Gate 2016: Jobghost.blogspot.com
provides the Gate-2016 important study materials for Civil engineering for the candidates
who are going to appear for the GATE-2016 examination. Here the “ENVIRONMENTAL ENGINEERING-Water
Purification” topic is explained. And also the GATE-2016 practice sets, Example
problems, previous year Question papers are available in jobghost.blogspot.com.
Water
purification:
The water collected from the sources
has to be treated before used for drinking or domestic purpose. The treatment process sequence is given
below…!
1. Surface Treatment ====> 2.Plain
Sedimentation ====> 3.Coagulation ==== > 4.Filteration =====>
5.Disinfection =====> 6.Distribution
The above sequence of process was done
to make polluted water to potable water.
1. Screening:
Screening is the first process in
treating the polluted water. This process used to remove the large floating
particle such as debris, trees, branches, animals, bushes, etc. Screening
plates are used in various sizes to increase the efficiency of the screening.
For the most efficient screening the screen should be 45o inclined
to the surface.
2.Sedimentation tank:
In this process the suspended coarser
particles are removed from the water which tend to settle down due to gravity
when stored for a period of time or flowing at settling velocity.
There are two sedimentation tanks
available.
a) Intermittent tank:
This tank is also called quiescent
tank. In this tank the water is completely brought to rest. The flowing of
water completely arrested. So the suspended particles settles down due to
gravitational force.
b) Continuous tank:
In this tank the water is not brought
to complete rest. But the velocity of water is reduced. In such a velocity the
suspended particles tend to settle down while travelling from one end to other.
The efficiency of sedimentation is based on this flow length. As travel length
increases the efficiency increases.
So the reduced velocity in which the
suspended particles tend to settle down is called settling velocity.
3.Coagulation:
The coarser size suspended particles
settles down in the plain sedimentation process. But the finer particle will not
settle. So the coagulation is introduced in the sedimentation process.
Coagulation is nothing but the addition of coagulants to the water which forms
a gelatinous participate called “floc” which attracts the fine particles and
make it coarser. So the process of removing the fine suspended particles by
adding flocculating agents is called coagulation. Alum is generally used
coagulants for treating the water. The main disadvantage of this method is it
imparts hardness to the water.
a) Alum: Al2(So4).18H2O
(Aluminum sulphate )
b) Copperas (Ferrous Sulphate+Lime)
c) Chlorinated Coppers (Ferric Sulphate)
d) Sodium sulphate
The water is kept in a tank for 2 to 4
hours(detention time) after the addition of coagulants.
The overflow rate of the coagulation
tank is 1000 to 1250 lit/hr/m2
Floc chamber:
This chamber is used to mix the water
with coagulating agents. After the addition of alum in the mixing basin, it is
gently stirred by paddles fitted in the floc chamber to build up floc.
Generally it mixed up for up to 15 to 40 minutes.
Average dose for normal water=
14mg/lit
pH value for effective function= 6.5
to 8.5
4.Filtration:
The process of removing the very fine
suspended particles, colloidal particles, bacteria, odour, and taste from the
water is called filtration. The filtration is done by using the filter bed
consists of granular material i.e. Filter media.
The various types of filters and
important points are given below…!
Slow Sand Filter: (SSF)
Slow sand filter consist of single
filtering media which is fine sands. And
in this method the filtration is done by gravity. The slow sand filtration is
highly acceptable and it produces well filtered and quality water. So it
requires no further treatment after filtration. Filter media will not allow the
flocculated water through it. So the coagulation process should be skipped
before filtration. It requires cleaning for 1 to 3 months. Cleaning of filter
is done by scrapping and removal of top sand layer of about 1.5 to 3 cm
thickness.
Rate of filtration is very low as 100
to 200 lit/hr/m2
Efficiency of filter is about 98 to
99%
Wash water required is 0.2 to 0.6 of
filtered water
Depth of filter is 2.5 to 3.5 m
Rapid Sand Filter: (RSF)
Rapid sand filters are used to remove
the suspended particles, colour, bacteria etc. The filter media consists of
coarse sand and the size varies from 10 m2 to 80 m2.
Rapid sand filter is highly efficient in colour removal. Filter is operated
till the turbidity break-through reaches the value of 3 NTU. And it removes about
80 to 90% of bacterial load. The pores between sands are large so it must need
coagulation for effective removal of fine particles. Cleaning is done by
agitation and back washing with or without compressed air.
Rate of filtration is 3000 to 6000
lit/hr/m2.
Amount of wash water required is 2 to
5 % of filtered water.
Bed slope of 1 in 100 is provided.
Thickness of Filter bed is 60 cm to 90
cm
Effective size of sand grain is 0.35
mm to 0.50 mm
Uniform coefficient is 1.2 to 1.8
Pressure Filters:
These filters are used for the less
populated area like individual industries, colonies, Private estates, etc. In
this filter the water is forced through the filter media at 3 to 7 kg/cm2
pressure. And the process and working is similar to Rapid sand filter.
Rate of filtration is 6000 to 15000
lit/hr/m2.
Efficiency of filter is about 60 to 70
%.
Uniform Coefficient is 1.2
5.Disinfection:
Disinfection is the process of killing
of disease causing organism like pathogenic bacteria present in the water. The
consumption of these infected water cause water born disease to the human
beings. The killing of these pathogenic bacteria is done in disinfection. It
also assure against future contamination.
Important methods of disinfection are
given below…!
a) Boiling Of water:
It is the most effective method of
disinfection of water. But these cannot be used for huge quantity of public
water supplies. And it should not assure about future contamination. These are
used only for the domestic purposes and emergency purposes.
b) Treatment with Excess lime:
In this method the pH is increased by
adding excess quantity of lime thereby the most of the bacteria will be killed.
The process of removal of excess lime before using it is supplied to the public
is known as re-carbonation.
c) Ozone:
Ozone is used to treat the lesser
quantity of water which such as swimming pool waters. This method of treatment
is more costly than other. The efficiency of disinfection is high.
This method effectively kills bacteria
and as same as above will not safe guard the future population.
d) Ultra Violet rays:
This is very costly technique of
disinfection of water. It is adopted for private buildings, Institutions.
e) Chlorination:
The commonly used method of
disinfection of water for public water supplies. And it ensures the future
re-contamination of water. The main disadvantage is it imparts bad taste to the
water.
Chlorination is not effective if the
water contains more alkalinity.
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