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Osmotic equilibrium in the forward osmosis process: Modelling, experiments and implications for process performance
... The π (osmotic pressure, bar) and D (diffusion coefficient, m 2 /s) are function of solute concentrations (C, molar). The values of π and D can be obtained using empirical equations 5 and 6 (Phuntsho et al., 2014), when using NaCl as the solute and under constant temperature of 25 C. The equations were derived using OLI Stream Analyser 3.2 (5) ...
... 13 and 14). The empirical relations between ρ and µ and C and solute concentration are as follow (Phuntsho et al., 2014): ...
... Counter-current flow orientation shows slightly higher average flux in comparison to the co-and the cross-currents in ALFS-mode (Figure 2 and Table 3), which is in line with other findings (Jung et al., 2011;Phuntsho et al., 2014;Gu et al., 2011). Similar trend is also found for the ALDS-mode, with rather higher flux magnitudes. ...
In forward osmosis (FO), a semi-permeable membrane separates a concentrated draw and a diluted feed solution. FO has emerges as a promising alternative for various applications. To support further development of FO process, a larger scale optimization is required to accurately envisage the most critical factors to be explored. In this study, we applied a mass-transfer model coupled with the mass conservation and area discretization to simulate the performance of plate-and-frame FO modules (10 sheets of 1x1m). Effects of numerous parameters were simulated: modes, flow orientations (co-, counter-and cross-currents), spacers and spacer properties, membrane parameters and operational parameters. Results show that counter-current flow orientation offers the highest flux with minimum spatial distribution. Module performance can be improved by developing FO membrane through reducing membrane structural (S) parameter and increasing water permeability (A): increasing A-value only significant at low S-value, and vice versa (i.e., for A-value of 1 LMH/atm, S-value must be below 50 µm). Furthermore, inclusion of spacer in the flow channel slightly increases the flux (merely up to 2%). Module performance can also be enhanced by increasing feed flow rate, lowering solute in the feed and increasing solute in the draw solution.
... The π (osmotic pressure, bar) and D (diffusion coefficient, m 2 /s) are function of solute concentrations (C, molar). The values of π and D was obtained from empirical Eqs. 5 and 6 ( Phuntsho et al., 2014), when using NaCl as the solute and at constant temperature of 25 o C. The equations were derived using OLI Stream Analyser 3.2. ...
... 13 and 14). The empirical relations between ρ and µ to C are as follow ( Phuntsho et al., 2014): Because the thermodynamics properties are given as a function of C, it is possible to capture the effect of feed concentration and draw solute dilution as a result of water flux and reverse salt flux inside the module. p-ISSN 2528-1410 e-ISSN 2527-8045 ...
... A clear advantage is shown by the counter-crosscurrent flow orientation. However, the magnitude of flux advantage is not as high as reported elsewhere ( Jung et al., 2011;Phuntsho et al., 2014;Gu et al., 2011;Bilad, 2016). For the corresponding flow orientation, the spatial flux ranges are about the same. ...
Forward osmosis (FO) is an attractive technology that offers advantages especially for treatment of challenging feeds in comparison to other membrane technologies. Substantial developments of membrane material have been shown recently. To support further development of FO process, a larger scale study via membrane module development is required to accurately envisage the most critical factors to be exploited to realize the promises. In this study, we applied a mass-transfer model coupled with the mass conservation and area discretization to simulate the performance of modified spiral-wound (MSW) modules (10 sheets of 1x1m). The study focuses on the spatial flux profile in a full-scale module as function of operational mode: co- vs counter cross current and membrane orientations (active-layer facing feed (ALFS); solution and active layer facing draw solution, (ALDS)). Results show that all modes offer almost similar average flux of about 9-10 L/m2h, but the co-current flows have much higher flux ranges (≈43%). The latter is expected to worsen membrane fouling resistant due to mal distribution in hydraulic loading. An operation with counter current and ALFS and counter current flow is then recommended because it offer similar flux but lower spatial flux ranges (7%).
... Membrane characteristics for the FO and RO processes [20,26,47,52,53]. FO [44][45][46]. ...
... Although the RO module employed in the present study has been commercialized, the FO module is still under development. Therefore, in the present study, the FO module design was based on a plate-and-frame FO membrane module with a cellulose triacetate-based membrane used to simulate the FO process [47][48][49][50][51]. The RO membrane design was also taken from a past study [26]. ...
... In practical terms, the FO process operates by concentration (or osmotic pressure) differences between the feed solution (FS), which features a low-salinity solution (i.e., low osmotic pressure), and the draw solution (DS), which feature a high-salinity solution (i.e., high osmotic pressure) [12,13]. As a consequence, FO requires a highly concentrated draw solution to induce the driving force for separation, and hence, the water molecules will start moving from the feed to the draw solution [14,15]. During this process, the majority of the dissolved molecules or multivalent ions already present in the feed water are retained by the membrane. ...
... Whereas DS is diluted as water continues to be transported across a membrane, that is, the osmotic pressure of DS decreases and the osmotic pressure of FS increases until it reaches its osmotic equilibrium point. This will determine the final concentration of the dilute DS, which needs an additional process to remove the draw solute and produce pure water, as the product (i.e., dilute DS) cannot be consumed directly as freshwater [15]. A schematic representation of a FO process concept is shown in Figure 3. Thereby, membrane FO and draw solution are certainly the heart of the FO system and play an important role in moving FO forward to commercialization. ...
Forward osmosis (FO) is a low-energy treatment process driven by osmosis to induce the separation of water from dissolved solutes/foulants through the membrane in hydraulic pressure absence while retaining all of these materials on the other side. All these advantages make it an alternative process to reduce the disadvantages of traditional desalination processes. However, several critical fundamentals still require more attention for understanding them, most notably the synthesis of novel membranes that offer a support layer with high flux and an active layer with high water permeability and solute rejection from both solutions at the same time, and a novel draw solution which provides low solute flux, high water flux, and easy regeneration. This work reviews the fundamentals controlling the FO process performance such as the role of the active layer and substrate and advances in the modification of FO membranes utilizing nanomaterials. Then, other aspects that affect the performance of FO are further summarized, including types of draw solutions and the role of operating conditions. Finally, challenges associated with the FO process, such as concentration polarization (CP), membrane fouling, and reverse solute diffusion (RSD) were analyzed by defining their causes and how to mitigate them. Moreover, factors affecting the energy consumption of the FO system were discussed and compared with reverse osmosis (RO). This review will provide in-depth details about FO technology, the issues it faces, and potential solutions to those issues to help the scientific researcher facilitate a full understanding of FO technology.
... K D and K f can be calculated using the following expressions [30,41,42]: ...
... The module specification and operating specification of the HFFO.6 membrane are tabulated in Table 2 [31]. D D and D F are the diffusion coefficients of the draw and feed solution, respectively, which can be calculated using the following formulae [30,31,37,41,42,[44][45][46]. 33 (for laminar flow) ...
The design and operation of FO systems require accurate prediction of performance according to the operating parameters. Performance parameters such as water flux, reverse solute flux (RSF), specific reverse solute flux (SRSF), recovery and rejection are influenced by operating parameters including temperature, concentration difference (CD), and flow rates of feed solution (FS) and draw solution (DS). Whereas previous studies analysed the effect of only one or two operating parameters on one or two output parameters, this study provides a complete investigation into the effects of all operating parameters on all output parameters important for performance. A bench-scale FO setup using an advanced FO membrane and NaCl draw solution has been studied and compared to a predictive model, giving agreement within 10 %. Water flux and RSF increased with temperature, CD and flow rate. SRSF decreased with temperature, increased with CD, and increased as flow rates of FS and DS increased. Increasing temperature or CD increased the recovery; whereas increasing the flow rate of FS or DS reduced the recovery. Rejection dropped at higher CD and flow rates. Hence, it is recommended to operate at lower CD and flow rates to achieve higher recovery and rejection.
... It should be mentioned that for estimation of the TCF, equations applied to the RO process were used as an approximation as the temperature effect on the PRO process was not evaluated in this study. Eq. (4) [72] was used to calculate osmotic pressure from an NaCl concentration (mol L − 1 ). For the calculation of π D, m and π F, m , C D, m and C F, m were used, respectively, in Eq. (4). ...
... where C D, m and C F, m are the concentrations on the membrane surface on the draw and feed sides considering ECP and ICP [73,74], C D, av and C D, av are the average concentrations on the draw and feed side, J s the reverse solute flux, k D and k F the mass transfer coefficients on the draw and feed side, and C D, in and C D, out are the input and output concentrations on the draw side. β is the dimensionless Van't Hoff factor for strong electrolytes (β = 2 for NaCl) [72], R is the gas constant (8.31 J mol − 1 K − 1 ) and T is the absolute temperature (in K) of the solution, taken as 25 <circ>C for both solutions (DS and FS) in this study. It should be noted that the proposed methodology is able to simulate PRO systems with different temperatures by considering T dependent equations for D, ρ and μ. ...
Pressure retarded osmosis (PRO) is a process that is able to convert a salinity gradient into electrical energy through a turbine. This process has gained attention as a possible renewable energy technology for integration into desalination plants to improve their energy efficiency. Despite recent efforts, PRO is not yet commercially available due to drawbacks related to, among others, PRO membrane and module development. The aim of this study is to provide a simulation tool for full-scale PRO systems that allows accurate estimates of PRO-related energy generation to be made. The proposed tool enables analysis of single-stage systems with PRO modules in series and the setting of boundary conditions per module in terms of maximum flux recovery, and maximum and minimum feed/draw flow. The HTI OsMem™ 2521 spiral wound membrane module (SWMM) was evaluated considering an 8 in. diameter (high active area). Increasing the number of SWMMs in series was found to increase permeate flow and the energy that can be generated, even when considering the pressure drop on both draw and feed side and the effect of the dilution and concentration of the draw and feed solutions. The proposed tool allows to determine the safe operating windows and operating points for maximization of energy generation for fixed and variable operating conditions.
... The osmotic pressure is calculated using Eq. (5) which describes a non-linear relationship between the osmotic pressure and concentration of NaCl [26]. This relationship is obtained from fitting the simulated data in OLI STREAM software for a temperature of 25°C. ...
... The diffusivity coefficient was estimated using the following [26]: ...
... Using the concentration gradient across the selective layer, the actual osmotic pressure gradient across the selective layer (∆π s ) can also be calculated [39]. Then the water flux is evaluated by the solution diffusion equation as follows, ...
... According to this configuration, the effective membrane area per pressure vessel is 31.5 m 2 (which is the membrane area of a single CTA hollow fibre module used in this study). However, for a small membrane area (flat sheet membrane, width 1 m, area less than 40 m 2 ) performance is almost similar for both co-current and counter-current flow directions according to our previous study [39]. Since the membrane area of the current module is smaller than 40 m 2 , both flow directions will perform almost equally. ...
This study is aimed at developing system mathematical design models to simulate and optimize a full scale forward osmosis (FO) for a hollow fibre membrane module for energy efficient desalination. Experimental data from a commercial outer selective CTA hollow fibre FO membrane module was used for validation. Less than 10% difference between the simulation and experimental results were observed which validated the reliability of the models. Simulation and design were performed for a 1000 m³/day FO plant using 0.6 M NaCl as draw solution (DS) (~seawater) and 0.02 M NaCl feed solution (FS) (~MBR effluent) to produce 0.25, 0.2 and 0.15 M NaCl diluted seawater to reduce the energy consumption of downstream pressure driven desalination process. A single element parallel module arrangement was found more suitable for this commercial hollow fibre membrane element. Finally, the numerical simulations revealed that to achieve 0.25, 0.20 and 0.15 M final DS concentrations, the optimum number of modules required were 370, 435 and 555 respectively considering membrane cost and energy consumption. The FO system using the commercial CTA hollow fibre module was found more energy efficient than a commercial TFC spiral wound membrane module.
... Despite the increase in water permeates, the normalized feed volume at the outlet increased with increasing feed flow rates. This observation corresponded with previous findings, which slightly improved water flux by feed flow increase and did not result in higher water recovery due to a relatively high feed flow rate compared to the permeate improvement [22,33]. ...
... Despite the increase in water permeates, the normalized feed volume at the outlet Membranes 2020, 10, 318 9 of 17 increased with increasing feed flow rates. This observation corresponded with previous findings, which slightly improved water flux by feed flow increase and did not result in higher water recovery due to a relatively high feed flow rate compared to the permeate improvement [22,33]. ...
We compared two representative forward osmosis (FO) modules—spiral-wound (SW) and plate-and-frame (PF)—to provide practical information for the selection of FO element for a large-scale FO process. The FO operating performance of commercially available SW FO and PF FO was explored under different membrane area and flow rate conditions. The performance trend as a function of the membrane was obtained by adjusting the number of serially connected elements. Although SW FO and PF FO elements exhibited comparable feed pressure drops, SW FO demonstrated a significantly higher draw channel pressure drop than PF FO. Furthermore, the significant draw pressure drop in SW FO increased the draw inlet pressure, consequently limiting the number of serially connected elements. For example, the maximum number of serially connected elements for the normal operation was three elements for SW FO (45.9 m2) but nine elements for PF FO (63 m2) when the flow rate of 10 LMP was applied for feed and draw streams. Additionally, a footprint analysis indicated that SW FO module exhibited a slightly larger footprint than PF FO. Under investigated conditions, PF FO exhibited relatively better performance than SW FO. Therefore, this pilot-scale FO study highlighted the need to reduce the flow resistance of SW FO draw channel to take advantage of the high packing density of the SW element.
... For the single-stage concentration, the filtration was done without replenishment of the DS until the system reached osmotic equilibrium [32]. For the multi-stage concentration, the initial FS was set at 300 mL. ...
... Based on the value of the final water flux, it does not reach the osmotic equilibrium yet, hence further concentration of the PW is still possible. However, operation under low osmotic difference leads to low flux because this condition occurs near the osmotic equilibrium when the osmotic pressure of the FS is almost equals to the osmotic pressure of the DS [32]. If a full-scale FO system is operated under this condition at given capacity, the low flux must be compensated with large membrane area that inflates the membrane investment cost. ...
Produced water (PW) generated from oil and gas production is a threat to the environment if not treated properly. Conventional methods for PW treatment are often accompanied by a series of treatments to fulfill the discharge standard. Forward osmosis (FO) is a promising option due to its high solute retention, less irreversible fouling, low energy footprint and potentially used as a standalone unit. However, FO still suffers from the low flux and fouling when treating highly contaminated feeds. This study investigated fouling control in the FO system for concentrating PW by using seawater as a draw solution (DS). A multi-stage filtration system (via via replenishments of the DS) with an aeration and module inclination for fouling mitigation was proposed to improve concentration factor (CF) and flux. Results showed that the multi-stage concentration offered higher fluxes range of 1.72–15.48−1.72 L/(m²h) (LMH) and four times of CF than the single-stage one with fluxes range of 0.39–9.49 LMH corresponding to CF of 1.75. The aeration was effective to enhance the water flux and suppress the fouling, and showed a significant impact at the rate of 0.4 L/min, reaching flux increment by 11 times at a rate of 1 L/min. The impact of aeration was enhanced by inclining the filtration cell up to 5 times at the inclination angle (θ) of 90° due to the improved contacts of air bubbles with the membrane surface. The contribution of the aeration and cell inclination on the water flux can be explained through the forces acting on moving air bubbles.
... Such an approach of operating offers the double benefit of (i) avoiding elevated flux at the start of the concentration process, limiting the fouling propensity and (ii) to still maintain a flux at the end of the concentration process instead of having difficulties when reaching the osmotic equilibrium [248,249]. Phuntsho et al. demonstrated that operating in counter-current allows for a higher average permeation flux and water extraction rate for the same filtration area [250,254]. Even more interesting is the fact that this mode of operation allows us to shift the osmotic equilibrium to increase the extraction capacity of a draw solutes and therefore the concentration capacity of the feed stream [254]. ...
... Phuntsho et al. demonstrated that operating in counter-current allows for a higher average permeation flux and water extraction rate for the same filtration area [250,254]. Even more interesting is the fact that this mode of operation allows us to shift the osmotic equilibrium to increase the extraction capacity of a draw solutes and therefore the concentration capacity of the feed stream [254]. As a final step for concentration, pressure assisted osmosis may be used to overcome osmotic equilibrium limitations and reach a higher concentration level if required [255]. ...
In the past few years, osmotic membrane systems, such as forward osmosis (FO), have gained popularity as “soft” concentration processes. FO has unique properties by combining high rejection rate and low fouling propensity and can be operated without significant pressure or temperature gradient, and therefore can be considered as a potential candidate for a broad range of concentration applications where current technologies still suffer from critical limitations. This review extensively compiles and critically assesses recent considerations of FO as a concentration process for applications, including food and beverages, organics value added compounds, water reuse and nutrients recovery, treatment of waste streams and brine management. Specific requirements for the concentration process regarding the evaluation of concentration factor, modules and design and process operation, draw selection and fouling aspects are also described. Encouraging potential is demonstrated to concentrate streams more than 20-fold with high rejection rate of most compounds and preservation of added value products. For applications dealing with highly concentrated or complex streams, FO still features lower propensity to fouling compared to other membranes technologies along with good versatility and robustness. However, further assessments on lab and pilot scales are expected to better define the achievable concentration factor, rejection and effective concentration of valuable compounds and to clearly demonstrate process limitations (such as fouling or clogging) when reaching high concentration rate. Another important consideration is the draw solution selection and its recovery that should be in line with application needs (i.e., food compatible draw for food and beverage applications, high osmotic pressure for brine management, etc.) and be economically competitive.
... Whilst on another side, the DS is watered down as the transfer continues of water through a membrane until it reaches a state of osmotic convergence between two sides, at which time the osmotic pressure serving as a driving force vanishes. This would define the dilute DS's final concentration, which directly impacts how the water ultimately produced will be used [11]. Thereby, FO process efficiency primarily depends on two main criteria which are the existence of the FO membrane and DS with optimal specifications. ...
The environmental and economic costs are pivotal aspects that should be considered before the commercialization of any membrane separation-based process. Particularly, realizing this goal relies entirely on the key role played by fabrication and operational parameters. Herein, we introduced a systematic investigation for constructing thin-film composite (TFC) - forward osmosis (FO) membranes for direct fertigation applications. To the best of our knowledge, PVC membrane was rarely used as a porous top surface substrate for TFC-FO membranes. The PVC substrate porous structure and polyamide (PA) active layer were manipulated by variating their concentrations and reaction time to optimize the FO membrane performance. All necessary characterization tools were employed to visualize the fabricated TFC membrane's surface characteristics. The influence of feed solution concentration, membrane orientation, flow rate, and draw solutes on the FO work was also methodically tested. Results demonstrated that the porous structure-substrate membrane host polymer content had imparted a significant role in tailoring the TFC-FO performance concerning permeability and back-solute flux. The water flux was improved by 44.3 % with decreasing of the PVC amount, whereas water flux was improved by 48.13 % with decreasing of the M-phenylenediamine monomer (MPD) content from 3 to 1 wt% at constant tri-mesoylchloride monomer (TMC) content of 0.1 wt% in IP reaction conditions of a thin film. This was gone hand in hand with optimal operational conditions harnessed for optimizing the FO process.
... In addition, crossflow direction plays an important role in the FO process. Operating the FO process in a counter-current crossflow mode offers several advantages over the cocurrent mode such as gradual decrease in the water flux, higher average water flux and higher water extraction capacity of the DSs (Phuntsho et al., 2014). Thus, the countercurrent crossflow mode is exclusively applied throughout this thesis. ...
Resource recovery from wastewater has attracted significant attention in the circular economy. Among different technologies, forward osmosis (FO) as an emerging membrane technology has the potential to transform municipal wastewater treatment plants into factories for water, nutrients and energy production by concentrating very diluted municipal wastewater for direct nutrients recovery and subsequent water and energy recovery. Although FO membrane has much lower fouling propensity than reverse osmosis (RO) due to much lower pressure applied to membrane, fouling is still one of the barriers to the FO application. The first technical chapter of this thesis investigated the concentration of synthetic and real municipal wastewater to 90% water recovery rate by two different configurations, i.e., hollow fiber and flat sheet thin film composite (TFC) FO membranes, and their associated membrane fouling and cleaning. Results show that the FO membrane had high rejection rates of COD, phosphate, Ca<sup>2+</sup> and Mg<sup>2+</sup> with concentration factors at around 8 when achieving a 90% water recovery rate, which facilitated downstream phosphate recovery by direct precipitation and energy production by anaerobic digestion (AD). The significantly increased Ca<sup>2+</sup> and phosphate concentrations after FO filtration with 90% water recovery was found to be the main factor contributing to inorganic scaling, which was harder to be cleaned by physical cleaning compared with cellulose particles used to simulate suspended solids (SS) in synthetic wastewater. Both hollow fibers and flat-sheet configurations are found not to be suitable for treatment municipal wastewater without prior SS removal although flat sheet configuration was still able to achieve a 90% water recovery rate but with water flux of only 7.5 L/m<sup>2</sup>·hr at 90% water recovery rate while hollow fiber FO was completely clogged with operation failure. The use of a spacer in the flat sheet configuration did not alleviate membrane fouling during the membrane filtration process, but it improved the efficiency of the following physical cleaning by around 15%. This study highlighted the importance of the chemistry of feed solution (FS) and draw solution (DS) and FO membrane configuration on membrane fouling particularly at high water recovery rates and the necessity of pre-treatment of municipal wastewater. For wastewater treatment with FO, it has been widely reported that FO has very low ammonium rejection primarily due to the negative charge nature of membrane. This restricts the application of FO for wastewater treatment particularly with a purpose of ammonium and water recovery. The second technical chapter in this thesis thus aimed to enhance ammonium rejection in the FO process by selecting DSs with different physiochemical characteristics. Results show that under the same osmotic pressure divalent cation DS (i.e. ii Mg<sup>2+</sup>) with larger hydrated radius resulted in a higher ammonium rejection than monovalent cation DS (i.e. Na<sup>+</sup> ) with lower hydrated radius. For the same cation based DS, DS with lower diffusion coefficient showed higher ammonium rejection. These results imply that impeding the exchange of cation in DS with ammonium in FS by increasing cation radius or reducing diffusion coefficient could minimize the ammonium permeation to DS. Non-ionic DS such as glucose, glycine and ethanol are able to minimize cation exchange between feed and draw solutions, leading to a 98.5-100% ammonium rejection rate. This further validate that even with negative charge of membrane, cation exchange between FS and DS is critical for ammonium permeation. The treatment of filtered municipal wastewater and sludge digestate with glucose and NaCl as DSs, respectively, proved that the non-ionic nature of glucose resulted in a high rejection rate not only for ammonium, and other cations such as Ca<sup>2+</sup> and Mg<sup>2+</sup> but also for anion such as PO<sub>4</sub><sup>3+</sup>. This is for the first time that a generic guideline could be proposed based on cation exchange mechanism for the selection of DS in wastewater treatment especially when ammonium and water recovery is a concern. To recover water from municipal wastewater, FO is usually needed to combine with other technologies. Membrane distillation (MD) offers a promising solution for simultaneous water recovery from DS and DS regeneration when waste heat is easily available. Although the integrated FO-MD system was studied in literature, temperature effects and heat balance in such integrated system are unclear. In the third technical chapter, the effect of FS and DS temperatures was examined. In addition, ammonium permeated from FS to DS was studied in the MD configuration for its potential contamination to the recovered water. It is found that higher FS and DS temperatures resulted in a higher water flux and a higher RSF from either NaCl or glucose as DS due to the increased diffusivity of molecules. However, the water flux increased at a higher rate with glucose DS than with NaCl DS by 10-14.8%, while the RSF increase rate with NaCl DS was two times higher than glucose DS. In addition, the use of NaCl DS at higher DS and FS temperatures such as 50 and 42 °C, respectively, resulted in more ammonium permeation from the FS to the DS, whereas ammonium was completely rejected with glucose DS even at high temperature. These results are very important because in the integrated FO–MD system at higher temperatures such as 50 °C, the advantage of high flux of NaCl as DS was not increased significantly while the disadvantages of high RSF of NaCl and lower ammonium rejection were greatly amplified. On the contrary, the disadvantage of low water flux of glucose as DS was overcome at 50 °C while 100% ammonium rejection was still maintained. This implies that temperature needs to be an important factor for the consideration in selecting DS in FO-MD system. Furthermore, a simple heat balance calculation suggests that if there was no heat recovery in the FO-MD system, waste heat from power plants is far less than the heat iii demand in such systems for municipal wastewater treatment. An internal heat exchange between different streams was proposed to maximize the efficiency of heat utilization, but unarguably it would increase the complexity of the FO-MD system. Further study is needed regarding heat efficiency. Overall, this project shows that the FO membrane fouling, the associated cleaning methods and ammonium rejection can be enhanced by understanding the feed and draw solutions chemistry. In addition, this project highlights the importance of the operation temperature when selecting the DS and the necessity of internal heat recovery in the FO-MD system.
... The mode has a great potential for irrigation purposes with low energy consumption compared to other desalination processes. Additionally, FDFO processes can be integrated with solar energy or other renewable energy especially in coastal areas having water scarcity issues[29,210]. ...
Agriculture consumes about 70% of the total available freshwater withdrawals around the world. Further, rapid population growth and industrialisation leads to the increased demand for water and putting additional stress on the limited sources of water. To meet the increasing demand, the efficient use of water and forming alternative sources. Membrane technologies can probably play a significant role in recycling wastewater as well as creating an alternative source of water. However, these technologies require higher power to operate them. A significantly low energy consumption membrane technology will be beneficial for wastewater treatment and agriculture thereby reducing the impact on climate change.
Lately, forward osmosis (FO) has grown interest in membrane technologies that use the natural osmosis principle utilizing the osmotic gradient of the solutions which does not require hydraulic pressure. Thus, the FO process requires lower power necessary for recirculating the solutions across the membranes. Generally, in FO processes, clean water is extracted from a lower concentrated feed solution (FS) through a semi-permeable membrane to a highly concentrated draw solution (DS). The issues of fouling in the FO process are less challenging compared to reverse osmosis (RO) processes. Nevertheless, the lack of suitable DS for the FO processes in wastewater treatment is a challenge. The diluted DS still cannot be directly utilised for drinking which requires additional treatment requiring energy and resources making the FO process highly challenging compared to other membrane technologies.
The uniqueness in the FO process lies in the applications which can either directly use the diluted DS with the draw solutes or in applications that do not require total removal of draw solutes. Thus, the FO process for textile wastewater treatment is suggested with the use of fertilizers as DS in which the diluted fertilizer solution can be used for processes like fertigation. This concept can be applied for textile wastewater treatment which consumes large volumes of water thereby generating higher quantities of wastewater and the use of fertilizers as DS can avoid the requirement of additional treatment stages for the recovery of raw solutes. The aim of this study is hence to study the FO process for its applications in textile wastewater treatment and fertilizers as DS for fertigation, investigate the impacts of various process parameters influencing the performance of the FO process, model and optimise the FO process for various performance factors and to evaluate the techno-economic feasibility for pilot-scale studies. The study has been discussed in eight chapters which describe the FO process, modelling and optimisation of FO performance to identify the suitable fertilizer DS for textile wastewater treatment by investigating the various process variables through experimental design and modelling using machine learning techniques. Further to investigate the feasibility studies for pilot-scale studies.
The initial experimental investigation of the FO process was carried using response surface methodology (RSM) for prediction and optimization of FO process using low concentration DS. The process variables were FS temperature, DS concentration & FS concentration. These variables were subjected to regression models and had R2-values > 0.9. FS temperature had a negative impact on the weight transfer from DS, while FS/DS concentration had a direct correlation to the performance. At FS temperature of 29°C, DS at 13986 mg/L and FS at 1702 mg/L produced the best results thereby lowering the DS regeneration costs. At this condition, the models were experimentally validated confirming the predictive ability of the models.
Further study was carried by using Potassium Chloride (KCl) as DS for treating textile wastewater as FS. The effects of FS temperature, pH, FS, and DS concentrations were investigated. The water flux, reverse salt flux, and specific reverse salt flux was studied for these parameters. DS and FS properties, osmotic potential, and temperature played a vital role in the performance of FDFO. At the FS temperature of 30°C, the water flux of 5.5 LMH was found to be the highest. Reverse salt flux increased due to the increase in solute diffusivity. The highest water flux was obtained for a DS of 1.150M and an FS of 1000 mg/L. The water permeation improved due to the difference in DS and FS concentrations for the pH above 7. The results of FDFO suggested that KCl as DS has a higher potential for the treatment of textile wastewater at a higher temperature of 30 °C. The influences of these variables were considered for the modelling of the FO process using machine learning techniques.
From the observed influence of various factors on the performance of FO process was applied for the treatment of textile industry wastewater using fertilizer as DS by modelling and optimisation of FO process using machine learning techniques like Response Surface Methodology (RSM), Artificial Neural Network (ANN) and Adaptive Neuro-Fuzzy Inference System (ANFIS). To model the FO process, a central composite design was utilized to examine the effect of initial draw concentration, initial feed concentration, time, initial feed pH and temperature on the water flux and reverse salt flux. The optimum water flux (8.527 LMH) and reverse salt flux (7.246 GMH) was obtained using an initial draw concentration of 1.625 M, initial feed concentration of 1090 mg/L, the reaction time of 90 mins, initial feed pH of 7.33 and temperature of 35°C. Under these conditions, FO performance was carried out experimentally and validated with the models. The model developed for the FO process by ANN and RSM was considerably better than that of other models in terms of precision of predicting the water flux and the reverse salt flux, respectively. About six different chemical fertilizer solutions were screened and tested at optimum conditions to identify the best suitable fertilizer DS for the FO process using textile wastewater. The results indicate that Diammonium Phosphate (DAP) along with Potassium Chloride (KCl) fertilizer as DS gave a better performance concerning water flux.
In the techno-economic study, 1.625 M of KCl was used as fertilizer DS and textile wastewater as FS. The optimum FS concentration of 1091 mg/L and FS temperature of 35°C provided the highest water flux (8.527 L/m2h) thereby requiring $0.392/m3 low unit cost of treatment. Sensitivity analysis determined that increasing the FS temperature decreases the water cost. This determines that the FS temperature highly sensitive for unit treatment cost. Further, it was found that Nitrogen/ Phosphorus based fertilizers had lower water cost compared to Potassium/ Phosphorus based fertilizers.
Finally, this study recommended additional investigations using commercial fertilizers as DS to enhance the performance of the FO process. Further, a pressure-assisted osmosis process was recommended to improvise and enhance the water flux generation with lower power consumption than RO/NF processes. Also, other recommendations included the study on membrane fouling and real-time pilot-scale application using various membranes for the treatment of textile wastewater.
... Thus, this model needs to be developed for more complex processes. The two main polarizations that need to be included are internal and external concentration polarization [39,40]. The water flux model that considers the two polarizations is written in Eq. (2) as follows, ...
The need for lithium as a raw material for battery production in electric vehicles has triggered the growth of the lithium industry throughout the world, resulting in massive competition for the exploitation of lithium. Responding to these challenges, lithium recovery technology continues to be developed, one of which is membrane technology. This research focuses on the use of forward osmosis (FO) technology. The search for the best operating condition parameters for the process highlights a major concern. The condition parameters include temperature, draw solution concentration, and flow rate. The temperature varied from 30, 33, 36, 39 to 42 °C, the draw solution concentration varied from 1, 2 to 5 M, while the flow rate varied by 2, 3 and 4 L h−1. The best conditions were obtained at a temperature of 42 °C, a concentration of 5 M draw solution, a flow rate of 4 L h−1 with a flux of 68.47 L m−2 h−1, a normalized concentration ratio of 3.31, and an average solute rejection of 79.25%. Meanwhile, the most suitable osmotic pressure model to explain the phenomenon in the FO process is the Extended Pitzer.
... The selected membranes were applied in the FO performances of four various fertilizers DSs as examples for the desalination of brine H2O for irrigation, the chemical features for the chosen draw fertilizer were obtainable in Table 3. can be attributed to the higher diffusion coefficient of monovalent electrolytes as compared to bivalent electrolytes and trivalent electrolyte [Corzo et al., 2017]. DS can extract H2O from the FS tank when the osmotic pressure of the DS reaches stability with the osmotic pressure of the FS [Phuntsho et al., 2014]. The performance of fertilizer DSs in terms of RSF varied widely depending on the type of fertilizers also is shown in Fig. 19a The divalent fertilizer draw solutions (NH4)2SO4 and K2HPO4 have ionic species with a hydrated diameter of PO4 and SO4 relatively much greater than the hydrated diameter of the monovalent fertilizer species (NH4, Cl and K) and ...
... Another alternative to overcome this issue and optimize the process at full scale is to use modules in series operated in counter current mode so to operate at higher and constant osmotic driving force with the concentrated DS in contact with the concentrated LL [23,24]. ...
A forward osmosis (FO) – electrodialysis (ED) pilot was built and tested for landfill leachate (LL) concentration and draw and water recovery. 70% water extraction from high salinity (35mS.cm⁻¹) LL was achieved through FO in batch operation; no fouling or clogging were observed during the operation of the pilot. Operating with constant high salinity draw solution (>100 mS.cm⁻¹) was required to reach 70% water extraction especially for the final step of LL concentration. FO allowed for high rejection (>90%) of most LL compounds and their effective concentration in the concentrated LL except for ammonium which was partially lost through the membrane or by stripping. Lowering the pH of the LL (from 8.3 down to 6.5) allowed to reduce losses by stripping but led to higher overall losses due to longer filtration time required (higher salinity of the feed solution following hydrochloric acid addition for neutralization). Ultimately, tests in continuous mode (one pass through the FO module) demonstrated that ammonium losses can be reduced significantly (below 15%), thus allowing for efficient operation in counter current mode, resulting also in a better usage of the osmotic pressure. Regeneration of the draw solution and production of a dilute water stream within the discharge requirements (<6 ms.cm⁻¹) was achieved by ED. Energy consumption of the FO-ED system remain below 8 kWh.m⁻³ allowing the process to be energetically competitive in comparison with other LL concentration technologies and other FO hybrid systems.
... The dual FO-RO system overcomes the limitations related to the FO and RO each as a stand-alone process. Lower energy demand and lesser membrane fouling have increased research interests in FO for several applications such as pre-treatment for potable and non-potable water reuse, wastewater treatment, food processing, pharmaceutical applications, and pre-treatment and concentrate treatment for RO (Akther et al., 2015;Cath et al., 2006;Coday et al., 2014aCoday et al., , 2014bHickenbottom et al., 2013;Phuntsho et al., 2014;Shaffer et al., 2015;Zhao et al., 2012). The use of FO as a pre-treatment (Xie et al., 2012(Xie et al., , 2015 to RO can reduce the energy consumption of RO and protect RO membrane from fouling and scaling associated with the constituents in wastewater. ...
This study investigated the impact of seasonal variation and operating conditions on recovery of potable quality water from municipal wastewater effluent using an integrated algal treatment process with a dual forward osmosis (FO)-reverse osmosis (RO) membrane system. Pilot study of the algal process treating primary effluent validated the technical viability and seasonal performance during warm weather (May to October, 25–55 °C) using an extremophilic algal strain Galdieria sulphuraria, and during cold weather (November to April, 4–17 °C) using polyculture strains of algae and bacteria. Algal effluents from both seasons were used as the feed solution for the laboratory FO-RO study. In addition, pilot-scale FO-RO experiments were conducted to compare the system performance during treatment of algal effluent and secondary effluent from the conventional treatment facility. At 90% water recovery, the FO-RO achieved over 90% overall rejection of major ions and organic matter using the bench-scale system and over 99% rejection of all contaminants in pilot-scale studies. Detailed water quality analysis indicated that the product water from the integrated system met both the primary and secondary drinking water standards. This study demonstrated that the FO-RO system can be engineered as a viable alternative to treat algal effluent and secondary effluent for potable water reuse independent of seasonal variations and operating conditions.
... Therefore, the datasets should be classified according to the membrane material to prevent unnecessary errors. Previous studies reported that the effect of flow direction is less than 10% [71,110]. Therefore, the results from the dataset classified into TFC and CTA membranes are much more reliable than those from the dataset with all membranes. ...
Operating variables, such as flow direction, velocity, temperature, and osmotic pressure, affect the performance of forward osmosis (FO) systems. Water and salt permeabilities of the FO membrane have been regarded as membrane intrinsic parameters, and numerous studies have fixed the water and salt permeabilities regardless of the operating conditions except temperature. In this study, we collected approximately 700 experimental data entries on FO to analyze whether water and salt permeabilities are indeed membrane intrinsic parameters. The widely scattered water and salt permeabilities in the collected FO experimental data imply that the material of the FO membrane and the operating conditions influence the effective water and salt permeabilities significantly. To investigate the effects of operating conditions on the FO performance quantitatively, a principal component analysis (PCA) method was employed. The PCA results showed that osmotic pressure, temperature, and flow velocity are the main operating variables in the FO system, and the contribution of these variables on FO performance is similar. Unlike the common knowledge in the FO, we confirmed that the flow velocity was as important as the osmotic pressure or temperature. In this study, we revealed a new understanding of membrane intrinsic parameters and the extent of influence of operating conditions.
... At an early stage, the S parameter is called the intrinsic structural parameter or S int , and it is determined by measuring the geometric variables (i.e., membrane porosity, tortuosity, and thickness). This parameter value is constant and remains unaffected by operating conditions [2,6,11,12]. However, this model is questionable and challengeable and should be investigated by varying the operating pressure and temperature [2,13]. ...
The structural (S) parameter of a medium is used to represent the mass transport resistance of an asymmetric membrane. In this study, we aimed to fabricate a membrane sublayer using a novel composition to improve the S parameter for enhanced forward osmosis (FO). Thin film composite (TFC) membranes using polyamide (PA) as an active layer and different polysulfone:polyethersulfone (PSf:PES) supports as sublayers were prepared via the phase inversion technique, followed by interfacial polymerization. The membrane made with a PSf:PES ratio of 2:3 was observed to have the lowest contact angle (CA) with the highest overall porosity. It also had the highest water permeability (A; 3.79 ± 1.06 L m−2 h−1 bar−1) and salt permeability (B; 8.42 ± 2.34 g m−2 h−1), as well as a good NaCl rejection rate of 74%. An increase in porosity at elevated temperatures from 30 to 40 °C decreased Sint from 184 ± 4 to 159 ± 2 μm. At elevated temperatures, significant increases in the water flux from 13.81 to 42.86 L m−2 h−1 and reverse salt flux (RSF) from 12.74 to 460 g m−2 h−1 occur, reducing Seff from 152 ± 26 to 120 ± 14 μm. Sint is a temperature-dependent parameter, whereas Seff can only be reduced in a high-water- permeability membrane at elevated temperatures.
... One of the advantages of these DSs is the fact that NF instead of RO can be applied to regenerate most of these DSs. In addition, it has been claimed that FO-RO or FO-NF processes need less energy than FO alone to desalinate seawater (Cai, 2016); however, this has been called into question by other studies from at least the theoretical aspects (Elimelech & Phillip, 2011;Phuntsho et al., 2014). Organic and inorganic salts along with a wide range of water-soluble polymers have been applied as some of the non-responsive DSs in the FO process. ...
Renewable energy, water conservation, and environmental protection are the most important challenges today. Osmotic membrane bioreactor (OMBR) is an innovative process showing superior performance in bioenergy production, eliminating contaminants, and low fouling tendency. However, salinity build-up is the main drawback of this process. Identifying the microbial community can improve the process in bioenergy production and contaminant treatment. This review aims to study the recent progress and challenges of OMBRs in contaminant removal, microbial communities and bioenergy production. OMBRs are widely reported to remove over 80% of total organic carbon, PO43- and NH4+ and emerging contaminants from wastewater. The most important microbial phyla for both hydrogen and methane production in OMBR are Firmicutes, Proteobacteria and Bacteroidetes. Firmicutes' dominance in anaerobic processes is considerably increased from usually 20% at the beginning to 80% under stable condition. Overall, OMBR process has great potential to be applied for simultaneous bioenergy production and wastewater treatment.
... One of the advantages of these DSs is the fact that NF instead of RO can be applied to regenerate most of these DSs. In addition, it has been claimed that FO-RO or FO-NF processes need less energy than FO alone to desalinate seawater (Cai, 2016); however, this has been called into question by other studies from at least the theoretical aspects (Elimelech & Phillip, 2011;Phuntsho et al., 2014). Organic and inorganic salts along with a wide range of water-soluble polymers have been applied as some of the non-responsive DSs in the FO process. ...
Renewable energy, water conservation, and environmental protection are the most important challenges today. Osmotic membrane bioreactor (OMBR) is an innovative process showing superior performance in bioenergy production, eliminating contaminants, and low fouling tendency. However, salinity build-up is the main drawback of this process. Identifying the microbial community can improve the process in bioenergy production and contaminant treatment. This review aims to study the recent progress and challenges of OMBRs in contaminant removal, microbial communities and bioenergy production. OMBRs are widely reported to remove over 80% of total organic carbon, PO4³⁻, NH4⁺ and emerging contaminants from wastewater. The most important microbial phyla for both hydrogen and methane production in OMBR are Firmicutes, Proteobacteria and Bacteroidetes. Firmicutes' dominance in anaerobic processes is considerably increased from usually 20% at the beginning to 80% under stable condition. Overall, OMBR process has great potential to be applied for simultaneous bioenergy production and wastewater treatment.
... The driving force in forward osmosis (FO) is the osmotic gradient between a dilute feed solution and a concentrated draw solution. The most common transport models studied for the FO process are based on the film theory concept [2][3][4]. The development of these mathematical models requires extensive knowledge of the process as well as the evaluation of the membrane, solute, and solvent properties, which adds more complexity to the process [5,6]. ...
The forward osmosis (FO) process is an emerging technology that has been considered as an alternative to desalination due to its low energy consumption and less severe reversible fouling. Artificial neural networks (ANNs) and response surface methodology (RSM) have become popular for the modeling and optimization of membrane processes. RSM requires the data on a specific experimental design whereas ANN does not. In this work, a combined ANN-RSM approach is presented to predict and optimize the membrane flux for the FO process. The ANN model, developed based on an experimental study, is used to predict the membrane flux for the experimental design in order to create the RSM model for optimization. A Box–Behnken design (BBD) is used to develop a response surface design where the ANN model evaluates the responses. The input variables were osmotic pressure difference, feed solution (FS) velocity, draw solution (DS) velocity, FS temperature, and DS temperature. The R2 obtained for the developed ANN and RSM model are 0.98036 and 0.9408, respectively. The weights of the ANN model and the response surface plots were used to optimize and study the influence of the operating conditions on the membrane flux.
... Moreover, FDFO can be easily powered by renewable energy sources and therefore suitable for inland and remote applications [120]. The technology shows its potential for the irrigation in coastal areas with water scarcity [121]. ...
... In fact, the absence of hydraulic pressure in the FO process has resulted in minimal irreversible fouling and lower operating costs [12,13]. However, mass transfer effects, which appear in the form of concentration polarization (CP), challenge the water recovery in the FO process and causes some nonlinearity in operation [14]. The mass transfer effects mostly include the external concentration polarization (ECP) and the internal concentration polarization phenomena. ...
Forward osmosis (FO) is an energy-saving separation process that can be used in desalination applications. This work investigated the effect of mass transfer phenomenon on the FO desalination process. For this purpose, the water flux was studied through a bench scale system using a flat sheet FO membrane and feeds with various salinity. Then, the mass transfer resistances, which appear in the form of concentration polarization (CP) for the FO process, were evaluated qualitatively and quantitatively, using the collected experimental data and by employing a mathematical model. The results indicated that the increase in feed salinity led to a decrease in water flux due to the counteracted part of the draw solution osmotic pressure, thus leading to a lower effective osmotic pressure and driving force. Also, according to the results, there was a significant difference between the theoretical and experimental fluxes, indicating the influence of the mass transfer effects on the osmotic pressure drop. The modeling results showed that the internal concentration polarization (ICP) still held more contribution to the osmotic pressure loss. Furthermore, it was observed that as the feed solution concentration increased, both the ICP and dilutive external concentration polarization (DECP) decreased, whereas the concentrative ECP (CECP) intensified. Therefore, increasing the CECP led to a significant reduction in the effective osmotic pressure. In addition, increasing the draw solution concentration was accompanied by a much more severe ICP that limited the enhancement of effective flux.
... Where D s is the salt diffusivity in water (1:52 � 10 À 9 m 2 /s at 25 � C [40]), J FO w is pure water permeate flux in cross-flow FO process using a 1 M NaCl draw soluiton (DS). π D;b is the bulk osmotic pressure of the draw solution while π F;m is osmotic pressure at the membrane surface in the feed side which is assumed zero as feed solution (FS) is ultra-pure Milli-Q water. ...
The poor permeability of prevalent thin film composite (TFC) membranes impedes their application in forward osmosis (FO). In this study, a high-flux TFC membrane was prepared by incorporating dopamine (DA) into an aqueous solution with various concentrations of m-phenylendiamine (MPD) in interfacial polymerization with trimesoyl chloride (TMC) on a fabricated polysulfone (PSF) substrate. SEM, AFM, XPS, ATR-FTIR, and water contact angle measurement (WCA) and Zeta potential were exploited to characterize synthesized membranes. The optimized TFC membrane (TFC-2; MPD: 2 wt%, DA: 0.1 wt%) attained a nearly five-fold water flux improvement (33.3 LMH (L.m⁻².h⁻¹) versus 7.1 LMH for control membrane), an acceptable reverse salt flux of 4.1 g/m²h and a reduced structural parameter (125 μm) in FO with 1 M NaCl draw solution. Membranes were then employed for pesticide removal from water in both reverse osmosis (RO) and FO resulting in high rejection values of >92% in RO and >91% in FO by the optimal membrane for all studied pesticides. Furthermore, the optimized DA-incorporated sample exhibited a better performance compared to the control membrane in terms of anti-scaling behavior and the scaling propensity of RO and FO processes was studied through flux decline measurement in supersaturated solutions with different concentrations of gypsum as model scalant. This study suggests that the FO process, regardless of the membrane material, is per se more resistant against scaling with a threshold gypsum concentration of 25–30 mM compared to RO starting to scale within 20–25 mM of gypsum solution within 24 h.
... Osmotic gradient processes, such as FO, have potential for agricultural irrigation. Although the individual FO process requires lower energy input and less influenced by fouling, it has some disadvantages, like the separation of the draw solution and loss of nutrients [56,57]. To separate the draw solution effectively, a post-treatment strategy is required, which increases energy consumption. ...
... Moreover, FDFO can be easily powered by renewable energy sources and therefore suitable for inland and remote applications [120]. The technology shows its potential for the irrigation in coastal areas with water scarcity [121]. ...
As an emerging desalination technology, forward osmosis (FO) can potentially become a reliable method to help remedy the current water crisis. Introducing uncomplicated and precise models could help FO systems' optimization. This paper presents the prediction and evaluation of FO systems' membrane flux using various artificial intelligence‐based models. Detailed data gathering and cleaning were emphasized because appropriate modeling requires precise inputs. Accumulating data from the original sources, followed by duplicate removal, outlier detection, and feature selection, paved the way to begin modeling. Six models were executed for the prediction task, among which two are tree‐based models, two are deep learning models, and two are miscellaneous models. The calculated coefficient of determination ( R ² ) of our best model (XGBoost) was 0.992. In conclusion, tree‐based models (XGBoost and CatBoost) show more accurate performance than neural networks. Furthermore, in the sensitivity analysis, feed solution (FS) and draw solution (DS) concentrations showed a strong correlation with membrane flux.
Practitioner Points
The FO membrane flux was predicted using a variety of machine‐learning models.
Thorough data preprocessing was executed.
The XGBoost model showed the best performance, with an R ² of 0.992.
Tree‐based models outperformed neural networks and other models.
Polyacrylonitrile (PAN), with its unique chemical, electrical, mechanical, and thermal properties, has become a crucial acrylic polymer for the industry. This polymer has been widely used to fabricate ultrafiltration, nanofiltration, and reverse osmosis membranes for water treatment applications. However, it recently started to be used to fabricate thin-film composite (TFC) and fiber-based forward osmosis (FO) membranes at a lab scale. Phase inversion and electrospinning methods were the most utilized techniques to fabricate PAN-based FO membranes. The PAN substrate layer could function as a good support layer to create TFC and fiber membranes with excellent performance under FO process conditions by selecting the proper modification techniques. The various modification techniques used to enhance PAN-based FO performance include interfacial polymerization, layer-by-layer assembly, simple coating, and incorporating nanofillers. Thus, the fabrication and modification techniques of PAN-based porous FO membranes have been highlighted in this work. Also, the performance of these FO membranes was investigated. Finally, perspectives and potential directions for further study on PAN-based FO membranes are presented in light of the developments in this area. This review is expected to aid the scientific community in creating novel effective porous FO polymeric membranes based on PAN polymer for various water and wastewater treatment applications.
The objective of this research was to use a stepwise approach to develop a temperature-dependent prediction model for scale-up of forward osmosis. A Sobol sensitivity analysis confirmed that water permeability (A) of Forward Osmosis (FO) membrane has significant effect on water flux while salt permeability (B) has limited impact. Statistical analysis indicated that the stepwise model prediction provided improved agreement with experimental measurements of water and salt fluxes compared to prediction from a lumped parameter model over the range from 15 °C to 35 °C. The temperature predictions from the stepwise model were in close agreement with experimental temperatures; with deviations of less than 0.2 °C. When simulated with the increase in the length and width of the flow channel, the differences in predictions between the stepwise and lumped model increased. These results emphasize that parameter variations along the channel length should be accommodated by the stepwise model. In simulations using the stepwise model, the co-current flow mode was found to generate higher water flux than the counter-current flow mode. The results and analysis from this investigation demonstrate the advantages of the stepwise model for prediction of water flux for scale-up of FO operations.
Artificial neural networks (ANN) models are becoming more popular than mathematical and transport-based models due to their high performance and accuracy. Previous literature shows a lack of application of powerful ANN techniques for predicting forward osmosis (FO) performance. In this study, we developed a feedforward network to predict and analyze the permeate flux in the FO process. The ANN model was developed based on a lab-scale experimental database from various published articles. The permeate flux was modeled as a function of membrane-type, membrane orientation, feed and draw solution molarity, feed and draw velocity, molecular weight, feed solution temperature, and draw solution temperature. The influence of foulants on permeate flux has not been considered for developing the ANN model to avoid over-complication of the present work. The adj. R-squared values for train, unseen test and total datasets are 0.99, 0.92, and 0.95, respectively. These values are higher than those found in previously published literature (0.97, 0.85, and 0.82). Moreover, this is the first time that the effect of individual variables on permeate flux has been estimated quantitatively.
This work investigates the capability of multiple linear regression (MLR) and artificial neural networks (ANN) to model permeate flux in a thermodynamically complex forward osmosis (FO) process. Whey-permeate was concentrated to a dry matter content of more than 55 %, creating a highly supersaturated metastable solution and exceeding the established boundaries of conventional membrane technology. Different ANN architectures were trained and tested with a varying number of hidden layers and neurons to find an accurate structure.
Furthermore, the evaluated significance of the input parameters was used to reduce the model's complexity. This work shows that both approaches (MLR: R²test = 0.9718, ANN: R²test = 0.9849) were able to model the FO's permeate flux accurately, even with a reduced number of inputs. Finally, due to its slightly better performance, the ANN was used to outline the relation of FS inlet flow and process temperature.
Water is a critical commodity and a strategic compound for health and economic prosperity. Its availability is key for sustainable industrial activities. This paper presents a novel energy efficient Forward Osmosis-Freeze Concentration (FO-FC) hybrid process able to recover water as ice. In this process, FO operating with a soluble inorganic draw solution, can reduce the volume of aqueous streams by spontaneous osmotic dewatering. Water is in turn recovered as ice by FC, while simultaneously concentrating and regenerating the draw solution. Water recovery by freezing is up to 7 times more energy efficient thermodynamically than by distillation. The water recovered in FO-FC is not intended for drinking purposes, but rather for reuse in industrial operations, in an effort to minimize industrial fresh water intakes. We tested both DI water and a hydrometallurgical effluent as the feed solution in FO-FC. Our FO results, suggest an increase in membrane selectivity as the operating temperature decreases, using MgCl2 as the draw solution and DI water as the feed solution in FO. Further, a significant rejection, namely >90%, of the elements dissolved in the hydrometallurgical effluent used as the feed solution was achieved. Our FC results indicate >66% water recovery as ice with 80–96% recovery efficiency and 0.12 ± 0.05 mol/kg impurities in the ice. Finally, the regenerated concentrated draw solution concentration is 2.8-4× higher compared to the dilute draw solution and is therefore suitable to be reused in the FO step.
Forward osmosis (FO) has attracted growing attention in the field of membrane-based separation technology over the last two decades. Despite recent advancements in various osmosis-assisted processes, few studies have succeeded in commercialization. This paper reviews the state-of-the-art developments of FO membrane, limitation analysis, and commercial proper applications. First, the development of FO technology in terms of FO membranes, FO draw solution (DS), and FO systems is reviewed. Based on a literature database survey spanning 1965-2020, current limitations of FO, particularly in terms of DS regeneration, energy consumption, and scale-up implementation, are identified to overcome the obstacles to commercialization. The key applications of the FO membrane process in commercial sectors are further classified into three configurations (i.e., osmotic dilution, osmotic concentration, and simultaneous osmotic dilution and concentration), and their successful applications are discussed. Since industrial demonstrations of simultaneous osmotic dilution and concentration are in progress, we believe that FO technology with no need for DS regeneration will be commercialized soon in the future.
Forward osmosis (FO), considered as a promising separation process for nutrient enrichment in wastewater, is attracting increasing interest in integration with chemical precipitation and other technologies for recovering nutrients in wastewater treatment. In this chapter, the processes of nutrients recovery via FO‐based systems are introduced in terms of mechanisms and influencing factors. Additionally, the key challenges related to the recovery systems are discussed and some approaches are proposed to resolve these challenges. Roadmaps for future research and development on the nutrients recovery using FO‐based systems are identified. Compared to aerobic FO‐based systems, anaerobic FO‐based processes need more investigations into their integration's efficiency in the context of nutrient recovery from wastewater. Emphasis is given to carry out more economic assessment and pilot‐ and plant‐scale evolutions of the recovery systems, which makes the nutrients recovery via FO‐based technologies more sustainable in wastewater treatment.
This paper deals with various fertilizer influences to draw solutions to the neat CTA/CA, MA/CTA/CA, and the Al2O3/MA/CTA/CA nanocomposite (NC) modified membrane. Also, the applicability of the neat CTA/CA, MA/CTA/CA, and The Al2O3/MA/CTA/CA nanocomposite (NC) modified membrane display high water flux when it used to desalinate brine water sample collected from the brine mid-stream from Mersa Matruh area, NorthWestern Coast of Egypt. The salinity of the collected sample is 12760 mg/L and PH (8.5) and used as FS, and 1M from different fertilizer draw solutes (DFDS) include KCl, NH4Cl, (NH4)2SO4, and K2HPO4 used as DS. The results reveal that the flux was KCl and NH4Cl (17.8 L/m 2 .h) and followed by (NH4)2SO4 (17.1 L/m 2 .h) and K2HPO4 (16.6 L/m 2 .h) using the natural saline water as FS using Al2O3/MA/CTA/CA NC modified membrane. The reusability test of the synthesised Al2O3/MA/CTA/CA NC modified membrane showed good sustainability during the 1260 min continuous test. The FO application displayed a great potential to be interested in brine wastewater desalination and enhanced water source sustainability to use in agriculture fertigation.
The viability of the FO process for concentration of sugarcane juice by using a commercially available aquaporin embedded hollow fibre membrane module (HFFO) (active area, 2.3 m²) is presented. NaCl is used as a draw solution (DS), and the selection of suitable DS concentration, flow conditions and flow configuration are observed to be vital aspects to minimize the reverse solute flux (RSF) and energy consumption. Further, the one dimensional mathematical model for the batch and continuous FO process using HF module is developed and validated with experimental data within the error limit of 5% for water flux and RSF. Based on the flowsheet modelling, the method for establishing the optimised FO process is presented. The optimized FO flowsheet presented in this study can concentrate the sugarcane juice from 150 g L⁻¹ to 531 g L⁻¹ with energy consumption of 92.14 WL⁻¹ of permeate and specific RSF = 1.57 g L⁻¹. Further, to commercialize FO process for sugarcane juice concentration application, the selection of suitable food-grade draw solute that can provide high osmotic pressure and low RSF is a very important aspect.
Forward osmosis-based processes are low energy separation technology that can help in the recovery of fresh water. The design of such systems is approximated either from the simple mass conservations or using the analogy of heat exchanger design correlations. Such relationship has serious drawbacks, particularly at high recoveries, when the impact of mass transfer boundary layer is paramount. Unlike the reverse osmosis, the permeate flux (hence the recovery) is reduced with the convection (Peclet number, Pe). The membrane surface concentration and the thickness of concentration boundary layer on the draw side decreases with Pe. Since, permeate flux increases with the surface concentration, the recovery is dependent on Pe. Besides the relative concentration and flow-rates, the membrane area is influenced by the mass transfer boundary layer, manifested through Pe. The results reported in the Mondal and Field (2018) is a special case when Pe ≪ 1 (negligible convection), leading to maximum recovery. We evaluate the required membrane area and the performance of the forward-osmosis system as a function of the operating conditions, including the Pe. We have identified the range of operating conditions, for which the sizing of FO system is underpredicted in the case of negligible convection and the log-mean concentration difference.
Osmotically driven membrane processes such as forward osmosis and pressure retarded osmosis may hold key advantages when integrated with seawater reverse osmosis to form hybrid FO-RO and RO-PRO systems. In this work, module-scale modeling of these two processes was improved by accurately representing the features of a spiral-wound membrane. The model captures important characteristics such as the cross-flow stream orientation, membrane baffling, and channel dimensions unique to spiral-wound membranes. The new module-scale model was then scaled to the system-level to compare various system designs for FO-RO and RO-PRO systems, most notably, a multi-stage recharge design was defined. Results indicate that the multi-stage recharge design leads to an increase in wastewater utilization, as high as 90%, when compared to the single-stage designs. Additionally, the multi-stage recharge configuration can increase the specific energy recovery of pressure retarded osmosis by over 75%. The multi-stage recharge design is found to be not only advantageous but may be also necessary to the integration of osmotically driven membrane processes with seawater reverse osmosis.
Osmotic distillation (OD) membranes offer selectivity based on vapor pressure and therefore highly reject all non-volatile contaminants. Despite the advantage of high selectivity, OD membranes have not been widely implemented due to poor water fluxes and detrimental heat transfer effects. In this study, we use element- and module-scale computational modeling to examine how OD membrane optimization can improve water treatment performance and compare the productivity of simulated OD membrane modules to conventional FO systems. Several OD membrane parameters are optimized, including thickness, thermal conductivity, porosity, and pore diameter. Among the membrane properties explored, we identify that reducing the OD membrane thickness is the most crucial factor in enabling high performance, and a membrane thickness of approximately 0.1 μm is needed to achieve water fluxes exceeding those of current commercial FO membranes. Thin membranes are also critical to minimize detrimental heat transfer effects in large-scale systems. Comprehensive comparison of OD with FO membranes showed that optimized OD membranes can outcompete high-performance FO membranes in maximum achievable water flux (25.3 vs. 18.6 kg m⁻²h⁻¹ for OD and FO membranes, respectively) and module-scale water recovery (0.28 vs. 0.18). Overall, the results of this work demonstrate the promise of OD membranes to overcome the selectivity limitations of conventional polymeric membranes and offer guidelines for future OD membrane design.
This work reports on efforts to develop an integrated continuous forward osmosis system for the recovery of water from wastewater streams, highlighting critical process parameters to minimize energy consumption. Forward osmosis experiments were performed using NaCl draw solutions of various concentrations and the intrinsic membrane parameters (water permeability, draw solution permeability, and structural parameter) were then determined via nonlinear regression using MATLAB. The experimental data was then used to validate a theoretical water flux model, which was subsequently applied to simulate the FO performance under different hydrodynamic conditions using both NaCl and TMA-CO2-H2O draw solutions. Analysis of the energy efficiency of the TMA-CO2 draw solution regeneration stage revealed that the draw solution flow rate has a significant impact on energy consumption. More so, increasing the feed flow rate was found to slightly enhance the water flux up to 2.5%, while having a negligible impact on the downstream regeneration process energy consumption.
The support layer of an asymmetric thin-film composite membrane results in structural resistance (internal concentration polarization) that significantly undermines engineered osmosis. Increasing the porosity and reducing the thickness and tortuosity of the membrane support layer reduces structural resistance; however, internal concentration polarization still impacts membrane performance. A novel, ultrathin, free-standing and symmetric membrane has been synthesized using sulfonated polyether ketone and tested for forward osmosis applications. This membrane is composed of a protonic acid group containing an aromatic polyether resin with sulfonated structural units. Polyether ketone provides high mechanical strength essential for ultrathin free-standing membranes, while sulfonation enhances the membrane hydrophilicity. These sulfonated polyether ketone membranes show promising water flux performances with impressive mechanical strength under the hydraulic operating conditions used for a FO process.
This chapter explains the concept of the fertilizer-drawn forward osmosis (FDFO) desalination process and evaluates the potential of fertilizer solutions as draw solutions (DS). Different types of fertilizers are used to grow crops, and hence understanding what types of fertilizers are more suitable for the FDFO desalination process and how to screen and assess suitable fertilizer candidates for use as DS in the FDFO desalination process is important. FDFO desalination adds value to irrigation water and provides more opportunities for improving the efficiencies of water and fertilizer use. The major limitations of the FDFO desalination process are the challenge of meeting the irrigation water quality standards in terms of nutrient concentrations, which limits the direct use of FDFO product water for fertigation. Several options are discussed that could be integrated with the FDFO desalination process to reduce the final nutrient concentration closer to an acceptable limit.
With the world’s population growing rapidly, pressure is increasing on the limited fresh water resources. Membrane technology could play a vital role in solving the water scarcity issues through alternative sources such as saline water sources and wastewater reclamation. The current generation of membrane technologies, particularly reverse osmosis (RO), has significantly improved in performance. However, RO desalination is still energy intensive and any effort to improve energy efficiency increases total cost of the product water. Since energy, environment and climate change issues are all inter-related, desalination for large-scale irrigation requires new novel technologies that address the energy issues. Forward osmosis (FO) is an emerging membrane technology. However, FO desalination for potable water is still a challenge because, recovery and regeneration of draw solutes require additional processes and energy. This article focuses on the application of FO desalination for non-potable irrigation where maximum water is required. In this concept of fertiliser drawn FO (FDFO) desalination, fertilisers are used as draw solutions (DS). The diluted draw solution after desalination can be directly applied for fertigation without the need for recovery and regeneration of DS. FDFO desalination can make irrigation water available at comparatively lower energy than the current desalination technologies. As a low energy technology, FDFO can be easily powered by renewable energy sources and therefore suitable for inland and remote applications. This article outlines the concept of FDFO desalination and critically evaluates the scope and limitations of this technology for fertigation, including suggestions on options to overcome some of these limitations.
Forward osmosis (FO) is an emerging technology for low energy desalination. Amongst the many other factors, temperature of the draw solution (DS) and feed solution (FS) plays an important role in influencing the performance of the FO process. In this study, the influence of the temperature and the temperature difference on the performance of FO process has been studied in terms of water and solute fluxes. Temperature difference was maintained by elevating only one of the solutions (either DS or FS). The results indicate that, water flux on average increases by up to 1.2% for every degree rise in temperature from 25 °C to 35 °C while this rise is 2.3% from 25 °C to 45 °C. Providing a temperature difference by elevating only the DS also enhanced the water flux significantly, although it was lower than FO process operated at isothermal conditions. However, elevating only the temperature of FS did not significantly improve the water flux although it was higher than the FO process operated at 25 °C. This has significant implications in FO process because the total mass of the DS requiring heat energy is significantly less than the total FS used. The influences of temperature in the FO process such as through changes in the thermodynamic properties of the solutions and the various concentration polarisation effects are also explained in details.
Forward osmosis (FO) is a novel and emerging low energy technology for desalination. It will be particularly more attractive, if the draw solution separation and recovery are not necessary after FO process. The application of this new concept is briefly described here in this paper for the desalination of saline water for irrigation, using fertilizer as a draw agent. Instead of separating the draw solution from desalinated water, the diluted fertilizer draw solution can be directly applied for fertigation. We report the results on the commonly used chemical fertilizers as FO draw solution. Based on the currently available FO technology, about nine different commonly used fertilizers were finally screened from a comprehensive list of fertilizers and, their performances were assessed in terms of pure water flux and reverse draw solute flux. These results indicate that, most soluble fertilizers can generate osmotic potential much higher than the sea water. The draw solutions of KCl, NaNO3 and KNO3 performed best in terms of water flux while NH4H2PO4, (NH4)2HPO4, Ca(NO3)2 and (NH4)2SO4 had the lowest reverse solute flux. Initial estimation indicates that, 1kg of fertilizer can extract water ranging from 11 to 29L from sea water.
Recently, membrane pretreatment has been accepted as a technically and economically viable alternative to conventional pretreatment in SWRO processes. MF, UF and NF membranes were used in these pressure driven processes, with UF most commonly recommended. Amongst these, forward osmosis (FO) driven process has been used in different water treatment operations including membrane pretreatment of wastewater. In this study, the application of a FO driven membrane process in seawater pretreatment was evaluated. This application is particularly important for small autonomous RO desalination units as it eliminates the need for chemicals which are conventionally used in pretreatment steps, the concomitant need for disposal of the chemical laden waste resulting from the process, and the requirement of qualified expertise for unit operation. The performance of commercially available FO membrane cartridges (Hydration Technologies Inc.) in terms of water flux and salt flux was evaluated using tap water and seawater as feed. Refined sea salt was used for preparing highly concentrated osmotic draw solutions at three salinity levels up to 100,000 ppm. Profiles of water and salt fluxes versus osmotic draw solution concentrations were established and analyzed. A conceptual system design for an integrated desalination unit consisting of a closed FO and RO process was presented in which RO brine is used as the osmotic draw solution and thus the chemical energy stored in the RO brine is recovered and utilized by the FO membrane process. In addition, the energy contained in the excess RO brine pressure is used for brine circulation in the FO process loop. The main operating parameters and relationships of the conceptual FO-RO system are described.
The performance of FO is predicted numerically by one-dimensional model. Mass balance equation for the feed and draw side are coupled with the water flux model considering concentration polarization. Results of the present study showed the flow rate of the feed and draw solution should be determined by considering the water flux and the water recovery efficiency. Using the draw solution of as high concentration as possible is helpful to improve the water flux. As increasing the membrane module length, the averaged water flux per membrane length decreases but the water production increases. Therefore, in order to determine the membrane length, it is required to consider the water flux reduction, total water production, membrane size and the number of membrane. The water flux of counter-current flow is about 10% higher than that of co-current flow. Forming feed solution into series and draw solution into rows are effective in increasing water flux.
The concept of fertiliser drawn forward osmosis (FDFO) desalination lies in the premise that fertilisers that serve as draw solutions (DS) add value to the FDFO product water for fertigation. However, because FDFO desalination is concentration based, the process cannot continue beyond the concentration equilibrium, one of the major limitations of the forward osmosis (FO) process. This results in final FDFO product water that, unless subjected to substantial dilution with fresh water, exceeds the acceptable nutrient concentrations for direct fertigation. In this study, nanofiltration (NF) has been assessed as an integrated process to FDFO desalination, either as a pre-treatment or post-treatment, to reduce the nutrient concentrations in the final product water and thereby allow direct use of the product water for fertigation without further dilution. NF as pre-treatment or post-treatment was found effective in reducing the nutrient concentrations using brackish groundwater (BGW) sources with relatively low total dissolved solid (TDS). However, when using higher TDS BGW sources, the product water still required further dilution or post-treatment before fertigation. NF as post-treatment was found to be more advantageous both in terms of reduced nutrient concentrations in the final product water and energy consumption.
Forward osmosis (FO) is an emerging osmotically driven membrane process, and its applications are becoming diversified. As one important membrane configuration, hollow fiber modules have been applied to some innovative FO processes, e.g., osmotic membrane bioreactor (OMBR). Aside from the inherent concentration polarization (CP) phenomena, new challenges are posed by the coupled concentrating and dilution effects in the design of FO hollow fiber modules. In this paper, a mathematical model is therefore developed to account for the evolution of the local performances within the FO hollow fiber module and to evaluate the global performances of interest. Then, this model is employed to theoretically investigate the filtration behaviors of the FO hollow fiber module by using the well-defined dimensionless groups, which indicates the complex interplay among a variety of design parameters. Particularly, the optimization objectives are focused on enhancing the module-averaged FO efficiency and avoiding the severe concentration variations in the module channels. In terms of the simulation results, some criteria are obtained for optimizing the operating conditions (flow configuration, inlet concentration level, inlet flow rate), the hollow fiber characteristics (fiber length), and the FO membrane properties (active layer selectivity, support layer transport resistance). This work provides deep insights into the design of the FO hollow fiber module, and could be readily modified to accommodate more complicated cases.
Pressure retarded osmosis (PRO) is a potential technology to harvest the renewable osmotic power from the salinity-gradient resources. This study systematically investigated the effects of operating conditions (feed and draw solution concentration, membrane type, membrane orientation, and temperature) and reverse solute diffusion on PRO performance using commercially available osmotic membranes. The PRO performance was improved by decreasing the feed solution concentration, increasing the draw solution concentration, orientating the membrane with active layer facing draw solution (AL-DS), and increasing temperature. The membrane with higher water permeability, lower solute permeability and lower structure parameter performed better in PRO process. However, the experimentally obtained power densities for all the membranes used in this study were lower than the predictions from conventional ICP model that assumes membrane separation parameters are constant in PRO process. It was found that this was mainly caused by the severe reverse solute diffusion and thus the enhanced internal concentration polarization (ICP) in PRO. The specific reverse solute flux was found to increase with increasing the applied hydraulic pressure, but the increase of experimental results was much more drastic than the theoretic prediction especially under higher hydraulic pressure, probably due to the increased solute permeability caused by membrane deformation.
In the rapidly developing shale gas industry, managing produced water is a major challenge for maintaining the profitability of shale gas extraction while protecting public health and the environment. We review the current state of practice for produced water management across the United States and discuss the interrelated regulatory, infrastructure, and economic drivers for produced water reuse. Within this framework, we examine the Marcellus shale play, a region in the eastern United States where produced water is currently reused without desalination. In the Marcellus region, and other shale plays worldwide with similar constraints, contraction of current reuse opportunities within the shale gas industry and growing restrictions on produced water disposal will provide strong incentives for produced water desalination for reuse outside the industry. The most challenging scenarios for the selection of desalination for reuse over other management strategies will be those involving high-salinity produced water, which must be desalinated with thermal separation processes. We explore desalination technologies for treatment of high-salinity shale gas produced water, and we critically review mechanical vapor compression (MVC), membrane distillation (MD), and forward osmosis (FO) as the technologies best suited for desalination of high-salinity produced water for reuse outside the shale gas industry. The advantages and challenges of applying MVC, MD, and FO technologies to produced water desalination are discussed, and directions for future research and development are identified. We find that desalination for reuse of produced water is technically feasible and can be economically relevant. However, because produced water management is primarily an economic decision, expanding desalination for reuse is dependent on process and materials improvements to reduce capital and operating costs.
This paper focuses on steady state performance predictions and optimization of the reverse osmosis (RO) based desalination process utilizing a given spiral wound type membrane modules. A set of implicit mathematical equations are generated by combining solution-diffusion model with film theory approach to model the RO process which is used for simulation and optimization. The simulation results for a three-stage RO process are compared with the results from literature and are found to be in good agreement having relative errors of 0.71% and 1.02%, in terms of water recovery and salt rejection, respectively. The sensitivity of different operating parameters (feed concentration and feed pressure) and design parameters (number of elements, spacer thickness, length of spacer filament) on the plant performance are also investigated. Finally, for the same type of spiral wound membrane, two optimization problems are formulated and solved.In the first one, a non-linear optimization problem is formulated for the same three-stage RO process (fixed configuration) to minimize the specific energy consumption at fixed product flow rate and quality while optimizing the operating and design parameters. The results showed a 20% savings in specific energy consumption compared to the base case.In the second one, a mixed-integer nonlinear programming (MINLP) problem based on a superstructure is formulated for fixed freshwater demand and quality to minimize the total annualized cost while optimizing the design and operation of the RO network. A variable fouling profile along the membrane stages is introduced to see how the network design and operation of the RO system are to be adjusted to minimize the total annualized cost. Outer-approximation algorithm has been used to solve the MINLP problem. The results show that the fouling distribution between stages affects significantly the optimal design and operation of RO process.
Forward osmosis (FO) has attracted growing attention for its great promise in desalination, wastewater treatment, liquid food processing and power generation. However, there is no clear agreement on the selection of membrane orientation in these applications. This study investigates the effects of membrane orientation on FO performance in saline water desalination without fouling, and with inorganic or organic fouling. The results show that the feed solution component and the concentration degree could influence the selection of membrane orientation. When severe membrane fouling or scaling occurs, the isoflux point occurs relatively early and FO mode (active layer facing the feed) provides a more stable and higher water flux than that in the alternative membrane orientation, i.e. pressure retarded osmosis (PRO) mode (support layer facing the feed). Additionally, lower fouling but higher cleaning efficiency is observed in FO mode compared with PRO mode. Therefore, in the applications of treating feed solutions with higher fouling/scaling tendencies (e.g. wastewater treatment) or treating higher salinity water (e.g. seawater desalination), FO mode is more favourable. While PRO mode is preferred when using the solutions with lower fouling/scaling tendencies as the feed (e.g. brackish water desalination), or where intensive concentration is unnecessary (e.g. power generation).
In this investigation, a forward osmosis (FO) dewatering process is evaluated as an alternative for dewatering orange peel press liquor. The press liquor is concentrated by removing water via osmosis into a concentrated “draw solution” across a polymeric cellulose acetate membrane. For this investigation, the draw solution consisted of sodium chloride at two different concentrations (2M and 4M). These draw solutions were used to concentrate synthetic press liquor. Concentration factors up to 3.7 resulted when using a 4M NaCl draw solution and the synthetic press liquor. Also during this investigation, fouling behavior was observed and the mechanisms of fouling elucidated through a systematic approach. This approach identified the key components of the press liquor that were primarily responsible for fouling. Calcium, normally a critical component of organic fouling and ubiquitous to press liquor, was found to have little effect on fouling behavior, likely due to complexation with citric acid also present in the liquor. Pectin was found to be the dominant component contributing to fouling, which was found to reduce permeate flux by as much as 50%. Removing pectin through a pretreatment process would enhance dewatering.
The membrane structure parameter (S) is an intrinsic membrane parameter used to determine the degree of internal concentration polarization (ICP) in the porous support structure of forward osmosis (FO) membranes, and is crucial in evaluations of FO membrane performance. Although S values only depend on membrane properties, and should be close to a constant value, experiments to determine the S values produce various values, which vary with respect to the concentrations of the feed and draw solutions. In this study, we develop a numerical model based on the finite element method (FEM) to determine a constant membrane structure parameter. In contrast to other FO models, the developed model successfully simulates the performance of FO processes, and maintains a consistent S value. Here, the most influential factor causing inconsistent S values is found to be the assumption that the ratio of concentrations is approximately identical to the ratio of osmotic pressures, as is frequently used in FO modeling due to lack of availability of concentration profiles at the active layer–support layer interface. However, use of a constant diffusion coefficient had little influence on either the simulation result or the S value consistency.
Several osmotic processes, concentrate return reverse osmosis, osmotic sink reverse osmosis, and osmotic sink osmosis are proposed for the production or further concentration of solutions having a high osmotic pressure. These processes all involve countercurrent flow, on opposite sides of a semipermeable membrane, of the permeant donating, i.e. concentrating solution, and the permeant-receiving i.e. diluting solution.The processes depend on the fact that, at any point in the train of membranes, the permeant-receiving solution has a higher osmotic pressure than the permeant. This effect is achieved by the injection of a solution having a high osmotic pressure at the beginning of the permeant-receiving side of the countercurrent system.
The inherent challenge of the forward osmosis (FO) process is the severity of both external (ECP) and internal concentration polarization (ICP), which significantly reduces the water flux across the highly selective membrane. In this study, the impacts of concentration polarization on flux behavior were investigated. A modified-film model developed using the boundary layer concept described the ECP layer much better than previously used models. By including the diffusion coefficient into the derivative of the governing convective-diffusion equations, the predicted water flux due to ICP was in excellent agreement with experimental flux data. This was attributed to the usage of a better solute resistivity constant within the porous support layer, K*, which is independent of the diffusivity coefficient. Laboratory experiments were carried out to account for both ECP and ICP and the associated water fluxes were verified with the improved models. Previous models overestimated the water flux by as much as 15% of the experimental flux and the modified models showed significant improvements in flux prediction for the FO process, particularly at higher draw solution concentration. A better understanding of the effects of concentration polarization achieved from this study could allow us to further modify the FO membrane structure to improve water flux.
A membrane separation model for tubular module reverse osmosis and ultrafiltration processes was developed in this work. The membrane area of a process can be calculated by this model and the stream matrix of a process can be determined by a process simulation program. In this work the unicorn simulation program was used. The membrane separation model calculates permeate flux and rejection of the solute in small increments of the membrane tube over the entire range of the tube and the process. Calculation of the permeate flux and rejection can be performed by polynomial equations fitted to the experimental data, or by equations based on mass transfer models. The finely porous model and the statistical mechanical model were used. Some experimental data are also needed for determining the parameters of the mass transfer model equations by parameter fitting.The membrane separation model was tested by the simulation of reverse osmosis of aqueous ethanol and acetic acid solutions, and ultrafiltration of aqueous sodium carboxymethylcellulose (CMC) and poly(vinylpyrrolidone) (PVP) solutions. Multistage recycle separation processes with four or five recycle stages were used as test processes. The values of the calculation parameters (i.e., iteration accuracy of the recycle stream, iteration acceleration factor and the number of increments of the membrane tube) giving reliable results with minimum computing time were determined.
Performance of forward osmosis (FO) process is significantly affected by factors such as membrane properties, concentration polarization (CP), and fouling. In this study, FO performance of a plate and frame type membrane is investigated via a numerical simulation based on mass conservation theorem. To evaluate the FO membrane performance, permeate flux and recovery rate are simulated according to membrane orientation, flow direction of feed and draw solutions, flow rate, and solute resistivity (K). In the case of membrane orientation, all-inside case, in which the draw solution faces the active layer, displays a relatively higher performance than all-outside and all-up cases. Notably, the membrane performance is highly affected by K indicating the extent of the internal CP. During the simulation approach, the spatial variation of the concentration profile was observed on a 2-dimensional membrane area; it was expected to cause a high diffusion load on a particular area of membrane, due to the relatively higher flux at that location. Moreover, it can result in unexpected fouling in a specific area on a membrane. Accordingly, the findings in this study suggest that the numerical simulation can be applied to optimize both physical properties and operation conditions, thereby ensuring cost-effective operation of FO processes.
A model has been developed for obtaining the projected performance of membranes in pressure-retarded osmosis (PRO) from direct osmosis and reverse osmosis measurements. The model shows that concentration polarization within the porous substrate of the membrane markedly lowers the water flux under PRO conditions. The model has been used along with experimental data obtained with a variety of reverse osmosis membranes to project PRO performance with several water—brine sources. Some literature data on PRO have been similarly examined. Based on these results and a simple economic analysis we conclude that membranes with significantly improved performance will be needed if PRO is to become an economically feasible method for power generation using seawater—fresh water as the salinity gradient resource. However, the economics of a brine/fresh water system appear competitive with conventional power generation technologies.
Commercially available asymmetric membranes of the Loeb-Sourirajan (L-S) type comprise a support fabric, bonded to the porous substructure. The influence of this fabric on osmotic permeation flux was examined, mostly with a Toray CA-3000 membrane from which, with care, it was possible to remove the support fabric. In osmosis experiments with 12% MgCl2 solution on one side (either side) and 6% solution on the other, the permeation flux (J1) was of the order of 0.01 and 0.06 m3/m2 d with and without fabric, respectively. These results could be generalized by considering the resistivity to solute diffusion in the non-skin part of the membrane. This resistivity term averaged 104 and 17 d/m for membranes with and without fabric, respectively, and in further tests without fabric, it was between 15 and 25 d/m over a wide range of MgCl2 concentrations. Four other L-S membranes, all with support fabric, were tested in osmosis experiments. Their resistivity values were similar to or higher than those of the Toray membrane with fabric, but, with one of the four, the results were affected by switching the location of the high and low concentration solutions. It was concluded that existing commercially available L-S membranes are not appropriate for large-scale osmosis applications because their support fabric decreases permeation flux excessively.
The forward osmosis process is considered a promising desalination method due to its low energy requirement compared to other methods. In this study, modelling and simulations for a plate-and-frame and a modified spiral-wound module are carried out for the FO process. The mathematical models consist of mass balance, a permeate flux model, and concentration polarization equations. The plate-and-frame model is formulated with consideration of flow directions, and the modified spiral-wound model is formulated with consideration of its geometric characteristics. These two sets of model equations are numerically and iteratively integrated since they are implicit and highly non-linear. The simulation for both modules was conducted by varying 4 types of operating conditions: volumetric flow rate of the feed and the draw solution, the concentration of the draw solution, flow direction, and the membrane orientation. The results for various conditions are also compared. In future research, the developed model could be applied for designing FO modules and finding optimal operating conditions.Highlights► Two dimensional detail mathematical model of FO module is developed. ► Plate and frame and modified spiral wound modules are modelled. ► For modified spiral wound module, the structural characteristics due to winding of membrane envelope are also considered. ► Flow configurations are compared for FO modules. ► The developed model for a modified spiral wound module is validated by experimental data from a published paper.
A mathematical model for spiral wound Reverse Osmosis membrane module is presented in this work. The model incorporates spatial variations of pressure, flow and solute concentration in the feed channel and uniform conditions of pressure in the permeate channel. Assuming solution–diffusion model to be valid, explicit analytical equations were derived for spatial variations of pressure, flow, solvent flux and solute concentration on the feed channel side of the module. Analytical procedures for estimation of model parameters were presented. Graphical linear fit methods were developed for estimation of parameters Aw (solvent transport coefficient), Bs (solute transport coefficient) and b (feed channel friction parameter). The mass transfer coefficient k was assumed to vary along the length of the feed channel with varying conditions of flow, solute concentration and pressure. Explicit analytical equations for estimation of mass transfer coefficient were presented. In this paper (Part I), theoretical studies on development of mathematical model and methods for estimation of model parameters are presented.In Part II of this paper series [1], Studies on validation of this model with experimental data are presented. The studies cover experimental work on a spiral wound RO module with an organic compound namely chlorophenol as a solute.Research Highlights► A new analytical model for spiral wound RO modules is developed. ► Validity of Solution diffusion model with concentration polarization is assumed. ► Model predicts spatial variations of Pressure, Flow and Concentration in feed channel. ► New graphical methods for estimation of membrane transport parameters are developed. ► Explicit equations for the estimation of mass transfer coefficient are derived.
The hybrid FO/RO desalination is an innovative technology which provides many advantages such as reducing RO fouling and scaling, recovery of osmotic energy of RO brine and minimizing the use of chemicals required for conventional pretreatment steps. Conceptual FO–RO system design alternatives were presented in this paper. Either seawater or RO brine may be used as a draw solution to extract water from an impaired source through FO. An analysis of osmotic energy recovery in this system was given and the main operating parameters and relationships governing the operation of the conceptual system were also described. At concentration gradients of 15 g/l and 30 g/l, the osmotic energy recovered ranged between 1.1 kJ and 2.2 kJ per each liter of permeate water, respectively. The power density of the membrane at these concentrations was 1.45 and 4.35 W/m2, respectively. Future success of FO desalination depends on designing new membranes in terms of structure and configuration specifically tailored for FO. Four new membrane module configurations were suggested and described in this paper. The new configurations were intended to improve performance of the modules in terms of water flux and effectiveness of backwashing as well as to lower pressure drop in the membrane envelope.Research Highlights►Hybrid FO/RO system has potential for pretreatment of wastewater ►Osmotic energy can reach 1.1 kJ at concentration gradients of 15 g/l ►Power density of membrane is 1.45 W/m2 at concentration gradients of 15 g/l ►Four new membrane module configurations were suggested
The energy released from the mixing of freshwater with saltwater is a source of renewable energy that can be harvested using pressure retarded osmosis (PRO). In PRO, water from a low salinity solution permeates through a membrane into a pressurized, high salinity solution; power is obtained by depressurizing the permeate through a hydroturbine. The combination of increased interest in renewable and sustainable sources of power production and recent progress in membrane science has led to a spike in PRO interest in the last decade. This interest culminated in the first prototype installation of PRO which opened in Norway in late 2009. Although many investigators would suggest there is still lack of theoretical and experimental investigations to ensure the success of scaled-up PRO, the Norway installation has evoked several specialized and main-stream press news articles. Whether the installation and the press it has received will also boost competitive commercialization of membranes and modules for PRO applications remains to be seen. This state-of-the-art review paper tells the unusual journey of PRO, from the pioneering days in the middle of the 20th century to the first experimental installation.
Unusually high ultrafiltrate flux values have been observed by use of thin-channel ultrafiltration in the dewatering and purification of colloidal suspensions. Polymer latices, paints, metal oxides, starch, and even cellular suspensions have all exhibited higher flux values than would be predicted by the now recognized gel-polarization model. Theoretical reasons for these anomalies are discussed in conjunction with experimental data obtained with thin-channel devices utilizing anisotropic noncellulosic membranes.
The purpose of this short communication is to share our perspectives on future R & D for FO processes in order to develop effective and sustainable technologies for water, energy and pharmaceutical production.Research Highlights► A review of current and future FO technologies for water, energy and pharmaceuticals.
Preface 1. Models for diffusion Part I. Fundamentals of Diffusion: 2. Diffusion in dilute solutions 3. Diffusion in concentrated solutions 4. Dispersion Part II. Diffusion Coefficients: 5. Values of diffusion coefficients 6. Diffusion of interacting species 7. Multicomponent diffusion Part III. Mass Transfer: 8. Fundamentals of mass transfer 9. Theories of mass transfer 10. Absorption 11. Absorption in biology and medicine 12. Differential distillation 13. Staged distillation 14. Extraction 15. Absorption Part IV. Diffusion Coupled with other Processes: 16. General questions and heterogeneous chemical reactions 17. Homogeneous chemical reactions 18. Membranes 19. Controlled release and related phenomena 20. Heat transfer 21. Simultaneous heat and mass transfer Problems Subject index Materials index.
A novel forward (direct) osmosis (FO) desalination process is presented. The process uses an ammonium bicarbonate draw solution to extract water from a saline feed water across a semi-permeable polymeric membrane. Very large osmotic pressures generated by the highly soluble ammonium bicarbonate draw solution yield high water fluxes and can result in very high feed water recoveries. Upon moderate heating, ammonium bicarbonate decomposes into ammonia and carbon dioxide gases that can be separated and recycled as draw solutes, leaving the fresh product water. Experiments with a laboratory-scale FO unit utilizing a flat sheet cellulose tri-acetate membrane demonstrated high product water flux and relatively high salt rejection. The results further revealed that reverse osmosis (RO) membranes are not suitable for the FO process because of relatively low product water fluxes attributed to severe internal concentration polarization in the porous support and fabric layers of the RO membrane.
Forward (direct) osmosis (FO) using semi-permeable polymeric membranes may be a viable alternative to reverse osmosis as a lower cost and more environmentally friendly desalination technology. The driving force in the described FO process is provided by a draw solution comprising highly soluble gases—ammonia and carbon dioxide. Using a commercially available FO membrane, experiments conducted in a crossflow, flat-sheet membrane filtration cell yielded water fluxes ranging from 1 to 10 m/s (2.1 to 21.2 gal ft −2 d −1 or 3.6 to 36.0 l m −2 h −1) for a wide range of draw and feed solution concentrations. It was found, however, that the experimental water fluxes were far lower than those anticipated based on available bulk osmotic pressure difference and membrane pure water permeability data. Internal concentration polarization was determined to be the major cause for the lower than expected water flux by analysis of the available water flux data and SEM images of the membrane displaying a porous support layer. Draw solution concentration was found to play a key role in this phenomenon. Sodium chloride rejection was determined to be 95–99% for most tests, with higher rejections occurring under higher water flux conditions. Desalination of very high sodium chloride feed solutions (simulating 75% recovery of seawater) was also deemed possible, leading to the possibility of brine discharge minimization.
Forward osmosis (FO) is an osmotic process that uses a semi-permeable membrane to eff ect separa-tion of water from dissolved solutes by an osmotic pressure gradient. Unlike reverse osmosis (RO), FO does not require high pressure for separation, allowing low energy consumption to produce water. How-ever, the internal concentration polarization in FO is an important factor aff ecting the performance of FO processes. This paper was intended to investigate the characteristics of FO and RO processes. A simple fi lm theory model was applied to consider concentration polarization in FO and RO processes. This model allows the estimation of internal and external concentration polarization eff ects in FO process. A laboratory-scale FO device was used to fi nd the model parameters for further calculations. The calculated fl ux was compared with experimental fl ux under a variety of operating conditions. It was found that the combination of FO and RO may result in a higher fl ux than FO-only process under some operating conditions. Further research will be required to investigate the eff ect of membrane materials on energy effi ciency of FO and RO hybrid system.
A coupled model of concentration polarization and pore transport of multicomponent salt mixtures in crossflow nanofiltration rigorously predicts local variations of ionic concentrations, flux and individual ion rejections along a rectangular crossflow filtration channel by a coupled solution of the convective-diffusion and extended Nernst-Planck equations. Coupling the pore transport model with the multicomponent convective-diffusion equation in the concentration polarization layer provides a comprehensive understanding of the interplay between concentration polarization and salt rejection. The coupled model is used to predict the local variations of ion rejection, permeate flux and mixture composition in a rectangular crossflow filtration channel for three-component salt mixtures. The total membrane surface concentration of the ions and the ratio of different ions in the mixture (salt ratio) can change considerably along a crossflow filtration channel, and, consequently, cause remarkable variations in intrinsic ion rejections with axial position in the channel.
Osmotically-driven membrane processes, such as forward osmosis and pressure retarded osmosis, operate on the principle of osmotic transport of water across a semipermeable membrane from a dilute feed solution into a concentrated draw solution. The major hindrance to permeate water flux performance is the prevalence of concentration polarization on both sides of the membrane. This article evaluates the external and internal boundary layers, which decrease the effective osmotic driving force. By modeling permeate flux performance, the role that feed and draw concentrations, membrane orientation, and membrane structural properties play in overall permeate flux performance are elucidated and linked to prevalence of external and internal concentration polarization. External concentration polarization is found to play a significant role in the reduction of driving force, though internal concentration polarization has a far more pronounced effect for the chosen system conditions. Reduction of internal concentration polarization by way of membrane modification was found to improve the predicted flux performance significantly, suggesting that alteration of membrane design will lead to improved performance of osmotically driven membrane processes. © 2007 American Institute of Chemical Engineers AIChE J, 2007
We have developed new equations for general osmotic pressures based on a simple equation of state for solutions. The present method is a completely different approach (directly solving the osmotic equilibrium), compared with the conventional formulation. The model equations are analytical and valid for the entire range of solute concentrations, but require numerical iterations to solve the osmotic pressure, which can be easily made by a simple Newton-Raphson method. The model calculations reveal highly rich and complex behaviors including critical phenomena in the osmotic pressure. Then, we demonstrate that the present model can be applied successfully to actual osmotic data, such as aqueous polymer solutions and aqueous salt solutions of proteins using a single adjustable parameter, which has a physical meaning.
The Gibbs free energy of mixing dissipated when fresh river water flows into the sea can be harnessed for sustainable power generation. Pressure retarded osmosis (PRO) is one of the methods proposed to generate power from natural salinity gradients. In this study, we carry out a thermodynamic and energy efficiency analysis of PRO work extraction. First, we present a reversible thermodynamic model for PRO and verify that the theoretical maximum extractable work in a reversible PRO process is identical to the Gibbs free energy of mixing. Work extraction in an irreversible constant-pressure PRO process is then examined. We derive an expression for the maximum extractable work in a constant-pressure PRO process and show that it is less than the ideal work (i.e., Gibbs free energy of mixing) due to inefficiencies intrinsic to the process. These inherent inefficiencies are attributed to (i) frictional losses required to overcome hydraulic resistance and drive water permeation and (ii) unutilized energy due to the discontinuation of water permeation when the osmotic pressure difference becomes equal to the applied hydraulic pressure. The highest extractable work in constant-pressure PRO with a seawater draw solution and river water feed solution is 0.75 kWh/m(3) while the free energy of mixing is 0.81 kWh/m(3)-a thermodynamic extraction efficiency of 91.1%. Our analysis further reveals that the operational objective to achieve high power density in a practical PRO process is inconsistent with the goal of maximum energy extraction. This study demonstrates thermodynamic and energetic approaches for PRO and offers insights on actual energy accessible for utilization in PRO power generation through salinity gradients.
The design of various multistage RO systems under different feed concentration and product specification is presented in this work. An optimization method using the process synthesis approach to design an RO system has been developed. First, a simplified superstructure that contains all the feasible design in present desalination process has been presented. It offers extensive flexibility towards optimizing various types of RO system and thus may be used for the selection of the optimal structural and operating schemes. A pressure vessel model that takes into account the pressure drop and concentration changes in the membrane channel has also been given to simulate multi-element performance in the pressure vessel. Then the cost equation relating the capital and operating cost to the design variables, as well as the structural variables of the designed system have been introduced in the objective function. Finally the optimum design problem can be formulated as a mixed-integer nonlinear programming (MINLP) problem, which minimizes the total annualized cost. The solution to the problem includes optimal arrangement of the RO modules, pumps, energy recovery devices, the optimal operating conditions, and the optimal selection of types and number of membrane elements. The effectiveness of this design methodology has been demonstrated by solving several seawater desalination cases. Some of the trends of the optimum RO system design have been presented.
Direct osmosis is a non-thermal membrane process employed for the concentration of fruit juices at ambient temperature and atmospheric pressure, thereby maintaining the organoleptic and nutritional properties of fruit juices. In the present study, concentration of pineapple juice by direct osmosis was explored. Aqueous solution of sucrose (0–40%, w/w)–sodium chloride (0–26%, w/w) combination was investigated as an alternative osmotic agent. The sucrose–sodium chloride combination can overcome the drawback of sucrose (low flux) and sodium chloride (salt migration) as osmotic agents during direct osmosis process. The effect of the hydrodynamic conditions in the module and feed temperature (25–45 °C) on transmembrane flux was evaluated. For a range of hydrodynamic conditions studied, it was observed that transmembrane flux increases with Reynolds number. The increase in feed temperature resulted in an increase in transmembrane flux. The pineapple juice was concentrated upto a total soluble solids content of 60 °Brix at ambient temperature. The effect of direct osmosis process on physico-chemical characteristics of pineapple juice was also studied. The ascorbic acid content was well preserved in the pineapple juice concentrate by direct osmosis process.
The design and engineering of membrane structure that produces low salt leakage and minimized internal concentration polarization (ICP) in forward osmosis (FO) processes have been explored in this work. The fundamentals of phase inversion of cellulose acetate (CA) regarding the formation of an ultra-thin selective layer at the bottom interface of polymer and casting substrate were investigated by using substrates with different hydrophilicity. An in-depth understanding of membrane structure and pore size distribution has been elucidated with field emission scanning electronic microscopy (FESEM) and positron annihilation spectroscopy (PAS). A double dense-layer structure is formed when glass plate is used as the casting substrate and water as the coagulant. The thickness of the ultra-thin bottom layer resulted from hydrophilic–hydrophilic interaction is identified to be around 95 nm, while a fully porous, open-cell structure is formed in the middle support layer due to spinodal decomposition. Consequently, the membrane shows low salt leakage with mitigated ICP in the FO process for seawater desalination. The structural parameter (St) of the membrane is analyzed by modeling water flux using the theory that considers both external concentration polarization (ECP) and ICP, and the St value of the double dense-layer membrane is much smaller than those reported in literatures. Furthermore, the effects of an intermediate immersion into a solvent/water mixed bath prior to complete immersion in water on membrane formation have been studied. The resultant membranes may have a single dense layer with an even lower St value. A comparison of fouling behavior in a simple FO-membrane bioreactor (MBR) system is evaluated for these two types of membranes. The double dense-layer membrane shows a less fouling propensity. This study may help pave the way to improve the membrane design for new-generation FO membranes.
Concentrating sugar solutions is a common process used in the production of many food products for either dewatering a high value product or concentrating waste streams prior to disposal. Thermal and pressure-driven dewatering methods are widely used, but they are prohibitively energy intensive and hence, expensive. Osmotically driven membrane processes, like forward osmosis, may be a viable and sustainable alternative to these current technologies. Using NaCl as a surrogate draw solution, this investigation shows that forward osmosis processes can lead to sucrose concentration factors that far exceed current pressure-driven membrane technologies, such as reverse osmosis. For instance, a concentration factor of 5.7 was achieved by forward osmosis with a starting sucrose concentration of 0.29 M, compared to reported concentration factors of up to 2.5 with reverse osmosis. Water fluxes were found to be lower than those commonly obtained in reverse osmosis, which is a consequence of the significantly higher concentration factors in conjunction with internal concentration polarization. The latter is a common problem in forward osmosis processes that utilize current generation anisotropic polymeric membranes. Further advances in forward osmosis membrane technology would yield higher water fluxes and concentration factors.
Osmosis is a physical phenomenon that has been extensively studied by scientists in various disciplines of science and engineering. Early researchers studied the mechanism of osmosis through natural materials, and from the 1960s, special attention has been given to osmosis through synthetic materials. Following the progress in membrane science in the last few decades, especially for reverse osmosis applications, the interests in engineered applications of osmosis has been spurred. Osmosis, or as it is currently referred to as forward osmosis, has new applications in separation processes for wastewater treatment, food processing, and seawater/brackish water desalination. Other unique areas of forward osmosis research include pressure-retarded osmosis for generation of electricity from saline and fresh water and implantable osmotic pumps for controlled drug release. This paper provides the state-of-the-art of the physical principles and applications of forward osmosis as well as their strengths and limitations.
Osmosis through asymmetric membranes has been studied as a means of desalination via forward osmosis and power generation through a process known as pressure retarded osmosis. The primary obstacle to using asymmetric membranes for osmotic processes is the presence of internal concentration polarization, which significantly reduces the available osmotic driving force. This study explores the impact of both concentrative and dilutive internal concentration polarization on permeate water flux through a commercially available forward osmosis membrane. The coupling of internal and external concentration polarization is also investigated. A flux model that accounts for the presence of both internal and external concentration polarization for the two possible membrane orientations involving the feed and draw solutions is presented. The model is verified by data obtained from laboratory-scale experiments under well controlled conditions in both membrane orientations. Furthermore, the model is used to predict flux performance after hypothetical improvements to the membrane or changes in system conditions.
Vacuum-enhanced direct contact membrane distillation (VEDCMD) and forward osmosis (FO) were investigated for water recovery enhancement in desalination of brackish water. Past studies have demonstrated that both VEDCMD and FO can be effectively utilized in the treatment of a wide range of highly concentrated feed solutions. In the current study, two reverse osmosis (RO) brine streams with total dissolved solids concentrations averaging 7500 and 17,500 mg/L were further desalinated by VEDCMD and by FO. In both processes, high water recoveries were achieved; however, recoveries were limited by precipitation of inorganic salts on the membrane surface. Various cleaning techniques were able to remove the scale layer from the membrane and restore water flux to almost initial levels. FO achieved water recoveries up to 90% from the brines and VEDCMD achieved water recoveries up to 81% from the brines. Addition of a scale inhibitor during both processes was effective at maintaining high water flux for extended time. When considering the total water recovery (the recovery from the RO processes combined with the batch recovery from the VEDCMD or FO process), greater than 96 and 98% total recoveries were achieved for the two different brine streams.
The mechanisms governing internal concentration polarization (ICP) were studied using well-controlled forward osmosis experiments. The relationship between osmotic pressure and water flux was observed across a range of solute concentrations and molecular weights. The effect of membrane orientation on ICP was also studied. Two regimes of ICP — dilutive and concentrative — were described and characterized, and their governing equations were tested. Resistances to solute diffusion within the membrane porous support layer were calculated under each regime and found to be consistent across a wide variety of experimental parameters.
The improvement of an innovative dual membrane contactor process for treatment of combined hygiene and metabolic wastewater was investigated. Flux and solute rejection in the combined direct osmosis/osmotic distillation (DO/OD) process were enhanced by incorporating membrane distillation (MD) concepts into the process. Two new configurations were investigated: DO/MD, in which the driving force was temperature gradient only, and DO/membrane osmotic distillation (DO/MOD) in which the driving forces were temperature gradient and concentration gradient. Development of a temperature gradient across the membranes substantially enhances the flux of the dual membrane process. It was demonstrated that water flux could be increased by up to 25 times with only a 3–5 °C temperature difference across the membranes. Solutes in the feed wastewater, including urea, were completely rejected. It was demonstrated that complex wastewaters that cannot be treated by one process only could be well treated using a dual membrane process.
Pressure retarded osmosis (PRO) was investigated as a viable source of renewable energy. In PRO, water from a low salinity feed solution permeates through a membrane into a pressurized, high salinity draw solution; power is obtained by depressurizing the permeate through a hydroturbine. A PRO model was developed to predict water flux and power density under specific experimental conditions. The model relies on experimental determination of the membrane water permeability coefficient (A), the membrane salt permeability coefficient (B), and the solute resistivity (K). A and B were determined under reverse osmosis conditions, while K was determined under forward osmosis (FO) conditions. The model was tested using experimental results from a bench-scale PRO system. Previous investigations of PRO were unable to verify model predictions due to the lack of suitable membranes and membrane modules. In this investigation, the use of a custom-made laboratory-scale membrane module enabled the collection of experimental PRO data. Results obtained with a flat-sheet cellulose triacetate (CTA) FO membrane and NaCl feed and draw solutions closely matched model predictions. Maximum power densities of 2.7 and 5.1 W/m2 were observed for 35 and 60 g/L NaCl draw solutions, respectively, at 970 kPa of hydraulic pressure. Power density was substantially reduced due to internal concentration polarization in the asymmetric CTA membranes and, to a lesser degree, to salt passage. External concentration polarization was found to exhibit a relatively small effect on reducing the osmotic pressure driving force. Using the predictive PRO model, optimal membrane characteristics and module configuration can be determined in order to design a system specifically tailored for PRO processes.
Forward osmosis (FO) is attracting increasing interest for its potential applications in water and wastewater treatment and desalination. One of the major drawbacks of FO is internal concentration polarization (ICP), which significantly limits the FO flux efficiency. In addition, FO membrane flux can be adversely affected by membrane fouling. The effects of ICP and fouling on FO flux behavior were systematically investigated in the current study. Both theoretical model and experimental results demonstrated that the FO flux was highly non-linear with respect to the apparent driving force (the concentration difference between the draw solution and the feed water) as a result of ICP. ICP played a dominant role on FO flux behavior at greater draw solution concentrations and/or greater membrane fluxes due to the exponential dependence of ICP on flux level. Compared to the active layer facing draw solution (AL-facing-DS) configuration, more severe ICP was observed when the membrane active layer faced the feed water (AL-facing-FW) as a result of dilutive ICP in the FO support layer. Interestingly, the AL-facing-FW configuration showed remarkable flux stability against both dilution of the bulk draw solution and membrane fouling. In this configuration, any attempt to reduce membrane flux was compensated by a reduced level of ICP. The net result was only a marginal flux reduction. In addition, foulant deposition was insignificant in this configuration. Thus, the AL-facing-FW configuration enjoyed inherently stable flux, however, at the expense of severer initial ICP. In contrast, the AL-facing-DS configuration suffered severe flux reduction as porous membrane support faced the humic acid containing feed water. The flux loss in this configuration was likely due to the combined effects of (1) the internal clogging of the FO support structure as well as (2) the resulting enhanced ICP in the support layer. The latter was caused by reduced porosity and reduced mass transfer coefficient of the support. The pore clogging enhanced ICP mechanism probably played a dominant role in FO flux reduction at higher flux levels. To the authors’ best knowledge, this is the first study to systematically demonstrate the coupled effects of ICP and fouling on the FO flux behavior.