Desalination 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
|
Country |
Capital 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.
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