Water covers over 70% of the Earth’s surface, but the vast majority is ocean water, which is naturally unusable for human consumption. Seawater is approximately 96.5% pure water; the remaining 3.5% is composed of dissolved salts and other substances, primarily sodium chloride, that make it toxic to the human body. Making this abundant water safe and accessible for drinking requires overcoming significant biological, chemical, and economic hurdles.
The Fundamental Barrier: High Salinity
The core problem with consuming ocean water is its high salt concentration, roughly 35 grams of dissolved salts per liter, or about 3.5% salinity. This level is more than three times the salt concentration found in human blood. When consumed, the excess salt enters the bloodstream, immediately creating a severe osmotic imbalance in the body.
Osmosis is the natural process where water moves across a membrane from an area of lower salt concentration to an area of higher salt concentration to achieve equilibrium. Because saltwater has a higher solute concentration than the body’s cells, water is drawn out of the cells and tissues into the bloodstream to dilute the incoming salt. This leads to cellular dehydration, even while drinking, and rapidly increases the body’s overall toxicity.
The kidneys filter out excess salt and waste, but they cannot excrete salt at a concentration higher than that of seawater. To eliminate the high salt load, the kidneys require more fresh water than was initially consumed, forcing the body to pull water from its own reserves. This accelerates severe dehydration, strains the kidneys, and can ultimately lead to kidney failure, high blood pressure, and death.
How We Make Ocean Water Usable: Desalination Methods
Since the salt cannot be consumed, it must be removed through desalination, a process that separates saline water into a freshwater stream and a concentrated salt stream. Two primary technologies dominate large-scale desalination: thermal and membrane-based methods. These processes physically separate the water molecules from the dissolved salts and minerals.
Thermal desalination, such as Multi-Stage Flash Distillation, mimics the natural water cycle by heating seawater until it evaporates into steam, leaving the salt behind. The pure steam is then condensed back into liquid fresh water. Membrane-based methods, primarily Reverse Osmosis (RO), use high pressure to force saline water through a semi-permeable membrane that blocks the larger dissolved salt ions while allowing water molecules to pass.
Reverse Osmosis has become the most widely used desalination technology globally due to its energy efficiency and scalability. However, the RO process still requires significant energy to generate the pressure necessary to overcome the natural osmotic pressure of the seawater. Both the thermal and membrane methods are effective at producing high-quality fresh water from the ocean.
The Practical Limitations of Large-Scale Desalination
Although desalination technology is proven, its widespread deployment is constrained by three major practical limitations, primarily energy consumption and cost. Desalination is an energy-intensive process because it must break the molecular bonds between water and salt, setting a minimum energy threshold. Producing a cubic meter of desalinated water requires a substantial amount of power, making the resulting water more expensive than that from conventional sources.
The high energy demand translates into high capital investment and infrastructure requirements for new plants. Desalination facilities are complex industrial operations that require specialized equipment, extensive pretreatment systems, and large coastal land areas. These construction and maintenance costs, combined with the continuous high cost of energy, mean that desalinated water is generally only cost-effective near the coast and impractical for agriculture.
A significant environmental constraint is the safe disposal of the concentrated byproduct, known as brine. For every cubic meter of freshwater produced, roughly 1.5 cubic meters of brine is created—a highly concentrated saltwater solution that often contains pretreatment chemicals. If this hypersaline waste is discharged back into the ocean without proper mixing and dilution, it can form dense plumes along the seafloor, creating localized “dead zones” that harm or kill marine life.
Additional Hazards Beyond Salt
Even if the challenge of salinity were solved, untreated ocean water contains numerous other contaminants that prevent it from being potable. Seawater carries industrial pollution, including synthetic chemicals, heavy metals like mercury, and persistent organic pollutants. Although these substances are often present in trace amounts, they still require specialized purification steps to ensure human safety.
Ocean water also harbors a range of biological contaminants that can cause immediate illness. Pathogens, including bacteria, viruses, and protists, are naturally present, especially near coastal areas and river mouths where runoff and untreated sewage enter the marine environment. Therefore, even after desalination, the resulting fresh water must undergo extensive purification and disinfection processes to remove these invisible threats, just like any municipal drinking water source.