Making saltwater drinkable is a process utilized globally to secure fresh water supplies. This process, known as desalination, involves removing dissolved salts and minerals from saline water, such as seawater or brackish groundwater. Desalination transforms water with high total dissolved solids (TDS) into potable water that meets strict health and safety standards. The technology is increasingly relied upon in arid regions and coastal areas where natural freshwater sources are scarce.
The Physiological Danger of High Salt Concentration
The human body cannot safely process the high sodium chloride concentration found in seawater, which typically measures around 35,000 milligrams per liter (mg/L). Drinking this water triggers a physiological response governed by osmosis, the movement of water across a semipermeable membrane to balance solute concentration. Since seawater is saltier than human blood, consuming it increases the blood’s salinity, creating a state known as hypernatremia.
This high salt concentration draws water out of the body’s cells, including brain cells, as the body attempts to dilute the bloodstream. The kidneys, which excrete excess sodium, are unable to produce urine saltier than seawater. To flush out the high sodium load, the body uses more water than was consumed, accelerating severe dehydration. This places immense strain on the renal system and can ultimately lead to organ failure and death.
Industrial Scale Desalination: The Dominant Methods
To safely produce large volumes of drinking water, modern infrastructure relies on two primary industrial desalination technologies: membrane separation and thermal distillation. The most widely adopted method is Reverse Osmosis (RO), which employs mechanical pressure to separate water from salt. In the RO process, high-pressure pumps force the saline water against a semipermeable membrane, overcoming the natural osmotic pressure.
The membrane is engineered with pores so small—often less than a nanometer in size—that they allow water molecules to pass through while rejecting dissolved salts and other impurities. The resulting fresh water, called permeate, is collected, while the concentrated salt solution, known as brine, is discharged. RO is the most energy-efficient desalination method, contributing to its dominance in new plant construction worldwide.
The other major industrial approach is thermal distillation, which mimics the natural water cycle of evaporation and condensation. In this process, saline water is heated until it vaporizes, leaving the salts behind. The pure water vapor is then cooled, condensing it back into liquid freshwater.
Common large-scale thermal variants include Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED). MED uses a series of interconnected chambers, or “effects,” each operating at a progressively lower temperature and pressure. The steam generated in one effect is used to heat the water in the next, recycling heat and improving energy efficiency. Thermal methods are often more robust to poor feedwater quality compared to RO and can be powered by waste heat from industrial or power generation facilities.
Personal and Emergency Desalination Techniques
In situations where industrial infrastructure is unavailable, such as in a survival scenario, small-scale desalination can be achieved using basic techniques, though the yield is lower. Simple distillation involves boiling the saltwater and capturing the steam, which is free of salt. By collecting this steam and allowing it to cool and condense, a small amount of potable water can be gathered. This method requires a continuous heat source, which can be a limiting factor in an emergency.
A passive technique that requires only sunlight and simple materials is the solar still. This method uses a container of saltwater covered by a sheet of clear plastic, with a collection cup placed underneath. Solar energy heats the water, causing it to evaporate; the vapor then condenses on the underside of the cooler plastic sheet. The condensed fresh water runs down the plastic and drips into the collection cup.
While effective at removing salt, solar stills are slow and produce a low volume of water, typically yielding only one to two liters per day. For a more immediate, albeit still low-volume, solution, specialized hand-pumped Reverse Osmosis devices are available for marine survival kits. These manual pumps force water through a compact membrane, providing a quicker supply of drinking water than a passive still.