Making saltwater safe to drink requires desalination, which removes dissolved minerals and salts. Seawater contains an average salinity of about 35,000 parts per million, or 3.5% salt by weight. This high concentration must be reduced significantly to make the water potable, as the human body cannot process such a heavy mineral load. Purification is accomplished by separating the water molecules from the salt. This article explains the biological consequences of drinking seawater and details the most accessible methods for making it safe.
The Physiological Danger of Drinking Seawater
The primary hazard of drinking seawater is that it causes severe dehydration. Human cells maintain a delicate balance of water and dissolved salts. Seawater is a hypertonic solution, meaning it has a much higher salt concentration than blood. The body attempts to equalize this imbalance through osmosis, which draws water from inside the body’s cells into the bloodstream to dilute the excessive sodium. This cellular water loss leads to symptoms of severe dehydration, including muscle cramps and confusion.
The kidneys are unable to excrete urine saltier than seawater, forcing them to use more water than was initially consumed to flush out the excess sodium. For every liter of seawater ingested, the kidneys must produce approximately one and two-thirds liters of urine to eliminate the salt load. This net water loss rapidly depletes the body’s fluid reserves, accelerating dehydration. The resulting high sodium levels in the blood, known as hypernatremia, place strain on the kidneys and can lead to kidney failure and death.
Distillation: The Accessible Method of Desalination
Desalination works by exploiting the difference in boiling points between water and salt, a process known as distillation. When saltwater is heated, the water turns into steam, leaving all dissolved salts and impurities behind. The salt molecules are too heavy to evaporate, ensuring the resulting steam is pure water. This pure vapor is then collected and cooled, condensing back into liquid form free from the high mineral content of the original seawater.
Distillation requires creating a sealed system where the water vapor can be collected without re-contacting the untreated saltwater. The key to successful desalination is ensuring that only the steam is captured, not tiny droplets of boiling saltwater carried upward during the boil. This principle is applied in various setups, from simple stovetop methods to passive solar devices.
Simple Boiling and Condensation
One accessible method uses common kitchen items, often called the pot-in-pot method. Seawater is placed in a large pot, and a clean, smaller collection container is positioned inside, ensuring it does not float or tip over. The large pot is covered with an inverted lid, which acts as a condensation surface. The lid is placed so the center knob or high point is directly over the collection container. As the seawater gently boils, steam rises and condenses on the cooler inverted lid, dripping down the center into the clean container.
A cold compress, such as ice or cold water, can be placed on the outside of the inverted lid to increase the temperature differential. This cooling accelerates the condensation rate, maximizing the freshwater collected. It is important to maintain a gentle simmer rather than a hard boil to prevent splashing, which could contaminate the collected water. The heat must be turned off and the collection container carefully removed before the saltwater level runs too low.
Solar Still Construction
In situations without a direct heat source, a solar still can passively use the sun’s energy for desalination. This method requires a container for the saltwater, a collection cup, clear plastic sheeting, and a small weight like a rock. The saltwater is poured into the container, and the collection cup is placed centrally inside, ensuring its lip is above the saltwater level. The plastic sheet is stretched tightly over the container’s opening, creating a sealed environment.
The final step involves placing the small rock in the center of the plastic sheet, positioned directly over the collection cup. The sun heats the saltwater, causing it to evaporate. The resulting vapor condenses on the underside of the cooler plastic sheet. The weight causes the plastic to sag slightly, directing the condensed freshwater droplets to run down to the center point and drip into the collection cup. This process is slower than boiling but provides a continuous, hands-off source of desalinated water.
Post-Desalination Treatment and Storage
Desalination removes salts and minerals, but it does not guarantee the removal of all biological contaminants or volatile organic chemicals. The collected water must undergo a final purification step to ensure it is safe to drink, especially if the source water was polluted. Even though the water came from steam, the collection process introduces a risk of recontamination from the equipment or the air.
For this reason, secondary purification is recommended, typically by boiling the collected water for a full minute to kill any remaining bacteria or viruses. If the collected water is visibly cloudy or contains fine particles carried over with the steam, a simple filter, such as a fine mesh cloth or activated charcoal, can be used to strain it. Finally, the purified water must be stored immediately in a clean, sealed container to prevent further contamination.