Page 165 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

When excreta is not properly managed, it can contaminate drinking water sources,

soil & food, leading to the spread of these diseases. Additionally, untreated excreta

attracts flies & other insects, which further transmit diseases.
3. Why are bacteria used in sewage treatment plants?

Ans: This is because they naturally break down organic waste in the sewage. They digest
harmful substances & convert them into less harmful products through biological

processes. This helps in reducing pollutants, eliminating pathogens & improving

water quality.
Using bacteria is an efficient, cost-effective & environment friendly method to treat

waste-water & make it safe for discharge or reuse.
4. Explain the relationship between sanitation and diseases.

Ans: Sanitation & diseases are closely related because poor sanitation facilitates the

spread of infectious disease.
Inadequate sanitation leads to the contamination of water, food & soil with pathogens

from human excreta. This can cause outbreaks of diseases such as cholera, dysentry,

typhoid & hepatitis.
Proper sanitation practices, including safe disposal of waste, clean water access &

good hygiene, significantly reduce the transmission of these diseases.

5. Describe the steps involved in getting clean water from wastewater.
Ans: The steps involved in getting clean water from wastewater are:

Physical process
• Filtration: In this process, wastewater is passed through bar screens for removal

of large objects. This process is used to remove rags, sticks, plastic bags etc.

• Reducing size of solid waste: Once filtered, the wasterwater is passed through
a grinder where the solid waste is broken down into small pieces, so it does not

clog the plant.
• Removal of grit and sand: The wastewater is then slowly passed through the

grit and sand removal tank. The flow of water is greatly reduced to allow the

solid waste to settle at the base of the tank.
• Sedimentation: Water is then sent to a sedimentation tank. Solid material, such

as faeces settles at the bottom and is called the sludge. Floatable impurities

like oil, grease, etc., float on the surface of water. A scraper removes the sludge
while the skimmer removes the impurities floating on water. The water is now

called clarified water.

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