As of July 2004, the two leading methods were Reverse Osmosis (47.2% of installed capacity world-wide) and Multi Stage Flash (36.5%). (Source: 2004 IDA Worldwide Desalting Plants Inventory Report No 18; published by Wangnick Consulting: [1].

Desalination of ocean water is common in the Middle East and the Caribbean, and is growing fast in the USA, North Africa, Spain, Australia and China. It is also used on ships, submarines and islands.

The traditional process used in these operations is distillation — 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.

In the last decade, membrane processes have grown very fast, and Reverse Osmosis (R.O.) has taken nearly half the world's installed capacity. Membrane processes use semi-permeable membranes to filter out dissolved material or fine solids. The systems are usually driven by high-pressure pumps, but the growth of more efficient energy-recovery devices has reduced the power consumption of these plants and made them much more viable; however, they remain energy intensive and, as energy costs rise, so will the cost of R.O. water.

Forward Osmosis (F.O.) employs a passive membrane filter that is hydrophylic (attracts water), slowly permeable to water, and blocks a portion of the solutes. Water is driven across the membrane by osmotic pressure created by food grade concentrate on the clean side of the membrane. Forward osmosis systems are passive in that they require no energy inputs. They are used for emergency desalination purposes in seawater and floodwater settings.

Co generation
There are circumstances in which it may be possible to use the same energy more than once. With co generation 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 if this concentrate does not exceed the normal ocean salinity gradients to which osmoregulators are accustomed. Reverse Osmosis, for instance, may remove 50% or more of the water, doubling the salinity of ocean waste. The community cannot accommodate such an extreme change and many filter feeding animals are destroyed when the water is returned to the ocean. 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 energy needed for desalination, particularly R.O. has declined but not as fast as energy has increased in price recently[citation needed]. A modern, large, efficient plant is within 20% of the cost of developing a new, local source of fresh water in some places[citation needed]. 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, and there is a pilot plant in the former USSR.

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[citation needed].

Environmental
From an environmental point of view, in some locations geothermal 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.

Aside from the energy costs of the process, desalination plants produce hyper saline brine that must be disposed of. These concentrates are classified by the U.S. Environmental Protection Agency as industrial wastes. The hyper saline brine has the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity. Examples of such locations are the Persian Gulf, the Red Sea and, in particular, coral lagoons of atolls and other tropical islands around the world.

Experimental techniques and other developments
In the past many novel desalination techniques have been researched with varying degrees of success. Some are still on the drawing board now while others have attracted research funding. For example, to offset the energetic requirements of desalination, the U.S. Government is working to develop practical Solar Desalination. This development has much potential, since the regions in which desalination is most needed often have an abundance of solar energy.

Other approaches involve the use of geothermal energy. An example would be the work being done by SDSU Center for Advanced Water Technologies. [2]