Everything you always wanted to know about desalination

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Fresh water being unequally distributed around the globe, it is a scarce resource in some regions. Fresh water demand for human consumption, agriculture and industry has also largely increased over the past years and is expected to continue to grow.

Desalination is an alternative solution to increase the available water resources.


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According to the International Desalination Association, in 2017, the total worldwide installed desalination capacity represents 92.5 million m3/day. Roughly 60% of desalination is devoted to human consumption. It is estimated that some 300 million people rely on this process for their daily water usage.

Kuwait for instance has an average rainfall of 110mm per year and produces almost 100% of its fresh water use through desalination.


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Thermal desalination processes produce distilled water, which is quite pure. Whereas seawater contains some 40 kilos of salt per ton, distilled water has only a few grams of salt per ton. In this sense it does not comply with WHO recommendations. However having a glass of it would not be a problem for most of us. The problem is preservation of such water since it will acidify, become corrosive and can easily be contaminated.

Therefore water desalinated for human consumption is mineralised and chlorinated before dispatch.


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The sun has done it forever ... by evaporation to provide rain. Steam powered ships were first equipped with evaporators for the production of boilers make-up water (1900). Desalination units were first used on board ships and were then widely developed with the growth of OPEC countries (1970). The capacity of the plants has grown drastically over that period, from a few hundreds cubic meters per day in the 1960's to hundreds of thousands cubic meters per day for the largest desalination plants nowadays.

Desalination is used for industrial, human consumption or agricultural purposes. Petroleum or chemical industries always need large quantities of pure water for boilers or process requirements. Wherever local supply cannot meet their own requirements they will go to desalination to free themselves from local constraints. This particular industrial use represents one third of the world wide installed capacity. 60% of desalination is devoted to human consumption: thus a reliable water resource is created and enables sustainable development of a population in a region.

Agricultural use is mainly a second hand use: in many Middle East countries, desalinated water is first devoted to human consumption, then after treatment it is re-used in irrigation.

A few percent of desalination capacity is for military, navy, tourism uses ...etc...


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The cost of desalinated water taken at the outlet of a plant may vary widely from one site to the other. It mainly depends on the cost of energy, the plant total capacity, the depreciation period, and on the production pattern (whether it is used seasonally only or throughout the year).

In the late 2000s, innovation helped in the dramatic decrease of the cost of desalinated water but as the cost of energy rose again, the cost of operating a desalination plant rose as well. Reducing the consumption of energy of the plant is the challenge faced by all the players of the desalination market.

For large desalination plants, the current trend is to reach less than 0.5 USD per cubic meter. 


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The desalination plant typically uses three kilograms of seawater to produce 1 kilogram of fresh water. The extracted salt dissolves in the excess sea water used in the process to form so-called brine. The brine is returned to the sea where it is diluted again in its natural medium.


The usual desalination processes do not provide for such recovery. Whereas they concentrate seawater 1.5 times, recovery of salt would require seawater to be concentrated ten times. Under such conditions the first crystals would appear in the brine. This would require a lot of energy and cannot be justified on an economic standpoint. Today whenever a large surface area is available close to a sunny seashore, salt pans, which make use of solar energy, are still the best method of salt production.


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The market is split into two major families: reverse osmosis and distillation.

Osmosis is a natural phenomenon that occurs when two solutions with different salt concentrations are separated by a semi-permeable membrane: fresh water migrates through the membrane from the lower concentration side to the higher concentration one. When putting pressure on the high concentration side, this natural flow will decrease. It will stop at a pressure so called the osmotic pressure. If the pressure increases beyond this point then the flow turns opposite: from higher concentration to lower. This is reverse osmosis.

This phenomenon is used for desalination. After a thorough pre-treatment aiming at full destruction of all biological life and material, and also at controlling scaling within the system, water is passed through osmotic membranes, which will let fresh water through and retain salt.

Distillation or thermal desalination reproduces the natural cycle of rain within an evaporator. There are two main types of seawater distillation processes: Multi Stage Flash (MSF) and Multiple Effect Distillation (MED). The latter has two alternate configurations: MED with thermal vapour compression (MED-TVC) and MED with mechanical vapour compression (MED-MVC).

In a MSF unit seawater is significantly heated and pressurised. Then it is introduced into different cells where pressure and temperature are lower and lower. In each cell a phenomenon occurs that can be compared to what would happen when opening a pressure cooker filled up with hot water under pressure: it would violently boil out (it is not advisable to try it!), it would flash. Vapour released by this boiling in each cell is condensed into fresh water.

In each cell of a MED unit seawater flows down by gravity in a thin film around horizontal tubes heated by an internal steam flow. Seawater partly evaporates and the vapour raised condenses inside the horizontal tubes of the next cell, thus turning to fresh water. This principle is repeated in multiple cells: the vapour raised in one cell heats up the tubes of the next one.


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Distillation processes are not sensitive to seawater quality. A 0.5mm filtration, a chlorination and a suitable anti-scale and anti-foam treatment will be sufficient to guarantee a steady water quality versus time, particularly with the low temperature MED process. Bacteria and pathogenic organisms are generally destroyed by these processes. Distilling pollutants are the only ones to create a problem when mixing with condensed vapour, if a seawater screening solution is not set up in due time. (see pollution by oil spills)

Membrane processes are more sensitive to seawater quality. Depending on seasonal conditions water quality may be affected by the weather or by biological activity. In such case the installed pre-treatment system may well no longer be fitted for such conditions. Bio-fouling which is the clogging of membranes by bacteria is a permanent threat for membranes. It generally results in a loss of production together with an increase of the produced water salinity. In such a case fresh water is contaminated with chloride and it will corrode the water distribution network : water is rusty at the consumer's tap. To enhance the reliability of such pre-treatment, the reverse osmosis systems involve more and more frequently additional ultra or nano filtration membranes, although this results in a price increase.


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Desalination plants reject slightly concentrated seawater called brine to the sea. The salt content of the brine is roughly 60g/l for a seawater at 40g/l. The temperature of the brine reject is slightly higher than that of the seawater - a few degrees Celsius. This excess salt concentration and temperature quickly vanish in the vicinity of the rejection point.

Seawater is chlorinated at the plant inlet to avoid the growth of seaweed and seashells inside the piping. Chlorine is accurately dosed so as to avoid active chlorine remaining at the outlet of the plant.

Antiscale and antifoam additives are also dosed in seawater entering the plant, dose levels being in the range of a few grams per ton. Once they have reacted inside the plant and due to their huge dilution, they present no risk of pollution at the plant outlet.

Desalination units are often set in environmentally protected areas with no particular problem raised.

As such, a desalination plant has no other reject to the environment. However it can be coupled with a boiler or a power plant. The rejects of such plants are controlled by environmental standards.

These combined plants provide the best efficiency and minimise the overall impact of power and water production on the environment.


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Oil in seawater can foul the piping, reduce exchange surfaces performance and even pollute fresh water if part of it distils with seawater. For a thermal plant this damage can be corrected by circulation of adequate dispersing agents within the units: after a while normal cleanliness will be restored. Reverse osmosis membranes are not so tolerant and must be kept away from oil pollution.

In the event of oil spill it is recommended to protect the sea water inlet system - with floating dams for example - and to reduce the load in accordance with the filtration capacity of such dams.


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Salted water is not always the same. Depending on whether they come from warm or cold oceans, closed or open seas, their salt content will vary. This is a short comparison between samples:

  •    Brackish water:0,5 à 3 g/l
  •    Northern Sea close to estuaries :21 g/l
  •    Atlantic Ocean :35 g/l
  •    Mediterranean Sea :38 g/l
  •    Arabian Sea :45 g/l
  •    Dead Sea :300 g/l

Open seawaters always contain the same proportion of different salts; they can be desalinated without problem.

For brackish waters, whether from underground or surface, a particular study is often required since they often contain very low solubility salts. These salts will impede concentration as they will immediately precipitate and disturb process operation.


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Thermal desalination is even more efficient when combined with a power plant. Considering the low overall efficiency of power generation whatever the process used, it will reject large quantities of low temperature heat to the environment. Since desalination processes work at low temperatures they can be efficiently supplied with heat recovered from power plant rejects.

The combination of power and water production gives the best economical performance and is therefore widely used in the Middle East. Such projects known as Independent Water & Power Production (IWPP) projects are usually subject to large public tenders.


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The combination of power and water generation systems results from a production cost optimisation by the investor. In some countries, this optimisation is difficult to make due to large variation of power demand between summer and winter.

Whereas water demand remains steady all year long, power demand may vary from 100% in summer to 30% in winter due to air-conditioning systems power consumption.

Combining reverse osmosis with thermal desalination on the same site allows the optimisation of the re-use of the steam coming from the power plant (through thermal distillation) with a steady water demand.

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