Purification of water

Is matter around us pure of Class 9

River water is normally used to supply drinking water to big cities. This water is unfit for drinking purposes as it contains a large amount of suspended impurities and harmful micro-organisms, such as bacteria and germs. The river water is purified in the following stages -

Sedimentation tank : The water from river or lake is first passed through the sedimentation tank. During this time, many suspended solids settle down at the bottom of the tank.
Loading tank: From sedimentation tank, water is sent to the loading tank. In the loading tank, some alum (phitkari) is added to the water. The colloidal particles of clay present in water are negatively charged. They do not settle down in the sedimentation tank. However, when alum is added, these negatively charged particles get neutralized by the positively charged aluminium ions present in alum. In this way, suspended clay particles get loaded with aluminium ions, become heavy and settle down at the bottom of the tank.
Filtration tank: After the removal of insoluble solids and other suspended impurities, the water is passed through a filtration tank. The filtration tank has three layers. coarse gravel is placed at the bottom of the tank. Above it is placed fine gravel and on the top of gravel is placed a thick layer of fine sand. These layers of sand and gravel act as filters. The impure water is introduced at the bottom. As the water rises above, all the impurities present in water are retained and the pure water leaves the tank at the top.
Chlorination tank: The water obtained from the filtration tank is introduced into the chlorination tank where it is treated with bleaching powder or some other germicides to kill bacteria.

The water obtained after the above treatments is free from all types of impurities and bacteria and is supplied to our homes for drinking purposes.