Screening

Filtration

Chlorination

Screening: large debris such as branches and twigs are removed from the water by passing it through large screens.

Filtration: smaller dirt particles are removed by filtering the water through beds of sand.

Chlorination: chlorine is added to the water to kill any bacteria.

(Other stages are sedimentation where the water is allowed to settle in order to allow dirt particles to fall to the bottom, and fluoridation where fluorine is added to the water to help prevent tooth decay.)

Because the water may not be treated properly or enough chlorine added and boiling will kill bacteria if they are present.