Water for drinking

Water treatment

The treatment needed to make potable water depends upon the source of the water.

Surface water

Surface water is removed from rivers, lakes, streams or ponds and then treated. There are many steps to this process, but these are the main ones:

  1. screening - removes large solid items.
  2. clarification - allows solids to settle to the bottom.
  3. filtration - this removes small particles that are suspended in the water.
  4. chlorination - kills any microorganisms in the water by adding chlorine.
Water is purified by filtration, sedimentation and the addition of chlorine.


Water that fell as rain thousands of years ago remains trapped underground. Some may be reached by digging wells or by boring into deep underground aquifers.

This water has already been filtered as it passed through the layers of sand and rock. To make it potable, the water just needs to be chlorinated to kill any microorganisms in it.


Water demand is very high in some parts of the UK. During a drought, there is a risk that demand might be greater than the supply, leading to water shortages.

Wastewater from the sewers can be made safe by treatment. It can then be returned to the river system, where it can later be extracted and used again.

  1. Screening - removes large solids.
  2. Sedimentation - allows solids to settle to the bottom.
  3. Aeration - adding air and using biological treatment with bacteria. Any organic matter is broken down by the bacteria. Aeration provides the oxygen needed by the bacteria.
  4. Final settlement - allows any remaining very fine particles to settle before the water is returned to the river.

Testing for chlorine

Chlorine gas turns damp-blue litmus paper red then white.

Chlorine gas is harmful so always use a fume cupboard when working with it.

A glass jar contains chlorine gas and a piece of damp-blue litmus paper.A test for chlorine