Pressure Retarded Osmosis Technology Research Articles

Role of permeability coefficients in salinity gradient energy generation by PRO systems with spiral wound membrane modules

Processes that can transform a salinity gradient into electrical energy have gained attention in recent years. One such process, which uses semipermeable membranes to generate electrical energy through a turbine, is pressure retarded osmosis (PRO). As a potential renewable energy technology, this process could also be integrated in desalination plants to reduce the energy consumption. However, principally because of certain drawbacks concerning membrane and module characteristics, PRO technology has not yet been fully exploited at commercial scale. This study aims to assess the impact of membrane permeability coefficients on the energy generated by full-scale single-stage PRO systems. This allow an evaluation of the performance of PRO modules in series considering variation of the permeability coefficients that may be due to the impact of fouling. An evaluation was made of the HTI OsMem™ 2521 spiral wound membrane module considering a diameter of 8 inches (high up-scaled active area) and different permeability coefficient ranges. The results showed that a 50% water permeability coefficient decrease would produce an approximately 25% decrease in the amount of energy that could be generated, while a 50% increase in the solute permeability coefficient would have virtually no effect when considering optimal operating points. Variation of the water permeability coefficient had more impact on the potential amount of generated energy than variation of the solute permeability coefficient.

Salinity gradient energy generation by pressure retarded osmosis: A review

Pressure retarded osmosis (PRO) has gained attention due to its use as a salinity gradient energy-generating membrane process. This process can convert difference in salinity between two streams into energy as it allows water transport through a semi-permeable membrane against the application of hydraulic pressure. This review provides a comprehensive look at the history and latest developments in preparation of membranes and modules for the PRO process, as well as the various applications of PRO. This review also explored the influence of feed characteristics and pretreatment strategies on water permeation and power generation during PRO operation. The current status and technological advancements of PRO as a process were reviewed, revealing how PRO can be operated as a stand-alone process or in integration with other hybrid processes. Despite the recent advancements in material and process development for PRO, membrane performance, wide-scale implementation, and commercialization efforts still leave much to be desired. Recognizing the current challenges facing the PRO technology, the advancements in PRO membrane and module development, and the various applications of the process, this review also draws out the future direction of PRO research and generation of osmotic salinity gradient energy as a viable energy source.

Toward scale-up of seawater reverse osmosis (SWRO) – pressure retarded osmosis (PRO) hybrid system: A case study of a 240 m3/day pilot plant

Pressure retarded osmosis (PRO) has been considered as a potential technique to harvest salinity gradient energy (or osmotic energy). The SWRO-PRO hybrid system is one of the viable ways of implementing PRO technology to resolve problems associated with seawater desalination. However, relatively few works have been attempted in pilot scales, which are inevitable for the scale-up and practical application of the SWRO-PRO technology. In this context, this paper reports the pilot study results of SWRO-PRO hybrid system (240 m3/day). To examine the long-term performance, the plant was operated for more than 2 years. The flux and energy consumption in the plant were analyzed to examine the potential of PRO as a secondary energy recovery device. The fouling propensity of the PRO membranes was examined. Results showed that the use of PRO can reduce the energy consumption of SWRO process by approximately 20%. Moreover, the SWRO brine was diluted up to 63%. This work will provide an insight into the researchers and scientific community by filling the gap between the bench-scale works and the commercial-scale application.

A novel approach for optimal energy recovery using pressure retarded osmosis technology: Chemical exergy pinch analysis – Case study in a sugar mill plant

In the present study, pinch analysis is extended taking chemical exergy concept into account. The novel chemical exergy pinch analysis is proposed for sustainable power production by an economic application of pressure retarded osmosis membranes in chemical industries. Chemical exergy composite curves and chemical exergy cascade tables are developed as graphical and numerical tools, respectively. The tools are used to obtain maximum waste energy recovery by achieving various targets and determining the pinch point in a salinity gradient network. Thus, maximum energy recovery and minimum waste treatment are targeted, simultaneously. Moreover, a mathematical model follows the chemical exergy pinch analysis for an economic evaluation of pressure retarded osmosis-retrofitted industries under three probable scenarios. A sugar mill plant is simulated as the case study to validate the model-based analysis. The results showed that chemical exergy pinch analysis could efficiently provide the optimal pressure retarded osmosis -retrofitted industrial networks for decision-making. Having analysed the complex chemical exergy streams by chemical exergy pinch analysis, 11.30 MW net power is recovered with 0.038 $/kWh levelized cost of energy in the case study.

Thin-film composite hollow fiber membrane with inorganic salt additives for high mechanical strength and high power density for pressure-retarded osmosis

Using high salinity draw solutions in pressure retarded osmosis (PRO) demands novel membranes with a higher withstanding pressure and a higher power density to take full advantage of the increased osmotic gradient for greater osmotic energy production. This study highlights novel strategies to design an inner-selective thin-film composite (TFC) PRO hollow fiber membrane with a high operating pressure and a high power density by adding an inorganic salt, CaCl2, to the dope solution and fine-tuning the spinning conditions. An optimal CaCl2 dosage can not only effectively increase the dope viscosity but also reduce the mean pore size and narrow the pore size distribution of the polyethersulfone (PES) hollow fiber substrates. A high flowrate ratio of dope solution to bore fluid greatly enhances the mechanical strength of the PES substrates. The combination of these two strategies results in a more permeable, better-supported and less defective polyamide selective layer after interfacial polymerization. The best newly developed TFC hollow fiber has a power density of 38 W/m2 at 30 bar by using 1.2 M NaCl and deionized water as the feed pair. To our best knowledge, these are the highest power density and operation pressure ever achieved by PRO hollow fiber membranes in the literature. The improved performance will advance the PRO technology closer to commercialization and make osmotic energy more competitive with other renewable energies.

Carbon Quantum Dots Grafted Antifouling Membranes for Osmotic Power Generation via Pressure-Retarded Osmosis Process.

Osmotic power generated by pressure-retarded osmosis (PRO) has attracted global attention as a clean, abundant and renewable energy resource. However, the substrates of PRO membranes are particularly prone to fouling because of their direct contact with various foulants in raw water. This leads to a significant decline in power density and impedes the commercialization of PRO technology. In this work, a facile surface modification method has been developed to obtain a new type of nanoparticle functionalized antifouling PRO membranes. Carbon quantum dots (CQDs), with an average size around 3.2 nm, are fabricated from citric acid via a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on the substrate surface of poly(ether sulfone) (PES) membranes via covalent bonding. The bacteria diffusion tests show that the CQD modified PRO membranes possess much enhanced antibacterial activity and antibiofouling propensity. The continuous PRO operations at 15 bar also confirm that the CQD modified membranes exhibit a much higher power density (11.0 vs 8.8 W/m2) and water recovery after backwash (94 vs 89%) than the unmodified ones. This study may open up a new avenue in the fabrication of nanostructure functionalized polymeric membranes for wastewater treatment and osmotic power generation.

Investigation of the fouling effect on a commercial semi-permeable membrane in the pressure retarded osmosis (PRO) process

Abstract One of the main challenges for generating osmotic power by using PRO technology is the membrane fouling which reduces the permeate flux and consequently increases the cost and decreases the osmotic power generation. In this research, different feed waters with various qualities were used to investigate the effect of fouling on a commercial membrane in PRO mode in continuous conditions. Also, the fouling mechanisms were investigated in order to study the cleaning of the membrane in PRO mode in the future research. In addition, four classic fouling models such as complete blocking model (CBM), intermediate blocking model (IBM), standard blocking model (SBM) and cake filtration model (CFM) were used. Cake enhanced osmotic pressure as a new mechanism for osmotically driven membranes was studied as well. According to the results, the fouling rate when the draw solution was synthetic salt water followed the order of: untreated river water > multimedia sand filter > microfiltration > ultrafiltration effluents. The fouling rate in ultrafiltration and microfiltration effluents using sea water occurred faster compared to the results for untreated synthetic salt water. Complete fouling (permeate flux was negligible) occurred after 580 h using feed water from ultrafiltration unit. It was observed that cake filtration and cake enhanced osmotic pressure were the main fouling mechanisms that governed the membrane fouling. These models could describe the membrane fouling in PRO processes.

Development of a SWRO-PRO hybrid desalination system: pilot plant investigations

Abstract Seawater reverse osmosis (SWRO) desalination technology accounted for 78% of annual new contracted desalination capacity from 2006 to 2012, due of its lower electrical power consumption (3–5 kWh/m3) versus thermal desalination technologies (up to 18 kWh/m3), such as multistage flash and multiple effects. However, the existing SWRO desalination process still needs further improvement to lower the energy consumption. Recently, a novel hybrid SWRO desalination system using pressure-retarded osmosis (PRO) technology has been studied, which can recover a large amount of osmotic power from the concentrated brine. In this study, GS Engineering & Construction Corp. (GS E&C) developed an advanced SWRO-PRO process to economically couple this PRO technology with a conventional SWRO desalination process. To investigate the SWRO-PRO process, pilot and demonstration plants, of 20 m3/d and 240 m3/d PRO treatment capacity, were constructed and thin-film composite spiral-wound PRO membrane modules (8 inches) were assessed. The operating parameters of the pilot plants, such as pressure, temperature, and flow rates of the draw solution and the feed solution, were found to be important factors determining the plant energy consumption and operating efficiency. An economic analysis of a large-scale SWRO-PRO hybrid desalination is also described.

Identification of the type of foulants and investigation of the membrane cleaning methods for PRO processes in osmotic power application

Pressure retarded osmosis (PRO) is a membrane based technology used for generation of osmotic power as a sustainable energy. The main challenge of PRO technology is the membrane fouling which reduces the permeate flux and osmotic power generation which consequently increases the cost. In this research, different quality feed waters were used to identify the main foulants on the surface of a commercial semi permeable membrane in the PRO process. In addition, two cleaning methods were investigated to determine the desirable cleaning methods. According to the results, the main inorganic foulants using synthetic salt water were iron, aluminum, calcium, sodium, and silica and when sea water was used as the draw solution, the same foulants, in addition to magnesium and potassium were detected. The organic foulants were humic substances, polysaccharides and proteins for both synthetic salt and sea water draw solutions. According to the results, the water flux recoveries were 98.4%, 98%, 97.5%, 97.3%, and 97.2% after each sequential chemical cleaning, respectively. Therefore, the most successful cleaning method was with acidic and basic cleaners. This research indicates that chemical cleaning can decrease the membrane fouling in PRO processes and consequently improve the osmotic power generation.

Design and fabrication of inner-selective thin-film composite (TFC) hollow fiber modules for pressure retarded osmosis (PRO)

Abstract Pressure retarded osmosis (PRO) is a promising technology to harvest the renewable osmotic energy from salinity gradients. There are great progresses in the fabrication of PRO membranes in the last decade. Thin-film composite (TFC) hollow fibers have been widely studied and demonstrated superior performance. However, the lack of effective TFC hollow fiber modules hinders the commercialization of the PRO technology. Knowledge and experiences to fabricate TFC hollow fiber modules remain limited in the open literature. In this study, we aim to reveal the engineering and science on how to fabricate TFC hollow fiber modules including the formation of inner-selective polyamide layers and the repair of leakages. TFC-PES hollow fiber modules with 30% and 50% packing densities have been successfully fabricated, showing peak power densities of 20.0 W/m2 and 19.4 W/m2, respectively, at 20 bar using 1 M NaCl solution and DI water as feeds. The modules may be damaged during handling and high pressure testing. The repaired modules have a power density of 18.2 W/m2, 91% of the power densities of the undamaged ones. This study would make up the gap between TFC membrane fabrication and TFC membrane module fabrication in the membrane industry.

Influence of macromolecular additive on reinforced flat-sheet thin film composite pressure-retarded osmosis membranes

The salinity gradient is a renewable and sustainable energy source that can be utilized by a pressure-retarded osmosis (PRO) process. Development of the PRO technology was impeded by shortage of efficient membranes until recent surge on PRO research. In this work, flat-sheet thin film composite (TFC) PRO membranes with reinforced support layer were developed. The membrane properties were optimized by blending high molecular weight polyvinylpyrrolidone (PVP) into the polysulfone (PSf) casting solution. Influences of PVP addition on the kinetic and thermodynamic properties of the casting solution, resultant membrane properties as well as PRO performance were systematically investigated. It is showed that PVP can effectively improve the membrane morphology and pore structure. By adjusting the PSf/PVP composition in the substrate recipe, the reinforced TFC membrane is able to achieve a power density of 12.9W/m2 at 22bar using 1M NaCl draw solution and pure water feed. Long-term PRO test at the peak power density and multi-pressure cycle tests were performed on the membranes. Consistent power density and low salt flux/water flux ratio were observed in the whole tests. The results demonstrated that the reinforced TFC membrane in this study had excellent mechanical strength and stability. It would be promising for future engineering application.

Recent Advances in Osmotic Energy Generation via Pressure-Retarded Osmosis (PRO): A Review

Global energy consumption has been highly dependent on fossil fuels which cause severe climate change and, therefore, the exploration of new technologies to produce effective renewable energy plays an important role in the world. Pressure-retarded osmosis (PRO) is one of the promising candidates to reduce the reliance on fossil fuels by harnessing energy from the salinity gradient between seawater and fresh water. In PRO, water is transported though a semi-permeable membrane from a low-concentrated feed solution to a high-concentrated draw solution. The increased volumetric water flow then runs a hydro-turbine to generate power. PRO technology has rapidly improved in recent years; however, the commercial-scale PRO plant is yet to be developed. In this context, recent developments on the PRO process are reviewed in terms of mathematical models, membrane modules, process designs, numerical works, and fouling and cleaning. In addition, the research requirements to accelerate PRO commercialization are discussed. It is expected that this article can help comprehensively understand the PRO process and thereby provide essential information to activate further research and development.

Experiment and modeling for performance of a spiral-wound pressure-retarded osmosis membrane module

Pressure-retarded osmosis (PRO) process utilizes the transport of water through a semipermeable membrane to generate electricity from salinity gradient resources. Recent PRO research has shown the feasibility of PRO technologies in laboratory-scale experiments, but there is currently a lack of experimental pilot-scale investigations to ensure the success of PRO technology. This study was conducted to predict the power density of a PRO module using PRO membrane transport properties such as water permeability, salt permeability, and membrane structure parameter. The performance of an 8040 spiral-wound PRO module was experimentally investigated, and the results were compared with the simulated prediction. The maximum power density of the investigated PRO module was 1.8 W m−2 at 10.4 bar using 35 g L−1 of NaCl as a draw solution. At the outlet of the module, the concentration changes of the draw and feed solutions were observed, suggesting a gradual decrease of membrane power density inside the PRO module. The simulation model, which considered concentration changes of draw and feed solutions, reverse salt flux, and mass transport coefficient inside the module, closely estimated the performance of the PRO module. However, the model overestimated the power density at high hydraulic pressure difference. It was concluded that severe increase of reverse salt flux at a high hydraulic pressure difference negatively contributed to the performance of the PRO module.

Three pressure retarded osmosis processes

Pressure retarded osmosis (PRO) processes can be implemented on a number of water types, using different technologies and achieving various power outcomes. The first one is seawater with river water, necessitating movement at seawater level of huge water volumes with extremely low power losses. The pressure exchanger has to consume significantly less power than it does in RO technology. Pretreatment CAPEX and OPEX expenses have to be considerably lower than those implemented by RO technology, while water quality must be as good as that required for RO. The second one is SWRO brine with wastewater, using already filtered and pressurized brine from an SWRO plant. This option is the most economical, as gauge and osmotic pressure are recovered in the same block of pressure exchangers. The osmotic pressure of the SWRO brine is directly converted to the gauge pressure of the SWRO feed. A proportionally diminishing flow is required from the high pressure pump. The third one is Dead Sea or salt lake with river water, implementing natural exotic resources with extremely high osmotic pressures. The PRO technology in this option requires operation in several stages.

Structural optimization of osmosis processes for water and power production in desalination applications

Osmotic pressure is a separation barrier during the reverse osmosis (RO) desalination operation. Hydraulic pressure must be inserted on the membrane surface to overcome the osmotic pressure and to facilitate the desalination process. Pressure retarded osmosis (PRO) is a renewable source for power production. Power generation through PRO is achieved through salinity gradient generated across an asymmetric membrane. In this study, we seek an optimal integrated system of RO and PRO for seawater desalination and power generation. A mixed integer nonlinear programming (MINLP) model is developed to find optimal osmosis processes arrangement with auxiliary equipment, and their optimal operation conditions for seawater desalination and power generation. Several case studies were analyzed to show the applicability of the proposed model. In general, the results show that the operation of RO can be a feasible source of salinity gradient for power generation by PRO technology while producing desalinated water streams. The process layouts show arrangement of RO and PRO stages with recycled streams in order to increase the salinity gradient in the PRO stages. This is an undesirable condition for the RO operation and it is suggested from our results that high performance PRO membrane with high water permeation and salt rejection may eliminate this condition.

Pressure retarded osmosis in closed circuit: a new technology for clean power generation without need of energy recovery

Power generation from salinity gradients by means of Pressure Retarded Osmosis (PRO) is currently preformed by conventional methods, wherein Energy Recovery (ER) is an essential feature without which such a process is impossible. The newly conceived Closed Circuit (CC) Pressure Retarded Osmosis technology (CC-PRO) described herein is based on a continuous consecutive sequential batch process; wherein, pressurized high salinity feed (HSF) is continuously supplied to the inlets of PRO modules and pressurized effluent removed from their outlets through internal circulation means by the alternating engagement/disengagement of two side conduits (SC). This process proceeds with high energy conservation without need of ER means. The new CC-PRO technology enables high operational flexibility achieved by set point selection of the permeation flux; the circulation flow; the applied pressure of power generation; and the flow ratio circulation/permeation which defines the salinity and osmotic pressure gradients inside the PRO modules. The high flexibility is manifested by the availability of an infinite number of set point combinations for the optimization of the process with respect to maximum power production under lower membrane detrimental effects and reduced fouling factors.

Future of the osmotic processes

Osmotic processes are widely utilized in water-ion separation applications. These include purification of low salinity water, desalination of brackish and seawater and reclamation of wastewater. The advantage of application of pressure driven membrane separation processes is obvious when comparing energy requirements of membrane processes with other desalination technologies. It is possible that future improvement of water permeability of current reverse osmosis (RO) membranes could bring some additional reduction of energy use of membrane desalination. However, for seawater desalination, any significant future reduction of energy requirement is limited by osmotic pressure of the concentrate and apparent coupling of water and salt transport. For current commercial RO membranes the increase of water permeability is associated with increase of salt transport and increased permeate salinity. The nominal salt rejection of commercial seawater membranes is about 99.85%. In order to maintain the same permeate salinity at lower feed pressure, membranes with higher water permeability have to maintain the same salt transport rate, which translates to a proportionally reduced salt passage, i.e. increased salt rejection. It is not very likely that current membrane manufacturing methods could bring additional improvement of salt rejection, which is today frequently above 99.9% for the flat sheet membranes. A new osmotic process that could bring a meaningful reduction of energy of seawater desalination is forward osmosis (FO). FO requires low pressure for recirculation of seawater and draw solution, but does not require high feed pressure to generate sufficient net driving pressure (NDP) to drive water through the membrane. FO utilizes osmotic pressure gradient to extract low salinity water from seawater. Under conditions of availability of low temperature waste heat, FO could desalt seawater at energy requirement less than 50% of the energy required by the current RO seawater desalination technology. The major obstacle for commercial implementation of FO to water desalination is lack of suitable commercially available membranes. The same obstacle, unavailability of suitable membranes, is facing another promising osmotic process: pressure retarded osmosis (PRO). Potentially, PRO could be a significant source of renewable energy, utilizing low salinity-high salinity water mixing energy to generate hydraulic gradient that would drive turbine and generate electric power. Detailed energy generating system configurations have been developed for PRO by S. Loeb and others in the past. However, due to difficulty of manufacturing of suitable membranes, the PRO technology verification experiments have been limited to laboratory conditions only. This paper includes information on current status of reverse osmosis as applied to desalination and wastewater treatment. Process configurations and areas of potential future improvement are discussed. Forward osmosis and pressure retarded osmosis process configurations and projected operating parameters of commercial systems are presented. Performance and configuration requirements of suitable membranes for FO and PRO are discussed.