Seawater Reverse Osmosis Desalination Plant Research Articles

Reverse osmosis desalination system and algal blooms part II: seawater intake technologies

While thermal desalination processes require minimum pretreatment (mainly screening and chemical additions to prevent scaling), seawater reverse osmosis (SWRO) desalination plants require extensive pretreatment of the feedwater before entering the membranes. As the Arabian Gulf (AG) countries depend on seawater desalination, there is a strategic decision to move gradually to SWRO desalination technologies. The algal bloom (AB) events that have happened in the AG countries raise more concerns about seawater pretreatment. A seawater intake is a key limiting factor and is a real part of pretreatment for high performance desalination process. This paper (second part of a series of three parts) reviews several intake options and their effects on the quality of feed seawater and the major parameters causing membrane fouling, especially bio-fouling. These include the concentrations of algae, bacteria, total organic carbon, particulate and colloidal transparent exopolymer particles (TEP), and the biopolymer fraction of natural organic carbon. Several forms of algal organic matter (AOM) are produced by ABs with varying concentrations and include intracellular organic matter formed due to autolysis consisting of proteins, nucleic acids, lipids and small molecules; and extracellular organic matter formed via metabolic excretion and composed mainly of exopolysaccharides. Being comparatively large macromolecules, exopolysaccharides are most often insoluble in water. A significant fraction of these exopolysaccharides, known as TEP, are highly surface-active, sticky, and play a significant role in the aggregation dynamics of algae during AB events. This paper reviews the different seawater intake technologies and highlights advantages and disadvantages of each. It aims at recommending the best intake technology for the site-specific conditions of a given desalination project.

Energy-exergy analysis of seawater reverse osmosis plants

In this paper, a seawater reverse osmosis desalination plant with various energy recovery systems is studied using exergy analysis. These energy recovery devices include turbines and pressure exchangers as well as infinite area based single and two-stage pressure retarded osmosis units. The appropriate exergetic efficiency definition for such systems is mentioned. The effect of pump and turbine efficiency, salinity, temperature and mass ratio is studied using a validated program. In this regard, modified Van't Hoff constants for a large range of seawater surface temperatures are also determined. The best efficiency was obtained using the pressure exchanger for all systems investigated. Use of pressure retarded osmosis units as energy recovery devices provided efficiencies nearly equal to or less than the hydro-turbine. Thus, for the range investigated, it does not seem to be a viable energy recovery method for reverse osmosis units with seawater feed since constraints such as concentration polarization and finite area would further decrease performance.

Cost assessment in SWRO desalination plants with a production of 600 m3/d in Canary Islands

Water resources available in the island of Fuerteventura come mostly from small-scale capacity seawater desalination plants. Desalted water demand in the island has grown considerably in the last decade forcing managers to adapt desalinated water supply constantly. Additionally, the operating cost of the plants is relevant. The staff, chemical consumption, cartridge filter, and membrane replacement cost are essential in order to establish more efficient operation conditions of a reverse osmosis desalination plant. This article aims to study and compare the mentioned cost of six different seawater reverse osmosis desalination plants with the same production of 600 m3/d in the island of Fuerteventura. The results show for each of the six cases how costs can be reduced and the essential role of automation dealing with the staff cost with the particular capacity of 600 m3/d.

Economics of Renewable Energy for Water Desalination in Developing Countries

Freshwater shortage in many areas around the world presents a constraint to sustainable development. Desalination of saline water, especially seawater which accounts for more than 97% of the total water quantity on earth, represents a feasible option in many cases. The need for sustainable sources of energy to operate high energy - consumption desalination systems is mandatory. Future, in remote coastal and arid areas, the use of renewable energy for powering desalination plants could be a feasible option in view of several technical, economic and environmental considerations. In this paper, the issue of integration of desalination technologies and renewable energy from specified sources is addressed. The features of Photovoltaic (PV) system combined with reverse osmosis desalination technology, which represents the most commonly applied integration between renewable energy and desalination technology, are analyzed. Further, a case study for conceptual seawater reverse osmosis (SW-RO) desalination plant with 1000 m3/d capacity is presented, based on PV and conventional generators powered with fossil fuel to be installed in a remote coastal area in Egypt, as a typical developing country. The estimated water cost for desalination with PV/ SW-RO system is about $1.21 m3, while ranging between $1.18-1.56 for SW-RO powered with conventional generator powered with fossil fuel. Analysis of the economical, technical and environmental factors depicts the merits of using large scale integrated PV/RO system as an economically feasible water supply with an autonomous power supply.

Mass balance for a novel RO/FO hybrid system in seawater desalination

Seawater Reverse Osmosis (SWRO) desalination is being used by several countries to aid the current demand for fresh water, hence numerous large scale and small scale desalination plants have been built during last decade. Despite major advancements in SWRO technology, the desalination industry is still facing significant practical issues. Two of the major issues are (1) generation of higher volumes of pre-treatment sludge, and (2) overall water recovery. This paper proposes a novel hybrid reverse osmosis (RO) – forward osmosis (FO) system to overcome the above two drawbacks. Mass balance calculations based on laboratory experiments have been used to predict increased water recovery and reduced pre-treatment sludge volume arising from large scale (340,000m3/day of intake) and small scale (15,000m3/day of intake) hybrid SWRO desalination plants. The percentage reduction of pre-treatment sludge volume, increase in overall RO water recovery, FO membrane area required and dilution in RO reject have been estimated.

The integration of desalination plants and mineral production

A noticeable increase in the number of seawater reverse osmosis (SWRO) desalination plants has been observed during the last decade. This is due to the low energy consumed by reverse osmosis technology and the high freshwater recovery when compared to other desalination technologies. The desalination of seawater is the main source of freshwater in Kuwait. However, SWRO desalination plants generate huge volumes of concentrated brine, which is considered a potential source of mineral content. Some of these minerals are expensive and scarce on earth. The recovery of these minerals from brine can help in increasing the freshwater recovery, reducing the overall cost of desalinated water, offering a new source of raw materials, and decreasing the environmental impact from discharging brine to the sea. This study gives an overview of the different technologies used for the treatment of SWRO brine and explores the benefit to recover and reuse valuable potential minerals in brine. The study is motivated by the strong need to establish a local industry in Kuwait based on the extraction of potential valuable minerals from SWRO brine. Such industries can be combined with desalination plants to form a complex cogeneration of plants that accommodate desalination and power plants.

Energy consumption assessment of 4,000 m3/d SWRO desalination plants

The Canary Islands has been pioneer on desalination in Europe since the 1960s and has continued to use intensively the desalination technologies in order to provide water resources. However, one of the most important problems related to the operating efficient of seawater reverse osmosis (SWRO) desalination plants is dealing with the high-energy consumption. The present article aims to study, define, and compare the energy consumption cost of five SWRO desalination plants with the same production of 4,000 m3/d in the island of Fuerteventura (Canary Islands). The methodology for carrying out the work is based on analyzing the energy consumption data that directly affect to the operating cost. A comparison between different energy recovery systems was carried out using the data collected over 5 years of operation. The article presents an assessment of different ways to get more efficient operating conditions in order to reduce the operating cost.

Characterization of microbial communities in water and biofilms along a large scale SWRO desalination facility: Site-specific prerequisite for biofouling treatments

Biofouling impacts seawater reverse osmosis (SWRO) desalination plants by hindering module performance, increasing energetic demands, and incurring further costs. Here we investigated the spatial–temporal dynamics of microbial communities along the feedwater, pretreatment, and reverse osmosis stages of a large-scale SWRO desalination facility. While the composition of water-based microbial communities varied seasonally, the composition of biofilm microbial communities clustered by locations. Proteobacteria dominated throughout the water and biofilm communities while other dominant phyla varied seasonally and spatially. The microbial community composition significantly differed along the pathway locations of feedwater, rapid sand filtration (RSF), cartridge filters (CF), and the reverse osmosis (RO) membranes. Biofilms on the RSF and CF were composed of more diverse microbial populations than RO biofilms as determined by the effective number of species. Biofilms that developed along the treatment pathway (CF) served as inocula enhancing biofouling downstream on the RO membranes. Subsequently, we believe that prior to the development of advanced antibiofouling treatments for the desalination industries, the site-specific microbial community of feedwater, pretreatment and RO biofouling should be characterized. Site specific identification of these communities will enable optimization of pretreatment and cleaning procedures and can ultimately reduce chemical usage and incurred costs.

Stand-alone seawater RO (reverse osmosis) desalination powered by PV (photovoltaic) and PRO (pressure retarded osmosis)

A novel RO seawater desalination plant powered by PV (Photovoltaic) and PRO (PVROPRO) is proposed and the feasibility of two stand-alone schemes, SSRO (salinity-solar powered RO) operation and SRO (salinity powered RO) operation, are investigated. First, the stand-alone feasibility of the plant is thermodynamically analysed. In doing so, on the basis of mathematical models describing RO, PRO and the PV array, the stand-alone feasibility is numerically investigated and the feasible operational windows for the two operation schemes, SSRO and SRO, are identified. In addition, the detrimental effects, CP (concentration polarization) and RSP (reverse salt permeation) in the mass transfer, on the operational windows are investigated. Finally, a case study of the proposed PVROPRO plant is developed based on the hourly solar data of Perth Australia in a year. The highest weekly production rate is found to be almost 20 times the rate in PVRO in the same week. Annual production is increased more than nine times compared to the stand-alone PVRO plant. Furthermore, it is found that, due to detrimental effects the weekly PW (product water) production rate is decreased in the range of 16–20% and the overall annual reduction is 18.07%.

Evaluation of multivariate statistical analyses for monitoring and prediction of processes in an seawater reverse osmosis desalination plant

Our aim was to analyze, monitor, and predict the outcomes of processes in a full-scale seawater reverse osmosis (SWRO) desalination plant using multivariate statistical techniques. Multivariate analysis of variance (MANOVA) was used to investigate the performance and efficiencies of two SWRO processes, namely, pore controllable fiber filter-reverse osmosis (PCF-SWRO) and sand filtration-ultra filtration-reverse osmosis (SF-UF-SWRO). Principal component analysis (PCA) was applied to monitor the two SWRO processes. PCA monitoring revealed that the SF-UF-SWRO process could be analyzed reliably with a low number of outliers and disturbances. Partial least squares (PLS) analysis was then conducted to predict which of the seven input parameters of feed flow rate, PCF/SF-UF filtrate flow rate, temperature of feed water, turbidity feed, pH, reverse osmosis (RO)flow rate, and pressure had a significant effect on the outcome variables of permeate flow rate and concentration. Root mean squared errors (RMSEs) of the PLS models for permeate flow rates were 31.5 and 28.6 for the PCF-SWRO process and SF-UF-SWRO process, respectively, while RMSEs of permeate concentrations were 350.44 and 289.4, respectively. These results indicate that the SF-UF-SWRO process can be modeled more accurately than the PCF-SWRO process, because the RMSE values of permeate flowrate and concentration obtained using a PLS regression model of the SF-UF-SWRO process were lower than those obtained for the PCF-SWRO process.

Environmental and economic assessment of beach well intake versus open intake for seawater reverse osmosis desalination

This paper presents a comparative life cycle assessment (LCA) and levelised cost (LC) analysis of two scenarios: an open intake scenario in which a seawater reverse osmosis (SWRO) desalination plant employs an open intake and membrane pre-treatment prior to RO, and a beach well scenario in which feedwater is extracted from the subsurface using beach well intake and cartridge filtration prior to RO. In both scenarios, desalination plants with 35,000m3/day capacities were modelled. Results indicate that the beach well intake plant life cycle environmental burdens and LC were as much as 31% and 13% lower respectively, compared with the open intake plant. A detailed contribution analysis revealed that the better environmental performance of the beach well intake plant was significantly influenced by its comparatively low electricity use in the simplified pre-treatment process. The better economic performance of the plant with beach well intake was mostly due to savings in chemical use. The results are based on site specific assumptions. However, the LCA and LC framework developed herein could be used to determine the optimum SWRO seawater intake and pre-treatment configuration at plant sites with different characteristics to those modelled herein, provided sufficient data is available.

Preliminary experimental analysis of a small-scale prototype SWRO desalination plant, designed for continuous adjustment of its energy consumption to the widely varying power generated by a stand-alone wind turbine

Given the significant water-energy problems associated with many remote and arid areas of the planet, most studies, projects and developments of installations for the production of fresh water using desalination technologies powered by renewable energy sources have focussed on small-scale stand-alone systems. The most commonly used energy sources have been solar photovoltaic and wind and the most widely applied desalination technology that of reverse osmosis (RO). Most of the systems use batteries as a means of mass energy storage and the RO plants normally operate at constant pressure and flow rate. This paper presents a small-scale prototype SWRO (seawater reverse osmosis) desalination plant designed to continuously adapt its energy consumption to the variable power supplied by a wind turbine (WT), dispensing with mass energy storage in batteries and proposing the use of a supercapacitor bank as a dynamic regulation system. A description is given of the tests performed to date with the SWRO desalination plant connected to the conventional grid while controlling the number of pressure vessels that are connected/disconnected to/from the system and regulating their operating pressures and flow rates (within predetermined admissible limits) to maintain a constant permeate recovery rate and adapt the energy consumption of the plant to a widely varying simulated wind energy supply.One of the most important conclusions that can be drawn from the studies undertaken is the feasibility of adapting the consumption of the prototype of the SWRO desalination plant to widely varying WT-generated power. Despite using various time interval lengths in which it was assumed that the WT output power remained constant, a perfect fit was not obtained between the theoretical WT-generated power and the power consumed by the SWRO desalination plant, nor was it possible to maintain a constant permeate recovery rate at each instant.

Las Palmas’ ERD experience

A reverse osmosis seawater desalination plant on the island of Las Palmas in the Canary Islands has been operating successfully for more than 20 years. The original installation featured Calder™ Energy Recovery Turbines, which helped to drive the membrane feed pumps. At that time, this was the most efficient energy recovery method available, while offering the ease of operation because of the direct drive connection with the membrane feed pumps. The opportunity arose to retrofit the installation using isobaric machines in order to improve the efficiency and capacity of the installation. The DWEER™ was used because it had the highest efficiency available, and also fit in the restricted footprint available at the existing site. Flowserve provided a turnkey installation for the retrofit of all the relevant pumps and the Energy Recovery Device (ERD). The compact nature of the site and the overall length of the DWEER required a clever solution for the installation’s packaging. This paper reviews the engineering solutions provided and compares the operating results after the conversion was completed, using data for three full years of operation incorporating some of the latest improvements of the DWEER.

Corrosion of stainless steel components in seawater reverse osmosis desalination plants—investigations on adapted internal cathodic protection

Stainless steel is widely used in seawater reverse osmosis units (SWRO) for both good mechanical and corrosion resistance properties. However, many corrosion failures of stainless steel in SWRO desalination units have been reported. These failures may often be attributed to un-adapted stainless steel grade selection and/or to the particular aggressive seawater conditions in “warm” regions (high ambient temperature, severe biofouling, etc.). Cathodic protection (CP) is a well-known efficient system to prevent corrosion of metallic materials in seawater. It is successfully used in the oil and gas industry to protect carbon steel structures exposed in open-sea. However, the specific service conditions of SWRO units may seriously affect the efficiency of such anti-corrosion system (high flow rates, large stainless steel surfaces affected by biofouling, confinement limiting protective cathodic current flow, etc.). Hence, CP in SWRO units should be considered with special care and modeling appears as useful tool to assess an appropriate CP design. However, there is a clear lack of CP data that could be transposed to SWRO service conditions (i.e. stainless steel, effect of biofouling, high flow rate, etc.). From this background a Join Industry Program was initiated including laboratory exposures, field measurements in a full scale SWRO desalination plant, and modeling work using PROCOR software. The present paper reviews the main parameters affecting corrosion of stainless steel alloys in seawater reverse osmosis units. CP on specific stainless steel devices was investigated in order to assess its actual efficiency for SWRO units. Severe environmental conditions were intentionally used to promote corrosion on the tested stainless steel products in order to evaluate the efficiency of CP. The study includes a modeling work aiming at predicting and designing adapted CP protection to modeled stainless steel units. An excellent correlation between modeling work and field measurements was found.

Technical feasibility of a seabed gallery seawater intake at Ras Abu Ali Island, Arabian Gulf, Saudi Arabia

Open-ocean intake systems require extensive and advanced pretreatment unit operation to produce feed water with low membrane fouling potential in seawater reverse osmosis (SWRO) facilities. Alternatively, subsurface intake systems tend to produce high quality raw seawater even before pretreatment. Subsurface intakes extract seawater indirectly through the geological structure of shoreline or nearshore sediments. Water percolation through geological units provides physical and biological treatment, so that the raw seawater is microbiologically stable with relatively low particulate and organics content. Overall, utilization of subsurface intakes will reduce the intensity of pretreatment, which reduces operating cost, lowers chemical and energy consumption, and reduces environmental impacts. An important aspect in the feasibility of a subsurface intake is the compatibility of the local geological environment. In this study, a field investigation was conducted at Ras Abu Ali Island in the Arabian Gulf. This location currently contains an of existing oil company facilities and a proposed governmental marine fish hatchery facility. Recreational, commercial, and domestic potable water uses require the need to use the SWRO process to meet demands. Characterization of the shoreline and marine offshore bottom were performed as well as observation of tidal fluctuations and wave heights. A specific grid area was chosen where 35 sediment samples were collected from the seabed floor for laboratory analysis of grain size distribution, sediment porosity, and hydraulic conductivity. Onsite observation showed that the marine bottom has a low slope creating a wide intertidal area. The lowest tidal zone is more than 150 m from the shoreline defining a far seaward boundary for the intake construction point. A relatively thin layer of mixed-type sediment (carbonate and siliciclastic) covers the marine hardground bottom. The unlithified bottom sediment contains a low mud percentage (less than 1%) with porosity ranging between 0.29 and 0.41 and hydraulic conductivities up to 22.5 m3/d. It was determined that seabed gallery development is suitable at this location. Preliminary design for a seabed gallery filter was developed using a series of cells. Each gallery cell represents a unit that can be simply duplicated to meet the overall intake capacity requirement. Each gallery cell is designed to have minimum 8 m/d infiltration rate through five layers of engineered sand and gravel. The total thickness of the filter bed is 4.5 m (2 m top layer). The dimensions of the proposed cells are 100 × 30 m and each cell will conservatively provide 24,000 m3/d of filtered water. The design is flexible to meet the required capacity. For example, a SWRO desalination plant which produces 54,000 m3/d product water from 38,000 mg/L salinity seawater at a 45% conversion rate will require a minimum of 6 cells using the preliminary design.

Influence of site-specific parameters on environmental impacts of desalination

Many metropolitan areas have a high dependency on Seawater Reverse Osmosis (SWRO) desalination plants for bulk water supply. Location and scale decisions are important for SWRO desalination plants owing to the significant environmental costs associated with long distance water pumping. The aim of this study is to introduce a Geographical Information System (GIS)-based method to assist such site-specific decisions. The method has 3 stages. Stage 1 uses GIS to identify feasible plant locations and water demand areas. Stage 2 develops a range of scenarios that balance plant size and number with water demand. In stage 3, the preferred scenario is selected based on environmental life cycle assessment. The method’s applicability was tested using data for the northern corridor of Perth, Western Australia (WA). Spatial water demand and suitable vacant land for accommodating SWRO plants in the case study are obtained in Stage 1. Based on these spatial data, two water planning options are designed in order to supply desalinated water to the demand area. The first option consists of a large SWRO desalination plant and its connected trunk main which supplies water into the demand centrally (centralized scenario). Second option consists of five medium-sized SWRO desalination plants integrated within the demand area (distributed scenario). The best scenario for environmental performance was found to be the distributed scenario which has 18% less GHG emission compared to centralized scenario. This method is adaptable to other case studies for identifying optimal SWRO plant sizes and locations based on environmental criteria.

Operational Optimization of Large-Scale Parallel-Unit SWRO Desalination Plant Using Differential Evolution Algorithm

A large-scale parallel-unit seawater reverse osmosis desalination plant contains many reverse osmosis (RO) units. If the operating conditions change, these RO units will not work at the optimal design points which are computed before the plant is built. The operational optimization problem (OOP) of the plant is to find out a scheduling of operation to minimize the total running cost when the change happens. In this paper, the OOP is modelled as a mixed-integer nonlinear programming problem. A two-stage differential evolution algorithm is proposed to solve this OOP. Experimental results show that the proposed method is satisfactory in solution quality.

Optimal planning and design of seawater RO brine outfalls under environmental uncertainty

Increasing demand for water in urban areas and agricultural zones in arid and semi-arid coastal regions has urged planners and regulators to look for alternative renewable water sources. Seawater reverse osmosis desalination plants have become an essential supply source for the production of freshwater in such regions. However, disposal of hypersaline wastes from the plants in many of these regions has not been fully and properly addressed. A simulation–optimization approach is proposed to design a system for safe disposal of brine wastes. We use a hydrodynamic model to assess the initial dilution of hypersaline effluent discharged into coastal waters. A regression model is developed to relate the input and output parameters of the simulation model. We then formulate an optimization model to determine the design of a brine disposal system with multiport diffusers in which the regression model replaces the simulation model. The design parameters are the length, diameter and number of ports of the disposal system. Given the parameter uncertainty, a chance-constrained programming model is used. This simulation–optimization framework provides planners with effective tools that meet environmental permitting requirements and restrictions, while achieving cost savings and adequate hydrodynamic performance. A case study demonstrates the applicability of the proposed methodology.

Thermodynamic and thermoeconomic analyses of seawater reverse osmosis desalination plant with energy recovery

This paper investigates the performance of a RO (reverse osmosis) desalination plant at different seawater salinity values. An energy recovery Pelton turbine is integrated with the desalination plant. Thermodynamic analysis, based on the first and second laws of thermodynamics, as well as a thermo-based economic analysis is performed for the proposed system. The effects of the system components irreversibilities on the economics and cost of product water are parametrically studied through the thermoeconomic analysis. The exergy analysis shows that large irreversibilities occur in the high pressure pump and in the RO module. Both thermodynamic and thermoeconomic performances of the overall system are investigated under different operating parameters. For the base case; the system achieves an exergy efficiency of 5.82%. The product cost is estimated to be 2.451 $/m3 and 54.2 $/MJ when source water with salinity of 35,000 ppm is fed to the system.

Environmental life cycle assessment of seawater reverse osmosis desalination plant powered by renewable energy

This paper evaluates life cycle Greenhouse Gas (GHG) emissions of a Seawater Reverse Osmosis (SWRO) desalination plant and assesses its performance under three power supply scenarios. A Life Cycle Assessment (LCA) analysis is conducted for a plant located in Perth, Western Australia (WA). Input and output flows of SWRO plant are based on literature and Perth desalination plants. The Simapro Australian and Ecoinvent databases are used for operational phase Life Cycle Inventory (LCI). An LCI for the construction phase of the plant is developed using economic input–output analysis. Electricity supply scenarios are “100% WA grid”, “100% wind energy” and “92% wind energy plus 8% Photovoltaic (PV) solar energy”. Results indicate that renewable energy powered desalination plants achieve GHG emissions reduction of ∼90% compared to the plant powered by WA grid scenario. For the plant powered by fossil based grid electricity, electricity use in the operational phase is found to be responsible for more than 92% of its GHG emissions. On the other hand, for the plants powered by renewable energy, the highest contribution belongs to chemical use in the operational phase (60%) followed by the construction phase (17%). Indirect emissions due to the electricity consumption in the chemical, wind turbine and PV solar panel manufacturing are found to contribute the lion's share (36–39%) of the life cycle emissions for the renewable energy powered desalination plants. Any improvement in fuel mixes in grid electricity towards cleaner energy sources can be beneficial by reducing impacts associated with upstream electricity use in manufacturing. This work provides the first reference to identify and quantify supply chain contributions to the overall environmental impact associated with renewable energy powered desalination plants.