Download - WATER & ITS TREATMENT

04/10/23 Dr.Priy Brat Dwivedi,NMIMS University, Mumbai

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WATER & ITS TREATMENT


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“ALTHOUGH WATER IS NATURE’S

MOST WONDERFUL, ABUNDANT &

USEFUL COMPOUND, YET IS ALSO

THE MOST MISUSED ONE”


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Lesson Plan

• Various sources of water• Characteristics imparted by impurities in water• Hardness of water & various types of hardness• Disadvantages of using hard water• Boiler feed water & its problems (Scale, Sludge, Caustic

Embrittlement, Boiler corrosion, Priming & Foaming)• Internal treatment of water (discussed as above)• External treatment (softening of water)• Lime soda method• Zeolites method• Ion exchange method


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Water

• Water occupies a unique position in industries. Probably its most important use as an engineering material is in the steam generation

• It is used as a coolant in power & chemical plants

• It is widely used in other fields such as textiles, atomic energy, chemicals , ice , drinking bathing, sanitary, washing, irrigation etc


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Various Sources of Water

Sources

Surface Water Under ground water

Rain water

River water

Lake water

Sea water

Spring & Well water


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• Rain Water: Purest form of water. Since it is obtained

as a result of evaporation from surface water.

• But during journey downwards through the

atmosphere, it dissolves a considerable amount of

industrial gases like CO2, SO2, NO2 etc & suspended

particles.

• Lake water: has a more constant composition.


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• River water: contains minerals of soil such chlorides,

sulphates, carbonates etc. It also contains organic &

inorganic matter.

• Sea water: the most impure form of water.

• Underground water: in form of spring & well water.

It is clearer in appearance due to the filtering action

of soil, but contains more of dissolved salts.


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Characteristics Imparted by Impurities in Water

Characteristics

Physical Chemical Biological

Color

Turbidity

Taste

Odour

Acidity

Gases

Mineral Matters

Micro organisms

Water bodies


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Physical• Color :in water is due to dissolved substances &

substances present as fine colloids. e.g.

(i) metallic substances like salts of Fe, Mn

(ii) human materials, tannins

(iii) Peat, algae, weeds, protozoa

(iv) Industrial effluents (from paper & pulp, textile & tanneries etc)

• Color in water is not harmful unless it is associated with toxic chemicals.


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Turbidity

• It is an Optical property. It means water sample is scattering more light than transmitting in straight line.

• It is due to Colloidal &extremely fine suspension .e.g. clay, silt, finely divided matter (organic & inorganic), micro-organisms, plankton etc.

• It is eliminated by sedimentation, coagulation & filtration.


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Turbidity

• Measured by Jackson Turbitimeter. It is expressed in terms of JTU (Jackson Turbidity Unit). Silica has been chosen as the arbitrary standard

1JTU = 1mg SiO2/litre• Now a days suspension of clay or standard

suspension of Formazin are used &it is expressed in Formazin Turbidity Unit

• FTU ≈ JTU.


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Taste

• is usually interlinked with Odor. It is due to the

presence of dissolved minerals.

• Bitter: Fe, Al, Mn & sulphate salts, excess of lime

• Soapy: Excess amount of NaHCO3

• Brackish: Unusual amount of salts

• Palatable: Dissolved gases CO2 & nitrate minerals


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Odor

• It is most disagreeable & due to

i. Presence of living organism

ii. Decaying vegetation-algae, bacteria, fungi, weeds

iii. Sewage & industrial effluents

• The most common disagreeable odor in water bodies

is due to presence of small quantity of Sulphides.


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Chemical

• Acids : released by DDT, high explosives, battery

industries.

• It is caused by the presence of CO2, mineral

acids(H2SO4),weakly associated acids.

• Acidity is not a specific pollutant. It may be defined

as the power of water to neutralize hydroxyl ions &

is expressed in terms of ppm (mg/L) of CaCO3

equivalent.


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Gases

• Water contains dissolved gases eg.

i. CO2: It is not assumed as an impurity, but water used in industries some times needed to get rid of it.

All natural water has it from atmosphere & biological oxidation of organic matter.

CH4 + O2 → CO2 + H2O

ii. O2: All natural water has dissolved oxygen. It is very imp for aquatic lives but it is a great nuisance in industrial purposes.


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Gases

The test for oxygen is carried out for determining Dissolved Oxygen [DO] of polluted water & industrial effluents & constitutes a means of controlling water pollution.

iii. NH3: it is acquired from decomposition (aerobic &anaerobic) of Nitrogenous organic matter (in polluted & sewage water) and urea.


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Mineral matter

• Origins from rocks & industrial effluents

• These includes Mineral, acids, Ca2+, Mg2+, Na+, K+, Fe2+, CO3

2- etc.

• Industrially Hardness & alkalinity are important.


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Biological Impurities

• There are some biological impurities which

contaminate water, e.g. pathogenic bacteria, certain

fungi, viruses, parasitic worms etc. The imp source of

this contamination is from domestic & sewage waste,

solid excreta from warm blooded animals including

man, wild & domestic animals.


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Biological Impurities

Micro-organism: Algae, fungi,& bacteria form slime

& there by causing fouling & corrosion. This slime

clogs the spray nozzle & screens of the circulating

pipes of AC & other industrial plants. In order to

control these, chemical treatment like chlorination is

done.


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Water bodies

• Water bodies: includes (i) bacteria, organisms

inhabitating the bottom sludge & (ii) organism &

plankton developed at the water surface. E.g. flat

worms ,round worms, rotifers etc.

• Usually most worms are parasitic in nature these are

not only harmful to fishes but also to human health.


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HARDNESS OF WATER

• Hardness of water is that characteristic which prevents the lathering of soap. This is due to presence of certain salts of Ca, Mg & other heavy metals dissolved in water. This water when treated with soap (Na or K salts of higher fatty acids like, oleic palmitic, or stearic) forms a white scum or ppt of insoluble soaps of Ca & Mg as follows-


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HARDNESS OF WATER

2C17H35COONa + CaCl2 → (C17H35COO)2Ca ↓ + 2NaCl

2C17H35COONa + MgSO4 → (C17H35COO)2Mg ↓ + Na2SO4

• The hardness of water is of two types:


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Temporary

It is due to the presence of soluble salts of bicarbonates of Ca & Mg & other heavy metals & the carbonate of iron. It can be easily removed by mere boiling of water, when bicarbonates are decomposed , yielding insoluble carbonates or hydroxides-

• Ca(HCO3)2 → CaCO3 + H2O + CO2

• Mg(HCO3)2 → Mg(OH)2 + 2CO2


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Permanent

• Non-carbonate hardness: is due to the presence of Cl-

SO42- of Ca, Mg, Fe & other heavy metals. It can not

be removed by boiling. [Softening methods are used]

• The alkaline hardness is due to the presence of

bicarbonate & carbonate of metals, while non-

alkaline hardness is due to the presence of Cl- , SO42-


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Disadvantages of using hard water

Domestic use:

i. Washing : no lather formation ,wastage of soap

ii. Bathing: no lather formation. Also the resulting ppt stick on body

iii. Cooking: due to dissolved salts boiling point of water is elevated, causing unnecessarily wastage of time & fuel.

iv: Drinking: bad effect on metabolic system. Calcium oxalate stones may develop in urinary tracts, if used regularly. Also it causes deposition of Ca in the bone joints.


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Industrial use:

i: Textiles: loss of soaps during washing of yarn, & fabrics. Ppt sticked on fabric& when it is colored. Also Fe, Mn etc salts leave colored spots on fabrics

ii. Sugar: crystallization of sugar is affected.

iii. Dyeing: react with costly dyes, ppt occurred. it may impure shades.

iv: Paper: react with chemicals to provide smooth & glossy finishing to paper. Iron salts add unwarranted color in paper.

v. Pharmaceuticals: undesirable products may be produced in medicines


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Problems of Boiler feed water

• If the boiler feed water contain the excess of

impurities then it causes the following defects:

(a) Scale & sludge formation

(b) Corrosion

(c) Caustic Embrittlement

(d) Priming & foaming


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Sludge

• It is a soft, loose & slimy ppt

• Collected in colder parts of the boiler, where the flow of water is slow & in area of bends.

• Substances which have greater solubility in hot water than in cold water form sludge e g MgCl2, MgSO4, MgCO3 &CaCl2.


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Sludge

(a) Disadvantages:

i: it is a bad conductor of heat so a lot of energy is wasted.

ii: chocking & poor water circulation

(b) Prevention:

i: using soft water

ii: blow down operation i.e. drawing off a portion of the concentrated water.


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Scale

• It is hard & adhering coating ,which deposits & sticks firmly to the inner surfaces of the boiler.

• It is formed as follows-

(i) Decomposition of bicarbonate

• Ca(HCO)3 → CaCO3 ↓ + H2O + CO2

Though scale due to CaCO3 is soft & is the main cause of scale formation in low pressure boilers. But in high pressure boilers CaCO3 dissolves.


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Scale(ii) Deposition of CaSO4: It is soluble in cold water but

less soluble in hot water & almost insoluble in super heated water.

(iii) Magnesium salts: dissolved Mg salts undergo hydrolysis (at prevailing high tem inside the boiler) forming Mg(OH)2 ppt ;a soft type of scale

(iv) Silica: deposits as CaSiO3 &MgSiO3 & deposits very firmly on inner side & difficult to remove.


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Disadvantages

(i) Fuel wastage - due to low thermal conductivity of scale

(ii) Boiler safety : Regular overheating weakens the surface of boiler & material.

(iii) Decrease in efficiency: Sometimes scale may deposit in the valves & condensers of the boiler & choke them partially.

(iv) Explosion:


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Prevention

External: using soft water

Internal: As follows:

Colloidal conditioning: In low pressure boilers scale formation can be avoided by adding kerosene, tannin, agar-agar gel, which get coated over the scale forming ppt, thereby yielding non-sticky & loose deposits which can be easily removed by blow down operation.


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Phosphate conditioning

• In high pressure boilers scale formation can be

avoided by adding Na3PO4,which react with

hardness of water, forming a non-adherent & easily

removable soft sludge of Ca & Mg phosphates; which

can be removed by blow down operation.

• 3 CaCl2 + Na3PO4 → Ca3(PO4)2 + 6 NaCl


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Carbonate conditioning

• In low pressure boilers ;scale formation can be avoided by adding Na2CO3 which convert CaSO4 in a loose sludge of CaCO3 & that can be removed by blow down operation.

• Na2CO3 + CaSO4 → Na2SO4 + CaCO3 ↓


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Calgon conditioning

• It prevents both, sludge & scale formation by forming a soluble complex compound with CaSO4.

• Na2[Na4(PO3)6] 2Na+ +[Na4P6O18]2-

• 2CaSO4 + [Na4P6O18]-- → [Ca2P6O18]-- + 2Na2SO4


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Treatment with NaAlO2

• NaAlO2 + 2H2O → NaOH + Al(OH)3 ↓

• The NaOH so formed ,ppt some of Mg as Mg(OH)2

• NaOH + MgCl2 → Mg(OH)2 ↓ + 2NaCl

• The flocculent ppt of Mg(OH)2 & Al(OH)3 entraps

finely suspended & colloidal impurities including oil

drops & silica. The loose ppt can be removed by

blow-down opn.


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Complexometric method

(vi) It involves adding 1.5% alkaline (pH=8.5) solution

of EDTA to feed water. The EDTA binds the scale

forming cation i.e. Ca, Mg, to form stable & soluble

complex.

This conditioning is superior to phosphate conditioning

for preventing the formation of iron & cuprous

deposition.


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Removal of Scale

(i) with the help of scrapper or wire brush if loosely adhering.

(ii) thermal socks by heating & suddenly cooling with cold water, if they are brittle

(iii) dissolving them in appropriate chemicals; if adherent & hard. For CaCO3 scale,5-10% HCl & for CaSO4 scale, EDTA is used.

(iv) blow down operation; if loosely adhering


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Caustic Embrittlement

• Dissolution of boiler iron as sodium ferroate.

• In high pressure boilers Na2CO3 (usually present in

small proportion in softened water) decomposes to

give NaOH & CO2,

Na2CO3 + H2O → 2 NaOH + CO2

• And it makes the water “caustic”. This water flows

into the minute hair cracks, always present in the

inner side of the boiler by capillary action .


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Caustic Embrittlement

• Here water evaporates & the dissolved caustic soda

concentration increases progressively. This caustic

soda attacks the surrounding area thereby dissolving

iron of the boiler as sodium ferroate.

• Fe + 2NaOH → Na2FeO2 + H2

• This causes Embrittlement of boiler parts particularly

stressed parts like bends, joints, rivets etc.


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Prevention of Embrittlement

(i) by using Na3(PO4) as softening agent instead of

Na2CO3

(ii) by adding tannin or lignin to boiler water, since

these blocks the hair cracks

(iii) addition of Na2SO4 also blocks hair cracks. Caustic

cracking can be prevented if Na2SO4 is added in a

particular ratio.


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Boiler Corrosion• It is the decay of boiler material by a Chemical or

electro-chemical attack by its environment. Main reasons-

1. Dissolved Oxygen:

2Fe + 2 H2O + O2 → 2Fe(OH)2

4 Fe(OH)2 + O2 → 2[Fe2O3.2H2O]

Removal: by adding calculated quantity of Na2SO3,

Na2S etc.

2NaSO3 + O2 → 2Na2SO4


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2. Dissolved CO2 : is carbonic acid

CO2 + H2O → H2CO3

It which has a slow corrosive effect on the boiler material.

Removal: i. by adding a calculated quantity of Ammonia,

2NH4OH + CO2 → (NH4)2CO3 + H2O

ii: by mechanical de-aeration process along with oxygen


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3. Acids from dissolved salts: Water containing dissolved Mg salts liberate acids on hydrolysis e.g.

MgCl2 + 2H2O → Mg(OH)2 + 2 HCl

• The liberated acid reacts with iron of the boiler in chain like reactions producing acid again & again;

Fe + 2HCl → FeCl2 + H2

FeCl2 + 2H2O → Fe(OH)2 + 2 HCl


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Priming & Foaming• When a boiler is steaming rapidly , some particles

of the liquid water are carried along with the steam. This process of ‘wet steam’ formation is called Priming.

It is caused by-

• The presence of large amount of dissolved solid

• Sudden boiling

• Improper boiler design

• Sudden increase in steam production rate


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Priming can be avoided by-

• using mechanical steam purifier

• avoiding rapid change in steaming rate

• maintaining low water level in boiler

• efficient softening & filtration of boiler feed water


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Foaming

• It is the production of persistent foam or bubbles in boilers, which do not break easily. These bubbles are carried along with steam leading to excessive priming.

• Substances which increase the viscosity of the film & substances lowering the surface tension of the water will connect at the interface & thus increases the foaming tendency of the liquid.


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It is objectionable because-

• Dissolved salts, suspended solids etc in boiler water carried by wet steam may enter the parts of machinery & get deposited on cylinder walls, turbine blades, or in steam lines. This deposit reduces their efficiency.

• Actual height of water column can not be judged properly, there by making the maintenance of boiler pressure at a particular level is difficult.


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Foaming can be avoided by-

• Adding anti-foaming agents like castor oil, which

spreads on the surface & prevent foaming

• Removing oil from boiler water by adding NaAlO2.

• Addition of small amount of polyamide antifoamer

alters the surface tension & leads to the formation of

only large unstable bubbles at the same heat input.


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External Treatment

• External Treatment ( Softening methods): The process of removing hardness causing salts from water is called as softening of water. Mainly the following three methods are used for softening:

• Lime soda process

• Zeolite or permutit process

• Ion exchange or deionization or demineralization process


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Lime soda process

• Principle: In this method hard water is treated with

calculated amounts of slaked lime [Ca(OH)2] Soda

ash [Na2CO3] in reaction tanks, so as to convert

hardness producing chemicals into insoluble

compounds which are then removed by settling &

filtration.

• Usually 10% extra chemical is added for better

results.


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Lime Soda Process

• Lime removes hardness due to temporary Calcium &

all types of magnesium hardness.

• Lime react with free acids, Fe, Al salts, CO2 gas &

produce extra calcium hardness.

• Soda removes all the soluble permanent hardness due

to calcium salts (i.e. that which is present originally

as well as that which is introduced during removal of

Mg2+ , Fe2+ , Al3+ , HCl, H2SO4, CO2 etc by lime )


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• Amount of Lime required = 74/100 [temp Ca

hardness + (2 × temp. Mg hardness) + perm Mg

hardness + Salts like Fe, Al + CO2 + Acids + HCO3-

- NaAlO2 ] × volume of water × 100/ % purity

• Amount of Soda = 106/100 [perm Ca hardness +

perm Mg hardness + Salts like Fe, Al + Acids -

HCO3- ] × volume of water × 100/ % purity


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• The chemical reactions taking place during lime soda treatment are slow & ppt of CaCO3 & Mg(OH)2 are fine & produce super-saturated solution. As a result after deposition occurs in pipes, boiler tubes etc clogging the valves & leading to corrosion. To avoid this;

• Thorough mixing of chemicals & hard water.

• Sufficient time allowed to complete reactions


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• Accelerators i.e. Substances that bring down the fine particles of precipitates e.g. Activated charcoal

• Coagulants i.e. Substances which help in the formation of coarse precipitates are added e.g. alum

• Proper sedimentation chamber for precipitation to settle, before filtration being carried out

• Process is carried out at room temperature & at temperature between 500C- 1500C


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Cold lime Soda process

When the chemicals are added at room temperature. At this temperature the precipitates are finely divided & do not settle easily, nor they can be filtered. It is necessary therefore to add coagulants like alum, sodium aluminate etc

• NaAlO2 + 2H2O → NaOH + Al(OH)3

• This process provides water containing a residual hardness of 50-to 60 ppm.


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Hot Lime Soda Process

• When the chemicals are added at higher temperature 80 0C to 150 0C the process is known as Hot process. At higher temperature reactions are fast

• precipitation is more complete

• settling rate & filtration rates are increased

• So less amount of chemicals are needed.


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Cold lime Soda process chamber


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Advantages of Soda lime process

• Very economical• pH is increased , there by corrosion is reduced • to a certain extent iron & manganese are also

reduced.• Disadvantages:• Hardness after Cold process is about 50 ppm & 30

ppm by Hot process. These values are high for pressure boilers.

• Careful operation & skilled supervision is required


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Zeolite Permutit Process

• Hydrated sodium alumino silicate Na2O.Al2O3.x SiO2.yH2O

• Zeolites is capable of exchanging reversibly their sodium ions for hardness producing ions in water.

• Natural Zeolites : are non-porous more durable & are derived from green sands.

• Synthetic Zeolites : porous & possess gel structure & have higher exchange capacity per unit weight.


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• Theory: When hard water is passed over a bed of sodium zeolite , Ca2+ , Mg2+ ions are taken up by the zeolite simultaneously releasing equivalent Na+ ions in exchange for them.

• CaCl2 + Na2Z → CaZ + 2NaCl

• MgSO4 + Na2Z → MgZ + Na2SO4


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• Regeneration: When Zeolite is completely converted into calcium & magnesium Zeolites, it ceases to soften water i.e. it gets exhausted. It is generated by treating with 10% brine solution.

• CaZe + 2NaCl → Na2Ze + CaCl2

• MgZe + 2NaCl → Na2Ze + MgCl2


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• Process: Hard water enters from top at a specified rate & passes over a bed of sodium zeolite kept in a cylinder. Softened water is collected at the bottom of cylinder & is taken out from time to time.


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Limitations:

• Turbid water should not be admitted, otherwise it will block the pores of zeolite & make them inactive.

• Any colored Fe ions must be removed earlier because, it is difficult to regenerate it from iron zeolite.

• Mineral acid present in water must be neutralized earlier with soda otherwise that may destroy zeolite bed.


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Advantages:

• Water of about 10 ppm hardness is produced.

• Process automatically adjusts itself for different hardness of incoming water.

• Requires less skill in maintenance as well as operation.


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Disadvantages:

• Treated water contains more sodium salts

• The method only replaces Ca2+ & Mg2+ ions by Na+ ions, but leaves all acidic ions (HCO3 - & CO3 --)in soft water. Such soft water containing NaHCO3 , Na2CO3 etc when used in boilers , NaHCO3 decomposes to give CO2 which cause boiler corrosion & Na2CO3 hydrolyses to NaOH causing Caustic Embrittlement.


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Ion Exchange Process

• In this process, a reversible exchange of ions occur

between the stationary ion-exchange phase & the

external liquid phase.

• Ion exchange resins are insoluble , cross-linked, long

chain organic polymers which are permeable due to

their micro porous structure & functional group

attached to the chain are involved in the ion

exchanging process.


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Cation exchange resins

• These are capable of exchanging rapidly cations like Ca2+ & Mg2+ by H+ ions. E.g. sulphonated coals, tannins, formaldehyde resins, amberlite IR-120 etc

• RH2 + Ca2+ / Mg2 + → RCa/Mg + 2H+


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Anion exchange resins

• These are capable of exchanging rapidly

anions (Cl - , SO4 --) by OH- ions e.g. amino-

formaldehyde resins, copolymer of styrene &

di vinyl benzene, amberlite-400, Zeolite-FF

etc

• R’(OH)2 + H2SO4 → R’SO4 + 2H2O


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Process• Hard water is first passed through cation exchange

bed which removes all cations like Ca2+ , Mg2+ , Na+ & release H+ ions. Thus Cl - , SO4 --, OH- are converted into corresponding acids HCL, H2SO4, & H2CO3

• After this the acidic hard water is passed through an anion exchange bed which removes all anions present in water & equivalent amount of OH- ions is released from it to form water.


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• H+ & OH- ions combine to give ion- free water molecules called De-ionized or De-mineralized water.

• The water is finally freed from dissolved gases by passing it through a de- gasifier the water obtained by this process is very near to distill water.


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Regeneration

• When ion exchange capacities are lost ( resins are exhausted) the supply of water is stopped. The exhausted cation exchanger i.e. regenerated by passing dilute HCl or H2SO4.

• RCa or RMg + 2HCl → 2RH + CaCl2 or MgCl2

• The exchange bed is washed with de-ionized water & washings( contained Ca2+ , Mg2+ Cl - , SO4 --,) is passed to sink or drain.


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• The exhausted anion exchanger is regenerated by treating it with a dilute NaOH solution.

• R’Cl2 + 2NaOH → R’(OH)2 + 2NaCl

• The exchanged bed is washed with de ionized water & washings( containing NaCl, Na2SO4 ) is passed to sink.

• The regenerated ion-exchange resins are used again.


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• Advantages:

(i) The process can be used to soften highly acidic or alkaline water,

(ii) Water of low hardness(2 ppm) thus very good for high pressure boilers.

• Disadvantages:

(i) The equipment is coastally

(ii) Turbidity should be below 10 ppm


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