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Mr.K. Prabhakaran

Assistant Professor

Kongu Engineering College

DEPARTMENT OF CHEMISTRY

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Sources of Water

1. Surface Water

Rain Water - Pure but contaminated with gases

River Water - High dissolved salts moderate organics

Lake Water - High organics

Sea Water - High salinity, pathogens, organics

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2. Underground Water

Spring/Well Water - Crystal clear but high dissolved salts and free from organics

Classification of Impurities in water

1. Colour
2. Turbidity
3. Taste
4. Odour
5. Conductivity

1. Acidity (pH)
2. Gases (CO₂, O₂, NH₃)
3. Minerals
4. pH
5. Salinity
6. Alkalinity
7. Hardness

1. Microorganism
2. Water Bodies

Physical Impurities

Chemical Impurities

Biological Impurities

MAJOR IMPURITIES OF WATER

Cationic Calcium

Magnesium

Anionic Bicarbonate

Carbonate Hydroxide

Nonionic and undissolved Turbidity, silt, mud, dirt and

other suspended matter

Gases CO₂

H₂S NH₃ CH₄ O₂

Sodium Potassium Ammonium Iron Manganese

Sulfate Chloride Nitrate Phosphate

Color, Plankton Organic matter Colloidal silica Microorganisms Bacteria

Types of water

Soft Water

When water is mixed with soap it forms lather or foam is called as soft water.

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Hard Water

When water is mixed with soap it does not form lather or foam, it forms white scum or precipitate is called as hard water.

Hardness of Water

• Hardness of water is a characteristic that prevents the ‘lathering of soap’, thus water which does not produce lather with soap solution readily, but forms a white precipitate is called hard water.

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• Type of Hardness

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• Temporary or Carbonate Hardness
• Permanent Hardness or Non-carbonate Hardness.

Temporary Hardness

Temporary Hardness is caused by the presence of dissolved bicarbonate of calcium, magnesium and other heavy metals and the carbonate of iron. It is mostly destroyed by more boiling of water. While boiling bicarbonates are decomposed yielding insoluble carbonates.

Ca(HCO₃)₂

Calcium bicarbonate

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Mg( HCO₃)₂

CaCO₃ + H₂O + CO₂

Calcium Carbonate

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Mg(OH) ₂ + 2CO₂

Magnesium hydroxide

Magnesium Bicarbonate

Calcium/Magnesium Carbonates thus formed being almost insoluble, are deposited as a scale at the bottom of vessel, while carbon dioxide escapes out.

Heat

Heat

2C₁₇H₃₅COONa + CaCl₂

Sodium

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2C₁₇H₃₅COONa + MgSO₄

Sodium stearate

stearate

Hardness

Hardness

(C₁₇H₃₅COO)₂Ca + 2NaCl

Calcium stearate (Insoluble)

(C₁₇H₃₅COO)₂Mg + 2Na₂SO₄

Magnesium stearate (Insoluble)

Permanent Hardness

Non Carbonate Hardness is due to the presence of chlorides, sulfates of calcium, Magnesium, iron and other heavy metals

Units of Hardness

Expression of CaCO₃ equivalent hardness

Calcium carbonate equivalent =

Mass of hardness

producing substance

_X Molecular weight

of CaCO ₃

Molecular weight of hardness

producing substances

Problem 1

1. Calculate the calcium carbonate equivalent hardness of a water sample containing 204 mg of CaSO₄ per litre

Solution:

Calcium carbonate equivalent hardness =

240 100

120

= 150 mg of CaCO₃

= 150 ppm

2. Calculate the calcium carbonate equivalent hardness of a water sample containing 240 mg of MgSO₄ per litre

Solution:

Calcium carbonate equivalent hardness =

= 200 mg of CaCO₃

= 200 ppm

204 100

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Expression of Hardness

Estimation of Hardness of Water by EDTA Method

• The hardness of water can be estimated by methods such as gravimetric analysis, EDTA titration, atomic absorption, etc.,
• In the above methods, EDTA titration is the most inexpensive and simple way of determining the hardness.
• Hardness is usually determined by titrating it with a standard solution of ethylenediamminetetraacetic acid, EDTA.
• The EDTA is a complexing, or chelating agent used to capture the metal ions. This causes the water to become softened, but the metal ions are not removed from the water.
• This method includes a series of titrations to determine the total, permanent and temporary hardness of the given water sample.

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Principle

EDTA forms metal complexes with hardness producing metal ions (Ca²⁺, Mg²⁺) in water.

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pH is maintained between 8 to 10 using ammonia buffer solution (NH₄Cl + NH₄OH) and indicator EBT is added to the sample water, it forms indicator-metal complexes of wine red colour.

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Ca²⁺/ Mg²⁺ + EBT [Ca/Mg-EBT]

(Unstable complex)

wine red colour

This wine red colour solution is titrated against EDTA , then EDTA replaces the EBT indicator from [Ca/Mg-EBT] complex. The colour of the solution changes from wine red to steel blue at the end point.

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Total hardness is

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[Ca/Mg-EBT] + EDTA [Ca/Mg-EDTA] + EBT

(Unstable complex) (Stable complex) Steel blue colour

wine red colour

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Burette solution - Unknown EDTA solution

Pipette solution - 20 ml of Standard Hard water

Condition - Room Temp.

Reagents to

be added - 5 ml of buffer solution

Indicator - 2 drops of EBT (Eriochrome Black-T)

End point - Colour change from wine red to steel blue

Let the volume of EDTA consumed be V₁ ml.

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V₁ ml of EDTA consumes 20 ml of standard hard water

(since 1ml of standard hard water contains = 1 mg of CaCO₃)

20 ml of standard hard water contains = 20 * 1 mg of CaCO₃ eq. hardness

V₁ml of EDTA consumes = 20 mg of CaCO₃ eq. hardness

1ml of EDTA consumes = 20/V₁ mg of CaCO₃ eq. hardness

(ii) Estimation of Total Hardness

Calculation

Burette solution - Standardised EDTA solution

Pipette solution - 20 ml of Sample Hard water

Condition - Room Temp.

Reagents to

be added - 5 ml of buffer solution

Indicator - 2 drops of EBT (Eriochrome Black-T)

End point - Colour change from wine red to steel blue

Let the volume of EDTA consumed be V₂ml.

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V₂ ml of EDTA consumes 20 ml of sample hard water

1ml of EDTA consumes = 20/V₁ mg of CaCO₃ eq. hardness

V₂ ml of EDTA consumes = V₂ * 20/V₁ mg of CaCO₃ eq. hardness

20 ml of sample hard water consumes = V₂ * 20/V₁ mg of CaCO₃ eq. hardness

1 ml of sample hard water consumes = V₂/20 * 20/V₁ mg of CaCO₃ eq. hardness

V₂ 20

Therefore 1000 ml of sample hard water contains = *1000 mg/lit or ppm

20 V₁

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Total Hardness = V₂ /V₁ * 1000 mg/lit or ppm

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(iii) Estimation of Permanent Hardness

Burette solution - Standardised EDTA solution

Pipette solution - 20 ml of Boiled Hard water

Condition - Room Temp.

Reagents to

be added - 5 ml of buffer solution

Indicator - 2 drops of EBT (Eriochrome Black-T)

End point - Colour change from wine red to steel blue

Let the volume of EDTA consumed be V₃ ml.

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V₃ ml of EDTA consumes with 20 ml of boiled sample hard water

1ml of EDTA consumes = 20/V₁ mg of CaCO₃ eq. hardness

V₃ ml of EDTA consumes = V₃ * 20/V₁ mg of CaCO₃ eq. hardness

20 ml of boiled hard water consumes = V₃ * 20/V₁ mg of CaCO₃ eq. hardness

1 ml of boiled hard water consumes = V₃/20 * 20/V₁ mg of CaCO₃ eq. hardness

V₃ 20

Therefore 1000ml of boiled hard water contains = * 1000 mg/lit or ppm

20 V₁

Permanent Hardness = V₃/V₁ * 1000 mg/lit or ppm

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Temporary Hardness = Total Hardness – Permanent Hardness

OR

Total Hardness = Temporary Hardness + Permanent Hardness

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Temporary Hardness = V₂ /V₁ * 1000 - V₃/V₁ * 1000 mg/lit or ppm

= 1000 [ V₂/V₁ – V₃/V₁] mg/lit or ppm

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Temporary Hardness = 1000 V₂ – V₃ mg/lit or ppm

V₁

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Advantages:

• More accurate
• Convenient and more rapid process
• Most reliable method

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Estimation of Temporary Hardness

Industrial use:

(i) Textile industry: Hard water causes much of the soap (used in washing yarn, fabric etc.,) to go as waste, because hard water cannot produce good quality of lather. Moreover, precipitated of calcium and magnesium soaps adhere to the fabrics. These fabrics, when dyed latter on, do not produce exact shades of colour. Iron and manganese salts-containing water may cause coloured spots on fabrics, thereby spoiling their beauty.

(ii) Sugar industry: Water containing sulphates, nitrates, alkali carbonated, etc., if used in sugar refining, causes difficulties in the crystallization of sugar. Moreover, the sugar so-produced may be deliquescent.

(iii) Dyeing industry: The dissolved calcium, magnesium and iron salts in hard water may react with costly dyes, forming undesirable precipitated, which yields impure shades and give spots on the fabrics being dyed.

(iv) Paper industry: Calcium and magnesium salts tend to react with chemicals and other materials employed to provide a smooth and glossy (i.e., shining) finish to paper. Moreover, iron salts may even affect the colour of the paper being produced.

(v) Laundry: Hard water, if used in laundry, causes much of the soap used in washing to go as waste. Iron salts may even cause coloration of the clothes.

(vi) Concrete making: Water containing chlorides and sulphates, if used for concrete making, affects the hydration of cement and the final strength of the hardened concrete.

(vii) Pharmaceutical industry: Hard water, if used for preparing pharmaceutical products (like drugs, injections, ointments, etc.,) may produce certain undesirable products in them.

Disadvantages of using hard water in Industries

Boiler Feedwater

• One of the chief use of water is generation of steam by boilers.

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Essential requirements of Boiler Feed Water :-

It should be free from

• Turbidity, oil, dissolved salts
• Hardness & scale forming constituents
• Dissolved O₂ & CO₂
• Caustic alkali

Boiler Troubles due to Hard Water

• If hard Water is directly fed into boiler there arise many problems such as

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• Sludge & Scale Formation
• Boiler corrosion
• Caustic Embrittlement
• Priming & Foaming

1.Sludge

Slimy loose precipitate called sludge suspended in water

water

Boiler wall

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• Sludge is a soft, loose and slimy precipitate formed within the boiler.
• It can be easily scrapped off with a wire brush.
• It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends.
• It is formed by substances which have greater solubility in hot water than in cold water,

e.g. MgCO₃, MgCl₂, CaCl₂, MgSO₄ etc.,

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Remedy: Sludges can be removed using wire brush or mild acid

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2. Scale

Hard adherent coating on inner walls of boiler

water

Boiler wall

Scales are hard substances which sticks very firmly to the inner surfaces of the boiler wall.

Scales are difficult to remove even with the help of a hammer and chisel.

Examples: CaSO₄, CaCO₃, Mg(OH)₂

Reasons for formation of scale

1. Presence of Ca(HCO₃) ₂ in low pressure boilers

Ca(HCO₃) ₂ Ca CO₃ + H₂O + CO₂

Calcium bicarbonate Calcium Carbonate (scale)

Low pressure boilers but in high pressure boilers it is soluble by forming Ca(OH) ₂

2. Presence of CaSO₄ in high pressure boilers

MgCl₂ + 2H₂O

Magnesium

chloride

Mg(OH)₂

scale

+ 2HCl

Mg(OH)₂ can also be generated by thermally decomposing of Mg(HCO₃)₂

4. Presence of SiO₂

It forms insoluble hard adherent CaSiO₃ and MgSiO₃ as scales

3. Presence of MgCl₂ in high temperature boilers

Disadvantages of scale formation

1. Fuel wastage – scales have low thermal conductivity

2. Degradation of boiler material and increases of risk of accident

3. Reduces the efficiency of the boiler and deposit on the valves and condensers
4. The boiler may explode – if crack occurs in scale

Remedies: Removal of scale

1. Using scrapper, wire brush often
2. By thermal shock- heating and cooling suddenly with cold water
3. Using chemicals – 5- 10% HCl and by adding EDTA

II. Caustic Embrittlement

• Excess sodium carbonate used up for removing hardness which result in the formation of NaOH in high pressure boilers.
• NaOH has better mobility and can percolate into fine cracks present in boiler walls.

Na₂CO₃ + H₂O → 2 NaOH + CO₂

• NaOH gets concentrated in the fine cracks present in the boiler walls.

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• Concentrated NaOH region behaves as anode thus resulting in
• corrosion of boiler leading to the formation of sodium ferroate.

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Fe + 2NaOH Na₂FeO₂ + H₂

Remedies:

^{Sodium Ferroate}

• (i) Use phosphate salts instead of sodium carbonate
• (ii) Use Na₂SO₄ or agar-agar gel compounds to fill the fine cracks.
• (iii) Adjusting the pH of the feedwater between 8 and 9.

III. Priming and Foaming

Foaming

It is the production of continuous foam or hard bubbles in boiler. Foaming is due to the presence of substance like oil in boiling water.

Priming

It is the process in which some particles in water are carried along with the steam. The resulting process is called as wet steam or carry over. The process of formation of wet steam in boilers is called as priming.

Causes of Priming,

1. Presence of dissolved salts
2. High velocity steam due to sudden boiling
3. Improper boiler design

Foaming

Priming

Normal bubble

Carry over bubble

Prevention

1. Adding coagulants like sodium aluminate, aluminium hydroxide,

ferrous sulphate, etc.,

2. Adding antifoaming chemicals such as castor oil and

synthetic polyamides

IV. Boiler corrosion

Degradation or destruction of boiler materials (Fe) due to the chemical or electrochemical attack of dissolved gases or salts is called boiler corrosion.

Boiler corrosion is of three types

1. Corrosion due to dissolved O₂
2. Corrosion due to dissolved CO₂

3. Corrosion due to dissolved salts

1. Corrosion due to dissolved oxygen (DO)

^{2 Fe} + 2 H₂O + O₂ 2 Fe(OH)₂

4 Fe(OH)₂ + O₂

Ferrous hydroxide

2 [Fe₂O₃.2H₂O]

Rust

Removal of Dissolved Oxygen (DO)

1. By the addition of chemicals

The dissolved oxygen present in the boiler feed water can be removed by the addition of sodium sulphite or hydrazine and the reactions can be written as below

2 Na₂SO₃ + O₂ 2 Na₂SO₄

Na₂S + 2O₂

_{Sodium sulphide}

N₂H₄ + O₂

Na₂SO₄

N₂ + 2H₂O

Sodium sulphite

Dissolved Oxygen

Sodium sulphate

Hydrazine

Nitrogen

2. By mechanical deaeration

It comprises of a tall stainless tower with different layers capped with baffles to facilitate multiple equilibration.

The entire chamber is vacuumized and also maintained at high temperature using perforated heating plates on the walls.

Feed Water

To vacuum

Steam jacket

Perforated plate

Deaerated water

Cap

2. Corrosion due to dissolved CO₂

Presence of bicarbonate salts of either magnesium or calcium also causes the release of CO₂ inside the boiler apart from the dissolved CO₂

Ca(HCO₃)₂

CO₂ + H₂O

CaCO₃ + H ₂O + CO₂

H₂CO₃

_{Carbonic acid}

(causes slow corrosion)

1. It can be removed by the addition of ammonium hydroxide

NH₄OH + CO₂ (NH₄)₂CO₃ + H₂O

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3 . Corrosion due to dissolved salts

MgCl₂ + 2 H₂O Mg(OH)₂ + 2HCl

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Fe + 2 HCl FeCl₂ + H₂

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FeCl₂ + 2 H₂O Fe(OH)₂ + 2HCl

Removal of CO₂

2. Mechanical Deaeration

Alkalinity in water analysis

Is a measure of the ability of water to neutralize the acid

(or)

It is the tendency of water to accept H⁺ ions.

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In water analysis it is often desirable to know the kinds and amounts of the various of

alkalinity present in water.

The major portion of alkalinity in natural water is caused by presence of bicarbonates, carbonates and hydroxides of Ca, Mg, Na and K.

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CaCO₃ + CO₂+H₂O → Ca(HCO₃)₂

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Classification:

1. Bicarbonate ion alkalinity

2. Carbonate ion alkalinity

3. Hydroxide ion alkalinity

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Experimental Determination of Alkalinity

Principle

OH^- + H⁺ → H₂O ----- ----- 1 (phenolphthalein end point)

CO₃^2- + H⁺ → HCO₃^- --- ---- 2 (phenolphthalein end point) (methyl orange end point)

HCO₃^- + H⁺ → H₂O + CO₂ ---- 3

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The results are summarized in table from which the amount of OH^-,CO₃^2-, HCO₃^- present in sample can be calculated.

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All the three ions cannot exists together. OH^- and HCO^-₃ cannot be present at the same time, because

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OH^- + HCO₃^- → CO₃^2- + H₂O

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Short Procedure

Titration – I

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Standardization of acid

Burette solution - Unknown HCl

Pipette solution - 20 ml of standard sodium hydroxide

Condition - Room Temp.

Reagents to be

added - Nil

Indicator - 2 drops of phenolphthalein

End Point - colour change from pink to colourless

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Short Procedure :

Titraion – II

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Estimation of Alkalinity

Burette solution - Standardized HCl

Pipette solution - 20 ml of water sample

Condition - Room Temp.

Reagents to be

added - Nil

Indicator - 2 drops of phenolphthalein and 2 drops of methyl orange

End Point - pink to colourless (V₁ ml) and yellow to reddish orange (V₂ ml).

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Calculation of hydroxides, carbonates and bicarbonates

Reference Table

Calculation

Volume of acid used up to phenolphthalein end point = V₁ ml

Normality of acid = N₁

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phenolphthalein alkalinity(P) in terms of calcium carbonate equivalent V₁*N₁

= * 50 * 1000 mg/lit

20

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Additional volume of acid used up to methyl orange end point = V₂ ml

Normality of acid = N₁

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(V₁+V₂) * N₁

Methyl orange alkalinity (M) in terms of CaCO₃ Equation = *50*1000 mg/lit

20

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Then the calculation of OH^-, CO^2-₃, HCO₃^- is made with the help of above table.

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Softening of water

Methods of softening

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• Internal treatment process

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• External treatment process

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“Process of removing hardness”

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“Hardness is due to the presence of calcium, magnesium ions”

Internal treatment process

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Colloidal conditioning

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Phosphate conditioning

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Carbonate conditioning

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Calgon conditioning

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Sodium aluminate conditioning

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Electrical conditioning

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Radioactive conditioning

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• Softening of water can be done at the boiler itself

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• Addition of chemicals to the boiler water

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To precipitate scale forming impurities which can be removed

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To convert the impurities into soluble compounds

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Carbonate conditioning

Scale

Loose sludge

Calgon conditioning

External treatment process

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• Removal of Ca, Mg and other salts which would form insoluble metallic soaps �externally

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• Before feed into boilers, its softened

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Lime soda process

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Zeolite process

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Ion-Exchange (or) Demineralisation

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Ion-Exchange (or) Demineralisation (or) Deionization process

Ion exchange resins are insoluble, cross linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chain is responsible for the “ion-exchange” properties.

Cation exchange Resin

Resin after treatment

In general the resins containing acidic functional groups (-COOH, -SO₃H etc) are capable of exchanging their H⁺ ions with other cations, which comes in their contact; whereas those containing basic functional groups ( -NH₂, =NH as hydrochlorides) are capable of exchanging their anions with other ions, which comes in their contact.

Based on the above fact the resins are classified into two types

1. Cation exchange resin (RH⁺) –

Strongly acidic (SO₃^-H⁺) and weakly acidic (COO^-H⁺) cation exchange resins

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2. Anion Exchange resin (ROH^-) –

Strongly basic (R₄N⁺OH^-) and weakly basic (RNH₂⁺OH^-) anion exchange resins

Resins..

Structure of Cation and Anoin resins

R = CH₃

Cation exchange resin

Anion exchange resin

Ion exchange purifier (or) softener

Cation exchange Resin

Anion exchange Resin

Gravel bed

Hard water

Injector

Injector

Alkaline solution for regeneration of resin

Wastages to sink

Wastages to sink

Acid

solution for

regeneratio n of resin

pump

Soft water

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Reactions occurring at Cation exchange resin

Reactions occurring at Anion exchange resin

At the end of the process

Process or Ion-exchange mechanism involved in water softening

Regeneration of ion exchange resins

Advantages

1. The process can be used to soften highly acidic or alkaline waters
2. It produces water of very low hardness of 1-2 ppm. So the treated waters by this method can be used in high pressure boilers

Disadvantages

1. The setup is costly and it uses costly chemicals.
2. The water should not be turbid and the turbidity level should not be more than 10ppm.

Regeneration of Cation exchange resin

Regeneration of Anion exchange resin

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Treatment of water for municipal water supply

Requirements of drinking water

• Clear, odourless
• Good taste
• Suspended particle < 10 ppm
• pH = 7-8
• Dissolved salts < 500 ppm
• Fluoride < 1.5 ppm
• Free from dissolved gases like H₂S, ….
• Should not have heavy metals
• Free from pathogens

• Removal of suspended particles

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• Disinfection of pathogens

Major steps involved in treatments

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• Screening

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• Aeration

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• Settling in big tank to remove suspended particles

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• Sedimentation occurs by adding coagulants (alum, etc). Precipitates contains aluminium, ferrous, ferric hydroxides…

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Steps involved

Removal of suspended particles

Chemical coagulants

• Alum

Flocculent precipitate

• Sodium aluminate

• Ferrous sulphate

Heavy floc

Gelatinous floc

• Finally filter through sand / gravity filters

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Disinfection of pathogens

Sterilization

“Process of removing, killing, deactivating microorganisms” such as fungi, bacteria, viruses, spores, unicellular eukaryotic organisms such as Plasmodium, etc.) and other biological agents..

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Sterilization by physical methods

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Sterilization by chemical methods

Commonly used Disinfectants

- Chlorine (Cl₂)

- Chlorine dioxide (ClO₂)

- Hypo chlorite (OCl^-)

- Ozone (O₃)

- Halogens: bromine (Br₂), iodine (I)

- Bromine chloride (BrCl)

- Metals: copper (Cu²⁺), silver (Ag⁺)

- Potassium Permanganate (KMnO₄)

- Alcohols

- Soaps and detergents

- Hydrogen peroxide

- Several acids and bases

Roles of Disinfectants

• Minimization of DBP (Disinfection By-Products) formation (strong oxidants, including potassium permanganate and ozone, may be used to control DBP)

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• Oxidation of iron and manganese

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• Prevention of regrowth in the distribution system and maintenance of biological stability

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• Removal of taste and odours through chemical oxidation

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• Improvement of coagulation and �filtration efficiency

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• Prevention of algal growth in �sedimentation basins and filters

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• Removal of colour

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By adding bleaching powder

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Some examples of chemical sterilization methods

By adding Chloramine

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HOCl + Bacteria

Bacteria are killed

Hypochlorous acid

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By doing Ozonisation

Advantages

• Sterilisation, bleaching, decolourisation, de-odourisation done at same time
• Ozone does not impact unpleasant taste, odour, no change in pH
• Dosage, 2-3 ppm; time 10-15 mins

Disadvantages

• Expensive

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Breakpoint chlorination

• Process involving in adding chlorine or chlorine compounds such as sodium hypochlorite to water which kills bacteria, viruses and other microbes in water

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• Addition of chlorine to a water decreases the initial chlorine level. However, the organic matters (containing ammonia or nitrogen) present in the water starts decomposition which produces an increased combined chlorine content which is used to kill pathogens.

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• In a stage where no further chlorination , and the oxidation of chloramines / other impurities starts which results in fall in combined chlorine content.

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• After complete oxidation, residual �chlorine starts increasing at a certain �point which is termed as “Break point �chlorination”

• De-chlorination

Advantages

• Completely oxidises organic compounds, ammonia,…..
• Removes colour
• Destroy bacteria 100% completely
• Removes odour, bad taste
• Prevents growth of weeds