Unit 1 - Water TreatmentSources of water

1. Rainwater. It is the purest form of natural water. But unfortunately itdissolves the toxic gases like CO2, SO2, NO2 etc. and other solids.

2. Sea water. It is the most impure form of water containing about 3.5%dissolved salts of which about 2.6% is NaCl. Other salts present includesulfates, bicarbonates, bromides of sodium, potassium, magnesium etc.

3. River water. The sources of river water are the springs (A small stream ofwater flowing naturally from the earth) and the rainwater. River water whileflowing through the land collects lots of organic matters from falling trees andnearby habitats and also other soluble and suspended matters from the lands,soils etc.

4. Lake water. It is much purer than river water, dissolved impurities are lessbut contains lots of organic matter.

5. Underground water. The rainwater and other surface water percolate downthrough the soil and rocks and get filtered and finally collected on rockysurface or again come out as spring. Though it contains less suspended matterbut the dissolved mineral content is quite high and is of high organic purity.

Characteristics Imparted by impurities in water

Physical Impurities

(a) Color: Eg, Fen+, Mnn+, humus materials, tannins, peat, algae, weeds protozoa,

(b) Turbidity: Collodial, fine suspension, microorganism plankton

(c) Taste: (i) Bitter taste: iron aluminium, manganese, sulphate

(ii)Soapy taste: large amount of sodium bicarbonate

(iii) Brackish: unsual amount of salts

(iv) palatable: dissolved gases CO2 and minerals like nitrates

(d) Odour: (i) Presence of living organisms, decaying vegetations algea, bacteria..

(ii) inorganic and organic compounds N,S, & P; Sewage

(iii) industrial effluents – organic and inorganic

Chemical Impurities

• Acidity: dissolved CO2• Gases: Dissolved O2• Dissolved ammonia: decomposition of nitrogeneous organic matter

Sources of Drinking water

Deep Groundwater

Shallow Groundwater

Upland Lakes and Reservoirs

Rivers, canals & Low land reservoirs

Contaminants in Raw water

Suspended particles, including colloids

Dissolved inorganic salts

Dissolved organic campounds

Micro-organisms

Dissolved gases

(a) Palatable (i.e. no unpleasant taste);

(b) Safe (i.e. does not contain pathogens or chemicals harmful to the consumer);

(c) Clear (i.e. free from suspended solids and turbidity);

(d) Colourless and odourless (i.e. aesthetic to drink);

(e) Reasonably soft (i.e. allows consumers to wash clothes, dishes, themselves,

without use of excessive quantities of detergents or soap);

(f) Non-corrosive (i.e. to protect pipework and prevent leaching of metals from

tanks or pipes);

(g) Low organic content (i.e. high organic content results in unwanted biological

growth in pipes and storage tanks that often affects quality).

Water Analysis

• Hardness

• Alkalinity

• Chlorides

• Nitrate

• Sulphate

• Fluoride

• Dissolved Solids (TDS)

• Dissolved gases

Hardness of Water

Hardness of water is the characteristic of preventing lather formation of water with soap. Generally salts like chlorides, bicarbonates and sulfates of Ca2+, Mg2+ and Fe2+ make water hard.

This hard water on treatment with soap which is stearic or palmitic acid salts of sodium or potassium causes white precipitate formation of calcium or magnesium stearate or palmitate.

Thus the cause of hardness is the precipitation of the soap and hence prevents lathering at first. When the hardness causing ions are removed as insoluble soaps, water becomes soft and forms lather.

Types of Hardness

The hardness is of two types:

(i) Temporary hardness is due to the bicarbonates of Ca2+ and Mg2+ andcarbonate of Fe2+. Temporary hardness can be destroyed by mereboiling of water, when bicarbonate decomposed, yielding insolublecarbonates or hydroxides, which are deposited as crust at the bottom ofvessel

(ii) Permanent hardness is due to the presence of chlorides and sulfates ofCa, Mg, Fe, etc. Permanent hardness cannot be removed easily onboiling.

Soft: 0 – 20 mg/L as calcium

Moderately soft: 20-40 mg/L as calcium

Slightly hard: 40-60 mg/L as calcium

Moderately hard: 60-80 mg/L as calcium

Hard: 80-120 mg/L as calcium

Very hard: > 120 mg/L as calcium

Units of Hardness

Both temporary and permanent hardnesses are expressed in ppm as

CaCO3. The choice of CaCO3 is due to the fact that its mol. wt. is 100

and equivalent weight is 50 and it is the most insoluble salt in water.

Equivalent of CaCO3

Units of Hardness

Hardness is principally expressed in ppm unit. Other limits include

French degree of hardness, English degree of hardness or Clark, USA

degree of hardness and German degree of hardness.

Expressing Hardness, Alkalinity, and others

Hardness and Alkalnity are due to different ions, therefore, they are expressed in

unit weights of CaCO3 in order to facilitate for computations.

Both temporary and permanent harnesses are expressed in ppm as CaCO3. The choice of

CaCO3 is due to the fact that its mol. wt. is 100 and equivalent weight is 50 and it is the

most insoluble salt in water

The following equation can be used to express concentration as CaCO3

Example: The concentration of Ca(HCO3)2 was found to be 60 mg/l. Express the

concentration in units as CaCO3.

Solution: eq. wt. of Ca(HCO3)2= MW/Z = 162/2 = 81g.

conc. as CaCO3 = 60mg/l (50/81) = 37.0 mg/l as CaCO3

Units of Hardness

Total Dissolved Salts TDS

Drinking water 25-250 mg/L

- Should not exceed 500 mg/L

- Lake and steams 50-250mg/L

- River water – 100-20,000 mg/L

- Sea water 35,000 mg/L

- Amount of particles dissolved in the water.

Dissolved

Salt/ion Molar Mass

Chemical

Equivalent

Multipication factor for

converting into

Equivalents of CaCO3

Ca(HCO3)2 162 81 100/162

Mg(HCO3)2 146 73 100/146

CaSO4 136 68 100/136

CaCl2 111 55.5 100/111

MgSO4 120 60 100/120

MgCl2 95 47.5 100/95

CaCO3 100 50 100/100

MgCO3 84 42 100/84

CO2 44 22 100/44

Ca(NO3)2 164 82 100/164

Mg(NO3)2 148 74 100/148

HCO3- 61 61 100/122

OH- 17 17 100/34

CO32- 60 30 100/60

NaAlO2 82 82 100/164

Al2(SO4)3 342 57 100/114

FeSO4.7H2O 278 139 100/278

H+ 1 1 100/2

HCl 36.5 1 100/73

Disadvantage of hardwater

• In domestic use

– Washing: Hard water, when used for washing purposes, does

not lather freely with soap. Instead it produces sticky precipitates

of calcium and magnesium soaps. Similar problem exists in

bathing.

– Cooking: Due to the presence of dissolved hardness producing

salts the boiling point of water is elevated. Consequently more

fuel is and time are required for cooking.

– Drinking: Hard water causes bad effect on our digestive system.

The possibility of forming calcium oxalate crystals in urinary

tracks is increased (Kidney stones).

DO is determined by the titrimetric method developed by Winkler.

1. Dissolved molecular oxygen in water is not capable of reacting with KI,

therefore an oxygen carrier such as manganese hydroxide is used. Mn(OH)2is produced by the action of KOH on MnSO4.

2. Mn(OH)2 so obtained reacts with dissolved molecular oxygen to form a

brown precipitate of basic manganic oxide, MnO(OH)2.

3. MnO(OH)2 then reacts with concentrated sulphuric acid to liberate nascent

oxygen.

4. Nascent oxygen results in oxidation of KI to I2.

5. This liberated iodine is then titrated against standard sodium thiosulphate

solution using starch as an indicator.

6. Thiosulphate reduces iodine to iodide ions and itself gets oxidized to

tetrathionate ion.

Dissolved Oxygen

EDTA forms complexes with Ca2+, Mg2+ and many other metal cations.

Since EDTA is insoluble in water, the disodium salt EDTA is used as the

Permanent complexing reagent with Ca2+, Mg2+ ions.

Structure of EDTA-Metal complex

Ca2+

Estimation of hardness using EDTA method

PrincipleWhen the indicator EBT (Eriochrome black-T) is added to the water sample

the indicator forms a weak indicator-metal complex with Ca2+ and Mg2+ ions

giving a wine red color to it. When this is titrated against EDTA, latter forms a

complex with the remaining Ca2+, Mg2+ ions of water sample. Near the end

point the EDTA abstracts the Ca2+ and Mg2+ ions from the weak indictor-metal

complex thus releasing the free indicator. This results in a steel blue color.

EDTA-metal complex is found to be stable at pH 8-10

EBT + M2+ M-EBT + 2H+

Steel blue wine red

EDTA + M2+ M-EDTA + 2H+

stable complex

EDTA + M-EBT M-EDTA + EBT

Complex steel blue

EDTA is found to behave like a dicarboxylic acid, i.e. two of its carboxyl group

is found to be strongly acidic, the other two hydrogens are released during its complex

formation

Reagents Required

EDTA (0.01M) Dissolve 4 g disodium salt of EDTA in one litre distilled water

Standard hard water Dissolve 1 g of calcium carbonate with a small quantity of HCl and make it up toone litre using distilled water

EBT Dissolve 0.5 g of indicator in 100 ml of water

Ammonium chloride-ammonium hydroxide buffer (pH) Dissolve 67.5 g of pure ammonium chloride in 570 ml of pre-cooled conc. ammonium hydroxide and make it up to one litre with distilled water

Sample and boiled water• Ground water available in the area

• Ground water boiled for 30 min.

Structure of

EBT (Eriochrome black-T) - Blue

Procedure

Standardization of EDTA 20 ml of standard hard water (1 ml of this solution contains1 mg of CaCO3) is pipetted out into a clean conical flask. 5 ml of ammonium buffer

and a few drops of EBT indicator are added. This then titrated against EDTA until

the wine red color changes to steel blue color. The titration is repeated till a concordant

value is obtained

Let the volume of EDTA consumed be V1 ml

Estimation of total hardness 20 ml of a given sample water is pipetted into a clean concialflask . 5 ml of ammonium buffer and a few drops of EBT is added and titrated against EDTA

until the wine red color changes to steel blue color. The titration is repeated till the concordant

Value is obtained.

Let the volume of EDTA consmed be V2 ml

Estimation of permanent hardness 20 ml of given sample water is pipetted into a clean conical Flask and boiled. The boiled sample water is filtered and the filtrate is collected in another

conical flask. 5 ml of ammonia buffer and a few drops EBT indicator are added. This is then

titrated against EDTA until the wine red color changes to steel blue color. The titration is

repeated till the concordant value is obtained

Let the volume of EDTA consmed be V3 ml

Calculation

Standardisation of EDTA:1 ml of standard hard water = 1 mg of CaCO320 ml of standard hard water = 20 mg of CaCO320 ml of standard hard water takes up V1 ml of EDTA

1 ml of EDTA is consumed by of CaCO3 equivalent of hardness

One gram of CaCO3 was dissolved in dil. HCl and the solution was

diluted to one litre (1mg CaCO3 in 1 mL of solution).

50 mL of this solution required 45 mL of EDTA, while 50 mL of the sample

Water required 18 mL of EDTA solution

On the other hand, 50 mL of the boiled sample water, then titrated against EDTA

consumed 9 mL of Solution. Calculate each type of hardness in ppm

V1 = 45 mL

V2 = 18 mL

V3 = 9 mL

50 mL of a sample water consume 15 mL of 0.01 M EDTA before boiling

And 5 mL of the same EDTA after boiling.

Calculate the degree of hardness, permanent hardness & temporary hardness

1 mole of CaCO3 = 1 mole of EDTA

0.01 M CaCO3 = 0.01 M EDTA

50 mL of boiled sample = 5 mL of 0.01 M EDTA

Boilers

A boiler is a closed vessel in which water or other fluid is heated.

The heated or vaporized fluid exits the boiler for use in various processes or

heating applications

H2O evaporates –

dissolved concentration of salts

increases progressively

Reaches saturation point – salts

are thrown out of the water as precipitates in the walls of the boilers

http://en.wikipedia.org/wiki/Pressure_vesselhttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Fluid

1.Sludge and Scale Formation

2. Caustic embrittlement

3.Corrosion

4.Priming and Foaming

Sludge and Scale

Sludge formation in boilers

• Sludge is a soft, loose and slimy precipitate formed within the boiler.

• Sludge can be easily scrapped off with the wire brush.

• It is formed comparatively colder portions of the boiler and collects in the bends where the flow rate is slow

• Sludges are formed by substanes which have greater solubilities in hot water than in cold water, e.g., MgCO3, CaCl2, MgCl2, etc.

Disadvantage of sludge formation

• Sludges are poor conductor of heat, so they tend to waste a portion of heat.

• Excessive sludge formation disturbs the working of the boiler. It settle at the

bends thereby causing blocking

Prevention of sludge formation

By using well softened water and by frequently blow down operation

Scales

• Scales are the main source of boiler troubles. Scale composed

chiefly of calcium carbonate is soft and is the main cause of scale

formation in low-pressure boilers.

• But in high-pressure boilers, CaCO3 is soluble.

Scale

CaCO3+ H2O Ca(OH)2 (Soluble) + CO2

Decomposition of calcium bicarbonate

Disadvantage of scale formation

• Low thermal conductivity (poor conductor)

Thickness of scale in

(mm)

0.325 0.625 1.25 2.5 12

Wastage of fuel 10% 15% 50% 80% 150%

-Hinders the flow of heat from the source to the water

• Lowering boiler safety

-Reducing the life of the boilers

• Decrease in efficiency:- tubes of the boilers may be clogged by scales

• Corrode away the tubes

-Scales of certain salts like MgCl2

MgCl2 + Fe + 2H2O ----→ Mg(OH)2 + FeCl2 + H2

• The solubility of calcium sulphate in water decreases with increaseof temperature.

• The solubility of CaSO43,200 ppm at 15°C

55 ppm at 230°C and

27 ppm at 320°C

• CaSO4 gets precipitated as hard scale on the heated portion of theboiler. This is the main cause of scales in high-pressure boilers.

Decomposition of calcium sulphate

Calcium Sulphate — Most natural waters contain this salt combined with two

molecules of water which at a temperature of 260° F is dehydrated and deposited in

the anhydrous form as a hard crystalline scale. This scale is more destructive and

difficult to remove than any other excepting those due to silicates. Although not

originally present in the feed water this salt may be produced by the interaction of

other salts such as magnesium sulphate and calcium bicarbonate.

Hydrolysis of magnesium salts

Dissolved magnesium salts undergo hydrolysis forming magnesium

hydroxide precipitate which forms a soft type of scale

MgCl2 + 2H2O Mg(OH)2 + 2HCl

Presence of Silica

Presence of silica in small quantities deposits as calcium silicate

(CaSiO3) or magnesium silicate (MgSiO3). These deposits stick very

firmly on the inner side of the boiler surface and are very difficult

to remove

Silica is practically insoluble in water, but when combined as silicic acid or in

alkaline waters as sodium silicate may produce an intensely hard scale which is

removed with great difficulty

Removal of Scales

• By giving thermal Shock if they are brittle

(heating the boiler and then suddenly cooling with cold water)

• If they are adherent and hard, dissolving them with help of chemicals.

(chemical treatment)

- Calcium carbonate scales can dissolved by using 5-10% HCl.

- Calcium Sulphate scales can be dissolved by adding EDTA (ethylene diamine

tetraacetic acid) with which they form soluble complex.

• Frequent blow down operation

- If they scales are loosely adhering

Ca2+ EDTA Complex

Ca2+

Prevention of scale formation

External Treatment

- By feeding soft water - ‘softening of water’

(will be discussed later)

Internal Treatment

• Colloidal Conditioning - low pressure boilers - “kerosene, tannin, agar-agar)

• Phosphate conditioning – high pressure boilers

• Carbonate conditioning - low pressure boilers

3CaCl2 + 2Na3PO4 → Ca3(PO4)2 + 6NaClNaH2PO4 (acidic)

Na2HPO4 ( weak alkaline)

Na3PO4 (alkaline)

Na2P2O7 (alkaline)

CaSO4 + Na2CO3 CaCO3 + Na2SO4

• Calgon conditioning

-Addition of small amount of sodium hexametaphosphate (NaPO3)6(Calgon)

Calgon forms soluble complex compound with CaSO4

Na2[Na4(PO3)6] 2Na+ + [Na4P6O18]

2-

2CaSO4 + [Na4P6O18]2-→ [Ca2P6O18]

2- + 2Na2SO4

Soluble complex

Caustic Embrittlement- a type of boiler corrosion

Na2CO3 + H2O 2 NaOH + CO2

During softening of water (lime-soda processes) slight excess of NaCO3 is used

after water evaporation the conc. NaCO3 is left and undergo hydrolysis

Caused by a high concentration of NaOH in the boiler feed water.

It is characterized by the formation of irregular

intergranular cracks on the boiler metal,

particularly at places of high local stress

such as bends and joints

This NaOH flows into the minute hairline cracks present on the boiler material by capillary

action and dissolves the surrounding area of iron as Sodium ferroate, Na2FeO2

Fe + 2NaOH Na2FeO2 + H2

This type of electrochemical corrosion occurs when the

concentration of NaOH is above 100 ppm

Caustic embrittlement can be avoided

• By using sodium phosphate as softening reagent instead of sodium

carbonate

• By adding tannin or lignin to boilers water, since it blocks the hair-

cracks, thereby preventing infiltration of caustic soda

• By adding sodium sulphate to boiler water – It also blocks the hair-

cracks and preventing infiltration of caustic soda.

Iron at

stressed

parts

Conc.

NaOH

solution

Dil.

NaOH

solution

Iron at

plane

surfaces

+ __ cathodeanodeConcentration

cell

“Concentration cell corrosion occurs when two or more areas of a metal surface

are in contact with different concentration of the same solution”

Eventually Fe gets dissolved or corroded

Boiler CorrosionCorrosion – “constant tearing of the material”

- by chemical or electrochemical attack by its environment

(1) Dissolved oxygen in water at high temperature attack boiler material.

2Fe + 2H2O + O2 2Fe(OH)2

4Fe(OH)2 + O2 2[Fe2O3.2H2O]

(Ferrous hydroxide) (Rust)

Removal of dissolved oxygen

2Na2SO3 + O2 2Na2SO4

NH2NH2 + O2 N2+ 2H2O

Na2S + 2O2 Na2SO4

By adding calculated quantity

sodium sulphite

hydrazine

sodium sulphide

By Mechanical de-aeration

Water spraying in a perforated plate-fitted tower, heated from sides and

connected to Vacuum pump. High temperature, low pressure and large

exposed surface reduces dissolved oxygen in water

de-aerated water

Water feed

Dissolved Carbon dioxide

Removal of CO2

(a) by adding calculated quantity of ammonia

2NH4OH + CO2 (NH4)2CO3 + H2O

Source of CO2

• Dissolved CO2 in raw material

• Decomposition of Mg(HCO3)2

CO2 + H2O H2CO3

H2CO3 (carbonic acid) has slow corrosive effect on the boiler material

(a) by mechanical de-aeration

Mg(HCO3)2 MgCO3 + H2O + CO2∆

Acids from Dissolved Salts

Water containing dissolved magnesium salts liberate acids on hydrolysis

The liberated acid reacts with iron of the boiler in chain-like reactions

producing HCl again and again.

Fe + 2HCl FeCl2 + H2

FeCl2 + 2H2O Fe(OH)2 + 2HCl

MgCl2 + 2H2O Mg(OH)2 + 2HCl

presence of even a small amount of MgCl2will cause corrosion of iron to a large extent.

Stream sometimes may be associated with small droplets of water. Such steam

containing liquid water is called wet steam. These droplets of water carry with them

some dissolved salts and sludge materials present in water. This phenomenon is

called carry over. This occurs mainly due to priming and foaming

Priming and Foaming

Priming: Formation of wet steam by rapid boiling of the water at the heating surfaces

is called priming. It may be caused by

• very high water level

• high steam velocity

• sudden steam demands leading to sudden boiling

• improper boiler design

Prevented by

• keeping the water level lower

• good boiler design with a mechanical steam purifier

• avoiding rapid changes in steam rate

• using treated water

Foaming:

The formation of stable bubbles above the surface of water

Caused by

• the presence of oil or grease in water

• fine sludge particles

Prevented by

• Removing the foaming stabilizing agents such as soluble salts, clay, and organic

matter from water by using antifoaming chemicals like synthetic polyamides

• Adding coagulants such as sodium aluminate, ferrous sulphate etc. to remove

sludge particles

Softening methods

“process of removing the hardness producing substance from the

water” - softening of water

In Industry

• Lime soda process

• Zeolite process

• Ion exchange process

Lime-Soda process

Soluble calcium and magnesium salts in water are chemically converted into

insoluble compounds by adding calculated amount of lime [Ca(OH)2] and

Soda [Na2CO3].

Soluble Ca2+ and Mg2+ salts are precipitated as [CaCO3] and [Mg(OH)2] and

filtered.

• Removes Temporary Hardness

• Removes Permanent Hardness

• Removes Fe & Al salts

• Removes Free Mineral Acids

• Removes dissolved CO2 & H2S

• Removal of Bicarbonates from NaHCO3

CaSO4 and CaCl2 formed as byproduct in

lime treatment will react with calculated

amount of SODA (Na2CO3) to form CaCO3

Role of Lime Ca(OH)2 - Precipitates the following present in water

Removes Temporary Hardness

Mg(HCO3)2 + 2Ca(OH)2 → 2CaCO3 + Mg(OH)2 + 2H2O

Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O

Removes Permanent Hardness

MgCl2 + Ca(OH)2 → Mg(OH)2 + CaCl2

MgSO4 + Ca(OH)2 → Mg(OH)2 + CaSO4

Removes Fe & Al salts

FeSO4 + Ca(OH)2 → Fe(OH)2 + CaSO4

2Fe(OH)2 + H2O + ½O2 → 2Fe(OH)3

Al2(SO4)3 + 3Ca(OH)2 → 2Al(OH)3 + 3CaSO4

Removes Free Mineral Acids

H2SO4 + Ca(OH)2 → CaSO4 + 2H2O

2HCl + Ca(OH)2 → CaCl2 + 2H2O

Removes dissolved CO2 & H2S

CO2 + Ca(OH)2 → CaCO3 + H2O

H2S + Ca(OH)2 → CaS + 2H2O

Removal of Bicarbonates from NaHCO3

2HCO3- + Ca(OH)2 → CaCO3 + H2O + CO3

2-

Coagulants (NaAlO2)

NaAlO2 + H2O → Al(OH)3 + NaOH

(2NaOH is equal of Ca(OH)2)

CaSO4 and CaCl2 formed as byproduct in lime treatment will react with calculated

amount of SODA (Na2CO3) to form CaCO3

Role of SODA Na2CO3

CaCl2 + Na2CO3 → CaCO3 + 2NaCl

CaSO4 + Na2CO3 → CaCO3 + Na2SO4

(i) the permanent calcium hardness already present in water before lime treatment

(ii) permanent calcium hardness is introduced during the removal of Mg2+, Fe2+,

HCl and H2SO4 by lime in the water due to the formation of calcium salts

Wt of all substances are converted into CaCO3 equivalent, Then,

Since 100 parts of CaCO3 is equivalent to 74 parts of Calcium hydroxide,

Washing soda requirement parts of

Water sample gave the following constituents on analysis

Ca(HCO3)2 = 40. 5 ppm; Mg(HCO3)2 = 36.5 ppm; CaSO4 = 34.0 ppm

MgCl2 = 47.5 ppm, MgSO4 = 6.0 ppm & NaCl = 4.8 ppm

Calculate the amount of lime (95 % pure) and soda (90 % pure) needed

for the treatment of 20,000 litres of water, also calculate the cost of

chemicals if costs per 100 Kg of lime and soda are Rs.80 and Rs.2550 respectively

Calculate the amount of lime (purity 92%) and soda (purity 95%) required for softening

50,000 litres of water containing the following salts per litre

Ca(HCO3)2 = 8.1 mg Mg(HCO3)2 = 7.5 mg

CaSO4 = 13.6 mg MgSO4 = 12.0 mg

MgCl2 = 2.0 mg NaCl = 4.7 mg

Ca(HCO3)2 8.1 (mg)

Mg(HCO3 7.5

CaSO4 13.6

MgSO4 12.0

MgCl2 2.0

Ca(HCO3)2 = 8.1 mg (162) Mg(HCO3)2 = 7.5 mg (146)

CaSO4 = 13.6 mg (136) MgSO4 = 12.0 mg (120)

MgCl2 = 2.0 mg (95) NaCl = 4.7 mg

[Ca(HCO3)2 + 2(Mg(HCO3)2 + MgSO4 + MgCl2 as CaCO3 eq]

5 ppm

5.14

10

10

2.11

[5 + 2(5.14) + 10 + 2.11 as CaCO3 eq] for 1L

[CaSO4 + MgSO4 + MgCl2 as CaCO3 eq]

[10 + 10 + 2.11 as CaCO3 eq] for 1L

• Occurring at room temperature

• precipitate formed are finely

divided hence do not settle down

easily

• It is essential to add small amount

of coagulant (alum, sodium

aluminate)

• Coagulant hydrolyse to form

gelatinous ppt. and entraps the

fine ppt.

• NaAlO2 + H2O NaOH + Al(OH)3

• It provides water with a residual

hardness of 50 to 60 ppm

Cold – Lime Soda Process

H o t l i m e - s o d a p r o c e s s

• Occurring at 80 to 150 °C close to the boiling point of the solution

• Reaction proceed faster

• The precipitate and sludge formed settle down rapidly so no

coagulant needed

• Viscosity of the softened water is lower, so filtration of water

becomes much easier

• Produce water contain the residual hardness of 15 to 30 ppm

Hot-lime Soda Process

Advantage of Lime-Soda process

• It is very economical

• This process increases the pH value of the treated-water, thereby

corrosion of the distribution pipes is reduced

• Besides the removal of the hardness, the quantity of minerals in

water reduced

• Disposal of large amounts of sludge poses a problem

• This can remove hardness only up to 15 ppm, which is not good for

boilers

Disadvantage of Lime-Soda process

Zeolites are naturally occurring hydrated sodium aluminosilicate.

General Formula is Na2O.Al2O3.xSiO2.yH2O x = 2, y = 2 – 6

Natrolite - Na2O.Al2O3.3SiO2.2H2O – Green sand and

are usually non-porous

The synthetic form of zeolite is known as Permutit, which is porous.

Sodium Zeolite is represented as Na2Ze ; exchange Na+ with Ca2+ & Mg2+ in water

Zeolite or Permutit Process

Process

( 10 % brine)

Regeneration process

NaCl

Solution

Zeolite Softener

Advantages of Zeolite process:

• Hardness of the water can be removed completely upto 10 ppm

• Equipment is compact and occupies a small space. Easy to operate.

• The method is very cheap because the regenerated permutit is again used

and the process can be repeated a number of times

• The process can be made automatic and continuous

• The process automatically adjusts itself for different hardness of incoming

water

• No sludge is formed during the process

Limitations of zeolite process:

• Turbid water cannot be used - the suspended impurities will clog the

pores of zeolite bed

• Mineral acids should be removed/neutralized – mineral acids destroy the zeolite

• If water contains large amounts of coloured ions such as Mn2+ and Fe3+, it

must be pretreated because the corresponding manganese and iron zeolite

cannot be easily converted to proper zeolite

• Since lead is not changed to lead zeolite, the method cannot remove

lead salts

• Zeolite treatment replaces only the cations like Mg2+ and Ca2+, leaving all the

anions like HCO3- and CO3

2- in the soft water.

NaHCO3 → NaOH + CO2Na2CO3 → 2NaOH + CO2

Caustic and

CO2 corrosion

2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2 CO2 removed by de-aeration

Ion-exchange Process

polymer beads

Ion-exchange resins are soluble, cross-linked, long chain organic polymers

with a microporous structure having some ionisable groups responsible for

the ion-exchanging properties

(a)Cation exchange resin

SO3-H+

COO-H+

O-H+

SO3¯H+

SO3¯H+

SO3¯H+

SO3¯H+

Styrene-divinyl benzene copolymer, which on sulphonation or carboxylation,

Become capable to exchange their hydrogen ions with the cations in the water

CH2NMe2+OH-

styrene-divinyl benzene or amine-formaldehyde copolymers, which contain

amino or quaternary ammonium or quaternary phophonium or tertiary

sulphonium groups as an integral part of the resin matrix.

“These after treatment with dil. NaOH solution capable to exchange their OH¯ ions

with the anions in the water”

(b) Anion exchange resin

Process

@ cation exchanger resin

cation exchanger (hydrogen exchanger) absorbs all the cations

present in water and leaves behind the hydrogen ions

2 R·COOH + Ca2+ → (R·COO)2Ca + 2H+

2 R·SO3H + Mg2+ → (R·SO3)2Mg + 2H+

2 R·OH + Mg2+ → (RO)2 Mg + 2H+

@ anion exchanger resin

R’·OH + Cl- → R’·Cl + OH-

2 R’·OH + SO42- → R’·SO42- + 2OH-

Finally, H+ & OH- get combined to produce water molecule

H+ + OH- → H2O

Water coming out from the exchanger is free from cations as well as anions, is

Known as deionized or demineralise water

Regeneration of cation exchange column

Regeneration of anion exchange column

Advantage

• The process can be used to soften highly acidic or alkaline water

• it produces water of very low hardness (2 ppm)

Disadvantage

• The equipment is costly and more expensive chemical are needed

• If the water contains turbidity then the output of the process is reduced

(dil. NaOH)

Regeneration Process

(dil. HCl/H2SO4)

Hard water →

Soft

water

Ion exchange process

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The outgoing water from the mixed-bed contains even less than 1 ppm of

dissolved salts

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Drinking water or Municipal water

Specifications of different materials in drinking water (ICMR

and WHO)S.No. Parameter/Material WHO

Standards/ppm

ICMR/BIS

Standards/ppm

1 Colour Clear Clear

2 Odour Pleasant Pleasant

3 Turbidity 2.5 2.5

4 pH 6.0 – 8.5 6.0 – 8.5

5 TDS 300 500

6 Total Hardness as CaCO3 200 300

7 Calcium 75 75

8 Chlorides 200 200

9 Sulphates 200 200

10 Fluoride 0.5 1.0

11 Mercury 0.006 0.001

12 Cadmium 0.003 0.01

13 Arsenic 0.01 0.02

14 Chromium as hexavalent 0.01 0.1

15 Lead 0.01 0.01

16 E.Coli No colony Should be

present in 100 mL

water

No colony Should be

present in 100 mL

water

Purification of Treatment of water for Municipal Supply

1. Screening

2. Aeration

3. Sedimentation and coagulation

4. Filtration

5. Sterilisation and disinfection

6. Storage and distribution

Flow chart:

Screening

Floating matter in the water can be retained by passing the water through

Screens having a large number of holes

Aeration

Promotes taste and odour by exchange of gases between the water and atmosphere

• to add or increase the content of oxygen in water

• to remove CO2 , H2S and other volatile substance causing bad taste and odour in water

• to remove the impurities like Fe and Mn (which are precipitate as their respective hydroxides)

Sedimentation with Coagulation

Suspended and collodial impurities are separated in a sedimentation tank by gravitation

Fine particles take many hours or days to settle down. Coagulation are added

Salts of aluminium (alum, sodium aluminate) and salts of iron (ferrous sulphate, ferric sulphate,

ferric chloride)

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Filtration

Removal of collodial and suspended matter (remaining after sedimentation)

and bacteria by passsing the water through filter beds containing fine sand, coarse

sand and gravel Types: Gravity type or pressure type

Sterilization and Disinfection

Harmful bacteria are destroyed to make the water safe for drinking is call disinection

Chemical used – Disinfectants

Boiling: When the water is boiled for 15 - 20 minutes, the harmful bacteria are killed

Disadvantages:

i. Boiling changes taste of water

ii. It is impractical for large treatment plants

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Removal of micro-organisms

• By adding bleaching powder (hypo-chlorination)

– About 1kg of bleaching powder per 1000 kiloliters of water is mixed

– Produces hypochlorous acid (powerful germicide)

CaOCl2 + H2O Ca(OH)2 + Cl2

Cl2 + H2O HCl + HOCl

Germs + HOCl Germs are killed

Disadvantage

Introduces Calcium in water, thereby making it more hard

excess of it gives a bad taste and smell to treated water

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• Chlorination– Chlorine either gas or in concentrated solution form produces

hypochlorous acid, which is a powerful germicide

• Factors affecting efficiency of chlorine– Number of micro-organism destroyed by chlorine per unit time is

proportional to the number of micro-organism alive.

– The rate of reaction with enzymes increases with temperature

– pH values between 5-6.5, less contact time is enough

• Advantage

– (i) Effective and economical (ii) used low as well as high temp (iii) mostideal

• Disadvantage

– (i) should not exceed 0.1 – 0.2 ppm (ii) less effective in higher pH values(iii) excess chlorine produces unpleasant taste and odour.

Cl2 + H2O HCl + HOCl

Germs + HOCl Germs are killed

84

Break point chlorination or free-residual

chlorination

Involves addition of sufficient amount of chlorine to oxidize organic matters,

reducing substances, free ammonia leaving behind free chlorine to kill bacteria.

•Removes color of water – no organic matter

•Removes odour and taste from the water

•It prevents the growth of weeds

The addition of chlorine at the dip or break is called break-point chlorination

Chloramine (ClNH2)Chlorine is not stable in water. Hence it is sometimes mixed with ammonia in the

ratio 2:1 by volume to form stable compond called chloramines.

HOCl inactivates the enzymes responsible for Bacteria and are killed

Disinfection by ozoneOzone is an unstable isotope of oxygen. It contains three atoms of oxygen and

one easily breaks away. This produces nascent oxygen which is a powerful

Disinfectant. It does not produce any taste of adour

Cl2 + NH3 ClNH2 + HCl

(Chloroamine)

ClNH2 + H2O HOCl + NH3(Disinfectant)

3O2 2O3

O3 O2 + [O ]

Nascent oxygen

Electric

Discharge

.

Desalination

The process of removal of dissolved salts (NaCl) from water is called

desalination

High conc. Of dissolved salts/solid → brackish water

Fresh water (less than 100mg/L of dissolved salts)

Brackish water (1000 – 35000 mg/L of dissolved salts)

Sea water (greater than 35000 mg/L of dissolved salts)

• Reverse Osmosis

• Electrodialysis

• Distillation etc

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Desalination of Brackish water

ElectrodialysisSeparation of dissolved salts from saline water in the form of ions under the influence

of a direct current using particular type of membrane called ion selective membranes

90

91

When two solutions of different concentrations are seperated by semi-permeable

membrane, solvent (water) flows from a region of low concentration to a region

of high concentration. This spontaneous process called Osmosis

The excess pressure applied on the concentrated solution side to prevent

osmosis called Osmotic pressure (15-40 Kg/cm2)

Fresh water to move from the concentrated to dilute side through semipermeable

Membrane is called Reverse Osmosis (super-filteration/hyper-filteration)

Osmosis

Osmosis~ the diffusion of water across a

selectively permeable membrane

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➢Super filtration

➢15-40 kg cm2

Advantages

➢Removes colloidal silica

➢Long life

➢Can be replaced within

few minutes

Reverse Osmosis

Salt Molecularweight

Chemicalequivalent

MultiplicationFactor

Ca(HCO3)2 162 81 100/162

Mg(HCO3)2 146 73 100/146

CaSO4 136 68 100/136

CaCl2 111 55·5 100/111

MgSO4 120 60 100/120

MgCl2 95 47·5 100/95

CaCO3 100 50 100/100

MgCO3 84 42 100/84

CO2 44 22 100/44

Mg(NO3)2 148 74 100/148

HCO3 61 61 100/122

OH 41 17 100/34

CO23 60 30 100/60

NaAlO2 82 82 100/164

Al2(SO4)3 342 57 100/114

FeSO4.7H2O 278 139 100/278

H+ 1 1 100/2

HCl 36·5 36·5 100/73

CaCO3 equivalent of hardness causing impurity .= 100×wt. of the impurity/2×chemical equivalent of impurity

= Multiplication factor x wt. of impurity

http://www.transtutors.com/homework-help/engineering-chemistry/softening-of-

water/calculations-lime-soda.aspx

Factors influencing the level of Dissolved Oxygen in water

• Temperature

• Excessive Organic Matter

• Type of water

• Atmospheric pressure

• Seasonal changes

Dissolved oxygen (DO) determination measures the amount of dissolved

(or free) oxygen present in water or wastewater. Aerobic bacteria and

aquatic life such as fish need dissolved oxygen to survive. If the amount of

free or DO present in the wastewater process is too low, the aerobic

bacteria that normally treat the sewage will die

Example: 1

On a water analysis it is found that 100 ml of a hard water sample require 25 ml of 0.01 M EDTA

with NH4Cl-NH4OH buffer and EBT indicator. 100 ml of the sample water boiled for about half an

hour and after filtering the precipitate, the volume of the filtrate is made to 100 ml again by the

addition of distilled water, 20 ml of this boiled sample requires only 4 ml of 0.01 M EDTA following

the same procedure. Calculate the temporary and permanent hardness of the sample

Example: 2

A 100 ml of water sample requires 20 ml N/100 EDTA when titrated using NH4Cl-NH4OH

Buffer and EBT indicator. Calculate the hardness of the sample

Molarity of EDTA = 2 X Normality of EDTA