Waste water Story - Revision Notes
CBSE Class 7 Science
Revision Notes
Chapter – 18
Wastewater Story
Wastewater: Black-brown water which is rich in lather , mixed with oil that goes down the drains from skins, showers, toilets, laundries is called wastewater.- Wastewater is generated in homes, industries, agricultural fields and in other human activities. This is called sewage.
- Sewage is a liquid waste which causes water and soil pollution.
- Wastewater is treated in a sewage treatment plant.
- Treatment plants reduce pollutants in wastewater to a level where nature can take care of it.
- Where underground sewerage systems and refuse disposal systems are not available, the low cost on-site sanitation system can be adopted.
- By-products of wastewater treatment are sludge and bio gas.
- Open drain system is a breeding place for flies, mosquitoes and organisms which cause diseases.
- We should not defecate in the open. It is possible to have safe disposal of excreta by low cost methods.
- Sewage Treatment:
- Aeration: Air is bubbled through the wastewater while it is continuously stirred.
- Filtration: Aerated water passes through a deep filter of layered sand, fine gravel and medium gravel.
- Chlorination: Chlorine is added and mixed to the filtered water until water is clear.
- Wastewater Treatment Plant (WWTP):
- Wastewater passes through screens to remove large objects.
- To go to a grit and sand removal tank at low speed.
- Water is allowed to settle in large tank.
- Floating solids are removed with skimmer. Settled solids (sludge) are removed with scraper.
- Clear water is called clarified water.
- Water is then decomposed by anaerobic bacteria in a tank and air is passed.
- Microbes settled at bottom as activated sludge and water from top is removed.