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4. UPGRADING WATER QUALITY

4.1 Desalination

Technical Description

Desalination provides a means of upgrading poor quality, saline waters, and refers to a suite of technologies designed to produce freshwater from saline sources such as seawater or brackish groundwater. There are basically two types of desalination processes: reverse osmosis or RO technologies and electro-dialysis technologies. The techniques are similar in that both make use of membranes that allow passage of water molecules but exclude salts and other contaminants, which are discharged as a brine waste. In electro-dialysis, however, ions are transported through a membrane from one solution to another under the influence of a direct current electrical potential. The total concentration of soluble salts in water is expressed in terms of either salinity (35 parts per thousand or 35 000 mg/l of total salts being standard seawater) or electrical conductivity for diagnostic purposes. For example, saline water with conductivity of less than 2 250 microSiemens/cm are suitable for irrigation purposes. The major factors that determine which type of treatment is best suited to a particular application include the levels of salinity and temporary hardness; the presence of colloidal suspended matter, dissolved metal ions, oxidizing agents, and hydrogen sulphide; and temperature of the feed water, etc. (Crossley, 1983). The treatment of saline water using this technology began in the 1940s and continues with several hundred desalination plants operating throughout the Region. These range in capacity from a few cubic metres to more than 10 000 cubic metres per day (McRae, 1983). The largest plant built is in Baghdad, Iraq, which desalinates 24 000 cubic metres of water per day (Khan, 1986). In some areas of Asia, desalinization is vital to water supply.

The main components of a desalination plant consist of pretreatment and post treatment storage areas, power supply, membrane stacks, membranes, spacers, and pumps. Electro-dialysis plants also include electrodes. The brackish water is passed over a stack of several hundred ion-permeable membranes. In RO plants, the brackish water is passed across these membranes under pressure, while, in electro-dialysis plants, the brackish water is passed across these membranes at relatively low pressure as an electrical current is passed across the membrane stack. During the latter process, half the membranes allow positively charged ions to pass through and half allow negatively charged ions to pass through. Typical recovery rates using the electro-dialysis process range from 80% to 90% of the volume of feed water (Frederick, 1992). This process uses energy at a rate directly proportional to the quantity of salts to be removed and, for this reason, is usually used for the treatment of brackish water. This technology is very cost competitive for salinities up to 10 000 mg/l, but is not well suited for desalinating high salinity seawater. These technologies are relatively easy and simple to operate; are amenable to rapid delivery and installation; have fast start up and shut down capabilities; can be easily expanded to meet additional demand; and, require a relatively small amount of available space for installation. Small plants are often used for emergency water supply purposes.

Extent of Use

Desalination technologies were first conceived some 200 years ago and, by the 1930's, several small desalination systems were constructed in the Middle East. Since 1970, there has been significant commercial development using various desalination technologies, including distillation, reverse osmosis, and electrolysis, in the Middle East, where other options for freshwater augmentation are limited. Desalination technologies have been used to produce product waters with salinities below 500 mg/l (Khan, 1986). In Asia, a 9 072 m2 plant was built in Gwadar, Pakistan, to desalinate sea water and, in Awania, India, a 1 867 m2 plant was built to treat brackish water (Fenton, 1983). A United Nations study on the extent of use of desalination technologies and their potential use in developing countries, conducted in the early 1960s (Popkin, 1968), indicated considerable potential for adoption of these technologies in Asia, in India, Quatar, Saudi Arabia, Israel, and Kuwait.

Operation and Maintenance

The complexity of the operation and maintenance procedures for desalination technologies depends upon the type of technology used. Generally, however, an high level of expertise is required.

Level of Involvement

This technology has been implemented within the private sector at the level of individual industries.

Costs

The costs of reverse osmosis (RO) and multistage flash (MSF) desalination technologies have been tabulated by Tare et al. (1991) and are shown in Table 13. As shown, RO appears to be the least costly option for desalination, but, as their study concludes, the cost effectiveness will largely depend on the local specific conditions. For example, the cost of desalination using either technology will involve the costs of withdrawal of water from the sea and of pumping the seawater the plant. Nevertheless, in most cases, RO is usually the preferred means of desalinating brackish water (rather than sea water):

- RO with multi-stage flash (MSF) is a reliable alternative

- The capital cost of MSF is usually higher than RO due to high cost of importing the equipment

- At high capacities, the seawater intake volume is higher in MSF plants, and efficiency lower

- Power requirements are much higher in the case of RO plants because of their high pressure operation

- Though RO plants have lower initial investment and fixed costs, they have higher maintenance costs

- MSF plants require a greater land area, which can increase the cost due to high land prices.

TABLE 13. Comparison of Desalination Alternatives for Water Supply to Industry in Arid Areas.

TABLE 14. Comparison of Alternative Industrial Water Supply Technologies in Arid Areas.

TABLE 15. Component Costs of Production for MSF Desalination Plants at Various Capacities.

TABLE 16. Component Cost of Production for RO Desalination Plants at Various Capacities.

Effectiveness of the Technology

Using desalination technologies, water supply can be increased incrementally with demand, by installing modular equipment in stepwise fashion, whereas other technologies generally require that the total investment be made at the beginning of the water resources development project. Desalination technologies also can be quickly installed, providing a rapid augmentation of freshwater supply without conflicts over traditional water rights. On this basis, the US Department of the Interior concluded that desalination is an entirely viable means of increasing water supply, at least from the point of view of technological feasibility (Clawson and Landsberg, 1972).

Suitability

This technology is suitable for use in areas where freshwater is scarce, but saline water is available and energy is cheap. Advantages Compared to water recycling technologies, desalination presents fewer health risks. The modular nature of this technology, using "packaged plants", makes desalination useful in emergencies and for small-scale applications.

Disadvantages

Desalination is expensive. It is energy intensive and incurs high energy costs. Its technical complexity may limit the ability for the technology to be introduce without a major realignment of the institutions.

Cultural Acceptability

Desalination is a new technology, and requires further testing before its cultural acceptability can be determined.

Further Development of the Technology

The application of desalination principles in a less expensive and less complex form is being undertaken. For example, the solar still is a low energy technology which has the future prospect of providing inexpensive desalinated water. There is also the prospect of using desalination technologies in combination with other technologies, such as multistage flash evaporation, as a means of reducing the high energy costs associated with the technology. The selection of this technology to augment public water supply purposes depends on many factors, including product water quality and quantity required; source water characteristics, temperature and reliability of supply; energy availability; and, the relative location of the consumers to the source.

Information Sources

Crossely, I.A. 1983. Desalination by Reverse Osmosis, In: A. Porteous (ed.), Desalination Technology: Development and Practice, Applied Science Publishers, London.

Fenton, G.G. 1983. Solar Distillation, In: A. Porteous (ed.), Desalination Technology: Development and Practice, Applied Science Publishers, London.

Khan, A.H. 1986. Desalination Processes and Multistage Flash Distillation Practice. Elsevier, Amsterdam.

McRae, W.A. 1983. Electrodialysis, In: A. Porteous (ed.), Desalination Technology: Development and Practice, Applied Science Publishers, London.

Popkin, R .1968. Desalination: Water for the World's Future, Frederick A. Praeger Publishers, New York.

Tare, M.M., et al. 1991. Economics of Desalination in Water Resource Management - A Comparison of Alternative Water Resources for Arid/Semi-arid Zones in Developing Countries, In: M. Balaban (ed.), Proceedings (Wl.1) of the 12th International Symposium, Institute of Chemical Engineering (UK), London.

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