Desalination removes dissolved salts and minerals from saline water, converting unusable sources like seawater or brackish groundwater into freshwater for drinking, agriculture, or industry. This expands available freshwater resources and secures water supplies in areas with limited access to conventional sources.
Distillation: Evaporation and Condensation
Distillation creates fresh water from saline sources by heating saltwater until it vaporizes, leaving behind dissolved salts. The pure water vapor then rises, cools, and condenses into liquid freshwater.
For small-scale or emergency use, boiling saltwater and collecting steam that condenses on a cooler surface, like a lid, separates water from contaminants. This collected condensate is purified water.
Large-scale distillation processes, such as Multi-Stage Flash (MSF) and Multiple Effect Distillation (MED), refine this principle for greater efficiency. MSF heats seawater and introduces it into a series of chambers, each at progressively lower pressures. The reduced pressure causes hot water to “flash” into steam, which is then condensed to collect freshwater.
MED systems operate through multiple stages, or “effects,” reusing latent heat from steam generated in one stage to evaporate water in the next, lower-pressure stage. This cascading energy reuse improves overall energy efficiency. While effective at producing highly pure water, distillation processes require substantial energy input, primarily as heat.
Reverse Osmosis: Pressure Filtration
Reverse Osmosis (RO) is a widely used membrane-based desalination technology. It uses pressure to force saltwater through a semi-permeable membrane, which permits water molecules to pass while blocking larger salt ions and other dissolved impurities.
Natural osmosis involves water moving from lower to higher solute concentration across a semi-permeable membrane. RO applies external pressure to the concentrated saltwater side, overcoming this natural osmotic pressure. This force reverses the flow, pushing pure water molecules through the membrane.
RO membranes are thin-film composite membranes, often made of polyamide, offering high salt rejection and good water permeability. These membranes are configured in a spiral-wound design to maximize surface area. Pre-treatment filters remove larger particles and impurities from feed water, protecting the membranes from fouling.
Compared to thermal distillation, reverse osmosis requires less energy, as it does not rely on heating water to its boiling point. It uses electrical energy to power high-pressure pumps that overcome osmotic pressure. Energy consumption depends on feedwater salinity, with seawater requiring pressures between 40 and 82 bar (600–1200 psi).
Other Approaches and Their Use
Beyond distillation and reverse osmosis, other desalination methods offer different advantages and applications. Solar stills provide a simple, low-tech way to produce small quantities of fresh water, useful in remote areas or survival situations. These devices harness solar energy to evaporate water.
A basic solar still consists of a sealed, dark-bottomed basin containing saltwater, covered by a sloped transparent sheet. Sunlight heats the water, causing it to evaporate, leaving impurities behind. The water vapor condenses on the cooler underside of the cover and trickles into a collection channel.
Freezing desalination leverages the principle that salt separates from water during freezing. When saline water freezes, pure water molecules form ice crystals, while salt remains concentrated in the unfrozen brine. The ice crystals can then be separated and melted to yield fresh water.
This method has a theoretical advantage of lower energy consumption compared to thermal processes because the latent heat of fusion for freezing is less than the latent heat of vaporization for boiling. However, separating ice from concentrated brine and ensuring the ice is free of adhering salt can be challenging, limiting its widespread commercial application.
The applicability of these desalination methods varies based on scale, energy availability, and specific water needs. Large-scale municipal freshwater production often relies on reverse osmosis plants due to their energy efficiency and high output. Thermal distillation plants are also used, sometimes integrated with power generation facilities to utilize waste heat.
Smaller RO units are common for marine vessels or residential use, providing on-demand purified water. Distillation, while energy-intensive for large volumes, can be practical for smaller quantities or when high purity is paramount. Simple solar stills are best suited for individual or small-group survival, providing basic water purification without external power.