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How much water is there in the world?

Nearly 75 per cent of the surface of the Earth is covered by sea. This is salty water. There is also a great deal of water on land, but much of it is ice. Less than three per cent of the Earth's water is in lakes, rivers and underground wells.

How is water cleaned?

This involves: screening - to remove large objects; clarification - where chemicals are added to make small particles clump together; filtration - through sand and gravel beds; disinfection - using chlorine; storage - in enclosed concrete reservoirs before being piped to our homes.

Do people in rich countries use more water than people in third world countries?

Yes. In Kenya, tourists use ten times as much water as the people in the villages.

How dangerous is it to drink untreated water?

Very. More people in the world die from water-borne diseases than from any other cause, including wars and famine.

How long could we live without water?

Having water to drink is more important than having food. People have lived for a month without food, but we could only last three or four days without water.

In what ways can we make energy from water?

Twenty per cent of the world's electricity comes from rivers using hydro-electric power. Water wheels also use the energy from river water. Sea water also provides energy. Barrages make energy from the rise and fall of the tides, and wave energy collectors make energy from the sea's waves.