A framework for better understanding membrane distillation separation process

A Systematic Framework for Optimizing a Sweeping Gas Membrane Distillation (SGMD)

The present work has undertaken a meticulous glance on optimizing the performance of an SGMD configuration utilized a porous poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) membrane. This was carried out by conducting a systematic framework for investigating and optimizing the pertinent parameters such as sweeping gas flow rate, feed temperature, feed concentration and feed flow rate on the permeate flux. For this purpose, the Taguchi method and design of experiment techniques were harnessed to statistically determine optimum operational conditions. Besides that, a comprehensive surface and permeation characterization was conducted against the hand-made membranes. Results showcased that the membrane performance was ultimately controlled by the feed temperature and was nearly (~680) % higher when the temperature raised from 45 to 65 °C. Also, to a lesser extent, the system was dominated by the feed flow rate. As the adopted feed flow rate increases (from 0.2 to 0.6 L/min), around 47.5% increment was bestowed on water permeability characteristics. In contra, 34.5% flux decline was witnessed when higher saline feed concentration (100 g/L) was utilized. In the meantime, with raising the sweeping gas flow rate (from 120 to 300 L/h), the distillate was nearly 129% higher. Based on Taguchi design, the maximum permeate flux (17.3 and 17 kg/m²·h) was secured at 35 g/L, 0.4 L/min, 65 °C and 300 L/h, for both commercial and prepared membranes, respectively.

Recent Developments in Nanomaterials-Modified Membranes for Improved Membrane Distillation Performance

Membrane distillation (MD) is a thermally induced membrane separation process that utilizes vapor pressure variance to permeate the more volatile constituent, typically water as vapor, across a hydrophobic membrane and rejects the less volatile components of the feed. Permeate flux decline, membrane fouling, and wetting are some serious challenges faced in MD operations. Thus, in recent years, various studies have been carried out on the modification of these MD membranes by incorporating nanomaterials to overcome these challenges and significantly improve the performance of these membranes. This review provides a comprehensive evaluation of the incorporation of new generation nanomaterials such as quantum dots, metalloids and metal oxide-based nanoparticles, metal organic frameworks (MOFs), and carbon-based nanomaterials in the MD membrane. The desired characteristics of the membrane for MD operations, such as a higher liquid entry pressure (LEPw), permeability, porosity, hydrophobicity, chemical stability, thermal conductivity, and mechanical strength, have been thoroughly discussed. Additionally, methodologies adopted for the incorporation of nanomaterials in these membranes, including surface grafting, plasma polymerization, interfacial polymerization, dip coating, and the efficacy of these modified membranes in various MD operations along with their applications are addressed. Further, the current challenges in modifying MD membranes using nanomaterials along with prominent future aspects have been systematically elaborated.

Highly Saline Water Desalination Using Direct Contact Membrane Distillation (DCMD): Experimental and Simulation Study

The path for water molecules transported across a membrane in real porous membranes has been considered to be a constant factor in the membrane distillation (MD) process (i.e., constant tortuosity); as such, its effect on membrane performance at various operating conditions has been ignored by researchers. Therefore, a simultaneous heat and mass transfer model throughout the direct contact membrane distillation (DCMD) module was developed in this study by taking into account the hypothetical path across the membrane as a variable factor within the operating conditions because it exhibits the changes to the mass transfer resistance across the membrane under the DCMD run. The DCMD process was described by the developed model using a system of nonlinear equations and solved numerically by MATLAB software. The performance of the poly-tetra-fluoroethylene (PTFE) membrane was examined to treat 200 g/L NaCl saline at various operating conditions. The simulation results in the present work showed that the hypothetical proposed path across the membrane has a variable value and was affected by changing the feed temperature and feed concentration. The results estimated by the developed model showed an excellent conformity with the experimental results. The salt rejection remained high (greater than 99.9%) in all cases. The temperature polarization coefficient for the DCMD ranged between 0.88 and 0.967, and the gain output ratio (GOR) was 0.893. The maximum thermal efficiency of the system was 84.5%.

Mercury, Arsenic and Lead Removal by Air Gap Membrane Distillation: Experimental Study

Synthetic industrial wastewater samples containing mercury (Hg), arsenic (As), and lead (Pb) ions in various concentrations were prepared and treated by air gap membrane distillation (AGMD), a promising method for heavy metals removal. Three different membrane pore sizes (0.2, 0.45, and 1 μm) which are commercially available (TF200, TF450, and TF1000) were tested to assess their effectiveness in combination with various heavy metal concentrations and operating parameters (flow rate 1–5 L/min, feed temperature 40–70 °C, and pH 2–11). The results indicated that a high removal efficiency of the heavy metals was achieved by AGMD. TF200 and TF450 showed excellent membrane removal efficiency, which was above 96% for heavy metal ions in a wide range of concentrations. In addition, there was no significant influence of the pH value on the metal removal efficiency. Energy consumption was monitored at different membrane pore sizes and was found to be almost independent of membrane pore size and metal type.

Covalent organic frameworks for separation applications

Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers with highly tuneable structures and functionalities. COFs have been proposed as ideal materials for applications in the energy-intensive field of molecular separation due to their notable intrinsic features such as low density, exceptional stability, high surface area, and readily adjustable pore size and chemical environment. This review attempts to highlight the key advancements made in the synthesis of COFs for diverse separation applications such as water treatment or the separation of gas mixtures and organic molecules, including chiral and isomeric compounds. Methods proposed for the fabrication of COF-based columns and continuous membranes for practical applications are also discussed in detail. Finally, a perspective regarding the remaining challenges and future directions for COF research in the field of separation has also been presented.

Separation and Recycling of Concentrated Heavy Metal Wastewater by Tube Membrane Distillation Integrated with Crystallization

Tube membrane distillation (MD) integrated with a crystallization method is used in this study for the concurrent productions of pure water and salt crystals from concentrated single and mixed system solutions. The effects of concentrated Zn2+ and Ni2+ on performance in terms of membrane flux, permeate conductivity, crystal recovery rates, and crystal grades are investigated. Preferred crystallization and co-crystallization determinations were performed for mixed solutions. The results revealed that membrane fluxes remained at 2.61 kg·m-2·h-1 and showed a sharp decline until the saturation increased to 1.38. Water yield conductivity was below 10 μs·cm-1. High concentrated zinc and nickel did not have a particular effect on the rejection of the membrane process. For the mixed solutions, membrane flux showed a sharp decrease due to the high saturation, while the conductivity of permeate remained below 10 μs·cm-1 during the whole process. Co-crystallization has been proven to be a better method due to the existence of the SO42- common-ion effect. Membrane fouling studies have suggested that the membrane has excellent resistance to fouling from highly concentrated solutions. The MD integrated with crystallization proves to be a promising technology for treating highly concentrated heavy metal solutions.

Permeate Flux and Rejection Behavior in Submerged Direct Contact Membrane Distillation Process Treating a Low-Strength Synthetic Wastewater

The effects of operational conditions such as permeate recirculation velocity, mixing intensity, and trans-membrane temperature on the performances of hydrophobic polyethylene (PE) hollow-fiber membrane were investigated by operating the submerged direct contact membrane distillation (SDCMD) process treating a synthetic low-strength wastewater. Permeate flux of the membrane increased with increasing a permeate recirculation velocity through the fiber lumen. However, the effectiveness was less pronounced as the velocity was higher than 0.5 m/s. Increasing rotational speed to 600 rpm, which can lead to mixing intensity from a bulk wastewater toward hollow-fiber membrane, enhanced permeate flux. Feed temperature played a more significant role in enhancing permeate flux rather than a permeate temperature under constant trans-membrane temperature. The SDCMD process treating a synthetic low-strength wastewater achieved an excellent rejection efficiency which is higher than 97.8% for both chemical oxygen demand (CODCr) and total phosphorus (T-P) due to the hydrophobic property of membrane material which can allow water vapor through membrane. However, the rejection efficiency of the ammonia nitrogen (NH3-N) was relatively low at about 87.5% because ammonia gas could be volatized easily through membrane pores in SDCMD operation. In a long-term operation of the SDCMD process, the permeate flux decreased significantly due to progressive formation of inorganic scaling on membrane.

Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process.

Removal of fluoride in membrane-based water and wastewater treatment technologies: Performance review

The presence of excess fluoride in aqueous media above local environmental standards (e.g., the U.S. Environmental Protection Agency (EPA) standard of 4 mg/L) affects the health of aquatic life. Excess fluoride in drinking water above the maximum contaminant level (e.g., the World Health Organization (WHO) standard of 1.5 mg/L) also affects the skeletal and nervous systems of humans. Fluoride removal from aqueous solutions is difficult using conventional electrochemical, precipitation, and adsorption methods owing to its ionic size and reactivity. Thus, new technologies have been introduced to reduce the fluoride concentration in industrial wastewater effluents and various drinking water sources. Membrane technology is one of the newer technologies found to be very effective in significantly reducing fluoride to desired standards levels; however, it has received less attention than other technologies because it is perceived as a costly process. This study critically reviewed the performance of various membrane process and compared it with effluent and zero liquid discharge (ZLD) standards. The performance review has been conducted with the consideration of the theoretical background, rejection mechanisms, technical viability, and parameters affecting flux and rejection performance. This review includes membrane systems investigated for the defluoridation process but operated under pressure (i.e., reverse osmosis [RO] and nanofiltration [NF]), temperature gradients (i.e., membrane distillation [MD]), electrical potential gradients (i.e., electrodialysis [ED] and Donnan dialysis [DD]), and concentration differences (i.e., forward osmosis [FO]). Moreover, the study also addressed the advantages, limitations, & applicable conditions of each membrane based defluoridation process.

Concentration of 1,3-dimethyl-2-imidazolidinone in Aqueous Solutions by Sweeping Gas Membrane Distillation: From Bench to Industrial Scale

Sweeping gas membrane distillation (SGMD) is a useful option for dehydration of aqueous solvent solutions. This study investigated the technical viability and competitiveness of the use of SGMD to concentrate aqueous solutions of 1,3-dimethyl-2-imidazolidinone (DMI), a dipolar aprotic solvent. The concentration from 30% to 50% of aqueous DMI solutions was attained in a bench installation with Liqui-Cel SuperPhobic^® hollow-fiber membranes. The selected membranes resulted in low vapor flux (below 0.15 kg/h·m²) but were also effective for minimization of DMI losses through the membranes, since these losses were maintained below 1% of the evaporated water flux. This fact implied that more than 99.2% of the DMI fed to the system was recovered in the produced concentrated solution. The influence of temperature and flowrate of the feed and sweep gas streams was analyzed to develop simple empirical models that represented the vapor permeation and DMI losses through the hollow-fiber membranes. The proposed models were successfully applied to the scaling-up of the process with a preliminary multi-objective optimization of the process based on the simultaneous minimization of the total membrane area, the heat requirement and the air consumption. Maximal feed temperature and air flowrate (and the corresponding high operation costs) were optimal conditions, but the excessive membrane area required implied an uncompetitive alternative for direct industrial application.

Membrane distillation as post-treatment for anaerobic fluidized bed membrane bioreactor for organic and nitrogen removal

To observe feasibility of membrane distillation (MD) as post-treatment for anaerobic fluidized bed membrane bioreactor (AFMBR), removals of organic and total nitrogen were investigated by using the commercial polyvinylidene difluoride (PVDF) membrane for direct contact membrane distillation (DCMD) at various operational conditions. Test solutions for MD experiments were permeate produced by staged AFMBR (SAF-MBR), permeate from single AFMBR and synthetic wastewater fed to both reactors. Increasing in feed temperature improved permeate flux through PVDF membrane, but it decreased total nitrogen (TN) removal efficiency. Effect of chemical oxygen demand (COD) concentrations in feed solutions for DCMD on TN removal efficiency was almost negligible. However, the COD removal efficiency was lower at lower feed concentration in DCMD operation. At constant feed temperature, TN removal efficiency was improved by increasing a recirculation flow rate on PVDF membrane across DCMD system. Both organic and inorganic fouling were observed on PVDF membrane surface and pore matrix after conducting DCMD operation. The organic fouling on PVDF membrane consisted mainly of protein and fatty acids, supporting that the permeate produced by AFMBR should have potentials to foul the membrane applied in DCMD system as post-treatment.

Desalination of Produced Water by Membrane Distillation: Effect of the Feed Components and of a Pre-treatment by Fenton Oxidation

The treatment of produced waters (by-products of oil and gas extraction) with the innovative process of membrane distillation is challenging, because these highly saline streams contain high concentrations of organic compounds and hydrocarbons that cause membrane wetting and impairment of performance. To design the most compact treatment scheme and with the aim of obtaining an easier management of produced water for reuse purposes, Fenton oxidation is here investigated as a feed pre-treatment that may produce an effluent easily handled by membrane distillation. In high-recovery membrane distillation tests, we systematically investigate the detrimental effects of individual contaminants in a synthetic produced water mimicking the composition of a real sample. The recovery rate depends strongly on the initial salinity, which eventually causes scaling and pore blocking. Surfactants are found to be mainly responsible for membrane wetting, but volatile and hydrophobic organics also spoil the quality of the product water. A Fenton oxidation pre-treatment is thus performed to degrade the target organics, with the aim of enhancing the effectiveness of the following membrane distillation and to improve the quality of the final product. The combined oxidation-membrane distillation scheme has both advantages and limitations, which need to be carefully evaluated and further investigated.

Impact of Operating Conditions on Measured and Predicted Concentration Polarization in Membrane Distillation

Concentration polarization (CP) occurs in almost all membrane-based separation processes. In this study, the concentration profile of the dissolved salt has been accurately characterized using a previously developed laser-based spectrophotometric method which had a spatial resolution of 4.5 μm. The objective of the current work was to probe the concentration profile of the solute and analyze the impact of operating parameters, such as feed concentration, hydrodynamic conditions, and feed temperature, on the solute concentration profile in the boundary layer. This study also examined the validity of the conventional approach, where semi-empirical models are used to estimate the boundary layer thickness (BLT) and concentration polarization coefficient (CPC)-based on experimental results. Nusselt correlations were developed specifically for the membrane cell and validated through experimental observations at the operating conditions used in this study. A key finding of this study is that the conventional approach of estimating the effect of CP severely underpredicts the BLT and CPC. The results of this study highlight the need to develop new methods to estimate the BLT and CPC as the conventional approach of using semi-empirical Nusselt and Sherwood correlations does not agree with experimental observations obtained for a membrane distillation system employed in this study.

Characterization and Assessment of a Novel Plate and Frame MD Module for Single Pass Wastewater Concentration-FEED Gap Air Gap Membrane Distillation

Membrane distillation (MD) is an up and coming technology for concentration and separation on the verge of reaching commercialization. One of the remaining boundaries is the lack of available full-scale MD modules and systems suitable to meet the requirements of potential industrial applications. In this work a new type of feed gap air gap MD (FGAGMD) plate and frame module is introduced, designed and characterized with tap water and NaCl–H₂O solution. The main feature of the new channel configuration is the separation of the heating and cooling channel from the feed channel, enabling a very high recovery ratio in a single pass. Key performance indicators (KPIs) such as flux, gained output ratio (GOR), recovery ratio and thermal efficiency are used to analyze the performance of the novel module concept within this work. A recovery rate of 93% was reached with tap water and between 32–53% with salt solutions ranging between 117 and 214 g NaCl/kg solution with this particular prototype module. Other than recovery ratio, the KPIs of the FGAGMD are similar to those of an air gap membrane distillation (AGMD) channel configuration. From the experimental results, furthermore, a new MD KPI was defined as the ratio of heating and cooling flow to feed flow. This R_F ratio can be used for optimization of the module design and efficiency.

Zero-liquid discharge (ZLD) technology for resource recovery from wastewater: A review

Water resources are becoming scarce meaning that reuse options are receiving more and more attention. In this perspective, zero-liquid discharge (ZLD) is considered as an emerging technique to minimize waste, recover resources, treat toxic industrial waste streams, and mitigate potential water quality impacts in receiving water streams. Although ZLD systems are capable of minimizing contamination of water sources and amplifying water supply, its industrial scale applications are restricted due to their high cost and intensive energy consumption. In ZLD systems, membrane-based technologies are an attractive future strategy for industrial wastewater reclamation. Therefore, this review examines why a greater focus on environmental protection and water security is leading to more widespread adoption of ZLD technology in various industries. We highlight existing ZLD processing schemes, including thermal and membrane-based processes, and discuss their limitations and potential solutions. We also investigated global application of ZLD systems for resource recovery from wastewater. Finally, we discuss the potential environmental impacts of ZLD technologies and provide some focus on future research needs.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.

Efficient Gas Adsorption Using Superamphiphobic Porous Monoliths as the under-Liquid Gas-Conductive Circuits

The gas-liquid membrane contactor forms a gas-solid-liquid interface and has a high potential for the applications in gas adsorption, catalysis, energy exchange, and so on. Porous superhydrophobic membranes show a great gas separation/adsorption ability. However, the complicated device architecture and the durability issue are normally concerned especially for the continuous circulation of gas and liquid. In this work, we present a free-standing gas-conductive circuit simply formed by connecting the superamphiphobic porous monoliths (SAPMs) to achieve an efficient under-liquid gas adsorption. The porous worm-like SAPM is prepared with low-temperature expandable graphite and polyvinylidenefluoride, exhibiting superamphiphobicity and superaerophilicity after fluoridation. The as-made SAPM circuits can be used as a reliable gas conductor under numerous liquids, such as water, alkaline, acidic, and oily solutions. In this work, the CO₂ adsorption capacities of the SAPM circuits are evaluated under NaOH and methyldiethanolamine solutions and the mass transfer rate can reach up to 9.61 mmol m^-2 s^-1. Moreover, the effective human blood oxygenation process is also demonstrated using SAPM circuits. Thus, the reported SAPM provides an alternative gas-liquid exchanging method and the simplified process could be of great benefit to the cost-effectively large-scale CO₂ capture or gas exchanging applications.

Application of direct contact membrane distillation for saline dairy effluent treatment: performance and fouling analysis

Membrane distillation is getting increasing attention thanks to its advantages in terms of energy consumption and final permeate quality in addition to its resistance against highly corrosive media which forms an appealing solution for industrial wastewater treatment. Despite its advantages, one of the most challenging issues in direct contact membrane distillation (DCMD) is membrane fouling and wetting. In the present research work, saline dairy effluent discharged from hard cheese industry was pretreated by macrofiltration (MAF) and ultrafiltration (UF) and processed by DCMD to investigate the extent of the aforementioned issues. Effluents pretreated by UF have led the best process performance with stable flux values at different operating conditions. Fouling has occurred in all the experiments, though their effect on the flux behavior and membrane wetting was different from one feed to the other. Changing the flow rate and the temperature difference have affected slightly the membrane wettability for all feed qualities. In all experiments, the permeate has maintained a good quality with low electrical conductivity that did not exceed 70 μS/cm and low total organic carbon < 2 mg/L.

Fouling and wetting in the membrane distillation driven wastewater reclamation process - A review

Fouling and wetting of membranes are significant concerns that can impede the widespread application of the membrane distillation (MD) process during high-salinity wastewater reclamation. Fouling, caused by the accumulation of undesirable materials on the membrane surface and pores, causes a decrease in permeate flux. Membrane wetting, the direct permeation of the feed solution through the membrane pores, results in reduced contaminant rejection and overall process failure. Lately, the application of MD for water recovery from various types of wastewaters has gained increased attention among researchers. In this review, we discuss fouling and wetting phenomena observed during the MD process, along with the effects of various mitigation strategies. In addition, we examine the interactions between contaminants and different types of MD membranes and the influence of different operating conditions on the occurrence of fouling and wetting. We also report on previously investigated feed pre-treatment options before MD, application of integrated MD processes, the performance of fabricated/modified MD membranes, and strategies for MD membrane maintenance during water reclamation. Energy consumption and economic aspects of MD for wastewater recovery is also discussed. Throughout the review, we engage in dialogues highlighting research needs for furthering the development of MD: the incorporation of MD in the overall wastewater treatment and recovery scheme (including selection of appropriate membrane material, suitable pre-treatment or integrated processes, and membrane maintenance strategies) and the application of MD in long-term pilot-scale studies using real wastewater.