treatment of water for the larger community use.

1. Prepared by
Innocent L Swai, Bsc EHS
inolswai@gmail.com
School Of Hygiene, Muhimbili.

2. Introduction
• Treatment is usually necessary for town water
supplies.
• Sufficient water for a whole town is not always
available from the ground, and so polluted
surface sources often have to be used.
• The larger scale of a town water supply makes
the quality of the water more important than for
a small village supply.
2

3. Intro…….
• A single source of pollution in an urban supply
could cause a water-borne epidemic in the
whole town, so that the consequences of poor
water quality are more serious.
• Treatment is of little use if it is only erratically
applied, and yet it is a major problem to ensure
continuous and reliable operation of water
treatment works in many countries.
3

4. Definitions
Water supply:
• Deals with all processes from water abstraction from
water sources to water uses at user (end) points
• Components of water supply system:
• Abstraction infrastructure (gates, wells, pumps)
• Treatment units (filtration, sedimentation, etc)
• Transmission lines
• Storage facilities
• Distribution lines / distribution points (e.g.
Kiosks)
4

5. WATER TREATMENT PLANT
› Location of water treatment plant
The selection for water treatment plant for
portable water supply to a town or city depends
on the following factors:
• Location of the water-source
• Quantity of water
• Quality of water
• The cost of water supply scheme
5

6. Cont…..
› Layout of water treatment plant
The different units in the treatment plant are so
arranged that the following would be achieved:
• The usual sequence of operation can be followed
directly.
• The necessary hydraulic fall is available between
different units and no intermediate pumping is
required.
• The layout is close and compact requiring lesser area
and short lengths of supplying pipes and conduits.
• Necessary space for future extension is available. 6

7. Pure water supply
Washwater
Sludge
Alkalies
Coagulants
Sludge
Source
High lift
pumps
Pure
water
tanks
Chlorine
Filters
Main
settling
basins
Flocculat
ion
Mixing
chamber
Pretreat
ment
Raw
water
pumps
Coarse
screens
screen
Lagoon
7

8. Cont…..
› Classification of water treatment methods
The raw water contaminants are removed by
physical, chemical and biological means. The
individual methods usually are classified as:
• Physical unit operations
• Chemical unit processes
• Biological unit processes
8

9. Physical Unit operations
• The treatment methods in which the
application of physical forces predominate.
• These methods were the first to be used for
water treatment. Methods are screening,
mixing, flocculation, sedimentation and
filtration.
9

10. Chemical Unit Processes
• The methods in which the removal or conversion
of contaminants is brought about by the addition
of chemicals or by the other chemical reactions
are known as chemical unit processes.
• Precipitation and disinfection are common
examples used in water treatment.
• In chemical precipitation, treatment is
accomplished by introducing a chemical
precipitate that will settle.
10

11. Biological Unit Processes
• Treatment methods in which the removal of
contaminants is brought about by biological
activity are known as biological unit processes.
• Biological treatment is used primarily to remove
the biodegradable organic substances (colloidal
or dissolved) in water example schmutzdecke
layer in slow sand filters.
11

12. Cont….
› Factors influencing the selection of treatment
processes are:
• Treated water specifications.
• Raw water quality and its variations.
• Local constraints.
• Relative cost of different treatment processes.
12

13. Treatment required
• A treatment plant consists of many processes-
screening, coagulation, flocullation, sedimentation,
filtration and sterilization/disinfection.
• Each of these processes is intended to perform one
main function although it may incidentally partially
assist with some other.
• The impurities are removed in order of size, the
bigger ones being eliminated first.
• Not every water contains all the impurities and
therefore not every water requires all the treatment
processes
13

14. Cont….
• The impurities are mainly removed as follows:
- Floating objects by screening;
- Algae (if present) by straining;
- Excessive iron, manganese and hardness in solution
by precipitation in basins after the addition of
chemicals;
- Normal suspended solids by settling;
- The remaining fines and some bacteria by filtration;
- Excessive bacterial pollution by pre-chlorination;
- Final bacterial surviving filtration by chlorination.14

15. INTAKE WORKS
• After the source has been fixed up, in any water
supply scheme, the next problem is to draw water
from this source including the provision of intake
devices and head works.
• Intake is a device or structure placed in a surface
water source to draw water from this source and then
discharge into an intake conduit through which it will
flow into the water works system. It consists of:
- a conduit with protective works
- Screen at open ends
- Gates and valves to regulate the flow. 15

16. Cont…
Source water intake structure
16

17. Cont….
• Location of intakes
- At the site, the best quality of water should be
available which will save time and purification cost
- Site must be easily accessible
- Site should never be selected at the downstream or
near disposal of waster water
- Site must be free from the effects of floods as far as
possible
- At the site, the velocity of flow in the source must be
gentle
- Site should be near the treatment plant 17

18. WATER QAULITY CONTROL
LABORATORY
• A well equipped water quality control laboratory
should be provided at the treatment plant for
checking the quality of raw water and treated
water.
• The parameters checked are physical tests,
chemical tests and biological tests. This is to
make sure that means employed for raw water
treatment are effective to achieve the required
standards for treated water.
18

19. 19
Water treatment processes
Preliminary Treatment
• Preliminary treatment is any physical, chemical or
mechanical process used on water before it undergoes the
main treatment process.
• The purpose of preliminary treatment processes is to remove
any materials which will interfere with further
treatment.
• Pretreatment may include screening, presedimentation,
chemical addition, flow measurement, and aeration.

20. 20
Preliminary Treatment / Screens
• The screens are used to remove rocks, sticks,
leaves, and other debris.
• Very small screens can be used to screen out algae
in the water.
• All objects are removed by physical size separation
• Screens on the outside of intakes are often cleaned
by flushing water from the treatment plant
backwards
• There are two primary types of screens - bar
screens and wire-mesh screens.
A wire-mesh
screen
A bar screen

21. 21
Preliminary Treatment / Screens
• A bar screen is used to remove large debris.
The spaces between the bars are two to four
inches wide.
• A wire-mesh screen is used to remove smaller
debris. The gaps are about half an inch wide.
• Water must be flowing slowly in order to pass
through a wire-mesh screen - velocities
should be no greater than 3.5 inches per
second.
A wire-mesh
screen
A bar screen

22. 22
Preliminary Treatment / Presedimentation -
Aeration
• Presedimentation is to settle out sand, grit, and gravel
which will settle rapidly out of the water without the
addition of chemicals at the beginning of the treatment
process.
• Presedimentation depends on gravity and includes no
coagulation and flocculation.
• Presedimentation will reduce the load on the
coagulation/flocculation basin and on the sedimentation
chamber, as well as reducing the volume of coagulant
chemicals required to treat the water.

23. • Presedimentation basins are useful because raw
water entering the plant from a reservoir is usually
more uniform in quality than water entering the plant
without such a holding basin
• Here in pretreatment, the purpose of sedimentation is
to make the chemical treatment phase of the water
treatment process more efficient by removing
sediment from the raw water.
23
Preliminary Treatment / Presedimentation -
Aeration

24. • In presedimentation basin, activated carbon may be
added to the basin for taste, odor, and color
problems, and some chemicals to control the growth
of algae.
• Aeration removes carbon dioxide and hydrogen
sulfide from the water. It also oxidizes the iron
and manganese.
24
Preliminary Treatment / Presedimentation -
Aeration

25. 25
Preliminary Treatment / Monitoring
• Flow Measurement : to adjust chemical feed rates,
calculate detention times, and monitor the amount of
water being treated.
• It is also monitored for a variety of characteristics
including pH, turbidity, total alkalinity, temperature,
and coliform bacteria.

26. 26
Preliminary Treatment / Monitoring
• The pH and total alkalinity of the water will influence
the amount of alkali to be added and can also influence
the flocculation conditions
• The level of turbidity will influence the amount of
polymer (coagulant) added to the water.
• Temperature is also measured since cold water does not
floc as well as warm water and requires the addition of
more polymer

27. 27
Primary Sedimentation
• Sedimentation is a treatment process in which
the velocity of the water is lowered below the
suspension velocity and the suspended
particles settle out of the water due to gravity.
• The process is also known as
settling or clarification
• Settled solids are removed as sludge, and
floating solids are removed as scum
• The efficiency or performance of the process is
controlled by: detention time, temperature, tank
design, and condition of the equipment.
Notes:
•sedimentation may not be
necessary in low turbidity
water of less than 10 NTU
•In this case, coagulation and
flocculation are used to
produce pinpoint (very
small) floc which is removed
from the water in the filters

28. 28
Primary Sedimentation / Location in the
Treatment Process
• The most common form of sedimentation follows
coagulation and flocculation and precedes filtration.
• This type of sedimentation requires chemical addition (in the
coagulation/flocculation step) and removes the resulting floc
from the water.
• sedimentation following coagulation/flocculation is meant to
remove most of the suspended particles in the water
before the water reaches the filters,

29. 29
Primary Sedimentation / Location in the
Treatment Process
• Sedimentation at this stage in the treatment process should
remove 90% of the suspended particles from the water,
including bacteria.
• The purpose of sedimentation here is to decrease the
concentration of suspended particles in the water,
reducing the load on the filters.
• Sedimentation can also occur as part of the pretreatment
process, where it is known as presedimentation.

30. 30
Types of sedimentation basins
Rectangular basins: have a variety of advantages - predictability,
cost-effectiveness, and low maintenance. In addition, rectangular
basins are the least likely to short-circuit, especially if the length is at
least twice the width. A disadvantage of rectangular basins is the large
amount of land area required.
Double-deck rectangular basins: This type of basin conserves land
area - has higher operation and maintenance costs.
Square or circular sedimentation basins with horizontal flow are
known as clarifiers. This type of basin is likely to have short-
circuiting problems.
Solids-contact clarifiers , also known as upflow solids-contact
clarifiers or upflow sludge-blanket clarifiers combine coagulation,
flocculation, and sedimentation within a single basin. found in
packaged plants and in cold climates where sedimentation must occur
indoors

31. 31
Sedimentation and flotation zones
• All sedimentation basins have
four zones - the inlet zone, the
settling zone, the sludge zone,
and the outlet zone.
• In a clarifier, water typically
enters the basin from the center
rather than from one end and
flows out to outlets located around
the edges of the basin. But the
four zones can still be found
within the clarifier
A rectangular sedimentation basin

32. 32
Sedimentation and flotation zones/Inlet Zone
Purposes of the inlet zone of a sedimentation basin are
• to distribute the water and to control the water's velocity as it
enters the basin.
• inlet devices act to prevent turbulence of the water.
• The incoming flow must be evenly distributed across the
width of the basin to prevent short-circuiting.

33. 33
Sedimentation and flotation zones/Inlet Zone
• Short-circuiting is a problematic circumstance in which
water bypasses the normal flow path through the basin and
reaches the outlet in less than the normal detention time.
• If the water velocity is greater than 0.5 ft/sec, then floc in the
water will break up due to agitation of the water.

34. 34
Sedimentation and flotation zones/Inlet Zone
Two types of inlets.
1. The stilling wall, also known as a perforated
baffle wall , spans the entire basin from top to
bottom and from side to side. Water leaves the inlet
and enters the settling zone of the sedimentation
basin by flowing through the holes evenly spaced
across the stilling wall.
2. The second type of inlet allows water to enter the
basin by first flowing through the holes evenly
spaced across the bottom of the channel and then by
flowing under the baffle in front of the channel.
The combination of channel and baffle serves to
evenly distribute the incoming water

35. 35
Sedimentation and flotation / Settling Zone
• water enters the settling zone where water velocity is
greatly reduced.
• the bulk of floc settling occurs and this zone will make
up the largest volume of the sedimentation basin.
• For optimal performance, the settling zone requires a
slow, even flow of water.
• The settling zone may be simply a large expanse of open
water. But in some cases, tube settlers and lamella
plates, are included in the settling zone.

36. 36
Traditional Circular
Clarifiers / Settling Zone

37. 37
Sedimentation and flotation / Outlet Zone
Outlet Zone is designed to:
• prevent short-circuiting of water in the
basin.
• ensure that only well-settled water
leaves the basin and enters the filter.
• control the water level in the basin.
37

38. 38
Sedimentation and flotation / Outlet Zone
• ensure that the water flowing out of the
sedimentation basin has the minimum
amount of floc suspended in it.
• A typical outlet zone begins with a baffle
in front of the effluent.
• This baffle prevents floating material
from escaping the sedimentation basin
and clogging the filters.
• The weirs serve to skim the water evenly
off the tank
38

39. 39
Sedimentation and flotation / Sludge Zone
• The sludge zone is found across the bottom of the
sedimentation basin.
• Velocity should be very slow to prevent resuspension of
sludge.
• The tank bottom should slope toward the drains
• Sludge removal by ( automated equipment or manually at
least twice per year).
• The best time of cleaning when water demand is low.
39

40. 40
Sedimentation and flotation / Sludge Zone
• Many plants have at least two sedimentation basins so
that water can continue to be treated while one basin is
being cleaned, maintained, and inspected.
• If sludge is not removed from enough,
the effective volume of the tank will decrease, reducing
the efficiency of sedimentation.
• Sludge built up on the bottom of the tank may
become septic (anaerobically).
• Septic sludge may result in taste and odor problems or
may float to the top of the water and become scum or
resuspended to be carried over to the filters.
40

41. 41
Aeration
Types of Aerators
• air into the water
• water into the air

42. 42
Aeration Efficiency
Surface contact between air and water
•Smaller bubble size, greater surface contact with
water.
•Smaller drop size, greater surface contact with the
air.

43. 43
Aeration
• Aeration is the process of
bringing water and air into close
contact.
• Aeration is the process to remove
dissolved gases, such as carbon
dioxide, hydrogen sulfide, and to
oxidize dissolved metals such as
iron. It can also be used to
remove volatile organic
chemicals (VOC).

44. 44
Aeration
It happened by:
• Exposing drops or thin sheets of water
to the air or
• introducing small bubbles of air and
letting them rise through the water.
the aeration is accomplished the desired
results by:
• Sweeping or scrubbing action caused by
the turbulence of water and air mixing
together
• Oxidizing certain metals and gases

45. 45
TASTE AND ODOR & DISSOLVED OXYGEN
TASTE AND ODOR
• Aeration is effective in removing tastes and odors that are
caused by volatile materials
• Volatile materials (e.g Methane and hydrogen sulfide)
have low boiling point and will vaporize very easily.
• Many taste and odor problems in surface water could be
caused by oils and by-products that algae produce.
• Since oils are much less volatile than gases, aeration is
only partially effective.

46. 46
TASTE AND ODOR & DISSOLVED OXYGEN
DISSOLVED OXYGEN
• Oxygen is injected into water through aeration to remove
the flat taste.
• The amount of oxygen that the water can hold is
dependent on the temp.
• The colder the water, the more oxygen the water can
hold.
• Water that contains excessive amounts of oxygen can
become very corrosive.
• Excessive oxygen can cause air binding of filters.

47. 47
Types Of Aerators
Aerators fall into two general categories.
• introduce air into the water or water into the air.
• The water-to-air method is designed to produce small
drops of water that fall through the air
• The air-to-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 the air and water to enhance the transfer
of the gases.

48. 48
Water Into Air
Cascade Aerators
• consists of a series of steps that the water
flows over.
• aeration is accomplished in the splash
zones.
• The aeration action is similar to a
flowing stream.
• Splash areas are created by placing
blocks across the incline.
• Cascade aerators used to oxidize iron
and to partially reduce dissolved gases.

49. 49
Water Into Air
• the oldest and most common type
of aerators.
Cone Aerators
• are used primarily to oxidize iron
and manganese prior to filtration.
• the water pumped to the top of the
cones and then allowed to cascade
down through the aerator.

50. 50
Water Into Air
Slat and Coke Aerators
• similar to the cascade and cone
types.
• They usually consist of three-to-five
stacked trays, which have spaced
wooden slats in them.
• The trays are filled with fist-sized
pieces of coke, rock, ceramic balls,
limestone, or other materials.
• The primary purpose of the
materials is provide additional
surface contact area between the air
and water.

51. 51
Water Into Air
Spray Aerators
• spray aeration is
successful in
oxidizing iron and
manganese and is
successful in
increasing the
dissolved oxygen of
the water.

52. 52
Water Into Air
Draft Aerators: the air is induced by a blower.
Types:
• external blowers mounted at the bottom of the
tower to induce air from the bottom of the
tower.
• Water is pumped to the top and allowed to
cascade down through the rising air.
• The other, an induced-draft aerator, has a top-
mounted blower forcing air from bottom vents
up through the unit to the top.
• Both types are effective in oxidizing iron and
manganese before filtration.

53. 53
Air Into Water
• These are not common types used in water
treatment.
• The air is injected into the water through a series
of nozzles submerged in the water.
• It is more commonly used in wastewater
treatment for the aeration of activated sludge.
Air-into-water
• Diffuser
• Draft tube

54. 54
Air Into Water
Pressure Aerators
• Uses a pressure vessel.
• The water to be treated is sprayed into the
high-pressure air, allowing the water to
quickly pick up dissolved oxygen.
• A pressure aerator commonly used in
pressure filtration is a porous stone
installed in a pipeline before filtration.

55. 55
Air Into Water
• The air is injected into the stone and
allowed to stream into the water as a fine
bubble, causing the iron to be readily
oxidized.
• The higher the pressure, the more readily
the transfer of the oxygen to the water.
• more O2 is available, more readily the
oxidation of the Fe or Mn.

56. 56
Coagulation and Flocculation
• Coagulation refers to all the reactions and mechanisms
that result in particle aggregation in the water being
treated, including in situ coagulant formation (where
applicable), particle destabilization, and physical
interparticle contacts
• Coagulant formation, particle destabilization,
typically occur during and immediately after chemical
dispersal in rapid mixing;

57. 57
Coagulation and Flocculation
• inter particle collisions that cause aggregate (floc)
formation begin during rapid mixing but usually occur
predominantly in the flocculation process.
• The physical process of producing inter-particle
contacts is termed flocculation.
• Flocculation defined as the uses gentle stirring to bring
suspended particles together so they will form larger
more settleable clumps (groups) called floc.

58. 58
A common classification of particles
• Molecules sizes smaller than 1
nm
• Colloids generally with
dimensions between 1 nm - 1 μm
• Suspended matter having sizes
larger than 1 μm.
• Colloids: humic acids, proteins,
colloidal clay, silica and viruses.

59. 59
A common classification of particles
• Suspended matter: Bacteria,
algae, silt, sand and organic debris.
• Suspended matter-when it is larger
than 5-10 μm can be removed
quite easily by filtration or
sedimentation and filtration.
• The removal of colloids is possible
by slow filtration in cases the
water is not strongly polluted.

60. 60
Stability Of Particle Suspensions
• Coagulation process is used to increase the rate or kinetics
of particle aggregation and floc formation
• The objective is to transform a stable suspension [i.e., one
that is resistant to aggregation (or attachment to a filter
grain)] into an unstable one.
• there are forces that tend to pull the interacting surfaces
together
• The most important attractive force is called the London–
van der Waals force.

61. 61
Stability Of Particle Suspensions
• It arises from spontaneous electrical and magnetic
polarizations that create a fluctuating electromagnetic field
within the particles and in the space between them
• The most well-known repulsive force is caused by the
interaction of the electrical double layers of the surfaces
(“electrostatic” stabilization).
• As particles approach one another on a collision course, the
fluid between them must move out of the way.
• The repulsive force caused by this displacement of fluid is
called hydrodynamic retardation.

62. 62
Coagulation Process Description
Purpose to aid in the removal of nonsettleable solids from
water.
Coagulation is defined as:
• the destabilization of colloidal solids;
• the water treatment process which causes very small
suspended solids to attract one another and form larger
particles.
Suspended particles in water resist settling for two primary
reasons:
1. Particle size; and,
2. Natural forces between particles.

63. 63
Coagulation Process Description
Suspended particles in water normally have a negative (-) charge.
• Since these particles all have the same charge, they repel each
other, keeping each other from settling.
• This natural repelling force is called the zeta potential.
• Coagulation neutralizes the forces (zeta potential), which cause
suspended solids in water to repel each other and resist settling.
• Once the repulsive forces have been neutralized these particles
can stick together (agglomerate) when they collide.
• The force which holds the floc together is called the van der
Waals force.

64. 64
Flocculation
• After coagulation the destabilized particles can collide,
aggregate so flocs can be formed. This step is called
flocculation.
• Flocculation: The process of agglomeration of the
destabilized particles to such a size that separation by
sedimentation and filtration is possible.
• In flocculation one can make a distinction between peri-
kinetic and ortho-kinetic flocculation.
• Brownian motion is the driving force in the agglomeration
of destabilized particles up to 1μm-level peri-kinetic
flocculation).

65. 65
Flocculation
• Above ~ 1 μm the influence of Brownian motion on the
collision rate of the particles can be neglected, then artificial
mixing is necessary to get an efficient flocculation. That part
of the flocculation process is called ortho-kinetic
flocculation.
• Flocculation uses gentle stirring to cause the particles to
collide so that they can stick together, for a particle (floc)
large enough and heavy enough to settle

66. 66
Types of Flocculation Tanks
Mechanical Flocculators
Paddle wheel Type (vertical and Horizontal Types)
Foil Type Mixing Blade

67. 67
Types of Flocculation Tanks
Hydraulic Flocculators
• The axial flow flocculators are typically used because they impart a
nearly constant gradient in each compartment.
• Flocculators are designed to have a minimum of three
compartments to provide for tapered (to make smaller gradually)
mixing.
• The velocity gradient, G is tapered so that it is larger in the first
compartment and less is the other compartments as the floc grows.

68. 68
Chemicals used to neutralize the zeta potential
• These chemicals are coagulants, sometimes called primary
coagulants, and coagulant aids.
• Since most suspended particles in water carry a negative (-)
charge, coagulants consist of chemicals that provide
positively (+) charged ions.
Common coagulants are:
1. Metal Salts
a. Aluminum Salts (Alum (aluminum sulfate) - PACs
(polyaluminum chlorohydrate, and other variations)
b. Iron Salts (Ferric Chloride - Ferric Sulfate - Ferrous
Sulfate)
2. Polymers (polyelectrolytes)

69. 69
Common coagulants / Polymers
• Polymers (polyelectrolytes) are extremely large molecules
which produce thousands of charged ions when dissolved in
water.
1. Cationic Polyelectrolytes - Have a positive (+) charge. Used
as either a primary coagulant or as a coagulant aid. Cationic
polymers:
• allow reduced coagulant dose;
• improve floc settling;
• are less sensitive to pH;
• improve flocculation of organisms such as bacteria and
algae.

70. 70
Common coagulants / Polymers
2. Anionic Polyelectrolytes- Have a negative (-) charge. Used
primarily as a coagulant aid. Anionic polymers are used to:
• increase floc size;
• improve settling;
• produce a stronger floc;
• They are not materially affected by pH, alkalinity, hardness
or turbidity.
3. Nonionic Polyelectrolytes- Balanced or neutral charge.
• Used as a primary coagulant or coagulant aid.

71. 71
Coagulant aids
Coagulant aids are chemicals which are added to water
during coagulation to improve coagulation by:
• building a stronger, more settleable floc;
• overcoming slow floc formation in cold water;
• reducing the amount of coagulant required;
• reducing the amount of sludge produced.
• The key reason coagulant aids are used is to reduce the
amount of alum used, which, in turn, decreases the
amount of alum sludge produced.
• Alum sludge is difficult to dewater and to dispose of.

72. 72
Types of Coagulant Aids
Activated Silica
• increases the coagulation rate;
• reduces the amount of coagulant needed;
• widens the pH range for effective coagulation;
• strengthens floc
Weighting agents (Bentonite Clay, Powdered Limestone;
Powdered Silica) provide additional particles that can enhance
floc formation.
They are used to treat water that is:
• high in color; or,
• low in turbidity; or,
• low in mineral content.

73. 73
Factors Which Affect How Well a Coagulant Work
Factors Which Affect How Well a Coagulant Work
(1) Mixing Conditions
(2) pH
(3) Alkalinity
(4) Water Temperature
(5) Turbidity
• If the alkalinity concentration in the water is not high enough,
and effective floc will not form when either alum or ferric
sulfate is used. Metal salts (alum, ferric sulfate, ferric
chloride) consume natural alkalinity.

74. 74
Factors Which Affect How Well a Coagulant Work
• Each mg/L of alum will consume 0.5 mg/l total alkalinity (as
CaCO3).
• Each mg/L ferric sulfate will consume 0.75 mg/L total
alkalinity (as as CaCO3).
• Each mg/L ferric chloride will consume 0.92 mg/L total
alkalinity (as CaCO3).
• It may be necessary to add alkalinity to the water (lime, soda
ash, caustic soda) to the water in order for the metal salts to
work properly. The doses should be confirmed with jar
testing.

75. 75
Coagulation/Flocculation Facilities
Coagulation/Flocculation Facilities
• Flash Mix - purpose is to distribute the coagulant rapidly and
evenly throughout the water.
• Water should be stirred violently for a brief time to encourage
the greatest number of collisions between particles as possible.
• Types of Mixers: Mechanical - Pumps and Conduits
• Detention time should be 30 seconds or less (Design Criteria).
• Flocculation - provides for gentle mixing to encourage floc
formation.
• Detention time of at least 30 minutes, with a detention time of
45 minutes preferred.

76. 76
Process Control
A. Chemical Selection
B. Chemical Application / Solution Preparation
C. Monitoring Process Effectiveness

77. 77
Process Control / Chemical Selection
A. Chemical Selection - These raw water characteristics should
be monitored in order to do a thorough job of chemical
selection.
1. Temperature
• Low water temperatures slow chemical reactions, causing
decreased efficiency and slow floc formation.
• Higher coagulant doses may be required to maintain
acceptable results.
2. pH
• Extremes can interfere with the coagulation/flocculation
process.
• The optimum pH depends on the specific coagulant.

78. 78
Process Control / Chemical Selection
3. Alkalinity
• Low alkalinity causes poor coagulation.
• May be necessary to add alkalinity (lime, caustic soda,
soda ash).
4. Turbidity
• Difficult to form floc with low turbidity water, may need
to add weighting agents.
5. Color
• Indicates presence of organic chemicals which can react
with the coagulant, and with chlorine to form disinfection
byproducts.

79. 79
Process Control / Chemical Application
B. Chemical Application:
Solution Preparation
• For Example when preparing potassium permanganate solutions, a
three percent solution is best. Potassium permanganate has a
limited solubility of about five percent at normal temperatures.
In order to prepare the solution needed the following information is
required:
• Chemical required
• Volume of water required
• Specific gravity
• Weight of solution
• Concentration

80. 80
Process Control / Monitoring Process
Effectiveness
C- Monitoring Process Effectiveness
• (1) Jar Test
• (2) pH
• (3) Turbidity
• (4) Temperature
• (5) Alkalinity

81. 81
Jar Test
• Coagulation/flocculation is the process of binding small
particles in the water together into larger, heavier clumps
which settle out relatively quickly.
• The larger particles are known as floc.
• changing water characteristics require the operator to
adjust coagulant dosages at intervals to achieve optimal
coagulation.
• Different dosages of coagulants are tested using a jar
test, which mimics the conditions found in the treatment
plant.

82. 82
Jar Test
• The first step of the jar test involves adding coagulant to
the source water and mixing the water rapidly (as it would be
mixed in the flash mix chamber) to completely dissolve the
coagulant in the water.
• Then the water is mixed more slowly for a longer time
period (as flocculation basin conditions and allowing the
forming floc particles to cluster together).
• Finally, the mixer is stopped and the floc is allowed to settle
out, as it would in the sedimentation basin.

83. 83
Jar Test
• A major goal of water treatment is turbidity removal.
• The jar test is a simulation of the treatment processes that
have been developed to accomplish turbidity removal
• Alum, ferrous sulfate, and ferric chloride are three common
coagulants
• The best dose will also be a function of pH. The optimum pH
for alum coagulation is usually between 5.5 and 6.5.
• There is no way to “calculate” the best dose. It must be
determined by trial and error; hence, the jar test.
• The reaction chemistry varies according to the pH and
alkalinity of the test sample.

84. 84
Jar Test
Alum coagulation proceeds according to the following equation
• if there is enough alkalinity in the water to react with the
amount of alum dosed:
Al2(SO4)3 • 14H2O + 6HCO3
- ↔ 2Al(OH)3(s) + 6CO2 +
14H2O + 3SO4
-2
• If there is insufficient alkalinity, the reaction will proceed
according to the equation:
• Al2(SO4)3 • 14H2O ↔ 2Al(OH)3 + 3H2SO4 + 8H2O
• An alkalinity test is usually performed before initiating a jar
test to determine whether alkalinity supplements might be
required.

85. 85
JAR TEST

86. 86
Filtration
• After separating most floc, the water is filtered as the final
step to remove remaining suspended particles and unsettled
floc
1. Rapid sand filters (RSF)
• use relatively coarse sand and other granular media to remove
particles and impurities that have been trapped in a floc
through the use of flocculation chemicals-typically salts
of aluminium or iron.
• Water and flocs flows through the filter medium under
gravity or under pumped pressure

87. 87
Filtration
• Water moves vertically through sand which often has a layer
of activated carbon or anthracite coal (a hard, compact variety of
mineral coal).
• The top layer removes organic compounds
• Most particles pass through surface layers but are trapped in pore
spaces or adhere to sand particles
• To clean the filter, water is passed quickly upward through the
filter, opposite the normal direction
(called backflushing or backwashing)
• compressed air may be blown up through the bottom of the filter to
break up the compacted filter media to aid the backwashing
process

88. 88
Rapid sand filters

89. Backwashing of RSF
• Treated water from storage is used for the backwash
cycle. This treated water is generally taken from
elevated storage tanks or pumped in from the clear well.
• The filter backwash rate has to be great enough to
expand and agitate the filter media and suspend the floc
in the water for removal.
• However, if the filter backwash rate is too high, media
will be washed from the filter into the troughs and out
of the filter.
89

90. When is backwashing needed
The filter should be backwashed when the
following conditions have been met:
• The head loss is so high that the filter no longer
produces water at the desired rate; and/or
• Floc starts to break through the filter and the
turbidity in the filter effluent increases; and/or
• A filter run reaches a given hour of operation.
90

91. 91
Rapid sand filters / Advantages and disadvantages
Advantages
• Much higher flow rate than a slow sand filter;
• Requires relatively small land area
• Less sensitive to changes in raw water quality, e.g. turbidity
• requires less quantity of sand
• Disadvantages
• Requires greater maintenance than a slow sand filter. For this
reason, it is not usually classed as an "appropriate
technology,".

92. 92
Rapid sand filters / Advantages and disadvantages
• Generally ineffective against taste and odour problems.
• Produces large volumes of sludge for disposal.
• Requires on-going investment in costly flocculation reagents.
• treatment of raw water with chemicals is essential
• skilled supervision is essential
• cost of maintenance is more
• it cannot remove bacteria

93. 93
Slow sand filters
• Slow "artificial" filtration (a variation of bank filtration)
to the ground, Water purification plant
• The filters are carefully constructed using graded layers
of sand with the coarsest sand, along with some gravel, at
the bottom and finest sand at the top.
• Drains at the base convey treated water away for
disinfection

94. 94
Slow sand filters
• effective slow sand filter may remain in service for many
weeks or even months
• produces water with a very low available nutrient level
and low disinfectant levels
• Slow sand filters are not backwashed; they are
maintained by having the top layer of sand scraped off
• A 'large-scale' form of slow sand filter is the process
of bank filtration in a riverbank.

95. 95
Slow sand filters

96. 96
Slow sand filters / Advantages and disadvantages
Advantages
• require little or no mechanical power, chemicals or
replaceable parts,
• require minimal operator training and only periodic
maintenance,
• often an appropriate technology for poor and isolated
areas.
• simple design

97. 97
Slow sand filters / Advantages and disadvantages
Disadvantages
• Due to the low filtration rate, slow sand filters require
extensive land area for a large municipal system.
• Many municipal systems in grown cities
installed rapid sand filters, due to increased demand
for drinking water.

98. Differences between SSF an RSF
98

99. 99
Disinfection
99

100. 100
Background: Current Methods
of Disinfection
• Large-Scale:
– Chlorination
– Ozone
– UV irradiation
• Small Scale:
– Boiling
– Iodine tablets
– Filters
100

101. Why disinfection?
• In the water treatment processes like sedimentation,
coagulation, filteration, etc considered so far, all the
bacteria from the water can not be removed.
• Moreover there is every chance of getting the water
contaminated during it flow through the water
distribution system especially in case of intermittent
supply, where the pipes remain empty for a
considerable period.
• Therefore water is disinfected as soon as it leaves by
Chlorine or Bleaching powder. 101

102. Requirements of Disinfectants
• The requirement of good disinfectants may be:
- They should be able to destroy all the harmful
pathogenic bacteria and make the water perfectly safe
- They should be economical and easily available
- They should be able to kill all pathogenic germs within
required time at normal temperature
- After their treatment the water should not become
objectionable and toxic to the customer
- The disinfectant dose should be such that, it may leave
some concentration for protection against contamination
in water. 102

103. 103
Use of Disinfectants as Chemical Oxidants
Oxidation is a chemical reaction where electrons are
transferred from one species (the reducer) to another species
(the oxidant)
Disinfectants are used for more than just disinfection in
drinking water treatment. While inactivation of pathogenic
organisms is a primary function, disinfectants are also used
oxidants in drinking water treatment for several other
functions:
1. Minimization of Disinfection Byproducts formation :
Several strong oxidants, including potassium permanganate
and ozone, may be used to control DBP 103

104. 2. Prevention of re-growth in the distribution system and
maintenance of biological stability;
– Removing nutrients from the water prior to distribution;
– Maintaining a disinfectant residual in the treated water;
and
– Combining nutrient removal and disinfectant residual
maintenance
3. Removal of color: Free chlorine is used for color removal. A
low pH is favored. Color is caused by humic compounds,
which have a high potential for DBP formation
104
Use of Disinfectants as Chemical Oxidants

105. 105
Continue: Use of Disinfectants as Chemical Oxidants
4. Improvement of coagulation and filtration efficiency;
a. Oxidation of organics into more polar forms;
b. Oxidation of metal ions to yield insoluble complexes such
as ferric iron complexes;
c. Change in the structure and size of suspended particles.
5. Oxidation is commonly used to remove taste and odor
causing compounds. Because many of these compounds are
very resistant to oxidation, advanced oxidation processes
(ozone/hydrogen peroxide, ozone/UV, etc.) and ozone by
itself are often used to address taste and odor problems. The
effectiveness of various chemicals to control taste and odors
can be site-specific. 105

106. 106
Continue: Use of Disinfectants as Chemical Oxidants
6. Prevention of algal growth in sedimentation basins and
filters: Prechlorination will prevent slime formation on
filters, pipes, and tanks, and reduce potential taste and
odor problems associated with such slimes.
106

107. 107
Factors affecting disinfection effectiveness
• Time
• pH
• Temperature
• Concentration of the disinfectant
• Concentration of organisms
• Nature of the disinfectant
• Nature of the organisms to be inactivated
• Nature of the suspending medium 107

108. 108
CT Factor
• One of the most important factors for determining or
predicting the germicidal efficiency of any disinfectant is the
CT factor, a version of the Chick-Watson law (Chick, 1908;
Watson, 1908).
• The CT factor is defined as disinfectant contact time, the
mathematical product of C x T, where C is the residual
disinfectant concentration measured in mg/L, and T is the
corresponding contact time measured in minutes.
• CT values for chlorine disinfection are based on a free
chlorine residual.
108

109. 109
CT Factor
• Chlorine is less effective as pH increases from 6 to 9.
• In addition, for a given CT value, a low C and a high T is
more effective than the reverse (i.e., a high C and a low T).
• For all disinfectants, as temperature increases, effectiveness
increases.
109

110. 110
Chlorine
Chlorine has many attractive features that contribute to its wide
use in the industry. Four of the key attributes of chlorine are that
it:
• Effectively inactivates a wide range of pathogens
commonly found in water;
• Leaves a residual in the water that is easily measured and
controlled;
• Is economical; and
110

111. 111
Chlorine
• Has an extensive track record of successful use in improving
water treatment operations
There are, however, some concerns regarding chlorine usage
that may impact its uses such as:
• Chlorine reacts with many naturally occurring organic and
inorganic compounds in water to produce undesirable DBPs;
• Hazards associated with using chlorine, specifically
chlorine gas, require special treatment and response
programs; and
• High chlorine doses can cause taste and odor problems.
111

112. 112
Chlorine purposes in water treatment
• Taste and odor control;
• Prevention of algal growths;
• Maintenance of clear filter media;
• Removal of iron and manganese;
• Destruction of hydrogen sulfide;
112

113. 113
Chlorine purposes in water treatment
• Bleaching of certain organic colors;
• Maintenance of distribution system water quality by
controlling slime growth;
• Restoration and preservation of pipeline capacity;
• Restoration of well capacity, water main sterilization; and
• Improved coagulation by activated silica.
113

114. 114
Chlorine Chemistry
• Chlorine gas hydrolyzes rapidly in water to form
hypochlorous acid
• Hypochlorous acid is a weak acid (pKa of about 7.5),
meaning it dissociates slightly into hydrogen and
hypochlorite ions
114

115. 115
Chlorine Chemistry
• Between a pH of 6.5 and 8.5 this dissociation is
incomplete and both HOCl and OCl- species are present to
some extent (White, 1992). Below a pH of 6.5, no
dissociation of HOCl occurs, while above a pH of 8.5,
complete dissociation to OCl- occurs.
• As the germicidal effects of HOCl is much higher than
that of OCl-, chlorination at a lower pH is preferred.
115

116. 116
Commonly Used Chlorine Sources
Sodium hypochlorite and calcium hypochlorite are the most
common sources of chlorine used for disinfection of onsite
water supplies.
• Sodium Hypochlorite (common household bleach)
• Sodium hypochlorite is produced when chlorine gas is
dissolved in a sodium hydroxide solution.
116

117. 117
Commonly Used Chlorine Sources
• Clear to slightly yellow colored liquid with a distinct chlorine
odor.
• Common laundry bleach - 5.25 to 6.0 percent available
chlorine, when bottled.
• Do not use bleach products that contain additives such as
surfactants, thickeners, stabilizers, and perfumes.
• Always check product labels to verify product content and use
instructions.
117

118. 118
Commonly Used Chlorine Sources
• Calcium hypochlorite is formed from the precipitate that
results from dissolving chlorine gas in a solution of
calcium oxide (lime) and sodium hydroxide.
• Dry white powder, granules, or tablets - 60 to 70 percent
available chlorine - 12-month shelf life if kept cool and dry
- If stored wet, looses chlorine rapidly and is corrosive.
• A chlorine test kit should be used to check the final
chlorine residual in a prepared chlorine solution to assure
that you have the concentration intended.
118

119. 119
Which is Best, Sodium Hypochlorite or
Calcium Hypochlorite?
• Sodium hypochlorite is more effective
• This may be associated with the quality of the ground water
in the well being treated rather than with the source of the
chlorine itself.
• If there is an abundance of calcium based materials in both
bedrock wells. Calcium hypochlorite already has a high
concentration of calcium.
119

120. 120
Which is Best, Sodium Hypochlorite or
Calcium Hypochlorite?
• At 180 ppm of hardness, water is saturated with calcium to
the point that it precipitates out of the solution, changing
from the dissolved state to a solid state.
• Introducing a calcium hypochlorite solution into a calcium
rich aquifer can cause the formation of a calcium carbonate
(hardness) precipitate that may partially plug off the well
intake.
• Sodium hypochlorite does not have the tendency to create the
precipitate.
• If the calcium carbonate concentration in the ground water is
above 100 ppm (mg/l), the use of sodium hypochlorite is
recommended instead of calcium hypochlorite. 120

121. 121
Chlorine Added
Initial chlorine concentration added to water
Chlorine Demand
Reactions with organic material,
metals, other compounds present
in water prior
to disinfection
Total Chlorine
Remaining chlorine
concentration after chlorine
demand of water
Free Chlorine
Concentration of chlorine
available for disinfection
Combined Chlorine
Concentration of chlorine
combined with nitrogen in the
water and unavailable for disinfection
Chlorine Addition Flow Chart
121

122. 122
DISINFECTANT DEMAND REACTIONS
Reactions with Ammonia
• In the presence of ammonium ion, free chlorine reacts in a
stepwise manner to form chloramines
• monochloramine (NH2Cl), dichloramine (NHCl2 ), and
trichloramine (NCl3), each contribute to the total (or
combined) chlorine residual in a water. 122

123. 123
DISINFECTANT DEMAND REACTIONS
• The terms total available chlorine and total oxidants refer,
respectively, to the sum of free chlorine compounds and
reactive chloramines, or total oxidating agents.
• Under normal conditions of water treatment, if any excess
ammonia is present, at equilibrium the amount of free
chlorine will be much less than 1 percent of total residual
chlorine.
123

124. 124
Chlorine residual
• Chlorine persists in water as ‘residual’ chlorine after dosing
and this helps to minimize the effects of re-contamination by
inactivating microbes which may enter the water supply after
chlorination. It is important to take this into account when
estimating requirements for chlorination to ensure residual
chlorine.
• The level of chlorine residual required varies with type of
water supply and local conditions.
• In water supplies which are chlorinated there should always
be a minimum of 0.5mg/l residual chlorine after 30 minutes
contact time in water. 124

125. 125
Chlorine residual
• Where there is a risk of cholera or an outbreak has
occurred the following chlorine residuals should be
maintained:
– At all points in a piped supply 0.5mg/l
– At standposts and wells 1.0mg/l
– In tanker trucks, at filling 2.0mg/l
• In areas where there is little risk of a cholera outbreak, there
should be a chlorine residual of 0.2 to 0.5 mg/l at all points
in the supply. This means that a chlorine residual of about
1mg/l when water leaves the treatment plant is needed.
125

126. 126
Combined Chlorine
What is it?
• Free chlorine that has combined with ammonia (NH3) or
other nitrogen-containing organic substances.
• Typically, chloramines are formed .
Where does NH3, etc come from?
• Present in some source waters (e.g., surface water).
• Contamination; oxidation of organic matter
• Some systems (about 25% of U.S. water supplies) actually
Add ammonia.
126

127. 127
Combined Chlorine
Why would you want to Add ammonia?
• Chloramines still retain disinfect capability (~5 % of FAC,
Free Available Chlorine)
• Chloramines not powerful enough to form THMs.
• Last a lot longer in the mains than free chlorine,
– Free chlorine + Combined chlorine = Total Chlorine
Residual
• Can measure “Total” Chlorine
• Can measure “Free” Chlorine
• Combined Chlorine can be determined by subtraction
127

128. 128
pH Effect on Chlorine
• Chlorine is a more effective disinfectant at pH levels between
6.0 and 7.0, because hypochlorous acid is maximized at these
pH levels
• Any attempt to disinfect water with a pH greater than 9 to 10 or
more will not be very effective.
• The pH determines the biocidal effects of chlorine.
• Chlorine will raise the pH when added to water.
• By increasing the concentration of chlorine, and subsequently
raising the pH, the chlorine solution is actually less efficient as a
biocide.
• Controlling the pH of the water in the aquifer is not practical.
However buffering or pH-altering agents may be used to control
pH in the chlorine solution being placed in the well.
128

129. 129
Temperature Effect on Chlorine
• As temperatures increase, the metabolism rate of
microorganisms increases.
• With the higher metabolic rate, the chlorine is taken into
the microbial cell faster, and its bactericidal effect is
significantly increased.
• The higher the temperature the more likely the
disinfection will produce the desired results.
129

130. 130
Temperature Effect on Chlorine
• Virus studies indicate that the contact time should be
increased by two to three times to achieve comparable
inactivation levels when the water temperature is lowered
by 10°C (Clarke et al., 1962).
• Steam injection has been used to elevate temperatures in
a well and the area surrounding the Well bore
130

131. 131
Contact time
• Time is required in order that any pathogens present in the
water are inactivated.
• The time taken for different types of microbes to be killed
varies widely.
• it is important to ensure that adequate contact time is
available before water enters a distribution system or is
collected for use
• In general, amoebic cysts are very resistant and require
most exposure. Bacteria, including free-living Vibrio
cholerae are rapidly inactivated by free chlorine under
normal conditions. 131

132. 132
Contact time
• For example, a chlorine residual of 1mg/l after 30 minutes
will kill schistosomiasis cercariae, while 2mg/l after 30
minutes may be required to kill amoebic cysts.
• Contact time in piped supplies is normally assured by
passing the water, after addition of chlorine, into a tank from
which it is then abstracted.
• In small community supplies this is often the storage
reservoir (storage tank). In larger systems purpose-built
tanks with baffles may be used. These have the advantage
that they are less prone to "short circuiting" than simple
tanks. 132

133. 133
Germicidal Efficiency of Chlorine
• The major factors affecting the germicidal efficiency of the
free chlorine residual process are: chlorine residual
concentration - contact time – pH - water temperature.
• Increasing the chlorine residual, the contact time, or the water
temperature increases the germicidal efficiency. Increasing
the pH above 7.5 drastically decreases the germicidal
efficiency of free chlorine.
133

134. 134
• Chlorine dissolved in water, regardless of whether sodium
hypochlorite or calcium hypochlorite is used as the source of
the chlorine, generally exists in two forms, depending on the
pH of the water:
- HOCl - hypochlorous acid (biocidal)
- OCl - hypochlorite ion (oxidative)
• Hypochlorous acid is the most effective of all the chlorine
residual fractions
• Hypochlorous acid is 100 times more effective as a
disinfectant than the hypochlorite ion
Germicidal Efficiency of Chlorine

135. 135
Breakpoint Chlorination
• The type of chlorine dosing
normally applied to piped
water supply systems is
referred to as breakpoint
chlorination. Sufficient
chlorine is added to satisfy all
of the chlorine demand and
then sufficient extra chlorine
is added for the purposes of
disinfection.
Distilled water and rainwater (no Cl2
demand) will not show a breakpoint.
Breakpo
int
135

136. 136
The “Breakpoint”…another look
Chlorine is
reduced to
chlorides by
easily
oxidizable
stuff (H2S,
Fe2+, etc.)
Chloramines
broken down
& converted to
nitrogen gas
which leaves
the system
(Breakpoint).
Cl2 consumed
by reaction
with organic
matter. If
NH3 is
present,
chloramine
formation
begins.
At this
point,THM
formation
can occur
136

137. 137
1. Chlorine is a health concern at certain levels of exposure.
2. Drinking water containing chlorine well in excess of
drinking water standards could cause irritating effects to eyes
and nose.
3. Some people who drink water containing chlorine well in
excess of standards could experience stomach discomfort.
4. Drinking water standards for chlorine protect against the risk
of these adverse effects.
5. Little or no risk with drinking water that meets the drinking
water standard level and should be considered safe with
respect to chlorine.
Can we have too much Chlorine?
137

138. 138
Chlorine Residual Testing
The presence of chlorine residual in drinking water indicates
that:
• a sufficient amount of chlorine was added initially to the
water to inactivate the bacteria and some viruses that
cause diarrheal disease; and,
• the water is protected from recontamination during
storage. The presence of free residual chlorine in drinking
water is correlated with the absence of disease-causing
organisms, and thus is a measure of the potability of
water.
• The following accounts for the methods which can be
employed to test residual chlorine: 138

139. Chlorine Residual Testing contd
- Orthotolidine test
- D.P.D test
- Chlorotex test
- Starch-iodide test
139

140. 140
Restricted Water Use During Chlorination
1. Do not drink the water and avoid all body contact.
2. Water use should be minimized to assure that chlorine remains in the
well during the minimum contact period.
3. If strong chlorine odors are detected, ventilate the effected area
immediately, and minimize exposure to the fumes.
4. Avoid doing laundry, filling fish tanks, watering plants and using
water for other purposes where the chlorine may have an adverse
effect.
140

141. Special Methods of Chlorination
Chlorine is generally applied after all other treatment
have been given to the water supply. The special
methods of chlorination may be as follows:
Post-chlorination
• When chlorine is added in the water after all
treatments, it is known as post chlorination, it is
generally done after filteration. The chlorine is
commonly added in the clear water reservoir. The
minimum contact period should be 30 min, before
use of water. 141

142. Special Methods of Chlorination
Plain chlorination
• When only chlorine treatment is given to raw
water, the process is called plain chlorination. The
amount of chlorine required is 0.5 mg/l
Prechlorination
• It is the application of chlorine before filtration. It
may be added in the suction pipes or in the
miximing basins.
142

143. Special Methods of Chlorination
• It reduces bacterial load on filters, this results
increased filter runs and oxidizes excessive organic
matter. This helps in removing taste and odour and
makes the water fit for use.
Super-chlorination
• It is application of excessive amount of chlorine to
water. The amount of chlorine may vary from 5 to
15 mg/l of water. This is not ordinarily employed
but is practised only during the epidermic of water
borne diseases. 143

144. Special Methods of Chlorination
Double-chlorination
• It is application of chlorine at two points in the
treatment process. It is also prechlorination with an
added treatment to the final effluent from the
filters.
Break-point chlorination
• This term gives an idea of the extent of chlorine
added to water.
144

145. Special Methods of Chlorination
• It represents a dose of chlorination beyond which
any further addition of chlorine will appear as free
residual chlorine
Dechlorination
• The process of removing excess chlorine from
water.
• It is done in such a way that some residual chlorine
remains in water. Dechlorinating agents or
chemicals used are:
– Potassium permanganate 145

146. Special Methods of Chlorination
– Sodium bisulphate
– Sodium thiosulphate
– Sodium sulphite
– Sulphur dioxide etc
146

147. CHLORINATION BY-PRODUCTS
• By-products created from the reactions between
inorganic compounds and chlorine are harmless
and can be easily removed by filtration.
• Other by-products such as chloramine are
beneficial to disinfection process.
• Other by-products are:
 TRIHALOMETHANES
 Formed by reaction between chlorine and organic
material such as humic acid and fulvic acid to
create haloginated organics.

148. • Trihalomethanes are carcinogenic.
• The trihalomethane of most concern is
chloroform.
• Chronic exposure may cause damage to liver
and kidneys.
 TRICHLOROACETIC ACID
• Produced commercially for use as a herbicide
and is also produced in drinking water.

149.  DICHLOROACETIC ACID
• It is an irritant ,corrosive and destructive
against mucous membrane.
 HALOACETONITRILES
• Used as pesticide in the past ,but no longer
manufactured.

150. • They are produced as a result of reaction
between chlorine ,natural organic matter and
bromide.
 CHLOROPHENOLS
Cause taste and odor problems.
• They are toxic when present in higher
concentrations.
• Affect the respiration and energy storage
process in the body.

151. References
 ALAN C. TWORT, DON D. RATNAYAKA &
MALCOLM J. BRANDT. (2000) Water Supply.5th ed.
London: Eliane Wigzell, pp 267-317.
 A.K. UPADHYAY. (2009) Water Supply and Waste
Water Engineering. India: Sanjeev Kataria, pp 59-
94,120-128.
 Water Quality, Control and Treatment notes by Dr.
Khamis AL-Mahallawi
151

152. References
 http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-
KANPUR/wasteWater/Domestic water treat.htm
 http://resources.jorum.ac.uk/xmlui/handle/123456789/
1015
152