Desalination is critical to efforts to address water shortages, particularly in coastal areas with a high population density because only 1% of the water on Earth is liquid freshwater, while 97% of all water resources are saltwater.
As a result, the world faces nowadays severe challenges to our ability to meet our future water needs.
So the world will need to make additional water resources available to all segments of a nation and provide additional water resources at a cost and in a manner that supports urban, rural and agricultural prosperity and environmental protection.
Meeting these challenges may lead us to use saline water for water conservation.
Desalination definition
Desalination is a process that removes salts and other dissolved solids from brackish Water or seawater.
Brackish water is saltier than freshwater, but not as salty as seawater.
Brackish water usually has a salt concentration between 5 and 20 parts per thousand (ppt) and seawater generally has a concentration of salt greater than 20 ppt. Brackish waters may also be found in aquifers.
Water type and total dissolved solids
| Water type | TDS(mg/l) |
| Sweet waters | 0-1000 |
| Brackish waters | 1000-5000 |
| Moderately saline waters | 5000-10000 |
| Severely saline waters | 10000-30000 |
| Seawater | More than 30000 |
The value of desalination
Desalination technologies will contribute significantly to ensuring a safe, sustainable, affordable and adequate water supply.
• Provide safe water: A safe water supply meets all drinking water standards, meets all standards for use by agricultural and industrial interests, and that strives to move toward greater water security during drought, natural disasters and transport.
• Ensure the sustainability of the nation’s water supply: A sustainable water supply meets today’s needs without jeopardizing the ability to meet the needs of future generations.
• Keep water affordable: An affordable water supply provides water to the nation’s future citizenry at rates comparable to that of today.
• Ensure adequate supplies: An adequate water supply guarantees local and regional availability of water.
Desalination techniques
Distillation
Multi-stage flash distillation
With the use of many stages of what are countercurrent heat exchangers, the multi-stage flash distillation (MSFD) method of water desalination distills salt water.
The majority of desalinated water in the world—64%—is produced by multi-stage flash distillation plants, however, reverse osmosis units are more prevalent.
Principle
The plant is divided into stages, each of which has a heat exchanger and a condensate collection.
The series has a chilly end and a hot end, while the temperatures in the middle stages are in the mid-range.
The boiling points of water at each stage’s temperatures are reflected in the stages’ varying pressures.
The brine heater is a container that is located after the hot end.
When the plant is in a steady state, feed water at the cold intake temperature is pushed through the stages’ heat exchangers or otherwise circulated through them to warm up.
It is already close to the maximum temperature when it reaches the brine heater.
There is some additional heat provided to the heater.
The water flows back through valves into the stages that have progressively decreasing pressure and temperature after the heater.
To distinguish it from the inflow water, the water is now referred to as brine as it runs back through the stages.
As the brine enters each stage, its temperature is higher than the boiling point at that stage’s pressure.
A little portion of the brine water then boils (or “flashes”) to steam, bringing the temperature down until equilibrium is attained.
The steam that results is a little hotter than the heat exchanger’s supply water.
As previously mentioned, the steam cools and condenses against the heat exchanger tubes to heat the feed water.
Multiple-effect distillation (MED)
A common distillation method for desalinating saltwater is multiple-effect distillation.
It has numerous “effects” or stages. The feed water is heated by steam in tubes at each stage.
When more water is heated and evaporated, a stream of evaporating water pours into the tubes of the following step. The energy from the preceding stage is essentially recycled at each stage.
A bank of horizontal tubes can have their tops sprayed with feed water, which then drips from tube to tube until it is collected at the bottom of the stage.
Alternatively, the tubes can be submerged in the feed water.
Principle
A heat source and a heat sink are located at opposite ends of a series of closed chambers that are separated by tube walls in the plant.
The outside of the tubes in stage n and the interior of the tubes in stage n+1 make up the two communicating subspaces that make up each space.
The temperatures and pressures in each area are lower than in the one before it, and the temperatures of the fluids on either side of the tube walls are similar.
A space’s pressure and the temperatures of its walls in both subspaces cannot be in equilibrium.
It has a medium level of pressure.
The water then evaporates because the pressure is too low or the temperature is too high in the first subspace.
The vapor condenses in the second subspace because of excessively high pressure or low temperature.
From the warmer first subspace to the cooler second subspace, this transfers evaporation energy.
In the second subspace, the tube walls transfer energy to the subsequent, colder region by conduction.
Vapor-compression desalination
The VC operates mainly on a small scale, in small locations.
The main mechanism is similar to MED except that it is based on the compression of the vapor generated by evaporating water to a higher pressure, Which allows the reuse of the vapor for supplying heat for the evaporating process.
Membrane desalination
Electrodialysis reversal
It is an electrodialysis reversal water desalination membrane process that has been commercially used since the early 1960s.
An electric current migrates dissolved salt ions, including fluorides, nitrates and sulfates, through an electrodialysis stack consisting of alternating layers of cationic and anionic ion-exchange membranes.
Periodically, the direction of ion flow is reversed by reversing the polarity applied electric current.
Reverse osmosis
Reverse osmosis (RO) is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane.
As a result, the pure solvent is let cross to the other side of the membrane while the solute is kept on the pressured side.
Smaller elements of the solution, like the solvent, should be able to pass through the pores (holes) of this membrane without restriction for it to be considered “selective.”
Advantages and disadvantages of desalination techniques
| Desalination type | Usage | Advantages | Disadvantages |
| Multi-stage flash distillation (MSF) The desalination process distills seawater by flashing a portion of the water into steam in multiple stages of what are essentially regenerative heat exchangers. |
Accounts for 85% of all desalinated water; used since the early 1950s | MSF plants, especially large ones, produce a lot of waste heat and can therefore often be paired with cogeneration | High operating costs when waste heat is not available for distillation. High rates of corrosion |
| Multiple-effect evaporator (MED|ME) Using the heat from steam to evaporate water. Water is boiled in a series of vessels in a multiple-effect evaporator, each held at a lower pressure than the one before it |
Widely used, since 1845 | High efficiency, while relatively inexpensive | A large heating area is required |
| Vapor-compression evaporation (VC) Evaporation method by which a blower, compressor, or jet ejector is used to compress, and thus, increase the temperature of the vapor produced. |
Mainly used for wastewater recovery | The technique copes well with high salt content in water | – |
| Evaporation/condensation Evaporation of seawater or brackish water and consecutive condensation of the generated humid air, mostly at ambient pressure. |
Widely used | The easiest method of distillation | Time-consuming and inefficient in comparison to other techniques |
| Electrodialysis reversal (EDR) Electrochemical separation process that removes ions and other charged species from water and other fluids. |
Widely used, since the early 1960s | Long membrane lifetime and high efficiency (up to 94% water recovery, usually around 80%) | High capital and operational costs |
| Reverse osmosis (RO) The separation process uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. |
Widely used, the first plant was installed in Saudi Arabia in 1979 | Water purification effectively removes all types of contaminants to some extent | Requires more pretreatment of the seawater and more maintenance than MSF plants |
| Nanofiltration (NF) Nanofiltration membranes have a pore size in the order of nanometers and are increasingly being used for water desalination. |
Emerging technology | Very high efficiency | Very high-efficiency High capital cost, an unknown life of the membrane, no large-scale plant built yet |
| Membrane distillation (MD) In membrane distillation, the driving force for desalination is the difference in vapor pressure of water across the membrane, rather than total pressure. |
Widely used | Low energy consumption, low fouling | – |
To take into consideration
1. Cogeneration
Cogeneration is the process of using excess heat from power production to accomplish another task.
Theoretically, any form of energy production could be used.
However, the majority of desalination plants use either fossil fuels or nuclear power as their source of energy.
Most plants are located in the Middle East or North Africa, due to their petroleum resources.
2. Economics
Several factors determine the capital and operating costs for desalination: capacity and type of facility, location, feed water, labor, energy, financing and concentrate disposal
Desalination still now controls pressure, temperature, and brine concentrations to optimize water extraction efficiency.
In places far from the sea, like New Delhi, or high places, like Mexico City, high transport costs would add to the high desalination costs.
One needs to lift the water by 2,000 meters (6,600 ft) or transport it over more than 1,600 kilometers (990 mi) to get transport costs equal to the desalination costs.
Therefore, it can be more cost-effective to bring fresh water from another location rather than desalinate it.
In locations that are both slightly inland and fairly high, like Riyadh, desalinated water is also pricy.
Desalinating water in Singapore costs $0.49 per cubic meter of water.
3. Environmental
One of the primary environmental considerations of ocean water desalination plants is the impact of the open ocean water intake, especially when co-located with plants.
These intakes are no longer viable without reducing mortality, by ninety percent, of the life in the ocean; the plankton and fish eggs.
Other environmental concerns include air pollution and greenhouse gas emissions from power plants.
To limit the environmental impact of returning the brine to the ocean, it can be diluted with another stream of water entering the ocean.
Discharges of brine into seawater have the potential to harm ecosystems, especially marine environments in regions with low turbidity and high evaporation that already have elevated salinity.
Examples of such locations are the Persian Gulf, and the Red Sea, and The UAE, Qatar, Bahrain, Saudi Arabia, Kuwait and Iran have 120 desalination plants between them.
These plants flush nearly 24 tons of chlorine, 65 tons of algae-harming antiscalants used to descale pipes, and around 300kg of copper into the Persian Gulf every day.
