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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Small Island Developing States>


PART B - ALTERNATIVE TECHNOLOGIES

4. TECHNOLOGIES APPLICABLE TO SMALL ISLANDS WITH SPECIFIC PROBLEMS OR CIRCUMSTANCES

4.2 Water Quality Improvement Technologies

4.2.1 Desalination

Technical Description

Desalination is a water treatment process that separates salts from saline water to produce a water that is low in total dissolved solids (TDS). The use of this process on small islands has obvious benefits as it opens up a wide range of raw water sources, ranging from the ocean itself to brackish waters located on-island or underground. The various processes considered in this chapter are distillation; electrodialysis; reverse osmosis; and, solar desalination.

Distillation

The distillation process mimics the natural water cycle in that saline water is heated, producing water vapour that, in turn, is condensed to form fresh water. In a laboratory or industrial plant, water is heated under ambient conditions to the boiling point to produce water vapour. However, for this to be done economically in a desalination plant, the boiling point is altered by adjusting the atmo-spheric pressure on the water being boiled to produce the maximum amount of water vapour under controlled conditions. The tempera-ture required to boil water decreases as the pressure above the water decreases. The reduction of the boiling point is important in the desalination process for two major reasons: multiple boiling and scale control. These two concepts, boiling temperature reduction and multiple boiling, have made various forms of distilla-tion successful in locations around the world. Three types of thermal distillation units are used commercially; namely, Multistage Flash (MSF, Figure 32); Multiple Effect Distillation (MED); and, Vapor Compression (VC).

Figure 32

Figure 32. Diagram of a multi-stage flash distillation plant (Buros, 1990).

Electrodialysis

The basic electrodialysis unit consists of several hundred cell pairs bound together with electrodes on the outside and referred to as a membrane stack. Feedwater passes simultaneously through the cells to provide a continuous, parallel flow of desalted product water and brine that emerge from the stack (Figure 33). Depending on the design of the system, chemicals may be added to the streams in the stack to reduce the potential for scaling. Electrodialysis (ED) is only an economical process when used on brackish water, and tends to be most economical at TDS levels of up to 4 000 to 5 000 mg/l. Most of the electrodialysis units sold are built by one company, Ionics, and use a variation of the basic process known as electrodialysis-reversal (EDR). Electrodialysis units have a waste discharge of brackish water ranging in volume from 10% to 50% of its output of freshwater. The feedwater must be pretreated to prevent mate-rials from entering the membrane stack that could harm the membranes or clog the narrow channels in the cells. Post-treatment consists of stabilizing the water and preparing it for distribution by removing gases such as hydrogen sulfide and adjusting the pH.

Figure 33

Figure 33. Movement of ions in the electrodialysis process (Buros, 1990)

Reverse Osmosis

Reverse osmosis (RO) is a membrane separation process in which water in a pressurized saline solution is separated from the solutes (the dissolved material) by a membrane. No heating or phase change is necessary for this separation, and the major energy requirement is for pressurizing the feedwater. In practice, the saline feedwater is pumped into a closed vessel where it is pressurized against the membrane (Figure 34). As a portion of the water passes through the membrane, the salt content of the remaining feedwater increases since there is less water containing the same total amount of dissolved salts. At the same time, a portion of this saltier feedwater is discharged without passing through the membrane. Reverse osmosis units have a waste discharge of brackish water or brine which could range from 35% to 100% of its output of fresh water, depending on the feedwater being treated. Two improvements have helped reduce the operating costs of RO plants during the past decade are the develop-ment of membranes that can operate efficiently at lower pressures, and the use of energy recovery devices. Low-pressure membranes are being widely used to desalinate brackish water as they save on the energy costs associated with pumping.

Figure 34
(larger image)

Figure 34. Schematic diagram of a reverse osmosis plant for sea water (Smith and Shaw, 1994).

Solar Desalination

There are three basic ways in which solar energy is used to desalinate saltwater. These are humidification, distillation, and photovoltaic separation.

Figure 35

Figure 35. Section of a typical solar still (Smith and Shaw, 1994)

Solar Humidification: This technology imitates a part of the natural hydrologic cycle by using the Sun=s rays to heat a saline water source to produce water vapour. This vapour, or humidity, is then condensed on a cooler surface and the condensate collected as product water. An example of this type of process is the greenhouse solar still, in which the saline water is heated in a basin on the floor of the greenhouse, and the water vapor condensed on the sloping glass roof that covers the basin (Figure 35). Application of this type of solar humidification unit (if affordable at all) is best suited for small scale use by a single family or small village where labour can be organized to maintain the units.

Solar Distillation: In this process, a solar collector is used to concentrate solar energy to heat the feedwater so that it can be used in the high temperature end of a standard thermal desalination process. This is usually a multiple effect or multistage flash process (see Figure 32). These units tend to be very capital intensive and require specialized staff to operate them over a long period of time. In addition, they require additional energy inputs to pump the water through the process.

Desalination with Photovoltaics: Desalting units that use photovoltaics to provide electrical energy have also been built to operate standard desalting processes like reverse osmosis (Figure 34) or electrodialysis (Figure 33). Batteries are used to store energy and inverters are needed to supply alternating current when necessary. Since solar energy is usually not available throughout the 24 hours of a day, without extensive battery systems, the desalting units themselves need to be sized to produce water for daily consumption during only a portion of the day. Therefore, a commercial unit must be oversized to produce the quantity of water needed for most applications. The use of photovoltaics adds a further degree of complexity to the desalination process, and it has generally been found that a gas-driven generator can supply the power for far less capital cost.

Extent of Use

Distillation accounts for about 65% of the world's installed desalination capacity, with the MSF process making up the highest proportion of distillation units. The MSF and MED processes are often used as part of a dual purpose facility where the steam to run the desalination unit is taken from the low pressure end of a steam turbine that is used to generate electricity. The remaining steam and condensate is then returned to the boiler to be reheated and reused. Individual MSF or MED units generally have a capacity of 1 000 to 20 000 m3/d. Several of these units can be grouped around an electrical generating plant to utilize low pressure steam. Facilities with a total water output of 200 000 m3/d or more are not uncommon in the Middle East, while smaller facilities, consisting of several 5 000 m3/d units, are used in island locations like St. Thomas and St. Croix in the Caribbean. VC units are also widely used but, individually, have much smaller capacities, and, hence, a lower overall total capacity than that of the MSF and MED plants. The VC units are usually built in the 20 to 2 000 m3/d range, and are often used for supplying water to island re-sorts, industries, and off-shore drilling sites. Distillation plants installed on small islands (UNESCO, 1991) include the first land-based, multi-effect distillation process desalination plant, installed on Curacao, Netherlands Antilles, in 1928, with a capacity of 60 m3/d (0phir and Manor, 1987), subsequently increased by other MED and multi-stage flash (MSF) plants to a total capacity of 36 000 m3/d; one of the first MSF plants, built on Guernsey in the English Channel, in 1960, with a capacity of 2 300 m3/d (Silver, 1987); a number of MSF plants, installed in the Canary Islands, beginning in the late 1960s, and ranging in size from 2 000 to 20 000 m3/d, and a number of vapour compression plants with capacities ranging from 500 to 2 000 m3/d (Torrest et al., 1985); a 19 000 m3/d MSF plant, installed on Aruba, Netherland Antilles (Smith, G., 1986); a low temperature MED (LT-MED) plant, with a capacity of 17 500 m3/d, one of a number of MED plants, in the U.S. Virgin Islands (Matz and Zimerman, 1985); a 6 000 m3/d MED plant in Singapore (Hori, 1984); two 4 500 m3/d multi-effect VC plants in Antigua (Lucas, 1987); and, VC plants with a combined capacity of 2 600 m3/d in the Cayman Islands (Beswick, 1987).

Electrodialysis makes up about 5% of the world's installed desalination capacity. Electrodialysis units are used in applications requiring smaller volumes of water and can be purchased in units with individual capacities ranging from 10 to 4 000 m3/d. They are used by individual homes, resorts, manufacturers, and small municipalities. A large ED facility would have a capacity of about 40 000 m3/d made up of many 2 000 to 4 000 m3/d units. Electrodialysis and electrodialysis reversal plants on small islands include a 21 000 m3/d ED plant on Gran Canaria used for brackish water desalination; a 15 000 m3/d ED plant on Corfu, Greece, which became fully operational in 1978 to supply additional water to meet peak tourist demands during the summer months; an 1 800 m3/d EDR plant on Bermuda; a number of ED plants for brackish water desalination on Japanese islands with capacities ranging from 10 to 1 000 m3/d; and, a 200 m3/d ED plant for seawater desalination on Sisha Yongxingdao Island, China.

Reverse osmosis makes up about 30% of the world's installed desalination capacity. Reverse osmosis units are also small relative to thermal distillation units, and can be purchased in units with individual capacities from 10 to 4 000 m3/d. They are used by individual homes, resorts, manufacturers, and small municipalities. Because they are modular in nature, many individual units can be used conjunctively. The largest plants are in the range of 40 000 m3/d made up of a number of 2 000 m3/d individual units. Reverse osmosis plants on small islands (UNESCO, 1991) include a 20 000 m3/d plant in Malta for seawater desalination, commissioned in 1983 as the largest seawater RO plant in the world (World Water, 1984), which, in 1986, produced about 30% of the island's total water supply (Spiteri Staines, 1989); a 30 000 m3/d plant on Gran Canaria (World Water, 1986b; see also Part C, Case Studies) which supplements the water supply of the capital, Las Palmas, and a 7 500 m3/d plant (with an ultimate capacity of 30 000 m3/d) which serves a tourist complex on the island; a 1 700 m3/d plant in Bermuda for brackish water desalination; and, many smaller RO plants installed on the Cape Verde Islands, Cayman Islands, and a number of small islands of Japan, with typical plant capacities of 10 to 500 m3/d.

Solar desalination is not used extensively in the world and remains largely experimental. There are no large-scale installations, generally because of the large solar collection area requirements; high capital cost; vulnerability to weather-related damage; and, complexity of operation. An inventory of known wind- and solar-powered desalting plants (Wangnick, 1990) listed about 100 plants scattered over 25 countries. Most of these installations had capacities of less than 20 m3/d. However, this inventory did not account for the many small solar stills used by individual families in many parts of the world.

Operation and Maintenance

MSF and MED units are large and complex, and due to the particular economies of scale for MSF and MED, tend to come in units of 20 000 and 5 000 m3/day, respectively, when combined with a steam-operated electrical generating plants. These industrial sized units require competent operators, mechanics, electricians, etc. The VC units, which are much smaller, can be run by one or two persons. For all three types of distillation plants, it is necessary to have a ready supply of spare parts and chemicals (to prevent scaling), and reliable instrumentation. The larger part of the MSF and MED plants are usually completed at the factory where they are produced due to the need for specialized factory equipment and metallurgical controls in their manufacture. For this reason, and due to their large size and mass, MSF and MED plants are usually installed near the shore so that the units, built in factories in France, Israel, Italy, Japan, Korea, the UK or the USA, can be barged to the site and then moved only a short distance to their foundations within the facility. At that time the remaining pipes, pumps, control wiring, and other parts are installed to make it operational. Likewise, VC units are almost always completed in the factory and shipped to the site. They are smaller, and, with proper handling equipment, can be readily unloaded from a ship or plane and moved to the selected site. These units tend to be unforgiving in their operation, and, if mistakes are made, can be costly to repair. However, one of the important features of thermal plants is that they are not as sensitive to the quality of the feedwater as membrane plants, and can operate with raw water obtained directly from the sea.

Electrodialysis units are relatively easy to operate. However, they do require that the operator has a knowledge of electricity, pumps, and plumbing, and be able to repair the instrumentation. The feedwater for an electrodialysis plant is usually taken from a groundwater source using a well, which limits the amount of particulates and microorganisms in the feedwater and reduces the need for maintenance. Electrodialysis units also require a reliable source of electricity, and this is the most significant component of their operational costs in addition to the chemicals needed for pretreatment of the raw water.

Since Reverse Osmosis (RO) units come in smaller sizes than the other types of desalination equipment, they are easier to install. The membranes can be shipped separately ,making the pieces easier to move to a site. The heaviest portions of the RO units are generally the high pressure pumps. Although operation of the units is relatively easy, the operator must have a knowledge of electricity, pumps, plumbing, and instrumentation, and be able to mix the chemicals needed for pretreatment of the raw water. Pretreatment of the feedwater prior to it reaching the membranes is the critical operational element: the suspended solids content of the raw water (including colloids, potential scaling constituents and microorganisms) must be reduced to avoid fouling the membranes. While membranes can be cleaned by the use of special chemicals, the membrane units cannot be disassembled for cleaning, and, if a membrane is clogged due to misuse, it is usually irreparable and must be replaced. RO units require a reliable source of electricity which is the most significant component of their operational costs. The membranes, which are usually a third of the cost of the units, must be replaced every 3 to 5 years under normal operation (or considerably more frequently if poorly operated).

Simple solar stills which work by humidification can be operated and maintained by people without extensive technical skills. However, such stills require constant maintenance if they are to efficiently produce freshwater. The ponds must be kept filled with feedwater, to prevent scale formation caused by the basins drying out, and the glass and collection troughs kept clean and in good repair, to minimise vapour leaks caused by broken glass panes. In contrast to these simple stills, hybrid units, employing solar collectors to raise the temperature to achieve true boiling, tend to be complex and their operation needs specially-trained technicians. Units using photovoltaics are probably midway between the other two in effort and complexity, and specialized assistance required. All units are likely to require some imported materials. For example, even simple stills require glass and sealer that may have to be imported; the extent to which weather (wind, blowing sand, etc), wandering animals, and birds can affect the quality of the glass needed for the solar stills should not be underestimated. This is especially critical on islands which are subjected to high winds, hurricanes, etc.

Level of Involvement

The use of MSF and MED distillation units needs to have the full commitment of the community as they are large and will require considerable funds to purchase and operate. A VC unit, being smaller, requires less community commitment but will require that the owner be financially solvent to provide the necessary funds to maintain and operate the unit.

The level of involvement required for an electrodialysis unit is dependent on the capacity of the facility installed. A large facility with a capacity of 20 000 to 40 000 m3/d would be considered a major industrial installation requiring community acceptance and financing. Smaller units, in the 10 m3/d capacity range, could be installed and maintained by a large family or small business.

The level of involvement of the community in the operation and maintenance of a reverse osmosis unit is dependent on the capacity of the facility installed. A large facility would have a capacity of about 20 000 m3/d and would be a major industrial installation requiring community acceptance and financing. However units in the 10 m3/d capacity could be installed and maintained by a large family or small business.

The level of involvement of the community in the use of solar distillation units depends on the type and size of the units. They all require effort to maintain and operate, and some organization of the effort and ability to collect moneys for future repairs and maintenance (system administration) is also required. Due to the complexities and responsibilities inherent in the use of solar desalination technologies, there should be a real local understanding and commitment to this type of installation before proceeding.

Costs

The capital cost of the MSF and MED distillation units tends to be in the range of $1 000 to $2 000/m3/d of installed capacity, exclusive of the steam supply and site preparation. If the units are built as part of a dual purpose, electricity and water production facility, then the cost of the electricity and steam plants must be added to that of the distillation plant (however, the potential income from these ancillary operations should also be included in the costing). The capital cost for VC units tends to be around $2 500 to $3 000/m3/d of installed capacity. These units require less site preparation. In general, production costs tend to be in the range of $1 to $4/m3/d of water produced, depending on the size of the unit. Both the capital and operating costs are very site-dependent.

The 1995 capital cost of electrodialysis units tends to range from $250 to $750/m3/d of installed capacity, exclusive of the site preparation, buildings (it usually requires one), and development of the raw water supply. Production costs, including depreciation, tend to be in the range $0.25 to $1/m3/d of water produced depending on the size. Both the capital and operating costs are very site-dependent.

The capital cost of brackish water reverse osmosis units ranges from $250 to $750/m3/d of installed capacity, exclusive of the site preparation, utilities, buildings (it usually requires one), and development of the raw water supply. The capital cost of a seawater RO unit could range from $800 to $1 250/m3/d of installed capacity. Production costs for a brackish water plant, including depreciation, range from $0.25 to $1/m3/d of water produced, depending on the size. Both the capital and operating costs are site dependent. Similarly, for a seawater plant, production costs could range from $1 to $4/m3/d.

Since there is limited commercialization of solar units, the capital cost and operating cost are not as well established as for the other processes. For the hybrid plants (distillation and photovoltaic) it can be assumed that the capital costs of the solar generating system will significantly exceed that of the desalination unit. The economics of operating photovoltaic, solar desalting units tend to be related to the cost of producing energy with these alternative energy devices. At this time, the cost tends to be high, but may be expected to decline as the further development of photovoltaic devices reduces their capital cost. The capital cost of an 80 m3/d solar-assisted MED facility installed at Umm Al Nar in Abu Dhabi has recently been estimated at about $2 million, or about $25 000/m3/d of installed capacity (El-Nashar and Samad, 1995). In partial contrast, solar stills are expensive to construct correctly, and, although the thermal energy used in the distillation process may be free, additional energy is usually needed to pump the water to and from the facility.

Effectiveness of the Technology

The effectiveness and efficiency of distillation units is generally measured by the amount of water produced per unit of steam delivered to the plant. The higher the efficiency, the smaller the plant tends to be. This is related to the top temperature at which the plant is run as well as to the type of heat transfer surfaces used. Higher-efficiency plants tend to have an higher risk of scaling and require more care in operation.

The effectiveness and efficiency of electrodialysis units is generally measured by the amount of water produced per kilowatt hour (kWh) of electricity used. Usually more than one membrane stack in series is required to achieve adequate reduction of the dissolved salts. Each stack reduces the TDS by about half. Thus a properly designed 4-stack array could reduce a feedwater of 4 000 mg/l total dissolved solids content to about 250 mg/l.

The effectiveness and efficiency of reverse osmosis units is generally measured by the amount of water produced per unit of steam delivered to the plant. Typically 9.7 kWh/m3 of electric power is consumed in a 30% recovery. If an energy recovery turbine of 80% efficiency is used, the energy requirement will fall to 6.5 KWH/m3.

Although solar desalination technology has been in existence for a long time, it is difficult to find successful, long-term applications. The technology can work, especially the simple solar stills, but it must be carefully matched to the application. A general rule of thumb for solar stills is that a solar collection area of about one square metre is needed to produce 4 litres of water per day. Thus a facility with a solar collection area of one hectare should produce about 40 m3/d.

Suitability

Distillation or thermal desalination is suited to any climate. MSF and MED plants are cost-effective methods for producing large amounts of water when they are used as part of a dual-purpose (water-electricity) production facility. They are more thermally-efficient with a source of cooler water to provide a wider temperature differential between the hot end (brine heater) of the plant and the heat rejection section (coldest part maintained by circulating cooling water). It is necessary for any thermal installation, especially the MSF and MED units, to have trained staff and access to foreign exchange for parts and chemicals. It is important to remember that thermal plants will have a significant discharge of heated water from the plant that must be disposed of.

The electrodialysis process also can be used in any climate, although it can be damaged if the lines or membrane stacks are frozen. It also works better in warmer weather as the efficiency of electrodialysis process increases as the temperature of the feedwater increases. Electrodialysis is best suited for feedwater with a TDS of 4 000 mg/l or less, and in situations where feedwater are high in silica (which tend to create problems with reverse osmosis plants). It is also suited for very complex brackish feedwater where the TDS exceeds 10 000 mg/l, but is not economically viable for seawater desalination.

Reverse osmosis is suitable for desalting brackish or sea water. It can be used in any climate, although the membranes can be ruined if water is frozen in them. Also, the process works more efficiently with warmer feedwater, but the membrane units begin to have mechanical stability problems if the feedwater temperature exceeds 35°C.

At present, the use of conventional energy sources to operate desalting devices is generally more cost-effective than using solar and wind-driven devices, although appropri-ate applications for solar and wind-driven desalters do exist.

Advantages

Distillation is a proven technology and has been used in many parts of the world to provide a good quality product water. Distillation is generally more tolerant of poor quality feedwater than other desalting processes.

The electrodialysis process has the advantage of being simple to use, and, since the product water does not go through the membrane and the passages through the membrane stack are larger, the process is less apt to scale or be plugged with debris. This is a good technology to use when the feedwater is likely to contain an high concentration of suspended solids. The process requires little chemical pretreatment of the source water, and is quiet compared to thermal and reverse osmosis units. Electrodialysis has the capability of achieving high recovery volumes (more product and less brine), with the amount of energy used being proportional to the mass of salts removed. It is also not affected by non-ionic substances such as silica. Reverse osmosis also has a very simple design: the plants can be broken into small pieces for shipment which make them ideal for use as emergency water supplies. There are also many manufacturers of reverse osmosis equipment components, which tends to keep the prices in balance. Energy usage is not as sensitive to the proportion of the salts removed as ED, and RO units will remove both ionic and non-ionic substances. Energy can be recovered from the pressurized waste stream.

Solar desalination technologies can significantly reduce energy costs.

Disadvantages

The long-term success of distillation units is directly related to the use of appropriate and high-quality construction materials. These, in turn, increase the capital costs of the plants. Balancing the capital cost against the long-term performance of the unit requires care in specification of the type of equipment to be supplied. High capital costs can lead to high production costs. The MSF and MED processes generally are cumbersome to ship and install.

Electrodialysis units are supplied by only one significant manufacturer. Thus, there is a lack of significant competition (unlike the manufacture of reverse osmosis units by numerous companies, which generally keeps the prices of those units in check) to moderate the capital cost of ED units. For optimal operation, the units require a reliable source of electricity, which may not be available on islands. The process will only remove ions or charged particulates from the source water, and problems with clogging have been experienced when the raw water contains high levels of bacteria. Since the removal efficiency of the plant is a design feature determined by the number of stages in the ED plant, the use of ED with a feedwater of variable or increasing TDS may create problems that can be more readily handled with a reverse osmosis unit.

Reverse osmosis membranes are very sensitive to suspended solids, including colloids, in the feedwater, and good pretreatment is very important for maintaining the life of the membranes. Skilled operators and a ready source of chemicals and spare parts are essential.

Solar desalination has high capital costs and the operation of solar systems can be complex. In addition, a major production facility would take up a large land area, which could create problems if the facility was located on an island where land was scarce and/or expensive.

Cultural Acceptability

Desalination plants should only be installed after the capacity of the community to finance, operate and maintain the units is established. In the case of solar desalination, land ownership issues could become a problem.

Further Development of the Technology

There probably will not be major significant technical improvements in distillation processes, but a continual and gradual improvement in the use of more suitable materials for construction, the use of scale control chemicals, and operational experience.

Electrodialysis is a mature technology with efforts being concentrated on producing components which are more effective and last longer.

There is continuing work in developing better reverse osmosis membranes that work at lower pressures or are more selective in their removal characteristics

Solar desalination should benefit from the development of a process called membrane distillation for direct or solar-assisted desalination. This involves using a membrane which allows water vapour to pass through but retains water in its liquid form. Further development of this and other technological advances should help to make solar devices more cost effective.

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