Desalination is the process of removing salt and other minerals from seawater or brackish water to make it suitable for human consumption. Seawater is unusable because its high concentration of dissolved salts, primarily sodium chloride, is far beyond what the human body can safely process. Drinking highly saline water rapidly dehydrates the body as the kidneys use more water to flush out the excess salt than is consumed. Desalination involves specialized techniques that separate water molecules from dissolved salt ions, providing a reliable source of drinking water independent of natural freshwater reserves.
Separating Salt from Water: The Scientific Challenge
The reason standard water filters cannot purify seawater lies in the fundamental difference between suspended solids and dissolved ions. Conventional filtration media, such as those found in pitchers or camping filters, are designed to trap larger particles like dirt, sediment, bacteria, and cysts. These filters operate based on size exclusion, physically blocking anything larger than their pore size, which is typically measured in microns.
Salt does not exist as a solid particle in water; instead, it dissolves completely, dissociating into individual sodium and chloride ions. These ions are extremely small, existing at the atomic level, and are measured in the nanometer range. To capture these dissolved ions, a filter would need pores approximately 0.0001 microns in size, far finer than any standard filter. Desalination must rely on processes that either exploit a phase change, like turning water into vapor, or employ a highly specialized barrier, such as a semipermeable membrane, to separate the water molecules from the salt ions.
Thermal Desalination: Simple Distillation Methods
Thermal desalination, a process that mimics the natural rain cycle, is the oldest and simplest method for separating water from salt. This technique, known as distillation, involves heating the saline water until it evaporates, leaving behind the non-volatile salt and mineral residue. The resulting water vapor is collected and cooled, condensing back into purified liquid water. The energy required for this phase change is the primary operational cost.
A basic solar still is a practical, low-tech application of thermal distillation, often used in survival or remote settings. This device uses solar energy to heat a basin of seawater covered by an angled, transparent surface, such as plastic sheeting or glass. Water vapor rises and contacts the cooler surface, where it condenses as fresh water and runs down a collection channel. While this method is effective at removing nearly all dissolved solids, its output is very low, making it impractical for large-scale needs.
For larger operations, industrial thermal methods like Multi-Stage Flash (MSF) or Multi-Effect Distillation (MED) optimize energy use by repeatedly reusing the heat of condensation. These systems often utilize a vacuum to lower the boiling point of water, requiring less energy input to create steam. The core principle remains consistent: the water changes phase from liquid to gas, leaving the salt behind, and then changes back from gas to liquid, resulting in highly pure water.
Membrane Desalination: Reverse Osmosis
Modern, large-scale desalination relies heavily on membrane technology, most notably reverse osmosis (RO). This method operates by applying immense pressure to saline water to force it through a semipermeable membrane. The membrane’s structure is designed to allow the passage of water molecules while physically rejecting the salt ions and other dissolved solids.
The process is named for its reversal of natural osmosis, where water would normally move from a low-salt concentration to a high-salt concentration side to balance the solute levels. In RO, a high-pressure pump must overcome the natural osmotic pressure of the seawater, requiring a force between 40 to 82 bar (600 to 1200 psi) for typical ocean water. This pressure pushes the water molecules against their natural tendency and through the membrane pores, which are less than a nanometer in diameter.
Reverse osmosis is highly efficient, typically removing over 99% of dissolved salts, and is the technology used in municipal desalination plants worldwide. Smaller, portable RO units are also used on marine vessels and in disaster relief, though they still require significant energy to maintain the necessary high pressure. The system produces two streams: the purified water, called permeate, and a concentrated brine solution containing the rejected salts and contaminants, which must be safely discharged.
Ensuring Water Safety and Purity
Once the salt has been removed, the desalinated water requires further treatment before it is considered safe and palatable for consumption. The water produced by both thermal distillation and reverse osmosis is nearly pure, meaning it lacks the natural minerals found in freshwater sources. This pure water is chemically aggressive, or corrosive, and can leach materials from the pipes and containers in a distribution system.
To counteract this corrosivity and improve flavor, remineralization is necessary, which involves adding back specific minerals like calcium and magnesium. This is often accomplished by introducing carbon dioxide and lime into the water, which restores the water’s chemical stability and helps meet public health standards. Blending the desalinated water with a small amount of treated, mineral-rich source water is another common method to achieve the desired balance.
Finally, the product water must be disinfected to guarantee that any microorganisms or pathogens are neutralized. While the salt removal processes are effective, a secondary disinfection step, such as chlorination or ultraviolet (UV) light exposure, provides a final barrier against biological contamination. This post-treatment ensures the water is not only salt-free but also non-corrosive, mineral-balanced, and microbiologically safe for distribution and drinking.