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How Does Desalination Work?

The desalination of seawater or brackish water provides less than 1% of the world’s drinking water. In 2018, there were more than 20,000 desalination plants worldwide, primarily in the Gulf, the United States, China and Israel. Originally invented in 1965, desalination has grown rapidly since 2000 and is closely intertwined with energy issues.

The micro-filtering room of the Palma de Mallorca plant ©JAIME REINA - AFP

Desalination is the process of obtaining fresh water from either seawater (30-44 grams of salt per liter) or brackish water from estuaries (less salty). These sources are inexhaustible – they account for nearly 98% of the Earth’s available water – but desalination is expensive and requires much more energy than either wastewater, surface water or groundwater treatment.
There are three principle methods of seawater desalination. Sometimes they are used in combination with one another.
 

  • Thermal Distillation

Thermal distillation is the oldest and simplest method. Seawater that has been drawn from the sea or ocean is filtered to remove the larger impurities. It is then heated to produce vapor in a vessel that collects the salts. The vapor is subsequently condensed and converted to a mineral-free liquid.
 

  • Reverse Osmosis

1%: The percentage of the world’s drinking water that comes from seawater desalination.

The seawater is carefully filtered through several layers of sand or charcoalCharcoal is carbon produced by the pyrolysis of wood in the absence of oxygen... to eliminate microalgae and suspended particles, leaving only the salts behind. The water is then fed under high pressure (50 to 80 bar) through a very fine, semi-permeable membrane, whose pores measure one millionth of a millimeter. The membrane barrier lets the water molecules through but traps the salt. This method is called reverse osmosis1. The membranes are enclosed in cylinders that are arranged side by side. The world’s largest reverse osmosis plant is in Israel. It has nearly 50,000 membranes and produces 600,000 cubic meters of water daily. For every two liters of seawater, reverse osmosis produces one liter of fresh water and one liter of brine, whose salt concentration is two to three times that of the feed water. The brine is discharged into the sea. In both the thermal distillation and reverse osmosis processes, minerals (calcium, magnesium, potassium, etc.) are added to the desalinated water to make it suitable for consumption.
 

  • Electrodialysis

Electrodialysis is a membrane-based process that uses an electric field to filter out the salt. It uses very little energy but is limited to the treatment of low-salinity water.

Salt can be removed from seawater by evaporating and condensing the water or by pushing the water through an ultra-fine membrane that does not let the salt pass through.


The Energy Cost of Desalination

For this reason, distillation was initially adopted in regions with abundant, cheap energy, like the oil-and-gas-rich countries of the Gulf. Desalination plants are often built close to thermal powerIn physics, power is the amount of energy supplied by a system per unit time. In simpler terms, power can be viewed as energy output... plants since large quantities of heatIn the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter... are needed to evaporate the seawater. Waste heatLike waste energy, waste heat is heat whose release during a process or the manufacture of a product is unavoidable... produced as a byproduct during the generation of electricityForm of energy resulting from the movement of charged particles (electrons) through a conductor... can therefore be recovered by the desalination plant.

In reverse osmosis, electricity is required to operate the pumps and compressors that draw and move the seawater through the membranes. The rise of renewable electricity has opened up new opportunities, as illustrated by the development of combined solar farm desalination plants.
 

The Cost of a Liter of Desalinated Water

The future of desalination hinges largely on the liter cost of the desalinated water. Running a large-scale reverse osmosis facility that can treat several hundred thousand cubic meters of  water per day can lower the cost of producing a cubic meter of water by €0.40 to €0.80. But desalination plants of this size call for heavy investment, and the water is almost twice as expensive as that produced by a traditional river or lake water purification process.

It is possible to build smaller-scale systems, but the cost of the water is higher. The Jülich Solar Institute in Germany is developing small desalination units for remote villages in arid regions that harness the heat of the sun2. They produce 10 cubic meters of fresh water per day at a cost of €20 per cubic meter (€0.20 per liter), which represents almost €1.00 a day for a family of eight. That is still far too expensive for local standards.

 

(1) In natural osmosis, the pure water from a dilute solution flows through the membrane to the saltier side until the salt concentration is the same on each side. In the desalination process, pressure is required to push the salt water through the membrane in the opposite direction.

(2) Read the Deutsche Welle article – https://www.dw.com/en/making-seawater-into-drinking-water-with-the-help-of-the-sun/a-39924334