Pressure-driven Membrane Processes Research Articles

Electrospun Membranes for Desalination and Water/Wastewater Treatment: A Comprehensive Review

Polymeric nanofbers, specifcally fabricated by electrospinning, offer viable means useful for a wide range of applications such as health, energy and environmental issues. However, among the mentioned sectors, desalination and water/wastewater treatment applications have been highlighted during the past decade. This article focuses on the present status and recent development of electrospun nanofbrous membranes and their potential impact in two major areas, i.e., desalination and water/wastewater treatment. Specifc applications for desalination and high-quality water/wastewater treatment, including pressure-driven and osmotic membrane processes (MF, UF, NF, FO, etc.), thermal-driven membrane processes, coalescing fltration and adsorptive application of nanofbers, are described. Also, benefts, limitations and challenges are discussed, comprehensively. Electrospun membranes can play a critical role in improving membrane-based desalination and water/wastewater treatment systems. These fltration elements with 3D inter-connected structures will be shown to have interesting and crucial advantages over conventional polymeric membranes in terms of performance, cost and energy savings. This article also highlights the prospects of electrospun membranes and specifcally provides the state-of-the-art applications in the water industry.

Processing of Xoconostle fruit (Opuntia joconostle ) juice for improving its commercialization using membrane filtration

The aim of this study was to propose a membrane process for processing of a native Mexican fruit with limitations for its commercialization. The clarification of Xoconostle juice was performed using a hollow fiber membrane. The operating conditions were determined (T = 25 °C, Qf = 58 L/hr, TMP = 138 kPa); the process presented high productivities in terms of permeate flux (12.0–19.7 kg/m2/hr). The proposed method achieved high retention of turbidity (87.50%); whereas low retentions of pH (0.98%), total soluble solids (10.17%), carbohydrates (33.17%), polyphenols (2.10%), betalains (5.52%), and antioxidant activity (15.74%) were observed. Finally, a clear juice (3.79 ± 1.3 NTUs) was obtained with high antioxidant activity (21.40 ± 1.1 TEAC), high phenolic (27.50 ± 0.2 mg gallic acid/L) and betalain contents (18.80 ± 0.6 mg/L). An enhancement on color properties was observed (redness grade, h° = 10.14 ± 2.0). Practical applications Nowadays, the interest of food industry is focusing on the development of functional foods as well as looking for new valuable sources rich in bioactive compounds. Until now, many native Mexican fruits have not been processed in agro-food industry, that is, Xoconostle fruit is one of the natural products not strongly processed. The processing of its derivative products, as fresh juice, can offer benefits to exploit a bit more its well-known functional properties to human health. The enhancement of the quality of this natural juice preserving its nutritional and biological properties can be carried out using pressure-driven membrane processes. Based on (a) the high productivity of the applied methodology for juice processing, (b) the high removal of turbidity, (c) the enhancement of physical characteristics (colorimetric), and (d) the preservation of its natural chemical substances, the membrane filtration seems to be a good alternative to going further in the commercialization of the fruit.

A Short Review of Membrane Fouling in Forward Osmosis Processes.

Interest in forward osmosis (FO) research has rapidly increased in the last decade due to problems of water and energy scarcity. FO processes have been used in many applications, including wastewater reclamation, desalination, energy production, fertigation, and food and pharmaceutical processing. However, the inherent disadvantages of FO, such as lower permeate water flux compared to pressure driven membrane processes, concentration polarisation (CP), reverse salt diffusion, the energy consumption of draw solution recovery and issues of membrane fouling have restricted its industrial applications. This paper focuses on the fouling phenomena of FO processes in different areas, including organic, inorganic and biological categories, for better understanding of this long-standing issue in membrane processes. Furthermore, membrane fouling monitoring and mitigation strategies are reviewed.

Reuse of discarded membrane distillation membranes in microfiltration technology

Membrane distillation (MD) technology is being implemented in industry thanks to the incessant progress made so far in membrane modules, membrane design and their fabrication. The main drawback of MD is the irreversible membrane pore wetting. Special care must be made in order to maintain the membrane pores dry. Once the pores are getting wet and/or blocked the efficiency of the MD process is reduced and the membrane is finally discarded. It is to be noted that MD membranes are more expensive than the membranes used in other membrane processes (e.g. pressure driven membrane processes microfiltration, MF; ultrafiltration, UF or reverse osmosis, RO). Reuse of disposed MD membranes is a possible solution to prevent membrane disposal and save costs. In this study, polytetrafluoroethylene (PTFE) membranes were used for the treatment of synthetic (i.e. 65g/L NaCl aqueous solutions) and RO brines (~50g/L TDS) at different feed temperatures (between 318 and 348K) by air gap membrane distillation (AGMD) until the membrane pores were blocked or wetted. These membranes were proposed for MF considering as a feed solution example humic acid (HA) aqueous solutions (15mg/L HA). Fouling of the discarded PTFE membranes for use in AGMD was studied and compared with that of new PTFE membranes. Depending on the membrane characteristics and its initial state, MF results are different in terms of the permeate flux, which is higher for the TF450 membranes and for those previously used in synthetic brine concentration, the rejection factor, which is lower for the TF450_ROB_318K membrane (70.76%) or the performance index. It was observed that the reused AGMD membranes could exhibit even better MF performance than the new ones being the best case, 189.2kg/m2h, for the membrane TF450_ROB_318K compared to 123.9kg/m2h exhibited by the new TF450 membrane at the same temperature.

Analysis of enhancing water flux and reducing reverse solute flux in pressure assisted forward osmosis process

Pressure assisted osmosis (PAO) has been recently suggested as a way to overcome the current limitations of forward osmosis (FO), since water flux can be increased by additional hydraulic driving force. To validate its feasibility more fundamentally, the effect of hydraulic pressure combined with osmotically driven FO process was evaluated experimentally, and compared with theoretical modeling. Four different FO and NF membranes were selected and their performance characteristics were determined by both RO- and FO-based methods, since PAO is a simultaneous osmotic- and pressure-driven membrane process. The degree of enhancing water flux and reducing reverse solute flux (RSF) in the PAO process clearly differed according to the membrane type and their performance parameters. Modeling PAO performance, using the values of A, B, and S determined from both RO- and FO-based methods, often failed to exactly match experimental observations, particularly for thin film composite (TFC) FO membranes, suggesting that PAO membrane performance parameters are apparently pressure-dependent. Discernible compaction in the support layer of TFC FO membranes was identified and confirmed by FO tests at different operating modes and SEM analysis, partially explaining large variations in S values under pressurized PAO operation and thus resulting large deviation from theoretical predictions.

Conduction Properties of Polyethersulfone/Carbon Nanotubes (CNTs)-Based Membranes for Water Treatment: Forecast By Electrochemical Impedance Spectroscopy

Pressure-driven membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are widely used for water treatment1-2. Recently, attention has been focused on the use of carbon nanotubes (CNTs) electrical and chemical properties to produce conductive membranes with the goals of preventing membrane fouling and enhancing mechanical properties3-4. Fouling is one of the main issues causing the decline of filtration performance over time. The application of an external electric field on or near the membrane has been used to reduce fouling5. This may produce back transport phenomena that reduce the accumulation of foulants near the membrane, produce reactive oxidant species that may inhibit the development of biofilms and/or remove biofilms and adsorbed organic matter on membrane surfaces. These latter processes may be enhanced by formulating electrically conductive membranes. A strategy to design such membranes requires insight into the physico-chemical properties and electrochemical characterization that are relevant to forecasting filtration performance. The application of an electrical potential across membranes may also be used to characterize membranes and diagnose membrane fouling. In this work, structural and physico-chemical characteristics of polyethersuflone-based membranes were studied where electrically conductive carbon nanotubes (CNTs) were added to the polymer. Among other characterization techniques, electrochemical impedance spectroscopy (EIS) was used to gain insight into the membrane properties. Resistance, diffusion and pore accessibility were systematically analyzed for different electrolyte concentrations. Results indicate that the addition of CNTs into the polymeric membrane changes the electronic conductivity and the ion transport within the porous structure, in part due the enhanced hydrophilicity provided by the CNTs. From the perspective of generating reactive oxidant species at the membrane interface or within the membrane matrix, charge transfer properties were also studied in the presence of a redox probe.

Fouling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation

Various membrane separation processes are being used for seawater desalination and treatment of wastewaters in order to deal with the worldwide water shortage problem. Different types of membranes of distinct morphologies, structures and physico-chemical characteristics are employed. Among the considered membrane technologies, membrane distillation (MD), osmotic distillation (OD) and osmotic membrane distillation (OMD) use porous and hydrophobic membranes for production of distilled water and/or concentration of wastewaters for recovery and recycling of valuable compounds. However, the efficiency of these technologies is hampered by fouling phenomena. This refers to the accumulation of organic/inorganic deposits including biological matter on the membrane surface and/or in the membrane pores. Fouling in MD, OD and OMD differs from that observed in electric and pressure-driven membrane processes such electrodialysis (ED), membrane capacitive deionization (MCD), reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), etc. Other than pore blockage, fouling in MD, OD and OMD increases the risk of membrane pores wetting and reduces therefore the quantity and quality of the produced water or the concentration efficiency of the process. This review deals with the observed fouling phenomena in MD, OD and OMD. It highlights different detected fouling types (organic fouling, inorganic fouling and biofouling), fouling characterization techniques as well as various methods of fouling reduction including pretreatment, membrane modification, membrane cleaning and antiscalants application.

Separation of Biologically Active Compounds by Membrane Operations.

Bioactive compounds from various natural sources have been attracting more and more attention, owing to their broad diversity of functionalities and availabilities. However, many of the bioactive compounds often exist at an extremely low concentration in a mixture so that massive harvesting is needed to obtain sufficient amounts for their practical usage. Thus, effective fractionation or separation technologies are essential for the screening and production of the bioactive compound products. The applicatons of conventional processes such as extraction, distillation and lyophilisation, etc. may be tedious, have high energy consumption or cause denature or degradation of the bioactive compounds. Membrane separation processes operate at ambient temperature, without the need for heating and therefore with less energy consumption. The "cold" separation technology also prevents the possible degradation of the bioactive compounds. The separation process is mainly physical and both fractions (permeate and retentate) of the membrane processes may be recovered. Thus, using membrane separation technology is a promising approach to concentrate and separate bioactive compounds. A comprehensive survey of membrane operations used for the separation of bioactive compounds is conducted. The available and established membrane separation processes are introduced and reviewed. The most frequently used membrane processes are the pressure driven ones, including microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF). They are applied either individually as a single sieve or in combination as an integrated membrane array to meet the different requirements in the separation of bioactive compounds. Other new membrane processes with multiple functions have also been developed and employed for the separation or fractionation of bioactive compounds. The hybrid electrodialysis (ED)-UF membrane process, for example has been used to provide a solution for the separation of biomolecules with similar molecular weights but different surface electrical properties. In contrast, the affinity membrane technology is shown to have the advantages of increasing the separation efficiency at low operational pressures through selectively adsorbing bioactive compounds during the filtration process. Individual membranes or membrane arrays are effectively used to separate bioactive compounds or achieve multiple fractionation of them with different molecule weights or sizes. Pressure driven membrane processes are highly efficient and widely used. Membrane fouling, especially irreversible organic and biological fouling, is the inevitable problem. Multifunctional membranes and affinity membranes provide the possibility of effectively separating bioactive compounds that are similar in sizes but different in other physical and chemical properties. Surface modification methods are of great potential to increase membrane separation efficiency as well as reduce the problem of membrane fouling. Developing membranes and optimizing the operational parameters specifically for the applications of separation of various bioactive compounds should be taken as an important part of ongoing or future membrane research in this field.

Optimising the recovery of EDTA-2Na draw solution in forward osmosis through direct contact membrane distillation

Abstract Ethylenediaminetetraacetic acid disodium (EDTA-2Na) has been demonstrated as an excellent draw solution in the forward osmosis (FO) process because of its high osmotic pressure together with low reverse salt flux but its application is hindered by difficulties in the recovery of draw solution. Hence, in this study, microporous hydrophobic membranes were used in direct contact membrane distillation (DCMD) to concentrate the diluted EDTA-2Na draw solution. The MD was found to require lower operating pressures than do all other widely applied pressure-driven membrane processes, particularly in RO. This study systematically investigated the effect of different polytetrafluoroethylene membranes under various cross flow velocities of 2.67–14.67 cm/s, feed temperatures of 35–60 °C, and distillate temperatures of 10–20 °C in DCMD process for regeneration of diluted EDTA-2Na. The results revealed that DCMD system could achieve a salinity rejection rate exceeding 99.99%; furthermore, the conductivity of the permeate distillate was consistently below 6.4 µS/cm for all of the EDTA-2Na feed concentrations. More importantly, the water flux slightly decreased from 8.27 to 7.04 L/m2 h when the concentration of the EDTA-2Na feed increased from 0.1 to 0.5 M, corresponding to increased osmolality from 300 to 1411 mOsm/kg, indicating that water flux in DCMD is not significantly influenced by the osmotic pressure gradient across the membrane. This study demonstrated that MD could be an effective method for EDTA-2Na recovery in FO–MD systems and could economically utilize the wasted heat from industrial sources.

Research on flux decline in nanofiltration of lactic acid solutions with ZRIV/PAA membranes application

Abstract The efficiency of lactic acid solutions nanofiltration with dynamically formed zirconium (IV) hydrous oxide polyacrylate (ZrIV/PAA) membranes application were considered in this paper. The results of investigations on flux decline in nanofiltration of lactic acid solutions under conditions resulting in low and high lactic acid rejection are reported. In the long term experimental research on pressure driven membrane processes the main reason of permeate flux reduction is an accumulation of concentration polarization and fouling effects. The experimental permeate flux versus time curves were analyzed in the frame of resistance-in-series model with the aim to develop the characteristic fouling and concentration polarization resistances. The analysis of experimental data and results of calculations showed that both: concentration polarization and fouling phenomena in investigated system depend on hydrodynamic conditions and properties of filtered solutions and (ZrIV/PAA) membrane.

Implications of inhomogeneous distribution of concentration polarization for interpretation of pressure-driven membrane measurements

A number of CFD studies have demonstrated that there is a considerable inhomogeneity of extent of Concentration Polarization (CP) over the membrane surface especially in spacer-filled feed channels. However, the consequences of this inhomogeneity for the interpretation of measurements of solute rejection in pressure-driven membrane processes have received little attention.This study uses a simple model of locally-1D CP combined with a postulated probability distribution of unstirred-layer thickness over the membrane thickness. In this way, we obtain transparent analytical results and can consider qualitative consequences of inhomogeneous distribution of CP over membrane surface. Our analysis shows that disregarding the CP distribution under-estimates the CP of strongly positively-rejected solutes and over-estimates the CP for the negatively-rejected ones. This observation is especially important for the interpretation of ion rejection from multi-ion solutions in nanofiltration where strong positive and pronounced negative rejections can occur simultaneously for solutes of different charges.We conclude that for reliable interpretation of pressure-driven membrane measurements it is desirable to reduce the inhomogeneity of CP distribution to a minimum in membrane-testing devices.

Pressure-driven membrane processes for the recovery of antioxidant compounds from winery effluents

Winery effluents are substantial sources for the recovery of antioxidant compounds, namely polyphenols and polysaccharides. In this study, aqueous extraction associated with microfiltration was used as a pretreatment for the recovery, by membrane processes, of polyphenols and polysaccharides present in effluents (wine lees) from the first racking of red winemaking. The study was carried out in total recirculation mode using a flat-sheet membrane and in concentration mode using a hollow fiber membrane. Although the most diluted permeates presented lower contents of polyphenols and polysaccharides, the solutes recovery did not follow this behavior. Higher dilution factors of the effluent resulted in higher permeate fluxes as well as in higher polyphenols recovery. At the best conditions, a solution diluted 50 V/V followed by microfiltration led to the achievement of a limpid permeate, representing a recovery of 1 g of polyphenols and 1 g of polysaccharides per liter of raw effluent.

The potential of direct contact membrane distillation for industrial textile wastewater treatment using PVDF-Cloisite 15A nanocomposite membrane

This work demonstrates the feasibility of employing direct contact membrane distillation (DCMD) for treating industrial textile wastewater for clean water production. Experimental results showed that the in-house fabricated polyvinylidene fluoride-Cloisite 15A polymer–inorganic nanocomposite membrane is robust and able to treat the industrial effluent by reducing at least 89% of the initial values of the water quality parameters measured. However, the membrane permeate flux was reported to decline almost 50% in the first few hours of the 40-h treatment process before reaching water flux of 13–22kg/m2h. It is believed that the initial flux decline is mainly caused by the foulants accumulated on the membrane outer surface that increases mass transfer resistance of water molecules and reduces water productivity. With respect to separation characteristics, the DCMD process has shown better performance for COD and color removal in comparison to the other commonly used pressure-driven membrane processes. Further improvement on the membrane surface properties is necessary to reduce fouling propensity and pore wetting caused by the surfactants and other foulants in the textile wastewater. This is of particular importance for long-term operation of DCMD process.

Concentration of contaminated single-phase detergents by means of unit and integrated membrane processes

ABSTRACTUnit and integrated pressure-driven membrane processes were investigated in a semi-pilot scale for the purification and concentration of contaminated cleaning solutions. For the tests, liquid acidic and alkaline single-phase detergents tailor-made for CIP systems in the dairy industry were selected. The effect of the permeate recovery rate on the quality of recovered solutions was evaluated.Integrated purification processes provided a very high separation of the milk components with a slight weakening of the detergency properties of the permeate. The least effective integrated treatment option (PM5+AFC30) enabled 100% retention of proteins, and over 98% retention of low molecular weight lactose.

Separation of Peptides and Interaction with Forward Osmosis Biomimetic Membranes: A Solution Diffusion Model

Recently, a membrane based technique, forward osmosis (FO), has gained interest with respect to the separation and purification of biomolecules. Forward osmosis relies on concentration gradients to draw water across the membrane and is as such a gentle technique compared to the traditional pressure driven membrane processes. The challenge within FO is to operate at a high water flux while maintaining a high rejection and low reverse flux of salt that may otherwise interfere with the biomolecules. By adding water-transporting aquaporin protein channels to the membrane matrix to create biomimetic membranes, selective transport of water is possible. These kinds of membranes could as such be ideal for separation of biomolecules.In this study, a biomimetic forward osmosis membrane was tested for the separation of two peptides (416.48 Da and 691.71 Da). The membrane was found to exhibit high, but not complete, rejection rates of both peptides (>98%). It is interesting that even relatively large molecules are able to pass the otherwise dense active layer of the forward osmosis membrane. Therefore, to further understand the transport mechanisms underlying the filtration process, a custom made cell was constructed and used to model the process with filtration models. It was found that the filtration process could be modeled using the solution diffusion approach, with the diffusion coefficient being related to the radius of gyration of the peptides.

A study of membrane distillation and crystallization for lithium recovery from high-concentrated aqueous solutions

Membrane distillation is emerging as an interesting solution to address the difficulties and limits of conventional pressure driven membrane processes to treat highly concentrated solutions. Mineral depletion, new environmental regulations and emerging opportunities emphasize the treatment of waste streams with extraordinary high concentrations. In this current study, comparative analysis of membrane crystallization in direct-contact, osmotic and vacuum configurations have been carried out with the aim to recover lithium chloride from aqueous solutions. To precipitate LiCl from single salt solutions, a concentration above 14M is required. The osmotic pressure associated with this concentrated solution is very high and its treatment is challenging for direct-contact membrane distillation and osmotic membrane distillation. The phenomena of osmotic pressure has been avoided by using vacuum membrane distillation. Only the application of vacuum membrane distillation, among the studied configurations, ensures the achievement of supersaturation required for crystallization. Moreover, studies on crystal morphology has been performed. Crystals can be recovered in cubic or orthorhombic polymorphic structures depending on the operative conditions. In general, this study provides an interesting comparative analysis of various membrane distillation configurations to treat highly concentrated solutions, which can contribute to the future integrated membrane desalination systems for simultaneously water, energy and minerals production.

Reverse osmosis applications: Prospect and challenges

Reverse osmosis (RO) is a pressure driven membrane process which has been widely applied and recognized as the leading technology of desalination process. Improvement in RO technology including advanced membrane material, module and process design, and energy recovery has led to cost reduction which in turn gaining interest to its commercial applications. RO is now being used in various applications including selective separation, purification, and concentration processes. In food industry, RO is applied for concentration of fruits and vegetable juices, pre-concentration of milk and whey, and dealcoholization of alcoholic beverage. For area which has large source of natural humic water or peat water, RO can be applied to produce clean water for community water supply. RO was also investigated for organic mixture separation and CO2 regeneration from essential oil extraction using supercritical fluid. The application of RO as a final step of wastewater treatment for water reuse and valuable component recovery seems to be promising in wastewater reclamation. In this paper, the applications of RO, its advantages, and limitations are discussed. In addition, challenges and perspective of RO membranes are pointed out.

Membrane fouling and long-term performance of seawater-driven forward osmosis for enrichment of nutrients in treated municipal wastewater

Forward Osmosis (FO) has been designed and studied for various applications due to the advantages it offers when compared to traditional pressure-driven membrane processes. In our previous study, seawater-driven forward osmosis was successfully applied to concentrate nutrients in treated municipal wastewater. In this study, the long-term operational stability and performance of the seawater-driven FO process was investigated by a 2-month continuous operation of bench-top FO reactors. Nitrification inhibition was recommended to maintain the concentration performance of nitrogen by preventing ammonia from being oxidized by microorganisms. With nitrification inhibition, concentration performances of 3- to 4-fold of total phosphorus and 2.1-fold of ammonia were stably achieved at a feed volume reduction of 75%. Membrane fouling, with the active layer of the FO membrane facing the feed solution, had only a mild impact; there was approximately 5% water flux reduction but no significant effect on nutrient enrichment performance. The analytical results by FTIR and SEM-XMA suggested that bio-fouling with a combined structure of bacterial clusters, biopolymers, and slight inorganic scales was the main FO membrane fouling pattern. Furthermore, pre-disinfection performed in feed solution effectively retarded the occurrence of membrane fouling in the FO process.

Energy consumption for desalination — A comparison of forward osmosis with reverse osmosis, and the potential for perfect membranes

Reverse osmosis (RO) is now the most ubiquitous technology for desalination, with numerous seawater RO plants being built in water-stressed countries to complement existing water resources. Despite the development of highly permeable RO membranes, energy consumption remains a major contributor to total cost. Forward osmosis (FO) is receiving much attention as a potentially lower energy alternative to RO. However, the draw solution (DS) recovery step in FO requires significant energy consumption. The present study is a modelling approach, simulating FO and RO desalination under various process conditions and process flow schemes using the Aspen Plus environment. Results suggest that there is practically no difference in specific energy consumption (SEC) between standalone RO, and FO with nanofiltration (NF) DS recovery; this can be generalised for any pressure-driven membrane process used for the DS recovery stage in a hybrid FO process. Furthermore, even if any or all of the membranes considered, FO, RO or NF, were perfect (i.e. had infinite permeance and 100% rejection), it would not change the SEC significantly. Hence, any advantage possessed by the FO with NF recovery process derives from the lower fouling propensity of FO, which may reduce or eliminate the need for pre-treatment and chemical cleaning.

Pressure-driven membrane and ion exchange hybrid system for the treatment of wastewater containing sulfite and nitrite

Denitrification of flue gas can generate a large amount of wastewater. Depending on the denitrification technology used, the water could contain a fair amount of sulfite and nitrite that need to be removed for water reuse or discharge. An integrated pressure-driven membrane and ion exchange (IX) process is therefore designed and tested for this purpose in this work. The process contains a combination of nanofiltration (NF), reverse osmosis (RO), and a mixed bed IX. NF was performed on a DL1812 membrane module at 2.0 MPa using a simulated wastewater containing 63.0 g/L Na2SO3 and 49.5 g/L NaNO2. Sulfite was separated from nitrite with a retention rate of 92.0%, and its overall recovery efficiency achieved 91.09%. The recovery rate of nitrite in the permeate of NF was 71.06%, which was further elevated by RO using a SG1812 membrane module. A four-pass RO process was shown to be capable of recovering 98.15% of NaNO2 in the final retention. Finally, it was demonstrated that followed by IX treatment, sulfite and nitrite were successfully removed to meet the water surface discharge criteria, proving the feasibility of the hybrid NF‐RO‐IX system as a viable alternative for treating and reusing denitrification wastewater.