Desalination

Shevchenko BN350 desalination unit situated on the shore of the Caspian Sea.

Desalination means any process that removes the excess salt and other minerals from water in order to obtain fresh water suitable for animal consumption or irrigation. Most desalination is by distillation. Some is by reverse osmosis or other methods.

The main purpose is to make water for people to use. The salt is usually carried away as brine but in some places the solid salt is extracted.^[1]

Desalination of brackish water is done in the United States in order to meet treaty obligations for river water entering Mexico. Several Middle Eastern countries have energy reserves so great that they use desalinated water for agriculture. Saudi Arabia's desalination plants account for about 24% of total world capacity. The world's largest desalination plant is the Jebel Ali Desalination Plant (Phase 2) in UAE. It uses multi-stage flash distillation, dual-purpose and it is capable of producing 300 million cubic meters of water per year.^[2]

Methods

Distillation
1. Multi-stage flash (MSF)
2. Multiple-effect (MED|ME)
3. Vapor compression (VC)
4. Evaporation/condensation
Membrane processes
1. Electrodialysis reversal (EDR)
2. Reverse osmosis (RO)
3. Nanofiltration (NF)
4. Forward osmosis (FO)
5. Membrane distillation (MD)
Cold (freezing)
thermal
Solar humidification
Methane hydrate crystallisation
High grade water recycling

The traditional process used in these operations is vacuum 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.

Desalination Media

The El Prat Desalination Plant, a seawater reverse osmosis facility near Barcelona, Catalonia.
Schematic of a multistage flash desalinatorA – steam in B – seawater in C – potable water outD – brine out (waste) E – condensate out F – heat exchange G – condensation collection (desalinated water)H – brine heater*The pressure vessel acts as a countercurrent heat exchanger. A vacuum pump lowers the pressure in the vessel to facilitate the evaporation of the heated seawater (brine) which enters the vessel from the right side (darker shades indicate lower temperature).
Schematic representation of a typical desalination plant using reverse osmosis. Hybrid desalination plants using liquid nitrogen freeze thaw in conjunction with reverse osmosis have been found to improve efficiency.
Scheme of the desalination machine: the desalination box of volume V_{box} contains a gel of volume V_{gel} which is separated by a sieve from the outer solution volume V_{out} = V_{box} - V_{gel}. The box is connected to two big tanks with high and low salinity by two taps which can be opened and closed as desired.
Mangrove leaf with salt crystals