In a survival scenario, being stranded at sea or on a remote coastline presents a threat due to the lack of drinkable water. The vast ocean, while seemingly an endless supply of liquid, is unusable because of its high salt content. All successful methods for making ocean water drinkable rely on distillation, which separates pure water from dissolved minerals.
The Danger of Salt Consumption
Seawater is approximately 3.5% dissolved salt, primarily sodium chloride, a concentration nearly four times higher than the salt content in human blood. When consumed, this hypertonic fluid creates an imbalance in the body’s chemistry. The body attempts to restore balance through osmosis, drawing water out of cells to dilute the salt in the bloodstream.
The kidneys are forced to work overtime to eliminate the massive sodium load. Human kidneys can only produce urine that is less salty than seawater. To flush out the excess salt ingested, the body must use more water than was consumed, resulting in a net loss of fluid. This process accelerates dehydration, leading quickly to dry mouth, muscle cramps, organ failure, and death.
Passive Desalination Using a Solar Still
A solar still uses the sun’s energy for a continuous, low-effort method of desalination based on evaporation and condensation. This technique requires minimal resources, making it suitable for survival where fuel is scarce. The process begins by digging a pit in the ground, about two feet across and one and a half feet deep, or by using a large, dark container.
A smaller, empty collection vessel, such as a cup or can, is placed in the center of the pit or container. Seawater is poured into the outer area, ensuring it does not splash into the collection vessel. A sheet of clear plastic is stretched taut over the opening, with the edges secured by dirt, sand, or rocks to create an airtight seal.
A small stone or weight is placed on the plastic sheet directly above the center of the collection cup, creating a downward sag or funnel. Sunlight heats the water, causing it to evaporate; the water molecules turn into vapor, leaving the dissolved salt and other impurities behind. This pure water vapor rises until it contacts the cooler underside of the plastic sheet, where it condenses back into liquid droplets. These droplets follow the funnel created by the weight and drip directly into the collection vessel below.
This passive method provides a slow but steady yield, often producing between 0.5 to 2.0 liters of fresh water per day under good solar conditions. To increase production, non-poisonous plant matter or urine can be added to the pit, as the still purifies moisture from these sources. A tip is to insert a small tube under the plastic’s edge and into the collection cup, allowing water to be siphoned out without breaking the still’s seal.
Active Method: Boiling and Condensation
When a reliable heat source like a fire is available, an active method of desalination can produce a higher volume of water more quickly than a solar still. This process uses distillation, where boiling the seawater forces the water to become steam, leaving all dissolved solids behind in the boiling vessel. The challenge lies in capturing and condensing the steam before it escapes.
A basic apparatus requires a boiling container, a cover, and a separate vessel to collect the purified water. Seawater is placed in a pot or can over the heat source, and a lid is placed over the opening, often slightly ajar or tilted to direct the steam. The steam must be captured and cooled to condense it back into a liquid.
For a more efficient setup, a tube can be sealed or fitted to the boiling container’s opening, directing the steam away from the heat. This tube must be angled downward and run through a cooling medium, such as wet cloth or cold seawater, to rapidly lower the steam’s temperature. As the steam cools, it turns back into liquid water and drips into a clean receiving container at the end of the tube. This method requires constant management of the heat source and careful monitoring to avoid boiling the container dry, which leaves a hard salt crust inside.
What Does Not Remove Salt
In an urgent situation, people often turn to familiar water purification methods that are ineffective against dissolved salt. Standard physical filtration, such as passing seawater through cloth, sand, or charcoal, removes only suspended debris and large particles. Salt, specifically sodium and chloride ions, is dissolved at an atomic level and is too small to be blocked by mechanical filters.
Similarly, chemical purification methods, including iodine tablets or chlorine drops, are designed to neutralize biological pathogens like bacteria and viruses. These chemical treatments have no effect on the molecular structure of dissolved minerals, meaning the water remains just as salty as before treatment. Relying on these common methods to treat seawater wastes precious time and resources, providing a false sense of security while dehydration progresses. Only distillation, which involves a phase change from liquid to vapor and back to liquid, or specialized reverse osmosis systems, can effectively remove the dissolved salt.