Multi stags flash (MSF). With installed capacity estimated at 295 million gallons a day (g/d), MSF technology accounts for 92 per cent of the UAE’s total desalination capacity and is used at Umm al-Nan, Mirfa, Taweelah, Jebel Ali and Layyah. The principal suppliers have been France’s Sidem, Ansaldo Industria and Italimpianti, both of Italy, and the UK’s Weir Westgarth. The technology operates by passing seawater in stages through a series of metal tubes, where it is heated and on which the steam is condensed. The sea water is further heated to a temperature of between 90- 100 C by taking steam directly from a boiler or turbine. It is then transferred back into the original vessels in a vacuum condition, allowing the seawater to boil or ‘flash’ to form steam. The secondary steam condenses on the metal tubes to form distilled water Multi effect (ME). The ME technology is used in six small units located in more remote areas of Abu Dhabi, such as Jebel Dhanna and Delma island. Total ME capacity stands at 5.5 million g/d.

The main supplier has been Sidem. Like MSF, ME is a thermal process. Steam enters the distillation plant in the first stage. It passes through a bank of metal tubes, onto which seawater is sprayed.

This takes some of the heat out of the steam which causes condensation and some evaporation. The secondary steam is then taken into the next stage, where a proportion condenses on the metal tubes to form distilled water. The number of stages in ME plants ranges between two and eight. To improve efficiency, some multi-effect plants have a vapour compressor added on. There are 12 such units in Abu Dhabi.

Reverse osmosis (RO). The 27 RO units located in the federation have a combined design capacity of 20.6 million g/d. The largest user of the technology is the federal Ministry of Electricity & Water, which operates installations using either seawater or brackish water. The main suppliers are Aqua Engineering, Italimpianti, Hydrotechnik and Metito. The RO principle, as the name suggests, reverses the natural osmosis process. Seawater. is pumped at high pressure into a pressure chamber where a manufactured membrane is located. Molecules of pure water then pass through the membrane to form a dilute solution on the low pressure side.