Newsletter and Technical Publications
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 The water vapor is withdrawn from the evaporation chamber with a compressor.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.
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
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:
|United Arab Emirates
Effectiveness of the Technology
This technology is considered very effective, as it can produce desalinated water with less
than 30 ppm.
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.
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.
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.
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.