The world’s oceans hold nearly 97% of all water on Earth, presenting a seemingly limitless resource for a planet facing increasing fresh water scarcity. However, this vast supply is unusable for drinking or agriculture due to its high salinity, averaging about 35 grams of dissolved salt per liter. This concentration of dissolved minerals is the primary obstacle separating the ocean’s massive volume from the small fraction of fresh water required for human survival. Overcoming this barrier requires understanding the biological consequences of consuming salt water, the complex technology needed to remove the salt, and the significant economic and environmental trade-offs of purification.
The Physiological Impact of High Salinity
The human body cannot process the high salt concentration found in ocean water, which is approximately four times saltier than human blood. When seawater is consumed, the salt is absorbed into the bloodstream, immediately raising the blood’s osmolarity (concentration of solutes). This hypertonic state triggers the body’s natural response to restore balance through osmosis.
The kidneys must work overtime to excrete the influx of sodium and chloride ions. To excrete salt, the kidney’s filtration system requires water to dilute the concentrated urine it produces. Since the urine’s maximum achievable salt concentration is significantly lower than that of seawater, the kidneys must pull water from the body’s cells and tissues to flush the excess salt.
This attempt to normalize the salt balance results in a net loss of water, accelerating dehydration instead of alleviating thirst. The body uses more water to excrete the salt consumed than the water gained from drinking the seawater itself. Continued consumption leads to severe dehydration and electrolyte imbalance, which can cause kidney failure, organ damage, and eventually death.
The Process of Desalination
Since the body cannot remove the salt, technology performs this separation through desalination. The two primary global methods are thermal distillation and membrane separation, with membrane separation being the modern standard. Thermal methods, such as Multi-Stage Flash Distillation (MSF), mimic the natural water cycle by heating seawater until it evaporates, leaving impurities behind, and then condensing the pure water vapor.
The most widespread method is Reverse Osmosis (RO), which relies on a physical mechanism rather than a phase change. Normal osmosis involves water moving across a semi-permeable membrane from a low-salt area to a high-salt area. RO reverses this flow by applying immense pressure, typically between 40 and 82 bar for seawater, to force the water molecules through the membrane.
This pressure overcomes the natural osmotic pressure, pushing only water molecules through the membrane’s microscopic pores while rejecting dissolved salt ions. The output consists of two streams: purified, low-salinity product water and a highly concentrated salt solution known as brine. Although RO efficiency has made it the preferred technique, it introduces practical challenges.
Energy Consumption and Brine Disposal
The energy required to overcome the ocean’s natural osmotic pressure is the primary factor limiting desalination’s universal application. Modern Reverse Osmosis plants still require around 3 kilowatt-hours of energy to produce one cubic meter of fresh water. This high energy intensity translates into substantial operational costs, making desalinated water significantly more expensive than traditional freshwater sources like rivers or aquifers.
The second major challenge is the disposal of brine, the concentrated waste product containing rejected salt and pretreatment chemicals. For every two gallons of seawater processed, roughly one gallon of highly saline brine is produced. When this hypersaline effluent is discharged back into the ocean, it creates a dense, oxygen-depleted plume that sinks to the seafloor.
This dense plume harms local marine ecosystems by increasing the salinity of the receiving waters, which is detrimental to benthic organisms like mussels and crabs. The brine is often warmer than the surrounding ocean water and may contain residual treatment chemicals like copper and chlorine. Safe disposal requires costly infrastructure, such as submerged outfalls and diffusers, to mix the brine effectively and mitigate its ecological impact.
Non-Saline Contaminants in Ocean Water
While salt is the main obstacle, ocean water contains numerous other contaminants requiring complex treatment steps beyond simple desalination. Industrial and agricultural runoff introduce heavy metals like mercury, lead, and cadmium, which are toxic and bioaccumulative. These require specific pre-treatment filtration stages to prevent damage to the delicate RO membranes and ensure the final product water is safe.
Pathogens, including bacteria, viruses, and protozoa from sewage runoff, must be neutralized through chemical disinfection methods like chlorination or ultraviolet light treatment. A growing concern is the presence of microplastics, minute plastic particles that can be as small as a few nanometers. These emerging contaminants require additional, specialized filtration and purification technologies to ensure the final water quality meets stringent drinking standards.