Water sources across the globe often contain dissolved salts, a characteristic known as salinity. This refers to the total concentration of all dissolved salts, with sodium chloride being the most common. This dissolved salt content significantly influences the water’s chemical and physical properties, including its density and electrical conductivity. High levels of dissolved salts make water unsuitable for drinking or many other uses. Desalination is a process designed to remove these mineral components from saline water, making it suitable for human consumption, agriculture, or industrial applications.
Understanding Salt in Water
Salt, such as common table salt (sodium chloride), is an ionic compound where positive sodium ions are bonded to negative chloride ions. Water molecules (H₂O) possess polarity: the oxygen atom has a slight negative charge, and hydrogen atoms carry a slight positive charge. This polarity makes water an effective solvent.
When salt is introduced to water, water molecules are strongly attracted to the oppositely charged ions in the salt crystal. The positive hydrogen ends surround negative chloride ions, while the negative oxygen end attracts positive sodium ions. This electrostatic attraction overcomes the ionic bonds holding the salt crystal together, causing the ions to separate and disperse evenly.
Because salt dissolves at a molecular level, forming individual ions, conventional filtration methods cannot remove it. A standard filter traps larger impurities but allows these microscopic dissolved ions to pass through. Therefore, separating dissolved salts requires processes that address these molecular interactions, typically involving a change of state for the water or specialized membranes.
Desalination Methods for Home Use
One practical method for removing salt from water at home is distillation, which mimics Earth’s natural water cycle. This process involves heating water until it turns into steam, leaving salts and other impurities behind, then collecting the condensed, purified water. To perform this, place a smaller collection bowl inside a larger pot filled halfway with saline water.
Heat the larger pot to a simmer, allowing steam to rise. An inverted lid, often with ice on top, causes the steam to condense into pure water droplets. These droplets trickle down the lid’s concave surface and collect in the smaller bowl, yielding distilled water. This method effectively removes minerals, heavy metals, and microorganisms because they do not evaporate with the water.
Another accessible method is solar distillation, which uses the sun’s energy to evaporate water. A solar still consists of a basin containing impure water, covered by a transparent material like glass or plastic. Sunlight passes through the cover, heats the water, causing it to evaporate. The water vapor then condenses on the cooler underside of the transparent cover and flows into a collection channel, leaving contaminants in the basin. Solar stills are useful in remote areas with ample sunlight and limited access to freshwater.
Industrial Scale Water Purification
For large-scale water purification, such as municipal or industrial use, more advanced technologies are employed. Reverse Osmosis (RO) is a widely adopted method, accounting for approximately 85% of global desalination plants. This process works by applying high pressure to force saline water through a semi-permeable membrane. The membrane allows water molecules to pass through while blocking dissolved salts and other impurities.
RO systems require pressure to overcome natural osmotic pressure, which is water’s tendency to move from a less concentrated solution to a more concentrated one across a membrane. The result is two separate streams: purified freshwater and a concentrated brine solution containing rejected salts. This method is favored for its efficiency and ability to treat water for various industrial needs, including power generation and mining.
Multi-Stage Flash (MSF) distillation is another industrial method, though its market share has decreased compared to RO due to higher energy consumption. MSF involves heating seawater and introducing it into a series of chambers, or stages, each maintained at progressively lower pressures. As hot water enters each stage, the sudden pressure reduction causes a portion to “flash” into steam. This steam is then condensed to produce freshwater, while the remaining concentrated brine moves to the next stage where the process repeats.
The Global Importance of Desalination
Desalination has emerged as a solution to address water scarcity in many parts of the world. Regions with limited natural freshwater resources, particularly arid coastal areas, rely on desalination plants to provide drinking water for their populations. For instance, the Middle East and North Africa account for a substantial portion of global desalination capacity.
Beyond drinking water, desalinated water supports agricultural activities, especially where traditional irrigation sources are insufficient. While historically expensive for agriculture, technological advancements have made it more viable for high-value crops and in regions facing severe drought. Desalination also plays a role in various industrial processes, providing a reliable water supply for sectors like power generation, petrochemicals, and pharmaceuticals. This technology ensures operational continuity for industries, reduces their reliance on diminishing groundwater, and supports sustainable industrial development.