Page 177 - Science Class 07

Some mes, waste water enters the pipes of these problems we can adopt sewage treatment
water supply and contaminate this water. This
contaminated water, if consumed, can cause
severe health hazards.
Waste water also percolates through the
surface and reaches the ground water. In this
way, many harmful organic and inorganic
ma er mix up with ground water which leads
to pollu on of ground water. This water cannot
be used for drinking purpose.
Pollu on of a lake or pond by sewage or
fer lisers make it eutrophic. It means disposal
of sewage into lakes or ponds causes high
concentra on of nutrients in them. This process
is referred to as eutrophica on. This lead to
excessive growth of algae. These algae
consume a large part of dissolved oxygen which
results in deple on of O in water. Due to acute
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shortage of O many aqua c plants and animals
2 aera on to reduce pathogens. The process of
die. This water cannot be used for household
purposes.
Eutrophica on
Run-off from fer lisers, animals wastes and
silage contains nitrates and other nutrients
which causes eutrophica on.
SEWAGE TREATMENTS
Over popula on in ci es has given rise to water
consump on. This has created two major
problems— shortage of drinking water and
disposal of sewage. Therefore, to get rid of
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methods. It is done in different phases called
primary treatment, secondary treatment and
ter ary treatment.
Primary Treatment
In this treatment, solid contents of the sewages
are removed by the method of filtera ons by
se ng the sludge by sinking it in large holding
tanks. In this stage, bacterial reduc on is
normally 90% and the viral reduc on is even
more as the viruses mainly a ach themselves
to the solids. Besides faecal solids, oils and
grease floa ng on the top are skimmed off.
Waste water containing solid wastes is called
sludge.
Secondary Treatment
A er the removal of sludge and other solid or
semi-solid wastes, we follow the process of
aera on is done in daylight in large storage
tanks or it may be trickled or sprayed over
banks of stones open to the air and day light.
The pathogen reduc on is a further 99% of
total reduc on.
Water Treatment Process
Ter ary Treatment
This is the final stage of water treatment. In this
method, water obtained, a er primary and
secondary treatment, is passed through clear
quartz tubes exposed to intense ultra violet
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