Removing dissolved salt from water presents a unique challenge because simple filtration methods, such as those using activated carbon or sediment cartridges, cannot separate the minuscule salt particles. Traditional industrial desalination relies on energy-intensive processes, typically involving high-pressure membrane systems or large-scale thermal distillation that requires significant fuel. These methods are impractical for home use or in low-resource situations where external power is limited. Fortunately, accessible, low-energy alternatives exploit the physics of water’s phase changes to effectively separate water molecules from salt ions without needing to reach the boiling point. These methods allow for the purification of small quantities of water using basic household tools or readily available solar energy.
Understanding Salt and Water Separation
When common table salt, sodium chloride (\(text{NaCl}\)), dissolves in water, it undergoes dissociation into positively charged sodium ions (\(text{Na}^{+}\)) and negatively charged chloride ions (\(text{Cl}^{-}\)). These individual ions are instantly surrounded by water molecules, which form a protective structure called a hydration shell due to strong electrostatic attraction.
Because these charged, hydrated ions are fully integrated into the solution, they are far too small to be physically blocked by the microscopic pores of standard mechanical filters. Standard filtration works by trapping larger contaminants, but the dissolved salt ions are effectively part of the solvent itself. Therefore, effective salt removal must rely on a process that forces the water to physically transition into a different state, such as a solid or a gas, to leave the salt behind.
Desalination Using the Freezing Method
The freezing method, formally known as fractional crystallization, capitalizes on the fact that salt ions do not fit into the highly ordered crystalline structure of ice. When water freezes, the molecules arrange themselves into a hexagonal lattice that structurally rejects impurities like sodium and chloride ions. This exclusion principle makes the process energy efficient compared to boiling, as the latent heat of fusion for ice is significantly lower than the heat of vaporization for water.
To perform this at home, one should partially freeze the saltwater in an appropriate container, allowing only a fraction of the liquid to solidify from the top down. The remaining unfrozen liquid, known as brine, becomes highly concentrated with the rejected salt. After physically separating the purer ice from the concentrated liquid, melting the ice yields desalinated water. For a significant reduction in salinity, this separation and freezing cycle must be repeated multiple times, as the initial ice still contains some trapped brine pockets that lower the overall purity.
Desalination Using Passive Solar Evaporation
Passive solar evaporation utilizes a simple device called a solar still to mimic the Earth’s natural water cycle. The core mechanism involves using the sun’s thermal energy to cause evaporation and then collecting the resulting condensation. The still’s basin, which should be constructed from or lined with a dark material to maximize solar absorption, holds the saltwater and heats it well below the boiling point. This low-temperature heating causes the pure water to slowly evaporate, leaving all non-volatile contaminants, including salt, minerals, and heavy metals, behind in the basin.
The resulting pure water vapor then rises until it contacts the still’s sloped, clear cover, which is cooler than the air inside the chamber. Upon contact, the vapor condenses back into liquid water droplets that trickle down the angled surface into a separate collection trough or cup. A basic solar still can be constructed using a dark container for the basin, a clear plastic sheet or glass pane for the angled cover, and a small cup positioned beneath the cover’s apex to catch the purified distillate. This method is characterized by a relatively slow output, typically producing only about 1 to 4 liters of purified water per square meter per day, depending on sun intensity. The advantage of this low-temperature process is that it avoids vaporizing certain volatile organic compounds that might be present in the water, unlike high-heat distillation.
Practical Limitations of Simple Filters
The primary reason simple pitcher filters and whole-house sediment filters fail to remove salt is their reliance on mechanical trapping or chemical adsorption. Activated carbon and mesh filters are designed to capture larger particles, chemicals like chlorine, or organic compounds, but they cannot physically block the hydrated sodium and chloride ions.
The only non-thermal filtration technology effective for salt removal is Reverse Osmosis (RO). RO systems apply significant external pressure to force water through a semi-permeable membrane that contains pores as small as 0.0001 microns. This pressure overcomes the natural osmotic flow and actively pushes water molecules past the membrane while rejecting 95% to 99% of the dissolved salts. While highly effective and available for residential use, RO requires specialized equipment and a high-pressure pump to operate, making it a complex engineered solution rather than a simple, low-energy method like freezing or solar evaporation.