Ocean water, or seawater, has an average salinity of about 3.5%, meaning every liter contains approximately 35 grams of dissolved salts, predominantly sodium chloride. This concentration is far too high for human consumption and cannot be made safe simply by heating it over a fire. The process required to remove these dissolved solids is much more complex than standard water purification.
Why Simple Boiling Fails
When water is heated to its boiling point, the liquid turns into steam, a process called vaporization. This action is effective for purifying water by killing harmful bacteria, viruses, and other pathogens that cause disease. However, boiling does not eliminate the salt content present in the water.
The dissolved solids (TDS) have a much higher boiling point than water. As the pure water evaporates away as steam, the solids are left behind in the remaining liquid. Seawater contains over 35,000 milligrams of TDS per liter, which is far beyond the safe limit for drinking.
Because the volume of water decreases while the mass of salt stays the same, the concentration of salinity in the remaining boiled water increases. This resulting liquid is more concentrated than the original seawater, making it more dangerous to consume. Simple boiling purifies the water of biological contaminants but simultaneously worsens the problem of high salinity.
The Health Risks of Drinking Seawater
Consuming highly saline water leads to hypernatremia, an excess of sodium in the bloodstream. This surge of sodium disrupts the delicate balance of fluids surrounding the body’s cells.
The body attempts to restore balance through osmosis, where water moves across cell membranes to dilute the higher solute concentration. Since the blood has a significantly higher salt level than the cells, water is pulled out of the cells and into the bloodstream to equalize the concentration. This action causes cellular dehydration throughout the body, including in the brain.
The kidneys are put under extreme strain as they try to excrete the sodium load. Human kidneys can only produce urine that is slightly less salty than seawater, meaning they must use more water than was initially consumed to process the excess salt. This results in a net loss of body water, worsening the dehydration and leading to kidney failure, coma, and death.
How to Safely Remove Salt
The only way to safely remove salt from seawater is through distillation. This method mimics the natural water cycle by separating the water from the dissolved solids. Distillation involves boiling the saltwater to create steam, which is then captured and condensed back into a liquid form.
A simple survival method involves a basic solar still, which uses the sun’s heat to promote evaporation. Seawater is placed in a dark container under a clear plastic sheet, which traps the heat and encourages the water to evaporate. The resulting vapor condenses on the underside of the plastic and runs down to a separate collection point, leaving the salt behind.
A stovetop still can be created using household items for a more immediate method. This involves boiling the seawater in a covered pot, with the lid positioned slightly ajar to direct the steam into a clean collection vessel. The steam is pure, fresh water, while the original pot retains all the salts and minerals. While industrial processes like reverse osmosis (RO) exist, distillation is the most accessible method for individuals.