Pressure Retarded Osmosis Modes Research Articles

Computational Fluid Dynamics Modeling of Pressure-Retarded Osmosis: Towards a Virtual Lab for Osmotic-Driven Process Simulations

Pressure-Retarded Osmosis (PRO) is an osmotically driven membrane-based process that has recently garnered significant attention from researchers due to its potential for clean energy harvesting from salinity gradients. The complex interactions between mixed-mode channel flows and osmotic fluxes in real PRO membrane modules necessitate high-fidelity modeling approaches. In this work, an efficient CFD framework is developed for the 3D simulation of osmotically driven membrane processes. This approach is based on a two-way coupling between a CFD solver, which captures external concentration polarization (ECP) effects, and an analytical representation of internal concentration polarization (ICP). Consequently, the osmotic water flux and reverse salt flux (RSF) can be accurately determined, accounting for all CP effects without any limitations on the geometrical complexity of the membrane chamber or its flow mode/regime. The proposed model is validated against experimental data, showing good agreement across various PRO case studies. Additionally, the model’s flexibility to simulate other types of osmotically driven processes such as forward osmosis (FO) is examined. Thus, the contributions of ECP and ICP effects in local osmotic pressure drop along the membrane chamber are comprehensively compared for FO and PRO modes. Finally, the capability of the CFD model to simulate a lab-scale PRO module is demonstrated across a range of Reynolds numbers from low-speed laminar up to turbulent flow regimes.

A multifunctional desalination-osmotic energy storage (DOES) system for managing energy and water supply

Net-zero carbon emission target for mitigating climate change accelerates the exploitation of renewable energy, such as solar and wind, as power origin in utilities sector. However, the intermittency of renewable energy escalates the supply-demand mismatch in not only electricity sector but also water sector, as freshwater supply increasingly relies on unconventional, energy-intensive desalination that is increasingly powered by renewables. To enhance the energy-water resilience, we propose a desalination-osmotic energy storage (DOES) system, which alternates the operation of reverse osmosis (RO) for desalination and pressure retarded osmosis (PRO) for electricity generation, achieving multiple functions including freshwater production and storage, grid energy storage, and eventually bulk-scale management of freshwater and energy supply. Via innovative system design and operation integrating semi-closed (SC) and closed-circuit (CC) configurations in RO and PRO modes in the proposed DOES, energy loss arising from over-pressurization in RO and under-pressurization in PRO could be substantially reduced. As a result, DOES can achieve practical maximum energy efficiencies of >75 % for both RO desalination and PRO electricity generation, and a round-trip efficiency of >68 % when used as a grid-scale energy storage system (ESS). Apart from the quantitative analysis of energy performance, a qualitative comparison of various other performance metrics between DOES and other grid ESSs is also conducted. Given its various performance advantages as well as multi-functionality, the DOES system could be an important complement to, though not replacement of, existing grid-scale ESSs.

High-performance thin-film composite forward osmosis membranes with hydrophilic PDA@TiO2 nanocomposite substrate for the treatment of oily wastewater under PRO mode

The applications of thin-film composite (TFC) forward osmosis (FO) membranes are limited by low flux and strong internal concentration polarization (ICP). In this study, a simple, facile, and eco-friendly approach to improve the wettability of the FO membrane substrate using hydroxyl- and amino-functionalized TiO2 nanocomposites by polydopamine (PDA) coating on TiO2 particles is demonstrated. In our research, a composite membrane substrate was fabricated by incorporating PDA@TiO2 into polyethersulfone (PES) through the non-solvent induced phase separation (NIPS) process. Then, a selective active polyamide layer was formed by interfacial polymerization on top of the PES-PDA@TiO2 membrane substrate to obtain the resulting TFC-FO membrane. The composite membrane substrate exhibited higher hydrophilicity (about 25% lower contact angle) and higher porosity than the pristine membrane substrate. The water flux (Jw) of 34.3 L/m2h and 60.1 L/m2h was achieved for the resulting TFC-PDA@TiO2 FO membrane under FO and pressure retarded osmosis (PRO) modes which was ~174% and ~183% higher than that of the pristine TFC-FO membrane under FO and PRO modes respectively. Furthermore, the TFC-PDA@TiO2 membrane exhibited enhanced selectivity, as expressed by the decrease in the specific salt flux (Js/JW) values (from 0.48 g/L and 0.53 g/L of pristine TFC to 0.27 g/L and 0.28 g/L for TFC-PDA@TiO2 under the FO and PRO modes respectively). More importantly, the TFC-PDA@TiO2 membrane exhibited better fouling behavior with the organic foulant solution. These results demonstrate that the fabricated TFC-FO membranes have significant potential for sustainable water reclamation from oily wastewater via FO under the PRO mode.

Synthesis and characterization of novel thin film composite forward osmosis membrane using charcoal-based carbon nanomaterials for desalination application

In this study, charcoal-based carbon nanomaterial (CNM) was used to modify thin film composite (TFC) membranes for forward osmosis (FO) process. Different amounts of CNM were incorporated into polyethersulfone (PES) and the effects of CNM addition on hydrophilicity, roughness, and morphology of the prepared substrates were investigated. To fabricate the TFC-FO membranes, m-phenylenediamine (MPD) and trimesoyl chloride (TMC) were used for polyamide (PA) layer formation on the substrate surface. FO performance of the prepared TFC membranes was evaluated in both FO and pressure retarded osmosis (PRO) modes using deionized (DI) water as feed solution (FS) and 1 M NaCl solution as draw solution (DS). Also, to study the TFC membranes capability for desalination purposes, the Caspian Seawater was used as FS. It was found that incorporation of CNM into the TFC membrane substrate leads to the reduced structural parameter (S), significantly. The membrane containing 0.5 wt% CNM showed the best performance with water flux of 12.08 and 32.25 L.m−2.h−1 (LMH) in FO and PRO modes, respectively (approximately 3 times higher than that for the neat membrane). Also, Js/Jw value significantly reduced from 1.40 to 0.25 g/L. This research demonstrated that charcoal-based CNM is a proper candidate for modification of the TFC-FO membranes to achieve high water flux and selectivity.

Combined effects of Pseudomonas quinolone signal-based quorum quenching and graphene oxide on the mitigation of biofouling and improvement of the application potential for the thin-film composite membrane

Biofouling caused by the growth of the biofilm is the main bottleneck that limits the effective operation of thin-film composite (TFC) membrane in the forward osmosis (FO) process. This study investigated the combined effects of graphene oxide (GO) immobilized thin-film nanocomposite (TFN-S) membrane and Pseudomonas quinolone signal (PQS)-based quorum quenching on biofouling mitigation, especially under the operation of pressure-retarded osmosis (PRO) mode, and the influence of methyl anthranilate (MA) inhibitor on the composition and structure of biofilm was also evaluated. Synthetic wastewater was used as the feed solution, in which the model strain Pseudomonas aeruginosa was added to simulate biofouling. The results showed that GO modification and MA addition both efficiently mitigated flux decline and EPS secretion, but the interference of PQS pathway on biofouling control was better than GO embedding. TFN-S membrane with MA addition exhibited superior anti-biofouling performance based on the combined effects of GO and MA. The alleviated concentration polarization and enhanced hydrophilicity of the TFN-S membrane reduced the flux decline in the early stage. Additionally, the antibacterial property of GO inhibited the viability of the attached bacteria (under PRO mode) and MA further mitigated the EPS secretion and biofilm development in the later stage. In the presence of PQS inhibitor MA, live/total cells ratio was 15% and 13% higher than that of TFC membrane in FO and PRO modes, respectively. Furthermore, exogenous addition of MA led to a relatively loose biofilm structure, resulting in high membrane permeability in the biofouling formation process.

Fabrication of high-performance nanofiber-based FO membranes

Being partially commercialized and has specific application areas, where the reverse osmosis technology cannot serve, forward osmosis (FO) technology is continually receiving extensive research to promote its performance. In this study, high-performance FO nanofiber-based substrate membrane was fabricated for potential application of saline water desalination. Sulfonated polysulfone (sPSU) with definite sulfonation level was used to fabricate support layer. Tubular beadless fiber network owning scaffold-like structure with a fiber diameter of 247 nm was formed. Polysulfone was sulfonated by heterogeneous method using chlorosulfonic acid as a sulfonation agent. The substrate and FO membranes were characterized mainly by means of scanning electron microscopy (SEM), water permeation flux, porometry, contact angle, Fourier transform infrared (FTIR), as well as other tests, while the characterization of thin-film composite separation layer was restricted to SEM and FTIR. The characterization illustrates that the sPSU support layer is highly porous with a narrow pore size distribution. FO performance evaluation of two commercial and newly developed membranes was probed using FO and pressure-retarded osmosis (PRO) modes with cocurrent and counter-current flow scheme. The active layer presents excellent intrinsic properties with A/B of 17.31 and a high salt separation ratio of 99.54%. The newly developed membrane can achieve a high FO and PRO water flux of 65.7 and 313 L m(-2) h(-1), respectively, using a 1 M NaCl draw solution and deionized water feed solution. The corresponding salt flux is only 2.5 and 5.3 g m(-2 )h(-1). The reverse flux selectivity represented by the ratio of water flux to reverse salt flux (J(w)/J(s)) was kept as high as 26.3 and 58.8 L g(-1) for FO and PRO modes. To the best of our knowledge, the performance of the current work-developed membrane is superior to all FO membranes previously reported in the literature.

Effect of fouling on performance of pressure retarded osmosis (PRO) and forward osmosis (FO)

A layered model is developed to describe mass transport through fouled membranes in pressure retarded osmosis (PRO) and forward osmosis (FO) processes. This resistance-in-series model accounts for salt and water transport through the active layer, support layer, external boundary layers, and the cake layer formed by foulants. Closed-form algebraic expressions for the water flux, salt flux, salt concentration profile, and overall transport coefficients are presented for FO membranes in both membrane orientations. Organic fouling experiments using alginate are used to validate the model and observe the effects of feed salinity, cross-flow velocity, and membrane orientation on foulant accumulation rates. Increasing feed salinity and cross flow velocity both lead to a decrease in foulant accumulation in PRO orientation. Under identical operating conditions, foulant accumulation rates are comparable in FO and PRO modes. The model is then evaluated to show how foulant accumulation affects FO system performance in both membrane orientations. The model is also used to elucidate the effect of fouling on power production in PRO: fouling affects the optimal PRO operating pressure, and a 0.5 mm-thick cake layer can reduce power production by 50%. The mathematical expressions developed serve as a simple tool to predict the performance of fouled membranes in both PRO and FO processes.

Forward osmosis for oily wastewater reclamation: Multi-charged oxalic acid complexes as draw solutes

Forward osmosis (FO) has demonstrated its merits in hybrid FO seawater desalination. However, FO may have a potential for other applications if suitable draw solutes are available. In this study, a series of novel draw solutes based on oxalic acid (OA)-transitional metal complexes are presented. Influential factors of FO performance have been systematically investigated by varying the transitional metals, cations of the complex draw solutes as well as the experimental conditions. Compared to NaCl and other recently synthesized draw solutes, the OA complexes show superior FO performance in terms of high water fluxes up to 27.5 and 89.1 LMH under the respective FO and PRO (pressure retarded osmosis) modes, both with negligible reverse solute fluxes. The features of octahedral geometry, abundant hydrophilic groups and ionic species are crucial for the OA complexes as appropriate draw solutes with satisfactory FO performance. Among the synthesized OA complexes, the ammonium salt of chromic complex (NH4-Cr-OA) outperforms others due to the presence of more ionic species in its complex system. NH4-Cr-OA also performs better than the typical NaCl draw solute in FO oily wastewater treatment with higher water recovery and negligible reverse fluxes. Dilute solutions of OA complexes have been reconcentrated through membrane distillation (MD) and reused to new round of FO processes. The OA complexes have demonstrated their suitability and superiority as a novel class of draw solutes for the FO process in this study.

Influence of different ion types and membrane orientations on the forward osmosis performance

This study investigated the influence of different ions on forward osmosis (FO) performance in terms of water flux. NaCl, NaNO3, KCl, KNO3, NH4Cl, NH4NO3, CaCl2 and Ca(NO3)2 were selected as representatives of draw solutions (DSs). Data were obtained from laboratory-scale experiments under well controlled conditions in both membrane orientations operated in FO mode pressure retarded osmosis (PRO) mode. The basic properties of DS (osmotic potential, viscosity, diffusion coefficient) changed the water and reverse salt transmission. The results indicated that the performance of one solution in different modes was different. The behavior of Na+ and K+ salts was quite satisfactory in the PRO mode; specifically NaCl had the greatest water flux (16.96L/(m2h)), and the following largest was KCl (16.55L/(m2h)). K+ salts had the highest water flux among all the selected positive ions in the FO mode which was KCl (10.30L/(m2h)). To the negative ions, Cl− salts showed higher water flux than nitrates in both PRO and FO modes, such as the water flux of NaNO3 was 7.96L/(m2h), while that of NaCl was 8.86L/(m2h) in the FO mode. Overall, K+ and Cl− were regarded as optimal candidates of cation and anion, respectively.

Removal of dissolved organics from produced water by forward osmosis

This study investigated the performance of the forward osmosis (FO) process for treating produced water. Water permeate flux and reverse salt flux (RSF) were examined at different feed pH values and operating configurations (i.e. FO, pressure retarded osmosis (PRO), and reverse osmosis (RO) modes). Acetic acid was selected as a model organic acid to present the dissolved organic fraction in produced water. Results reported here indicate that only membranes specifically designed for FO applications can be used in the FO and PRO modes. Due to the internal concentration polarization phenomenon, the PRO mode resulted in a higher water permeate flux and RSF than those in the FO mode. Acetate rejection was pH dependent in both the FO and RO modes. Furthermore, in the RO mode, acetate rejections by the FO membranes were higher than their nanofiltration counterparts. Results reported here suggest that FO can be a viable treatment option for the removal of dissolved organics from produced water.

A sulfonated polyphenylenesulfone (sPPSU) as the supporting substrate in thin film composite (TFC) membranes with enhanced performance for forward osmosis (FO)

The new sulfonated polyphenylenesulfone (sPPSU) materials synthesized via direct route with various content of sulfonated units, i.e., 2.5 and 5mol% 3,3′-di-sodiumdisulfate-4,4′-dichlorodiphenyl sulfone (sDCDPS) monomer, have been effectively implemented as supporting layers of the thin film composite (TFC) membranes for forward osmosis (FO) applications. Not only does the hydrophilic nature of membrane substrates essentially facilitate the water transport across the membrane during the FO process, but also possibly provide anti-fouling characteristics as well as induce the formation of fully sponge-like structures. Compared to TFC-FO membranes made of hydrophobic non-sulfonated PPSU supporting layers, those made of hydrophilic sPPSU supporting layers comprising 2.5mol% sDCDPS can achieve a 4.4-fold increment on water flux up to 54 LMH with 8.8 gMH salt reverse flux under the pressure retarded osmosis (PRO) mode using 2M NaCl as draw solution. Surprisingly, the newly developed TFC-FO membranes show a much smaller difference in water flux between PRO and FO modes compared to previous works, indicating much lower ICP, particularly at low draw solution concentrations, i.e. 0.5–2M NaCl. When tested for seawater desalination using 3.5wt% NaCl as the feed and 2M NaCl as the draw solution, the aforementioned membrane show a water flux up to 22 LMH under the PRO mode, which is the highest ever reported. Furthermore, the structural parameter indicating the internal concentration polarization (ICP) can be remarkably decreased with an increase in sulphonated material contents in membrane substrates.

Removal of trace organic contaminants by the forward osmosis process

The rejection of trace organic contaminants (TrOCs) by an osmotically driven membrane filtration process was investigated. A set of 40 compounds representing major groups of TrOCs of concern was selected for this study. The rejection of the TrOCs by a commercial cellulose acetate asymmetric forward osmosis membrane, as well as a “tight” commercial thin-film composite nanofiltration (NF) membrane, was systematically investigated and compared under three different operating modes: forward osmosis (FO), pressure retarded osmosis (PRO) and reverse osmosis (RO). Results revealed that the cellulose acetate membrane had considerably smaller water and salt permeabilities as well as less negative surface charge compared to the NF membrane. However, the cellulose acetate membrane resulted in considerably higher water flux than the NF when operated in FO and PRO modes. Nevertheless, the NF membrane displayed consistently better TrOC rejection than the HTI membrane. In RO mode, electrostatic interactions played a dominant role in governing the rejection of charged TrOCs. In FO and PRO modes, the rejection of charged TrOCs was governed by both electrostatic interaction and size exclusion, while rejection of neutral compounds was dominated by size exclusion, with rejection increasing with TrOC molecular weight. Operating in PRO mode resulted in a higher water flux but a notably lower TrOC rejection as compared with FO mode, because of more severe internal concentration polarization (ICP) phenomenon. Another important observation from this study is that rejection of neutral TrOCs in FO mode was higher than that in RO mode. This could be attributed to the retarded forward diffusion of TrOCs resulting from reverse salt flux of the NaCl draw solution, a phenomenon that takes place in FO mode but is not possible in RO mode.

Effect of draw solution concentration and operating conditions on forward osmosis and pressure retarded osmosis performance in a spiral wound module

Forward osmosis (FO) and pressure retarded osmosis (PRO) are concentration-driven membrane processes. While they can be potentially used in water, wastewater, and energy applications, these processes suffer from the concentration polarization inside the porous membrane support resulting in severe flux decrease, a phenomenon known as internal concentration polarization (ICP). Researchers have investigated the effect of ICP both in theoretical and experimental studies. The current study extends the existing ICP model to include the effect of draw solution dilution by membrane permeate flow in a spiral wound FO module (SWFO). FO and PRO experiments were performed using a Hydrowell ® SWFO under both submerged and cross-flow conditions. The effect of draw solution concentration, draw solution flow rate, feed water flow rate, and membrane orientation on FO and PRO water flux performance was systematically investigated. Permeate flow increased with greater draw solution concentration in both FO and PRO modes. ICP was found to drastically limit the available membrane flux in the concentration-driven membrane processes, and its adverse effect was more severe at greater draw solution concentration. Membrane flux was also affected by the dilution of draw solution when the permeate flow rate was comparable or greater than the draw solution flow rate. The submerged FO configuration performed nearly as good as the cross-flow configuration with feed water circulating outside of the membrane envelope (shorter flow path). In this case, the feed water flow rate only had limited effect on membrane flux likely due to its low mass transfer resistance. In contrary, the membrane flux can be adversely affected at low feed water flow rate when it was circulated inside of the membrane envelope (longer flow path).