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Seawater Desalination Plants

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Overview
Water is vital to all life. However, of all the water on the Earth’s surface, only 2.53% is fresh water that can be utilized in daily living. The environmental destruction and population growth that are byproducts of global warming, urbanization, and industrialization have caused the increasing desertification of the Earth’s land surface. Water shortage is now one of the most serious environmental problems that we face.

It is for these reasons that seawater desalination plants have become crucial systems in overcoming chronic water shortages. Going forward, issues such as reducing desalination costs, stabilizing plant performance, and developing more efficient systems of operation, maintenance and management must be tackled, but plants already making use of Japan’s significant technological achievements in this field are showing some promising results.

Of all the seawater desalination systems commercially available today, the following two are most important.

1) Multi-Stage Flash Method
In this system, seawater is heated, flashed to steam, then cooled to produce fresh water. In other words, desalinated water is produced by distilling seawater. In order to improve heat efficiency, vacuum distillation is used. At commercial plants, there are multiple decompression chambers, accounting for the name ‘Multi Stage Flash Distillation’. The salt content of the water generated is low, at less than 5 ppm. This method allows for water distillation at volume, and has no prerequisites as to the quality of the seawater used. However, the thermal efficiency of this system is extremely low, and requires large amounts of energy.

Ocean Thermal Energy Conversion
Thermal energy conversion is an innovative energy generation system that uses the relatively small temperature difference between heat and cold (above 15°C) to generate electricity. It is possible to use surface seawater (approx. 25–30°C), hot spring water, waste heat from factories, and cooling water from ship engines as the heat source. When seawater is used as the heat source, the system is referred to as Ocean Thermal Energy Conversion, or OTEC. This system generates zero carbon dioxide emissions, and is an extremely environmentally friendly energy generation system. It is currently attracting significant global attention because of its two notable merits: it can help to prevent further global warming and can generate electricity.

Xenesys Inc. is currently carrying out OTEC based on the research findings of Professor Haruo Uehara, formally a Professor at Saga University, who is the pioneer of OTEC here in Japan.

2) Reverse Osmosis Method
In this method, seawater is pressurized then forced through a Reverse Osmosis (RO) Membrane, a type of filtration membrane. The salt content of the seawater becomes concentrated and can be removed, produced desalinated water. Although it consumes less energy than the multi-stage flash method, it does have drawbacks, such as the need for preparation to ensure that microorganisms and deposits contained within the seawater do not clog the membrane, and considerable maintenance costs. The desalinated water generated has a slightly higher salt content than flash method-generated water, at 100 ppm. Until the 1990s, this method was only applied on a relatively small scale. Recently, however, most of the large-scale desalination plants, producing more than 10,000 tons of desalinated water a day, being constructed today are adopting this method of desalination.

Reverse Osmosis Membrane (RO Membrane)
By forcing together a concentrated aqueous solution and a dilute aqueous solution through a semi-permeable membrane, water will move from the dilute solution side to the concentrated solution side as a result of the osmotic pressure brought about by the difference in concentration of the two solutions. Apply a stronger pressure than the osmotic pressure to the concentrated solution side, and the water will pass to the dilute solution side through the semi-permeable membrane. The desalination method that makes use of a membrane isolation process based on this phenomenon is known as the reverse osmosis (RO) method, and the membrane used in process is known as a RO membrane. The RO membrane removes metal ions including sodium and calcium, anions including chlorine ions and sulfate ions, and low-molecular-weight organic compounds including agrichemicals. In 2006, Toray Industries, was the first company in the world to succeed in quantifying the sub-nanometer pore-size distribution of the reverse osmosis membrane, and in verifying the correlation between the removal ratio of boron, said to be harmful to humans, and pore-size distribution.

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http://www.hitachizosen.co.jp/english/index.html
(Hitachi Zosen Corporation)
http://www.toray.com
(Toray Industries, Inc.)
http://www.mhi.co.jp/en/index.html
(Mitsubishi Heavy Industries, LTD.)