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A Reverse-osmosis desalination
B Physical geography of Jordan and Israel
C Historical review of the political riparian issues in the development of the Jordan River and basin management
D Recommendations for future joint development and management: the Mediterranean-Dead Sea canal and Al-Wuheda dam projects
Desalting techniques are primarily intended for the removal of dissolved salts that generally cannot be removed by conventional treatment processes. Distillation units have been used on some American ships for more than 100 years. Desalting was used on a limited scale for municipal water treatment in the late 1960s. The past four decades can be divided into three phases of desalting: The 1950s were a time of discovery; the 1960s were concerned with research; and the 1970s and 1980s have been the time of commercialization. Beginning in the 1970s, the industry began to concentrate on commercially viable desalination applications and processes (Burgs 1989).
The first commercial plant for the production of potable water from a saline source using electrodialysis (ED) and ion-exchange membranes was put into operation in 1954 (Powell and Guild 1961). In 1968, use of membranes for brackish-water treatment started with the construction of an ED plant in Florida, in the United States. This process was not favourably received in view of its inability to adequately reduce dissolved solids. The first reverseosmosis (RO) water treatment plant was constructed in 1970 for a condominium project on Longboat Key, Florida (Dykes and Colon 1989). Significant advances in membrane technologies in the last 20 years have improved the cost effectiveness and performance capabilities of the processes. RO membrane processes are increasingly used worldwide to solve a variety of water treatment problems.
A.2 World desalination
The arid region, with its very limited fresh-water potential, has generally used high-salinity waters such as seawater as major water supply sources. More than two-thirds of the world's desalting capacity is located in the arid, oil-rich areas of North Africa and Western Asia, or the Middle East (Burgs 1989):
Fig. A.1 Typical feed-water TDS operating ranges for desalting processes (Source: AWWA 1989)
A.2.1 Desalting technology and processes
The major desalting technologies used today are distillation (several types of evaporative process), reverse-osmosis (RO), electrodialysis (ED), electrodialysis reversal (EDR), and ion-exchange demineralization. The typical concentration ranges of total dissolved solids (TDS) in the feed water for distillation, RO, ED, and EDR demineralization are: between 10,000 and 100,000 mg/l for distillation and other thermal (non-membrane) processes, up to about 35,000-45,000 mg/l (seawater concentrations) for RO membrane, up to approximately 10,000 mg/l for ED and EDR membrane (AWWA 1989) (see fig. A.1). Ion exchange, in which anion and cation resins are used to exchange ions for hydrogen and hydroxide, is primarily used in industrial applications for which very pure water is required and the feed-water TDS is relatively low. Distillation and other thermal processes are used primarily for seawater conversion and special industrial applications, such as brine concentration.
A.2.2 Desalination capacity by process
As shown in table A.1, 70% of the world's desalination capacity is dependent on the distilling process. In the Middle East and North Africa, distillation of seawater is the main process being used, while the processes favoured in the United States and other countries are quite different, reflecting the numerous applications for the desalination of brackish water (Burgs 1989).
Table A.1 Distribution of desalination capacity by process
|Share of capacity (%)|
Source: Buros 1989.
In 1985 the total worldwide installed capacity of land-based desalting plants with a capacity exceeding 100 m³ (25,000 gallons) per day was more than 11.4 million m³ (3 x 109 gallons) per day, which is more than three times the capacity in 1975. Seawater and brackish-water sources with salinity in a range between 1,000 and 40,000 ppm of TDS account for nearly all of this installed capacity, comprising approximately 75% of seawater and 23% of brackishwater sources. Membrane processes represent about 30% of this total capacity and nearly all of the brackish-water treatment capacity (AWWA 1989).
A.2.3 Economics and environment
The cost of desalination has generally decreased from more than US$3/m³ to as low as US$0.50/m³ over time as a result of both technological advances and market processes (Burgs 1989). The historical cost of desalting brackish water and seawater with available technologies is shown in fig. A.2, which is based on a recent cost assessment study by the Office of Technology Assessment for the US Congress.
Fig. A.2 Desalination cost ranges over time. Cost of producing potable water, by distillation or reverse osmosis, including both capital and operating costs (at 1985 prices), for plants producing 3,700-18,000 m³ per day. The increasing cost of distillation from the early 1 1970s reflects international crude-oil prices. (Source: Buros 1989)
Improvements in RO membranes have been the main technological change in desalination in recent years. The United States and Japan are the world's leading countries in innovative research to develop the membrane industry. A high level of competition on both a national and international basis has also played a significant role in containing prices for capital equipment. The following percentages illustrate the distribution of overall costs for the operation of a brackish-water desalting plant (Dykes and Colon 1989):
As can be seen, the main item of cost is capital recovery, which represents almost half of the overall cost. The next most significant cost items are energy, membranes, and labour.
The dominant cost element in seawater RO desalination is the energy, or electricity, which accounts for 50% or more. Substantial cost reduction can be achieved by taking into account the following:
>> technology improvement and research to increase membrane life- from three years to up to now five to seven years;
>> the development of low-pressure-type or low-energy-requirement membranes;
>> the development of a pumped-storage-type RO desalination system using off-peak cheap electricity from nuclear power plants or other thermal power plants; hydro-powered RO desalination is another option (Murakami 1991,1993a).
Generally seen as benign, desalination is not without environmental concerns of its own. From a regulatory standpoint, the big advantage to desalination is that it takes out water that is unusable, leaving fresh water available for other uses. But there is some question about how to dispose of the concentrated brine that is the by-product of desalination. The very highly concentrated brine from distilling plants poses a major problem to be solved, including a sufficient analysis of the potential impact on the marine environment, while less saline brine from RO plants may have a relatively minor adverse impact on the marine ecology.
Table A.2 Installed capacity of desalination plants en Arabian Gulf countries (million m³ per day)
Source: Akkad 1990.
A.3 Seawater desalination in the Arabian Gulf countries
In extremely arid countries, where good-quality water is not available, seawater desalination is commonly used to supply water for municipal and industrial uses. In spite of the high cost of desalinated water, a vast quantity is produced to meet the demand for domestic water in the Gulf countries.
A.3.1 Installed capacity of desalination plants
The installed capacity of desalination plants in Saudi Arabia, Kuwait, the United Arab Emirates, Qatar, Bahrain, and Oman is estimated at 5.08 million m³ per day in total, including 2.4 million m³ per day in Saudi Arabia, which is approximately half of the total for the Gulf countries. Dual-purpose multi-stage flash (MSF) is the most commonly used technique to desalt seawater, representing 97% of the total installed capacity, as shown in table A.2 (Akkad 1990).
A.3.2 World's largest water pipeline and seawater distillation for municipal and industrial water supply
Saudi Arabia has commissioned several desalination plants to meet the rapidly increasing demand for domestic water since 1970. They are located mainly along the Red Sea and Arabian Gulf coasts and produce 2,183,607 m³ of potable water per day and 4,147 MW of electricity (see fig. A.3). The Al-Jubail-Riyadh pipeline, whose water source is wholly dependent on seawater distillation, is one of the world's largest high-pressure pipeline systems. It has a diameter of 1.5 m (60 inches), a length of 465 km, a differential head of 690 m, and a design flow rate of 830,000 m³ day. There are six pumping stations with 430 MW capacity in total and a terminal reservoir with 300,000 m³ storage at Riyad (Abanmy and Al-Rashed 1992).
Fig. A.3 Desalination plants in Saudi Arabia (Source: SWCC 1988)
A.3.3 Remarks on seawater distillation
The problem with seawater distillation is the high cost of producing fresh water by the MSF process, which is the most prevalent type of thermal distillation in the Middle East. The process is largely dependent on the rate of energy consumption, which is high and influenced by the unstable world market price of crude oil (fig. A.2).
A.4 Reverse-osmosis desalination
The membrane processes will probably be the key technological approach to the desalination of brackish water and seawater over the next ten years. Although the RO, ED, and EDR membrane processes are used worldwide to solve a variety of water treatment problems, it is likely that RO will continue to have the greatest market share. Where fresh water supplies are limited or must be imported over long distances, RO desalting of nearby brackish water can be cost-effective. Most of the countries in the Middle East and North Africa have rather long sea coasts, with a total length of about 25,000 km. Seawater desalination will continue to increase in these countries, either by distillation or RO, depending on site-specific conditions and technology development.
A.4.1 Brackish water
Good-quality water is neither abundant nor available to meet the growing demand in most of the coastal areas in arid to semi-arid countries. However, sufficient brackish water is normally available on site to support development. Since the early 1970s, advances in desalination have mostly been directed towards improving the abundant sources of brackish water rather than towards the comparatively expensive conversion of seawater. Significant advances in membrane technologies in the last twenty years have improved the cost effectiveness and performance capabilities of the RO process. Brackish water desalination usually costs only one-fifth to one-third as much as seawater desalting (Burgs 1989).
For environmental reasons, thermal distillation plants, which are largely dependent on high specific energy consumption, are likely to be replaced progressively by low-specific-energy-consumption types of RO desalination plants. At the end of 1989, the world's largest seawater RO desalination plant, with an installed capacity of 56,500 m³ day, was constructed at Jeddah, on the west coast of Saudi Arabia, to replace a 20-year-old MSF distilling plant that was being phased out (fig. A.3). It may be noted that a case study on the Doha desalination plant in Kuwait in 1989 estimated the unit cost of seawater desalination at US$2.70/m³ by MSF and US$1.70/m³ by the RO membrane process (Darwish et al. 1989).
A.4.3 Reclamation of treated waste water
The application of membrane-separation technology to salvage treated waste water includes three alternative processes: (1) micro filtration, (2) ultra filtration, and (3) loose reverse osmosis. Both the micro-filter and ultra-filter are used to control better water quality in the tertiary treatment process and/or the final water purification process. Another application is being tested to be used directly in the primary or secondary waste-water treatment process. The loose RO process is being developed to reclaim treated waste water for strategic reuse in the agricultural and/or industrial sector.
Although it is clear that membrane desalting is a cost-effective alternative to importing fresh water over long distances, the use of the membrane process to remove turbidity, organic matter, and hardness has typically been more expensive than conventional treatment. However, with the continuing improvements in membrane-separation technology, increasing competition among manufacturers of capital equipment, and the ever-escalating cost of meeting increasingly stringent water quality standards by conventional approaches, the cost gap is decreasing (AWWA 1989).
A.4.4 Key applications in the twenty-first century
Membrane-separation technology, using low-energy and/or energy saving types of membrane, are expected to continue to be popular in arid coastal regions and other areas wherever saline waters are available but good-quality fresh water is limited or not available. Application of the membrane-separation technology in the arid countries will be mainly for seawater and brackishwater desalination. Reverse osmosis, ultra-filter, and micro-filter membranes are being used to reclaim either saline water or waste water for reuse, which could indeed be the technology to take the drinking-water and sanitation industries gradually into the twenty-first century.
A.5 Principle, method, and process of reverse osmosis
Osmosis is a natural process whereby a solvent (water) diffuses through a semi-permeable membrane from a solution of lower concentration to one of higher concentration (part A in fig. A.4). The membrane readily passes the solvent but acts as a barrier to the solutes (dissolved solids). At equilibrium conditions, the pressure differential across the membrane is called the osmotic pressure (part B. fig. A.4). For example, the osmotic pressure of brackish water containing about 2,000 ppm TDS at a typical water temperature of 25°C is only about 1.6 kg/cm², whereas it is 27.7 kg/cm² for standard seawater of 35,000 ppm TDS at 25°C. In reverse osmosis, a pressure greater than the osmotic pressure is applied to the concentrated solution (saline water), and a dilute permeate (product water) is produced (part C, fig. A.4).
Fig. A.4 Simplified concept of osmosis, osmotic pressure, and reverse osmosis (Source: AWWA 1989)
Fig. A.5 is a flow schematic of a simplified RO unit. The pressure of the feed water, pre-treated to meet certain established RO membrane feed-water quality guidelines, is boosted before the water enters the RO membranes. Two flows exit the membranes: (1) the combined product (permeate), and (2) the combined concentrate (reject). The fraction of the feed water that results as permeate is called the recovery and is usually expressed as a percentage. The maximum allowable ratio of permeate to reject depends on the water's scaling potential, which is a function of the feed-water quality. This ratio is maintained by the use of a control valve on the reject piping, which controls the flow rate of the reject, thus forcing the permeate flow rate to the desired value.
A.5.2 RO membranes
The first commercially available membranes, developed in the mid 1960s, were made of cellulose acetate (CA) manufactured in flat sheets. Modern CA membranes are modifications of the cellulose acetate structure, including blends and different surface treatments, and are called cellulosic or symmetric membrane. Non-cellulosic membranes, called thin-film composite membranes, have been developed since the 1970s. These include polyamide membranes with relatively thick asymmetric polyamide support structures and composite membranes with thin-film polyamide or other membrane materials on a porous support structure.
Fig. A.5 Flow diagram and schematic of typical reverse-osmosb system (Source: AWWA 1989)
Each membrane material has advantages and disadvantages. The CA-based membranes are now generally the least expensive per gallon of installed capacity (first cost). The price difference between CA and composites is decreasing, however, as the number of manufacturers supplying the composite-type membranes increases and with new developments in the manufacturing process. Use of CA membranes generally requires chlorinated feed water and higher operating pressures than those needed by the composite membranes. Composite membranes generally operate over wider pHand temperature ranges than CA membranes. In some cases these operating characteristics of composite membranes result in savings in electric power and chemical costs. Their greater pH tolerance provides additional advantages in cleaning for some applications.
Sensitivity to chlorine and other strong oxidants in the feed water is a disadvantage of polyamide-based membranes. New developments in membrane research to produce chlorine-tolerant composite membranes are overcoming this limitation.
A.5.3 Membrane elements and RO units
RO membranes are placed inside pressure vessels in several different configurations: (1) hollow-fibre, (2) spiral-wound, (3) tubular, and (4) plateand-frame. In the past twenty years, hollow-fibre and spiral-wound configurations (figs. A.6 and A.7) have become the industry standard for RO water treatment. The predominance of the spiral wound configuration has resulted from recent advances in membrane technology which have been more easily translated into commercial flat-sheet membranes than into the hollow-fibre configuration.
Fig. A.6 Typical hollow-fibre RO membrane element (Source: AWWA 1989)
Fig. A.7 Typical spiral-wound RO membrane element and pressure vessel (Source: AWWA 1989)
Depending on the desired capacity of an RO system, one or more pressure vessels containing RO membranes are used to form a modular block. Pressure vessels within an RO block can be arranged in parallel, in series, or both, depending on the design requirements. Often this membrane-pressure-vessel arrangement is called a membrane array or a pressure-vessel array. For example, a 2:1 pressure-vessel array indicates a two-stage system with two pressure vessels in the first stage and one vessel in the second stage. In a reject-stage arrangement, the membranes in the second-stage vessel would treat the waste concentrate (reject) water from the first stage, thus recovering more product water from the feed-water supply.
A.5.4 Potential water sources
Although some locations have a shortage of water of any quality, the most common situation is a shortage of water of potable quality. Desalting processes can expand the availability of potable water supplies by converting previously unusable supplies to potable water. The potential sources of water for membrane desalting include brackish groundwater, brackish surface water, hard water, municipal waste water, high-nitrate groundwater, irrigation return flows, and seawater.
A.5.5 Feed-water quality
The composition of raw water from the supply source must always be considered in the design of both conventional water treatment and desalting processes. However, the design of desalting systems and their operating economics are much more interrelated with feed-water composition and the required product composition than most conventional treatment processes. The composition of raw water is probably the most important component in desalting-process design The typical water-quality parameters needed for the process design of membrane desalting systems are:
Pre-treatment is usually required to protect the membrane system, to improve performance, or both. The type of pre-treatment required depends on the feed-water characteristics, membrane type, and system design parameters. Pre-treatment requirements can be minimal, such as cartridge filtration of well water, or extensive, such as conventional coagulation, sedimentation, and filtration of surface water supply.
For RO systems, standard pre-treatment usually consists of adding chemicals for scale control, followed by cartridge filtration (usually 1-, 5-, 10-, or 20-microm nominal rating) for membrane protection. The feed water is often acidified to lower its pH; this step is nearly always required for cellulosic membranes. Scale inhibitors such as sodium hexametaphosphate or proprietary chemicals are also added to reduce carbonate and sulphate scale potential.
A.5.7 Pump system
The pump system raises the pressure of the pre-treated feed water to the level required for operation of the desalting system. For RO, the pump system discharge pressure typically is 8.8-28.1 kg/cm² (125-400 psi) for low-TDS and brackish-water systems and 56.2-84.3 kg/cm² (800-1,200 psi) for seawater systems. The pump system for RO might also include energyrecovery devices, particularly for seawater systems.
Post-treatment to supply drinking water commonly includes product-water pH adjustment for corrosion control and chemical addition for disinfection. Typically entrained gases such as carbon dioxide and hydrogen sulphide (if present) are removed before final pH adjustment and disinfection.
Removal of these gases is normally accomplished by stripping in a forceddraft packed column. In the most cases, carbon dioxide must be removed to stabilize the RO product water. If hydrogen sulphide is present, degassing of the product water is usually done to control odour and minimize the amount of disinfectant (e.g., chlorine). The final product-water pH is often adjusted by caustic soda, soda ash, or lime. A non-corrosive water can be produced by using these alkaline chemicals and, in some cases, other chemicals and blending with raw or other water supplies that may also feed the distribution system.
Post-treatment disinfection is normally accomplished with chlorine. However, if the desalting process allows the passage of trihalomethane (THM) precursors, chlorine dioxide, or chloramines, some additional post-treatment may be required to comply with THM drinking-water quality standards.
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