Assessing the major factors affecting the performances of forward osmosis and its implications on the desalination process

Forward osmosis (FO) is an emerging membrane separation technology. It is different from the well-studied pressure-driven membrane separation processes. The FO process is based on water transport under an osmotic pressure difference across a semi-permeable membrane. The distinct operating conditions lead to unique technical challenges during the exploitation of FO technology. According to a comprehensive literature investigation, one of the stringent barriers is lacking of effective FO membranes. The objectives of this research were to develop high performance FO membranes, and furthermore, to systematically study the mass transport and the governing mechanisms in FO process. Thin film composite (TFC) FO membranes with a tailored support structure were developed in this study. The membranes consisted of a highly porous substrate with finger-like pore structure, which was prepared via phase inversion, and a polyamide rejection layer synthesized by interfacial polymerization. The TFC FO membranes had small structural parameters due to the thin cross-section, low tortuosity, and high porosity of the substrates. The membrane rejection layers exhibited superior separation properties (higher water permeability and excellent selectivity) relative to commercial FO membranes. Under FO testing conditions, these membranes achieved high water flux while maintaining relatively low solute reverse diffusion. Comparison of the synthesized TFC FO membranes with commercial FO and reverse osmosis (RO) membranes revealed the critical importance of the substrate structure, with a straight finger-like pore structure preferred over a spongy pore structure to minimize internal concentration polarization (ICP), a unique and critical problem resulting in low water flux in the osmotically driven membrane processes. In addition, membranes with high water permeability and excellent selectivity are preferred to achieve both high FO water flux and low solute flux. The results proved that TFC membranes with a tailored porous substrate and rejection layer are promising for FO applications. In the study of polyamide rejection layer synthesis, the influence of monomer concentrations (i.e., m-phenylenediamine (MPD) and trimesoyl chloride (TMC) concentrations) on the membrane separation properties as well as the FO performance was systematically investigated. A strong trade-off between the water permeability and salt rejection of the membranes was observed, where reducing the MPD concentration or increasing the TMC concentration may result in a higher membrane permeability but a lower salt rejection. In FO tests, membranes with poor salt rejection had severe solute reverse diffusion, which enhanced the severity of ICP. It was found that the FO water flux was governed by both the membrane water permeability and solute rejection. For a membrane with higher water permeability but lower solute rejection, the reduced membrane frictional resistance was compensated simultaneously by the more severe solute-reverse-diffusion-induced ICP. The net effect on the FO water flux depends on the competition of these two opposing mechanisms. Under conditions where solute reverse diffusion may cause severe ICP (e.g., high draw solution concentration and high water flux level), membranes need to be optimized to achieve a high salt rejection even if this is at the expense of lower water permeability. In view of the importance of the water permeability and salt permeability on FO performance, a systematic comparison study of prevailing semi-permeable FO membranes with nanofiltration (NF)-like and RO-like separation properties in terms of flux performance and fouling behavior was conducted. Due to the crucial influence of solute reverse diffusion on FO water flux, the high-rejection RO-like FO membranes generally performed better than the NF-like counterparts in sodium chloride based FO tests. On the other hand, the high permeability of NF-like FO membranes could achieve higher water flux, when proper draw solutes were used to minimize draw solute leakage. Fouling tests suggested that the NF-like TFC FO membranes tended to be more fouling resistant due to their relatively smooth membrane surface. This work further elucidated the major mechanisms that govern the FO performance. These mechanisms were summarized as a frictional resistance loss mechanism (MR), solute-reverse-diffusion-induced ICP (MICP-Js), concentration of feed solutes (concentrative ICP or MICP-feed in the active-layer-facing-draw-solution orientation) and dilution of draw solutes (dilutive ICP or MICP-draw in the active-layer-facing-feed-solution orientation). These mechanisms are related to the properties of membrane, draw and feed solutions. This work led to a set of systematic criteria for the selection of FO membranes, draw solution and optimization of other operating conditions, of which the practicability was demonstrated in potential FO applications.

Forward osmosis (FO) is a membrane process that occurs when solutions of different osmotic pressures are separated by a membrane which is selectively permeable to water. It is a process that drives water permeation across the membrane spontaneously even in the absence of hydraulic pressure difference across the membrane. FO has attracted lots of attention over the last decade and has been explored as a potential alternative to desalination, wastewater treatment and liquid food processing. Significant progress has been made in the development of high-performance FO membranes with high water flux and low reverse solute flux, particularly cellulosic membranes, thin-film composite (TFC) membranes and polyelectrolyte-based membranes. Yet, a few major challenges continue to hamper the widespread implementation of the process in the industry, mainly internal concentration polarization, reverse solute diffusion, membrane fouling, mechanical durability and draw solution regeneration. Most of these challenges are associated with membrane characteristics, which has significantly limited the efficiency of the FO process. To address these challenges, firstly, hollow fiber ultrafiltration membranes were fabricated from polyethersulfone (PES) via a non-solvent induced phase separation (NIPS) process and were used as substrates to prepare inner-selective TFC hollow fiber membranes via an interfacial polymerization (IP) process. The effect of the hollow fiber substrate fabrication conditions on the properties of the substrate and TFC membranes were briefly investigated. The FO performance of the TFC membranes were characterized by using 0.5 M NaCl and DI water as the draw and feed solutions. when the membrane was operated in the active layer-facing-feed solution (AL-FS) and active layer-facing-draw solution (AL-DS) configurations, water flux as high as 41.2 L/m2/h and 74.9 L/m2/h were achieved, while specific reverse solute flux were 0.11 g/L and 0.10 g/L, respectively. Subsequently, a novel double-skinned hollow fiber TFC FO membrane has been successfully fabricated. The FO membrane consisted of a one-step dual-layer substrate and a thin inner selective layer formed via the IP process. The substrate comprises a dense ultrafiltration (UF) outer layer and a relatively porous UF inner layer, both of which were constructed from PES by using a dual-layer co-extrusion technique. The fouling resistance of the double-skinned hollow fiber membrane was evaluated under various testing conditions to verify the viability of double-skinned hollow fiber membranes as a solution to membrane fouling in the FO process. Compared to the commercial and reported double-skinned FO membranes, the FO membrane developed in this thesis exhibited a higher permeate flux with humic acid solution as a feed solution. Furthermore, the double-skinned FO membrane was applied in concentrating activated sludge using 0.5 M NaCl as a draw solution. A permeate flux at 5.4 L/m2/h was achieved after 5-hour operation, which was higher than, or comparable to, those of the reported FO membranes. Membrane autopsies and foulant analysis suggested that the dense UF skin layer helped to reject larger-sized organic foulants (> 300 Da), which shed light on the importance of fabrication features and promising application of the double-skinned hollow fiber TFC FO membrane in sludge concentration. On the other hand, a series of characterization revealed that TFC hollow fiber membranes may experience significant compaction during the FO process despite the lack of applied pressure. Three TFC hollow fiber membranes were fabricated with varied water permeability to study the effect of the osmotic pressure on the TFC membranes. The TFC membranes were continuously tested in FO experiments for 24 h using DI water as feed and varied concentration of NaCl solutions as draw solutions, and their performances were evaluated again using fresh feed solutions. At the end of the FO experiments, all TFC membranes experienced water and salt flux decline to different extents. Visible changes in the cross-sectional morphology and surface topography of the TFC membranes were observed. These observations suggested that the occurrence of membrane compaction is strongly associated with the characteristics of the hollow fiber substrates that were used to prepare the TFC membranes and may be attributed to “negative pressure” build-up within the support layer of the TFC membranes.

Higher boron rejection with a new TFC forward osmosis membrane

Sustainable management of saline oily wastewater via forward osmosis using aquaporin membrane

The role of phase transfer catalysts on properties of polyamide thin-film composite forward osmosis membranes

In order to ensure long-term sustainability of the reservoir, the gas industry in Qatar is faced with the challenge of reducing the volume of produced and process water (PPW) sent to disposal wells by 50% [1-3]. Recently, Qatargas initiated a project to recycle process water and thus, reduce disposal volumes using commercial advanced water treatment technologies [4]. One emerging technology, “osmotic concentration” (OC) has been identified that offers a low-energy alternative to conventional thermal or membrane volume reduction methods. Osmotic concentration is a membrane filtration process that mimics first step in a forward osmosis (FO) system. It requires a high salinity draw solution (DS) which passes on one side of a semi-permeable FO membrane while the feed passes on the other side. Water from the feed is drawn through the membrane, via natural osmosis, reducing the feed volume and increasing the volume of the draw solution. This paper summarizes the results of bench-scale volume reduction tests wit...

Water is crucial for enhancing the yield of agricultural land to meet the growing demand. Forward osmosis (FO) is a developing technology that utilizes the natural osmotic gradient of solutions. In this study, fertilizer drawn FO setup was considered by using potassium chloride (KCl) as the draw solution (DS) for treating textile wastewater as the feed solution (FS). This study investigated the effects of FS temperature, pH, and FS and DS concentrations. The performance investigation involved the study in terms of water flux, reverse salt flux, and specific reverse salt flux. DS and FS properties, osmotic potential, and temperature played a vital role in the performance. At 30°C FS temperature, the highest water flux (5.5 LMH) was observed. Reverse salt flux increased due to the increase in solute diffusivity. The highest value of water flux was obtained at a DS of 1.150 M and FS of 1000 mg/L. The permeation of water improved due to the difference in DS and FS concentrations at pH values above 7. The results of this study suggest that KCl as DS has a higher potential for the treatment of textile wastewater at a temperature of 30°C. Additionally, the functional groups attached to the FO membrane were identified through Fourier-transform infrared (FTIR) spectroscopic study. PRACTITIONER POINTS: Treatment of textile wastewater with the use of fertilizer draw solution (KCl) by forward osmosis process as carried out. The performance was assessed in terms of water flux, reverse salt flux, and specific reverse salt flux. The effects of feed and fertilizer draw solution concentrations; pH and temperature were evaluated on the performance of FO process.

In this study, robust and defect-free thin film composite (TFC) forward osmosis (FO) membranes have been successfully fabricated using ceramic hollow fibers as the substrate. Polydopamine (PDA) coating under controlled conditions is effective in reducing the surface pores of the substrate and making the substrate smooth enough for interfacial polymerization. The pure water permeability (A), solute permeability (B), and structural parameter (S) of the resultant FO membrane are 0.854 L·m–2·h−1·bar−1 (LMH/Bar), 0.186 L·m–2·h−1 (LMH), and 1720 µm, respectively. The water flux and reverse draw solute flux are measured using NaCl and proprietary ferric sodium citrate (FeNaCA) draw solutions at low and high osmotic pressure ranges. As the osmotic pressure increases, a higher water flux is obtained, but its increase is not directly proportional to the increase in the osmotic pressure. At the membrane surface, the effect of dilutive concentration polarization is much less serious for FeNaCA-draw solutions. At an osmotic pressure of 89.6 bar, the developed TFC membrane generates water fluxes of 11.5 and 30.0 LMH using NaCl and synthesized FeNaCA draw solutions. The corresponding reverse draw solute flux is 7.0 g·m–2·h−1 (gMH) for NaCl draw solution, but it is not detectable for FeNaCA draw solution. This means that the developed TFC FO membranes are defect-free and their surface pores are at the molecular level. The performance of the developed TFC FO membranes is also demonstrated for the enrichment of BSA protein.

In this study, robust and defect-free thin film composite (TFC) forward osmosis (FO) membranes have been successfully fabricated using ceramic hollow fibers as the substrate. Polydopamine (PDA) coating under controlled conditions is effective to reduce the surface pores of the substrate and make the substrate smooth enough for the interfacial polymerization. The pure water permeability (A), solute permeability (B) and structural parameter (S) of the resultant FO membrane are 0.854 L·m-2h-1bar-1 (LMH/Bar) 0.186 L·m-2h-1 (LMH) and 1720 µm, respectively. The water flux and reverse draw solute flux are measured using NaCl and proprietary ferric sodium citrate (FeNaCA) draw solutions at low and high osmotic pressure ranges. With increasing the osmotic pressure, higher water flux is obtained but its increase is not directly proportional to the increase in the osmotic pressure. At the membrane surface, the effect of dilutive concentration polarization is much less serious for FeNaCA draw solutions. At an osmotic pressure of 89.6 bar, the developed TFC membrane generates water fluxes of 11.5 and 30.0 LMH using NaCl and synthesized FeNaCA draw solutions. The corresponding reverse draw solute flux is 7.0 g·m-2h-1 (gMH) for NaCl draw solution but it is not detectable for FeNaCA draw solution. This means that the developed TFC FO membranes are defect free and their surface pores are at molecular level. The performance of the developed TFC FO membranes are also demonstrated for the enrichment of BSA protein.

Synthesis and characterization of flat-sheet thin film composite forward osmosis membranes

Forward Osmosis (FO) process is driven by osmotic energy, which is arisen from the osmotic pressure difference between the draw solution (high concentration) and the feed solution (low concentration) separated by a semi-permeable membrane. Combining anaerobic digestion with FO membrane to retain influent organic waste, this research aims to develop the integrated biological wastewater treatment technology: Anaerobic Osmosis-Membrane Reactor (AnOMBR). In the preliminary study, mixed organic fouling of the FO membrane in submerged mode was systematically investigated. Fouling behavior of cellulose triacetate (CTA) FO membrane and thin film composite (TFC) polyamide FO membranes were studied and compared. It was interesting to find under mild FO fouling conditions, TFC FO membranes could have greater fouling tendency as compared to CTA FO membranes due to their greater surface roughness. Although FO is believed to have superior fouling resistance in the AL-FS orientation, severe fouling could occur even at moderate flux levels, especially for TFC membranes or for unstable feed solutions. In this case, solution chemistries such as pH and presence of calcium ions posed remarkable effect on the cake layer composition due to the effect of foulant-foulant interaction(s); In contrast, the foulant composition was not strongly affected by the membrane type (CTA versus TFC) nor the testing mode (pressure-driven NF mode versus osmosis-driven FO mode). With the understanding of FO organic fouling mechanisms, a novel submerged AnOMBR utilizing CTA FO membrane in anaerobic bioreactor was developed and feasibility of using the AnOMBR to treat low-strength synthetic wastewater at mesophilic temperature was evaluated. Flux declined under the effect of both feed conductivity build-up and membrane fouling. Generally fouling on membrane was mild, while both organic fouling and inorganic scaling could still be observed at the edge of membrane. Bulk pH could be sustained within neutral to slightly alkaline due to the retention of alkalinity by FO membrane. The AnOMBR showed good and stable soluble chemical oxygen demand (sCOD) removal and perfect total phosphorous removal. However the removal of total nitrogen and ammonia still needed improvements. The elevated salt environment had marginal effect on bioactivity of methanogens and methane production of AnOMBR system was stable. Based on the promising results, the AnOMBR was operated at both mesophilic temperature and room temperature to compare the performance in terms of membrane flux level and mixed liquor conductivity, nutrient removal and methane production. At room temperature, the flux decreased and conductivity increased both at a slower speed than at mesophilic temperature. The membrane durability was also better and tap water cleaning was practical at room temperature with 90% flux recovery. At both temperatures, the AnOMBR showed good rejection to nutrients. However, at higher temperature, the nutrient concentration in supernatants was relatively lower, indicating the faster and efficient nutrient degradation by microbial at higher temperature. Methane production rate at mesophilic temperature was also significant higher than at room temperature.

Modification of polyacrylonitrile TFC-FO membrane by biowaste-derived hydrophilic N-doped carbon quantum dots for enhanced water desalination performance

Reverse solute flux(RSF) is a big challenge of forward osmosis(FO) technology. This experiment was based on the study of the performance of apple juice concentration by FO technology and the solute diffusion law of functional draw solution(sodium acetate, sodium bicarbonate and sodiumcitrate). Firstly, the basic properties of FO system were studied, including water flux, RSF and rejection rate, by varying the concentration of NaCl draw solution, influent flow rate and membrane operation mode. The concentration ability of deionized water and apple juice and solute diffusion law were analyzed. To achieve the purpose of converting the RSF into advantages, the effect of different functional draw solutions on apple juice concentration and RSF were compared. The results showed that the concentration efficiency and RSF were affected by the concentration of draw solution and membrane operation mode. The membrane mode of pressure retarded osmosis(PRO) led to the higher RSF and the concentration degree of apple juice than the FO mode. In the PRO mode, the RSF reach to 87.34±6.32 g·m−2·h−1 with the NaCl draw solution concentration of 5 mol·L−1. The ability of concentrating apple juice by different concentration of functional draw solution was various. The order of water extraction capacity from high to low was sodium bicarbonate, sodium chloride, sodium acetate, sodium citrate and the order of RSF from high to low was sodium citrate, sodium bicarbonate, sodium chloride, sodium citrate. The RSF was 29.61±2.19 g·m−2·h−1 with 2 mol·L−1 NaCl draw solution, which was only half of the same concentration of NaCl draw solution. Compared with traditional NaCl draw solution, sodium citrate draw solution could effectively control RSF.

Rejection of nutrients contained in an anaerobic digestion effluent using a forward osmosis membrane

Forward osmosis (FO), is membrane separation technology in its infancy with various applications such as desalination, power generation, in food industry and others. The technology has shown growing interest due to its numerous advantages and its potential for energy-saving. FO process is, however, still facing important limitations related to efficient membrane and draw agent to achieve higher performances. Water diffusion in FO is driven by the osmotic pressure gradient from a highly concentrated stream (draw solution) to a lower concentration solution (the feed stream) across a semi-permeable membrane. Concentration polarization (CP) phenomena have been reported to be the most important factor causing water flux decline in FO process. Reverse solute flux (RSF) is also considered as a major issue in FO. In this work, an experimental study has been carried out for the evaluation of CP phenomena and RSF in FO process. Ammonium bicarbonate and sodium chloride have been used respectively as draw and feed solutions. Results have shown that dilutive internal concentration polarization phenomena (ICP) induced a draw solution osmotic pressure reduction of 42%. Concentrative external concentration polarization (ECP) effects have shown to affect driving force to a lesser extent, the combined effect of ICP and ECP caused a water decline of 51.5%. A concentration profile based on the obtained results across the FO membrane has been defined.