driven water purification

Waste heat capture from systems such as photovoltaics (PV) and refrigerators can lower their energy efficiency by increasing their operating temperature. In this study, we evaluate the potential of final-effect evaporative cooling and internal heat recovery in a multi-effect diffusion distillation (MEDD) to produce pure water without negatively impacting the energy efficiency of the waste-heat source. Lab-scale experimental results from a multi-effect membrane distillation module validate the concept, showing that water production can be enhanced by >20 % while simultaneously pulling down the module's operating temperature. A detailed numerical model of a solar-MEDD is implemented and validated. The incorporation of sensible heat recovery and evaporative cooling increase pure water production by approximately 10 % each. Although the pure water production of a standalone MEDD increases with increasing effects N, when coupled to a solar PV module, increasing N also decreases PV electricity production. Therefore, a PV-MEDD with fewer effects (≤4) is preferable and such a system can produce sufficient water for electrolysis (green H2 production) while maintaining or improving PV electrical energy production throughout the year under varying climatic conditions.