CHAPTER – 1

INTRODUCTION TO WTP

1.0 INTRODUCTION

1.1 Potable water purification

Water purification is the removal of contaminants from untreated water to produce drinking water that is pure enough for the most critical of its intended uses, usually for human consumption. Substances that are removed during the process of drinking water treatment include suspended solids, bacteria, algae, viruses, fungi, minerals such as iron, manganese and sulphur, and other chemical pollutants such as fertilizers. Measures taken to ensure water quality not only relate to the treatment of the water, but to its conveyance and distribution after treatment as well. It is therefore common practice to have residual disinfectants in the treated water in order to kill any bacteriological contamination during distribution. World Health Organization (WHO) guidelines are generally followed throughout the world for drinking water quality requirements. In addition to the WHO guidelines, each country or territory or water supply body can have their own guidelines in order for consumers to have access to safe drinking water.

1.2 Processes for drinking water treatment

A combination selected from the following processes is used for municipal drinking water treatment worldwide:

1. Pre-chlorination - for algae control and arresting any biological growth. 2. Aeration - along with pre-chlorination for removal of dissolved iron and manganese. 3. Coagulation - for flocculation.4. Coagulant aids, also known as polyelectrolyte - to improve coagulation and for

thicker flock formation.5. Sedimentation - for solids separation, that is, removal of suspended solids trapped in

the floc. 6. Filtration - removing particles from water.7. Desalination - Process of removing salt from the water. 8. Disinfection - for killing bacteria.

1.3 Objectives of Water Treatment Plant

1. To remove colour, unwanted gases and merciless of water.2. To remove objectionable taste and odour.3. To remove diseases producing microorganisms.

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4. To remove hardness of water.5. To remove iron and magnesium present in water.6. To remove settle able suspended impurities and non settle able colloidal impurities.7. To make water suitable for a wide range of industrial purpose.8. To make the water potable so that all the characteristics of water are within the

permissible limits.

1.4Unit Operations and Unit ProcessesThe term ‘unit operation’ is applied to procedures in which changes produced are

essentially physical. However, in water and waste water treatment, unit operations are those

procedures in which the changes produced are physical, chemical or biological. The unit

operation has following advantages:

1. It gives better understanding of the processes inherent in the treatment, and of the

capabilities of these processes in attaining objective.

2. It helps in the development of mathematical and physical modes of treatment

mechanisms and the consequent design of treatment plants.

3. It helps in the coordination of effective treatment procedure to attain desired plant

performance and effluent quality.

The analysis of the physical, chemical and biological unit operations is generally achieved

by:

i. Development of mathematical models and

ii. Construction of simple physical model to produce the desired reaction.

1.5 Important Unit operations

The following are the important unit operations employed in water and waste water

treatment.

1. Gas Transfer: Aeration.

2. Ion Transfer:

a) Chemical coagulation

b) Chemical precipitation

c) Ion exchange

d) Adsorption.

3. Solute Stabilization:

a) Chlorination

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b) Liming

c) Recarbonation

d) Super-chlorination

4. Solids Transfer:

a) Straining

b) Sedimentation

c) Floatation

d) Filtration

5. Nutrient or molecular Transfer

6. Interfacial contact

7. Miscellaneous operations:

a) Disinfection

b) Copper sulphating

c) Fluoridation

d) Thermal desalination.

8. Solid concentration and stabilization

a) Thickening

b) Centrifuging

c) Chemical conditioning

d) Elutriation

e) Biological floatation

f) Vacuum filtration

g) Air drying

h) Heat drying

i) Sludge digestion

j) Incineration

k) Wet combustion

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1.6 Layout of Water Treatment Plant

1.7 Functions of Units of WTP:

1. Screening: It is used to remove all the floating matters from the surface water such as plastic, polyethylene, etc. It is generally provided at the intake point.

2. Aeration: It is the process of bringing water in intimate contact with air so as to increase the oxygen concentration in water. It works on the Principle of gas exchange process.

3. Coagulation: The process of adding and mixing certain chemicals with water in order to form an insoluble gelatinous precipitate for removal of very fine, suspended and colloidal matter is called coagulation.

4. Sedimentation: To remove inorganic suspended impurities such as sand, silt, etc. Particles under the action of gravity without using coagulant. This process is also known as Plain Sedimentation.

5. Sedimentation with Coagulation: To remove very fine suspended impurities and colloidal impurities and also some bacteria with the help of adding coagulant in sedimentation tank. This process is also known as Clarification.

6. Filtration: It removes the fine particles with the help of filter media which are left in the water after the complete process of sedimentation. In this process microorganisms are also removed.

7. Disinfection: To remove pathogenic microorganism present in the water and the future possible contamination of water during long process of distribution.

8. Miscellaneous Processes: Many processes are carried out depending upon the chemical characteristics of water like water softening, removal of iron, magnesium, excess fluoride, etc.

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1.8 Requirements of Water Treatment Facility:1. It is preferably to locate the plant as near as possible to the area of distribution

in order to avoid the risk of water getting contaminant in transient. 2. If the sources is at lower level than the distribution area and the plant may be

located near the source.3. In very large cities, small disinfection unit may be installed and scattered over

the city to guard against the possible contamination during conveyance.4. Various units of the plants should be located in a proper sequence to ensure

that the flows by gravity without pumping.5. The treatment layout should be compact so that less space and less piping is

required.6. There should be sufficient space for future expansion.7. Provision must be made at the plant for storage, laboratories and shelter for the

working person.8. The Plant should be provided with essential services like approach road,

power supply, toilets, telephone facility, etc.

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CHAPTER – 2

AERATION

2.0 AERATION It is the process of bringing water in intimate contact with air so as to increase the oxygen concentration in water. It works on the principle of gas exchange process.

2.1 Objectives

1. It removes tastes & odours caused by gases due to organic decomposition.

2. It remove the unwanted gases like H2S gases from water, hence odour due to this is also removed also the presence of CO2 can be removed up to 70%

3. It removes Iron, Manganese up to certain limit.

4. It increases the dissolved oxygen content of the water.

5. Due to agitation of water bacteria may be killed to some extent.

6. Also its purpose is to reduced the thickness of liquid by continues agitation

Of liquid & enable it to come in contact with atmospheric air.

2.2 Types of Aerators

a) Gravity Aerators:

i. Cascade Aerators

ii. Inclined plane Aerators

iii. Vertical Stack Aerator or Tray Aerators

b) Diffused Air Aerator

c) Trickling Bed Aerator

d) Spray Aerator

2.2.1 Cascade Aerator

1. In this method the water is allowed to fall over a series of concrete steps or over a weir in a thin film, during the fall, water gets thoroughly mixed with Atmospheric air and gets aerated.

2. Aeration method can remove iron, manganese, CO2, H2S and oxidize certain other organic impurities present in water.

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3. But it cannot remove 100% CO2 and its residue always remains in small quantity of 3.5 mg/lit.

2.2.1.2 Inclined Apron Aerator

In this type of aerator, water is allowed to fall along an inclined plane/apron which is fixed

with riffle plates. The breaking up to the sheet of water will cause agitation of water and

consequent aeration.

2.2.1.3 Slat Tray Aerator

It consists of a closed round or square structure containing a series of closely stacked

superimposed wood slat tray.

Water enters from the top of aerator and evenly distributed over the topmost layer.

Water is transferred from the top layer to the bottom layer.

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Air is supplied from the bottom of the aerator with the help of blower, which blows it

upward.

A ventilator is provided at the top, which discharges unwanted gases upward to the

atmosphere.

Water is then collected at the collector pan and further discharges.

2.2.1.4 Gravel Bed Aerator (Trickling Beds)

In this aerator water is applied at the top and trickles down through the bed of gravel

while air is applied in upward direction.

The thickness of gravel bed may be from 1 to 1.5m.

In another form i.e. trickling beds 3 to 4 layers are filled with coke, slag or lime stone.

Thickness of bed is generally 0.50 to 0.60m while the vertical distance between each

tray is kept about 0.50m.

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2.2.2 Air Diffusion

In this method perforated pipes are fixed in bottom of settling tanks. The Compressed

air is blown through pipes which comes out in the form of Bubbles & stir the whole water at

greater speeds.

During upward moment of air it is thoroughly mixed with water and does it’s aeration. The aeration tanks are usually made 2.5 to 3.0m deep and work on principal of continuous flow, having minimum detention period of 15. (At average flow)

The quantity of air consumed varies from 0.3 to 0.6m per 100lit of water.

2.2.3 Spray Nozzle Aerator

In this type water is thrown up in the air into fine spray of a height of 2 - 2.5m under water pressure of 0.7 to 1.15kg/cm2

When the small particles of water come in contact with air in a greater Surface area, they absorb in it, and the water is aerated.

The dissolved gasses like H2S, CO2, etc. escape into atmosphere & oxidation of various substance and organic matter takes place.

2.3 GAS TRANSFER

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In this unit operation gases are either released/desorbed from water or are

dissolved/absorbed in water. This is achieved by exposing the water through spray or bubble

aeration to either air or other atmospheres, under normal, increased or reduced pressures. This

is an important unit operation which is carried out for water purification to achieve the

following.

1. Removal of objectionable gases such as carbon dioxide, hydrogen sulphide and

other volatile odorous substances, by spray or bubble aeration.

2. Deferrization and/or demagnetisation of water, through the addition of oxygen by

spray or bubble aeration.

3. Addition of ozone from ozone generators or chlorine gas from chlorine dispensers,

either for the disinfection of waters or for the destruction of odours and tastes in

water.

4. Addition of carbon dioxide from the gas or carbon dioxide generators to

recarbonate lime softened water.

5. Removal of corrosion-prompting oxygen as well as other gases (degasification) by

spraying water into vacuum chamber at ordinary temperatures or at elevated

temperatures.

The first two operations of gas transfer, mentioned above, are commonly called

aeration. In most instances, the shared engineering objective of aeration is either removal of

gas and other volatile substances from the water or their addition to water, or both at the same

time.

2.4 Factors governing aeration

1. Time of exposure

2. Area of exposure

3. Rate of gas absorption

4. Rate of gas desorption

2.5 Limitations of aeration

1. It is not an efficient method of removal of taste & odours caused by relatively non- volatile substances like oils & algae etc.

2. Odour removal is 50% only when symura was causative organism.

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3. Taste & odours caused by chemicals due to industrial waste discharged into receiving waters are not satisfactorily reduced.

4. Aeration may add more oxygen in water making it more corrosive while removing iron & manganese.

5. Iron & manganese can be precipitated by aeration only when organic matter is not present.

6. Addition lime may be required to neutralize the CO2 that would be removed by aeration.

7. Aeration is economical only in warmer months.

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