Desalination

Section 1 of 6What desalination is and where it is used

Definition

• Definition Desalination is the removal of dissolved salt from seawater to produce fresh water that is suitable for drinking.

Where it makes sense

• Where it makes sense It is used in countries with little rainfall and few rivers or lakes but with a coastline, for example Saudi Arabia.
• These places have plenty of seawater but very limited natural fresh water.

Two main methods

• Two main methods Distillation evaporates the water and condenses it again, leaving the salt behind.
• Reverse osmosis uses membranes and high pressure.

Section 2 of 6How reverse osmosis works

1

Seawater is pumped at very high pressure towards a semi-permeable membrane.

2

The membrane has pores so small that water molecules can pass through but larger particles, including salt ions, cannot.

3

Fresh water collects on the far side of the membrane and is drawn off for use.

4

The concentrated salty water left behind on the other side is removed as waste.

Section 3 of 6Osmosis and reverse osmosis compared

Feature	Osmosis	Reverse osmosis
Direction of water movement	From a dilute solution to a concentrated one	From a concentrated solution to a dilute one
What drives it	Natural movement across the membrane	High pressure applied from outside
Energy input needed	None	Large amounts to run the pumps
Use	Occurs in living cells	Makes fresh water from seawater

Section 4 of 6Problems with desalination

Drawbacks

Energy and cost

• Energy and cost Both distillation and reverse osmosis use large amounts of energy,
  • so the water is expensive and the process is poorly suited to producing huge quantities.

Fairness

• Fairness The high cost means poorer countries often cannot afford desalination plants.

Greenhouse gases

• Greenhouse gases If the energy comes from fossil fuels, desalination releases more greenhouse gases than conventional water treatment.

Location and pipes

• Location and pipes Plants may be far from towns, so long pipelines must be built, which adds to the cost.

Waste brine

• Waste brine The very salty water left over is a pollutant and must be disposed of carefully to avoid harming sea life.

Section 5 of 6Key terms

Term	Meaning
Desalination	Removing salt from seawater to make fresh water.
Reverse osmosis	Forcing water through a semi-permeable membrane at high pressure so salt is left behind.
Semi-permeable membrane	A thin barrier that lets small molecules such as water through but blocks larger particles and ions.
Distillation	Evaporating a liquid and condensing the vapour to separate it from dissolved solids.
Brine	Very concentrated salt solution left after fresh water has been removed.

Section 6 of 6Exam technique

1

Describing reverse osmosis as the same as osmosis - the water is forced from concentrated to dilute by pressure, the opposite of natural osmosis.

2

Saying the membrane filters out water; it lets water molecules through and blocks the ions.

3

Forgetting the waste brine as an environmental problem.

4

Suggesting desalination is cheap - the main limitation is its energy use and cost.