Energy advantages of reverse osmosis in seawater desalination

The very rapid increase in energy costs during the past three years is causing a change in the preferred process technology for seawater desalination. The phase changes, evaporation, and condensation, required in the distillation processes make them more energy intensive than the ambient temperature liquid separation that occurs in the reverse osmosis (RO) process. This paper describes the RO process and how to calculate its energy consultation.The RO process requires only 5–7 KWh/m³ of product water compared to 15–16 KWh/m³ required by the most efficient distillation process. The productivity of a large dual purpose electricity/RO water plant is compared to the productivity of a commercially purchased state-of-the-art dual purpose electric/distillation water plant that is currently under construction. The RO potable water productivity is about 2X the distillate flow at the same fuel rate     

Distillation vs. membrane filtration: overview of process evolutions in seawater desalination

The worldwide need for fresh water requires more and more plants for the treatment of non-conventional water sources. During the last decades, seawater has become an important source of fresh water in many arid regions. The traditional desalination processes [reverse osmosis (RO), multi stage flash (MSF), multi effect distillation (MED), electrodialysis (ED)] have evoluated to reliable and established processes; current research focuses on process improvements in view of a lower cost and a more environmentally friendly operation. This paper provides an overview of recent process improvements in seawater desalination using RO, MSF, MED and ED. Important topics that are discussed include the use of alternative energy sources (wind energy, solar energy, nuclear energy) for RO or distillation processes, and the impact of the different desalination process on the environment; the implementation of hybrid processes in seawater desalination; pretreatment of desalination plants by pressure driven membrane processes (microfiltration, ultrafiltration and nanofiltration) compared to chemical pretreatment; new materials to prevent corrosion in distillation processes; and the prevention of fouling in reverse osmosis units. These improvements contribute to the cost effectiveness of the desalination process, and ensure a sustainable production of drinking water on long terms in regions with limited reserves of fresh water.     

Energy and exergy analyses of seawater desalination system integrated in a solar heat transformer

Among the numerous options to improve the energy efficiency of desalination plants stands out the absorption heat transformer. A heat transformer is a device, which can deliver heat at a higher temperature than the temperature of the fluid by which it is fed. Solar thermal energy can be used as heat input for single effect heat transformer while the high grade thermal energy delivered by the heat transformer can be used as heat source for water desalination.In this paper, an attempt has been made to study the combination: flat plate solar collectors, a single effect heat transformer and desalination system (distillation process) used to provide a beach house located in Skikda (East of Algeria; Latitude 36.52°N, Longitude 6.57°E) with drinking water. This system produces about 500 l of drinking water per day in July.Mathematical models of the solar flat plate collectors (FPC), absorption heat transformer (AHT) operating with the water/lithium bromide solution and the overall desalination system (WP) were developed to simulate the performance of this combination system. The energy and exergy analyses are carried out for each component of the system. All exergy losses that exist in this solar desalination system are calculated. Energy and exergy efficiencies are estimated.     

Development of a marine osmotic solar still for sea water desalination

This paper describes the principals of an approach to desalt sea water via exploitation of solar energy and membrane technology. The approach has been under test aimed at commercialization of the process in the near future. The process is a combination of diffusion across a salinity gradient established by evaporation and distillation in the form of a solar still-type arrangement. An osmotic membrane is used to separate the liquids of different solid concentration. In other words, the desalination devise is a hybrid of a solar still and osmosis unit which could operate in an economical and efficient utilization of solar energy.The marine osmotic solar still does not have the complexity of a solar powered reverse osmosis that draws power from an active solar collection system. Solar collectors and associated energy delivery systems, if used with an RO unit, will be rather expensive, and will occupy a large space. Conventional solar stills occupy large space and are rather inefficient in water production. The marine osmotic solar still is a stand alone solar energy system which is capable of providing distilled water of high purity. It can also be constructed offshore or on land near the seashore.The offshore scheme involves floating units in contact with the sea water. Each is capable of producing 150 gallons of water containing less than 50 ppm of dissolved solid. A conceptual design has been developed and described here of a pilot plant that produces 6,000 gpd.Material tests have been conducted in Singapore and Malysia to assure proper selection of structural material that are suitable for the highly corrosive environment. Measurements on flow rate have shown values of 15 to 20 gallones per square foot per day. The measurements have been conducted in Long Island Sound off Southport, Connecticut.In this paper, the actual design of the plant is described and results of material tests and simulation system analysis are reported. A comparison is made with conventional solar stills and solar powered membrane desalination processes. Results of an extensive marketing research are reported for various applications of the process.     

Performance study on an acrylic mirror boosted solar distillation unit utilizing seawater

A solar still is a simple desalination unit, but typical configuration is a sealed box with an angled glass top. Sunlight shining into the box heats water or liquid, causing it to evaporate. The moisture condenses on the relatively cool glass cover and runs down the sloped surface for collection. In this paper, the performance of solar still in terms of collection of distilled water have been analyzed and a booster mirror (acrylic) is attached with just above the glass cover of solar still, which will reflect solar radiation in excess to water and it is possible to adjust the booster mirror for perfect reflection depending upon the sun moving angle. Low rates of distillation have been observed with the existing unit. A notable result has been observed with a boosted distillation unit (4.2 L/m²/d at 890 W/m² max.). The arrangements have been made by commercial Al sheet material and insulated with a thermocol sheet.     

Enhancement of solar water distillation process by surfactant additives

The presence of surfactant additives in water is found to enhance the boiling heat transfer significantly. The effect of using a surfactant as sodium lauryl sulfate (SLS) in relatively small dosages of concentration with a small size unit of solar water distillation process is investigated. Sun simulator (electric heater and variac transformer) is used to quantify the same input power for each experiment for the system instead of the sun. The experimental results show that a small amount of surfactant additive makes the top brine temperature (TBT) considerably higher, hence the freshwater product. Temperature curves for various surfactant concentrations are obtained and compared. The percentage of the increase in the system productivity is 0.7%, 2.5%, 4.7% and 7% at additive concentration equal to 50, 100, 200 and 300 ppm respectively. It is found that increasing the surfactant concentration more than 300 ppm not affecting the system daily productivity (DP) and TBT. Using surfactant concentration more than 400 ppm will decrease the DP by 6%.     

Evaluation of an ion exchange system regenerated with seawater for the increase of product recovery of reverse osmosis brackish water plant

An experimental ion exchange unit was operated on the Red Sea shore. Softening of the reject containing 1,400 ppm calcium from a nearby RO brackish water plant was performed using seawater as regenerant. The ex- perimental results reported show that the calcium concentration in the softened reject was reduced to a level which enables further RO desalting of the reject at 50% product recovery. A preliminary economic evaluation of 4,000m³/day RO plant indicates that desalting of softened reject would be more economically advantageous than the continued operation of the existing thermal seawater desalting plants and should precede the commercial RO seawater desalting at this location.     

Effect of different operation pressures for various membranes on the performance of RO plants

The aim was to study the effect of different operation pressures on the performance of reverse osmosis (RO) plants for various types of membranes. The study was conducted in a pilot plant at the University of Basrah, College of Engineering, which has a capacity of 9 m³/h. The plant is comprised of two parallel vessels containing five elements for each vessel, 8″ in diameter and 40″ in length. The first vessel has Saehane membranes, type RE8040BE-400 ft² manufactured in South Korea. Koch membranes, type 8822-XR-365 ft² made in the US, were used in the second vessel. The pilot plant uses brackish water from the Tigris River with TDS <600 ppm. The new type of RO membranes (Saehane) were used for the first trial for production of desalted water from brackish water less than 600 ppm and the results were compared with performance of the Koch membranes. It was found that over 180 days of continuous operation, the amount of permeate for Saehane membranes is larger than Koch membranes by about 26%. It was also observed that the quality of permeate water stream for Koch membranes is less than for the Saehane membranes by about 11%.     

An experiment with a plastic solar still

A solar still is a device which allows obtaining fresh water from seawater or brackish water. It utilizes the greenhouse effect by using solar energy. In a conventional solar still the production of fresh water in bright sunny weather and with warm air temperature is about 5-5.5 L m⁻² d⁻¹, according to the depth of the water in the solar still. In some devices it is possible to obtain efficiencies of up to 0.50 and 0.60. The aim of this research is to increase distillation productivity by utilizing the latent heat released by the condensing water steam. For this purpose the author built a solar still characterized by two basins (B₁ and B₂) superimposed upon each other. The building materials were a sheet of black Plexiglas for the bottom of the solar still, a sheet of transparent Plexiglas for all boxes, and a sheet of expanded polystyrene, used as insulating material. The solar still was hermetically sealed to reduce the leakage of vapor to the surroundings. The greatest quantity of fresh water obtained by the tested solar still was 1.7-1.8 L m⁻² d⁻¹. This result was achieved in the third week of July when solar radiation was 27-28 MJ m⁻² d⁻¹. The efficiency of the tested solar still was about 0.16. This low efficiency is probably due to the low temperature of the water contained in the still (about 50°C). The solar still has only been used in experiments for some months, during which it has not been possible to study the deterioration of the material (Plexiglas). These results show that an elaborate design and the increased costs for such design and construction do not always improve the water yield.     

Wind and solar powered reverse osmosis desalination units - design,start up,operating experience

To demonstrate the operation of RO units with GKSS plate modules coupled with unconventional energy supply systems, two projects are presently carried out by the GKSS research center applying a photovoltaic solar generator and a wind energy converter for a site in the Northern part of Mexico and a small island at the German coast of the North Sea, respectively. The solar generator with a designed peak power of 2.5 kW is developed by AEG and powers a RO unit for the desalination of brackish well water. The plant has been installed in Concepcion del Oro, Zacatecas, in spring 1980 and has been operated by the Mexican company DIGAASES in cooperation with AEG and GKSS since May 1980. The second project is a 6 kw-wind energy converter supplying 3 RO-modules with a total membrane area of 30 m². After comprehensive component tests at the GKSS research center at Geesthacht the plant was installed at the very small island of Sūderoog at the German coast of the North Sea. Test operation was started in December 1980. For both plants a measurement program has been started.     

Removal of heavy metals from wastewater by membrane processes: a comparative study

Wastewater containing copper and cadmium can be produced by several industries. The application of both reverse osmosis (RO) and nanofiltration (NF) technologies for the treatment of wastewater containing copper and cadmium ions to reduce fresh water consumption and environmental degradation was investigated. Synthetic wastewater samples containing Cu²⁺ and Cd²⁺ ions at various concentrations were prepared and subjected to treatment by RO and NF in the laboratory. The results showed that high removal efficiency of the heavy metals could be achieved by RO process (98% and 99% for copper and cadmium, respectively). NF, however, was capable of removing more than 90% of the copper ions existing in the feed water. The effectiveness of RO and NF membranes in treating wastewater containing more than one heavy metal was also investigated. The results showed that the RO membrane was capable of treating wastewater with an initial concentration of 500 ppm and reducing the ion concentration to about 3 ppm (99.4% removal), while the average removal efficiency of NF was 97%. The low level of the heavy metals concentration in the permeate implies that water with good quality could be reclaimed for further reuse.     

Optimal design of hybrid RO/MSF desalination plants Part III: Sensitivity analysis

This is the last paper in a series of three parts entitled “Optimal design of hybrid RO/MSF desalination plants”. This research is concerned with exploring the feasibility of hybridization of multi-stage flash (MSF) and reverse osmosis (RO) technologies in order to improve the performance characteristics and process economics ofthe conventional MSF process. The research project involved an optimization study where the water cost perunit product is minimized subject to a number of constraints. In the first part, the design and cost models were presented, the optimization problem formulated and solutions for a number of cases were outlined. In the second part, results were presented and discussed. In this paper we discuss the sensitivity of water cost from the alternative plant designs to variations in some cost elements and operating conditions. In general, it is concluded that, for the same desalting capacity, hybrid RO/MSF plants can produce desalted water at a lower cost than brine recycle MSF plants, while hybrid plants are characterized, by lower specific capital costs and higher water recovery fractions. Reduction in steam cost allows MSF to compete more with hybrid RO/MSF plants. This result explains the advantage of coupling MSF plants and steam power plants where the exhaust steam from the back pressure turbine represents a relatively cheaper source of heat for the MSF process. Results showed that the RO technology exceeds all other designs over the whole range of energy, chemicals and membrane costs studied here. However, water cost of the RO process was the most sensitive to variations in membrane and electricity costs compared to other hybrid configurations.     

Fresh water production by means of solar concentration: the AQUASOLIS project

In 2006, the European Commission approved the specific support action (SSA) “AQUASOLIS” as an instrument for assessing the use of solar trough concentration plants for applications other than heating and cooling, in particular for the production of fresh water for human consumption and for agriculture for Mediterranean countries. The capability of solar trough plants of producing heat at temperatures over ca. 150° permits indeed to envisage the use of solar energy for water purification by distillation. At the same time the high temperature fluid generated by the plant can be used to drive adsorption chillers that can extract water from the atmosphere. The AQUASOLIS project was performed as a parallel investigation to the STREP project REACt which is aimed at the actual development of a heating/cooling solar trough system for Mediterranean countries. The results obtained in the AQUASOLIS project show a high promise of the system to help alleviate the conditions of drought in Mediterranean countries and at the same time boosting the use of renewable energy.     

Status of reverse osmosis desalination technology

Reverse osmosis (RO), a relatively new technology, is gradually becoming an established and economical method for demineralization of saline waters. Over 50 commercial plants ranging in size from 50,000 gpd to 2 million gpd (2 mgd) are producing fresh quality water for municipal and industrial uses from brackish water sources. The U.S. Congress has authorized construction of a 100 mgd plant in Yuma, Arizona to demineralize otherwise unusable high salinity irrigation return flows as part of the Colorado River Salinity Control. Engineering design and operation details together with cost information on some commercial plants and the planned 100 mgd plant will be presented.A review of the plant operation data indicates that is imperative for the plant owners and equipment suppliers to place due emphasis on providing adequate feed water pretreatment facilities and trained plant operation personnel to ensure trouble-free operation and to achieve furthur economy in desalting costs.Significant advances have been made in the development of RO process for sea water desalination. Soaring energy costs are providing incentive for plant owners to prefer RO plants (up to 100,000 gpd) over vapor compression distillation hardware. Results of the Federal Government Desalting R & D Programs clearly indicate that RO desalting costs will be at least 20–30% lower than distillation.     

A small solar desalination plant for the production of drinking water in remote arid areas of southern Algeria

The common methods of desalination salt water for production of fresh water by distillation, reverse osmosis and electrodialysis are intensive energy techniques. However, in remote arid areas, the desalination needs not exceed a few cubic meters per day. This decentralised demand favours local water production by developing other desalination processes, especially those using renewable or recovered energy (solar, geothermal, etc.). Solar desalination process is one of these methods used to resolve the scarcity of fresh water. Several reviews have been published by different authors. Small production systems as solar stills can be used if fresh water demand is low and the land is available at low cost. To supply the population of remote arid lands of South Algeria with drinkable water, solar distillation of brackish waters is recommended. It satisfies some of theses demands. Solar stills are used to produce fresh water from brackish water by directly utilising sunshine. These stills represent the best technical solution to supply remote villages or settlements in South Algeria with fresh water without depending on high-tech and skills. The production capacity indicates a possible daily production of far more than 15 l/m²d. Therefore, the still has a place in the upper range of known comparable products with regards to production output. This depends on the material used and the price of the solar stills and their accessories. The best working temperature solves most problems. Small, modular high-performance stills with features like the possibility of decentralised use, less maintenance and robust construction can help to reduce fresh water scarcity. The recent development of stills based on new concepts and heat recovery has been successful. The technical optimization is still in process today, it aims to improvement of the efficiency of these distillers. In our research work, a plant for brackish water distillation by directly sunshine and heat recovery was constructed and investigated experimentally and theoretically in South Algeria. This study aims the improvement of the performance of this solar distillation plant, conducted under the actual insulation, for brackish underground geothermal water desalination.     

Evaluation of technologies for a desalination operation and disposal in the Tularosa Basin, New Mexico

According to the United Nations Environment Programme, one-third of the world's population lives in a situation of water stress. In the case of New Mexico, about 90% of the 1.8 million inhabitants depend on ground brackish water as their only source of potable water in many areas of the state. This report presents a technically-supported, economically-feasible and environmentally friendly proposal to desalinate brackish water to supply potable water to inland, isolated communities in southwest New Mexico. Several existing technologies were reviewed to identify opportunities for optimization by combining them to provide potable water and reduce the waste stream. Alternatives were studied and experimentation was conducted for some of them. The alternatives proposed were the use of natural coagulants for pretreatment, various solar collectors' arrangements for energy supply, reverse osmosis (RO), low temperature multi-effect distillation (LT-MED), multi-stage flash distillation (MSF), solar distillation (SD), and electrodialysis for desalination process; Spirulina cultivation and SD for waste treatment, and deep well injection (DWI) for waste disposal. Some alternatives were eliminated because they are either technologically or economically not feasible for this case and present high environmental impact. Three plant configurations were analyzed. Option A involves using the linear Fresnel systems (LFS) to produce steam for the first effect of a nine-effect evaporation plant. The number of effects was determined to achieve the optimal relation between equipment investment costs and steam production cost. This plant operates 8 h per day with solar energy and the rest of the 24 hour operating time is provided with fossil fuels. The waste produced will be further evaporated with SD to minimize its flow and the concentrated brine will be injected into a deep well. Option B has the same elements as option A, except that it does not consider the SD, but direct brine injection into a deep well. Option C considers the use of SD as the only process for distillation with DWI as the waste disposal method. The selection criteria for the best configuration were optimal use of solar energy resources, minimization of fossil fuel consumption and waste stream generation and disposal. Operation requirements and economic analysis were considered to select a proposal easy to implement and operate in rural isolated communities. For the following reasons option A is the best configuration to cover the necessity of potable water in New Mexico: (A) the plant is easy to construct and operate. In addition, it can handle different ranges of brackish water flow. (B) The 76% water recovery of the system almost matches the recovery achieved in a RO plant (80%), with the advantage that maintenance costs are reduced and treatment flowrates cannot be matched by the RO plant. (C) Use of the LFS reduces the emission of combustion gases to the atmosphere by 33%. This manifests as a positive point in a LCA evaluation. (D) The minimum environmental impact of the process facilitates the public involvement plan (PIP) because it gives the plant an environmentally responsible image in terms of avoiding greenhouse gases emissions. (E) The return on investment (ROI) is 10.2% at a price of $5.00/m³ of desalinated water, which is superior to the estimated minimum attractive rate of return (MARR) used for LT-MED plants as 9.5% annually.     

Solar thermal desalination technologies

The use of solar energy in thermal desalination processes is one of the most promising applications of the renewable energies. Solar desalination can either be direct; use solar energy to produce distillate directly in the solar collector, or indirect; combining conventional desalination techniques, such as multistage flash desalination (MSF), vapor compression (VC), reverse osmosis (RO), membrane distillation (MD) and electrodialysis, with solar collectors for heat generation. Direct solar desalination compared with the indirect technologies requires large land areas and has a relatively low productivity. It is however competitive to the indirect desalination plants in small-scale production due to its relatively low cost and simplicity. This paper describes several desalination technologies in commercial and pilot stages of development. The primary focus is on those technologies suitable for use in remote areas, especially those which could be integrated into solar thermal energy systems.     

A highly productive basin-type-multiple-effect coupled solar still

A newly designed multiple-effect solar still with a triangle cross-section consisting of a horizontal basin liner, a tilted double glass cover and the vertical parallel partitions in contact with saline-soaked wicks is presented and theoretically analyzed. Solar radiation is absorbed in the basin and in the first partition, and the partition section recycles the energy from the basin as well as solar energy, which is directly absorbed by the first partition. A single distillation cell with a 5-mm diffusion gap between vertical partitions has been experimentally explored. No contamination of distillate with saline water was experimentally detected in the single distillation cell, and the experimental observation of the wick and the measurements of the temperature drop through the cell with the 5-mm gap showed that dry patches hardly appear on the wick. The proposed still with 5-mm diffusion gaps is theoretically predicted to produce distillate of 15.4kg⁻² m⁻²d⁻¹ on a sunny day of G_ti=22.4 MJ/m⁻²d⁻¹ solar radiation, and its efficiency is about 3.5 times larger than the average experimental value for the conventional basin type stills by Cooper [20] and 1.2-1.6 times larger than the experimental maximum values of the conventional multiple-effect stills [3,9].