We can now discuss the future of our water supplies, and consider some recent warnings that before the next century there will be a widespread shortage. Taking the inhabited world as a whole, there is sufficient precipitation to support a far larger world population than exists at present. The problem is rather one of storage and transport, for, as we have seen, many countries have adequate water resources only in one region, or at one time of the year. It is for these reasons that only a very small fraction of the world’s fresh water is economically available. As future demands increase, however, it will become more and more necessary to overcome this unequal distribution and timing of rainfall. More reservoirs will have to be built to store the wet season’s surplus, although in some regions it may well be cheaper to store surplus water in aquifers by some method of artificial recharge. Most of the sources and reservoir sites that are close to big cities are already exploited, so we shall have to seek out more distant catchment areas in which to build reservoirs; this implies even longer aqueducts than have been built so far.
Schemes that involve distant reservoirs and long aqueducts take many years to design and build, and it is no good starting them only when the supplies run short. The safe way is to anticipate demand by as much as 30 years ahead, otherwise very serious shortages are bound to occur. One particularly ambitious and romantic plan that attempts to provide for the needs of the 21st century is called NAWAPA , and was conceived by the Ralph M. Parsons Company of America. The huge water surplus of north-west Canada and the U.S.A. , which at present pours into the sea, would be stored in reservoirs at high elevation. From these, 53 million million gallons would flow annually to 7 provinces of Canada, 33 states of the Union, and 3 states of Mexico, through a system of tunnels, pipes, canals, and rivers. As well as helping industry and the domestic consumer, this scheme would irrigate 40 million acres of the arid north-western states of the U.S.A., triple the agricultural land of Mexico, feed a huge waterway from the Great Lakes of America across Canada and thus benefit transport, and at the same time maintain the levels of the Great Lakes. In addition to all this, the scheme would generate enormous quantities of electrical power.
The scheme is estimated to cost at least £33,000 million. A preliminary period of 10 years is provided for discussion leading to interstate treaties, and construction is estimated to take at least 30 years. The go-ahead has not yet been given, one of the many reasons being that Canada may not agree to share its water, even though no more than 20 per cent of the Canadian surplus would be exploited. But some large project will have to be decided on fairly soon if the western United States is to continue its rapid expansion and high standard of living into the next century, and the same considerations apply to other countries.
The transport of water from distant lakes and reservoirs should be considered only after every effort has been made to use existing supplies several times over. Already it is impossible to meet demands in many countries without using some water more than once, and this practice will have to be encouraged if water is not to become scarce. Not all water, of course, is reusable: that used for irrigation and by some industries is lost by absorption, evaporation, or both. But most water used by industry and the domestic consumer can be treated and then returned to rivers and lakes for further use.
Most industrial effluents contain pollutants, and the natural process of purification of rivers and lakes rarely restores the water to its original quality. Even the most careful treatment of sewage, for example, does not remove sodium chloride, and if several sewage works discharge into one river, the salt content progressively increases downstream. This may not matter with large rivers, where pollutants are much diluted, but they may reach dangerous concentrations when the river level falls. Water undertakings naturally try to abstract water upstream of sewage works, but this is not possible for long rivers such as the Rhine, Thames, and Danube, with towns along much of their length. Thus the maximum re-use of water is economic only when industrial effluents and sewage are properly treated before discharge and when they are adequately diluted by the natural river flow.
One way of re-using water is to treat sewage and then pass the effluent to a storage reservoir at the water treatment works, where it is made suitable for consumption. One American town has already been forced to do this, going through the cycle of water-sewage-water as many as 18 times before finally returning it to the river. The main problem here is the elimination of tastes, but new techniques may overcome this.
The re-use of water by industry is particularly important because of the enormous quantities involved. Water may be used once by a factory, or it may be recycled as many as 30 times, at each stage undergoing treatment to remove impurities. Without this re-use within the factory, undertakings would have to find new sources of water and build larger treatment works much sooner than they otherwise would do. Furthermore, industrial effluents are often discharged into sewers, and so more sewage works would have to be built as well. By placing greater demands on both treatment and sewage works, the price of water to industry would increase, so it is in their interest to re-use water.
One recent and exciting means of increasing our fresh-water resources is, by desalination, which by-passes the normal hydrological cycle. The natural conversion of salt water to fresh has been known for over two thousand years; the Greeks knew that the sun evaporated fresh water from the sea, and that this later fell to earth as rain. But only recently has man been able to desalinate sea water in large quantities at an acceptable cost. The simplest method of desalination is solar distillation, where solar energy is harnessed to evaporate sea water. The sea water is pumped into a reservoir that has a black lining to absorb the sun’s rays, and that is covered with an inverted U-shaped transparent plastic sheet. Fresh-water vapour condenses on the underside and runs down into a trough. Even though solar energy is free, the cost of a thousand gallons to one Greek island is supposed to be about 17s.6d., but this is still very cheap compared with the previous cost of about £2.17s.0d. for water shipped from the mainland. Perhaps this is an appropriate moment to mention the cost of water before we discuss desalination further. The acceptable cost depends on the alternative sources of supply; if there is no alternative, the price does not enter the argument, for the simple reason that water is essential to life.
Solar distillation is an unusual method of desalination and is economic only in regions with very long periods of sunshine and no other source of water. Other methods require an artificial source of energy, because when salts dissolve in water, heat energy is given off, and in order to separate water and salts, a certain definite amount of energy must therefore be put back. This quota of energy fixes the theoretical minimum cost of desalination at about 4d. per thousand gallons, but in practice the cost is very much higher because heat escapes at all stages and no machine works at 100 per cent efficiency. A more realistic price, allowing for the capital outlay, running costs, and expected life of the plant, is about 8s.6d. per thousand gallons, compared with about ls.6d. for conventional methods of supply.
The commonest method of desalination is by a process called multistage flash distillation. Sea water is heated to between 90 and 120°c and then sprayed into a vessel maintained at reduced pressure. Now the lower the atmospheric pressure, the lower the temperature at which water boils; therefore in this first vessel water boils, or flashes, instantaneously. The remaining brine, which is now cooler because of heat loss, then passes to another vessel, where the water again boils immediately because this vessel is at an even lower pressure than the first. Sea water may pass on through as many as 40 vessels at progressively lower pressure until the residual brine reaches such a low temperature that it will no longer boil quickly enough. The fresh-water vapour is condensed by water-cooled pipes; as it does so it gives out latent heat, returning some of the original heat to the fresh, incoming brine. The most costly part of flash distillation lies in heating the water. It is not really economical for a desalination plant to heat its own water, except in ships, where the very high cost of 16s.0d. per thousand gallons is still less than the loss of revenue that would occur if cargo space were given over to fresh-water storage. On land the usual practice is to use the residual heat in steam that has passed through the turbines of a power station; this ‘waste heat,’ which is about 120°c, is no use to the power station but is suitable for heating sea water. In any case, it would be impracticable to use steam above 120=c without causing excessive scale in the heat exchange pipes. The most efficient desalination thus combines the production of electricity and fresh water. Recently, nuclear energy has entered the scene in that Israel and California, for 90°C example, are planning to build atomic reactors to produce electricity, using waste heat for converting salt water to fresh.
Several other methods of desalination exist, but none competes with flash distillation for the production of large quantities of water at reasonable cost. Freeze-separation, for example, makes use of the fact that ice crystals from frozen sea water consist of fresh water. Another method, called electrodialysis, is the opposite of distillation in that it removes the salt from sea water, leaving fresh water behind. Such methods are only suitable for water of low salt content and may prove very useful in arid lands where there is a supply of brackish water.
It is impossible to predict exactly what part desalination will play in the future. It will never replace existing sources, and it is certainly too expensive for irrigation and for inland communities. Desalination is appropriate only for those situations in which a special combination of circumstances make it the cheapest—and often the only—method. Kuwait in the Persian Gulf, for instance, has no natural fresh water, an abundance of very cheap oil, and the highest average income per head in the world, so their plant is the obvious answer. Then we have Guernsey, where the main income comes from horticulture, and where there is little underground water and not enough space to impound surface water. In this situation it was worth while converting sea water to supplement the upland supply during the brief period of dry summer weather. Undoubtedly, more and more places with special problems will install desalination plants as the demands of industry and the domestic consumer increase.
If we have not mentioned in this article any prospect of discovering new ways of increasing our future water supplies, it is because these probably do not exist. There is no means of manufacturing water in large quantities without destroying some other equally valuable commodity, so we have to rely on trapping natural precipitation, or, if we are prepared to pay for it, on desalination. In other words, we can do no more than develop and perfect our present methods of obtaining fresh water.
We have seen that by re-using water as much as possible, and by reducing wastage and leakage, water can be made to go a surprisingly long way. We have also seen that there is abundant fresh water available to store and transport. As living standards improve and as populations increase, we shall sooner or later be faced with paying, directly or indirectly, for new and costly schemes. And we are in no position to resent the prospect; for most of us a constant supply of clean water when and where we want it, costing only a few pence per ton, is a real bargain, and we can count our blessings. It is in the underdeveloped countries that the problems are more serious. Water undertakings require massive capital expenditure, and capital is just what these countries do not have. The necessary finance can come to them only through foreign aid, and it is the duty of the developed countries to provide the means whereby every man, woman, and child on earth has an adequate water supply.