Desalination (Cambridge International IGCSE Environmental Management)

Desalination: distillation

• Desalination is the process of removing salt from seawater to produce potable (safe-to-drink) water

• Two main methods are used:

  • Distillation
  • Reverse osmosis

• Distillation involves heating seawater until it boils

  • Salt and impurities do not evaporate and remain in the liquid
  • The water turns into steam, which is pure water vapour
  • Steam rises and leaves the salty water behind
  • The steam is then cooled and condensed
  • Turns back into liquid fresh water

• Produces potable water, safe for drinking

Salt and water can be separated by distillation

Desalination: reverse osmosis

• Reverse osmosis involves forcing seawater through a partially permeable membrane using high pressure

  • The membrane allows small water molecules to pass through
  • It blocks salt ions and most other dissolved substances

• Produces fresh water on one side of the membrane

• Leftover salty water (brine) must be safely disposed of

*Only water molecules can get through the partially-permeable membrane *

Benefits & limitations of desalination

Benefits of desalination

• Provides a reliable source of drinking water in dry or water-poor regions

  • Useful where rainfall is low or rivers and lakes are limited

• Not affected by drought, because the ocean is a constant supply

• Can supply large populations, including major cities

• Helps countries reduce dependence on imported water or overused rivers

The advantages of desalination

Limitations of desalination

• Requires large amounts of energy, making it expensive to run

  • Especially for distillation, which involves heating water

• Produces brine, a very salty waste product

  • Must be disposed of carefully to avoid harming marine ecosystems

• Building and maintaining desalination plants is costly

• Not all countries can afford the infrastructure needed

• Reverse osmosis membranes need regular cleaning or replacing, increasing costs

The disadvantages of desalination