Direct Contact Membrane Distillation Process Research Articles

Dairy wastewater treatment with direct-contact membrane distillation in Oman

Direct contact membrane distillation (DCMD) process was implemented for treating dairy wastewater (DWW) from an Omani dairy industry. Commercial polytetrafluoroethylene (PTFE) and polypropylene (PP) membranes were tested under different temperatures. The aim of this study in to investigate the potential of pure water production rejecting salts, volatile and non-volatile organic compounds present in DWW. Using laboratory techniques such as gas chromatography, we inspected permeate composition, identifying both volatile and non-volatile organic matter. Comprehensive characterization of membrane surface properties via Fourier-transform Infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Thermal Gravimetric analysis (TGA) provided critical insights. Findings revealed the denaturation of proteins at 60 °C, leading to significant pore blockage and a consequential 50.3 % reduction in flux. Impressively, the membranes achieved rejection rates of 89–99 % for non-volatile organic matter, while completely rejecting D-limonene. However, traces of volatile fatty acids were detected in the permeate. Elemental analysis exposed the presence of various dairy-derived elements at the membrane surface. FTIR peaks highlighted distinct functional groups of dairy components, while TGA affirmed the accumulation of dairy aggregates with rising temperature. These findings underscore membrane distillation's competence in dairy wastewater treatment, presenting a compelling case for its widespread application in environmental remediation endeavors.

Ammonia recovery via direct contact membrane distillation: Modeling and performance optimization

Ammonia recovery from wastewater has positive environmental benefits, avoiding eutrophication and reducing production energy consumption, which is one of the most effective ways to manage nutrients in wastewater. Specifically, ammonia recovery by membrane distillation has been gradually adopted due to its excellent separation properties for volatile substances. However, the global optimization of direct contact membrane distillation (DCMD) operating parameters to maximize ammonia recovery efficiency (ARE) has not been attempted. In this work, three key operating factors affecting ammonia recovery, i.e., feed ammonia concentration, feed pH, and DCMD running time, were identified from eight factors, by a two-level Plackett-Burman Design (PBD). Subsequently, Box-Behnken design (BBD) under the response surface methodology (RSM) was used to model and optimize the significant operating parameters affecting the recovery of ammonia though DCMD identified by PBD and statistically verified by analysis of variance (ANOVA). Results showed that the model had a high coefficient of determination value (R2 = 0.99), and the interaction between NH4Cl concentration and feed pH had a significant effect on ARE. The optimal operating parameters of DCMD as follows: NH4Cl concentration of 0.46 g/L, feed pH of 10.6, DCMD running time of 11.3 h, and the maximum value of ARE was 98.46%. Under the optimized conditions, ARE reached up to 98.72%, which matched the predicted value and verified the validity and reliability of the model for the optimization of ammonia recovery by DCMD process.

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.

Surface modification for enhanced Anti‐Wetting properties of electrospun omniphobic membranes in membrane distillation

AbstractMembrane wetting caused by substances in the feed solution with low surface tension has posed serious issues for membrane distillation (MD). Omniphobic membranes could be helpful when dealing with this problem because they strongly repel liquids with a wide range of surface tensions. We fabricated electrospun omniphobic membranes containing PVDF‐TEOS‐PFDTMS, PVDF‐HFP‐TMOS‐PFDTMS, and PVDF‐HFP‐TEOS‐PFDTMS. All electrospun membranes have shown contact angles >90°, but the electrospun membrane containing PVDF‐HFP‐TEOS‐PFDTMS has shown super repellence with a contact angle >150° and maintains stable salt rejection and water flux in direct contact MD processes in the presence of sodium dodecyl sulfate surfactant (up to 0.3 mM). Furthermore, the morphology and crystallinity of the electrospun membranes were studied by scanning electron microscopy and Fourier‐transform infrared spectroscopy, and the percentage composition of elements was analyzed by x‐ray photoelectron spectroscopy. liquid entry pressure, mechanical strength, and the porosity of the electrospun omniphobic membranes were also studied.

Evaluating the feasibility of direct contact membrane distillation and nanofiltration in ground water treatment through a techno-economic analysis

This study delves into the realm of water treatment by conducting a comprehensive techno-economic evaluation of direct contact membrane distillation (DCMD) and nanofiltration (NF) processes. While previous research has explored the technical aspects of membrane distillation (MD) and nanofiltration, there remains a notable gap in economic analyses. Our research aims to bridge this gap by assessing the financial feasibility of employing MD and NF technologies for water desalination. Specifically, we scrutinize the performance of hydrophobic microporous flat sheet membranes crafted from polytetrafluoroethylene (PTFE) supported by non-woven polypropylene (PP) in desalinating brackish water through DCMD and NF processes. By varying operating conditions such as flow rate and feed temperature, we evaluate the membrane's efficacy. Employing an analytical model based on heat and mass transfer equations, we predict process performance across diverse scenarios. Our model demonstrates a high level of accuracy, with flux predictions deviating by less than 10% when utilizing the Knudsen-molecular mechanism model. Furthermore, through a detailed design and economic analysis of industrial-scale units for both processes, we reveal that the cost of permeated water is lower with NF compared to DCMD. Specifically, our calculations indicate a water cost of 1.34 USD/m3 for DCMD at a feed temperature of 65 °C with an 80% recovery rate, positioning it as a competitive option among conventional desalination methods. Notably, our financial assessment highlights that steam cost constitutes the primary expense in DCMD operations, contingent upon heating value and fuel prices. Noteworthy findings suggest that natural gas emerges as the most cost-effective fuel for steam production in a DCMD plant. This study underscores the economic viability and potential cost efficiencies associated with NF over DCMD in water treatment applications.

Membrane hydrophobicity enhanced by non-solvent induced separation strategy for improving membrane distillation performance

Development of high-performance hydrophobic layer using green and economic method is still urgently on demand for wide applications of membrane distillation (MD). In this study, a novel Janus membrane was prepared by electrospinning polystyrene with the water addition on the surface of hydrophilic electrospun nanofiber substrate. With increasing water content to 3 wt%, the hydrophobic layer is completely converted to be nanofiber-dominate morphology that could be beneficial for water vapor permeation during MD process. The surface roughness, water contact angle value and porosity of the hydrophobic layer are enhanced by the water addition, and the optimized Janus membrane with the hydrophobic layer prepared at the water content of 2 wt% could achieve a high water flux of 33.1 L/m2h and superior salt rejection of >99.9 % when 3.5 wt% NaCl solution was used as the feed solution. The as-prepared Janus membrane shows better performance stability and reusability than the control membrane (without the water addition) within the 24-h continuous operation duration. Our work demonstrates that the proper water addition could be used as a green strategy for fabricating efficient electrospun nanofiber membranes for the direct contact membrane distillation (DCMD) processes.

A CFD Analysis of the Desalination Performance of Ceramic-Based Hollow Fiber Membranes in Direct Contact Membrane Distillation

In this numerical study, the performance of ceramic-based mullite hollow fiber (HF) membranes in a direct contact membrane distillation (DCMD) process was evaluated. Three types of membranes were tested: (i) hydrophobic membrane C8-HFM, (ii) rod-like omniphobic membrane (C8-RL/TiO2), and (iii) flower-like omniphobic membrane (C8-FL/TiO2). The CFD model was developed and validated with experimental results, which were performed over a 500 min period. The initial mass flux of C8-HFM was 30% and 9% higher than that of C8-FL/TiO2 and C8-RL/TiO2, respectively. However, the flower-like omniphobic membrane C8-FL/TiO2 had the lowest drop in flux, around 11%, while the rod-like omniphobic membrane C8-RL/TiO2 had a 15% reduction, both better than the 23% reduction in the hydrophobic membrane C8-HFM over the 500 min. The study also analyzed the impact of fouling by examining the variation in mass transfer coefficient (MTC) over time. The results indicated that the ceramic-based mullite HF membranes with TiO2 flowers and rods demonstrated a high resistance to fouling compared to C8-HFM. The modified membranes could find applications in the desalination and handling of seawater samples containing organic contaminants. The CFD model’s versatility can be utilized beyond the current investigation’s scope, offering a valuable tool for efficient membrane development solutions, particularly for challenges such as the presence of organic contaminants in seawater.

SAN-including PVDF membranes with improved permeability for hypersaline water desalination using DCMD process

Styrene-acrylonitrile (SAN) was added to polyvinylidene fluoride (PVDF)/N-methyl-2 pyrrolidone (NMP) system to fabricate SAN-modified PVDF membranes using non-solvent induced phase separation (NIPS). By increasing the SAN concentration in casting solution, number of microbeads started to grow. However, by further increasing the amount of SAN, size of microbeads was also increased. Surface hydrophobicity of neat ethanol-induced composite membrane was increased from 139.2 ± 1 to 148.7° ±1, since SAN polymer incorporation bestowed another texture to whole structure. Different salty feed solutions, i.e., 35 and 105 g/L, were used as feed by direct contact membrane distillation (DCMD) process. Permeate flux and salt rejection were high enough with stable permeation using 35 g/L feed solution. Neat and well-modified PVDF/SAN membranes were then applied for hypersaline DCMD test. During 38 h desalination test using SAN-modified membrane, permeate flux and salt rejection of 29.74 ± 1 kg/m2 h and >99.9 % were achieved, respectively. Also, due to multi-textured structure, permeate flux enhancement and better scaling resistance were observed using SAN/PVDF membrane. A sufficient way was proposed to improve desalination performance using a straightforward fabrication process.

Efficient lithium recovery from simulated brine using a hybrid system: Direct contact membrane distillation (DCMD) and electrically switched ion exchange (ESIX)

Seawater reverse osmosis (SWRO) brine is a readily available resource hub in many countries, fulfilling the country's freshwater need by SWRO, yet lower in a concentration of high-demand elements like Li. This study outlines developing a novel hybrid system that combines direct contact membrane distillation (DCMD) and electrically switched ion exchange (ESIX) to facilitate simultaneous SWRO brine enrichment followed by selective Li recovery. The DCMD process concentrates the SWRO brine utilizing electrospun nanofiber membranes (ENMs) composed of polyvinylidene fluoride (PVDF). Incorporating reduced graphene oxide (rGO) nanoparticles augments the morphological, thermal, and mechanical stability of the PVDF ENMs. The water contact angle (WCA) of the 1-rGO/PVDF ENM stands at 142.08°, a testament to an enhanced hydrophobic property which resulted in a 12 % freshwater recovery from simulated SWRO brine and a 2.4-fold increase in Li+ concentration. The durability of the 1-rGO/PVDF ENM is evident in its minimal 11 % reduction in WCA after 15 h of brine concentration. In the context of hybrid operation, a Li-selective LiAlO2 electrode, coupled with an activated carbon counter electrode, demonstrated remarkable Li recovery for Li capture solutions enriched by the rGO-PVDF membrane in the DCMD phase. Compared to the Li concentration in the DCMD feed, sequential Li capture and release cycles recovered 91.8 % of Li, thereby underlining the critical role of the hybrid mode operation in concentrating Li from simulated brine solutions.

Modeling the flux of volatile fatty acid in a membrane distillation with the effect of pH

This study aims to develop a mathematical model to estimate the initial flux of volatile fatty acids (VFAs) in the direct contact membrane distillation (DCMD) process. VFAs are toxic and emit odors. However, they have recently been recognized as valuable raw materials. Water and VFAs can be separated together using membrane distillation (MD). Water vapor and volatile compounds can pass through the membrane pores. They move toward the permeate side. Flux occurs not only for water but also for VFAs in DCMD process due to the volatility of VFAs. Parameters affecting the flux of VFAs were identified. Their relationship was confirmed experimentally. Acetic acid, butyric acid, and valeric acid were selected as target compounds because they have different Henry's constants and hydrophobicity. The ionization of acid solution varies depending on pH. MD tests were performed under various pH conditions. The flux of VFAs was observed to vary depending on the pH. The flux of VFAs was found to be proportional to the ionization fraction (α0) of the feed solution. Therefore, the model was developed to reflect the ionization fraction. An empirical formula was derived to predict the flux of VFAs using experimental data. A statistical analysis supported that the flux predicted by the model fitted very well with the observed flux. Additionally, experiments were performed with acid solutions of various concentrations. The concentration range applicable to the derived empirical formula was validated. The developed model can provide further insight into the relationship between flux and pH.

Evaluation of anti‐wetting nano‐fumed silica/PVDF hollow‐fiber membrane by central composite design optimization towards efficient desalination performance via membrane distillation

AbstractBACKGROUNDMembrane distillation (MD) is considered a viable technology to overcome the issue of water scarcity by desalination of seawater. However, the application of MD has been severely restricted by membrane‐wetting phenomena. This study aims to optimize the desired mixed‐matrix hollow‐fiber (HF) membrane conditions that have anti‐wetting characteristics for efficient desalination. Response surface methodology based on central composite design was used to predict and optimize the permeate flux and salt rejection (SR). The combined effects of polymer (polyvinylidene fluoride, PVDF) concentration, nano‐fumed silica (NFS) particle loading, coagulation bath temperature (CBT) and air gap distance (AG), as well as their interactions in terms of desalination performance, have been investigated.RESULTSThe linear factors (NFS, CBT), the quadratic effects of all variables, as well as the interacting terms between (PVDF, NFS), (PVDF, CBT) and (NFS, CBT) had significant effects for both pure water and permeate fluxes. For SR, only PVDF presented a linear effect, while a quadratic effect was obtained in (NFS, AG) and an interaction effect was obtained between PVDF and NFS. Under optimal conditions of PVDF (15.03 wt%), NFS (1.21 wt%), CBT (54.85 °C) and AG (9.9 cm), the optimal mixed‐matrix HF membrane demonstrated higher experimental average values for pure water and permeate fluxes, which reached up to 11.95 and 11.61 kg m−2 h−1 respectively, and an SR of 99.96% as compared to the pristine membrane.CONCLUSIONOur research provides an efficient anti‐wetting NFS/PVDF mixed‐matrix HF membrane, indicating that the optimal membrane possesses high potential for the direct contact membrane distillation (DCMD) process. © 2023 Society of Chemical Industry (SCI).

MOF incorporated adsorptive nanofibrous membranes for enhanced ammonia removal by membrane distillation

This study presents the design and fabrication of a new adsorptive nanofibrous membrane incorporating metal-organic framework (MOF) by electrospinning technique for removing ammonia-nitrogen from wastewater via direct contact membrane distillation (DCMD) process. Three types of MOF particles (HKUST-1, AlFu, and UiO-66-NH2) were used as functional nanoadditives and incorporated into the nanofibrous membrane matrix. The ammonia removal performance of these nanocomposite membranes was comprehensively investigated and compared with an activated carbon (AC) incorporated nanofibrous membrane. Various parameters such as ammonia removal efficiency, ammonia separation factor, overall ammonia mass transfer coefficient, ammonia flux, and membrane stability were evaluated. Among the tested MOFs, UiO-66-NH2 demonstrated excellent performance and was selected for further optimization. The 5.0 wt.% UiO-66-NH2 incorporated membrane exhibited the best performance for ammonia removal. Additionally, the mechanism of MOF incorporated nanofibrous membranes for ammonia removal from aqueous solution by DCMD was schematically analyzed. In overall, this study confirmed the feasibility of fabricating MOF incorporated electrospun adsorptive nanofibrous membranes with outstanding ammonia removal performance, demonstrating the potential of membrane distillation to remove ammonia-nitrogen from the aquatic environment.

A luminescent thermosensitive coating for a non-invasive and in-situ study of thermal polarization in hollow fiber membranes

A new non-invasive method to evaluate the thermal polarization is herein proposed for hollow fiber (HFs) membranes during the Direct Contact Membrane Distillation (DCMD) process. The goal was reached by using a temperature-sensitive phosphorescent molecular probe such as Tris(phenanthroline) ruthenium(II) chloride (Ru(phen)3) and an infrared camera (IR) camera, to map, in-situ, the thermal profile directly at the membrane surface which is in contact with the hot feed water solution. The molecular probe was immobilized, by means of a perfluoropolyether (PFPE)-based coating, on the surface of a polypropylene HF in order to obtain a thermosensitive polymerized coating. The produced HFs were fully characterized in order to demonstrate the successful embodiment of the molecular probe on the surface of the membrane and their suitability, in terms of pore size, hydrophobicity and stability, for DCMD application. The luminescent activity of the immobilized Ru(phen)3 complex was measured by a bifurcated optical fiber, one branch was used to excite the molecular probe and the other branch was able to constantly monitor the molecular emission which was dependent on the temperature along the HF surface. The HFs were tested in DCMD at different feed temperatures (40 °C, 50 °C, 60 °C) and constant conditions of streams flow rate and distillate temperature (Qf = 90 L/h; Qp = 24 L/h; Tp = 13 °C). The obtained results clearly demonstrated the possibility of studying the thermal polarization phenomenon occurring during DCMD, with the temperature profile developed on the HF surface monitored, for the first time, at molecular level using a non-invasive technique. This approach can shed the light on better understanding this phenomenon and the possible strategies for its mitigation and counteraction.

A numerical analysis of the electromagnetic field effect on direct contact membrane distillation performance

Using an advanced CFD model, this study attempts to determine how electromagnetic field (EMF) affects the performance of the direct contact membrane distillation (DCMD) process. The influence of electromagnetic fields (EMF) on the permeate flow, evaporation efficiency (EE), and temperature polarization coefficient (TPC) of the DCMD process are studied using a thorough theoretical model linked with a CFD model. Experiments conducted in-house with and without the EMF influence are used to verify the CFD model. High magnetic potency propagates the turbulence in the feed channel, hence boosting the driving force and transport phenomena for vapor transport to the permeate side. Permeate flux increased by 30%, and EE increased by 15% when the magnetic field strength was raised from 0 to 0.5 T, particularly at a lower intake feed velocity (0.02 m/s) and temperature of 333.05 K. In contrast, with the same magnetic potency (0.5 T) at a higher velocity at an input feed temperature, there is less improvement in EE (5%) and permeate flux (12%) due to the elevated turbulence intensity at high feed velocity. This research contributes to studies aimed at elucidating how electromagnetic fields affect separation processes such as MD, opening the door for more tuning and use of the fascinating phenomenon.

Thermal modeling of fluid flow and heat transfer in direct contact membrane distillation

In this paper, a mathematical model was developed, and numerical simulations were conducted to analyze the direct contact membrane distillation process (DCMD) utilizing solar energy. The objective is to identify the optimal operating point, flow parameters, dimensions, and membrane properties of membrane module in terms of permeate water flux and GOR. The novel aspect of this paper is to analyze the flow patterns in the feed and permeate regions on performance of DCMD to maximize water production by optimizing the balance between permeate water flux and GOR. Additionally, the study will identify the most suitable location for installing the MD system based on factors such as solar energy availability and average annual temperature. The results show that the polyethylene (PE) membrane provides the highest permeate water flux, while the polytetrafluoroethylene (PTFE) membrane provides the highest GOR. A significant increase in permeate water flux and GOR is observed when the flow changes from laminar to turbulent. The study also examines the effect of membrane module dimensions, where increasing the length decreases flux but increases GOR, while increasing the width increases flux but decreases GOR. The study also reveals that countries with high annual average temperatures produce higher permeate water flux and GOR. The paper also delves into the effect of membrane properties on water flux and GOR. It is observed that a membrane with high porosity, low thickness, and having a pore size greater than 0.14 µm is desirable.

Using the Log Mean Temperature Difference (LMTD) and ε-NTU Methods to Analyze Heat and Mass Transfer in Direct Contact Membrane Distillation.

In direct contact membrane distillation (DCMD), heat and mass transfers occur through the porous membrane. Any model developed for the DCMD process should therefore be able to describe the mass transport mechanism through the membrane, the temperature and concentration effects on the surface of the membrane, the permeate flux, and the selectivity of the membrane. In the present study, we developed a predictive mathematical model based on a counter flow heat exchanger analogy for the DCMD process. Two methods were used to analyze the water permeate flux across one hydrophobic membrane layer, namely the log mean temperature difference (LMTD) and the effectiveness-NTU methods. The set of equations was derived in a manner analogous to that employed for heat exchanger systems. The obtained results showed that the permeate flux increases by a factor of approximately 220% when increasing the log mean temperature difference by a factor of 80% or increasing the number of transfer units by a factor of 3%. A good level of agreement between this theoretical model and the experimental data at various feed temperatures confirmed that the model accurately predicts the permeate flux values for the DCMD process.

Surface Modification of UiO-66 on Hollow Fibre Membrane for Membrane Distillation.

The hydrophobicity of metal-organic frameworks (MOFs) is critical in enhancing the separation process in membrane distillation. Herein, a new superhydrophobic University of Oslo 66 (UiO-66) MOFs was successfully constructed on the top of alumina hollow fibre (AHF) membrane for desalination purposes. The fabrication methodology of the membrane involved in situ growth of pure crystalline UiO-66 on top of AHF and post-synthetic modification by fluorosilane grafting. The resultant membrane was characterised to study the physicochemical properties of the pristine and modified membrane. A superhydrophobic UiO-66 with a contact angle of 163.6° and high liquid entry pressure was obtained by introducing a highly branched fluorocarbon chain while maintaining its crystallinity. As a result, the modified membrane achieved 14.95 L/m2∙h water flux and 99.9% NaCl rejection with low energy consumption in the direct contact membrane distillation process. Furthermore, the high surface energy contributed by UiO-66 is maximised to produce the maximum number of accessible sites for the grafting process. The synergistic effect of surface hydrophobicity and porous UiO-66 membrane in trapping water vapour shows great potential for desalination application.

Janus hollow fibre membranes with intrusion anchored structure for robust desalination and leachate treatment in direct contact membrane distillation

High-performance Janus membranes have been intriguing for membrane distillation due to its distinctive features for improving fouling and wetting resistance. However, long-term structural stability remains challenging owing to delamination caused by the opposite affinity between the two different kinds of materials. Herein, a physically-anchored hydrophilic layer containing graphene oxide (GO)/poly(vinyl alcohol) (PVA)/sulfosuccinic acid (SSA) i.e., GOSP layer was formed on the surface of a microporous polypropylene (PP) hollow fibre membrane. Meanwhile, the nanostructure of the GOSP layer could be tuned from 2-dimensional lamellar to 3-dimensional mixed matrix, exhibiting various water transport properties. As a result, the fabricated Janus membrane with the partially intruded pore channels exhibited superior antiwetting and antifouling performance for desalination and wastewater treatment in the direct contact membrane distillation process. For surfactant containing saline water, the GOSP/PP Janus membrane with optimised PVA/GO ratio possessed noticeably enhanced anti-wetting property compared to pristine PP membrane. In particular for leachate treatment, the GOSP/PP membrane produced high quality permeate with <1 mg/L ions and 10 mg/L total organics and maintained stable water flux while the leachate was concentrated by 5 times (80 % water recovery), providing great potential for using membrane distillation towards zero liquid discharge.

Enhanced properties of the PVDF membrane with carboxylated MWCNT and sodium alginate for membrane distillation

The use of nanomaterials to modify membrane distillation membranes enhances their performance by avoiding wetting and fouling. This study plans a critical analysis of the advancement made in the use of multi-walled carbon nanotubes and sodium alginate for PVDF membrane modification and further, the developed membrane has been analyzed for water desalination through the direct contact membrane distillation process. Using the phase inversion method polyvinylidene fluoride (PVDF, N1) and polyvinylidene fluoride/ multi-walled carbon nanotubes/sodium alginate (PVDF/MWCNT/SA, N2) membranes were prepared. Taguchi L16 array has been developed to find out optimum conditions. In this Taguchi method, four parameters with four levels are mainly; Feed temperature (55 °C, 60 °C, 65 °C, 70 °C), feed flow rate (0.8 LPM, 1 LPM. 1.2 LPM, 1.4 LPM), feed concentration (5 wt%, 10 wt%, 15 wt%, 20 wt%), and permeate temperature (9 °C, 12 °C, 15 °C, 18 °C) and optimum conditions (feed temperature-70 °C, feed flow rate-1.4 LPM, feed concentration-5 wt%, permeate temperature-15 °C) were found. The changes in hydrophobicity, morphology, surface roughness and chemical composition of the pure and modified membranes before the experiment and after fouling were analyzed by the contact angle, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier transformation infrared microscopy (FTIR), respectively. The effect of four operating parameters on permeate flux and rejection has also been evaluated. It was determined that carboxylated MWCNT and sodium alginate incorporated membrane gave the highest permeate flux, 6.38 kg/m2.h for MgSO4 salt at operating parameters, which is 25 % higher than the pure PVDF membrane (flux - 5.13 kg/m2.h). A 100 h long run experiment was conducted on the N1 and N2 membranes. The modified N2 membrane showed high stability.

Minerals recovery from a rare earth extraction wastewater by a combined chemical precipitation and membrane distillation process

Rare earth extraction wastewater (REEW) derived from rare earth (RE) extraction contains high value-added resources, such as magnesium. Therefore, it is highly desirable to recover the minerals from REEW to reuse in the RE extraction process. This study investigated the combined chemical precipitation and direct contact membrane distillation (DCMD) process to treat REEW for minerals recovery. Results showed that 99.7% Mg2+ in REEW was precipitated as Mg(OH)2 by adjusting pH of the solution. The MgO was obtained through the calcination of Mg(OH)2. The specific surface area (SSA) and reactivity of the calcined MgO decreased with the increase of calcination duration. MgO calcined at 500 °C for 2 h exhibited higher reactivity and SSA of 188.2 m2/g. DCMD performance decline was mainly caused by gypsum during treatment of the Mg-separated stream and membrane fouling was effectively mitigated by almost complete removal of Ca2+, resulting in the concentrated Na2SO4 solution. Subsequently, Na2SO4 was successfully recovered via freeze crystallization by controlling its solubility. The recovered Na2SO4 exhibited narrow crystal size distribution with mean size of 43.3 μm. With this combined process, it was possible to successfully recover 20.1 g/L MgO, 49.7 g/L Na2SO4 along with 72.6% of the water from REEW. The recovered MgO and Na2SO4 with purity of 98.5% and 99.2%, respectively, achieving industrial grade. The results suggest that the combined technologies, chemical precipitation and DCMD process, can be used as a viable engineering solution for minerals recovery from REEW.