Seawater Reverse Osmosis System Research Articles

Simulation-Based Assessment of Safe Operating Windows and Optimization in Seawater Reverse Osmosis Systems

Simulation-Based Assessment of Safe Operating Windows and Optimization in Seawater Reverse Osmosis Systems

A water-heat combined supply system based on waste heat from a coastal nuclear power plant in northern China

• A new water-heat combined supply system based on waste heat is proposed. • The system realizes the simultaneous delivery of water and heat. • The heating and water supply costs are 34.27 ¥/GJ and 4.08 ¥/m 3 , respectively. • The system has a higher exergy efficiency and a better economic benefit. Based on the existing waste heat recovery technology and seawater desalination technology, a water-heat combined supply system based on waste heat from a costal nuclear power plant is proposed. The proposed system can improve the heating capacity of district heating systems and reduce the energy consumption of water-generating seawater reverse osmosis systems by recovering the waste heat from exhaust steam in a nuclear power plant. Through the simultaneous delivery of water and heat, the long-distance conveyance cost is reduced. The thermodynamic characteristics, energy-saving properties, environmental benefits, and economic efficiency of the proposed system are discussed and analysed. The results show that the heating capacity of the proposed system is increased by 55.5% compared to the original extraction steam heating load. The exergy efficiency is 16.07% higher and the heating cost is 12.32 ¥/GJ lower than those of traditional cogeneration district heating systems. The water supply cost of the proposed system is 2.95 ¥/m 3 lower than that of the conventional desalination water supply system. The current results of this study will be helpful in solving problems related to heat and water resource shortages and the high costs of long-distance heat and water conveyance in coastal areas.

Performance evaluation and boron rejection in a SWRO system under variable operating conditions

It is well known that reverse osmosis (RO) is the leading desalination technology. As an energy intensive technology, the exploitation of renewable energy sources (RES) to power RO systems is a attractive option. A strategy to take advantage of all the available energy of an off-grid renewable system is to work with the RO system under variable operating conditions. This implies additional challenges in terms of water production and permeate quality, among others. Boron rejection is one of the main concerns in seawater RO (SWRO) systems. The aim of this work was to evaluate the performance and boron rejection of a single-stage SWRO system with 7 membrane elements per pressure vessel under variable operating conditions. The initial permeability coefficients of two SWRO membranes (TM820L-440 and TM820S-400) were calculated from experimental data of a full-scale SWRO desalination plant. These coefficients and the characteristics of the membranes were introduced in a simulation algorithm to estimate the behavior of the SWRO system. The results show that, compared with the TM820S-400 membrane, the TM820L-440 performed better in terms of boron rejection in the form of boric acid, but worse in terms of water production. When RES-powered SWRO systems are designed to work under variable operating conditions, consideration needs to be given to the safe operation window in terms of boron concentration in the permeate and to variation of the permeability coefficient of the membranes.

Performance evaluation of a Sea water reverse osmosis system at different operating scenarios

Performance evaluation of a Sea water reverse osmosis system at different operating scenarios

Life cycle assessment for algae-based desalination system

To clearly understand the environmental impacts of algae-based desalination system, a life cycle assessment is undertaken by comparing four different scenarios: (1) multi-stage microalgae based desalination system without algal biomass reuse; (2) multi-stage microalgae based desalination system with algal biomass reuse; (3) hybrid desalination system with microalgae as the pre-treatment for Low Pressure Reverse Osmosis; and (4) conventional Seawater Reverse Osmosis system. It is suggested that scenarios 2 and 4 have the overall lowest and highest environmental impacts, respectively. This phenomenon is probably caused by the lower energy consumption and lower operational and maintenance requirements of high rate algal pond system. Furthermore, scenario 2 considers the algal biomass reuse to generate electricity through anaerobic digestion, which could offset the environmental impacts. It is also identified that the operational phase contributes approximately 50–60% of the total environmental impact in most impact categories for algae-based desalination system, however, the operational phase contributes around 70–80% of the total environmental impact for membrane based desalination system. A sensitivity study is undertaken to evaluate the effects of three key design parameters on environmental impact in global warming impact category. It is identified that the total dissolved solids removal efficiency of high rate algal pond plays the most important role in determining the potential environmental impacts of different scenarios. This study clearly shows that scenario 2 of multi-stage microalgae based desalination system with algal biomass reuse is the most environmentally friendly desalination approach.

Simultaneous Optimization of Size and Operation for Seawater Reverse Osmosis Network with Permeate Split and Interstage Permeate Split Designs

Simultaneous optimization of size and operation for the seawater reverse osmosis (RO) system is proposed through a superstructure-based mixed-integer differential-algebraic programming approach. The influences of operating conditions on the performance of pumps, RO membrane, and the properties of seawater are considered. The permeate split (PS) design, interstage permeate split (ISPS) design, and on–off model could give more operability and cost saving. For constant feed conditions, nearly constant profiles of control variables are obtained with a larger water tank. The maximum level in a water tank is an indicator to measure its buffer capacity. For the variation of temperature and time-of-use electricity price, although energy consumption can be effectively saved by one time shut off at a large water demand time period with high electricity price, not much water cost is reduced due to the increase of the plant size. Low load operation is an alternative solution. The ISPS design could balance the flux distributions inside the pressure vessel and extract more permeates with high quality; thus, the water cost and energy consumption could be reduced, compared with the interstage design (ISD) and the PS design. The uncertainties of the maximum concentration of product water and the membrane fouling factor are considered in a two-stage stochastic program. It is useful for identifying a robust optimal design and operation solutions for membrane-based seawater desalination.

Numerical methodology and CFD simulations of a rotary vane energy recovery device for seawater reverse osmosis desalination systems

• Novel analytical grid generation methodology for rotary vane machines. • Removal of backflows and torque peaks through flow port optimisation. • Sensitivity analysis to reduce cavitation and improve volumetric efficiency. • Volumetric efficiency with optimal speed and blade design equal to 91.6%. Energy recovery devices in Seawater Reverse Osmosis Systems (SWRO) reduce energy consumption and may facilitate the large-scale deployment of desalination systems. In this paper, a Rotary Vane Energy Recovery Device (RVERD) is analysed and optimised by aiming at weakening cavitation and improving the volumetric performance of the machine. An innovative analytical methodology based on user defined nodal displacement is proposed to address the need to discretise the rotating and deforming computational domain of double-acting vane machines. The generated grids are interfaced with the ANSYS FLUENT solver for multi-phase computational fluid dynamics simulations. The flow topology is analysed to reveal the flow and cavitation features especially in the blade tip regions. A port optimisation is then carried out followed by a sensitivity analysis on the design parameters to improve RVERD performance. The results show that delaying the discharge angle at the high-pressure outlet port by 3° and an optimal port to stator length ratio of 70% helped to prevent backflows and eliminate torque peaks. The sensitivity analysis has identified the rotational speed and the blade tip clearance as the two most influential factors affecting cavitation and, in turn, the volumetric efficiency of the machine. With respect to the baseline design configuration, at the optimal rotational speed of 1000 RPM and with a tip clearance gap of 50 μm, the volume-averaged vapour volume fraction in the core decreased from 20.6 × 10 −3 to 0.6 × 10 −3 while the volumetric efficiency increased from 85.7% to 91.6%. The axial clearance gap of 70 μm contributed to 2.9% of the volumetric losses.

The feasibility survey of an autonomous renewable seawater reverse osmosis system with underwater compressed air energy storage

Water scarcity issue is increasingly obvious due to rapid population growth and flourished industrialization. As an energy intensive technology, the desalination has been gained more attention nowadays, which is transformed to renewable sources driven for a sustainable future. In this paper, an autonomous renewable seawater reverse osmosis system with the support of underwater compressed air energy storage is proposed. As the primary power source, the wind turbine and solar photovoltaic are applied to generate electricity. Meanwhile, the underwater compressed air energy storage system acts as an energy buffer to manage the stochastic power generation and consumption. The simulation results show that the loss of power supply probability and the loss of water supply probability in 1% maximum loss of power supply probability threshold condition are 0.9993% and 1.3667%, respectively. Meanwhile, the energy demand can be matched well by renewable supply, despite some deficient exists. In addition, the power source configuration, the volume and pressure of flexible energy bag, the reverse osmosis membrane area and the wind turbine hub height have effect on system performance. This work can provide some useful information about the potential of compressed air energy storage in seawater reverse osmosis plant for future development.

Evidence of solution-diffusion-with-defects in an engineering-scale pressure retarded osmosis system

An engineering-scale seawater reverse osmosis-pressure retarded osmosis (SWRO-PRO) system was designed and deployed to evaluate the energy recovery during seawater desalination through salinity gradient energy. Experimental data on energy recovery and consumption were collected from a PRO system composed of up to five 4040 spiral-wound membrane elements, utilizing freshwater and RO concentrate to drive permeation. During experimental testing, specific energy recoveries as high as 0.14 kW h/m3 were achieved; however, energy consumption due to pressure losses in the system reduced the net specific energy recovery to a maximum of −0.07 kW h/m3. A 100% increase in energy recovery or 50% decrease in energy consumption would be necessary to yield a positive net energy recovery. Also, a combined approach of simultaneously increasing energy recovery and decreasing energy consumption would be a more desirable path for SWRO-PRO to become an energy positive technology. Experimental data were then paired with modeling software utilizing a solution-diffusion-with-defects model to demonstrate the presence and extent of defects in the membrane structure that allow for pressure-driven pore flow. The solution-diffusion-with-defect model explains the lower than expected water permeability and salt rejection often seen in experimental PRO results. Integrating this model into future software will allow for more accurate simulation of larger systems and aid in investigations of PRO scale-up.

Measuring Biofouling Potential in SWRO Plants with a Flow-Cytometry-Based Bacterial Growth Potential Method.

Measuring the bacterial growth potential of seawater reverse osmosis (SWRO) feed water is an issue that is receiving growing attention. This study developed and demonstrated the applicability of the flow-cytometry (FCM)-based bacterial growth potential (BGP) method to assess the biofouling potential in SWRO systems using natural microbial consortium. This method is relatively fast (2–3 days) compared to conventional bioassays. The effect of the potential introduction of nutrients during measurement has been studied thoroughly to achieve the lowest measure value of about 45,000 cells/mL, which is equivalent to about (10 µg-C glucose/L). The BGP method was applied in two full-scale SWRO plants that included (i) dissolved air flotation (DAF) and ultra-filtration (UF); (ii) dual-media filtration (DMF) and cartridge filter (CF), which were compared with the cleaning frequency of the plants. A significant reduction (54%) in BGP was observed through DAF–UF as pre-treatment (with 0.5 mg Fe3+/L), while there was a 40% reduction by DMF–CF (with 0.8 mg Fe3+/L). In terms of the absolute number, the SWRO feed water after DAF–UF supports 1.5 × 106 cells/mL, which is 1.25 times higher than after DMF–CF. This corresponds to the higher cleaning-in-place (CIP) frequency of SWRO with DAF–UF compared to DMF–CF as pre-treatment, indicating that the BGP method has an added value in monitoring the biofouling potential in SWRO systems.

Development and operational stability evaluation of new three-cylinder energy recovery device for SWRO desalination system

The energy recovery device (ERD) plays an important role in reducing energy consumption of seawater reverse osmosis (SWRO) system. However, for reciprocating-switcher energy recovery device (RS-ERD), stroke switching will cause flowrate and pressure pulsations of high and low pressure fluids, which will affect the operation stability of SWRO system. In this paper, we develop a three-cylinder energy recovery device (TC-ERD) and a matching control strategy with instantaneous overlap function to simultaneously reduce the flowrate and pressure pulsations of high and low pressure fluids. Under design capacity (30 m3/h) and 5.0 MPa operating pressure, the pressure pulsation amplitude of HP brine is only 0.0675 MPa, which is 86.5% lower than that of RS-ERD. The flowrate pulsation amplitude (0.954 m3/h) of LP seawater decreases by 68.2% compared with RS-ERD, while that (1.336 m3/h) of HP brine is approximately doubled. The doubling of HP brine flowrate pulsation arises from the fact that the actual operating point of the boost pump is much lower than its rated operating point, which can be improved by optimizing pump selection in future work. The energy recovery efficiency of the TC-ERD is as high as 97.1% at design capacity and 5.0 MPa operating pressure.

Exergoeconomic and exergoenvironmental analyses of an off-grid reverse osmosis system with internal combustion engine and waste heat recovery

Recognizing the intensive energy consumption of desalination and the importance of the water-energy nexus, integrated energy schemes can reduce energy costs and carbon emissions while simultaneously increasing energy efficiency. This study presents exergoeconomic and exergoenvironmental assessments for a stand-alone, off-grid desalination plant, constituted by an internal combustion engine, a Rankine-based heat recovery unit, and a seawater reverse osmosis system. The use of fossil diesel is compared to soybean biodiesel. The exergoeconomic analysis follows the SPECO method, and the exergoenvironmental analysis employs environmental data obtained from a Life Cycle Assessment using the Eco-indicator 99 impact assessment method. The exergy efficiency of the plant is 6.9% and 6.7% for the fossil diesel and biodiesel scenarios, respectively. The highest destructions of exergy occurred in the engine, followed by the reverse osmosis system, for both fuel scenarios. Results of the exergoeconomic and exergoenvironmental assessments suggest that the reverse osmosis system and the engine should be the first focuses of actions aimed at reducing overall costs and environmental impacts. The levelized cost of freshwater is 1.312 $/m³ and 2.164 $/m³ (within literature values), and the environmental impact of freshwater is 207.5 mPt/m³ and 70.4 mPt/m³ for the fossil diesel and biodiesel scenarios, respectively. The environmental impacts associated with freshwater are 75% lower for biodiesel because the impacts associated with its production are significantly lower. The system proposed herein is technically viable and an alternative for off-grid seawater desalination.

Monitoring Biofouling Potential Using ATP-Based Bacterial Growth Potential in SWRO Pre-Treatment of a Full-Scale Plant.

Several potential growth methods have been developed to monitor biological/organic fouling potential in seawater reverse osmosis (SWRO), but to date the correlation between these methods and biofouling of SWRO has not been demonstrated. In this research, the relation between a new adenosine triphosphate (ATP)-based bacterial growth potential (BGP) test of SWRO feed water and SWRO membrane performance is investigated. For this purpose, the pre-treatment of a full-scale SWRO plant including dissolved air flotation (DAF) and two stage dual media filtration (DMF) was monitored for 5 months using BGP, orthophosphate, organic fractions by liquid chromatography coupled with organic carbon detection (LC-OCD), silt density index (SDI), and modified fouling index (MFI). Results showed that particulate fouling potential was well controlled through the SWRO pre-treatment as the measured SDI and MFI in the SWRO feed water were below the recommended values. DAF in combination with coagulation (1–5 mg-Fe3+/L) consistently achieved 70% removal of orthophosphate, 50% removal of BGP, 25% removal of biopolymers, and 10% removal of humic substances. Higher BGP (100–950 µg-C/L) in the SWRO feed water corresponded to a higher normalized pressure drop in the SWRO, suggesting the applicability of using BGP as a biofouling indicator in SWRO systems. However, to validate this conclusion, more SWRO plants with different pre-treatment systems need to be monitored for longer periods of time.

Application of two-stage reverse osmosis system for desalination of high-salinity and high-temperature seawater with improved stability and performance

Owing to the significant increase in seawater desalination, seawater exhibiting extreme conditions of high salinity and high temperature cannot be avoided in the production of fresh water. Although seawater reverse osmosis (SWRO) is one of the most energy-saving desalination technologies, it might not be favorable for treating extreme seawater due to the operational limitations associated with the single-stage SWRO configuration. To overcome these limitations, the applicability of two-stage SWRO was examined. The recovery of single-stage SWRO was limited to 31% for stable operation, whereas two-stage SWRO, utilizing pressure exchangers as energy recovery devices, achieved 34% recovery for the same number of pressure vessels. Furthermore, the maximum water fluxes in the front SWRO elements were reduced under the design constraints of the two-stage configuration than that of the single-stage configuration. The two-stage SWRO system was energy-efficient with high recovery; however, under extreme conditions, the permeate quality was similar to that of single-stage SWRO. To improve the applicability of two-stage SWRO, internally staged design (ISD) and split partial second pass (SPSP) can be adopted. ISD can reduce water fluxes in the front elements, increasing system recovery, whereas SPSP design enables the reverse osmosis system to produce high-quality permeate with low additional energy.

Separation of mono- and di-valent ions from seawater reverse osmosis brine using selective electrodialysis.

As water scarcity has become a serious global issue, seawater reverse osmosis (SWRO) is considered as a promising technique to expand traditional water supplies. However, the reject brine from SWRO systems is still a major environmental concern. In this research, the monovalent selective electrodialysis (S-ED) was used to separate and recover one of the primary components, i.e., sodium chloride, from the SWRO brine, thereby avoiding the direct discharge of the brine and achieving the brine valorization. The permselectivity of selective ion-exchange membranes (IEMs) was elucidated by comparing with the standard IEMs in structure-property via membrane characterization techniques. Results indicated that the permselectivity of Selemion CSO membrane was attributed to the positive-charged layer with a low sulfonate/ammonium ratio of 1.28. Whereas the permselectivity of Selemion ASV membrane resulted from the highly cross-linked layer, according to the similar content of the fixed quaternary amines and the shift of the C‑N absorption peak. In addition, the effects of the current density and temperature on the membrane performance were studied, including permselectivity ([Formula: see text] and [Formula: see text]), Na+ recovery, and specific energy consumption (ESEC). Finally, the NaCl-rich brine with the total dissolved solid (TDS) value of 167.5 ± 3.3g/L was obtained using SWRO brine with the initial TDS of 76.8g/L. The Na+/Mg2+ mass ratio of the concentrate was 222.4, compared with the initial value of 9.7 in SWRO brine.

Performance Assessment of SWRO Spiral-Wound Membrane Modules with Different Feed Spacer Dimensions

Reverse osmosis is the leading process in seawater desalination. However, it is still an energy intensive technology. Feed spacer geometry design is a key factor in reverse osmosis spiral wound membrane module performance. Correlations obtained from experimental work and computational fluid dynamics modeling were used in a computational tool to simulate the impact of different feed spacer geometries in seawater reverse osmosis spiral wound membrane modules with different permeability coefficients in pressure vessels with 6, 7 and 8 elements. The aim of this work was to carry out a comparative analysis of the effect of different feed spacer geometries in combination with the water and solute permeability coefficients on seawater reverse osmosis spiral wound membrane modules performance. The results showed a higher impact of feed spacer geometries in the membrane with the highest production (highest water permeability coefficient). It was also found that the impact of feed spacer geometry increased with the number of spiral wound membrane modules in series in the pressure vessel. Installation of different feed spacer geometries in reverse osmosis membranes depending on the operating conditions could improve the performance of seawater reverse osmosis systems in terms of energy consumption and permeate quality.

Optimization of two-stage seawater reverse osmosis membrane processes with practical design aspects for improving energy efficiency

While single-stage is the general configuration for seawater reverse osmosis (SWRO), the two-stage design can increase the overall recovery of an SWRO system. Due to its high-recovery operation, the specific energy consumption (SEC) of two-stage SWRO is higher than that of single-stage. Thus, the two-stage configuration has not been extensively applied in the current desalination market. In contrast, recent studies have reported that the two-stage design can lower the SEC of SWRO compared to that of single-stage. However, the analyses were biased towards SEC, and the practical design aspects (e.g., permeate quality, water flux, and design ratios) were not systemically considered. Thus, this study examines the applicability of a two-stage SWRO system with a capacity of 100,000 m3/d that employs 1200 pressure vessels (PVs). Two-stage SWRO actually consumed a greater amount of energy than that of single-stage for typical SWRO recovery with the same number of PVs. In contrast, single- and two-stage SWRO produced permeate similar in quality, while the two-stage exhibited superior water-flux distribution along the PVs. Additionally, optimal ratios of permeate flow rate and number of PVs were determined by energy recovery devices type, where the ratio of 1:2 was selected for the reverse osmosis system with a pressure exchanger and 2:1 for that with a Pelton turbine. Considering SEC and other operational aspects, the use of two-stage SWRO was feasible at a 50–70% recovery rate.

Optimum arrangement of two-stage plug and concentrate recycling RO systems using thermodynamic and exergy analysis

Purpose This paper aims to determine the optimum arrangement of a reverse osmosis system in two methods of plug and concentrate recycling. Design/methodology/approach To compare the optimum conditions of these two methods, a seawater reverse osmosis system was considered to produce fresh water at a rate of 4,000 m3/d for Mahyarkala city, located in north of Iran, for a period of 20 years. Using genetic algorithms and two-objective optimization method, the reverse osmosis system was designed. Findings The results showed that exergy efficiency in optimum condition for concentrate recycling and plug methods was 82.6 and 92.4 per cent, respectively. The optimizations results showed that concentrate recycling method, despite a 36 per cent reduction in the initial cost and a 2 per cent increase in maintenance expenses, provides 6 per cent higher recovery and 19.7 per cent less permeate concentration than two-stage plug method. Originality/value Optimization parameters include feed water pressure, the rate of water return from the brine for concentrate recycling system, type of SW membrane, feedwater flow rate and numbers of elements in each pressure vessel (PV). These parameters were also compared to each other in terms of recovery (R) and freshwater unit production cost. In addition, the exergy of all elements was analyzed by selecting the optimal mode of each system.

Assessing pretreatment and seawater reverse osmosis performance using an ATP-based bacterial growth potential method

Various bacterial growth potential (BGP) methods have been developed recently to monitor biofouling in seawater reverse osmosis (SWRO) systems such as assimilable organic carbon and bacterial regrowth potential. However, the relationship between these methods and biofouling in SWRO desalination plants has not yet been demonstrated. In this research, an attempt is made to investigate if a correlation exists between BGP of SWRO feed water and the chemical cleaning frequency in SWRO plants using an ATP-based BGP method employing an indigenous microbial consortium. Using ATP-based BGP method at 5 different seawater locations showed low variations of bacterial yield.The BGP method was applied to assess the pretreatment performance of three full-scale SWRO plants with different pretreatment processes. Dual media filtration (DMF) showed the highest BGP removal (>50%) in two SWRO plants. Removal of BGP and hydrophilic organic carbon in dissolved air floatation combined with ultrafiltration was similar to the removal achieved with DMF in combination with inline coagulation. For the three SWRO plants investigated, a higher BGP in SWRO feed water corresponded to a higher chemical cleaning frequency. However, more data is required to confirm if a real correlation exists between BGP and biofouling in SWRO plants.

CFD Simulation on Cavitation in a Rotary Vane Energy Recovery Device

The rotary vane energy recovery device (RVERD), as a kind of rotary vane machines, has a potential application in seawater reverse osmosis (SWRO) system to decrease the system energy consumption, which can significantly improve the system efficiency and make the production of fresh water economical with low carbon emission. Cavitation can occur in RVERD where local pressure falls below the vapor pressure, which may decrease the device efficiency and erode the internal surface. In this work, the internal flow in working chamber and the vane slot of RVERD is fully simulated by ANSYS Fluent software. The stand k-ɛ turbulence model, together with the Schnerr-Sauer cavitation model, is adopted. A dynamic mesh method coupled with the User Defined Functions (UDF) to control the vane movement is proposed for grid generation in the moving and deforming fluid domain. According to the simulation results, the deforming grid generation can be achieved to perform numerical simulation of RVERD by the proposed method. Cavitation tends to occur in the vane tip gap zone, the downstream zone of vane, and the vane slot bottom zone, corresponding to the local low pressure zones within RVERD. This study presents the implementation of the mesh motion and the obtained cavitation results of RVERD, which can help for optimal design of rotary vane machines.