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Technology Description

This technology is based on filtering separate ions, with the use of some membranes that are permeable to positive ions and others that are permeable to negative ions. An electric current is introduced to the water to be desalinated, ionizing the water and allowing the migration of the ions through the membranes. The positive ions (cations) pass through the membrane from one side and the negative ions (anions) pass through it from the other side. However, the membrane does not allow both types of ions to pass through the membrane together, resulting in desalination of the water. The membranes are alternatively positioned, with one membrane attracting the negative ions followed by a membrane that attracts the positive ions. The electrodes are charged and, therefore, negative ions pass through the membrane to the high-concentration solution cell (the saline solution). This ionic movement allows the formation of two solutions, one concentrated and one diluted, in the spaces between the alternate membranes (called cells), resulting in a continuous flow of produced freshwater and saline water. The two types of continuous flowing water reach tanks in separate pipelines.

This method has been further developed by reversing the charge of the electrodes during 15-20 minute intervals, thereby allowing freshwater cells to become saline cells and vice versa. This process of reversing the electrode charges reduces the associated problem of corrosion flakes, increasing the desalination efficiency.

This technology is used to remove salt from saline well water for small communities to the extent that it can be used for human purposes. It is typically used to treat water with salinities ranging between 1,500-4,000 ppm.

Extent of Use

The use of this technology is limited primarily to treat saline water in relatively small-scale desalination plants (e.g., hotels). This technology is suited primarily for low-salinity brackish water, although it also can treat medium-salinity water if it is pre-treated. It is estimated that about 120,000 m³/day of desalinated water are produced with this method, which represents about 16.6% of the total water produced in the region by desalination. The greatest level of use is in Saudi Arabia, with an estimated production of 94,638 m³/day, followed by Bahrain (14,000 m³/day), Kuwait (5,000 m³/day) and United Arab Emirates (5,000 m³/day). The other countries in the region produce negligible quantities of water with this method (ESCWA, 1996). This technology is generally used in local units, either in industry or small communities.

Operation and Maintenance

Operation of the electrodialysis units requires qualified staff to run and maintain the pumps, engines, re-distillation equipment, valves, automatic activation instruments, and the assembling and disassembling of the tank membranes.

Level of Involvement

Interest in this technology is limited primarily to desalination of medium-salinity water, for the purpose of providing water to rural communities and tourist resorts. It is considered one of the most economic of the other desalination technologies. It also is easy to operate (can operate for several hours without monitoring), can withstand power losses, and does not need skilled supervision.

Costs

This technology is considered one of the most economic desalination technologies. Past experience with its use has helped reduce the use of chemicals during operation, thereby also reducing chemical transportation costs and the costs of electricity and filtering equipment.

Effectiveness of the Technology

It is not feasible to produce water with salinity less than 250 ppm with this technology. The quality of the desalinated water is more sensitive to the quality of the feed water with this method than in the case of the reverse osmosis (RO) technology. In general, it is considered an effective technology for water with a salinity less than 3,000 ppm.

Suitability

This technology is suitable for regions with limited water resources, but which does have available groundwater with salinities ranging between 2-3 gm/L.

Advantages

The advantages of this technology are as follows:

• It does not have high operational or investment costs;
• It exhibits flexibility in regard to the type of energy requirements;
• It has a high conversion ratio of about 80%;
• It exhibits low energy consumption;
• It does not require a large space or large-size equipment.

Disadvantages

The disadvantages of this technology are as follows:

• It is only efficient for desalination of low-to–medium-salinity water (less than 3,000 ppm);
• It requires good initial treatment of the feedwater;
• It has low productivity;
• The quality of the desaliniated wateis dependent on the quality of the feedwater.

Cultural Acceptance

The disadvantages of this technology are sufficient to discourage its acceptance and wide use in the West Asia region at the present time.

Further Development of the Technology

This technology needs further research and development efforts to overcome its significant disadvantages.

The Future of Desalination Technology

Although its development has progressed over the past 30 years (Figure 47), it has not yet become a main method of providing potable water. This is attributed to the high cost of producing desalinated water, compared to water produced from more conventions sources. As a result, desalination technology is limited mainly to the Gulf countries (where no other alternatives exist, and where the energy and financial requirements for the high investment costs exist), rather than the other countries in the West Asia region. The costs to invest in conventional water resource has increased significantly, especially for groundwater, due to falling water levels in most of the countries. In such cases, the production of water via desalination has become economically equal, or even more economical in some cases, than building a dam or extending pipelines to provide water from conventional sources to distant regions. This is especially the case in the Gulf countries. It is noted that the groundwater in the Gulf countries has an average salinity of about 1,000 ppm, and consists primarily of fossil water (and therefore not readily renewable), facilitating the development and use of desalination technology.

The investment cost for groundwater in the shallow aquifers in Saudi Arabia is about US$ 0.2-0.6/m³ (with subsidized energy costs), whereas the investment cost in the deep aquifers is about US$ 0.4-1.1/m³ (including treatment costs). The investment cost in Jordan is about US$ 0.4/m³ for shallow aquifer groundwater, whereas for deep aquifers it approximately equal to the cost of desalinated water in the Gulf countries.

Figure 47. Distillation and reverse osmosis systems in the Arab world,
compared to global desalination production capacity in 1995

It it noted that most desalination plants also contribute at the same time to the production of large quantities of electrical energy. Under such conditions, the cost of producing desalinated water is about US$ 0.5-2.02m³. It is likely that this technology will continue to develop to the degree that it will become an economic competitor to other water supply techologies. The cost may eventually reach US$ 2.5-3.0/m³ that reaches consumers (ESCWA, 1996).

In addition to energy costs, the main limitation in several countries in the region to expanding the use of this technology is the high cost of investment. The current direction of dual production of water and electricity from desalination plants has reduced the production costs for desalinated water, as well as the possibilities for expanding the use of this technology, even outside the oil-producing Gulf countries. This is particularly the case in view of the observation that the investment cost of conventional water sources has become high, since most of the low-cost conventional water sources has already been exploited. The cost of producing desalinated water also has become competitive with the cost of providing water from conventional sources.

It is anticipated that the use of desalination technology will increase in the West Asia region, as a means of attempting to satisfy the region’s increasing water demands. To this end, Table 12 illustrated the anticipated expansion in freshwater production for the countries of the Arabian peninsula, where the desalination technology will likely show significant increases in supplying water resources. In examining Table 12, it is pointed out that most of the currently operating plants in the Gulf countries have exceeded their operational lifetime. Thus, the cost of renovating these plants (estimated to be about US$ 20 billion) must be added to the cost of constructing a new plant (estimated to be US$ 30 billion). Recent studies show that the capacity of desalination plants constructed in 1995 in the United Arab Emirates is 693.5 million m³/year, whereas the actual production only reached 445.5 million m³/year in 1997. The water production in Qatar reached 156 million m³/year in 1997 (Proceedings of 4^th Gulf Water Conference, 1999)

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