Sources Of Potable Water

Section 1 of 7What potable means

Safe to drink

• Safe to drink Potable water has been treated so that it is safe for people to drink;
  • it is not the same as pure water.
• It may contain dissolved minerals, which are harmless and give water its taste.

Pure water

• Pure water Pure water contains only H₂O molecules and nothing dissolved.
• It can be tested because it boils at exactly 100 °C and freezes at exactly 0 °C.

Requirements

• Requirements pH between about 6.5 and 8.5.
• Only very small, regulated amounts of dissolved salts.
• Free from harmful bacteria and other microbes.

Section 2 of 7Where fresh water is found

Rain

• Rain Rainwater has few dissolved substances, but it collects dust and pollutants as it falls.

Surface water

• Surface water Collects in lakes, rivers and reservoirs, where it is stored for later use.

Groundwater

• Groundwater Held underground in aquifers, which are porous rocks.
  • This is the main source in drier areas such as south-east England.

Section 3 of 7Treating fresh water

1

Choose a suitable source for the local climate and rock type.

2

Pass the water through a wire mesh to remove large debris such as twigs.

3

Run it through beds of sand and gravel to filter out fine particles.

4

Sterilise it with chlorine, ozone or ultraviolet light to kill microbes.

5

Test the water and, where needed, adjust the pH before it is pumped to homes.

Section 4 of 7Desalination of salt water

Method	How it works	Drawback
Distillation	Water is boiled, the steam is condensed and collected, and the salt is left behind	Needs a lot of energy
Reverse osmosis	Salt water is forced at high pressure through a membrane that holds back salt ions	Requires pressure, energy and costly membranes

Section 5 of 7Choosing a method

Providing potable water

Plenty of rain and rivers

• Plenty of rain and rivers Filtration and sterilisation of fresh water is cheap and uses little energy.

Dry coastal country

• Dry coastal country Desalination is the only large supply, so the energy cost is accepted.

Waste water

• Waste water Sewage is treated separately by screening, sedimentation and microbial digestion before being released into rivers.

Section 6 of 7Key terms

Term	Meaning
Potable water	Water that is safe to drink.
Aquifer	A layer of porous rock that stores groundwater.
Sterilisation	Killing microorganisms, for example with chlorine.
Desalination	Removing dissolved salts from sea water.
Reverse osmosis	Forcing water through a membrane to separate it from dissolved salts.
Distillation	Boiling a liquid and condensing the vapour to separate it from dissolved solids.

Section 7 of 7Exam technique

1

Using 'potable' and 'pure' as if they mean the same. Drinking water usually contains dissolved minerals.

2

Thinking filtration removes dissolved salts. It only removes insoluble particles.

3

Forgetting that desalination is expensive because of its energy use.

4

Saying chlorine filters the water. It kills microbes.

5

Writing that rainwater is pure. It contains dissolved gases and dust.