http://www.arvanitakis.com/en/sw/desalination_process.htm

desalinationprocess of removing soluble salts from water to render it suitable for drinking, irrigation, or industrial uses. In distillation, salt water is heated in one container to make the water evaporate, leaving the salt behind. 

The desalinated vapor is then condensed to form water in a separate container. The high fuel costs involved in vaporizing salt water can be reduced by using a vacuum to lower the boiling point or by exposing a water spray or film to high heat, a process known as flash distillation. 

In Hawaii the vacuum method also produces electricity in a process that vaporizes warmer, near-surface water, uses the steam to run a turbine, and condenses the steam with cold water from the ocean depths. 

Other desalination techniques includ¨:

electrodialysis,the use of porous membranes to filter out negatively and positively charged salt ions; 

freezing, based on the principle that water excludes salt when it crystallizes to ice;  and reverse osmosis, in which pressure, generated by the presence of salt in the water, forces water

through a membrane permeable only by pure water.

http://www.mcwd.org/html/desal_process.htm

MCWD Seawater Desalination Project Process Flow

For details and descriptions of the process, click the numbers to the right. Move the resulting pop-up window to the left so both windows will remain visible as you click on other numbers.

Saltwater Desalination by Bennett Thomas and Michael Cuccinellontroduction

http://ewr.cee.vt.edu/environmental/teach/wtprimer/desalt/desalt.htmlWater is indisputably the most essential resource the earth has to offer to the human race. Unfortunately, it is distributed throughout the world as follows:

Oceans: 97.23%

Ice Caps and Glaciers: 2.14%

Groundwater: 0.61%

Freshwater Lakes: 0.01%

Other: 0.01%

Of these categories,agulation and filtration are the pretreatment processes which can be used to remove suspended solids and other particles in the feedwater. After pretreatment, the water is pressurized and sent through semipermeable membranes which separate the salt from the water. In general, the membranes do not allow ions, large organics, partcicles, and bacteria to pass through them. RO membranes can even retain small ions such as sodium, chlorine, calcium, and magnesium (Gabler, 1988). The potable water then enters a post-treatment process and is sent through the distribution system. The highly concentrated salt solution which is separated by the membrane is discarded.

RO membranes do vary somewhat in their ability to remove impurities from water. An excellent membrane, operating in a well functioning plant, is capable of removing as high as 99% of the bacteria and up to 90% of simple inorganic ions. However, RO membranes are not as effective at removing organic compounds. This may be a problem if the feedwater contains high levels of trihalomethanes (THM's) (Gabler, 1988). The efficiency of the reverse osmosis system is variable. It depends primarily on the quality of the feedwater, the pressure of the water as it is pushed through the membranes, and the the porosity of the membranes. Sometimes as high as 90% of the feedwater can be wasted. The following figure shows a standard Reverse Osmosis treatment system:

Distillation The distillation process consists of heating the influent saltwater until it boils. This will separate out the dissolved minerals resulting in a purified and salt-free product. This product is then captured in its gaseous state, with high efficiency, and piped out to the distribution system. The three main distillation processes are separated according to their heat source. These processes are multistage flash distillation (MSF) in which the latent heat comes from the cooling of the liquid being evaporated, multiple-effect distillation (MED) in which the latent heat comes from a solid surface, and vapor compression distillation (VC) in which the latent heat is obtained regeneratively (van der Leeden, et al., 1990). Each process results in the same product, just by different means. The following schematic highlights the basic process of distillation:

Wastewater from Treatment ProcessThe picture below is of a saltwater desalination plant located on the Outer Banks of North Carolina (Photo by Dr. Daniel Gallagher). The plant can be seen at the top of the picture.

On the bottom of the picture is the wastewater effluent leaving the plant by way of a creek which leads out into the bay. In general, wastewater effluent from desalination plants may have the following types of potentially adverse constituents and qualities:

Increase in salt concentrations from influent (influent concentration is about 35,000 ppm, desalination plants discharge brine ranging from 46,000 to 80,000 ppm).

Increase in temperatures from influent (about 5° F increase at the point of discharge) for discharges from distillation plants.

Increase in turbidity levels from influent water levels. Decrease in oxygen levels (distillation plants only). Chemicals from pre-treatment of the feedwater (these may

include biocides, sulfur dioxide and coagulants). Organics and metals that are present in the feedwater and concentrated in the

desalination process. Metals that are picked up by the brine in contact with plant equipment and

pipelines.

Environmental ImpactsThe use of desalination plants, by both Reverse Osmosis and Distillation, has the potential for adversely affecting the environment. Impacts on the environment can result from the discharge of chemicals used in the desalination process. Certain plants, for example, may use biocides such as chlorine to clean pipes or to pretreat the water. These chemicals must be treated before released to the ocean. Besides chemicals used in the plant, the wastewater from desalination plants is another concern because the effluent is a heavily concentrated brine solution. The brine solution, after it is discharged, has the potential to kill marine organisms. Although there are adverse environmental impacts from desalination plants, they can be avoided. For example, by using source water of a higher quality (i.e. from beach wells or infiltrat two desalination systems, one must examine the different costs involved. Both the Reverse Osmosis system and the Distillation system are expensive. The membranes used in the Reverse Osmosis process have a short life and the cost for replacement of these membranes can account for approximately half the cost of desalinating seawater. Because of this, it is suggested that reverse osmosis be used primarily with brackish waters as opposed to seawater, thus increasing the life of the membranes. Distillation plants consume more energy than Reverse Osmosis plants and are therefore more expensive to operate. The high energy cost for both types of plants can be alleviated by using solar energy or by using a cogeneration process. Cogeneration is a process in which the waste products from one system are used as a power source for another system. In this case, the exhaust steam from power plants could be used as a power source in both Reverse Osmosis and Distillation plants in order to reduce the energy needed. It should be noted that through the advancement of technology, desalination processes have become more efficient over the years, thereby requiring less energy. The following table represents desalination costs in the United States in 1985 dollars (van der Leeden, et al., 1990).

Reverse Osmosis

PLANT SIZE(MGD)

OVERALL COST(1985 dollars/1,000 gal)

.01 13.42

.1 9.88

1 7.40

3 6.64

5 6.36

10 6.03

25 5.96

Multi-Stage Flash Distillation

PLANT SIZE(MGD)

OVERALL COST(1985 dollars/1,000 gal)

1 9.73

5 6.78

10 6.50

25 6.10

Legal AspectsWithin the planning process of a desalination plant, there are three main federal acts with which one must comply. They are the Coastal Zone Management Act, the Clean Water Act, and the National Environmental Policy Act. The Coastal Zone Management Act states that any state with a coastal zone (including the Great Lake states) shall develop a management program to address all federal coastal water projects. If the program is approved by the Secretary of Commerce, the state can then determine if the proposed project is or is not compatible with their program. This gives the state a potential chance to veto the project. However, this act is usually utilized on a commenting level as opposed to a vetoing level. So in the case of implementing a desalination plant, the agency involved with the planning must check to see if there exists any incompatability. However, it is very unlikely for a desalination plant to be incompatible with a state's management program. The Clean Water Act applies in several ways. Its major method of application is through the National Pollutant Discharge Elimination System (NPDES). This system applies to any project through which there is a discharge from a point source into navigable waters. It is administered by the Environmental Protection Agency (EPA). It is the EPA's responsibility to decide if a permit will or will not be granted for the proposed project.

While this section of the Clean Water Act usually deals with wastewater treatment facilities and dams, a desalination plant would also require a permit. However due to its mild environmental impacts, resistance in obtaining a permit is highly unlikely. The National Environmental Policy Act (NEPA) is one that had a massive effect on all water resource projects. It created Environmental Impact Statements (EIS's). An EIS requires the federal agency to put into writing all of the environmental impacts of their project. It must include all other possible alternatives, short term needs versus long term productivity, and sustainability. It must also include all of the adverse effects on the environment that could not be avoided if the projects it built. The discharge of the waste brine solution is the main area of interest for a desalination plant. Perhaps the waste might threaten the habitat of an endangered species of fish. However, concerns like this must be investigated for each and every project. ReferencesGabler, Raymond. Is Your Water Safe to Drink?. Consumers Union. 1988.Peavy, Howard and Rowe, Donald and Tchobanoglous, George. Water Resources and Environmental Engineering. McGraw-Hill, Inc. 1985.van der Leeden, Frits and Troise, Fred L. and Todd, David Keith. 2nd Ed. Geraghty & Miller Groundwater Series: The Water Enclyclopedia. Lewis Publishers, Inc. 1990.

http://www.oas.org/dsd/publications/Unit/oea59e/ch20.htm

2.1 Desalination by reverse osmosisDesalination is a separation process used to reduce the dissolved salt content of saline water to a usable level. All desalination processes involve three liquid streams: the saline feedwater (brackish water or seawater), low-salinity product water, and very saline concentrate (brine or reject water). The saline feedwater is drawn from oceanic or underground sources. It is separated by the desalination process into the two output streams: the low-salinity product water and very saline concentrate streams. The use of desalination overcomes the paradox faced by many coastal communities, that of having access to a practically inexhaustible supply of saline water but having no way to use it. Although some substances dissolved in water, such as calcium carbonate, can be removed by chemical treatment, other common constituents, like sodium chloride, require more technically sophisticated methods, collectively known as desalination. In the past, the difficulty and expense of removing various dissolved salts from water made saline waters an impractical source of potable water. However, starting in the 1950s, desalination began to appear to be economically practical for ordinary use, under certain circumstances. The product water of the desalination process is generally water with less than 500 mg/1 dissolved solids, which is suitable for most domestic, industrial, and agricultural uses. A by-product of desalination is brine. Brine is a concentrated salt solution (with more than 35 000 mg/1 dissolved solids) that must be disposed of, generally by discharge into deep saline aquifers or surface waters with a higher salt content. Brine can also be diluted with treated effluent and disposed of by spraying on golf courses and/or other open space areas. Technical Description There are two types of membrane process used for desalination: reverse osmosis (RO) and electrodialysis (ED). The latter is not generally used in Latin America and the Caribbean. In the RO process, water from a pressurized saline solution is separated from the dissolved salts by flowing through a water-permeable membrane. The permeate (the liquid flowing through the membrane) is encouraged to flow through the membrane by the pressure differential created between the pressurized feedwater and the product water, which is at near-atmospheric pressure. The remaining feedwater continues through the pressurized side of the reactor as brine. No heating or phase change takes place. The major energy requirement is for the initial pressurization of the feedwater. For brackish water desalination the operating pressures range from 250 to 400 psi, and for seawater desalination from 800 to 1 000 psi. In practice, the feedwater is pumped into a closed container, against the membrane, to pressurize it. As the product water passes through the membrane, the remaining feedwater and brine solution becomes more and more concentrated. To reduce the concentration of dissolved salts remaining, a portion of this concentrated feedwater-brine solution is withdrawn from the container. Without this discharge, the concentration of dissolved salts in the feedwater would continue to increase, requiring ever-increasing energy inputs to overcome the naturally increased osmotic pressure.

A reverse osmosis system consists of four major components/processes: (1) pretreatment, (2) pressurization, (3) membrane separation, and (4) post-treatment stabilization. Figure 16 illustrates the basic components of a reverse osmosis system. Pretreatment: The incoming feedwater is pretreated to be compatible with the membranes by removing suspended solids, adjusting the pH, and adding a threshold inhibitor to control scaling caused by constituents such as calcium sulphate. Pressurization: The pump raises the pressure of the pretreated feedwater to an operating pressure appropriate for the membrane and the salinity of the feedwater. Separation: The permeable membranes inhibit the passage of dissolved salts while permitting the desalinated product water to pass through. Applying feedwater to the membrane assembly results in a freshwater product stream and a concentrated brine reject stream. Because no membrane is perfect in its rejection of dissolved salts, a small percentage of salt passes through the membrane and remains in the product water. Reverse osmosis membranes come in a variety of configurations. Two of the most popular are spiral wound and hollow fine fiber membranes (see Figure 17). They are generally made of cellulose acetate, aromatic polyamides, or, nowadays, thin film polymer composites. Both types are used for brackish water and seawater desalination, although the specific membrane and the construction of the pressure vessel vary according to the different operating pressures used for the two types of feedwater. Stabilization: The product water from the membrane assembly usually requires pH adjustment and degasification before being transferred to the distribution system for use as drinking water. The product passes through an aeration column in which the pH is elevated from a value of approximately 5 to a value close to 7. In many cases, this water is discharged to a storage cistern for later use. Figure 16: Elements of the Reverse Osmosis Desalination Process.

Source: O.K. Buros, et. Al., The USAID Desalination Manual, Englewood, N.J., U.S.A., IDEA Publications.Extent of Use

The capacity of reverse osmosis desalination plants sold or installed during the 20-year period between 1960 and 1980 was 1 050 600 m3/day. During the last 15 years, this capacity has continued to increase as a result of cost reductions and technological advances. RO-desalinated water has been used as potable water and for industrial and agricultural purposes. Potable Water Use: RO technology is currently being used in Argentina and the northeast region of Brazil to desalinate groundwater. New membranes are being designed to operate at higher pressures (7 to 8.5 atm) and with greater efficiencies (removing 60% to 75% of the salt plus nearly all organics, viruses, bacteria, and other chemical pollutants). Industrial Use: Industrial applications that require pure water, such as the manufacture of electronic parts, speciality foods, and pharmaceuticals, use reverse osmosis as an element of the production process, where the concentration and/or fractionating of a wet process stream is needed. Agricultural Use: Greenhouse and hydroponic farmers are beginning to use reverse osmosis to desalinate and purify irrigation water for greenhouse use (the RO product water tends to be lower in bacteria and nematodes, which also helps to control plant diseases). Reverse osmosis technology has been used for this type of application by a farmer in the State of Florida, U.S.A., whose production of European cucumbers in a 22 ac. greenhouse increased from about 4 000 dozen cucumbers/day to 7 000 dozen when the farmer changed the irrigation water supply from a contaminated surface water canal source to an RO-desalinated brackish groundwater source. A 300 l/d reverse osmosis system, producing water with less than 15 mg/1 of sodium, was used. In some Caribbean islands like Antigua, the Bahamas, and the British Virgin Islands (see case study in Part C, Chapter 5), reverse osmosis technology has been used to provide public water supplies with moderate success. In Antigua, there are five reverse osmosis units which provide water to the Antigua Public Utilities Authority, Water Division. Each RO unit has a capacity of 750 000 l/d. During the eighteen-month period between January 1994 and June 1995, the Antigua plant produced between 6.1 million l/d and 9.7 million l/d. In addition, the major resort hotels and a bottling company have desalination plants. In the British Virgin Islands, all water used on the island of Tortola, and approximately 90% of the water used on the island of Virgin Gorda, is supplied by desalination. On Tortola, there are about 4 000 water connections serving a population of 13 500 year-round residents and approximately 256 000 visitors annually. In 1994, the government water utility bought 950 million liters of desalinated water for distribution on Tortola. On Virgin Gorda, there are two seawater desalination plants. Both have open seawater intakes extending about 450 m offshore. These plants serve a population of 2 500 year-round residents and a visitor population of 49 000, annually. There are 675 connections to the public water system on Virgin Gorda. In 1994, the government water utility purchased 80 million liters of water for distribution on Virgin Gorda. In South America, particularly in the rural areas of Argentina, Brazil, and northern Chile, reverse osmosis desalination has been used on a smaller scale. Figure 17: Two Types of Reverse Osmosis Membranes. Source: O.K. Buros, et. al.. The USAID Desalination Manual, Englewood, N.J., U.S.A., IDEA Publications

Operation and Maintenance Operating experience with reverse osmosis technology has improved over the past 15 years. Fewer plants have had long-term operational problems. Assuming that a properly designed and constructed unit is installed, the major operational elements associated with the use of RO technology will be the day-to-day monitoring of the system and a systematic program of preventive maintenance. Preventive maintenance includes instrument calibration, pump adjustment, chemical feed inspection and adjustment, leak detection and repair, and structural repair of the system on a planned schedule. The main operational concern related to the use of reverse osmosis units is fouling. Fouling is caused when membrane pores are clogged by salts or obstructed by suspended particulates. It limits the amount of water that can be treated before cleaning is required. Membrane fouling can be corrected by backwashing or cleaning (about every 4 months), and by replacement of the cartridge filter elements (about every 8 weeks). The lifetime of a membrane in Argentina has been reported to be 2 to 3 years, although, in the literature, higher lifespans have been reported. Operation, maintenance, and monitoring of RO plants require trained engineering staff. Staffing levels are approximately one person for a 200 m3/day plant, increasing to three persons for a 4 000 m3/day plant. Level of Involvement The cost and scale of RO plants are so large that only public water supply companies with a large number of consumers, and industries or resort hotels, have considered this technology as an option. Small RO plants have been built in rural areas where there is no other water supply option. In some cases, such as the British Virgin Islands, the government provides the land and tax and customs exemptions, pays for the bulk water received, and monitors the product quality. The government also distributes the water and in some cases provides assistance for the operation of the plants. Costs The most significant costs associated with reverse osmosis plants, aside from the capital cost, are the costs of electricity, membrane replacement, and labor. All desalination techniques are energy-intensive relative to conventional technologies. Table 5 presents generalized capital and operation and maintenance costs for a 5 mgd reverse osmosis desalination in the United States. Reported cost estimates for RO installations in Latin American and the Caribbean are shown in Table 6. The variation in these costs reflects site-specific factors such as plant capacity and the salt content of the feedwater. The International Desalination Association (IDA) has designed a Seawater Desalting Costs Software Program to provide the mathematical tools necessary to estimate comparative capital and total costs for each of the seawater desalination processes. Table 5 U.S. Army Corps of Engineers Cost Estimates for RO Desalination Plants in Florida

Feedwater Type

Capital Cost per Unit of Daily Capacity ($/m3/day)

Operation & Maintenance per Unit of Production ($/m3)

Brackish water 380 - 562 0.28 - 0.41

Seawater 1341 - 2379 1.02 - 1.54

Table 6 Comparative Costs of RO Desalination for Several Latin American and Caribbean Developing Countries

CountryCapital Cost ($/m3/day)

Operation and Maintenance ($/m3)

Production Cost* ($/m3)a

Antigua 264 - 528 0.79 - 1.59

Argentina 3.25

Bahamas 4.60 - 5.10

Brazil 1454 - 4483 0.12 - 0.37

British Virgin Islands

1190 - 2642 b3.40 - 4.30

Chile 1300 1.00

a Includes amortization of capital, operation and maintenance, and membrane replacement.b Values of $2.30 - $3.60 were reported in February 1994. Effectiveness of the Technology Twenty-five years ago, researchers were struggling to separate product waters from 90% of the salt in feedwater at total dissolved solids (TDS) levels of 1 500 mg/1, using pressures of 600 psi and a flux through the membrane of 18 l/m2/day. Today, typical brackish installations can separate 98% of the salt from feedwater at TDS levels of 2 500 to 3 000 mg/1, using pressures of 13.6 to 17 atm and a flux of 24 l/m2/day - and guaranteeing to do it for 5 years without having to replace the membrane. Today's state-of-the-art technology uses thin film composite membranes in place of the older cellulose acetate and polyamide membranes. The composite membranes work over a wider range of pH, at higher temperatures, and within broader chemical limits, enabling them to withstand more operational abuse and conditions more commonly found in most industrial applications. In general, the recovery efficiency of RO desalination plants increases with time as long as there is no fouling of the membrane. Suitability This technology is suitable for use in regions where seawater or brackish groundwater is readily available. Advantages · The processing system is simple; the only complicating factor is finding or producing a clean supply of feedwater to minimize the need for frequent cleaning of the membrane. · Systems may be assembled from prepackaged modules to produce a supply of product water ranging from a few liters per day to 750 000 l/day for brackish water, and to 400 000 l/day for seawater; the modular system allows for high mobility, making RO plants ideal for emergency water supply use. · Installation costs are low. · RO plants have a very high space/production capacity ratio, ranging from 25 000 to 60 000 l/day/m2. · Low maintenance, nonmetallic materials are used in construction.

· Energy use to process brackish water ranges from 1 to 3 kWh per 1 0001 of product water. · RO technologies can make use of use an almost unlimited and reliable water source, the sea. · RO technologies can be used to remove organic and inorganic contaminants. · Aside from the need to dispose of the brine, RO has a negligible environmental impact. · The technology makes minimal use of chemicals.Disadvantages · The membranes are sensitive to abuse. · The feedwater usually needs to be pretreated to remove particulates (in order to prolong membrane life). · There may be interruptions of service during stormy weather (which may increase particulate resuspension and the amount of suspended solids in the feedwater) for plants that use seawater. · Operation of a RO plant requires a high quality standard for materials and equipment. · There is often a need for foreign assistance to design, construct, and operate plants. · An extensive spare parts inventory must be maintained, especially if the plants are of foreign manufacture. · Brine must be carefully disposed of to avoid deleterious environmental impacts. · There is a risk of bacterial contamination of the membranes; while bacteria are retained in the brine stream, bacterial growth on the membrane itself can introduce tastes and odors into the product water. · RO technologies require a reliable energy source. · Desalination technologies have a high cost when compared to other methods, such as groundwater extraction or rainwater harvesting.Cultural Acceptability RO technologies are perceived to be expensive and complex, a perception that restricts them to high-value coastal areas and limited use in areas with saline groundwater that lack access to more conventional technologies. At this time, use of RO technologies is not widespread. Further Development of the Technology The seawater and brackish water reverse osmosis process would be further improved with the following advances: · Development of membranes that are less prone to fouling, operate at lower pressures, and require less pretreatment of the feedwater. · Development of more energy-efficient technologies that are simpler to operate than the existing technology; alternatively, development of energy recovery methodologies that will make better use of the energy inputs to the systems. · Commercialization of the prototype centrifugal reverse osmosis desalination plant developed by the Canadian Department of National Defense; this process appears to be more reliable and efficient than existing technologies and to be economically attractive.Information Sources Contacts John Bradshaw, Engineer and Water Manager, Antigua Public Utilities Authority, Post Office Box 416, Thames Street, St. Johns, Antigua. Tel/Fax (809)462-2761.

Chief Executive Officer, Crystal Palace Resort & Casino, Marriot Hotel, Post Office Box N 8306, Cable Beach, Nassau, Bahamas. Tel. (809)32- 6200. Fax (809)327-6818. General Manager, Water and Sewerage Corporation, Post Office Box N3905, Nassau, Bahamas. Tel. (809)323-3944. Fax (809)322-5080. Chief Executive Officer, Atlantis Hotel, Sun International, Post Office Box N4777, Paradise Island, Nassau, Bahamas. Tel. (809)363-3000. Fax (809)363-3703. Vincent Sweeney, Sanitary Engineer, c/o Caribbean Environmental Health Institute (CEHI), Post Office Box 1111, Castries, Saint Lucia. Tel. (809)452-2501. Fax (809)453-2721. E-mail: cehi@isis.org.lc. Guillermo Navas Brule, Ingeniero Especialista Asuntos Ambientales, Codelco Chile Div. Chuquicamata Fono, Calama, Chile. Tel. (56-56)32-2207. Fax (56-56)32-2207. William T. Andrews, Managing Director, Ocean Conversion (BVI) Ltd, Post Office Box 122, Road Town, Tortola, British Virgin Islands. Roberta Espejo Guasp, Facultad de Ciencias, Universidad Católica del Norte, Departamento Física, Av. Angamos 0610, Casilla de Correo 1280, Antofagasta, Chile. Tel. (56-55)24-1148 anexo 211-312-287. Fax (56-55)24-1724/24-1756. E-mail: respejo@socompa.cecun.ucn.cl. María Teresa Ramírez, Ingeniero de Proyectos, Aguas Industriales, Ltda., Williams Rebolledo 1976, Santiago, Chile. Tel. (562)238-175S. Fax (562)238-1199. Claudison Rodríguez, Economista, Instituto ACQUA, Rua de Rumel 300/401,22210-010 Rio de Janeiro, Rio de Janeiro, Brasil. Tel. (55-21)205-5103. Fax (55-51)205-5544. E-mail: solon@omega.encc.br. Joseph E. Williams, Chief Environmental Health Officer, Environmental Health Department, Ministry of Health and Social Security, Duncombe Alley, Grand Turk, Turks and Caicos Islands, BWI. Tel (809)946-2152/946-1335. Fax (809)946-2411. Bibliography Birkett, J.D. (1987). "Factors Influencing the Economics of Desalination." In Non-Conventional Water Resources Use in Developing Countries. New York, United Nations, pp. 89-102. (Natural Resources/Water Series No. 22) Boari, et al. 1978. "R.O. Pilot Plants Performance in Brackish Water Desalination.," Desalination, 24, pp. 341-364. Buros, O.K. 1987. "An Introduction to Desalination." In Non-Conventional Water Resources Use in Developing Countries. New York, United Nations, pp. 37-53. (Natural Resources/Water Series No. 22) ----, et al. 1982. The USAID Desalination Manual. Englewood, N.J., U.S.A., IDEA. (Originally published by USAID/CH2M Hill) Cant, R.V. 1980. "Summary of Comments on R.A. Tidball's 'Lake Killarney Reverse Osmosis Plant."' In P. Hadwen (ed.). Proceedings of the United Nations Seminar on Small Island Water Problems, Barbados. New York, UNDP. pp. 552-554. Childs, W.D., and A.E. Dabiri. 1992. "Desalination Cost Savings or VARI-RO." Pumping Technology, 87, pp. 109-135. de Gunzbourg, J., and T. Froment. 1987. "Construction of a Solar Desalination Plant (40 cum/day) for a Caribbean Island," Desalination, 67, pp. 53-58. Dodero, E., et al. 1983. "Tres Años de Experiencia en la Planta de Desalinación de Aguas de Selva, Provincia de Santiago del Estero." Paper presented at the 6° Congreso Argentino de Saneamiento, Salta, Argentina.

Eisenberg, Talbert N., and E. Joe Middlebrooks. 1992. "A Survey of Problems with Reverse Osmosis Water Treatment," American Water Works Association Journal, 76(8), p. 44. Furukawa, D.H., and G. Milton. 1977. " High Recovery Reverse Osmosis with Strontium and Barium Sulfate in a Brackish Wellwater Source," Desalination, 22(1,2,3), p. 345. Gibbs, Robert, 1982. "Desalinización en México: Uso de la Tecnología Existente Mas Innovación," Agua (Houston, Texas), 1, 3; 4, pp. 17-20. Gomez, Evencio G. 1979. "Ten Years Operation Experience at 7.5 Mgd Desalination Plant; Rosarito B. Cafa, Mexico," Desalination, 31(1), pp. 77-90. Hall, W.A. 1980. "Desalination: Solution or New Problem for Island Water Supplies." In P. Hadwen (ed.), Proceedings of the United Nations Seminar on Small Island Water Problems, Barbados. New York, UNDP, pp. 542-543. IDA. 1988. Worldwide Inventory of Desalination Plants. Topsfield, Mass., U.S.A. Lawand, T.A. 1987. "Desalination With Renewable Energy Sources." In Non-Conventional Water Resources Use in Developing Countries. New York, United Nations, pp. 66-86. (Natural Resources/Water Series No. 22) Libert, J.J. 1982. "Desalination and Energy," Desalination, 40, pp. 401-406. Niehaus, F. Guillermo. 1991. "Separación por membranas." In Segundo Seminario de Purificación y Tratamiento de Agua, Santiago, Colegio de Ingenieros de Chile, pp. 51-63. Office for Technology Assessment (OTA). 1988. Using Desalination Technologies/or Water Treatment. Washington, D.C., U.S. Congress. Toelkes, W.E. 1987. "The Ebeye Desalination Project: Total Utilization of Diesel Waste Heat," Desalination, 66, pp. 59-66. Torres, M., J.A. Vera, and F. Fernandez. 1985. "20 Years of Desalination in the Canary Islands, Was It Worth It?" Aqua, 3, pp. 151-155. Troyano, F. 1979. "Introductory Report (Desalination: Operation and Economic Aspects of Management)." In Proceedings of the. Seminar on Selected Water Problems in Islands and Coastal Areas with Special Regard to Desalination of Groundwater, San Anton, Malta. New York, Pergamon Press, pp. 371-375. Water and Sewage Works. 1988. "Reverse Osmosis Used for Water Desalination in Sea World," 124(3), p. 81. Wild, Peter M., and Geoffrey W. Vickers. 1991. "The Technical and Economic Benefits of Centrifugal Reverse Osmosis Desalination." In IDA World Conference on Desalination and Water Reuse. Topsfield, Mass., U.S.A., IDP. World Water. 1982. "Desalter Systems for Man-made Islands," July, pp. 39-42. ----. 1984. "RO Renewal Rate Could Be Critical," July, pp. 35-39. ----. 1986. "Reverse Osmosis Still Needs Careful Treatment," December, pp. 33-35.

http://www.coastal.ca.gov/desalrpt/dchap1.html

alifornia Coastal Commission

Seawater Desalination in California

CHAPTER ONE: BACKGROUND

 

Desalination Plants Worldwide Desalination Technologies

o Reverse Osmosis (RO) o Distillation

Input Water (Feedwater) Product Water Product Water Recovery Pretreatment Processes Filter Backwashing, Membrane Cleaning and Storage, Scaling Prevention and

Removal, and Pipeline Cleaning Waste Discharges Energy Use Comparison of Distillation and Reverse Osmosis Technologies Costs of Desalinated Water Costs of Other Water Sources

Desalination Plants Worldwide

Of the more than 7,500 desalination plants in operation worldwide, 60% are located in the Middle East. The world's largest plant in Saudi Arabia produces 128 MGD of desalted water. In contrast, 12% of the world's capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida. To date, only a limited number of desalination plants have been built along the California coast, primarily because the cost of desalination is generally higher than the costs of other water supply alternatives available in California (e.g., water transfers and groundwater pumping). However, as drought conditions occur and concern over water availability increases, desalination projects are being proposed at numerous locations in the state.

Desalination Technologies

Desalination is a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including reverse osmosis (RO), distillation, electrodialysis, and vacuum freezing. Two of these technologies, RO and distillation, are being considered by municipalities, water districts, and private companies for development of seawater desalination in California. These methods are described below.

Reverse Osmosis (RO)

In RO, feedwater is pumped at high pressure through permeable membranes, separating salts from the water (Figure 1). The feedwater is pretreated to remove particles that would clog the membranes. The quality of the water produced depends on the pressure, the concentration of salts in the feedwater, and the salt permeation constant of the membranes. Product water quality can be improved by adding a second pass of membranes, whereby product water from the first pass is fed to the second pass.

Figure 1. Flow Diagram of a reverse osmosis system (courtesy of USAID). (Kahn, 1986.)

Distillation

In the distillation process, feedwater is heated and then evaporated to separate out dissolved minerals. The most common methods of distillation include multistage flash (MSF), multiple effect distillation (MED), and vapor compression (VC) (Figure 2).

Figure 2. Common methods of distillation.

In MSF, the feedwater is heated and the pressure is lowered, so the water "flashes" into steam. This process constitutes one stage of a number of stages in series, each of which is at a lower pressure. In MED, the feedwater passes through a number of evaporators in series. Vapor from one series is subsequently used to evaporate water in the next series. The VC process involves evaporating the feedwater, compressing the vapor, then using the heated compressed vapor as a heat source to evaporate additional feedwater. Some distillation plants are a hybrid of more than one desalination technologies. The waste product from these processes is a solution with high salt concentration.

Input Water (Feedwater)

Desalination plants may use seawater (directly from the ocean through offshore intakes and pipelines, or from wells located on the beach or seafloor), brackish groundwater, or reclaimed water as feedwater. Since brackish water has a lower salt concentration, the cost of desalting brackish water is generally less than the cost of desalting seawater. Intake pipes for desalination plants should be located away from sewage treatment plant outfalls to prevent intake of discharged effluent. If sewage treatment discharges or other types of pollutants are included in the intake, however, the pre- and post-treatment processes should remove the pollutants.

Product Water

Distillation plants produce a high-quality product water that ranges from 1.0 to 50 ppm tds, while RO plants produce a product water that ranges from 10 to 500 ppm tds. (The recommended California drinking water standard for maximum tds is 500 mg/L, which is equivalent to 500 ppm.) In desalination plants that produce water for domestic use, post-treatment processes are often employed to ensure that product water meets the health standards for drinking water as well as recommended aesthetic and anti-corrosive standards.

Desalination product water may be used in its pure form (e.g., for make-up water in power plant boilers) or it may be mixed with less pure water and used for drinking water, irrigation, or other uses. The desalinated product water is usually more pure than drinking water standards, so when product water is intended for municipal use, it may be mixed with water that contains higher levels of total dissolved solids. Pure desalination water is highly acidic and is thus corrosive to pipes, so it has to be mixed with other sources of water that are piped onsite or else adjusted for pH, hardness, and alkalinity before being piped offsite.

Product Water Recovery

The product water recovery relative to input water flow is 15 to 50% for most seawater desalination plants. For every 100 gallons of seawater, 15 to 50 gallons of pure water would be produced along with brine water containing dissolved solids. A desalination

plant's recovery varies, in part because the particulars of plant operations depend on site-specific conditions. In several locations in California, pilot projects are being proposed to test plant operations before full-scale projects are built.

Pretreatment Processes

Pretreatment processes are needed to remove substances that would interfere with the desalting process. Algae and bacteria can grow in both RO and distillation plants, so a biocide (usually less than 1 mg/L chlorine) is required to clean the system. Some RO membranes cannot tolerate chlorine, so dechlorination techniques are required. Ozone or ultraviolet light may also be used to remove marine organisms. If ozone is used, it must be removed with chemicals before reaching the membranes. An RO technology has been developed recently that does not require chemical pretreatment.

In RO plants, suspended solids and other particles in the feedwater must be removed to reduce fouling of the membranes. Suspended solids are removed with coagulation and filtration. Metals in the feedwater are rejected along with the salts by the membranes and are discharged in the brine. With normal concentrations for metals in seawater, the metals present in the brine discharge, though concentrated by the RO process, would not exceed discharge limits. Some distillation plants may also need to remove metals due to potential corrosion problems.

Filter Backwashing, Membrane Cleaning and Storage, Scaling Prevention and Removal, and Pipeline Cleaning

The filters for pretreatment of feedwater at RO plants must be cleaned every few days (backwashed) to clear accumulated sand and solids. The RO membranes must be cleaned approximately four times a year and must be replaced every three to five years. Alkaline cleaners are used to remove organic fouling, and acid cleaners are used to remove scale and other inorganic precipitates. All or a portion of RO plants must be shut down when the membranes are replaced. When RO plants are not used continuously, the RO membranes must be stored in a chemical disinfection/preservation solution that must be disposed of after use. Distillation plants can also be shut down for tube bundle replacement, which is analogous to membrane replacement.

Desalination plant components must be cleaned to reduce scaling-a condition where salts deposit on plant surfaces, such as pipes, tubing or membranes. Scaling is caused by the high salt concentration of seawater and can result in reduced plant efficiency and corrosion of the pipes. In general, scaling increases as temperature increases; thus scaling is of greater concern for distillation plants, since RO plants require lower temperatures to operate. Scaling can be reduced by introducing additives to inhibit crystal growth, reducing temperature and/or salt concentrations, removing scale-forming constituents, or seeding to form particles. Once scales have formed, they can be removed with chemical or mechanical means.

In addition to scaling, both RO and distillation plant intake and outfall structures and pipelines can become fouled with naturally occurring organisms or corroded. Structures and pipelines may be cleaned by mechanical means or by applying chemicals or heat. Feedwater may also be deaerated to reduce corrosion.

Waste Discharges

Desalination plants produce liquid wastes that may contain all or some of the following constituents: high salt concentrations, chemicals used during defouling of plant equipment and pretreatment, and toxic metals (which are most likely to be present if the discharge water was in contact with metallic materials used in construction of the plant facilities). Liquid wastes may be discharged directly into the ocean, combined with other discharges (e.g., power plant cooling water or sewage treatment plant effluent) before ocean discharge, discharged into a sewer for treatment in a sewage treatment plant, or dried out and disposed of in a landfill. Desalination plants also produce a small amount of solid waste (e.g., spent pretreatment filters and solid particles that are filtered out in the pretreatment process).

For example, the capacity of the City of Santa Barbara's desalination plant is 7,500 AF/yr (about 7.16 MGD). In May 1992, the plant produced 6.7 MGD of product water and generated 8.2 MGD of waste brine with a salinity approximately 1.8 times that of seawater. An additional 1.7 MGD of brine was generated from filter backwash. Assuming that concentrations of suspended solids in the seawater feed range from 10 to 50 ppm, approximately 1.7 to 5.1 cubic yards per day of solids were generated, which is equivalent to one to two truck-loads per week. (Source: Woodward-Clyde Consultants, EIR for the City of Santa Barbara and Ionics, Inc.'s Temporary Emergency Desalination Project, March 1991.)

Energy Use

The energy used in the desalination process is primarily electricity and heat. Energy requirements for desalination plants depend on the salinity and temperature of the feedwater, the quality of the water produced, and the desalting technology used. Estimates for electricity use requirements for various technologies for seawater desalination are:

In addition to electricity requirements, MSF, MED, and some VC plants also use thermal energy to heat feedwater. (Because of the inefficiency of converting thermal energy to

electricity, there is a high energy "penalty" if electricity is used to heat feedwater.) For example, in addition to the 3,500 to 7,000 kWh/AF of energy required for electricity, the thermal energy needs for a MSF distillation plant is estimated at 270 million Btu/AF (about 26,000 kWh/AF); for MED plants, the estimated additional thermal energy requirements are 230 million Btu/AF (about 22,000 kWh/AF).[1] Consequently, the total energy needs for distillation technologies are higher than for RO technologies.

Energy use requirements for desalination plants are high. For example, an estimated 50 million kWh/yr would be required for full-time operation of the City of Santa Barbara's desalination plant to produce 7,500 AF/yr of water. In contrast, the energy needed to pump 7,500 AF/yr of water from the Colorado River Aqueduct or the State Water Project to the Metropolitan Water District (MWD) of Southern California is 15 to 26 million kWh/yr. These energy requirements may be compared to the energy use of a small- to medium-sized industrial facility (such as a large refinery, small steel mill, or large computer center) which uses 75,000 to 100,000 kWh/yr.

Both RO and distillation plants can benefit from cogeneration plants to reduce energy use. Since increased energy use may cause adverse environmental impacts, the individual and cumulative impacts of energy use and production at a proposed desalination plant will require case-by-case analysis.

Comparison of Distillation and Reverse Osmosis Technologies

One advantage of distillation plants is that there is a greater potential for economies of scale. Distillation plants also do not shut down a portion of their operations for cleaning or replacement of equipment as often as RO plants, although distillation plants can and have shut down for tube bundle replacement and cleaning. Pretreatment requirements are greater for RO plants, because coagulants are needed to settle out particles before water passes through the membranes. Unlike RO plants, distillation plants do not generate waste from backwash of pretreatment filters.

Advantages of RO plants over distillation include: RO plant feedwater generally does not require heating, so the thermal impacts of discharges are lower; RO plants have fewer problems with corrosion; RO plants usually have lower energy requirements; RO plants tend to have higher recovery rates-about 45% for seawater; the RO process can remove unwanted contaminants, such as trihalomethane-precursors, pesticides, and bacteria; and RO plants take up less surface area than distillation plants for the same amount of water production.

Costs of Desalinated Water

The cost to produce water from desalination depends on the technology used and the plant capacity, among other factors. For example, the cost of desalted water in Santa Barbara ($1,900/AF) results from the following: a write-off of the capital cost over a short five-year period, high financing costs, and high energy costs. The overall costs of water production are about the same for RO and some forms of distillation plants.

Price estimates of water produced by desalination plants in California range from $1,000 to $4,000/AF. Table 2 lists the estimated costs of producing water for existing and proposed plants, where the information is available. (Specific cost estimates for most existing and proposed California desalination plants are also included in Chapter 2 of this report.) The costs include capital and operating and maintenance costs. For long-term projects, capital costs would most likely be amortized over an assumed plant life of 20 to 30 years. Capital costs for RO plants tend to be lower than for distillation plants. Some of the proposals are for plants that would operate for only a few years. Operating a plant on a part-time, rather than full-time, basis may be more expensive in the long run because maintenance and capital costs must be paid while the plant is shut down.

http://www.southeastwater.co.uk/develop02.asp

New Developments

Proposed Desalination Plant at Newhaven Harbour

The FactsSouth East Water is to assess the potential use of desalinated sea water as an additional water resource to meet future demand. With this in mind, it has applied for planning permission to construct a trail desalination plant at Newhaven Harbour. This small plant will be used only to test the feasibility of the scheme and no water from it will be pumped into the supply. If the scheme proves successful, a full-scale plant may be constructed to supplement existing water supplies during long periods.

What is a desalination plant?A desalination plant removes the salt and other impurities from sea water, creating drinking water.

Does this mean we might be drinking sea water in the future?All our water supplies originate from the sea already. In a process known as the ‘water cycle’, water evaporates from the oceans, forms clouds and then falls as rain. This filters through the ground into rivers and reservoirs, is collected, cleaned and fed into the domestic water supply. By desalinating sea water we are merely accelerating a natural process.

Why is the plant needed?Housing development in the south east, and the growth in the use of appliances such as dishwashers and power showers, has created significant increased demand and pressure on existing water resources. In addition, in the past we have been used to having abundant rainfall in the UK evenly distributed throughout the year. However, changes in

our climate have meant that we are now experiencing more and more long, dry spells. Last year for example, we had nine consecutive months of below average rainfall, extending well into the autumn.

This meant that our reservoirs did not refill as they normally do, and there was a real danger of restrictions on water use being put in place. As part of South East Water’s long term strategy to ensure we all have the water we need, we are looking at alternative methods of supplementing existing water supplies at times of high demand.

How big will the plant be?The pilot plant which will be used for the test is only 12x2.5x2.5m (the size of a cargo container). If the trial proves successful, the full-scale plant would be fully contained in a building about the size of a small warehouse, and would be capable of producing up to 8 million litres a day. South East Water supplies 400 million litres per day on average, so it would represent a very small proportion of the overall supply.

Would all our drinking water come from this plant?No. If a full-scale plant were built, it would not be running at full capacity all the time. Output would be increased during dry periods when demand was high, such as was experienced last year. It would then be blended with water from other sources, and fed into the distribution system to go to areas of high demand and water shortage.

How does it work?The desalination process comprises two stages. The water is first filtered using either sand or an ‘ultra-filtration’ membrane, which is a bit like a sieve with very, very small holes. Both methods, which remove solid particles and bacteria from the water, will be evaluated in the pilot plant. The second part of the process involves something called ‘reverse osmosis’ in which the water is forced through an incredibly fine semi-permeable membrane (another sieve with even smaller holes) at high pressure. This membrane only allows the water molecules to pass through, removing all the dissolved salts and other pollutants. The result is pure, clean water.

Example: Sea water desalination. The physical principle of the osmosis allows the least concentrated solution (soft water) to migrate naturally towards the most concentrated solution (sea water) to dilute it. In order to desalinate, the direction of the water flow is reversed by applying pressure on the concentrated solution. The membrane than allows water molecules through but, in this new direction, holds back the salts and pollutants (organic materials, viruses, bacteria).

When will all this happen?If the plans are approved, the pilot plant will be installed in June 2004 and the trial will run for about 12-14 months. If the results show that the scheme is both economically and environmentally viable, then the full-scale plant could be in operation for the summer of 2006.

What environmental impacts are there likely to be?That is exactly what the trial will show us. We are working very closely with the Environment Agency to ensure that the design takes into account all possible environmental considerations.

Why Newhaven?Mid-Sussex is an area where water resources are particularly stretched and we already take a significant amount from the river and ground water supplies. The extra demand that we experience in long, dry periods cannot easily be met just by taking more and more water from these existing supplies without detriment to the environment, hence the need for an alternative. Newhaven is a suitable location from which to deliver water into the areas of Mid-Sussex where it is most needed.

Is desalination used anywhere else?Sea water desalination is an established process for drinking water production in desert coastal areas of the world. Desalination plants are a common sight in the Canary Islands and Saudi Arabia, though these are far larger than the sort we are proposing. Desalination is also already used in Jersey and the Scilly Isles. Similar projects are currently under consideration around the UK, but sea water desalination on this scale would be a first in this country.

If this is such a good idea, why hasn’t it been used before?Historically, desalination plants have required a large amount of power. Their installation costs are low but operating costs have been high. However, new membrane technology and pumping, coupled with the design that we are proposing, means that 40 per cent of the power used can be recovered and fed back into the plant, thereby significantly reducing the running costs and making it a sustainable and viable process.

http://www.unu.edu/unupress/unupbooks/80858e/80858E09.htm

2.9 Seawater desalination in the Arabian Gulf countries

Owing to the rapid increase in demand for water in the Arabian Gulf countriesSaudi Arabia' Kuwait, the United Arab Emirates, Qatar, Bahrain, and Omanwhere conventional water resources such as fresh surface water and renewable groundwater are extremely limited, other alternatives such as wastewater reclamation and desalination have been adopted since the 1960s. Countries such as Saudi Arabia, Kuwait, Qatar, and Bahrain all use nonrenewable groundwater resources in large quantity, causing depletion of these valuable resources and deterioration in the quality of water. Although conventional water resources such as renewable groundwater and surface runoff are available in countries like Oman, the United Arab Emirates, and Saudi Arabia, these resources still need to be properly developed in an integrated water-resources planning context.

In some of the more arid parts of the Middle East, in particular the Gulf states, where good quality water is not available or is extremely limited, desalination of seawater has been commonly used to solve the problems of water supply for municipal and industrial uses.

Kuwait was the first state to adopt seawater desalination, linking electricity generation to desalination. The co-generation station, as it is called, re-uses low pressure steam from the generator to provide energy for the desalination process. As a result, both energy and costs are minimized. Kuwait began desalinated water production in 1957, when 3.1 million m³ were produced per year. By 1987 this figure had risen to 184 million m³ per year.

In Qatar, too, an intensive programme of desalinated water production has been started, which should be supplying about 150 million m³ of water per year by the year 2000. This is believed to be about threequarters of the total water demand, with the rest to be supplied from groundwater sources, which are mostly brackish. About half of the country's demand will be generated in the urban/industrial centres.

Saudi Arabia entered the desalinated water field much later than Kuwait. The first plant was commissioned in 1970. It has, however, gone in for an ambitious programme of desalination plant construction on both the Red Sea and Gulf coasts. The Saline Water Conversion Corporation had installed 30 desalination plant projects by the end of the 1980s. The total production of desalinated water is estimated to be 2.16 million m³ (572 million [US] gal.) per day including a facility at Al-Jubail producing 1 million m³ per day, which is currently the world's largest distillation plant.

In spite of the high cost of seawater desalination, with unit water costs five to ten times as high as those of conventional water-resources development, a vast quantity has been

produced to meet the increasing demand for domestic water in the Arabian Gulf countries. As in Kuwait, however, there is increasing government concern about the production cost of desalinated water, and every effort is being made to ensure that water use is as efficient as possible.

2.9.1 Installed capacity of desalination plants

There are about 1,483 desalination units operating in the Arabian Gulf countries, which account for 57.9% of the worldwide desalting plant capacity. The dominant plant type is multi-stage flash (MSF) which accounts for 86.7% of the desalting capacity, while the reverse osmosis accounts for only 10.7%. The installed capacity of desalination plants in the Arabian Gulf countries is estimated at 5.76 million m³ per day in total, including 2.98 million m³ in Saudi Arabia, which is approximately half of the total desalination capacity of the Gulf countries (Al-Mutaz 1989). The installed capacity with shares of each process are shown in table 2.10.

MSF desalting has proved to be the simplest, most reliable, and most commonly used seawater system in large capacities. It has reached maturity with very little improvement in sight. This maturity is expressed in reliable designs of large units up to 38,000 m³ (10 million gal.) per day, long operation experience with high on-line stream factors (up to 95%), confidence in material selection, and very satisfactory water pre-treatment. However, there has been a recent trend towards the use of reverse osmosis in seawater desalination, both for new plants and in connection with the present MSF plants, taking into account the possible reduction in energy requirements and the lower operation and maintenance cost for RO.

Table 2.10 Installed capacity of desalting plants and share by process type in the Arabian Gulf countries

  No. of units Capacity (1,000 m³/day)

Share by process type (%)

MSF RO ED VC MED

Saudi Arabia 874 2,980 80.7 16.2 2.6 0.5  

Kuwait 279 1,090 95.5 1.8 0.55 1.6 0.25

U.A.E. 99 1,020 98.3 0.9 0.5 - -

Qatar 47 310 9 7.9 - - 0.7 0.9

Bahrain 143 260 56.7 37.2 4.9 0.8 0.4

Oman 41 100 91.1 1.9 0.9 1.7  

TOTAL 1,483 5,760 86.7 10.7 1.8 0.65 0.15

Source: Akkad 1990.MSF = multi-stage flash. RO = reverse osmosis. ED = electrodialysis. VC = vapour compression. MED = multi-effect distillation.

2.9.2 The world's largest seawater desalination with high-pressure pipeline system

To meet the water demands of the increasing population and water short regions in Saudi Arabia, the Saline Water Conversion Office (SWCO) under the Ministry of Agriculture and Water was made responsible for providing fresh water by desalination of seawater in 1965. The first seawater desalination plant was commissioned in 1970. With its increasing responsibilities to provide fresh water, the SWCO was changed in 1974 into an independent corporation, the Saline Water Conversion Corporation (SWCC), which then developed an elaborate plan to construct dual-purpose plants on both the east and west coasts of the kingdom.

The SWCC had constructed 24 plants by 1985, including 17 plants on the western coast along the Red Sea, from Haql on the Gulf of Aqaba in the north to the tiny Farasan island in the south, and 7 plants on the east coast along the Arabian Gulf from Al-Khafji to Al-Khobar (fig. 2.48). These plants were producing 1.82 million m³ (481 million gal.) of fresh water per day and 3,631 MW of electric power. By the end of the 1980s the total production of fresh water was estimated to have been increased to 2.17 million m³ (572 million gal.) of fresh water per day and 4,079 MW of electric power by the addition of six cogeneration plants (SWCC 1988).

Fig. 2.48 Desalinsation plants and water supply in Saudi Arabia

In addition to desalination and power plants, the SWCC provides water to inland regions by means of pipelines. The Al-Jubail-Riyadh pipeline is one of the world's largest water pipeline systems with seawater desalination plants. The pipeline has a diameter of 1.5 m (60 inches), a length of 466 km, a differential head of 690 m, and a pumping capacity of 830,000 m³ per day (SWCC 1988).

2.9.3 Cost constraints of seawater desalination

The MSF process has served very well during the past ten years, especially in the Middle East. During this period, operating experience has been developed that should result in substantial extensions to what was heretofore considered a reasonable operating life. Certainly this favourable experience will be a factor in the selection of future plants.

However, the lower capital and operating costs for the RO process should receive increasing attention in the selection of a desalination process in coming years. There are still opportunities for further lowering of costs through improved membrane technology, notably in increasing membrane life. Another new development with good potential for reducing costs for the RO process are membranes for operating at high pressures up to 1,500 psi (105 kg/cm²) and 50% conversion when operating on seawater with 45,000 mg of TDS per litre.

Another alternative process will be low-temperature multi-effect horizontaltube evaporators. If aluminium tubes and tube sheets can be shown to have a reasonable life in Middle East seawater, the capital cost can be reduced, or a higher performance ratio can be achieved.

Another factor which will favour reverse osmosis in coming years is that it is the most energy-efficient of all of the processes. This will be of increasing importance if in fact fuel-oil prices rise further as expected and environmental considerations increase in importance. The cost of energy consumption is also the largest single item in the cost of desalted water. It is significant that, for either a single-purpose or a dual-purpose plant, RO appears to be the most cost-effective. On the basis of world fuel costs in 1989, the RO process would save over 10% compared with multi-effect distillation and 32% compared with MSF (Leitner 1989).

2.9.4 Hybrid RO/MSF seawater desalination to compromise quality-cost constraints

It seems that the race for the second generation of seawater desalters has been settled, with RO and low-temperature multi-effect horizontal tube evaporators as front runners. Both systems are characterized by their low energy requirements compared with the MSF system. As shown in fig. 2.49, which gives the worldwide market shares of various desalination processes, RO accounted for 65% of market share in 1987 (Wangnick and IDA 1988). Beside these two options, there are combination possibilities of different desalting plant types. In the hybrid MSF/RO desalination-power process, a seawater RO plant is combined with either a new or existing dualpurpose MSF plant with the following advantages:

Fig. 2.49 World market share of various desalination processes (Source: Wangnick and IDA 1988)

>> The capital cost of the combined RO/MSF plant can be reduced.

>> A common seawater intake is used.

>> Product waters from the RO and MSF plants are blended to obtain suitable product-water quality. Taking advantage of the fact that the MSF product (25 mg of TDS per litre) typically exceeds potable water specifications (WHO standard: 500-1,000 mg/l the product water specification in the RO system can thereby be reduced.

>> A single-stage RO process can be used and the RO membrane life can be extended because of the reduced product-water specification. (The life of the RO membrane can be extended from three to five years, or the annual membrane replacement cost can be reduced by nearly 40%.)

>> Electric power production from the MSF plant can be efficiently utilized in the RO plant, thereby reducing net export power production. In addition, the electric power requirement to drive the high-pressure pumps of the RO system, which is a major factor of energy consumption, can be reduced by 30% by adding an energy recovery unit to the brine discharge in the RO system. (Power consumption for a single-stage seawater RO plant at 30% of recovery/conversion is estimated to be 9.24 kWh/m³ without or 6.38 kWh/m³ with energy recovery on brine discharge [Awerbuch et al. 1989].)

>> Blending with RO product water reduces the temperature of the MSF product water. A problem common in areas in the Middle East is the high temperature of the product water. RO for high pressure brine when no energy recovery is used can be used to cool the MSF product water with an eductor.

JEDDAH RO/MSF HYBRID PROJECT. The first large-scale MSF/RO hybrid project, the Jeddah I rehabilitation project in Saudi Arabia, is now in operation by the Saline Water Conversion Corporation. This 15 million gal. (56,800 m³) per day RO plant, the world's largest facility for seawater conversion, has demonstrated the attractiveness of the hybrid concept. The Jeddah I MSF desalination plant was completed in 1970, with an installed capacity of 5 million gal. (18,925 m³) per day. It was one of the world's largest plants in the early 1970s and therefore has a significant place in history. The installed capacity of the Jeddah desalting complex was expanded by steps to a nominal capacity of 85 million gal. (321,725 m³) per day, all by MSF.

In 1985 the operation and maintenance of the Jeddah I MSF plant had become increasingly costly. To keep pace with the increasing water demand, the 5 million gal. per day Jeddah I MSF plant was replaced by a 15 million gal. per day RO plant (phase I) in 1986-1989, which is incorporated in a hybrid RO/MSF desalination system. The RO unit has the following design criteria (Muhurji et al. 1989):

feed-water quality: TDS = 43,300 mg/l chloride as Cl- = 22,400 mall, pH = 8.2, water temperature 24.5-32.5°C;

operating pressure at 60 kg/cm² (maximum design pressure: 70 kg/ cm ); a single-stage design, including 10 RO trains, with each train including 148 RO

modules; hollow fine fibre (Toyobo Hollosep made of cellulose triacetate) RO module with

10 inch diameter; recovery ratio of 35% of product water; product-water salinity as specified at 625 mg of chloride per litre (= 1,250 mg of

TDS per litre).

Since MSF product water has a salinity as low as 25-50 mg of TDS per litre, the salinity of the permeate from the Jeddah I RO plant (phase I) was specified as 625 mg of chloride (1,250 mg of TDS) per litre, which is a major factor in minimizing the cost of the RO. In a cost analysis done by Bechtel (Muhurji et al. 1989), it was shown that the product water cost from the RO system in a hybrid MSF/RO plant can be reduced by 15% compared with a stand-alone RO plant.

2.10 Groundwater-hydro development in Chile and Libya

Groundwater-hydro has been studied in two development projects in the arid regions of north-west Chile and the Sahara desert in Libya. The Chilean plan will involve constructing a high-pressure pipeline to exploit the height difference between the wellfield in the Andes and the coastal terrain. The Libyan plan will involve installing a mini-hydro station at the end of the Great Man-Made River pipeline to exploit the height difference of 200 m.

2.10.1 Groundwater-hydro in multi-purpose Salar del Huasco scheme in Chile

The coastal plains in the northern part of Chile may be classified as arid to extremely arid (fig. 2.50). The extremely arid Iquique region is located in the northern corner of Chile, where rainfall is only 10 mm or less per year. No water resources are available in these arid coastal regions except for a very limited amount of groundwater, whose quality is likely to be saline or brackish. By contrast, huge renewable groundwater resources with excellent quality can be tapped in the Andes mountain ranges. The hydro-potential of the Andes mountain ranges in South America is one of the world's largest, and includes both surface water and groundwater.

The Salar del Huasco project is being planned to develop groundwater for water supply, irrigation, and hydroelectric power. The groundwater-hydro scheme would use the substantial head difference between the wellfield on the mountain range (3,750 m) and the irrigation area on the coastal terrain (1,400 m). The water will be supplied from a wellfield 76 km away by a pipeline that will cross the mountains using pumping stations. The project will assure adequate drinking water supplies to Iquique until the middle of the next century and will increase the local availability of irrigation water by 50%. This will suffice for the cultivation of 4,800 ha of land on the extremely arid terrain. The hydro units will have a combined capacity of 50 MW (WPDC 1988).

Fig. 2.50 Salar del Huasco groundwater development scheme and wafer pipeline system in Chile

Fig. 2.51 Schematic profile of the Salar del Huasco groundwater-hydro scheme

The scheme will comprise the extraction of 2.4 m³ of groundwater per second from 54 wells in the area of Lake Huasco, which is at an elevation of 3,785 m. The water will be piped through a central collector to Iquique and Pica, and the available head will be used to generate electricity. The first or upper station will be built between the wellfield and Pica, at an elevation of 3,000 m, and the second or the lower station will be built in Pica, at an elevation of 1,400 m (fig. 2.51). The theoretical hydro-power is estimated to be 50 MW in total, 16 MW at the first power station and 34 MW at the second. The installed capacities of the power stations are preliminarily estimated to be 42 MW in total, consisting of 13 MW at the first station and 29 MW at the second.

2.10.2 Groundwater-hydro in the Great Man-Made River project, Libya

A hydroelectric power station will be installed in part of the massive Great Man-Made River project, which will carry an eventual 6 million m³ of water per day from beneath the southern Sahara desert for agricultural, industrial, and domestic use in the heavily populated coastal regions in Libya (see section 2.7.3 above). This groundwater-hydro plant will be the first of its kind.

The second phase of the project, begun in 1986, includes an option for an 18 MW hydroelectric station to be built adjacent to a terminal reservoir with a planned capacity of 28 million m³ (WPDC 1986). The station would use a differential head of water of some 200 m, and power output would compensate for the energy used to pump the water to the coast.

Costs of Other Water Sources

A number of California coastal communities are facing water shortages. Although the communities may have relatively inexpensive existing supplies of water, the supplies are perceived as being insufficient to meet community needs. New water supplies are more expensive than existing supplies, and in some cases the prices are comparable to desalinated water. Table 2 summarizes the costs of various water supplies.

In 1991, the Metropolitan Water District (MWD) of Southern California paid $27/AF for water from the Colorado River and $195/AF for water from the California Water Project. New sources of water would have cost $128/AF from the Imperial Irrigation District and $93/AF from Arvin Edison Water Storage in Kern County (if water was available during the drought). (Source: pers. comm. with Bob Muir, MWD, 1991.)

Noninterruptible untreated water for domestic uses in San Diego is purchased from the MWD for $269/AF; treated water costs an additional $53/AF. The least expensive new supplies, other than desalination, would cost $600-$700/AF. (Sources: pers. comm. with Gordon Hess, SDCWA, 1991 and Robert Yamada, SDCWA, 1992.)

In Santa Barbara, untreated water from the Cachuma reservoir costs $35/AF. Development of new wells to use further the City's groundwater basins would cost $200/AF, while new groundwater wells in the mountains would cost approximately $600-700/AF. Enlarging Cachuma Reservoir, if feasible, is estimated to cost $950/AF. During the recent drought, the City purchased water from the State Water Project on a temporary, emergency basis at a cost of $2,300/AF. This water was made available through a series of exchange arrangements with water agencies between Santa Barbara and the MWD. Permanently tying into the State Water Project is estimated to cost $1,300/AF. (Source: pers. comm. with Dale Brown, City of Santa Barbara, 1992.)

ENDNOTES

1. British thermal unit (Btu) values are multiplied by 0.33 to compute a kWh-equivalent because the efficiency of conversion from thermal energy to electricity is about 33%.