Desalination transforms the vast saline resources of the sea into clean, usable freshwater. This process involves removing dissolved salts and minerals from seawater or brackish groundwater to meet the growing global demand for potable water. Desalination offers a reliable water source independent of unpredictable rainfall and is increasingly important in water-stressed regions around the world.
Reverse Osmosis: The Dominant Technology
Reverse Osmosis (RO) is the leading method used globally for desalination, accounting for the majority of the world’s installed capacity. The process begins with thorough pre-treatment to filter out large debris, suspended solids, and organic matter that could damage the sensitive membranes. High-pressure pumps are then used to overcome the natural osmotic pressure of the saltwater.
The principle of osmosis naturally drives fresh water through a semipermeable membrane toward the salty water to equalize the concentration. Reverse osmosis inverts this flow by applying immense pressure—often between 40 and 82 bar for seawater—to push the water molecules in the opposite direction. This external force drives the water through the membrane, leaving the dissolved salts and other impurities behind.
The semipermeable membrane is engineered to allow only water molecules to pass through its microscopic pores. Salt ions attract and hold surrounding water molecules, forming a combined structure effectively larger than a single water molecule. This size difference causes the membrane to block the salt while allowing the pure water to flow through.
Water that successfully passes through the membrane is collected as permeate, or pure water. The highly concentrated salt solution that remains is discharged as brine. The purified water then undergoes post-treatment to adjust its pH and reintroduce beneficial minerals, making it stable and safe for consumption.
Thermal Desalination: Using Heat and Evaporation
Thermal desalination represents the second major category of purification, utilizing heat to mimic the natural process of evaporation. This approach physically separates pure water from salt by converting the saline water into steam, leaving the salt and minerals behind. The two most prevalent thermal methods are Multi-Stage Flash (MSF) distillation and Multiple Effect Distillation (MED).
Multi-Stage Flash distillation involves heating the incoming seawater and then rapidly introducing it into a series of chambers, or stages, each maintained at a lower pressure than the previous one. This sudden drop in pressure causes a portion of the hot water to instantly vaporize, or “flash,” into steam. The resulting pure steam is then condensed back into liquid water on heat exchanger tubes.
Multiple Effect Distillation operates on a similar principle but reuses the latent heat of condensation across multiple evaporative effects to improve efficiency. Steam produced in the first effect heats the seawater in the second effect, which operates at a slightly lower temperature and pressure. This process is repeated across several stages, allowing the plant to produce a greater volume of fresh water for a given amount of initial energy input.
These thermal methods often consume significant heat energy, which is why they are frequently used in co-generation plants. By coupling the desalination process with a power plant, the waste heat generated during electricity production can be utilized to heat the seawater. This integrated approach makes the thermal methods economically viable where a large source of waste heat is readily available.
Managing Outputs: Energy Demands and Brine Waste
Desalination’s large-scale feasibility is directly linked to managing its two primary outputs: energy consumption and brine waste. Both the membrane and thermal processes require substantial amounts of energy, which significantly influences the operating cost and environmental impact. For instance, modern seawater RO plants typically require around 3 kWh of electricity to produce one cubic meter of fresh water.
The high energy demand for RO is mostly attributed to the powerful pumps needed to generate the extreme pressure required. To mitigate this, many modern plants incorporate sophisticated energy recovery devices, such as pressure exchangers. These devices capture energy from the high-pressure brine stream before it is discharged, substantially lowering the overall specific energy consumption of the plant.
The second major challenge is the management of brine, the highly concentrated saltwater by-product that is left over after the freshwater is extracted. This brine stream is hypersaline, containing up to twice the salt concentration of normal seawater. It may also contain residual chemicals used in the pre-treatment process.
Improper disposal of this brine can harm local marine environments, particularly if it is released directly into shallow coastal areas. The increased salinity and higher temperature can negatively affect sensitive ecosystems like seagrass meadows and benthic organisms. To minimize this impact, facilities often employ submerged outfalls equipped with diffusers, which help rapidly mix and dilute the brine with the surrounding seawater.