The Okinawa Prefecture Seawater Desalination Center (Okinawa Enterprise Bureau)

Exploiting Differences in Ocean Temperatures
According to surveys by international organizations, 97.5% of the Earth's water is seawater, and no more than about 2.5% is freshwater. What is more, with most freshwater held in glaciers, icebergs, and underground reservoirs, water that can be used easily by humans, such as that in rivers and lakes, accounts for just 0.01% of the total. Meanwhile, the global population topped the 6 billion mark in 2000 and is expected to reach 8 billion in 2025, of which 3.5 billion are likely to face water shortages.

Mindful of these problems, Japanese universities and companies are stepping up their efforts to research ways of harnessing water resources. One of the projects attracting attention is a system under development at Saga University in Kyushu for desalinating seawater by taking advantage of differences in ocean temperatures. This method of desalination is called the spray-flash method. In it, a fine spray of warm surface seawater is sucked into the system, which is depressurized to 0.05 atmospheres by a vacuum pump. In such low pressure the boiling point of water drops to about 20 or 30 degrees Celsius, so the seawater mist quickly evaporates into steam with its salt content removed. This steam is cooled by cold deep-sea water and condenses into freshwater.

This method has been taken a step further by combining it with a form of electricity generation that also uses differences in ocean temperature. This system involves changing ammonia, which has a low boiling point, to gas by heating it with warm water near the ocean surface and using this gas to power a turbine. The ammonia is condensed back to its liquid form by cold deep-sea water, and the warm seawater left over from the process is turned into freshwater using the spray-flash method.

There are plans to build a desalination plant using this system in Saudi Arabia. The country already produces drinking water in desalination plants built by Japanese firms that use heat generated by burning oil, and the idea is to use the warm wastewater from these facilities in the system developed by Saga University. Under the plan, spray-flash facilities will be constructed alongside three desalination plants operating in Saudi Arabia.

The Saga University system is both cheap and environmentally friendly, and there has been a flood of inquiries from dozens of countries afflicted by water shortages in the Middle East and the South Pacific.

Companies Developing New Desalination Technology
In order to tackle water shortages, countries in the Middle East, Europe, and elsewhere are stepping up efforts to desalinate seawater by filtering it through special fine membranes. Plants for this purpose are underpinned by technology developed by Japanese companies.

Major textiles maker Toray Industries Inc. has delivered a water-processing membrane to a seawater desalination plant in the Caribbean nation of Trinidad and Tobago, and the plant started operating in March 2002. This reverse osmosis membrane is made of a special polymer called a polyamide, and what makes it special is its spiral configuration. With holes just one two-millionth of a millimeter wide, it even filters out viruses and bacteria. This system has been welcomed for its ability to produce 0.6 liters of freshwater from one liter of seawater. The Trinidad and Tobago plant is the largest facility of its kind in the world and can produce 136,000 tons of water a day - enough to provide drinking water for 400,000 people.

This reverse osmosis membrane is the result of technology cultivated since the days when Japan was known as a major textile producer and has proved its worth domestically, too. The Okinawa Prefecture Seawater Desalination Center was completed in 1997 and produces 40,000 cubic meters of drinking water per day, meeting 10% of water demand on the prefecture's main island of Okinawa. Toray expects demand for reverse osmosis membranes to rise and plans to increase its production capacity by 150% from October 2002 levels by October 2003.

Meanwhile, major general contractor Obayashi Corp. recently started work on constructing Japan's largest seawater desalination plant in Fukuoka Prefecture. The plant employs a system in which the water intake is laid in sand at the bottom of the ocean. The deep-sea sand acts as a filter, so there is no danger of the system sucking in fish eggs or fry. The plant is due to be completed in 2005. Many other universities and companies are striving to produce water resources, and the role of Japanese technology in international water projects is sure to keep on growing.

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Copyright (c) 2004 Web Japan. Edited by Japan Echo Inc. based on domestic Japanese news sources. Articles presented here are offered for reference purposes and do not necessarily represent the policy or views of the Japanese Government.