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Inorganic fouling is one of the critical operational issues in reverse osmosis membrane. Few researches investigated effects of membrane surface characteristics on inorganic fouling and on anti-scaling techniques although the fouling occurs on the membrane surface. The objective of this paper was to examine whether different characteristics of deposition of calcium carbonate solids would occur on four membranes having distinctive surface properties. A lab-scale cell reactor with a crossflow velocity was installed and two coupons were used for one type of membranes. Two feed waters were examined: concentrated synthetic seawater simulating a 30% recovery and a concentrate from a seawater RO plant in operation at Changwon, Korea. The amounts of solid deposition on the attached membranes were increased in all four membranes but the degree of deposition on each membrane was different. Various types of calcium carbonate solids were clearly detected by both XRD and SEM analyses. In general, a membrane with greater roughness and negative surface charge appeared to form more scales. This implied that membrane surface characteristics such as roughness and surface charge affected inorganic fouling, presumably by providing favourable sites for precipitation and enhancing attraction of species to the membrane surface.

Fouling indices have been extensively investigated in membrane applications, for evaluating the feed-water fouling potential and providing guidelines for the pretreatment process. For the past few decades, the silt density index (SDI) has been adopted as the most mature fouling index in many seawater reverse osmosis desalination plants. Recently, a modified fouling index (MFI) has gained attention since it compensates for the defects of the SDI. Its publication by ASTM Standard promoted more settlements in practical applications. To gain insight into the use of a membrane-fouling index, this paper reviewed SDI and MFI applications in laboratory-, pilot-, and plant-scale verifications, under conditions likely to be encountered in seawater desalination. The main focus was on the historical development of the fouling indices from theoretical basis to full-scale applications and the identification of future opportunities to expand their reliability in real applications. In particular, the practical implications associated with plant operation were documented to support an in-depth understanding of the MFI values. [ABSTRACT FROM AUTHOR]

The increasing impact of climate change has worsened drought conditions, leading to a surge in the demand for seawater desalination. However, current seawater reverse osmosis (SWRO) desalination technologies are not energy-efficient for handling the high-salinity, high-temperature, and high-turbidity seawater found in the Arabian Gulf. Therefore, a pilot-scale SWRO desalination plant was established to evaluate the new desalination technologies. The pilot plant tested high-performance SWRO membranes, meshed tube filtration (MTF) as a low-energy pretreatment for the high turbidity induced by algal blooms, and membrane capacitive deionization (MCDI) to improve permeate quality. The high-performance SWRO membrane demonstrated exceptional salt rejection and produced permeate quality in the range of 250–455 mg/L from the feed with salinities of 47,500–53,500 mg/L. MTF was effective in controlling turbidity, and the energy consumption was reduced by 77 % compared to the existing pretreatment process. Furthermore, MCDI exhibited similar levels of energy efficiency as brackish water reverse osmosis and may have the potential for future deployment by enhancing process development. Based on the pilot plant results, it is expected that a 100,000–200,000 m3/d SWRO desalination plant will have a specific energy consumption of 3.11–3.13 kWh/m3 for final product water quality at or below 200 mg/L. [Display omitted] • A pilot-scale SWRO desalination plant was used to investigate low-energy technologies. • Developed SWRO membranes significantly decreased feed TDS under normal pressure. • MTF reduced feed turbidity effectively and consumed lower energy compared to DAF. • MCDI demonstrated potential as a substitute for BWRO in two-pass RO systems. • The SEC of SWRO desalination plants was evaluated as 3.11 kWh/m3 through scale-up. [ABSTRACT FROM AUTHOR]

Abstract Internally staged design (ISD) was introduced as a method to lower front reverse osmosis (RO) element fluxes, yielding operational benefits. Systematic selection of the best ISD combinations has not been conducted as it is dependent on operating conditions (i.e., RO recovery and average flux). This study aimed to optimize seawater reverse osmosis (SWRO) operations using ISD for better product water quality and energy efficiency. A total of 36 ISD combinations were configured with three commercial SWRO membranes and systematically examined under varying operating conditions which were simulations of typical SWRO operation. A Pareto-optimal front, a non-dominated feasible solution set, was created to understand the tradeoff relationship between permeate quality and specific energy consumption while considering all parameters. The Pareto-optimal front was classified into five phases, and an analysis of phase changes provided optimal parameter settings for SWRO operation. However, the occurrence of colloidal fouling altered optimal ISD combinations on the Pareto-optimal front. Particularly, when SWRO membranes were fouled at high-recovery and -flux conditions, ISD combinations employing three different membrane types exhibited superior performance in terms of energy efficiency. With these results, the best SWRO operation was suggested depending on its performance requirements, and the advantages of ISD were further delineated. Graphical abstract fx1 Highlights • SWRO with ISD was optimized to achieve better water quality and energy efficiency. • Efficient SWRO performance control was examined by Pareto-optimal front analysis. • Colloidal fouling changes the best ISD combinations on the Pareto-optimal front. • ISD with three SWRO membrane types is feasible at high-recovery and -flux operations. • The optimal SWRO operation mode is proposed based on its performance requirements. [ABSTRACT FROM AUTHOR]

The effects of discharged brine from desalination plants, particularly brine containing algal toxins during harmful agal bloom (HAB) occurrence, are a major concern for marine life and, as a result, human health. Therefore, removing algal toxins from desalination brine is critical for safeguarding both human health and the marine environment. The present work examined the mineralization of domoic acid (DA), an algal toxin, in highly concentrated seawater for the first time using an ultraviolet light activated peroxymonosulfate (UV/PMS) system. Mineralization efficiencies, effects of various anion concentrations, and reaction by-products were investigated. The presence of anions such as chloride, bromide, and bicarbonate reduces the mineralization efficiency of the UV/PMS system, with bromide being the most detrimental. Although DA can be effectively degraded in synthetic seawater (35,000 ppm), total mineralization is not possible within 180 min of reaction time, with only 38.4 %, 44.8 %, and 41 % TOC removal in pH 3, 5, and 7, respectively, when 1 mM of PMS was used. Analyses of treated hypersaline seawater using ultra-high-resolution liquid chromatography coupled with quadrupole time-of-flight mass spectrometry and ion chromatography revealed that DA was completely degraded in synthetic seawater and that, in the presence of algal organic matter, no toxic by-product was generated during the detoxification process. Overall, the UV/PMS system has the potential to be an effective treatment strategy for algal toxins in seawater during HAB events in the SWRO desalination process. [Display omitted] • Domoic acid (DA) mineralization efficiency by UV/PMS system was investigated. • DA can be effectively removed in deionized water and synthetic seawater. • Anions affected the mineralization efficiency with the most detrimental being Br −> Cl −>HCO 3 −. • No halogenation byproducts, chlorates, or bromates were detected after treating DA in hypersaline seawater. [ABSTRACT FROM AUTHOR]

The use of membrane-based technology has evolved into an important strategy for supplying freshwater from seawater and wastewater to overcome the problems of water scarcity around the world. However, the presence of natural organic matter (NOM), including humic substances affects the performance of the process. Here, we present a systematic report on the mineralization of humic acid (HA), as a model for NOM, in high concentration of salts using the ultraviolet light-activated peroxymonosulfate (UV/PMS) system as a potential alternative for HA elimination during membrane-based seawater desalination and water treatment processes. Effects of various parameters such as PMS concentration, solution type, pH, anions, and anion-cation matrix on HA mineralization were assessed. The results show that 100%, 78% and 58% of HA (2 mg/L TOC) were mineralized with rate constants of 0.085 min−1, 0.0073 min−1, and 0.0041 min−1 after 180 min reaction time at pH 7 when 0.5 mM PMS was used in deionized water, sodium chloride solution (35,000 ppm) and synthetic seawater, respectively. The reduced efficiency under saline conditions was attributed to the presence of anions in the system that acted as sulfate and hydroxyl radicals' scavengers. Furthermore, the safety of the treated synthetic seawater was evaluated by analyzing the residual transformed products. Overall, pretreatment with the UV/PMS system mitigated fouling on the RO membrane. [Display omitted] • The UV/PMS system mineralized humic acid (HA) in high salt concentrations. • Mineralization rate was in the order of DI water > NaCl solution > synthetic seawater. • Anions, particularly Cl − and Br − , retarded mineralization efficiency. • There were no halogenated disinfection byproducts produced. • On the RO membrane, significant fouling mitigation was obtained. [ABSTRACT FROM AUTHOR]

Pressure retarded osmosis (PRO) harnesses salinity gradient energy through the mixing of freshwater and saltwater, addressing the demand for sustainable energy sources. PRO typically utilizes river water or secondary wastewater as the feed solution, paired with seawater reverse osmosis (SWRO) brine as the draw solution. However, the limited availability of low-saline water presents a significant obstacle to energy generation. Therefore, the feasibility of a high-pressure PRO process utilizing seawater as the feed solution and hypersaline water as the draw solution was assessed to generate sustainable blue energy. Seawater has the potential to achieve the maximum extractable Gibbs-free energy through high-pressure PRO by maximizing the feed/draw ratio. The performance of the high-pressure PRO was theoretically evaluated, resulting in a tenfold increase in specific energy production compared to conventional SWRO-PRO due to the higher feed/draw ratio. However, high hydraulic pressure increased the membrane structural parameter and further reduced water flux and power density due to significant internal concentration polarization. Nevertheless, the high-pressure PRO process can achieve a power density exceeding 84 W/m2 when the structural parameter remains below 100 μm. The implications of the high-pressure PRO were further discussed to advance the concept of a blue circular economy, enhance environmental resilience, and promote sustainability. [Display omitted] • Feasibility of high-pressure PRO was investigated using seawater/hypersaline water. • Higher feed/draw ratio boosted energy production in the PRO process. • Seawater was assessed as a feed for high-pressure PRO for blue energy generation. • High-pressure operation raised the structural parameter reducing power density. • High-pressure PRO achieved >80 W/m2 at structural parameter <100 μm. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • SWRO performance was evaluated for high-salinity and high-temperature seawater. • Single-stage SWRO suffered from low recovery and high water flux for front elements. • Two-stage SWRO achieved higher recovery and exhibited lower front water flux. • ISD further increased recovery by reducing water fluxes of front elements. • SPSP improved product quality and energy efficiency by treating rear permeate. [ABSTRACT FROM AUTHOR]

Wetting by fouling is phenomenon specific to membrane distillation (MD) and are regarded as challenges to the seawater membrane distillation (SWMD) process. To understand fouling and wetting, the influence of Mg and Sr crystals, which can potentially cause scaling, as well as Ca crystals deposited on the membrane surface were investigated. Mg(OH) 2 and CaSO 4 had significant impact on fouling and wetting. Even if CaCO 3 and SrSO 4 had no effects on fouling and wetting as single salts, CaCO 3 and CaSO 4 were dominant in synthetic seawater without Mg(OH) 2. However, the occurrence of Mg(OH) 2 scales became a cause for concern if Ca ion was removed from seawater for the prevention of fouling and wetting. Therefore, Mg as well as Ca should be removed for proper fouling and wetting control. NaOH/Na 2 CO 3 softening was used for the removal of Ca and Mg ions. In addition, based on the inhibition effects of Mg ions on Ca scales, a new pretreatment method involving the injection of MgCl 2 to increase the Mg /Ca ratio was examined. Image 1 • Wetting in MD process using a seawater was observed by visualization system. • Wetting by Mg(OH) 2 occurred during the MD process was firstly suggested. • Retardation of the wetting in MD process was achieved by a Mg addition to the feed. • The SrCO 3 scalant was formed when major components of seawater were removed. • The CaCO 3 and CaSO 4 scalants resulted in the partial and full wettings, respectively. [ABSTRACT FROM AUTHOR]

Although seawater desalination is becoming an important technology for freshwater production, the presence of a high concentration of bromide in the seawater presents a major challenge. Bromide is one of the major inorganic precursors for the formation of disinfection by-products such as bromate, which is highly regulated due to its toxicity and carcinogenicity. Hence, a significant reduction of bromide ions is required prior to water disinfection. In Australia, all the desalination plants have to operate a two-stage reverse osmosis system to ensure effective bromide removal, which adds significant cost to the desalination system. In this study, a pilot-scale membrane capacitive deionisation (MCDI) was investigated as a potential alternative to the 2nd stage RO in seawater desalination. Moreover, strategies to enhance water recovery in MCDI was also carried out by using lower flow rates and shorter duration during the desorption stage. In order to reduce energy consumption in MCDI, a combined short-circuit and reverse polarity desorption is introduced. The results showed that MCDI can effectively remove bromide and dissolved salt at a much lower energy consumption compared with membrane process and that MCDI can be operated to achieve high water recovery without increasing the total energy consumption. Unlabelled Image • Bromide and TDS removal in pilot-scale MCDI was investigated • Use of lower desorption flow rate can significantly increase water recovery • Shorter desorption time can result in increased water recovery • A combined short-circuit followed by reverse voltage desorption can be effective [ABSTRACT FROM AUTHOR]

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. Image 1 • Two-stage SWRO is fundamentally analyzed and optimized for seawater desalination. • SEC of two-stage SWRO is higher than that of single-stage in typical recovery. • Water quality of single- and two-stage SWRO is similar at the same average flux. • Optimal ratios of permeate flow rate and number of PVs vary depending on ERD types. • Two-stage SWRO is more energy-efficient at a high recovery rate (e.g., 50–70%). [ABSTRACT FROM AUTHOR]

Preoxidation is gaining spotlight in the mitigation of ultrafiltration (UF) membrane fouling caused by algal organic matter (AOM). Although it is known to be beneficial in freshwater, its applicability in seawater treatment has barely been explored. This study first evaluated the effect of UV/permonosulfate (PMS) oxidation for UF process against marine harmful algal blooms (HAB). The transition of AOM released from Pseudo Nitzchia was investigated which produces a neurotoxin, domoic acid (DA). Specifically, UV light emitting diodes (LED) were employed as UV source. The results indicated UV-LED/PMS treatment effectively reduces the fouling potential of AOM. However, the participation of abundant chloride ions generated free chlorine and disinfection byproducts, thus, PMS addition below 0.5 mM was suggested for seawater applications. With respect to PMS 0.5 mM, TOC and cake layer resistance were reduced by 10% and 85%, respectively, and this was followed by significant improvements in water permeability, flux reversibility and permeate quality of UF process. Furthermore, PMS exhibited significant reactivity for decomposing DA achieving rapid disappearance of 1 ppm of DA within a minute. The aforementioned results encourage the use of PMS as a bulk oxidant in seawater treatment to simultaneously mitigate membrane fouling and improve permeate quality. Unlabelled Image • The impact of UV-LED/PMS preoxidation was assessed against marine AOM fouling. • Despite abundant Cl− ions, it was effective to be applied in seawater condition. • Preoxidation greatly mitigated both reversible and irreversible UF membrane fouling. • UV-LED/PMS preoxidation is also beneficial to reduce toxic substance, domoic acid. • To control moderate amounts of free Cl and THMs, [PMS] < 0.5 mM was suggested. [ABSTRACT FROM AUTHOR]

Most Australian surface and ground waters have relatively high concentration of bromide between 400 and 8000 μg/L and even higher concentration in seawater between 60,000–78,000 μg/L. Although bromide is not regulated, even at low concentrations of 50–100 μg/L, it can lead to the formation of several types of harmful disinfection by-products (DBPs) during the disinfection process. One of the major concerns with brominated DBPs is the formation of bromate (BrO 3 − ), a serious carcinogen that is formed when water containing a high concentration of bromide is disinfected. As a result, bromate is highly regulated in Australian water standards with the maximum concentration of 20 μg/L in the drinking water. Since seawater reverse osmosis (SWRO) desalination plays an important role in augmenting fresh water supplies in Australia, SWRO plants in Australia usually adopt 2nd pass brackish water reverse osmosis (BWRO) for effective bromide removal, which is not only energy-intensive to operate but also has higher capital cost. In this study, we evaluated the feasibility of membrane capacitive deionisation (MCDI) as one of the alternatives to the 2nd pass BWRO for effective bromide removal in a more energy efficient way. [ABSTRACT FROM AUTHOR]

Abstract: Recent studies have shown that the rougher, less negatively charged, and more hydrophobic membranes are prone to organic fouling. These surface characteristics of RO membranes, however, have been usually determined at very low TDS conditions, while seawater contains ten to thousand times more TDS than surface and even brackish waters. In this study, three aromatic polyamide thin-fi lm composite (TFC) RO membranes were characterized for chemical and physical properties. Membrane characterization was performed under both the low (i.e., 10 mg/L) and high (i.e., 35,000 mg/L) TDS conditions to see how these surface characteristics are aff ected by seawater-level TDS. Results showed that both the chemical and physical surface properties were altered signifi cantly under the high TDS condition with becoming more favorable to membrane fouling, namely, less negatively charged, more hydrophobic, and rougher. Mechanisms responsible for these changes such as charge screening and osmotic swelling are delineated. The way of changing in surface characteristics under the high TDS condition was substantially diff erent with respect to the surface characteristics determined at the low TDS condition. It was confi rmed that the chemical and physical properties were interrelated and, thus, variations in chemical properties with respect to the solution chemistry applied led to changes in physical properties and vice versa. [Copyright &y& Elsevier]

A new meshed tube filtration (MTF) process was developed and assessed as a low energy-consuming pretreatment process that withstands algal blooms (i.e., red tide). This process can potentially replace the dissolved air flotation (DAF) process in typical seawater reverse osmosis (SWRO) desalination plants. Two marine microalgae species, namely C. polykrikoides and T. suecica , were first cultivated and then utilized to simulate algal bloom conditions in the lab-scale filtration experiments. The algal matter was coagulated and circulated by aeration in the MTF system that was filled with cylindrical polypropylene meshed tubes with ciliary ends and had the ability to capture coagulated algae in an effective manner. The efficiency of the MTF process to remove algal cells was significantly enhanced by optimizing the coagulant dose and the aeration rate. In order to further verify the MTF performance, DAF experiments were conducted, under similar conditions, for the comparison of algal removal effectiveness and energy consumption rates. The results showed high removal rates of algal cells as well as turbidity for MTF, although only a limited reduction of algal organic matter (AOM) was observed. When DAF was operated at energy consumption levels equivalent to MTF, its algae removal rate was lower than that of MTF. Furthermore, under optimal operating conditions, MTF consumed approximately one-third of the energy needed for DAF operation, mainly due to the fact that MTF requires less aeration. For future improvements in the AOM removal process, an energy-efficient pretreatment process could be realized for SWRO desalination facilities facing potential algal blooms. Unlabelled Image • Novel meshed tube filtration (MTF) was developed for algal bloom control in SWRO. • C. polykrikoides and T. suecica were tested as model marine microalgae species. • Coagulant dose and aeration rate were optimized for enhancing algal removal by MTF. • Turbidity and cell removal rates of MTF were comparable to dissolved air flotation. • MTF required only 33–39% of the energy needed for dissolved air flotation. [ABSTRACT FROM AUTHOR]

Seawater reverse osmosis (SWRO) is usually combined with brackish reverse osmosis (BWRO) to improve the water quality in seawater desalination; however, this configuration often results in high energy consumption. Consequently, membrane capacitive deionization (MCDI) has been suggested as a lower energy alternative technology, but cyclic operation within the cell (i.e., discontinuous operation of ad/desorption) is a main disadvantage. On the other hand, flow electrode capacitive deionization (FCDI) can be continuously operated by using a flow electrode. In this study, the feasibility of a SWRO-FCDI system was examined by using a suitable electrolyte in the flow electrode: seawater or SWRO concentrate. In addition, by employing a selective ion exchange membrane (IEM), the FCDI was able to selectively remove monovalent ions rather than divalent ions, unlike BWRO, thus producing a final product water containing divalent ions, which is more suitable for corrosion control in the subsequent pipe distribution systems. As a result, the remineralization process can be minimized. To test the SWRO-FCDI concept, actual SWRO permeate was first treated in a single mode with an electrolyte comprising real seawater in a flow electrode, and the feasibility of long-term operation was proven by batch-mode, which allowed the optimization of the FCDI active area. Unlabelled Image • The feasibility of a SWRO-FCDI system for low-energy desalination was examined. • Simulated seawater and SWRO concentrate were tested as electrolytes in FCDI. • FCDI removed divalent ions over monovalent ions with 30% higher efficiency. • Actual SWRO permeate was treated by FCDI using with a real seawater electrolyte. • In the batch mode, over 95% removal efficiency was achieved at a 1.3 kWh/m3. [ABSTRACT FROM AUTHOR]

In this study, the FO/crystallization/RO hybrid process was analyzed comprehensively, including experimentation, modeling, and energy and cost estimation, to examine and improve its feasibility to seawater desalination. A new operating strategy by heating the FO process to 45 °C was suggested, and a detailed process design was conducted. A comparative analysis with the conventional seawater reverse osmosis (SWRO) process was performed in terms of specific energy consumption (SEC) and specific water cost (SWC). The hybrid process can produce fresh water with SWC of 0.6964 $/m3, electrical SEC of 2.71 kWh/m3, and thermal SEC of 14.684 kWh/m3. Compared to the conventional SWRO process (SWC of 0.6890 $/m3 and electrical SEC of 2.674 kWh/m3), the hybrid process can produce water with comparable cost and energy consumption. An economic feasibility study that utilized the waste heat and the developed FO technology was also carried out to investigate future developments of the hybrid process. The SWC can be reduced to 0.6435 $/m3 with free waste heat energy. The permeate water quality of the hybrid process was about half that of the conventional SWRO process on molar basis. The results revealed that the FO/crystallization/RO hybrid process can be utilized as a competitive process for seawater desalination with high recovery and high water quality. Image 1 • New operation mode of FO/crystallization/RO hybrid process suggested. • Applicability of FO/crystallization/RO hybrid process experimentally confirmed. • This hybrid process confirmed as cost-competitive for seawater desalination. • Utilizing waste heat, water produced at 7% lower cost than that with SWRO. • Permeate water quality approximately half of the conventional SWRO on molar basis. [ABSTRACT FROM AUTHOR]

Seawater is receiving consideration as an infinite water source in the current era of water shortage. The application of seawater reverse osmosis (SWRO) to prepare seawater for human use is increasing exponentially owing to its high energy efficiency among desalination technologies. However, SWRO is still energy-intensive, and thus needs to be improved further to become more environmentally sound (i.e., producing less CO 2). To tackle such a challenge, this study was conducted to propose strategies to lower the specific energy consumption (SEC) of the SWRO process. From the results of extensive review and theoretical analysis, three directions were proposed for improving the energy efficiency of the SWRO process: i) minimize the irreversible work of the high-pressure pump, ii) decrease the osmotic pressure of the feed, and iii) recover osmotic energy from reverse osmosis (RO) concentrate. After the feasibility of each approach was identified by assessing the theoretical minimum energy and the maximum available margin, strategies for each direction were presented with a technical description, then critically evaluated by considering their practical application. Because this paper includes theoretical perspectives for energy savings in SWRO, it provides comprehensive ideas for lowering the SEC of SWRO desalination plants in the future. Image 1 • Three improvement axes for low-energy reverse osmosis were analyzed theoretically. • Maximum margin and obtainable energy reduction in each axis were calculated. • Minimizing irreversibility is effective, but operating issues are challenging. • Decreasing osmotic pressure can be effective only with additional low-saline water. • Recovering osmotic energy may be effective, but needs to be further developed. [ABSTRACT FROM AUTHOR]

• Critical review of energy consumption of seawater reverse osmosis plants. • Collection of more than 70 datasets for large-size seawater reverse osmosis plants. • Investigation of trends in the application of seawater reverse osmosis plants. • Analysis of factors associated with energy consumption of seawater reverse osmosis. • Future directions to reduce energy consumption of seawater reverse osmosis plants. High specific energy consumption (SEC) is the main barrier for the expansion of seawater reverse osmosis (SWRO). Therefore, the main objective of current SWRO research is to lower the SEC of SWRO plants. However, SEC of SWRO plants has not been systemically explored or analyzed, despite the need for information to develop appropriate strategies to reduce SEC. Therefore, this study aims to review and analyze SWRO plants for a comprehensive understanding of their SEC. First, trends in SWRO application are investigated using more than 70 datasets on large-scale SWRO. The analysis explains the increasing number of large-size SWRO plants, the SEC reduction by isobaric energy recovery devices (ERDs), and the use of different SWRO configurations to meet the energy and quality requirements. Factors associated with SEC (i.e. , feed conditions, target conditions, and equipment efficiency) are also analyzed. High salinity increases energy demand, whereas the temperature effect on energy consumption is not entirely clear. High-efficiency ERDs and pumps can reduce SEC, but overall SEC cannot be explained by these factors alone. SEC is also affected by target water quality and quantity. Moreover, specific SWRO designs can improve the system to efficiently achieve the established goals. Furthermore, future directions to develop low-energy SWRO plants are discussed. [ABSTRACT FROM AUTHOR]