Prepared by

Innocent L Swai, Bsc EHS

inolswai@gmail.com

School Of Hygiene, Muhimbili.

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

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

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

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

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

Pure water supply

Washwater

Sludge

Alkalies

Coagulants

Sludge

Source

High lift

pumps

Pure water tanks

Chlorine

Filters

Main settling basins

Flocculat

ion

Mixing chamber

Pretreatment

Raw water pumps

Coarse screens

screen

Lagoon

7

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

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

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

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

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

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

processes13

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

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

Cont…

Source water intake structure16

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

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

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

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

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

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.

• 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

• 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

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

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

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

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

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

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

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

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

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

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

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

Traditional Circular

Clarifiers / Settling Zone

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

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

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

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

Aeration

Types of Aerators

• air into the water

• water into the air

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Types of Flocculation Tanks

Mechanical Flocculators

Paddle wheel Type (vertical and Horizontal Types)

Foil Type Mixing Blade

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

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

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

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

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

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

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

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

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

Process Control

A. Chemical Selection

B. Chemical Application / Solution Preparation

C. Monitoring Process Effectiveness

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

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

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

Process Control / Monitoring Process

Effectiveness

C- Monitoring Process Effectiveness

• (1) Jar Test

• (2) pH

• (3) Turbidity

• (4) Temperature

• (5) Alkalinity

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

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

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

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

JAR TEST

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

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

Rapid sand filters

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

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

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

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

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

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

Slow sand filters

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

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.

Differences between SSF an RSF

98

99

Disinfection

99

100

Background: Current Methods

of Disinfection

• Large-Scale:

– Chlorination

– Ozone

– UV irradiation

• Small Scale:

– Boiling

– Iodine tablets

– Filters100

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

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

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 DBP103

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

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

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

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 medium107

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

pH Effect on Chlorine

• Chlorine is a more effective disinfectant at pH levels between6.0 and 7.0, because hypochlorous acid is maximized at thesepH levels

• Any attempt to disinfect water with a pH greater than 9 to 10 ormore 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 subsequentlyraising the pH, the chlorine solution is actually less efficient as abiocide.

• Controlling the pH of the water in the aquifer is not practical.However buffering or pH-altering agents may be used to controlpH in the chlorine solution being placed in the well.

128

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

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

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

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

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

• 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

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 Cl2demand) will not show a breakpoint.

Breakpo

int

135

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

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

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

Chlorine Residual Testing contd

- Orthotolidine test

- D.P.D test

- Chlorotex test

- Starch-iodide test

139

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

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

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

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

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

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

Special Methods of Chlorination

– Sodium bisulphate

– Sodium thiosulphate

– Sodium sulphite

– Sulphur dioxide etc

146

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.

• 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.

DICHLOROACETIC ACID

• It is an irritant ,corrosive and destructive

against mucous membrane.

HALOACETONITRILES

• Used as pesticide in the past ,but no longer

manufactured.

• 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.

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

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