Desalination is the operation that aims to remove the saline fraction from brackish or marine waters, in order to make them suitable for industrial or potable uses. This is a particularly useful process for arid and semi-arid areas and for islands without adequate water supply. However, the climate and drought crisis we are witnessing is helping to re-discuss the value of desalination even in regions where the water supply is (for the moment) sufficient.
Different desalination processes
The equipment involved in this process is called desalinator. In reality, there are different types of systems, based either on the separation of water from salts or on the separation of salts from water. Without any claim to being exhaustive, the following can be listed:
- evaporative desalination → the separation of water from salts occurs due to the change in the physical state of the water itself that evaporates; the steam produced, subsequently condensed, is constituted by practically pure water.
- Desalination by reverse osmosis → the water resource to be desalted is put in communication with fresh water through a membrane permeable only to the solvent; by applying a higher pressure from the side of the saline solution than that generated by osmosis, the normal sense of diffusion is reversed and the solvent tends to leave the solution with the higher salt content.
- Desalination by ion exchange → the properties of some natural or synthetic substances are exploited to exchange ions, removing the ions Na + and Cl–.
- Freezing desalination → the water separates in the form of ice, leaving a more concentrated saline solution.
- Desalination by electrodialysis → electrolysis is used to accelerate the phenomenon of dialysis (separation of solutes based on their different speed of diffusion), through membranes consisting of ion exchange substances.
The current trend is the result of combining different desalination plants to achieve more satisfactory results and reduce waste.
How is the infrastructure for desalination composed?
The infrastructures necessary for a desalination plant are:
- the intake structure,
- the sea water filtration system;
- the medium or high pressure steam production plant;
- the electrical system to power the pump motors;
- the water purification plant, in case of civil use;
- the storage tanks;
- the pumping plant in the city water supply.
The price of desalinated water
Thanks to technological progress, the costs for desalination operations have decreased in recent years and they are set to decrease further. The price today fluctuates between 0.6 and 1.6 dollars per cubic meter, and the most performing plants manage to go down to 0.50 dollars. Consistent work is currently underway to reduce high energy consumption and improve waste disposal.
The role of renewables
A real turning point in desalination operations will take place thanks to the greater use of renewable energies. In fact, they can favor both sustainability and the reduction of operating costs. The most commonly used renewables in this area are solar and wind power, combined with each other or with traditional sources to ensure the continuous operation of the plant.
The Italian landscape
Thanks to the reduction of costs and the development of renewable energies, desalination in Italy may play a crucial role, also in light of the worsening of drought phenomena and desertification. The country has suitable characteristics to aim for the full development of this process, with several areas subject to chronic water scarcity and one of the largest coastline in the world.
An advantageous operation
To date, desalination in Italy accounts for only 0.1% of total water withdrawals. A fact that must not discourage, but must act as a stimulus for a clear improvement. In fact, it must be considered that in the islands desalination in situ is decisively cheaper than transport. The cost of desalinated water is around 2-3 euros per cubic meter, compared to about 13-14 euros per cubic meter of water transported by ship.
The need for institutional intervention
Desalination can therefore play a leading role in the fight against drought; however, it is necessary that the institutions take action to promote this solution. The measures taken in the summer to combat the climate crisis, including the Salvamare Law, have even penalized the desalination processes, with a worsening of the authorization procedure. The course must be changed to protect and finally promote the right to water throughout the world, which is nothing more than an extension of the right to life.
Sources: treccani, energiamercato.it, rinnovabili.it