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Case study Producing fresh water through desalination

Only 6% of the water on earth is fresh water and a large proportion of this fresh water is frozen in polar ice caps, mountain snow and glaciers or remains underground. Any water with dissolved solid content of up to 600mg/litre is considered to be drinkable. Water becomes significantly less palatable when total dissolved solids exceed 1,000mg/litre, so historically, this has led to the development of population centres around sources of fresh water such as rivers and springs.

Over the last 50 years, advances in desalination technology have made it possible to convert large quantities of saline water, from seas and oceans, into fresh water suitable for drinking and agriculture. Desalination of seawater has played a significant part in the growth and development of arid regions such as the Middle East and has offered security of water supply to its residents.

Water desalination technologies can be broadly divided into two processes, thermal and membrane. Thermal processes such as multi stage flash distillation (MSF) and multi effect distillation (MED) involve adding heat to seawater until part of it evaporates into water vapour. This, in turn, is condensed into high quality fresh water called distillate. The process is carried out over several stages and, by adjusting the pressure to keep the water boiling and vaporising in each cycle, the performance and efficiency of the process is improved. The distillate produced is so pure that to make it suitable for drinking it has to be re-mineralised by adding certain chemicals such as calcium carbonate.

Power stations usually have waste heat. Hence, combining power plants with desalination plants is a logical and economically attractive solution and this has been successfully applied, especially in the Middle East, thereby maximising the efficient use of fuel. The majority of our desalination plants use a thermal process to produce fresh water.

Reverse osmosis is the most common membrane process used for desalination. The technique involves pumping seawater at pressure through a semi-permeable membrane. The membrane allows the water to pass through whilst restricting any dissolved salts, which get left behind in the remainder of the seawater. Energy is required in the form of electricity for driving the pumps, but no heat is used as there is no evaporation process. The efficiency of the process is improved by employing energy recovery devices to draw energy from the pressurised return water stream.

International Power is actively engaged in seawater desalination across a number of plants in the Middle East such as Shuweihat S1 and Umm Al Nar in Abu Dhabi, Hidd power station in Bahrain and Ras Laffan B in Qatar. Currently International Power has interests in a total capacity to produce over 500 MIGD of fresh water, enough to fill approximately 550 Olympic-sized swimming pools per day.

We operate some of the largest desalination facilities around the world e.g. Umm Al Nar (143 MIGD) and Shuweihat S1 (100 MIGD). At Shuweihat the desalination units each have a capacity of 16.7 MIGD, making them the largest MSF units in commercial operation worldwide.