Pre and primary treatment of waste water – Equalization – Neutralization – Sedimentation – Oil separation-sour water strippers – Floatation – Coagulation, precipitation and metals removal– coagulation – Heavy metals removal – Aeration and mass transfer; mechanism of oxygen transfer – Aeration equipment – Air stripping of volatile organic compounds.

1. UNIT-II
WASTE WATER
TREATMENT PROCESSES

3. INTRODUCTION
• Wastewater treatment is the process of
improving the quality of wastewater and
converting it into an effluent that can be either
returned to the nature or incorporated to the
water cycle with minimum environmental issues
or that can be reused.
• The end user may be drinking, industrial water
supply, irrigation, river flow maintenance, water
recreation or many other uses, including being
safely returned to the environment.

4. INDUSTRIAL WASTEWATER
PRETREATMENT WORKS
Waste water Pretreatment
The term “pretreatment” means the treatment of
wastewater by commercial and industrial facilities
to remove harmful materials before being
discharged to a sewer system under the control
of a publicly owned wastewater treatment plant.
If you fail to properly treat your water or
improperly manage your discharge, you could
incur fines and possible legal action.

5. PRIMARY TREATMENT OF INDUSTRIAL
EFFLUENTS
• It is of general nature and is used for removing
suspended solids, odour, colour and to neutralize
It involves methods of:
(i) Screening
(ii) Neutralization
(iii) Equalization
(iv) Sedimentation
(v) Coagulation
the high or low pH.

6. SCREENING
• It is a process through which large materials
like wooden pieces, metal pieces, paper, rags,
pebbles, fibers etc. are removed.
TYPES OF WASTEWATER SCREENS
Coarse screens
fine screens
micro screens

7. COARSE OF SCREENING IN
WASTEWATER TREATMENT
Coarse screens have clear openings ranging
from 6 to 150 mm (0.25 to 6 in).
Coarse screens consist of parallel bars, rods or
wires, wire mesh or perforated plates with
openings generally of circular or rectangular
shapes.

8. COARSE OF SCREENING IN
WASTEWATER TREATMENT

9. Based on the wastewater screening
method used to clean them, coarse
screens are classified into two types
1. Hand cleaned coarse screens
• Used in the screening process in wastewater
treatment at small facilities, hand cleaned coarse
screens are hand raked. They are ideal to use as a
standby during periods of high flow, or when
more modern mechanical screening methods are
under repair or maintenance.
2. Mechanically cleaned screens
• Mechanically cleaned coarse screens increase
efficiency and reduce problems in the wastewater
treatment process.

10. Mechanically cleaned screens can be
classified into the following four main
categories
1. Chain Driven Screens
• These front and back chain driven screens can
rake from upstream or downstream. An
automatic chain cleans the stream, increasing the
functionality of the entire wastewater treatment
solution.
2. Catenaries Screens
• These front return, front cleaned chain driven
screens use impressive, yet straight forward
internal mechanics to prevent further jamming in
the presence of large or heavy objects.

11. 3. Reciprocating Rakes:
• Also known as a climber screen, these
wastewater treatment screening solutions use
one rake rather than multiple, making them less
efficient when facing heavy loads during the
screening process in water treatment.
4. Continuous belt screen:
• Ultra-high tech, functional and efficient, this type
of screening has many rakes and is continuous
and self-cleaning, whether facing fine or coarse
solid loads.

12. MICROSCREENS FOR WASTEWATER
SCREENING

13. MICROSCREENS FOR WASTEWATER
SCREENING
The smallest type of screening in wastewater
treatment is micro screening shown in fig .
These screens are typically low-speed drum
screens
The drums are lined with filtering fabrics with
openings of 10 to 35μm.
Wastewater enters the drum, and the
retained solid waste is collected and disposed
of waste.

14. FINE SCREENING IN WASTEWATER
TREATMENT PLANTS
The screening process in water treatment
plants employs screens that have clear
openings less than 6mm called fine screens.
They are made of wire cloth, wedge wire or
perforated plates.
Like micro screens, they are tools for
screening in wastewater treatment that are
used to remove fine solids.

15. Three common types of fine screening in
wastewater treatment are
• Drum Screens (rotating cylinders in the
flow channel)
• Step Screens (fixed and movable plates
across the width of the channel)
• Static Wedge Wire Screens (used by
large treatment plants with ample floor
space)

16. NEUTRALIZATION
• When pH of the industrial waste is too high or
too low then it should be neutralized by acid
or alkali and only neutral effluent should be
discharged into the public sewer.
(a) Lime stone treatment
• For acidic effluent, lime stone should be used
as it will form calcium compounds [CaCl2,
CaBr2, Ca (NO3) or CaSO4] depending upon
the presence and amount of acid.

17. (b) Caustic soda treatment
• Although it is costly method but it is also
utilized for neutralizing the acid.
• Here caustic soda is added in the effluent to
make the pH neutral. Only small amount of
caustic soda is needed for this work.

18. For neutralization of alkaline effluent
the following techniques are used
(a) Carbon dioxide treatment
• If factory is producing carbon dioxide then only this
method should be utilized for neutralizing the pH
otherwise it would be costlier affair. Here CO2 is
passed in alkaline effluent to make its pH almost 7.
(b) Sulphuric acid treatment
• This is a common method of neutralizing alkaline
effluent. Here sulphuric acid is added in the effluent till
pH becomes almost 7.
(c) Utilizing waste boiler – Flue gas
• The stack gas which contains about 12% carbon dioxide
is utilized to react alkaline effluent to make it neutral.

19. EQUALIZATION
• When effluent is discharged from factory then
its pH along with the quantity of suspended
solids, dissolved solids etc. vary from the
beginning to the last depending upon the
dilution, velocity and the amount of reactants
etc.

20. Equalization Tanks
The equalization tanks are provided
(i) to balance fluctuating flows or
concentrations
(ii) to assist self neutralization, or
(iii) to even out the effect of a periodic "slug"
discharge from a batch process.
Types of Equalization Tanks
Equalization tanks are generally of three types:
• Flow through type
• Intermittent flow type
• Variable inflow/constant discharge type

21. SEDIMENTATION
This treatment is only employed for the
settlement of suspended particles by gravity.
This technique is only used in the beginning to
settle down the solid particles in a high
suspension effluent
When a thick layer of sediment continues to
settle, this is known as consolidation.
When consolidation of sediment, or sludge, is
assisted by mechanical means then this is
known as thickening.

22. There is a variety of methods for applying
sedimentation and include:
Horizontal flow
Radial flow
Inclined plate
Ballasted flocculation and flocculation blanket
sedimentation.

23. OIL SEPARATION
An oil water separator is a piece of
equipment used to treat wastewater,
making it safe to discharge into an
approved discharge point, such as a
sewer.
It removes oils, grease and
hydrocarbons, leaving only the non-
hazardous water.
The wastewater can then dispose of
safely drainage.

24. The four common types of oil water
separators are
Coalescing plate separators -remove oils,
grease, and hydrocarbons from wastewater
Vertical gravity separators -Vertical gravity
separators operate by controlling both fluid
velocity and pressure.
Hydro cyclone separators -Hydro cyclone oil
separators work by sending wastewater through
a 'cyclone chamber', which applies extreme
centrifugal forces.
Petrol and oil interceptor pits -These systems
typically feature two to three compartment gravity
flow systems which work on the premise that
hydrocarbons, petrol, and diesel float above water

25. Sour water strippers
• The sour water stripper removes
hydrogen sulfide and ammonia from the
sour water generated in the refinery.
• The sour water is received from the
refinery in the flash drum, where light
hydrocarbons are flashed off.
• The sour water is then fed to the feed
prep tank, where the feed is mixed and
stabilized

26. Process of Sour water
strippers

27. • The sour water is then heated in the
feed/bottoms exchanger and fed to the stripper
column.
• Steam, generated in the re-boiler, heats the
water and strips the hydrogen sulfide (H2S) and
ammonia (NH3) from the water.
• The stripped water from the column is cooled in
the feed/bottoms exchanger and in the stripped
water cooler, and returned to the refinery.
• The H2S and NH3 removed from the sour water
is cooled in the pump-around cooler system or
in an overhead condenser system and sent to
the sulfur recovery unit for further processing

28. FLOATATION
• Flotation is a separation technique that
employs the use of gas bubbles as a transport
medium.
• Suspended particulate matter that is
hydrophobic or has been conditioned to be
hydrophobic attaches to the bubbles and flows
in the opposite direction of gravity towards the
aqueous solution surface

29. Removal of micro plastics
• Micro plastics (plastic particles less than 5
mm) are removed from effluent in four municipal
wastewater treatment plants using various
modern final stage treatment technologies,
including flotation.
• For the fast removal of antibiotics from water, a
coagulation-flotation approach (containing an
anionic surfactant and a cationic polyelectrolyte)
can be used.
• A common application for flotation is oily
wastewater (perhaps for density reasons).

30. Flotation of Metal Ions
• Ion flotation is the act of eliminating surface-
inactive ions from aqueous solutions using
surfactants or collectors, usually an ion with
the same charge as the metal ion to be
eliminated.
• Ion flotation is used to extract metal ions
from solutions containing low concentrations
of heavy metal, which can be produced by
any industrial process, including metal
working, semiconductor, and metal industries,
as well as mine water

31. Types of flotation
• Flotation can be divided into three categories: natural,
aided and induced flotation.
1. Natural Flotation
• Natural flotation is valid if the density has difference
which is naturally sufficient for separation.
2. Aided Flotation
• Aided flotation takes place when external means are
used to promote the separation of particles that are
naturally floatable
3. Induced Flotation
• Induced Flotation takes place when the density of
particles is artificially reduced to allow particles to
float. This flotation is based on the capacity for certain
solid and

32. • 4. Dissolved Air Flotation (DAF)
It is a method of induced flotation with very fine
air bubble or micro bubbles which are up to 40
to 70 microns.

33. COAGULATION
• Coagulation is the chemical water
treatment process used to remove solids
from water, by manipulating electrostatic
charges of particles suspended in water.
• This process introduces small, highly
charged molecules into water to
destabilize the charges on particles,
colloids, or oily materials in suspension

34. • For dealing waters with such impurities a
chemical process was evolved.
• This process removes all these impurities
within reasonable period of 3 — 4 hours.
• This chemical process is called coagulation
and the chemical used in the process is called
coagulant.

35. Principle of Coagulation
a. Floe formation
b. Electrical charges
a. Floe formation
When coagulant is added to the water and
thoroughly mixed, it produces a thick insoluble
gelatinous precipitate.
This precipitate is called floe.
The floe has the property of arresting the
suspended impurities in water during its
downward settlement towards the bottom of the
tank.

36. b) Electrical charges
• The flock ions are electrically charged
(positive) while all the colloidal particles have
negative charge.
• Therefore floes attract the colloidal particles
and cause their removal easily by settlement
at bottom of the vessel in which it is used.

37. Coagulants
The chemicals given below can be used as coagulants either alone or in combination
1. Sodium aluminates.
2. Sodium aluminates +
Aluminium sulphate.
3. Aluminium sulphate.
4. Sodium aluminate + Ferric
chloride.
5. Aluminium chloride (but used
under exceptional circumstances
only).
6. Aluminium sulphate + caustic
soda.
7. Ferric chloride alone.
8. Aluminium sulphate + hydrated
lime.
9. Polyelectrolyte.
10. Ferrous sulphate.
11. Copper sulphate.
12. Sodium aluminate + Magnesium
chloride.
13. Copper sulphate + hydrated lime.
14. Ferric sulphate.
15. Aluminium sulphate + Sodium
carbonate.
16. Ferric sulphate + hydrated lime.
17. Ferrous sulphate + hydraed lime.
18. Ferrous sulphate + chlorine.
19. Potassium permanganate +
ferrous sulphate.
20. Magnesium carbonate.

38. In water treatment plants following are the usual
coagulants most commonly used
1. Ferrous sulphate and lime.
2. Magnesium carbonate.
3. Polyelectrolyte.
4. Aluminium sulphate.
5. Sodium aluminate.
6. Chlorinated copper

39. Dry fed Devices

40. Dry powder of coagulant is filled in the conical
hopper.
The hoppers are fitted with agitating plates which
prevent the chemical from being stabilized.
At the bottom of the hopper a revolving helical
screw or the toothed wheel is fixed.
The rotation of the helical screw or the toothed
wheel is regulated through a venturi device in the
raw water pipe.
When more discharge is passed through the
venturi device, the rotation of the screw or
toothed wheel gets increased and more
coagulant is thrown in the water

41. Dry fed Devices

42. Mixing Channel of dry fed device

43. HEAVY METAL REMOVAL
• The presence of heavy metals in wastewater has
been increasing with the growth of industry and
human activities,
• e.g., plating and electroplating industry,
batteries, pesticides, mining industry, rayon
industry, metal rinse processes, tanning
industry, fluidized bed bioreactors, textile
industry, metal smelting, petrochemicals,
paper manufacturing, and electrolysis
applications.
• The heavy metal contaminated wastewater finds
its way into the environment, threatening human
health and the ecosystem.

44. Heavy metals are non-biodegradable and
could be carcinogenic thus, the presence of
these metals in water by improper amounts
could result in critical health issues to living
organisms.
The most popular heavy metals are lead (Pb),
zinc (Zn), mercury (Hg), nickel (Ni), cadmium
(Cd), copper (Cu), chromium (Cr), and arsenic
(As).
Although these heavy metals can be detected
in traces; however, they are still hazardous.

45. AERATION
Aeration brings water and air in close contact
in order to remove dissolved gases (such as
carbon dioxide) and oxidizes dissolved metals
such as iron, hydrogen sulfide, and volatile
organic chemicals (VOCs).
Aeration is often the first major process at the
treatment plant.
During aeration, constituents are removed or
modified before they can interfere with the
treatment processes.

46. • Aeration also helps remove dissolved metals
through oxidation, the chemical combination
of oxygen from the air with certain
undesirable metals in the water
• The efficiency of aeration depends on the
amount of surface contact between air and
water, which is controlled primarily by the size
of the water drop or air bubble

47. Chemicals Removed or Oxidized by
Aeration
Constituents commonly affected by aeration are
• Volatile organic chemicals, such as benzene (found
in gasoline), or trichloroethylene, dichloroethylene
(used in dry-cleaning or industrial processes)
• Ammonia
• Chlorine
• Carbon dioxide
• Hydrogen sulfide
• Methane
• Iron and Manganese

48. AERATION EQUIPMENTS
Aerators fall into two categories. They either
introduce air to water, or water to air.
The water-in-air method is designed to produce
small drops of water that fall through the air.
The air-in-water method creates small bubbles of
air that are injected into the water stream.
All aerators are designed to create a greater
amount of contact between air and water to
enhance the transfer of gases and increase
oxidation.

49. Water-Into-Air Aerators
Cascade Aerators
Cone Aerators
Slat and Coke Aerators
Draft Aerators
Spray Aerators

50. Cascade Aerators

51. Cone Aerators

52. Slat Aerators

53. Coke Trays Aerators

54. Type 1, Draft Aerators

55. Type 2, Draft Aerators

56. Spray Aerators

57. Air-Into-Water Aerators
a. Pressure Aerators
b. Centrifugal Aerators

58. Centrifugal Aerators

59. AIR STRIPPING
Air stripping is a process by which a liquid, usually
wastewater, is brought into intimate contact with
a gas, usually air, so that some undesirable
volatile substances present in the liquid phase
can be released and carried away by the gas.
Processes such as mechanical surface aeration,
diffused aeration, spray fountains, spray or tray
towers, and countercurrent packed towers are
encompassed by the term air stripping.

60. a. Corrosion

61. A certain amount of dissolved oxygen is present
in raw and treated waters.
However, dissolved oxygen can cause corrosion.
Corrosion can occur whenever water and oxygen
come into contact with metallic surfaces.
Generally the higher the dissolved oxygen
concentration, the more rapid the corrosion
The solution to this problem is to not over-aerate.
This may be difficult because no definite rule
exists as to what constitutes over-aeration.
The amount of aeration needed will vary from
plant to plant and will also vary with the season.

62. Air Stripping

63. False Clogging of Filters (Air Binding)
• Filters in water containing a high amount of
dissolved oxygen will have a tendency to
release the oxygen in the filter as it passes
through.
• The process can continue until the spaces
between the filter media particles begin to fill
with bubbles Called air binding, this causes
the filter to behave as though it is plugged and
in need of backwashing.

64. Slow Removal of Hydrogen Sulfide
Hydrogen sulfide is most efficiently removed,
not by oxidation, but by the physical scrubbing
action of aeration.
This removal is dependent on the pH of the
water. At a pH of 6 or less, the hydrogen
sulfide is easily removed.
If the water has a high pH, the hydrogen
sulfide will ionize, precluding removal by
aeration.

65. Three basic control tests are required
for aeration
1. Dissolved oxygen - The concentration of
dissolved oxygen can be used to determine if the
water is over or under-aerated. The pH test will give
an indication of the amount of carbon dioxide
removed.
2. pH - pH increases as carbon dioxide is removed.
pH can also be used to monitor the effective range
for hydrogen sulfide, iron, and manganese removal.
3. Temperature - The saturation point of oxygen
increases as the temperature decreases. As water
temperature drops, the operation.

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