CBSE Class 7 Answered

Main steps in sewage treatment:

• Screening 

1. Wastewater is passed through bar screens. Large and easily visible objects such as sticks, cans, plastic packets and napkins are removed.

• Grit and Sand Removal

1. Water then enters a grit and sand removal tank. Water is passed slowly to allow sand, grit, and pebbles to settle down.
2. Water is allowed to settle in a large tank which slopes towards the middle. Solids settle at the bottom and are removed with a scraper. A skimmer removes floatable solids.
3. The solid sludge is then transferred to a separate tank where anaerobic bacteria decompose it and produce biogas.

• Aeration                                                                            

1. Anaerobic bacteria are allowed to grow by pumping in air into clarified water. Bacteria consume the unwanted matter still present in the clarified water.
2. From the aeration tank, treated water is sent to another sedimentation tank, where, after several hours, the suspended microbes settle down as activated sludge and the water from the top is removed. The activated sludge is removed using sand drying beds or machines.