Pressure-driven Membrane Separation Processes Research Articles

A review on microfiltration membranes: fabrication, physical morphology, and fouling characterization techniques

Microfiltration is a commonly used pressure-driven membrane separation process for various applications. Depending on the manufacturing method, either tortuous or capillary pore structures are obtained. The structure plays an important role in controlling flux, selectivity, but most importantly, the fouling tendency of the membrane. This review attempts to cover past and current developments in physical morphology and fouling characterization methods, along with the manufacturing methods for microfiltration membranes. The limitations and advantages of direct microscopic techniques and gas-liquid displacement as an indirect method are discussed for physical characterization. Additionally, the current state of the art and technical challenges for various in-situ and ex-situ fouling characterization techniques are also discussed. Finally, some directions for future research are outlined.

Theory of expansion and compression of polymeric materials: Implications for membrane solvent flow under compaction

We extend the classical Flory-Rehner theory for the expansion and compression of porous materials such as cross-linked polymer networks and membranes. The theory includes volume exclusion, affinity with the solvent, and finite stretching of the polymer chains. We also modify this equilibrium theory, which applies to equal expansion of a material in all directions, to the situation where a material can only expand in a single direction, as is the case when a thin polymer layer is tightly bound to a support structure. We further extend this equilibrium model to the case where a pressure is applied across a thin polymer layer, such as a membrane, and liquid flows across the membrane. The theory describes how in the direction of liquid flow the membrane is increasingly compacted and becomes less porous, and this effect increases when the applied pressure goes up. We present results of example calculations for a thick membrane showing significant changes in compaction across its thickness, and a thin membrane where compaction due to flow is minor. Lastly, we model the dynamics of the change in size of a porous material in time after a step change in the solvent-polymer attraction parameter, which may be relevant, for instance, when the attraction parameter is highly temperature-dependent, and we change the temperature. The developed theory has direct implications for pressure-driven membrane separation processes ranging from reverse osmosis to organic solvent nanofiltration.

Review of Hybrid Membrane Distillation Systems.

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Non-Biological Slaughterhouse Wastewater Treatment with Membrane Processes-An Opportunity for Water Recycling.

The pressure-driven membrane separation processes ultrafiltration (UF) and reverse osmosis (RO) enable the effective purification of wastewater, in particular in combination, allowing organic and inorganic contaminants to be separated from the wastewater. Consequently, this work investigates the suitability of this technology for slaughterhouse wastewater (SWW) recycling. This was investigated by means of laboratory and bench-scale plant membrane experiments, whereby slaughterhouse wastewater (SWW) pre-treated by flotation was first treated with UF and then further purified with RO. Through the process combination UF + RO in the bench scale experiment, a reduction of the parameters total organic carbon (TOC), chemical oxygen demand (COD) of more than 98% and 97% for the parameter total nitrogen (TN) could be achieved. This means that wastewater reuse without product contact can be guaranteed. For direct process water reuse, only the concentration limit for ammonium could not be reached. In addition, scanning electron microscopy (SEM) images and energy dispersive X-ray spectroscopy (EDX) analyses of the RO membrane were carried out before and after the experiment, which did not indicate any scaling effects.

Brine minimization in desalination of the geothermal reinjection fluid by pressure-driven membrane separation processes

Brine obtained during water treatment by pressure driven membrane processes remains the major drawback. Therefore, it is of paramount important to find a lasting solution in order to minimize its production by both nanofiltration (NF) and reverse osmosis (RO) membranes. In this study, an experimental study with the aim of brine minimization during membrane desalination of the geothermal reinjection fluid using a mini-pilot scale membrane test system having spiral wound NF and RO membranes was conducted. The membranes employed for this task were TR-NF and BW30-RO membranes. First, studies with different brine to feed ratios of 1:4, 1:3, 1:2 and 2:3 represented as NF-F2, NF-F3, NF-F4 and NF-F5, respectively were investigated using TR-NF membrane. A control study with no brine recirculation was conducted as well in order to check the effect of brine recirculation on the membrane performance. Secondly, studies with BW30-RO membrane using same brine to feed ratios as in the case of NF membrane studies were carried out. An applied pressure of 15 bar, initial water recovery of 60% and 4 h of experimental time were employed as operational conditions for both NF and RO membrane studies. Based on the results obtained, it was found that the brine recirculation (with a brine to fresh feed ratio of 2:3) has a significant impact on the permeate flux. The product water can be utilized for the agricultural irrigation purposes. Nevertheless, the boron concentration in the product water was still high for the sensitive crops.

Membrane distillation bioreactor (MDBR) for wastewater treatment, water reuse, and resource recovery: A review

Membrane bioreactors (MBRs) have been well established as an advanced wastewater treatment technology that combines activated sludge with a pressure-driven membrane separation process. They have been widely applied in municipal and industrial wastewater treatment systems due to their advantages over conventional activated sludge systems. However, MBRs still face major issues hindering their widespread implementation such as poor effluent quality due to low retention of trace organic contaminants, instability due to changing operating conditions, and organic membrane fouling. Membrane distillation (MD) bioreactor (MDBR) is a novel wastewater treatment technology that combines MBRs with an MD unit either separated at a side stream or submerged in the bioreactor. The concept of hybrid MDBR configuration was firstly introduced in 2007 which was followed by numerous journal articles. These studies demonstrated the MDBR potential for water, nutrients, bioenergy, and other value-added product recovery from domestic and industrial wastewater; however, there is still a lack of a comprehensive study that shows the full MDBR potential. Hence there is a need for a dedicated review that summarize and critically analyse these research findings and highlight the research gaps to provide clear path for future research. This review presents system availability and applications of hybrid MDBR. In particular, the review presents brief introduction of MBR and MD technologies followed by covering the evolution of MDBR as a novel technology for municipal and industrial wastewater treatment. Several applications of MDBR systems are reviewed with a focus on system performance and removal efficiencies. The potential of MDBR to widen wastewater reuse and achieve nutrient and biogas recovery is also presented. Finally, the main challenges that hinder the development of MDBR are discussed while recommendations on future perspectives towards MDBR commercialization are suggested.

Forward osmosis to treat effluent of pulp and paper industry using urea draw-solute: Energy consumption, water flux, and solute flux

Wastewater treatment via suitable routes is necessary to protect the environment and fill the shortage of freshwater. An osmotic pressure-driven membrane separation process of forward Osmosis (FO) has gained attention as an alternative water treatment process over the conventional approaches. FO process needs a draw solution (DS). A commonly used fertilizer such as urea in water can be used as a DS, and after the FO run, diluted DS may use as fertilizer. Thus, it can simultaneously generate freshwater for agricultural purposes from industrial wastewater. FO process using thin-film composite flat sheet membrane has been investigated for various lab-made solutions and pulp & paper industrial effluent as feed solutions (FSs) under varying DSs of 0.25, 0.5, 1, and 2 M urea solutions. Limited FO studies examined the long-term and repetitive runs using the same membrane to access the fouling, water, and solute flux behavior with real industrial wastewater as feed solutions (FS). The long-term experimental results prove the strong influences of DS concentration on specific energy consumption (SEC), fouling behavior, and water flux. Increasing the cross-flow rate from 0.33 to 1 L/min has a minor effect on water flux, but SEC increases significantly from 0.17 ± 0.01 kWh/m3 to 0.50 ± 0.03 kWh/m3. FO runs show that the overall water flux enhances due to higher osmotic pressure difference and lesser membrane fouling at higher DS concentration using pulp & paper industrial secondary effluent as FS. This study demonstrates that a low-energy FO process can recover water from industrial wastewater while simultaneously produces a diluted fertilizer for fertigation.

Integrated pressure-driven membrane separation processes for the production of agricultural irrigation water from spent geothermal water

The application of different pressure-driven membranes to treat the spent geothermal water to be used for agricultural irrigation was explored in this study. Firstly, individual performances of different commercially available nanofiltration (NF) and reverse osmosis (RO) membranes attached to a mini-pilot membrane test system were studied. For the single membrane test TR-NF and NF90 as NF membranes and TR-BWRO, Vontrone, TR-SWRO and BW30 as RO membranes were employed while combination of TR-NF with TR-BWRO, Vontrone, TR-SWRO and BW30 was employed in the integrated study (with and without pH adjustment).When performances of individual membranes were investigated, the permeate fluxes obtained it was found that there was no significant flux drop with the experimental time (4 h) in all the studies. In terms of boron removals, boron was partially removed in single membrane study as 22.7–23.6% and 30.6–56.8% by NF and RO membranes, respectively. For the integrated NF + RO membrane configuration (without pH adjustment), boron removals were in the range of 42.1–59.4% while 91.3–95.4% of boron removal was obtained in the integrated NF + RO (at elevated pH). It was found that the produced water complied with class I quality with respect to irrigation water parameters with the exception of boron and sodium adsorption ratio (SAR), while the pH of the product water in the integrated NF + RO study at elevated pH was found to be in class III. Hence, produced water will be suitable (class II) for irrigation if 30% of well water should be blended with the product water in the integrated NF + RO studies (with and without pH adjustment) as well as pH adjustment when necessary.

Integrating Pressure-Driven Membrane Separation Processes to Improve Eco-Efficiency in Cheese Manufacture: A Preliminary Case Study.

Pressure-driven membrane separation processes are commonly used in cheese milk standardization. Using ultrafiltration (UF) or microfiltration (MF), membrane separation processes make it possible to concentrate the milk proteins and increase the yields of cheese vats. However, the contribution of membrane separation processes to the environmental impact and economical profitability of dairy processes is still unclear. The objective of this study was to evaluate the contribution of membrane separation processes to the eco-efficiency of cheddar cheese production in Québec (Canada) using process simulation. Three scenarios were compared: two included UF or MF at the cheese milk standardization step, and one did not incorporate membrane separation processes. The results showed that even if membrane separation processes make it possible to increase vat yields, they do not improve the eco-efficiency of cheddar cheese processes. However, membrane separation processes may benefit the eco-efficiency of the process more when used for byproduct valorization.

Roles of surfactants in pressure-driven membrane separation processes: a review

Surfactants widely exist in various kinds of wastewaters which could be treated by pressure-driven membrane separation (PDMS) techniques. Due to the special characteristics of surfactants, they may affect the performance of membrane filtration. Over the last two decades, there are a number of studies on treating wastewaters containing surfactants by PDMS. The current paper gives a review of the roles of surfactants in PDMS processes. The effects of surfactants on membrane performance were discussed via two aspects: influence of surfactants on membrane fouling and enhanced removal of pollutants by surfactants. The characteristics of surfactants in solution and at solid-liquid interface were summarized. Surfactants in membrane filtration processes cause membrane fouling mainly through adsorption, concentration polarization, pore blocking, and cake formation, and fouling degree may be influenced by various factors (feed water composition, membrane properties, and operation conditions). Furthermore, surfactants may also have a positive effect on membrane performance. Enhanced removal of various kinds of pollutants by PDMS in the presence of surfactants has been summarized, and the removal mechanism has been revealed. Based on the current reports, further studies on membrane fouling caused by surfactants and enhanced removal of pollutants by surfactant-aided membrane filtration were also proposed.

Art to use Electrospun Nanofbers/Nanofber Based Membrane in Waste Water Treatment, Chiral Separation and Desalination

The technique to fabricate nanofibrous mat by electro-spinning has been known for a long time. But the attempts to use the electospun nanofiber mats, also known as electro-spun nanofiber membranes (ENMs), for filtration purposes began only recently. Among many membrane filtration processes, air cleaning by the removal of dust particles has already been commercialized and the product has been in the market for some time. On the other hand, the application of ENMs for liquid separation has a much shorter history and its commercialization has not yet been achieved. Since a large number of researches are reported in the open literature each year, its commercialization looks only a matter of time. For example, many papers are now available on the pressure driven membrane separation processes such as RO, NF, UF, MF by ENMs, and as many papers have been published on the other membrane separation processes including pervaporation, membrane distillation, forward osmosis and membrane adsorption. It is needless to say that EMFs have gained popularity within a short period due to the facile fabrication, interconnectivity and large area/volume ratio. Despite these advantages, ENMs’ pore sizes are intrinsically very large (fractions of micrometer to few micrometer), which makes modification of surface chemistry and especially reduction of the ENM pore size indispensable for wider applications of ENMs for membrane separation processes.

The potential of organic solvent nanofiltration processes for oleochemical industry

Abstract Pressure-driven membrane separation processes bare the promise to significantly reduce energy requirements and operational cost compared to classical thermal separation processes. The ability of organic solvent nanofiltration (OSN) to operate at mild temperatures makes it especially interesting for oleochemical processes, such as the refinement of non-edible oils or waste oils. In this study, the potential of OSN for solvent recovery and deacidification is investigated by means of a model-based process analysis. On the basis of optimized membrane cascade configurations the OSN process is compared to the conventional reference process in terms of energy requirements and costs to judge on the competitiveness. It is shown that the energy demand for the recovery of extraction solvents can be reduced by more than 70% using an OSN-assisted evaporation process. While the operating costs are significantly reduced, the investment costs are increased in comparison to a classical evaporation process. The process analysis also shows that OSN is a promising alternative for deacidification of various low-quality oils using multi-stage membrane cascades. Optimal process design allows to upgrade the triglycerides to the necessary purity, e.g. for processing them in biodiesel production, while the free fatty acids are recovered as valuable by-product that enhance the profitability. Hence, OSN constitutes a valuable processing technology that can be integrated into upstream separation and recovery processes of the oleochemical industry.

Characterisation of Organic Solvent Nanofiltration membranes in multi-component mixtures: Process design workflow for utilising targeted solvent modifications

Organic Solvent Nanofiltration (OSN) is a pressure-driven membrane separation process for separating liquid organic solvents. It can potentially be applied in chemical, petrochemical and pharmaceutical industries. In contrast to well-established design approaches for conventional unit operations such as distillation, design methods for integrating OSN in chemical production processes are very limited. The reason is a lack of information on expected OSN membrane performance as well as missing general understanding of the complicated transport mechanism through OSN membranes. Therefore, this paper introduces a workflow that accounts for the current limited information and makes a general process design feasible. This includes a solvent/membrane selection in early stages of process design beyond state-of-the art OSN membrane screening experiments that so far do not address solvent variations. As the interaction between solute, solvent and membrane material on the separation behaviour of the membrane is substantial, an integrated selection of membrane and solvent for a targeted rejection of a solute might lead to large improvements. Within the developed four-step design workflow, also the integration of design tools such as membrane rejection maps (MRM), membrane modelling maps (MMM) and OSN membrane cascade based optimisation are addressed. Targeted solvent modifications/additions may therefore result in better flux and/or rejection properties of Puramem™280 and GMT-oNF-2 and hence, in reduction of production costs. The design approach is demonstrated on a case study of industrial importance which is the recycling of homogeneous catalysts.

Modeling of membrane separation and applying combined operations at biosystems

The importance of the treatment of water and wastewater has been steadily increasing because of the ever greater demands to eliminate environmental pollution. Pressure-driven membrane separation processes, including ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), have been widely used in water and wastewater treatment and are applied on an industrial scale worldwide. The aim of our paper is to introduce the results of our research team on this field. The main research area within the membrane separation was the reduction of resistances. The effect of ozonation, vibration and application of dolly particles were examined in our scientific works.

Separation Membranes Constructed from Inorganic Nanofibers by Filtration Technique

Nanofibrous materials have been extensively investigated and used as building blocks for various nanodevices, due to their unique one-dimensional structures. Recently, novel membranes constructed by using nanofibers have been reported by various techniques. Here, we will give a critical review of our recent research on the general solution processed unique sub-3 nm thin metal hydroxide nanofibers and their application for constructing ultrathin separation membranes via filtration technique. The superior separation performances of these membranes hold the promising future for pressure-driven membrane separation processes.

Mass transfer in pressure-driven membrane separation processes, Part II

Following on from the work presented in Part I, this Part II paper presents a new mathematical description for water and solute transfer in pressure-driven membrane separation processes, the Molecular Trap (MT) model. This model incorporates the outcomes from the Analytical Solution-Diffusion Pore-Flow (ASDPF) model, which explains the effects of the operational conditions on membrane microstructure and the permeated fluid physical properties, with the solute molecular properties. The new theory, which is found to be qualitatively in agreement with the experimental findings, may be useful for further process understanding and future development.

Mass transfer in pressure-driven membrane separation processes, Part I

An introduction to new theoretical approaches to describe the mass transfer in pressure-driven membrane separation processes is presented. This theory consists of two novel analytical models, namely the Analytical Solution-Diffusion Pore-Flow (ASDPF) model, presented in Part I, and the Molecular Trap (MT) model described in Part II from this work. These two models incorporate the membrane micro-structural parameters, the solute molecular properties, with the feed solution physical properties as well as the operational conditions into one rigorous mathematical description. The ASDPF model is proposed to explain the effects of the operational condition, namely the hydraulic pressure and the temperature, on membrane microstructure and the pure solvent physical properties, from which the permeability coefficient is obtained. The MT model further develops that by considering the molecular properties of the solute when introduced to the solvent in order to obtain the final system permeability. The new approaches are applied for several calculation examples, which are found to be qualitatively in agreement with the experimental findings.

Membrane mass transport by nanofiltration: Coupled effect of the polarization and membrane layers

The effect of the concentration boundary layer on the pressure-driven membrane separation like nanofiltration or ultrafiltration is well-known and extensively discussed in the literature. In most of these studies, the effect of the boundary layer and that of the membrane layer on the separation efficiency are discussed separately. This paper presents a general model to describe the convective and diffusive mass transport taking into account the simultaneous effect of both the concentration boundary layer and the membrane layer. It has been demonstrated how the Peclet number of the membrane, the ratio of diffusive mass transfer coefficient of the membrane to that of the polarization layer and the product of the steric partition coefficient and the convection hindrance factor affect the permeate concentration or the rejection coefficient. Comparing the measured and calculated data, it has been proven that the effect of the polarization layer can significantly affect the separation efficiency during practical nanofiltration of uncharged compounds. The model developed gives a general treatment method to evaluate pressure-driven membrane separation processes.

Magnetic resonance imaging and 3D simulation studies of biofilm accumulation and cleaning on reverse osmosis membranes

Reverse osmosis (RO) is one of the multiple pressure-driven membrane separation processes used primarily for the production of high purity water for various industries, including food processing. Biofilm growth in the spiral-wound membrane module, commonly referred to as biofouling, reduces the efficiency to produce water. Biofilm accumulation and removal using chemical cleaning on RO membranes were studied using magnetic resonance imaging (MRI) techniques. Additionally, a previously validated biofilm simulation model, which is based on a lattice Boltzmann platform, was modified to account for cleaning operations. The spatial and velocity MRI experimental results captured biofilm distribution and water flow within the fouled membrane modules and subsequent changes in the biofilm distribution and water flow due to cleaning. Cleaning was simulated by accounting for reductions in the biofilm cohesive strength in the numerical model. Qualitative and quantitative comparisons between the experimental and simulated images showed good agreement.

Experimental approaches to feed solution permeability in pressure-driven membrane separation processes

This study demonstrates the application of the solution-diffusion pore-flow fluid-resistance (SDPFFR) model, which is used to describe solvent transfer in pressure-driven membrane separation processes. The experimental work is carried out in a reverse osmosis (RO) setup by using two different membranes, TFC-HR and TFC-ULP, manufactured by Koch Membranes Systems, Inc. (USA). Characterization for these membranes by using the transmission electron microscopy (TEM) is also presented. Aqueous solutions of sodium chloride and ammonium bicarbonate were used as feed at 15, 25, and 35 °C. The results show that water transfer in the feed side of the membrane is controlled by the molecular properties of the solute in addition to the operational conditions. Water permeability in the feed solution has been observed to increase as the solute hydration strength or concentration decrease, and as the temperature and the hydraulic pressure increase. The validated model explains the interaction between water permeability in the feed solution and water permeability inside the membrane material. The model shows good agreement with the experimental results. It provides good description for solvent transfer in membrane separation processes and may be useful for future development and design applications, when based on experimental data.