Wind Energy - Water Desalination
Water desalination: an overview
Of all the water present on Earth, 96.5% consists of seawater. The remaining 3.5% is freshwater, but half of that is tied up in ice and consequently not usable. All in all, significantly less than 1% of the world’s water resources are exploitable as drinking water.
Drinking water is characterised by a high degree of purity, which among other things means a low salt content. There are often directives and regulations governing the permissible salt content in drinking water (such as the Drinking Water Ordinance in Germany). According to the limit defined by the World Health Organisation1, a salt content of up to 0.5 g/l is harmless to human health. Oc-casionally, though, the opinion is also heard that water with a salt content of up to 1 g/l is drinkable. The distinction between freshwater, brackish water and seawater is equally inconsistent. In Ger-man usage, the term brackish water usually relates to water with a salt content of 1 – 10 g/l, while in the Anglo-Saxon world water is still referred to as brackish if the salt content is as much as 18 g/l. Seawater has an average salt content of 35 g/l, although salinity can vary between 2 and 45 g/l depending on the locality. Whatever the case, obtaining drinking water or service water from salty seawater or brackish water calls for removal of salt (and other minerals) from the untreated raw water. This is done by separating the raw water into product water, with a low salt content, and brine, with a high salt content.
There are numerous desalination methods, some of which have been in use in large plants for over 100 years. The common, tried-and-tested desalination technologies can be divided into thermal methods (distillation or vaporisation processes) and membrane methods (see Table 1). In addition there are simple (solar) evaporation systems and complex techniques, not yet used commercially, such as freezing (water separation) and ion exchange (salt separation).
| Thermal methods |
Market share (2005) |
Membrane methods |
Market share (2005) |
|---|---|---|---|
| Multi-stage flash distillation (MSF) |
36% |
Reverse osmosis (RO)Reverse osmosis (RO) |
46% |
| Multi-effect distillation (MED) |
3% |
Electrodialysis (ED) |
5% |
| Vapour compression (VC) |
5% |
|
In early 2005 there were more than 10,000 desalination plants with a daily output of at least 100 m3 in service world-wide. The plants are situated in coastal locations where there is a shortage of drinking water, or in semiarid/arid areas where only brackish water is available. The principal consumers are towns, cities and local communities (63%). Desalination plants are also occasionally used on ships. However, the total installed desalination capacity of about 36 million m3/day met only just under 0.3% of global demand for fresh water. The majority of installed desalination plants (53%) are used for seawater desalination; 24% operate with brackish water, 9% treat surface water, and 6% are designed for treating wastewater (Pacific Institute, 2006). Roughly half the installed desalination capacity is located in the Middle East, the Persian Gulf and North Africa (Figure 1); thermal methods are preferred there, mostly in conjunction with combined heat and power plants (cogeneration stations). In countries such as Saudi Arabia, Kuwait and the United Arab Emirates, the main source of water is seawater. In the USA, which accounts for 17% of global desalination capacity, mainly brackish water and surface water is treated (70% on the basis of reverse osmosis).
