What happens to the salt?
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.
Can salt be recovered?
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.
Do desalination plants pollute?
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.
Is desalinated water drinkable?
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.
Who buys desalination plants?
The largest clients are located in the Middle East. This region is home to more than half of the world wide installed capacity, considering all processes, whereas the USA have 15%, Europe and Asia 10% each and Africa around 6%.
Beside these major clients the largest petroleum or chemical companies are clients of desalination in order to provide reliable water sources to their plants.
Desalination, what for?
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...
How much does desalinated water cost?
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). As a rule of thumb a large plant located in Europe with a continuous production pattern will produce water at a cost of 1 Euro per cubic meter. The water produced in a smaller plant ( a few thousand cubic meters per day) would cost 3 to 5 times higher.
Seawater or brackish water?
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.
Combined power and desalination plants
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. These particular arrangements provide the lowest water costs.
How much water comes from desalination?
According to the International Desalination Association the total worldwide installed desalination capacity represents 50 million m3/day, of which 46% is obtained by thermal distillation.
Roughly 70% of desalination is devoted to human consumption. Considering an average water daily consumption of 150 litres per capita, it means that some 230 million people rely on this process for their daily water usage.
For how long has seawater been desalinated?
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 perod, 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.
What are the main desalination processes?
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.
How much energy is required to retrieve the salt from the seawater?
In theory, calculation shows that 1 kWh is enough to produce one ton of fresh water from seawater. However this assumes a perfect thermodynamic and mechanical system which is not possible to build. In practical terms a desalination system will require 7 to 18 kWh/m3 depending on the corresponding investment.
Desalination systems vapour consumption is usually measured by giving how many kilos of fresh water are produced from one single kilo of steam entering the system. This number, the so-called Gain Output Ratio (GOR), will vary from 6 to 7 for classical plants up to 16 and more for plants equipped with the latest technology.
This figure however does not show properly how steam quality will impact on plant design: the higher the steam supply pressure the more velocity it will provide by expansion, the more suction it will generate and the bigger recovery it will enable thus enhancing overall efficiency. This is why the higher the steam pressure the lower the investment cost for a given GOR. A GOR 8 unit fed with 3 bar steam will cost some 20% more than the same GOR with 20 bar steam.
In addition to steam consumption one has to consider electrical power for pumps. The MSF process requires a large flow of seawater or brine to be circulated in condensers. This results in a specific electrical consumption of 3 to 4 kWh/m3 for MSF plants. This is to be compared to specific consumption of MED plants where no such circulation is required: 1,5 kWh/m3. This is the particular point for which MED now supersedes MSF. By going for MED an Independent Power and Water Company will save some 3 kWh per cubic meter produced: this additional power will be delivered to the network and will generate additional income. This will result in reduced cost of power and water produced by the plant when compared to MSF.
Are desalination plants vulnerable to oil spills?
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.
What is water-steam equilibrium?
Under atmospheric pressure (1000 hPa or 1 bar), pure water boils at 100°C and changes into steam. On top of the Mont-Blanc (Alt. 4807 m) where atmospheric pressure is 530 hPa it will boil at 83°C only. On the other hand in a sealed pressure cooker water must be heated to 120°C to start boiling, at that time internal pressure will be around 2000 hPa. For each pressure there is one boiling water temperature: when water is at the temperature corresponding to the pressure there is an equilibrium between water and steam.
An evaporator consists of several cells each working at different temperatures. Each of them is in an equilibrium state between steam and water. The lower the temperature, the lower the pressure. The coldest cells have a temperature in the range of 45°C and therefore a pressure of 100 hPa (which means a vacuum of -0,9 bar).
It is the main process for water production in the world, due to ocean evaporation. There are simple ways to reproduce this phenomenon such as for example having a small salted pond under a plastic cover heated up by the sun. Condensation droplets can be recovered from the cover. However this method is not very productive.
Distillation requires a heat source and a cold source. Heat may come from oil, gas, sun or nuclear. Each time distillation can adapt to the proposed energy source.
Today the economic advantage of solar desalination has not yet been proven. The main problem of solar energy is its high investment cost and uneven sunlight distribution. The lowest cost for water from the sea is still achieved by fossil fuel power plant combined with thermal desalination plants.
Has seawater quality any impact on desalinated water quality ?
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.