Desalination removes dissolved minerals (including but not limited to common salt) from seawater, brackish water or treated waste water. The amount of water in the sea is enormous, but before it can be used for water resources, dissolved salts in the water must be removed or substantially reduced. Desalination of seawater could produce unlimited supplies of fresh water and could solve many water resources problems if it were possible to do it inexpensively. Unfortunately, desalination is an expensive process, the water produced generally costing more than from other sources, as it has high capital costs and requires a lot of energy, so it is not usually the first choice for a water supply. However, it is used if there is no other source available. Other disadvantages are that a large amount of saline water is required, which generally restricts the process to coastal areas (although saline groundwater and water from inland seas can also be used as the raw material), and that disposal of the concentrated brine produced may be difficult.
There are various processes for desalinating water. The one most commonly used is distillation, which is similar to the natural evaporation of seawater in the hydrological cycle. Many arid countries receive large amounts of solar energy and this can be used as the energy source in a solar distillation process. However, solar distillation needs large areas of solar stills and produces only small quantities of water — a maximum of only 5 litres per day for each square metre of still area. It is usually used only in remote villages in arid developing countries. Larger quantities of water can be produced by distillation plants where the saline water is heated by more concentrated energy sources, and plants producing over 103 m3 per day are common. Most of the larger plants, such as the 106 m3 per day plant in Jubail, SaudiArabia, use the distillation method. The efficiency of distillation plants (ratio of usable output water to input water) ranges from 15-50%.
Another important desalination process is reverse osmosis (Figure 7). This uses high pressure to force saline water through a semipermeable plastic membrane, which filters out both suspended and dissolved substances. Reverse osmosis is more suitable for desalinating water with a lower salinity than seawater. The world's largest reverse osmosis plant is under construction and due to be commissioned in 2005 at Ashkelon, Israel: it will produce 3 × 105 m3 per day. The project is expected to cost US$200 million, and produce water at a production cost of around US$0.5 m-3.
Energy costs are a substantial part of the cost of desalinated water (except for solar stills, which use free solar energy but have relatively high capital costs per cubic metre and low outputs). It takes a considerable quantity of energy to desalinate a cubic metre of water by distillation, around 300 MJ m-3 for seawater, so the cost of desalination by this means depends directly on the energy cost. If a country has a cheap source of energy, desalination may be practical. In California, which has a number of desalination plants, the selling cost of the water is US$1-4 m-3. A desalination plant which began operation in 1992 in Santa Barbara in California had a capital cost of US$36 million for 12 × 106 m3 a year, at a selling price of US$2.4 m-3 . However, this plant has now been decommissioned (2004) as this water proved to be more expensive than other water sources.
Desalination is generally used only where there is no other possible source of water, as all other sources would be cheaper, with the possible exception of long-distance water transfer schemes. Desalination is used in wealthy but arid coastal areas, where it is economic to pay a higher price for water; Saudi Arabia produces 70% of its drinking water by desalination. The Arabian Peninsula and Iran, for example, have a greater desalination capacity than all the rest of the world, using energy from their abundant oil resources to produce water. The four countries with over 106 m3 per day desalination capacity are Saudi Arabia, the USA, the United Arab Emirates and Kuwait. The UK, for comparison, has a current capacity (2004) of about 105 m3 per day. However, this is likely to increase in the near future, as there are plans for London's first-ever desalination plant. The £200 million project is planned for completion in 2007/2008, to help with supplies during drought periods. This will convert water from the tidal River Thames (less saline than seawater) into drinking water. The plant will have a maximum treatment capacity of 1.5 × 105 m3 per day, using reverse osmosis.
Other areas where desalination is common include islands with limited amounts of water because of their necessarily limited catchment areas, and where desalination is the only method of increasing these resources. In many cases the desalination plants are used only as back-ups to the normal supply, or to meet seasonal demands. Jersey (capacity 6.8 × 103 m3 per day, installed 1969) and the Isles of Scilly (2.2 × 102 m3 per day, installed 1992), for example, have the only reasonably large-scale desalination plants in the UK at present (2004), used to meet the summer demand from holiday visitors.
If the technology improves, or the cost of other water resources increases, desalination is likely to become more economic and therefore used more. A distillation plant built in combination with a power station, using the waste heat from the power station to drive desalination, has much lower running costs, and these combined plants are likely to become more common. Desalination will probably become increasingly important for the richer arid coastal countries and as an emergency method of supply in other areas in times of drought, when there is no alternative to the high cost of desalinated water. But for irrigation use, and for poorer countries, it will be too expensive to use even in times of drought.
If the capital cost were repaid over 10 years, what would be the capital cost contribution per cubic metre to the selling cost of water from the desalination plants in (a) Santa Barbara, and (b) the Isles of Scilly (capital cost of £250 000) ignoring interest charges on the capital? Comment on the difference between the two values. Use the conversion US$1 = £0.55.
- a.Santa Barbara — capital cost US$36 million, output 12 × 106 m3 a year. Over 10 years, capital cost repayment = US$3.6 × 106 per year, which is US$3.6/12 per m3 = US$0.3 per m3. At US$1 = £0.55, this is £0.17 per m3.
b.Isles of Scilly — capital cost £250 000, output 220m3 per day. This plant is for seasonal demand, so it may work for about 200 days a year. Therefore:
- output per year = 220 × 200m3 = 4.4 × 104 m3
- capital cost repayment is £25 000 per year = £25 000/4.4 × 104 per m3
- = £0.6 per m3
The Isles of Scilly plant has a capital cost contribution per cubic metre about three or four times that of the Santa Barbara plant. This is probably due to the economies of scale for the Santa Barbara plant.