Ocean water can be distilled to make it drinkable. Distillation is a purification method that involves heating water into steam and then cooling it back into a pure liquid form. This process is necessary because seawater contains a high concentration of dissolved salts and other impurities that make it unsafe for human consumption. Purifying this abundant resource is a major factor in providing potable water, especially in arid coastal regions.
Why Ocean Water is Undrinkable
The reason ocean water is toxic to humans is its high salinity, averaging about 35 parts per thousand. This concentration is greater than the human body can safely process, leading to a biological imbalance. When consumed, the high sodium chloride increases the blood’s osmotic pressure, causing the body to draw water out of its cells. This results in cellular dehydration.
The kidneys attempt to excrete this excess salt, but they produce urine that is less salty than seawater. To process the salt load from one cup of seawater, the body must use more than one cup of its own stored water. Drinking ocean water causes a severe loss of bodily fluids, intensifying thirst and potentially leading to kidney failure.
Ocean water also contains a complex mixture of dissolved minerals, microorganisms, and pollutants. These can include heavy metals like mercury, various pathogens, and organic contaminants from industrial runoff.
The Basic Science of Water Distillation
Distillation is a thermal separation technique based on the differing boiling points of water and its dissolved contaminants. The process begins by heating the source water past its boiling point, causing it to transition from a liquid to a gaseous state (steam). This phase change leaves behind virtually all non-volatile substances, such as salts, minerals, and heavy metals, because their boiling temperatures are significantly higher than water’s.
The steam produced is essentially pure water molecules, having left the impurities in the boiling vessel. This purified water vapor is then channeled away into a separate, cooler chamber.
The final step is condensation, where the steam encounters a cool surface and returns to its liquid form. This collected liquid, known as distillate, is highly purified and free of the non-volatile contaminants that were in the original source water.
Practical Methods for Small-Scale Distillation
Individuals needing a small, reliable source of potable water commonly employ two primary distillation methods using simple equipment.
Solar Still
A survival solar still harnesses the sun’s energy, requiring no external fuel source. This method involves placing the source water in a black-lined basin and covering it with a slanted piece of clear plastic or glass. The sun’s heat causes the water to evaporate, and the vapor condenses on the cooler underside of the cover. Gravity causes the pure water droplets to collect in a separate container. While effective, a typical solar still is a slow process, often requiring a large surface area to produce enough water for one person per day.
Stovetop Setup
A stovetop distillation setup is suited for emergency or home use with a readily available heat source. This can be accomplished by boiling the seawater in a covered pot and offsetting the lid to allow the steam to condense into a separate collection vessel. Pre-made stovetop distillers are also available, often utilizing a kettle-like boiler connected to a cooled coil (condenser) to accelerate the phase change.
Large-Scale Thermal Desalination
On an industrial scale, distillation is employed in massive thermal desalination plants to produce millions of gallons of fresh water daily. These facilities utilize sophisticated techniques like Multi-Stage Flash (MSF) distillation.
Multi-Stage Flash (MSF)
In an MSF plant, pre-heated seawater is introduced into a series of chambers, or stages, each maintained at a progressively lower pressure. The sudden drop in pressure causes the hot water to instantaneously “flash” into steam without additional heating. This steam is then condensed and collected as fresh water in each stage, with the released energy used to preheat the incoming seawater.
Multiple-Effect Distillation (MED)
Multiple-Effect Distillation (MED) uses a sequence of heat exchanger tubes called “effects.” The steam generated in one effect is used as the heat source for the next, which operates at a slightly lower temperature and pressure. This sequential reuse of latent heat improves energy efficiency compared to simple boiling. Thermal processes like MSF and MED are still widely used, particularly where feedwater quality is poor or where excess heat from power generation is available.
Handling the Brine Byproduct
A consequence of distillation is the creation of a concentrated waste stream called brine, which is the remaining water containing all the separated salts and impurities. This byproduct is hyper-saline, meaning its salt concentration is significantly higher than the original ocean water. The brine also contains chemical additives used in the plant’s pre-treatment processes, such as chlorine and anti-scalants.
The disposal of this concentrated, chemically altered liquid presents an environmental challenge. The common practice is to discharge the brine back into the ocean through diffusers to aid rapid mixing and dilution. However, this discharge can still raise the local salinity and temperature of the receiving waters, while lowering the dissolved oxygen content. The alteration of these parameters can negatively impact marine ecosystems.