Desalination is the process of removing salts and other minerals from water, transforming saline water sources into freshwater suitable for various uses. This technology plays a significant role in addressing global water scarcity, especially in regions with limited natural freshwater resources. By making seawater and brackish groundwater potable, desalination enhances water supply resilience for communities worldwide.
Simple Methods for Removing Salt at Home
Boiling is an accessible method for removing salt from water. When saltwater is heated, water evaporates into steam, leaving salt behind. This steam then cools and condenses into liquid, which can be collected as freshwater. For example, boiling saltwater in a pot allows condensed steam on the lid to drip into a separate vessel.
Solar stills harness the sun’s energy for desalination. Solar radiation heats impure water, causing it to evaporate. The resulting water vapor rises and condenses on a cooler surface, such as a slanted cover, before pure water droplets slide into a collection trough. This method separates clean water from dissolved salts without requiring external power.
Freezing can also separate salt from water, as pure water freezes before salt does. As saltwater cools, pure water molecules crystallize into ice, excluding salt into the remaining liquid brine. The ice crystals can then be separated from the concentrated brine and melted to yield freshwater. This process requires significantly less energy than boiling methods.
Industrial Desalination Technologies
Reverse Osmosis (RO) is a common industrial desalination technology that uses pressure to separate water from dissolved salts. Saline water is forced at high pressure through a semi-permeable membrane. This membrane allows water molecules to pass through while blocking salt ions. The result is desalinated water and a concentrated brine stream containing the rejected salts.
Multi-Stage Flash (MSF) distillation is a thermal desalination method. It involves heating seawater and introducing it into a series of chambers, each at progressively lower pressures. As water enters each stage, the reduced pressure causes a portion to rapidly vaporize, or “flash,” into steam. This steam is then condensed to produce freshwater, while the remaining concentrated brine moves to the next stage.
Electrodialysis (ED) uses an electrical current and specialized membranes to remove salt ions from water. An electric field draws charged salt ions through selective ion-exchange membranes. This selective movement of ions creates a desalinated water stream and a separate concentrated brine stream.
Important Factors in Desalination
Desalination processes require significant energy, especially at industrial scales. Thermal distillation methods are more energy-intensive, while membrane-based technologies like reverse osmosis generally use less. Energy can account for 25% to 40% of the overall cost of producing desalinated water. Advances in technology have significantly reduced energy consumption for RO over recent decades.
The cost of desalinated water varies depending on factors like energy sources, plant size, and location, typically ranging from $0.80 to $2.50 per cubic meter. While desalination offers a reliable water source, the economic investment for construction and operation can be considerable.
Disposal of the concentrated salt solution, or brine, is an important consideration. This byproduct is saltier and warmer than ambient seawater. If improperly discharged, it can negatively impact marine ecosystems by increasing salinity and altering water chemistry. Effective management strategies, such as diluting the brine or selecting appropriate discharge locations, are necessary to minimize these environmental effects.
Desalinated water quality is high, but it often lacks beneficial minerals. Therefore, post-treatment processes adjust the water’s chemical composition, adding minerals like calcium and alkalinity. This ensures the water is stable, palatable, and safe for consumption. Re-mineralization and disinfection ensure the water meets potable standards and prevents issues like pipe corrosion.