space
About UNEP
space
space
United Nations Environment Programme
Division of Technology, Industry and Economics
top image
space
space space space
space
space

Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augmentation
in West Asia>


Vapor Pressure Desalination (VC)

This technology relies on mechanical energy, rather than direct heating, as the thermal energy source. The water vapor is withdrawn from the evaporation chamber with a compressor. Except for the first stage, condensation takes place outside the tubes in the same chambers. The heat of condensation is used to evaporate a saline water film that is passed through the tubes inside the evaporation chamber.

This type of desalination station has a capacity of less than 100 m3/day, and is usually used on the level of industrial facilities.

Extent of Use

The MSF technology is the major technology of this type used in this region for desalinating seawater. The largest seawater desalination center in the world using this technology is at Al-Jobil in Saudi Arabia, which has a capacity of about of 7.5% of the freshwater produced in the world in this manner. The site consists of 40 MSF units, producing about 2 million m3 of desalinated water per day. The produced water is used to provide potable water, and is usually mixed with the brackish water before being distributed into the potable water network. The water from the Al-Jobil plant, for example, is conveyed through pipes for distances up to 3,000 km, carrying a capacity of about 1.8x108 m3/day. One of these pipelines, with a length of 465 km, supplies Riyadh city with potable water, and the other supplies Al-Qosim city. Thus, the main use of this technology is for desalination of seawater to provide potable water. In contrast, no freshwater obtained with this technology is used to irrigate agriculture in this reigon. Some plants, however, have been constructed for some industrial zones (e.g., Gabal Aly in the United Arab Emirates). In general, more than 80% of the countries in the region use this technology to desalinate seawater.

The MED technology represent about 16.4% of the desalination plants in the Arabian Gulf region, with about 38,643 million m3 of water being produced with this method. For the vapor compression method (VC), about 97,367 million m3 of water is produced, with its use concentrated in the United Arab Emirates and Saudi Arabia.

Operation and Maintenance

Experience gained in the operation of these plants has demonstrated that the use of low-quality materials in their construction, or their improper operation (especially for units with acidic feeding) will result in problemsThese units also require effort to achieve an effective method that controls deposition and reduces corrision. The main problem in these units is that all the equipment (pumps, air vacuum, chemical feeding systems, saline water heaters) operate at high temperatures and in a high salinity environment, thereby increasing the complexity of the necessary maintenance. A high degree of skill, therefore, is required to operate the units, to carry out necessary and routine maintenance and to provide trained labor, spare parts and the necessary finances. Operational problems can be reduced by replacing the acids with other materials (e.g., polymers). The biggest problems the units face are the corrosion in the flash distillation chambers (which are made of carbon steel), and flakes that cause water pollution. Nevertheless, experiments have demonstrated that, if properly designed and constructed using corrosion resistant materials, this technology can achieve a productivity of about 95% of the plantís capacity.

Level of Involvement

This technology is one of the most popular in the region, especially in the Gulf countries. This is evident by the large expansion in its use, in spite of competition with reverse osmosis technology. The proponents of this technology note that it can unaffected by large changes in the quality of the raw seawater and does not need advanced technological components. This is in contrast to the reverse osmosis method, which faces the problem of sometimes damaging the mambrances after only a snall duration of operation. Because of the high cost of investment, the governments in the region are still responsible for constructing the plants, and there are plans to expand to construct more of these plants in virtually all the countries.

Costs

The cost of constructing a desalination plant depends mainly on the production capacity of this plant and the type of technology used, in addition to such factors such as the lifetime of the plant, its rate of usage and its energy costs. Overall, the cost of water desalination technology is still considered expensive. Nevertheless, provision of water from conventional sources (e.g., groundwater) has become so costly for some regions, especially if the water sources are located far from the areas where the water is consumed, that the cost of producing desalination water has become approximately equal to providing water from conventional sources.

The construction and operational costs of a desalination plant decrease considerably with increasing production capacity. After production of up to 5 million m3/day for seawater or 3 million m3/day for saline water (ESCWA, 1996), the desalination costs gradually decrease with increases in water production capacity. The operational and maintenance costs include energy, labor, chemicals and spare parts. The overall costs obviously depend on the salinity of the feed water and the desalination costs. The cost of desalination of seawater, for example, can be 7-8 times higher than for saline water. In regard to plant lifetime, it is noted that the average lifetime of plants using this technology is approximately 20 years, although some plants in Kuwait have been in operation for more than 26 years. Some reports have suggested that, with an increase in investment of about 20-30% over the initial investment, for the purpose of providing better equipment, can double the plant lifetime.

In general, the distribution of the costs for providing desalination water in the region is approximately 38% for capital, 20.5% for energy, 21.3% for labor, 16.2% for maintenance and 4% for chemicals.

Different technical sources indicate the cost of seawater desalination using the above-noted methods is estimated to vary between US$ 1-3.5/m3. For medium-salinity water, it is between US$ 0.4-1.5/m3. Based on data presented at the 4th Gulf Conference, held in Bahrain in 1999, the cost of desalination for a cubic meter of water using the different methods is between US$ 0.6-2.5. This high cost is attributed to high energy costs. According to ESCWA (1996), the cost of producing a cubic meter of water using this technology for some of the Gulf countries is as follows:

Bahrain US$ 0.56-2.2/m3
Saudi Arabia US$ 0.48-2.2/m3

Qatar

US$ 1.14-1.645/m3
United Arab Emirates US$ 1-1.45/m3

Effectiveness of the Technology

This technology is considered very effective, as it can produce desalinated water with less than 30 ppm.

Suitability

This technology is considered very suitable for seawater desalination, especially when the desalination plants have a large capacity, which reduces costs. Thus, these plants can be constructed on the coastline. Further, the water used as feedwater for the plants can be easily treated, compared to other technologies.

Advantages

The advantages of the MSF technology are:

  • It has relatively high flexibility in regard to the salinity of the feedwater;
  • It can produce very clean water;
  • It has a large production capacity of between 34,000-45,000 m3/day;
  • It does not require high operational skills;
  • It produces water and electricity;
  • It has relatively low energy requirements.

The advantages of the MED technology are:

  • It has high productivity;
  • It has low investment costs;
  • It produces excellent quality water (less than 30 ppm);
  • The required energy is independent of the feed water salinity;
  • It does not require high technical skills to operate.

The advantages of the VC technology are:

  • It produces excellent quality water (about 20 ppm)
  • It has a large operational load;
  • It requires a short construction period;
  • It does not require large space;
  • It has flexibility in operation and production.

Disadvantages

The MSF technology has the following disadvantages:

  • The feed water requires initial treatment;
  • Labor is needed to continuously clean all the chambers;
  • It has a low conversion ratio;
  • It has high operational costs (particularly chemicals);
  • It has large construction requirements (buildings + high quality equipment to reduce maintence costs from rapid corrosion);
  • There are limited opportunities for improvement.

The MED technology has the following disadvantages;

  • It has a long construction period;
  • It is difficult to monitor the water quality;
  • It has a low conversion of feed water (about 30-40%);
  • It is labor intensive;
  • It has large space requirements and manu on-site preparation needs.

The VC technology has the following disadvantages:

  • It has high operational costs;
  • It has high energy requirements;
  • It is impossible to monitor the water quality.

Cultural Acceptance

Despite competition between MSF and RO technology, the MSF technology still predominates in the countries of the region, especially for seawater desalination.

Further Development of the Technology

Technical and economic studies are still required to allow accurate comparisons between the MSF and RO technology systems for seawater desalination. Research into ways to reduce the desalination costs also is required. In addition, there is a need for further training and education of staff members in regard to operation, maintenance and development of the technology.

 

Back

Table of ContentsTable of Contents Next
  • Brochure
  • IETC Brochure


  • International Year of Forests
  • International Year of Forests


  • World Environment Day
  • ??????


  • UNEP Campaign
  • UNite to Combat Climate Change