The vast oceans and seas contain over 98% of the world’s water, but high salinity prevents its direct use for drinking, agriculture, or industry. Saltwater contains dissolved ionic compounds, primarily sodium chloride, making it unsafe for human consumption and corrosive to industrial systems. Desalination is the process of removing these dissolved salts and minerals from saline water to produce potable fresh water. While ancient civilizations used simple distillation, the widespread application of modern desalination began in the mid-20th century. Today, two primary methods dominate the industry: thermal separation, which uses heat, and reverse osmosis, which uses pressure and membranes.
The Principles of Thermal Separation (Distillation)
Thermal separation, commonly known as distillation, mimics the natural water cycle by heating saltwater to create pure water vapor, leaving the salt and other impurities behind. This process relies on the fact that water has a significantly lower boiling point than the dissolved salts, which do not vaporize at these temperatures. The resulting steam is then collected and cooled in a separate chamber, condensing into fresh, desalinated water.
Large-scale industrial applications of this principle include Multi-Stage Flash (MSF) and Multiple-Effect Distillation (MED). MSF involves heating saltwater and then introducing it into a series of chambers, or stages, each operating at a progressively lower pressure. The rapid drop in pressure causes the superheated water to “flash” or instantly vaporize into steam, which is then condensed.
Multiple-Effect Distillation (MED) uses a sequence of stages, reusing the heat from the steam of the previous stage to warm the next. In MED, incoming saltwater is sprayed over heated tubes, and the resulting vapor heats the next stage, which is kept at a lower temperature and pressure. This successive reuse of latent heat makes MED generally more energy-efficient than MSF. Both thermal methods produce very high-purity water, sometimes required for specific industrial applications.
The Mechanics of Reverse Osmosis
Reverse Osmosis (RO) is the most common modern method for large-scale desalination. Unlike natural osmosis, where water moves from a low-salt concentration to a high-salt concentration across a semi-permeable membrane, RO applies mechanical pressure to reverse this flow. This pressure must exceed the natural osmotic pressure of the saltwater, often requiring between 40 to 82 bar (600 to 1200 psi) for seawater.
The process relies on a semi-permeable membrane that acts as a highly selective barrier. The membrane’s pores are small enough to allow water molecules to pass through but reject larger dissolved salt ions, bacteria, and viruses. The high-pressure pump forces the water across the membrane, separating the feedwater into two streams: the purified fresh water (permeate) and a highly concentrated salt solution (brine).
Effective operation of an RO system requires extensive pre-treatment of the saltwater to prevent membrane fouling. Suspended solids, organic matter, and other particles must be removed through filtration steps, such as ultrafiltration, before the water reaches the RO membranes. A significant challenge is the disposal of the concentrated brine, which is typically returned to the ocean. This return stream must be carefully managed, often by diluting it before discharge, to prevent localized environmental harm to marine ecosystems.
Practical Small-Scale and Emergency Methods
For individuals, small communities, or emergency situations, desalination methods focus on accessibility and portability rather than industrial scale. One accessible technique is the solar still, which uses the same principles as thermal distillation. A solar still uses sunlight to heat saltwater held within a sealed container, causing it to evaporate. The water vapor then condenses on a cooler surface, such as a clear plastic sheet, and is collected in a separate trough.
While solar stills are highly feasible in sunny environments and require no external power source, they produce relatively small quantities of fresh water over a long period. Another practical solution involves emergency desalination kits, often based on hand-pumped Reverse Osmosis systems. These portable watermakers use manual effort to generate the high pressure needed to force seawater through an RO membrane.
Hand-pumped RO units are compact and produce potable water at a faster rate than solar stills, making them suitable for marine or survival use. These small-scale systems require regular maintenance, including cleaning and replacing the filters and membranes, to ensure effectiveness.