Fresh Water For The World

Press, Volume CVI, Issue 31071, 28 May 1966, Page 13

Fresh Water For The World

QVER 2,000 years ago Aristotle recorded that fresh water could he distilled from the sea, but it is only in modern times that the enormous demands of industry and growing populations have made the desalination of water a matter of major importance.

Nowadays half the desalinated water produced in the world today is processed by British equipment. Dr. Hugh Simpson, a senior desalination research engineer, in a recent 8.8. C. programme attributed this lead by a country until recently considered we'.’ endowed with water to Britain’s interest as a maritime power in desalting water for use on shipboard, and to the fact that she had been associated with many dry and desert areas of the world.

Today, her own water shortage is becoming more serious each year owing chiefly to the demands of industry. To make one ton of steel, for example, requires 65,000 gallons of water, and one ton of synthetic rubber calls for 600,000 gallons.

Plants In Ships The first steamships used the higher-quality distilled water for their boilers, and carried shore water for domestic purposes. But in large modern passenger ships this is uneconomic and nearly all have a desalination plant to provide fresh water for all purposes. The first land plant was built for a mining village on the Red Sea coast in 1910. Based on marine evaporation, it produced about 16,000 gallons a day—insignificant compared with a single modern plant in Kuwait which can produce nearly 1,500,000 gallons a day. A complex of several plants provides the area with over 3,000,000 gallon’- of fresh water each day. Dane Sinclair, of one of the major British desalination companies, described the efficient flash distillation technique used in a large proportion of British-designed plants. Water is heated in the input

section, and then transferred to another chamber at reduced pressure. This causes the process known as “flashing”—a sort of instantaneous boiling producing a quantity of vapour which rises and condenses on cooler tubes. The brine, or heated water, goes into another chamber at a rather lower temperature and a lower pressure again, and the flash process is repeated. This can go on for as many as 30 stages, some water being flashed off as vapour condensed on the tubes at each stage. The cooling water in the tubes is heated up by the condensing steam and returns to form the brine passing through the flashing process, which makes for a very economical use of the heat put into the system.

Cheapest Method Dr. Simpson said the evaporation process was the most common and cheapest method in use today, and was likely to remain the most reliable for the next eight to 10 years. But it had reached a stage of development where improvement was becoming increasingly difficult. The second main group of processes, he explained, involved a change from the liquid phase—seawater—to the solid phase—ice. The third group consisted of membrane processes, “in which either the sodium and chlorine in the water are caused to pass through a membrane preferentially to the fresh water, or the fresh water itself is caused to pass through the membrane, leaving the salt behind. The former case is an example of electrodialysis in which the sodium is caused to pass through one membrane preferentially while chlorine passes through another membrane preferentially under the action of an electric current: in the latter case we have the more recent process of reverse osmosis.”

Condensation Only the evaporation and electrodialysis processes had been of any practical consequence so far, although freezing processes had been studied extensively, and the reverse-osmosis process showed promise. All desalination processes need expensive plant and fuel, which makes the cost of the water produced consider-

ably higher than that of reservoir water, which in Britain costs an average of less than 3s a 1000 gallons. The new method of using the steam which has already passed through the turbines of nuclear-powered generating stations may bring more competitive prices.

Britain’s Reputation Dr. Hans Kronberger, of the United Kingdom Atomic Energy Authority, said that fresh water could be produced at about 4s a 1000 gallons in the case of a fairly large gas-cooled reactor. That cost was still far too high for irrigation purposes, but was approaching a figure that would be of commercial interest for domestic and industrial use.

“Cheaper fuel and larger plants are two of the factors involved,” summed up the narrator, Michael Pickstock, “but a third equally important one is the design and construction of plant which is efficient and reliable to run. British equipment already enjoys a reputation in this field and to ensure that it is maintained and to examine new desalination techniques for the future, the United Kingdom Atomic Energy Authority and a leading British company are to co-operate in the running of an experimental station on the Ayrshire coast in Scotland.”