The treatment needed to make potable water depends upon the source of the 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:
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.
Chlorine gas turns damp-blue litmus paper red then white.
Chlorine gas is harmful so always use a fume cupboard when working with it.