Page 166 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

Biological process

• The sludge collected in the physical process is transferred to a separate tank
where anaerobic bacteria (bacteria that proliferate or multiply in the absence

of air) decompose the sludge and convert it into biogas. This biogas is used as a
domestic fuel or to generate electricity.

• Aeration: Air is pumped into the clarified water so that bacteria can proliferate.
• Bacteria consume the human waste, food waste, soap and other unwanted

material in the water. The settled bacteria form the activated sludge. Dried
sludge is used as manure which returns the nutrients back to the soil.

• Water is then removed from the top. This water has very low levels of organic
impurities.

Chemical process
• Chlorination: Water purified through aeration or by natural means is not fit

for human consumption. It needs to be treated with chlorine to kill the germs
that may be left in the water. The treated water can now be released into the

nearest water body.
• Other methods of disinfecting of treated water includes treatment with ozone

and ultraviolet radiations. These also kill the germs present in the water and
make it fit for drinking.

6. How does factory waste contaminate freshwater and make it unfit for drinking?
Ans: Industries produce large amount of waste that contains pollutants and chemical

toxins. Factories dump waste, containing harmful chemicals, including lead, mercury,
sulphur, asbestos, nitrates, etc., into water bodies, without filtering or treating water

to remove toxins from the waste. This dumped waste contaminates the water.
It is harmful for marine life, such as fish and various sea animals. The spilled oil coats

the skin of the aquatic animals and makes it difficult for them to breathe. Feathers
of many birds and fur of mammals get coated with oil, making it heavy for them to

fly or move. It also causes them to freeze, since the feathers and fur cannot protect
them from the cold water temperatures. Plants growing near or underwater are also

harmed due to oil spills. The layer of oil on water blocks the sunlight from reaching
the plants due to which plants cannot perform photosynthesis and will eventually

die.
7. Enlist the various causes of water pollution.

Ans: Causes of Water Pollution:
Industrial waste: Industries produce large amount of waste that contains pollutants

and chemical toxins. Factories dump waste, containing harmful chemicals, including

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