Desalination techniques

In general terms, there are two main types of desalination techniques used worldwide: thermal processes and membrane desalination processes.

Thermal processes

In this case, the feed water is boiled and the vapour condensed as pure water (distillate) the main processes of this type are:

Multi-effect distillation (MED). This is a thermal desalination process that uses multiple stages of evaporation and condensation to convert seawater into freshwater. It exploits thermal efficiency by reusing the latent heat released during steam condensation to heat the seawater in a subsequent evaporation stage.

It is one of the most efficient methods for the production of freshwater from seawater and is used both in large-scale desalination plants, where energy efficiency is crucial and in smaller plants, making it versatile and scalable.

The multi-effect distillation process usually involves several evaporation and condensation units in series, called "effects". Each effect consists of an evaporator and a condenser. Seawater is fed into the first effect, where it is heated to evaporate some of the water and turn it into vapour. The steam generated flows into the condenser of the first effect, where it cools and condenses into fresh water. This process is repeated in multiple effects, and at each stage. Each effect operates at a lower temperature and pressure than the previous one, allowing efficient use of heat.

The novelty of multi-effect distillation is that the vapour condensed in the first effect is not discarded, but used as a heat source for the second effect, allowing for additional evaporation.

Multi-stage flash distillation (MSF). This process is based on the rapid expansion of hot, pressurised seawater in a set of distillation chambers or stages (called "flash chambers"), i.e. the rapid decrease in pressure causes the water to evaporate, separating salt and other contaminants. So each stage has a lower temperature and pressure than the previous one. The vapour generated is condensed in a condensation system and collected as fresh water.

MSF can be less energy efficient compared to MED, as it does not recycle as much latent heat from the steam repetitive pressurisation and depressurisation can lead to energy losses, and heat recovery is limited.

MSF is commonly used in large-scale desalination plants, especially in high-capacity applications requiring significant freshwater production.

Ultimately, MSF and MED are two thermal desalination approaches that use multiple evaporation and condensation stages but differ in terms of energy efficiency and specific applications. MED is known for its higher efficiency and versatility compared to MSF, but the choice between the two will depend on the requirements of the desalination plant and local conditions.

Thermal vapour compression (TVC). In this case, water vapour generated in an evaporation process (such as MED) is compressed using thermal energy instead of mechanical energy. The vapour cools and condenses, releasing latent heat. This latent heat is used to preheat the seawater before it enters the evaporator, thus reducing the amount of energy required to heat the water from the ambient temperature.

Thermal compression can have a significant effect on the overall efficiency of the desalination process, as it allows some of the latent heat to be recovered from the steam and reused rather than dissipated.

Mechanical Vapour Compression (MVC). A mechanical compressor is used to increase the pressure of the vapour generated in an evaporation process. This pressure increase raises the boiling temperature of the water, which facilitates distillation and reduces the amount of energy needed to evaporate the water. The compressed vapour cools and condenses, producing fresh water.

Mechanical vapour compression is common in reverse osmosis systems, where seawater needs to be pressurised to force it through a semi-permeable membrane. The mechanical compressor facilitates this process and improves the efficiency of the system.

Both CVT and CVM are used to increase efficiency in desalination processes by reducing energy consumption. These methods allow the latent heat of steam to be recycled and steam pressure to be increased, respectively, which reduces the energy required to produce freshwater from seawater or brackish water. The choice between one or the other process depends on the technology and the specific configuration of the desalination plant.

Membrane desalination processes

The feed water is pumped through semi-permeable membranes that filter out dissolved solids. The main processes of this type are:

Reverse osmosis (RO). This method is based on the principle of osmosis, which is the natural movement of water from a less concentrated (hypotonic) solution across a membrane to a more concentrated (hypertonic) solution to equalise the solute concentrations on both sides of the membrane.

Reverse osmosis reverses this natural process to remove salt and other contaminants from seawater or brackish water, producing fresh water. This process involves:

1. Pretreatment: Seawater first undergoes a pre-treatment process to remove solid particles, sediment, organic matter and suspended contaminants. This prevents these materials from clogging or damaging the membranes used in reverse osmosis.
2. Pressurisation: Seawater is pressurised to force it through a semi-permeable membrane that selectively retains salt ions and other contaminants, allowing only water molecules to pass through.
3. Reverse Osmosis: Pressurised seawater flows through the reverse osmosis membrane. Due to the high pressure applied, the water flows through the membrane, leaving behind salt and other contaminants on the inlet side of the membrane.
4. Permeate Recovery: The resulting fresh water, known as "permeate," is collected on the opposite side of the membrane and is suitable for use.

Reverse osmosis is widely used in desalination plants, especially in coastal regions where access to fresh water is limited, and is known for its efficiency and ability to produce high-quality fresh water. It is also suitable for applications of different scales, from domestic systems to large-scale desalination plants.

Electrodeionisation (EDI). EDI is a new technology that is a combination of electrodialysis and ion exchange. This desalination process uses membranes and electricity to remove dissolved ions and contaminants from water, producing high-quality fresh water.

EDI is an advanced technology used in desalination plants and industrial applications to treat seawater, brackish water and other types of saline water. This process involves:

1. Membranes: EDI uses ion-selective membranes that allow specific ions to pass through and block others, allowing the removal of dissolved salts and contaminants from the water. These membranes are designed to separate positive ions (cations) from negative ions (anions).
2. Electrodes: Electrodes that generate an electric field are placed in a chamber between the membranes. When an electric current is applied across the membranes, positive ions are attracted to the negative electrodes, while negative ions are attracted to the positive electrodes.
3. Water compression: Feed water, which is seawater or brackish water, is pressurised and made to flow through the membrane chambers. As the water flows through the membrane and electrode system, ions are attracted and accumulate in the concentrate chambers, reducing the concentration of ions in the water.
4. Freshwater production: The resulting freshwater is collected in the chambers between the membranes and can be used as drinking water or for other purposes.

EDI is known to be an efficient and continuous desalination technology, as it does not require periodic chemical regeneration, as in the case of traditional ion exchange desalination. It also has the advantage of producing high-purity freshwater without the need for additional chemicals.

The role of nuclear energy

Desalination consumes large amounts of energy. According to the IAEA (2015), "Only nuclear reactors are capable of supplying the copious amounts of energy needed for large-scale desalination projects". Thus, we can state that the main contribution of nuclear energy in desalination is to provide a constant and efficient source of heat needed to convert seawater into freshwater through thermal processes, such as distillation or reverse osmosis.

The key aspects of the role of nuclear power in desalination are explained below:

1. Constant thermal energy supply: Nuclear energy is used to generate constant heat through nuclear fission (in nuclear fission reactors) or in the future, nuclear fusion (although fusion is still under development). This constant heat source is essential to maintain the temperatures needed in desalination processes, as the production of fresh water from seawater requires the evaporation and condensation of water at high temperatures.
2. Efficiency and low carbon dioxide emissions: Nuclear power is known for its high efficiency and low carbon dioxide (CO2) emissions. This makes it attractive for desalination, as it contributes to reducing the carbon footprint of freshwater production compared to other energy sources that can be more polluting.
3. Large-scale desalination plants: Desalination plants that require large amounts of energy to produce freshwater, such as multiple effect distillation (MSF) and reverse osmosis (RO) facilities, often benefit from nuclear power. Continuous heat production in nuclear power plants is well suited to the operational needs of these large-scale desalination plants.
4. Energy stability and reliability: Nuclear power provides a stable and reliable source of energy that is not dependent on climatic factors, such as solar radiation or wind. This is especially important in arid and coastal regions where desalination is crucial to ensure a constant water supply.