Water (1)

Water

&

Its Treatment

IntroductionFor the existence of all living things water is essential. Without water we

cannot survive. Almost all human activities domestic, agricultural and industrial demand use of water.

Water from any source has to be treated before its use. The treatment to which it is subjected depends upon its use.

If it has to be used for drinking purposes, the treatment would include removal of objectionable colour, taste and pathogenic micro organisms, whereas the water for industrial use require the removal of dissolved salts if it is used for steam generation.

In this section we will discuss about the analysis of water and treatment of water for its industrial and domestic use.

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Sources of water

Surface water :-The water which comes from surface through rain. eg :- rain water, river water & sea water.

Underground water :- This water comes from rain that falls on earths surface & then goes into the ground water & travel down the impervious (cracks) layers of earth, thus forming ground water. eg :- spring water & well water.

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Impurities in Water• Silica, clay etc

• CO2, O2, H2S

• Bacteria & other Micro-organisms like algae & fungi

• Carbonates, Bicarbonates, Chlorides & Sulphates of Ca,

Mg.

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Types Of Water

HARD WATER SOFT WATER

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Hard water and Soft water

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HARD WATER SOFT WATERDoes not form lather with soap easily

Forms lather with soap easily

Contains dissolved salts of Ca & Mg

Does not contain dissolved salts of Ca & Mg

More wastage of time & fuel as boiling temp. of water gets increased due to impurities

Less wastage of time & fuel

More consumption of soap by hard water

Less consumption of soap by soft water

Hardness of water Hardness is the soap consuming capacity of water

Hardness of water is due to the presence of Ca and Mg salts in it. Other ions responsible for hardness are Al3+, Fe3+ and Mn2+

If Ca and Mg salts are present in water then they react with the soluble sodium soap to form insoluble salts calcium and magnesium.

2C17H35COONa + CaSO4--- (C17H35COO)2Ca + Na2SO4

Sodium stearate Insoluble salt 2C17H35COONa + MgCl2--- (C17H35COO)2Mg + 2NaCl

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Types Of Hardness

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TEMPORARY PERMANENT (Carbonate Hardness) (Non- Carbonate Hardness)

Temporary Hardness

Caused by the presence of dissolved bicarbonates of Ca, Mg and.. This hardness is also known as alkaline hardness.

Easily removed by heating :

HeatCa(HCO3)2--------- CaCO3 + H2O + CO2

HeatMg(HCO3)2--------- MgCO3 + H2O + CO2

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Permanent Hardness

Due to the presence of sulphates and chlorides of Ca & Mg

Cannot be removed simply by boiling.

Special methods like lime soda process, zeolite process, ion-exchange method are used for the removal of permanent hardness.

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Degree of Hardness in terms of CaCO3

equivalent

Hardness is expressed in terms of CaCO3 equivalents. Reasons for choosing CaCO3 as the reference standard for

calculating hardness of water is :

mol. wt. is 100 that makes mathematical calculation easier.

The most insoluble salt and can be easily precipitated in water treatment process.

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How to calculate harness in terms of CaCO3 equivalent

Hardness in terms of CaCO3 equivalents mol. mass of CaCO3= wt. of hardness producing sub. × mol. mass of hardness producing sub.

Units of Hardness

.

Relation between various units of hardness 1ppm = 1mg/l = 0.1°Fr = 0.07 °Cl

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a) parts per million ppm parts of CaCO3 equivalent hardness per 106 parts of water.

b) Milligrams per litre

Mg/l no. of milligrams of CaCO3 equivalent hardness per litre of water.

c) Degree Clark °Cl parts of CaCO3 equivalent hardness per 70,000 parts of water.

d) Degree French °Fr parts of CaCO3 equivalent hardness per 105 parts of water.

Boiler feed water For steam generations, boilers are used

if hard water is fed to the boiler, various problems are faced by boiler :

Scale and Sludge formation Priming and Foaming Boiler corrosion Caustic embrittlement

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Sludge & ScaleContinous evaporation of water takes place & conc. of dissolved salts gets

increased & at saturation point forms ppts. on the inner walls of the boiler.Sludges : If loose & slimy ppts formed.Scales : If sticky, hard & adhernt coat formed. Sludge Scale

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SludgeFORMATION:Where flow of water is slow At colder region By substances which have greater solubility in the hot water.MgCO3, MgCl2, CaCl2, MgSO4 etc.

DISADVANTAGES:Poor conductor of heat hence more consumption of time and fuel.Disturbs functioning of boiler & settles in the regions of poor water

circulation.

PREVENTION :By using soft water By using blow down pipe operation .

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ScaleFORMATION : Decomposition of Ca(HCO3)2 :

• Ca(HCO3)2 CaCO3 + H2O + CO2

Soft Scale

• CaCO3 + H2O Ca(OH)2 + CO2

Deposition of CaSO4 : • Soluble in cold water• As temp. solubility of CaSO4 hard scale

Hydrolysis of Mg salts• MgCl2 + 2H2O Mg (OH)2 + 2HCl Presence of (SiO2)

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ScaleDISADVANTAGES Fuel Wastage Lowering of boiler safety Decreased efficiency Danger of explosion

RemovalUsing wire brushBy using chemicals :• CaCO3 scales by 5-10% HCl • CaSO4 scales by EDTA Blow down pipe operationBy giving thermal shocks

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Scale PreventionExternal Treatment• By using soft water

INTERNAL TREATMENT• Colloidal conditioning : Addition of organic substances such as tannin, Agar- Agar• Phosphate conditioning :o CaCl2 + Na3PO4 Calcium phosphate + 6NaCl • Carbonate conditioningo CaSO4 + Na2CO3 CaCO3 + Na2SO4

• Calgon conditioningo CaSO4 + calgon Soluble complexes of Ca ions• Treatment with sodium aluminate :o NaAlO2 + 2H2O Al(OH)3 + NaOH

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Difference between Sludge & Scale

Sludge Scale

Soft, loose & slimy precipitates. hard deposits.

Non-adherent deposits & can be easily removed.

Stick very firmly to the inner surface of boiler and are very difficult to remove.

Formed by substances like CaCl2,

MgCl2, MgSO4 & MgCO3.

Formed by substances like CaSO4,

Mg(OH)2, CaCO3 & CaSio3.

Formed generally at colder portions of the boiler.

Formed generally at heated portions of the boiler.

Decrease the efficiency of boiler but are less dangerous.

Decrease the efficiency of boiler & chances of explosions are also there.

Priming and FoamingPrimingprocess of making wet steam when some of liquid particles are carried

along with steam

CausePresence of dissolved salts high steam velocity Sudden boilingSudden increase in steam production

FoamingFormation of bubbles in the boiler continuously

Causepresence of oil that reduces the surface tension

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Priming & FoamingDisadvantagesReduce the efficiency Difficult to maintain proper pressureWastage of fuelActual water level can not be accessed

PreventionRemoval of priming foaming substancesRemoval of Scale & sludgesAvoid rapid changes in steaming rateChange of boiler water from time to timeUsing antifoaming agents e.g. castor oilAddition of a chemical NaAlO2 to remove water

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Boiler CorrosionThe chemical or electro-chemical eating away of metal by its

environment in a boilerCauseDissolved Oxygen :2Fe +2H2O + O2 2Fe(OH)2 + O2 2(Fe2O3 .2H2O)Dissolved CO2 :CO2 + H2O H2CO3

Acids from dissolved salts :MgCl2 + 2H2O Mg(OH)2 + 2HClFe + HCl FeCl2 + H2

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

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Boiler Corrosion Disadvantages Shortening of boiler life Leakages of joints and rivets Increased cost of repairs and maintenance

Removal of boiler corroision : Removal of O2 : 2Na2SO3 + O2 2Na2SO4

N2H4 + O2 N2 + 2H2O Removal of CO2 : 2NH4OH + CO2 (NH4)2CO3 + H2O Removal of acids : By adding alkali

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Caustic Embrittlement

formation of brittle and incrystalline cracks in the boiler shell  due to the accumulation of caustic substances

Cause

Presence of alkali-metal carbonates and bicarbonates in feed water presence of sodium sulphate.Sodium carbonate is used in softening of water by lime soda process, due to

this some sodium carbonate may be left behind in the water.

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ProcessNa2CO3 used for softening of water & some of which remain unreacted

Na2CO3 + H2O → 2NaOH + CO2

As Conc. of NaOH increases, water flows into minute hair cracks by capillary action.

As water evaporates, conc. of NaOH increases further and react with iron of boiler, (thereby dissolving Iron of boiler as Sodium ferroate), hence cause Embrittlement.

This causes embrittlement of boiler parts such as bends, joints, reverts etc, due to which the boiler gets fail.etc.

prevention :Use of Na3PO4 instead of Na2CO3

By adding tanin & lignin that blocks the hair cracksBy adding NaSO4 that also blocks the cracks

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Softening Methods The following methods are used :

Lime soda Process

Zeolite softening process

Ion exchange process

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Lime – Soda ProcessTreatment of water with calculated amount of lime Ca(OH)2 & Soda

(Na2CO3) which results in the formation of insoluble ppts. of Ca & Mg that can be removed by filteration.

Cold Lime Soda Types

Hot Lime Soda

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Cold lime soda process

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Hot Lime Soda Process

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Cold Lime-Soda Method Hot Lime-Soda Method

Carried out at room temperature. Carried out at high temperature,almost at b.pt.of water.

Slow process Fast process

Coagulant AL2(SO4) is added No need of Coagulant.

Dissolved gases are not removed . Dissolved gases are removed .

water obtained is of hardness 60 ppm. water obtained is soft of hardness 15-20 ppm.

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Impurity Mol. Wt. Requirement

Ca(HCO3)2 162 L

Mg(HCO3)2 146 2L

CaCl2 111 S

CaSO4 136 S

MgCl2 95 L+S

MgSO4 120 L+S

Al2(SO4)3 342 3L+3S

H+ 1 L/2 + S/2

CO2 44 L

H2S 34 L

NaHCO3 84 L/2 - S/2

NaAlO2 182 -L/2

FeSO4.7H2O 278 L+S32

74 (Total Lime) (Vol. Of Water)(100% purity of lime)

Lime Requirement = 100

106(Total Soda) (Vol. Of Water)(100% purity of lime)

Soda Requirement = 100

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Zeolite or Permutit processNa2OAl2O3 xSiO2 yH2O Where x = 2 - 10 , y = 2 – 6Can be written as Na2Ze where Ze = OAl2O3 xSiO2 yH2O

exchanges reversibly its sodium ions with hardness producing ions( Ca & Mg) in water.

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Principle of Zeolite process

Softening Process :

Na2ze + Ca(HCO3)2 CaZe + 2 NaHCO3

Na2ze + Mg(HCO3)2 MgZe + 2 NaHCO3

Regeneration :

CaZe + 2 NaCl Na2ze + CaCl2

MgZe + 2 NaCl Na2ze + MgCl2

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Zeolite process

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Ion Exchange or De-ionization or De-mineralisation process

Hard water is allowed to pass through ion exchangers when soft water, free from all the minerals and hardness causing as well as the other ions is obtained.

Cation Exchange Column represented with its H+ ions

Types

Anion Exchange Column represented with its OH- ions

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Process

Cation Exchange column : 2RH+ + Ca+2 R2Ca+2 + 2H+

Cation exchange resin H. W. Cation exchange resin S. W.

Anion Exchange column :R’OH- + Cl- R’Cl- + OH-

Anion exchange resin H. W. Anion exchange resin S. W.

2R’OH- + SO4-2 R2’SO4-2 + 2OH-

Anion exchange resin H. W. Anion exchange resin S. W.

Regneration:

CationExchange column :R2Ca+2 + 2H+ 2RH+ + Ca+2 Saturated Cation exchange resin dil. acidic sol. Regenerated Cation exchange resin washings

Anion Exchange column :

R2’SO4-2 + 2OH- 2R’OH- + SO4-2

Saturated anion exchange resin dil. Basic sol. Regenerated Cation exchange resin washings 38

Ion Exchanger

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