The presence of dissolved salts in water, known as salinity, presents a significant barrier to its use for drinking, agriculture, and industry. Seawater, for instance, contains an average of about 35 grams of dissolved salt per kilogram of water, a concentration far too high for most applications. The process of removing these dissolved solids, called desalination, is an increasingly important method for securing freshwater supplies globally. Separating the salt ions from the water molecules is an energy-intensive undertaking, which is the primary challenge in making desalinated water widely accessible and affordable.
Understanding the Challenge of Salt Separation
The difficulty in separating salt from water stems from the strong molecular forces at play once the salt is dissolved. When common table salt, sodium chloride, enters water, the ionic bond between the sodium (Na+) and chloride (Cl-) breaks apart. The individual ions are immediately surrounded by water molecules in what is called a hydration shell. Standard physical filtration methods, such as pouring the water through a coffee filter or sediment screen, fail because the dissolved ions are far too small to be trapped. Effectively removing the salt requires either overcoming the attraction between the ions and the water through a phase change, which demands a large input of thermal energy, or forcing the water molecules away from the ions using high mechanical pressure and specialized membranes.
Thermal Methods: Distillation and Evaporation
Thermal desalination methods rely on the principle that water has a much lower boiling point than dissolved salts. Distillation works by heating saline water until it vaporizes into steam, leaving the non-volatile salt and impurities behind in the liquid residue. The pure steam is then collected and condensed back into liquid freshwater.
Modern industrial applications include Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) systems. MSF works by flashing a large volume of preheated seawater into steam by rapidly introducing it into chambers with progressively lower pressures. MED systems achieve efficiency by using the latent heat released from the condensing steam in one chamber, or “effect,” to heat the incoming brine in the next chamber.
Both MSF and MED produce water of high purity but are highly energy-intensive due to the significant heat required for the phase change. MSF plants typically consume 70 to 80 kilowatt-hours of thermal energy per cubic meter of water produced, in addition to electrical energy. Although MED is generally more energy-efficient than MSF, the necessity of a phase change means both methods demand a substantial energy supply.
Pressure-Driven Methods: Reverse Osmosis
Reverse Osmosis (RO) is the most widely adopted and energy-efficient large-scale method for desalination today, accounting for the majority of new installations. The process is named for its opposition to natural osmosis, where water naturally moves across a semi-permeable membrane from a low-salt concentration to a high-salt concentration. In reverse osmosis, a high-pressure pump is used to apply mechanical force to the saltwater side, overcoming the natural osmotic pressure. This force pushes the water molecules against their natural gradient and through a semi-permeable membrane. The membrane acts as a molecular sieve, with pores sized to allow the small water molecules to pass while physically rejecting the larger dissolved salt ions.
Seawater RO systems require immense pressure, often exceeding 55 to 80 bar (800 to 1,200 psi), to force the water through the membrane. Before reaching the membrane, the feed water must undergo extensive pre-treatment to remove suspended solids, bacteria, and other contaminants that could cause membrane fouling and degradation. The high-pressure brine concentrate rejected by the membrane still contains a vast amount of hydraulic energy.
To improve efficiency, modern RO plants incorporate Energy Recovery Devices (ERDs) that capture up to 95% of the energy from this high-pressure reject stream. Devices like pressure exchangers transfer the energy from the outgoing brine directly back into the incoming feed water, significantly reducing the load on the main high-pressure pump. This energy recovery allows modern RO systems to operate with an electrical energy consumption as low as 2 to 4 kilowatt-hours per cubic meter of water produced, making them significantly less energy-intensive than thermal methods.
Small-Scale and Emergency Desalination
For individuals needing to filter salt out of water in a survival or off-grid setting, simple methods based on evaporation and condensation are the most accessible. A solar still is a practical device that uses the sun’s energy for small-scale distillation. This device involves placing a collection cup in a pit, covering the pit with clear plastic, and anchoring the center of the plastic sheet with a small rock directly above the cup.
The sun heats the ground and the saline water, causing water to evaporate and condense on the cooler underside of the plastic sheet. The condensate runs down the plastic, guided by the rock, and drips into the collection cup, leaving the salt behind. While highly effective at purifying water, its output is very slow, typically yielding only a few hundred milliliters to one liter of water over a 24-hour period.
A more immediate, though still small-scale, method is simple pot distillation using a household stove or fire. Salty water is boiled in a pot, and a separate container is positioned to collect the steam as it condenses. This is achieved by placing a bowl inside the pot and inverting the lid to catch the steam, which then drips into the collection bowl. These low-tech approaches highlight the difference in scale and output compared to industrial desalination technologies.