Sodium chloride, commonly known as salt, is a naturally occurring compound that, when dissolved in water, results in salinity. Its presence can stem from natural sources like seawater or brackish groundwater, or from human activities such as road salting or water softening. Removing sodium chloride is necessary because elevated levels affect the taste of drinking water and pose health concerns for individuals on low-sodium diets. Effective purification methods are also required for specialized industrial processes and in regions facing water scarcity, where desalination is necessary to produce potable water.
The Challenge of Dissolved Salt
Removing dissolved salts is fundamentally different and more challenging than filtering suspended solids because of how sodium chloride interacts with water at a molecular level. When salt dissolves, the strong polarity of the water molecule overcomes the ionic bonds holding the sodium and chloride ions together. Water molecules surround the individual ions, a process called hydration, effectively separating them completely.
Each positively charged sodium ion (Na+) and negatively charged chloride ion (Cl-) becomes encased in a “hydration shell.” This shell stabilizes the ions, making them behave as individual, extremely small charged particles. Separating these ions from the water requires methods that either induce a phase change, like evaporation, or utilize specialized membranes that block particles at the ionic level.
Industrial Desalination Techniques
Industrial-scale removal relies on highly engineered processes that perform either a phase change or separation via pressure.
Reverse Osmosis (RO)
The most widespread method globally is Reverse Osmosis (RO), which uses high pressure to force water molecules through a semi-permeable membrane. This membrane has extremely small pores, allowing water molecules to pass while rejecting the hydrated sodium and chloride ions, achieving a salt removal rate of 95% to 99%. The pressure required is significant, needing to overcome the natural osmotic pressure. For seawater, operating pressure can range from 40 to 82 bar (600 to 1200 psi). This mechanical separation process is the preferred technology for producing large volumes of fresh water.
Thermal Distillation
Thermal distillation mimics the natural water cycle by evaporating water and then condensing the resulting steam. When water is boiled, the sodium and chloride ions do not vaporize, leaving them behind in the residual liquid. Modern industrial plants, such as Multi-Stage Flash (MSF) or Multi-Effect Distillation (MED), reuse thermal energy by operating in a series of vessels at progressively lower pressures and temperatures, which improves efficiency.
Electrodialysis (ED)
Electrodialysis (ED) uses an applied electrical field to remove salt ions. The system uses alternating anion-exchange and cation-exchange membranes placed between two electrodes. Under the influence of the electric field, positive sodium ions are drawn toward the negative electrode, and negative chloride ions are drawn toward the positive electrode, leaving behind desalinated water. ED is often more cost-effective than RO for treating brackish water with lower salt concentrations.
Small-Scale and Household Purification Methods
For individuals seeking to remove sodium chloride at home or in small-scale situations, options are limited to miniature versions of industrial methods.
Distillation is the most straightforward physical process, involving boiling water and collecting the condensed steam. This method effectively separates the salt from the water, but it is slow and requires significant energy, making it impractical for large daily volumes. Simple solar stills can also be constructed to harness the sun’s energy to evaporate water, providing a salt-free product in small quantities.
Household Reverse Osmosis (RO) systems are the most effective and popular option for point-of-use salt removal, typically installed under a kitchen sink. These compact units use a pump or household water pressure to force water through a specialized membrane, removing 95% or more of dissolved salts. Standard household filters, such as activated carbon blocks or pitcher filters, do not possess the fine pore size necessary to separate the dissolved sodium and chloride ions.
Managing the Brine Byproduct
All methods of sodium chloride removal inevitably result in a concentrated saline solution known as brine, which poses an environmental management challenge. Brine is the highly concentrated wastewater stream containing all the rejected salt and minerals from the original source water. Disposing of this hyper-saline waste stream without causing ecological damage is a major consideration in desalination.
For coastal desalination plants, the most common disposal method is discharging the brine back into the ocean, often mixing it with regular seawater to dilute the concentration before release. Inland plants must use alternatives such as deep-well injection into geological formations or evaporation ponds. Advanced strategies, collectively known as Zero Liquid Discharge (ZLD), involve processes like thermal evaporators or crystallizers to recover the remaining water and produce solid salts for industrial use, eliminating liquid waste discharge.