Desalination
Desalination refers to any of several processes that remove the excess salt and other minerals from water in order to obtain fresh water suitable for animal consumption or irrigation, and if almost all of the salt is removed, for human consumption, sometimes producing table salt as a by-product.
Desalination for brackish water is already commonplace in the U.S., where it is used to meet treaty obligations for river water entering Mexico. Indeed, desalination has spread into use in over a hundred countries, with Saudi Arabia accounting for about 24% of total world capacity. Kuwait built the world's first large-scale desalination plant in the 1960s. Kuwait's energy reserves are so great, that Kuwait is unique in using desalinated water for agriculture. The world's largest desalination plant is in Ashkelon, Israel. It began operating on August 4, 2005, and it is capable of producing 100 million cubic meters a year of water.
Methods of Desalination
- Distillation
- Multi-Stage Flash
- Multiple-Effect
- Vapor Compression
- Electrodialysis Reversal (EDR)
- Reverse osmosis (RO)
- Freezing
- Membrane Distillation
- Solar Humidification
- Ethane Hydrate Crystallisation
- High grade water recycling
- Experimental or proposed techniques
As of 1998, the two leading methods were Multi-Stage Flash Distillation (44%) and Reverse Osmosis (42%).
Distillation of ocean water is common in the Middle East, on ships, submarines and islands. The process used in these operations is essentially the boiling of water at less than atmospheric pressure, and thus a much lower temperature than normal. Due to the reduced temperature, energy is saved, however, bacteria are not killed. Therefore additional water processing is necessary before the water can be used by the public.
Considerations
Cogeneration
There are circumstances in which it may be possible to use the same energy more than once. With cogeneration this occurs as energy drops from a high level of activity to an ambient level. Distillation processes, in particular, can be designed to take advantage of co-generation. In the Middle East and North Africa, it has become fairly common for dual-purpose facilities to produce both electricity and water. The main advantage being that a combined facility can consume less fuel than would be needed by two separate facilities.
Concentrate Disposal
Regardless of the method used, there is always a highly concentrated waste product consisting of everything that was removed from the created "fresh water". With coastal facilities, it may be possible to return it to the sea without harm. It is more of a problem as you move inland, as one needs to avoid ruining existing fresh water supplies such as ponds, rivers and aquifers. As such, proper disposal of "concentrate" needs to be investigated during the design phase.
Economics
The price of desalination is rapidly declining. A modern, large, efficient plant is within 20% of the cost of developing a new, local source of fresh water in some places. Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency. Nuclear-powered desalination might be economical on a large scale.
A number of factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal. Generally the cost of removing salt from seawater will be about 3-5 times that of removing salt from brackish water.
Environmental
From an environmental point of view, in some locations desalination can be preferable to using fossil groundwater or surface water for human needs, as in many regions the available surface and groundwater resources already have long been under severe stress. However, desalination is not without all environmental drawbacks. Most desalination plants produce hypersaline brine that must be disposed. These desalting concentrates are classified by the U.S. Environmental Protection Agency as industrial wastes. The hypersaline brine has the potential to harm marine ecosystems, especially in sea regions with low turbidity, high evaporation and hence already increased salinity. Examples are the Persian Gulf, the Red Sea and in particular Coral Lagoons of Atolls and other tropical Islands around the world.
- See also: Soil salination