Wastewater needs to be treated before it can be released into a water body. Wastewater has many impurities and these impurities should be removed by water treatment.
Wastewater from households is treated at the treatment plant to remove the physical, biological and chemical matter. In the physical process, wastewater is filtered to remove large impurities.
In water treatment plants, the layers are substituted by:
Bar screens are screens that prevent the flow of large objects in wastewater.
The grit and sand removal tank may not remove some impurities, such as faeces.
A clarifier is a tank with its central part inclined downwards so as to allow faeces to settle down. This waste is termed as sludge. Sludge is transferred to a separate tank where it is decomposed using bacteria. Thus, if all the physical contaminants are separated from the sewage, then the sewage is termed to be clarified water.
Now this clarified water is passed into an aerator in waste water treatment. An aerator pumps air into the water. After many hours, the bacteria settle at the bottom of the tank as activated sludge. The water present in the top is 95% clean and is let out into a water source, while the activated sludge is dried in a sand bed and is used as manure.
In the chemical process, take a chlorine tablet and put it in a beaker full of water. Observe that water is clean when the tablet dissolves. That is why chlorine tablets or ozone is used to disinfect water in treatment plants.
