water purification process: screening, coagulation, sedimentation, filtration- rapid & slow sand filter, disinfection, aeration

2. 1) SCREENINNG
• Screening is the first unit
operation used at water supply
treatment plants .
• Screening removes objects such
as rags, paper, plastics, and
metals to prevent damage and
clogging of downstream
equipment, piping, and
appurtenances.
• Some modern water treatment
plants use both coarse screens
and fine screens

4. COARSE SCREEN (Trash Racks)
• Used to exclude the large sized particles, such as
debris, animals, tress branches, bushes etc.
• Coarse screen consist of parallel iron rods placed
vertically or at a slope(45 to 60 deggre), at about 2.5
to 5cm apart
FINE SCREEN
• The fine screen are usually made of woven mesh with
opening not more than 6mm sq.
• The coarse screen first remove the bigger floating
bodies and the organic solids and the fine screen
remove the fine suspended solids.
• The fine screen normally get clogged and are to be
cleaned frequently

5. • The material which is collected on the upstream side
of the screens is removed either manually or
mechanically.

6. 2) Sedimentation tank
• The basin in which the flow of the water is
retarded is called the settling tank or
sedimentation tank or sedimentation basin or
clarifier.
• Sedimentation tank are generally made up of
reinforced concrete and may be rectangular or
circular in plan.
• The theoretical average time for which the water
is detained in the tank is called the detention
period.

7. Principle of sedimentation tank
• Impurities remains suspended in moving water due
to turbulence.
• Turbulence is retarded by offering storage to the
water, the impurities whose specific gravity is higher
than the water get settled down under gravity

8. Factors affecting settling of particles in tank
• The velocity of the flow which carry the particle
horizontally.
• The grater the flow area, the lesser is the velocity,
and hence more easily the particle will settle
down.
• The viscosity of the water , the viscosity varies
inversely with the temperature. Less viscous
water offer less resistance to settlement.
• Size , shape and Specific Gravity of the water,
greater is the specificc gravity more readily the
particle will settle

9. Solution to these factors:
• 1) Viscosity : Not controllable
• 2) Velocity of the flow: Increase the length of
the basin, that leads to increase the detention
period and particle will settle down easily
• 3) Size of the particle: Addition of certain
chemicals (Coagulation), the size of the
particle can alter.

11. a) Rectangular tank
b) Circular tank

12. c) Hoppered bottomed tank

13. Parts of Rectangular Sedimentation tank (Horizontal Flow)

14. • Direction of the flow in the tank is substantially
horizontal.
• they may be provided with mechanical scrapping
devices to scrap the sludge to the sludge pit located
usually towards influent end, from where it is
continuously or periodically removed without
stopping the working of the tank.
• Such tanks are known as continuous flow type of
sedimentation tank.
• In such a tank the flow velocity is only reduced and
the water is not brought to complete rest.
• The velocity is so design that the time taken by the
sand particle to travel from one end to other is
slightly more than the time required for settlement
of that particle

15. Parts of Circular Sedimentation tank (Radial flow)

16. • Water enters at the centre of the tank into a
circular well provided with multiple ports ,
from which it spread out to flow radially
outward in all direction equally
• the water flow horizontally and radially from
the centre towards the periphery of the
circular tank
• the sludge is scrapped to the central sump
mechanically and continuously from where it
is withdrawn during operation.
• sludge removal mechanism consists of scraper
blades mounted on two or four arms revolving
slowly

17. Parts of Hopped Bottom Sedimentation tank
(vertical flow)
Blades of Scraper

18. • They may square or circular in plan and may
have hopper bottom
• The influent enters at the bottom of the unit
• The upflow velocity decreases with the
increase cross-section area of the tank
• the clarified water is withdrawn from the
circumferential or central weir

19. influent
effluent

20. influent
effluent
V0
L
B

21. Sedimentation with coagulation
• Coagulation is defined as the addition of a
chemical to a colloidal dispersion to which
result in particle destabilization by the
reduction in forces, which tend to keep
particles apart.
• This will increase the size of the particle by
changing them into flocculated particles and
make them easily to settle down in the
sedimentation tank during normal detention
period.
Process: The certain chemical compound called coagulants which on
thorough mixing form a gelatenous precipitate called floc. The very fine
colloidal particles present in water get attracted and absorbed in these flocs,
forming the bigger sized flocculated particles.
The use of coagulants is generally necessary for clarifying raw water containing
turbidity greater than 30 to 50mg/ltr.

22. Chemical Used for Coagulation
1. Alum
2. Iron salt (Copperas)
3. Sodium aluminate
4. Chlorinated copperas
5. Lime
These chemicals are highly effective when water is slightly
alkailine.
In the absence of such alkalinity external alkalies like
Sodium carbonate or lime is added to the water so as to
make it slightly alkaline to increase the effectiveness of
the coagulants.

23. Parameters for effective use of coagulants
• Ph:The optimal Ph range for coagulation is 6 to 7 when using alum
and 5.5 to 6.5 when using iron.
• For high alkalinity water, excessive amounts of coagulant may be
needed to lower the Ph to optimal pH range.
• It may be beneficial to use acid in addition to the coagulant to
reduce the amount of coagulant needed and effectively lower
chemical cost.
• Optimal dose: It is important to determine the optimal dose for
coagulation-
insufficint dose will not effectively destabilize the particles and
adding excessive doses can cause re-stabilization, excessive sludge
production or corrosion.
• Temperature: It also affects the coagulation process because lower
temperature water can decrease the hydrolysis and precipitation
kinetics.

24. Mixing Devices
• This violent agitation of water can be achieved
by means of mixing device, such as
– Centrifugal pumps
– Compressed air
– Mixing basins
Mixing basin with baffle walls Mixing basin equipped with
mechanical devices

25. Different mixing basin
Mixing basin with baffle walls

26. Flocculation tank or Flocculator
• The best floc will form when the mixture of water and
coagulant are violently agitiated followed by a relatively slow
and gentle stirring to permit build up and agglomeration of
the flocc particles.
• From the mixin basin, the water is therefore taken to a
flocculation tank called flocculator where it is given a slow
stirring motion.

27. Filtration
• Removal of fine suspended particles and bacteria to produce
potable water, the water is filtered through the beds of fine
granular material, such as sands .
• The process of passing the water through the beds of such
granular materials (called filter) is known as filtration.
• Filtration may be help in removing color, odour, turbidity, and
some pathogenic bacteria from the water.
1) The slow sand gravity filter
Types of filter:
2) The rapid sand gravity filter

28. Slow sand gravity filter

29. Slow sand gravity filter
• They can remove much larger percentage (98 to 99 %) of
impurities and bacteria from the water as compared to what
can be removed by rapid sand gravity filter.
• Slow sand filter yield a very slow rate of filtration
(100 to 200 liters per hour per m2 of filter area)
Therfore, Required large area and large volume of filtering
material for treating large scale supply
• Very costly and uneconomical for large supply
• Best suited for smaller plants
• These filter also removes odours and tastes, particularly those
caused by presence of organic impurities such as algae and
plankton.
• Less efficient in removing colors of raw water.
• Remove turbidity up to 50mg/ltr.

30. Rapid sand gravity filter

31. • The rate of filtration is 3000-6000 ltr/m2/hr
• Back wash system is provided to clear the filter.
• Cleaning of filter is easy, takes around 10 to 15
minutes and done after every 24 to 28hrs
• Rapid gravity filter compared to slow sand filter
are less efficent in removing bacteria and turbidity
• Efficiency with respect to bacteria removal is 80 to
90%, the remaining bacteria are removed in
disinfection
• It is highly efficient in colour removal when used
with coagulation sedimentation
Rapid sand gravity filter

32. Disinfection
Physical process
Ultraviolet radiation
heating
Chemical process
Adding chemicals
Disinfection
• Disinfection may be define as the process of killing of
inactivation of harmful micro- organisms in water
either by physical process or chemical process.

33. Chlorination (Major method)
• Chlorine in its various forms is invariably and almost
universally used for disinfection public water supplies.
• It is cheap
• Reliable
• Easy to handle
• Easily measurable
• It is capable of providing residual disinfecting effects
for long period in the distribution system.
• Disadvantage: Only When used in greater amount, it imparts
bitter and bad taste to the water.

34. Minor Method of disinfection
• Boiling of water
• Treatment with excess lime
• Treatment with ozone
• Treatment with iodide and bromine
• Treatment with ultraviolet rays
• Treatment with potassium permanganate
• Treatment with silver

35. Aeration
• It is The special treatment which may be given to the
water under special condition for removing colors,
taste and odours from it.
• Under the process of aeration, water is brought in
intimate contact with air, so as to absorb oxygen and to
remove carbon dioxide gas.
• It may also help in killing bacteria to certain extent.
• Method Of aeration:
• SPRAY NOZZEL
• AIR DIFFUSION
• CASCADES
• TRICKLING BEDS

36. Spray nozzles

37. Cascade Aerators

38. By air diffusion

39. TRICKLING BEDS