desalination - Methods, Considerations, Experimental techniques and other developments

The removal of salt from sea-water or brine to produce water that is potable, industrially or agriculturally usable, or suitable for ships' boilers. Distillation is the oldest process and, in revised efficient forms, still one of the most widely used. Membrane processes are useful with weak brackish water, the brine being forced under pressure against a membrane to produce a reverse osmosis, fresh water passing through to leave increasingly salt water behind. Electrodialysis and the freezing out of ice are also sometimes useful. About half of the world's desalinated water is produced in the Middle East, the rest in Africa, Asia, Australia, and the USA.

reverse osmosis) 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 of brackish water is already commonplace in the U.S., where it is used to meet treaty obligations for river water entering Mexico. Kuwait's energy reserves are so great that Kuwait is unique in using desalinated water for agriculture. The world's largest desalination plant is the Shoaiba Desalination Plant in Saudi Arabia. It uses multi-stage flash distillation, and it is capable of producing 150 million cubic meters of water per year.

Methods

Distillation Multi-stage flash (MSF) Multiple-effect (MED|ME) Vapor compression (VC) Evaporation/condensation Membrane processes Electrodialysis reversal (EDR) Reverse osmosis (RO) Nanofiltration (NF) Forward osmosis (FO) Membrane distillation (MD) Freezing Geothermal Solar humidification (HDH, MEH) Methane hydrate crystallisation High grade water recycling

As of July 2004, the two leading methods were Reverse Osmosis (47.2% of installed capacity world-wide) and Multi Stage Flash (36.5%).

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.

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.

In the last decade, membrane processes have grown very fast, and Reverse Osmosis (R.O.) has taken nearly half the world's installed capacity.

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.

Considerations

Cogeneration

There are circumstances in which it may be possible to use the same energy more than once. In the Middle East and North Africa, it has become fairly common for dual-purpose facilities to produce both electricity and water.

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". Reverse osmosis, for instance, may remove 50% or more of the water, doubling the salinity of ocean waste. The benthic 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.

Economics

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. Desalination stills now control pressure, temperature and brine concentrations to optimize the water extraction efficiency.

Environmental

Aside from the energy costs of the process, desalination plants produce hypersaline brine that must be disposed of.

Experimental techniques and other developments

In the past many novel desalination techniques have been researched with varying degrees of success. 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. From an environmental and economic point of view, in most 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.