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Biotechnology

Slaking the world's thirst

TOKYO -- More countries are turning to desalination to deal with fresh water scarcity.

     Creating enough fresh water for thirsty earthlings and their farms is not only critical to humanity's future, it will also be profitable. But first industry has to come up with cheap and efficient technologies.

     Japan Inc. is on the case. The Ministry of Economy, Trade and Industry estimates the global market for desalination will reach 4.4 trillion yen ($42.1 billion) by 2025, nearly four times the 1.2 trillion yen it was worth in 2007.

Better membranes

Many parts of the world are suffering serious shortages of potable water. One solution is to desalinate seawater, which makes up 97% of the earth's total water supply. But to be effective, the alchemy of turning saltwater fresh must be efficient and economical.

     Reverse osmosis uses a semipermeable membrane to trap the salt on one side and allow the water to pass through to the other. The process begins by pumping seawater through a filter to remove inorganic and organic foreign matter. The water is then forced through a membrane with countless tiny holes measuring less than 1 nanometer (one-billionth of a meter) in diameter. This effectively traps the salt.

     One of the largest desalination plants in the Middle East began operating in 2013 in Jiddah, Saudi Arabia, among the largest cities on the country's Red Sea coast. The plant can provide 260,000 cu. meters of fresh water a day, enough to provide drinking water for about 1 million people.

     The Jiddah desalination plant employs reverse osmosis and uses membranes made by Toyobo, a Japanese textile maker. The membranes are made by wrapping hollow fibers around a tube. Each of these fibers is 5 microns (millionths of a meter) thick. Water that has passed through the skin is then collected for use.

Freshen up

Reverse osmosis has been around for decades, but recently improved membranes and technology have renewed interest in the method. Today's membranes remove more salt and allow more fresh water to pass through, reducing the energy needed for desalination. The cost of the membranes themselves has also been declining.

     One drawback is that these membranes can become clogged with dirt and microorganisms. Toyobo's solution is a membrane made of cellulose triacetate, which makes it possible to use chlorine to purify the water.

     Nitto Denko commands a large share of the global market for reverse-osmosis membranes. The Japanese company has succeeded in expanding the surface area of its membranes with folds. The membranes can filter out about 99.8% of the salt from seawater, a company representative said.

     Toray Industries, another textile maker, offers a range of reverse-osmosis and other types of membranes. This allows the Japanese company to offer products suited to different localities' conditions, such as membranes that filter out coarse foreign objects before reverse osmosis.

New boiling technique

Another way of taking the salt out of seawater is evaporation. In essence, the water is boiled, leaving the salt behind when the water vapor cools. This method is popular in the Middle East, where energy is plentiful.

     Only a handful of companies around the world have the skills to operate facilities large enough to make evaporation economically feasible. One is Japan's Hitachi Zosen, which uses the multistage flash method of desalination.

    This involves passing seawater through multiple chambers to collect the vapor through tubes placed at the top. The water is heated to 110 C before it is fed into the first chamber. As the water passes through each chamber, the temperature drops.

     By lowering the air pressure from one chamber to the next, the water's boiling point gradually falls, keeping the water suspended as desalinated vapor. By the time it reaches the last chamber, the water temperature has fallen to around 40 C. This water is recycled to make the most of the remaining heat and boost the thermal efficiency of the system. This is key to lowering the cost of the evaporation method, which is more expensive than reverse osmosis.

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