Water Treatment - srmchemistry.wikispaces.comsrmchemistry.wikispaces.com/file/view/Water+Treatment+Whole... · Water Treatment The nature’s most ... Disadvantages of Hard Water

350

17Water Treatment

The nature�s most abundant supply i.e., water is essential for the survival of all the livingbeings on earth i.e., man, animals and plants. Water is not only essential for the survival oflife, but it is also used for the operation in a large number of industries as coolant, solvent, forsteam generation, for air conditioning, fire-fighting etc. Water is also used for all domesticpurposes like bathing, drinking, washing, sanitary, irrigation etc.Sources of Water

1. Rainwater. It is the purest form of natural water. But unfortunately it dissolves thetoxic gases like CO2, SO2, NO2 etc. and other solids, which pollute the atmosphere.

2. Sea water. It is the most impure form of water containing about 3.5% dissolved saltsof which about 2.6% is sodium chloride. Other salts present include sulfates, bicarbonates,bromides of sodium, potassium, magnesium etc. Other impurities are carried to sea throughrivers.

3. River water. The sources of river water are the springs and the rainwater. Riverwater while flowing 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. Thedissolved matters include the salts like sulfates, bicarbonates and bromides of sodium, potassiumand magnesium.

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

5. Underground water. The rainwater and other surface water percolate down throughthe soil and rocks and get filtered and finally collected on rocky surface or again come out asspring. Though it contains less suspended matter but the dissolved mineral content is quitehigh and is of high organic purity.

Thus we can say that the surface water collects lots of suspended materials, micro-organisms and other pollutants from the habitats and become unsuitable for direct humanconsumption and other usage.Effect of Water on Rocks

Water, while percolation, gets contaminated by the following processes:(a) Dissolution. Sodium chloride, gypsum (CaSO4.2H2O) and other soluble salts get

directly dissolved in the percolated water.(b) Hydration. While percolation, water causes hydration of the minerals like anhydrite,

olivine which leads to increase in volume and disintegration of such rocks, viz.CaSO4 + 2H2O → CaSO4.2H2O

Anhydrite Gypsum

WATER TREATMENT 351 Mg2SiO4 + xH2O → Mg2SiO4.xH2O

Olivine Serpentine(c) Oxidation. Dissolved oxygen brings about oxidation alongwith hydration.

2Fe3O4 →Oxidation 2Fe2O3 →

Hydration 2Fe2O3 .2H2O Magnetite Haematite Limonite

(d) Action of carbon dioxide. Insoluble carbonates of Ca, Mg and Fe alongwith silicatesof Na, K, Ca, and Fe are converted into soluble bicarbonates and soluble carbonates, viz.

CaCO3 + CO2 + H2O → Ca(HCO3)2 (Insoluble) (Soluble)

K2O.Al2O3.6SiO2 + CO2 + 2H2O → Al2O3.2SiO2.2H2O + K2CO3 + 4SiO2Hardness of Water

Hardness of water is the characteristic of preventing lather formation of water withsoap. Generally salts like chlorides, bicarbonates and sulfates of Ca2+, Mg2+ and Fe2+ makewater hard. This hard water on treatment with soap which is stearic or palmitic acid salts ofsodium or potassium causes white precipitate formation of calcium or magnesium stearate orpalmitate.

2C17H35 COONa + CaCl2 → (C17H35COO)2Ca + 2NaClSodium stearate Calcium stearate

(soap) (insoluble)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 softand forms lather.

The hardness is of two types:(i) Temporary hardness is due to the bicarbonates of Ca2+ and Mg2+ and carbonate of

Fe2+. Since bicarbonates readily get precipitated on boiling the water, the temporary hardnesscan be easily removed, viz.

Ca(HCO3)2 →heat CaCO3↓ + H2O + CO2

insoluble(ii) Permanent hardness is due to the presence of chlorides and sulfates of Ca, Mg, Fe,

etc. Permanent hardness cannot be removed easily on boiling.Units of Hardness

Both temporary and permanent hardnesses are expressed in ppm as CaCO3. The choiceof CaCO3 is due to the fact that its mol. wt. is 100 and equivalent weight is 50 and it is the mostinsoluble salt in water. Equivalent of CaCO3

= (Mass of hardness producing substance) (Chemical equivalent of CaCO )Chemical equivalent of hardness producing substance

3×

= Mass of hardness producing substance 50Chemical equivalent of hardness producing substance

×

Hardness is principally expressed in ppm unit. Other limits include French degree ofhardness, English degree of hardness or Clark, USA degree of hardness and German degree ofhardness.

1 ppm = 1part of hardness10 parts of water6

352 ENGINEERING CHEMISTRY

1 Fr. degree of hardness = 1 part of hardness10 parts of water5

1 Clark = 1 grain of hardness1 gallon or 70,000 grains

17 104=

×

1 USA degree of hardness = 1 grain1 USA gallon

1 grain58,300 grains

15.83 104= =

×

1 German degree of hardness = 1grain1German grain

1grain56,000 grains

15.6 104= =

×

Relation between various units of hardness1 ppm = 1 mg/l = 0.1° Fr = 0.07° Cl1 mg/l = 1 ppm = 0.1° Fr = 0.07° Cl1° Cl = 1.43° Fr = 14.3 ppm = 14.3 mg/l1° Fr = 10 ppm = 10 mg/l = 0.7° Cl.

Disadvantages of Hard Water(a) In domestic uses. For washing and bathing, hard water creates difficulties, since it

does not form lather freely with soap. It also creates sticky precipitates that deposit on bathtub, body, clothes etc. until all the Ca/Mg salts get precipitated. Thus a lot of soap get wastedalso.

For cooking hard water creates similar difficulties by producing scum on the bottom ofthe vessels. Due to the presence of hardness producing salts in hard water, boiling point getselevated and during cooking a lot of fuel is wasted. Pulses etc. do not cook in hard water. Tasteof tea, coffee becomes unpleasant. Drinking of hard water is also problematic since it affectsthe digestive system and at the same time the possibility of deposition of calcium oxalatecrystals in the urinary tract is alarming.

(b) In industrial uses. For textile industry and dyeing industry, hard water causes theusual problem of deposition of insoluble salts that interfere with the proper dyeing and printingof the fabrics. The stains of iron salts also are undesirable on fabrics. Hard water also hampersthe economy by wastage of soap as it does not form good lather.

• For sugar industry, the salts responsible for hardness create difficulties in sugar refiningand crystallization of sugar and the sugar becomes deliquescent.

• Calcium and magnesium salts also interfere with the smooth and glossy finish of thepapers in the paper industry. Iron salts interfere with the colour of the paper.

• In laundry, hard water causes wastage of costly soap and also interferes with thecoloration due to the staining of iron salts.

• The hydration of cement and final hardening of cement are affected by use of hardwater in concrete making.

• Hard water is not suitable for preparing drug solutions in pharmaceutical industry.• For steam generation in boilers, hard water creates many problems like (i) scale

formation, (ii) corrosion, (iii) priming and foaming and (iv) caustic embrittlement.Sludge and Scale Formation in Boilers

When hard water is used for boilers, on continuous evaporation, the salts present in thehard water gets saturated and are finally deposited in the areas where the flow is slow. When

WATER TREATMENT 353these precipitates are loose and slimy in nature, it is called sludge whereas when theseprecipitates are hard and they adhere strongly to the inner surface of the boilers, they arecalled scale.

Sludge can be easily removed by scrapping with a brush. Sludge is formed by the presenceof MgCO3, MgSO4, MgCl2, CaCl2 etc. These salts are more soluble in hot water.

Disadvantages of sludge formation are (i) poor heat conduction due to the presence ofsludge on the surface; (ii) difficulty in the operation of the boiler; (iii) if sludge is formed alongwith the scale and is trapped within the scale formed and so it is difficult to remove and (iv) itclogs the pipe lines and other connections to the vessel near the places where water circulationrate is slow.

Removal of sludges. Sludge formation can be prevented by (i) using soft water for boileroperation and (ii) removing the concentrated salty water from time to time so that depositionof sludge is prevented.

Scales are the hard deposits on the inner surface of the boilers which are difficult toremove. This scale formation takes place due to the following reasons:

(a) In low pressure boilers scale formation occurs due to the formation of CaCO3 fromCa(HCO3)2.i.e., Ca(HCO3)2 → CaCO3↓ + H2O + CO2

Scale(b) In high pressure boilers this CaCO3 gets converted to soluble Ca(OH)2. But here

CaSO4 forms the hard scale. Since the solubility of CaSO4 decreases with increase intemperature, and at high temperature the precipitated CaSO4 forms hard scale.

Similar hard scales are formed when SiO2 is present in the hard water. It deposits asCaSiO3 or MgSiO3. These calcium or magnesium silicate scales are very difficult to remove.

Dissolved magnesium salts also precipitate as Mg(OH)2 forming soft type of scale.Disadvantages of scale formation are similar to sludge formation but the severity is

more, since its removal is more difficult.Disadvantages include

(i) Poor heat transfer from boiler to water leading to increase in fuel consumption. Theincrease in thickness of the scale from 1.25 mm to 12 mm leads to increase in fuelconsumption from over 50% to 150%.

(ii) Due to the overheating of the boiler, different parts of the boiler become weak anddistorted and so the operation of the boiler becomes unsafe, particularly the highpressure boilers.The thick scales may sometimes lead to explosion due to sudden development of highpressure.

(iii) Valves and condensers of the boilers are chocked due to scale formation and boilerefficiency decreases.

Removal of scales can be done by:(i) Wooden scraper or wire brush, suitable for removing loose scales.

(ii) Blow down operations for loose scales. The operation actually involves the re-moval of very hard water from a tap at the bottom of the boiler and replenishingthe water with softened water called �make up� water.

(iii) Giving thermal shocks, which involve alternate heating and cooling to make thescales brittle.

(iv) Chemical treatment with 5-10% HCl for carbonates and EDTA treatment forCa/Mg salts forming complexes.


These are methods for the removal of the scales, when they are formed. There are alsosome methods for the prevention of scale formation by internal or external treatment.Internal treatment

Internal treatment involves addition of chemical to the boiler water either to (i) precipitatethe scale forming impurities in the form of sludges, which can be easily removed or (ii) convertthe impurities to soluble compounds, so that scale formation can be avoided. Important internaltreatments involve.

(a) Colloidal Conditioning: Organic substances like kerosene, tannin, agar-agar are addedto form gels and form loose non-sticky deposits with scale-forming precipitates, whichcan be easily removed by blow-down operations in low pressure boilers.

(b) Different sodium phosphates like NaH2PO4, Na2HPO4 and Na3PO4 are added to highpressure boilers to react with the hardness forming impurities to form soft sludge ofcalcium and magnesium phosphates and finally this can be removed by blow downoperation.

3CaCl2 + Na3PO4 → Ca3(PO4)2 + 6NaCl(c) Carbonate conditioning: Sodium carbonate is added to the water of low pressure

boiler whereby the scale forming CaSO4 gets converted to loose sludge of CaCO3,which can be easily removed by blow-down operation.

CaSO4 + Na2CO3 = CaCO3 + Na2SO4.(d) Calgon conditioning: Calgon i.e., sodium hexa meta phosphate when added to boiler

water, reacts with scale forming CaSO4 and forms soluble complex compound. 2CaSO4 + [Na4P6O8]2� → [Ca2P6O18]2� + 2Na2SO4

(e) Sodium aluminate conditioning: Sodium aluminate is hydrolysed yielding NaOHand gelatinous Al(OH)3. The NaOH formed reacts with magnesium salts to precipi-tate Mg(OH)2. This Mg(OH)2 and Al(OH)3 are flocculent and entraps the colloidal aswell as the finely divided impurities like silica in the boiler water and the looseprecipitate is finally removed by blow down operation.

NaAlO2 + 2H2O → NaOH + Al(OH)3↓

MgCl2 + 2NaOH → Mg(OH)2↓ + 2NaCl( f ) Electrical conditioning: Rotating mercury bulbs on heating by the boiling water emit

electrical discharges that prevent scale formation by the particles.( g) Radioactive conditioning: Tablets of radioactive salts placed inside a boiler emit

radiations, which prevent scale formation.Caustic Embrittlement

Embrittlement is the name that has been given to boiler failures due to development ofcertain types of crack resulting from excessive stress and chemical attack. In steam boileroperation, the chemicals that are believed to be responsible are NaOH and silica. Duringsoftening processes, Na2CO3 are added and it gives rise to NaOH at elevated temperatureaccording to the following reaction.

Na2CO3 + H2O → 2NaOH + CO2Steam boilers are made by rivetting constructions rather than welding and are

particularly liable to damage by caustic alkali solutions. This cracking is not due to corrosionand the cracks appear like brittle fracture and hence called caustic embrittlement. NaOHattacks and dissolves out iron of boiler forming sodium ferroate. This finally causes the stressedparts like bends, joints, rivets to lead to the boiler failure.

WATER TREATMENT 355Concentration cell develops between the boiler and the NaOH of different concentrations as

+ Fe atrivets, joints

etc.

ConcentratedNaOH

Solution

DiluteNaOH

Solution

Fe at �plane

SurfacesPrevention of Caustic Embrittlement

(i) Addition of sodium phosphate as softening agent instead of Na2CO3.(ii) Addition of sodium sulfate to ensure a weight ratio Na2SO4/NaOH > 2.5, whereby

the deposition of Na2SO4 prevents the penetration of NaOH into the cracks and stopscaustic embrittlement in high pressure boilers.

(iii) Addition of organic agents like tannin, lignin, querbracho etc. also prevents crackingsimilar to sodium sulfate in low pressure boilers.

(iv) Use of crack-resisting steels: Certain steels containing Al added during manufactureappear to be resistant against caustic cracking.

Corrosion in BoilersBoiler corrosion occurs by chemical or electrochemical attack of the contents of water.

Main types are of chemicals:(a) Dissolved oxygen: Oxygen is dissolved in water to the extent of 8 ml/l at room temperature

and it is responsible for corrosion in the boiler. Greater the pressure, higher the dissolved O2content. As the water is heated in the boiler the dissolved oxygen is liberated and iron is corroded.2Fe + 2H2O + O2 → 2Fe(OH)2 ↓ 4Fe(OH)2 ↓ + O2 → 2[Fe2O3.2H2O]

Dissolved oxygen can be removed by:(i) Mechanical deaeration methods using distillation, steam scrubbing, desorption, flash-

type deaeration, which reduce O2 concentration to about 0.01 ppm.(ii) Chemical treatment�oxygen concentration is virtually made zero by using reducing

agents like hydrazine, sodium sulfite etc.(iii) Ion exchange techniques also reduce oxygen concentration to very low level.(b) Dissolved CO2: Water contains some CO2 and the decomposition product of bicarbo-

nates present in water also produces CO2. This CO2 dissolves in water forming carbonic acidwhich causes local corrosion called pitting.

Fe + CO2 + H2O → FeCO3 + H24FeCO3 + O2 + 10H2O → 4Fe(OH)3 + 4H2O + 4CO2

4Fe(OH)3 → 2Fe2O3 + 6H2ORemoval of CO2 is done by:(i) Mechanical deaeration.

(ii) Adding lime or NH3.(iii) Heating.(c) Mineral acids: Water from industrial areas contains acidic wastes or inorganic salts

which on hydrolysis produce acids. These acids cause corrosion in boilers. The acids react withiron in chain reactions producing acids again and again and also produce H2 causing hydrogenembrittlement which leads to boiler failure.

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

FeCl2 + 2H2O → Fe(OH)2 ↓ + 2HCl


Priming and FoamingDuring rapid steam production, some liquid water drops are also carried along with the

steam. This wet-steam formation is called priming.Priming occurs due to (i) presence of large amount of dissolved solids, (ii) high steam

velocities, (iii) sudden boiling and (iv) sudden increase in steam production. Priming can becontrolled by (i) maintaining low water level in boiler, (ii) avoiding rapid change in steam rate,(iii) softening of boiler water and (iv) using mechanical device for steam purification.

Foaming is the production of bubbles and foams which do not break easily. Foamingoccurs due to the presence of oil in the water.

Foaming can be reduced by (i) removing oil from boiler-feed water and (ii) adding anti-foaming agents.

Priming and foaming occur together and they are undesirable since they wet othermechanical parts of the boiler and reduce their efficiency. Actual height of the water columncannot be judged due to foaming hence creating difficulty in the maintenance.

Highlights:• Chief sources of water: Sea water, rainwater, ground water and surface water.• Impurities present in water:

(i) Suspended, (ii) Colloidal, (iii) Dissolved impurities• Temporary hardness is due to Ca(HCO3)2 and Mg(HCO3)2 and can be removed by

boiling.• Permanent hardness is due to chlorides and sulfates of Ca2+, Mg2+, Fe2+ and other

heavy metals.• Hardness expressed in equivalent amount of CaCO3

= Mass of hardness producing substances 50Chemical equivalent of hardness producing substances

×

• 1 ppm hardness ≡ 1 part of CaCO3 equivalent hardness in 106 parts of water.• Hard water cannot be used for steam generation in boilers due to the problems

like scale and sludge formation, priming and foaming, boiler corrosion and failure.Softening of Water

Softening of water means the removal of calcium, magnesium, iron salts and similarother metallic ions, which would form insoluble metallic soaps. The three important industrialmethods employed for softening of water are:

1. Cold and hot lime-soda process.2. Permutit or zeolite process.3. Ion-exchange or demineralization process.1. Lime-soda process. By this process, soluble calcium and magnesium salts are rendered

insoluble by adding calculated amount of lime [(CaOH)2] and soda [Na2CO3]. The insolubleprecipitates of CaCO3 and Mg(OH)2 are removed by filtration. By this method, both temporaryand permanent hardness are removed.

For the removal of temporary hardness the reactions are: Ca(HCO3)2 + Ca(OH)2 → 2CaCO3 ↓ + 2H2OMg(HCO3)2 + Ca(OH)2 → 2CaCO3 ↓ + MgCO3 + 2H2O MgCO3 + Ca(OH)2 → Mg(OH)2 ↓ + CaCO3 ↓

Hence, to remove equivalent quantities of Ca and Mg hardnesses the amount of limenecessary is in the ratio of 1:2.

WATER TREATMENT 357Again for the removal of permanent hardness. The reactions are:

CaSO4 + Na2CO3 → CaCO3 ↓ + Na2SO4 MgSO4 + Na2CO3 → MgCO3 + Na2SO4

MgCO3 + Ca(OH)2 → Mg(OH)2 ↓ + CaCO3 ↓Hence, for the removal of permanent hardness due to Ca-salts, lime is not necessary,

but it is necessary for Mg salts. Extra addition of Ca(OH)2 causes hardness. So calculatedquantities of lime and soda are to be added after the determination of actual hardness.

In the actual process the water is thoroughly mixed with the chemicals and allowed toreact for sufficient time. Activated charcoal is added as activator, alum etc. are added ascoagulants. To avoid after-precipitation of CaCO3, sludge of the previous operation is added,which supplies the nuclei for the precipitation.Cold Lime-Soda Process

In this method, calculated quantity of chemicals and water, along with accelerators andcoagulators are added to a tank fitted with a stirrer (Fig. 17.1). On vigorous stirring, thoroughmixing takes place. After softening the soft water rises upwards and the heavy sludges settledown. The softened water passes through a filtering media ensuring complete removal of thesludge and finally the filtered water flows out through the top. Cold lime soda process is usedfor partial softening of municipal water, for softening of cooling water etc. In actual purpose,magnesium hardness is brought down to almost zero but calcium hardness remains about40 ppm.

Driving belt

Raw waterfeed inlet

Filtered softenedwater outlet

Outer chamber

Inner chamber

Motor

Chemicals(soda + lime+ coagulant)

feed inlet

Wood-fibrefilter

Stirrerpaddles

Stirrer

Sedimentedsludge[CaCO , Mg(OH) ]3 2

Sludge outlet

Fig. 17.1 Continuous cold lime-soda softener.


Hot Lime-Soda ProcessThis process is similar to the cold lime-soda process. Here the chemicals alongwith the

water are heated near about the boiling point of water by exhaust steam. As the reaction takesplace at high temperature, there are the following advantages:

(i) the precipitation reaction becomes almost complete.(ii) the reaction takes place faster.

(iii) the sludge settles rapidly.(iv) no coagulant is needed.(v) dissolved gases (which may cause corrosion) are removed.

(vi) viscosity of soft water is lower, hence filtered easily.(vii) Residual hardness is low compared to the cold process.

Hot lime-soda process consists of three parts:(a) �Reaction tank� in which complete mixing of the ingredients takes place.(b) �Ionical sedimentation vessel� where the sludge settles down and(c) �Sand filter� where sludge is completely removed.

The soft water from this process is used for feeding the boilers (Fig. 17.2).Advantages Include:

(i) Lime soda process is economical.(ii) The process improves the corrosion resistance of the water.

(iii) Mineral content of the water is reduced.(iv) pH of the water rises, which reduces the content of pathogenic bacteria.

Raw waterfeed inlet

Superheated

steam inlet

Reactiontank

Conicalsedimentation

tank

Precipitatedsludge

[CaCO , Mg(OH) ]3 2

Chemicals(lime + soda)feed inlet

Softenedwater

Finesandlayer

Sandfilter

Coarsesand layer

Gravels layer

Filteredsoftenedwater outlet

Precipitatedsludge outlet

Fig. 17.2 Continuous hot lime-soda softener.Disadvantages Include:

(i) Huge amount of sludge is formed and disposal is difficult.(ii) Due to residual hardness, water is not suitable for high pressure boiler.

WATER TREATMENT 359

Permutit or Zeolite ProcessZeolite is hydrated sodium alumino silicate capable of exchanging reversibly its sodium

ions for Ca2+ and Mg2+, having the general formula Na2O.Al2O3.xSiO2.yH2O. Common zeoliteis Na2O Al2O3.3SiO2.2H2O and is known as natrolith. Others gluconites, green sand etc. areused for water softening.

Artificial zeolite used for softening purpose is permutit. These are porous and glassyand have greater softening capacity than green sand. They are prepared by heating togetherwith china clay, feldspar and soda ash.

Method of Softening: Hardwater is passed through a bed of zeolite at a specific rate atordinary temperature; the hardness causing cations i.e., Ca2+ and Mg2+ are exchanged for Naand it is converted to CaZe and MgZe.

Reactions taking place are: Na2Ze + Ca(HCO3)2 = 2Na HCO3 + CaZe

Na2Ze + Mg(HCO3)2 = 2NaHCO3 + MgZe Na2Ze + CaSO4 = Na2SO4 + CaZe

Na2Ze + CaCl2 = 2NaCl + CaZeRegeneration of Zeolite: The process is also commercially successful since the Ca/Mg

zeolites formed by passing hard water through the bed can be easily regenerated into Na2Ze bypassing brine through the bed of inactivated zeolite.

CaZe or MgZe + 2NaCl = Na2Ze + CaCl2 (Or MgCl2)The washings containing CaCl2 or MgCl2 are wasted. The water softened by this process

can be used for laundry purposes.

Zeolite sand

Gravel

Top distributor

Hard water in

Injector

Salt storage

Waste

Soft waterout

Fig. 17.3 Softening of hard water by permutit process.Advantages. (i) Hardness of water can be removed completely upto about 10 ppm; (ii) The

equipment used is small and easy to handle; (iii) It requires less time for softening; (iv) Thereis no sludge formation, hence the process is clean; (v) Easy to regenerate; (vi) Any hardnesscan be removed without any adjustment of the process.

Disadvantages. (i) Coloured water or water containing suspended impurities cannot beused before filtration; (ii) Water containing acid cannot be used for softening since acid may


destroy the zeolite; (iii) Since on removal of Ca2+ and Mg2+ the soft water contains large amountof NaHCO3, this on heating liberates CO2, which causes corrosion in the boilers and hence thissoft water is not suitable for boilers.Ion Exchange or Demineralization

Ion exchange resins are organic polymers which are crosslinked having microporousstructure and the functional groups are attached to the chains which are responsible for theion exchange properties.

(i) Cation exchange resins (RH+) are phenol-sulfonic acid-formaldehyde resin, styrene-divinyl benzene copolymers which exchange their H+ ions with the cations present inthe water i.e., Ca2+ and Mg2+.

!CH CH CH CH2 2! !! ! !

SO H3 SO H3

n

cation exchange resin

(ii) Anion exchange resins (ROH�): The styrene divinyl benzene or amine formaldehydecopolymers contain quaternary ammonium tertiary sulphonium or amino group inthe resin. The resin on treatment with NaOH solution is capable of exchanging theOH� with different anions of water i.e., Cl�, SO4

2� etc.

CH CH CH CH2! !! !! !! !! !2 2 2CH CH CH

CH NMe OH2 2

–+

----CH ----CH2

CH CH CH CH----CH2! ! !2 2

CH NMe OH2 2+ –

CH NMe OH2 2+ – CH NMe OH2 2

+ –

Anion exchange resin

Method: The hard water is passed first through cation exchange resin similar to thepermutit process whereby the cations like Ca2+, Mg2+ are removed from the hard water andexchanged with H+ as follows:

2RH+ + Ca2+/Mg2+ → R2Ca2+/R2Mg2+ + 2H+

After this the hard water is again passed through anion exchange column, whichexchanges all the anions like SO4

2�, Cl� etc. present in the water with OH�

ROH� + Cl� → R+Cl� + OH�

These H+ and OH� combine to form water molecule. Thus the water coming out finallyfrom the two exchangers is ion free and called deioinized or demineralized water.

WATER TREATMENT 361

InjectorCation

exchangergravel

Acidregenerator

Wasteliquor

Wasteliquor Alkaline

regenerator

Injector

PumpAnion

exchangergravel

Fig. 17.4 Demineralisation of water.

SO4

––

C O

C C

COHO

C C

O

OCaC

C C

O

O

C C

OC

C C

C ONa+

Cl–

H+

H+

HO C

C C

OHO C

C C

OO C

CC

C

CC

O

Na O+

CO

OCa++

CC

CO

H+

H+

H+

Cl–

Cl–

H+

Cl–

H+

C H

C–N–H

C H–OHH

C–N–H

C H–OHH

C–N–H

C HH

SO4

–

C–N–H

CHH

C–N–H

C H–Cl–1

HO–H C

H–N–C

CH

HOH

H–N–C

CHH

H–N–C

HH C

H–N–C

CHHSO4

–

H–N–C

CH

Cationexchange

Filter bed

Hardwater

Anionexchange

D.M

. Water

Cation exchangereaction:

2R H + Ca CaR + 2H– + +2 +

2

Anion exchange reaction:

RNH + H O2 2 RNH OH3

–+

RNH OH + Cl RNH Cl + OH3 3

– – – –+ +

C

OH–

OH–

SO4

––

SO4

––

–

–

Fig. 17.5 Demineralisation of water showing the ion exchanges.


Regeneration: The inactivated or exhausted cation exchange resin is regenerated bydil. H2SO4/HCl.

R2Ca2+ + 2H+ → 2RH+ + Ca2+

Similarly, the exhausted anion exchange resin is regenerated by dil. NaOH R2SO42� + 2OH� → 2ROH� + SO42+

The columns are finally washed with deionized water and the washings are discarded.Mixed Bed Deionizer

It is actually a single cylinder containing and intimate mixture of cation exchanger andstrongly basic anion exchanger. Hence, water while passing through the column comes incontact with the two exchangers for a large number of times and the hardness is reduced to avery low level (1 ppm). Thus, it is actually equivalent to a series of cation and aninon exchangers.

Advantages of ion exchangers include (i) Easy regeneration; (ii) both acidic and alkalinewater can be softened; (iii) residual hardness is very low and hence the water is suitable forhigh pressure boilers also.

Disadvantages are (i) the equipment and the process is costly and (ii) turbid water cannotbe directly charged for softening. It needs prior filtration.Treatment of Water for Domestic Use

Municipalities supply potable water, which is suitable for drinking and it must satisfythe following requirements:Requirements for Drinking Water:

• It should be clear and odourless.• It should be devoid of any unpleasant taste.• Suspended matter should not exceed 10 ppm.• It should be devoid of heavy metals and arsenic.• pH should be about 8.• Content of dissolved salts should not exceed 500 ppm.• Lastly, it should be free from pathogenic (disease causing) microorganisms.

Natural water from river, lakes, canals are treated for the domestic purpose. The stepsare as follows:

• Aeration of the raw water by passing compressed air to remove mainly the obnoxiousodour and iron as Fe(OH)3.

• Settling in a large tank to remove suspended impurities.• Sedimentation through coagulation of the colloidal particles by adding a suitable

coagulant. Chemical coagulants are generally (i) Alum [K2SO4, Al2(SO4)3.24H2O](ii) Green vitriol [FeSO4.7H2O].

• Coagulants or flocculants are generally added in the form of solution and mixing iscarried out by stirring.

• The gelatinous precipitates mainly contain aluminium hydroxide, ferrous and ferrichydroxides (ferrous hydroxides are converted to ferric hydroxide by the dissolved oxygenof water).

• These heavy flocculant precipitates contain also some bacteria making the water freeof some of the microorganisms.

• These partially clarified water is filtered through sand gravity filters.

WATER TREATMENT 363

• These filters are rectangular tanks, which contain layers of fine sand at the top of onemeter thick, at middle of coarse sand of 0.5 meter thick and at bottom a gravel of 0.5meter thick.

Sterilization(i) By addition of bleaching powder (1 kg per 1000 kiloliters).Here sterilization is effected by the HOCl generated by the following reaction:

Ca(OCl)Cl + H2O → Ca(OH)2 + Cl2Cl2 + H2O → HCl + HOCl

and this HOCl kills the germs.Disadvantages of this process:(i) Bleaching powder makes water unnecessary hard and sometimes imparts a bad taste

to the water.(ii) By direct chlorination: Here also the generated HOCl kills the pathogenic bacteria.

Liquid chlorine is the most effective reagent, 0.3-0.5 ppm chlorine is sufficient.Advantage of using Chlorine. It is effective and economical; chlorine does not make

water hard and requires a little space for storage.Disadvantages. Excess chlorine imparts unpleasant taste and odour to the treated

water. Dissolved chlorine causes irritation to mucus membrane and lowers the pH of waterbelow 6.5. The treated water should contain chlorine less than 0.2 ppm.(ii) Break-point Chlorination

It is seen that when liquid chlorine is added or chlorine gas is passed through water theconsumption of chlorine makes the available chlorine less and after some time the availablechlorine increases. This is due to the consumption of chlorine in oxidising some oxidisableorganic matter present in water at first. Then available chlorine again increases due todecomposition of chloro-organic compounds.

This free chlorine is responsible for destruction of pathogenic bacteria in water. Thepoint at which the residual chlorine begins to appear is known as break point and at this pointwater is devoid of bad taste and odours and is bacteria free.

Advantages of break point chlorination• Organic matters present in water are completely oxidised leaving behind odour-free

water and sometimes colour-free water.• Pathogenic bacteria are destroyed completely.• It prevents the growth of weed in water in future.

(iii) Sterilization by ChloramineWhen chlorine and ammonia are mixed in the ratio 2:1 of volume chloramine is formed.

Cl2 + NH3 → Cl.NH2 + HClChloramine is much more effective than chlorine in its bactericidal (bacteria killing)

action. Here also HOCl is liberated. Cl.NH2 + H2O → HOCl + NH3

HOCl itself is bactericidal and here the liberated nascent oxygen is also bactericidal HOCl → HCl + [O].


The use of chloramine for sterilization is gaining importance as it does not impart anyirritating odour and does not affect the taste of the treated water.(iv) Sterilization by Ozonization

Ozone is an unstable gas and decomposes to give nascent oxygen. This nascent oxygenkills bacteria and oxidises the organic matter present in the water. The ozone treatment plantconsists of a tower made of enamelled iron, separated by perforated celluloid partition into alarge number of compartments, ozone is passed through bottom and the water is allowed topercolate through the celluloid partition. The perforated partition makes the water into minutebubbles resulting in a counter-current contact with ozone. This makes the treatment mosteffective.

Advantages of ozone sterilization are(a) Sterilization, bleaching, decolourisation and deodourisation take place at the same

time.(b) Ozone does not impart any unpleasant taste or odour to the treated water and does

not change its pH appreciably as it simply decomposes into oxygen. So it does notcause any irritation to the mucus membrane as is the case with chlorine or bleachingpowder treatment.

(c) Time of contact is only 10-15 minutes and dose strength is only 2-3 ppm.The only disadvantage of this process is that it is comparatively costly.

Sterilization through Physical Methods(i) Boiling, (ii) Exposure to the sunlight, (iii) Sterilization with UV light.

DechlorinationWhen during chlorination, break point chlorination is reached, it becomes sometimes

necessary to remove the excess chlorine from the treated water, this process is known asdechlorination. Sulphur dioxide is very common as an antichlor. Other common antichlorsare: sodium bisulphite, sodium thiosulphate and hydrogen peroxide.

Cl2 + H2O = HOCl + HCl H2O + SO2 = H2SO3 SO2 + Cl2 + 2H2O = 2HCl + H2SO4

HOCl + H2SO3 = HCl + H2SO4 H2O2 + Cl2 = 2HCl + O2↑

Dechlorination is needed to avoid the harmful effects of treated water.Desalination of Brackish Water

The water containing dissolved salts and having very salty taste is called brackish water,like the sea water and it is not suitable for drinking. The process of removal of common saltfrom this brackish water is called desalination. Desalination can be done by the followingmethods.

(a) Electrodialysis. This method involves the removal of ions from the brine solution byapplying direct current and employing pair of plastic membranes through which ions can pass.From Fig. 17.6 we can find that when direct current is passed through saline water, the Na+ions move towards the cathode while Cl� ions move towards the anode through the membraneand collect in the two side compartments while the central compartment contains puredesalinated water.

WATER TREATMENT 365Sea water

–

Cathode

Sea water Sea water

+

Anode

Negative ions

Positive ions

Membrane Membrane

Concentratedbrine

Concentratedbrine

Purewater

Fig. 17.6 Line diagram of electrodialysis.To make this process more effective, ions sensitive membranes are used that allow the

passage of either specific cations or specific anions. These membranes are selective as they aremade up of materials containing fixed functional groups.

An electrodialysis cell as shown in Fig. 17.7 contains a large number of pair of membranesand saline water is passed under pressure in electric field applied perpendicular to the directionof flowing water. Cations and anions pass through the membrane pairs and we obtain alternatestreams of pure water and concentrated salt solution.

Sea water feed

Cationselective

membrane

Cathode

Purewater outlet

Concentratedsea water

outlet

Anode

Anionselectivemembrane

C

C

A

A

C

C

A

A

C

C

A

A

Fig. 17.7 Electrodialysis cell.


Pressure

Sea water

Pure water

Piston

Stout semi-permeablemembrane

Purewater out

(b) Reverse osmosis. Osmosis describes the flow of solvent from dilute to concentratedsolution through a semipermeable membrane. Whereasreverse osmosis describes the flow of solvent in oppositedirection i.e., from concentrated solution to dilute solutionacross a semipermeable membrane by applying hydrostaticpressure in excess of osmotic pressure.

The cellulose acetate and the more recently usedpolymethacrylate and polyamide membranes do not allowthe solute pass, while the solvent is forced through andcollected as a pure solvent in a direction as shown inFig. 17.8. There are many advantages of using reverseosmosis as the purification process for water whichinclude (i) removal of ionic, non-ionic, colloidal and highmolecular weight solutes from water, (ii) regeneration ofthe process involves the easy replacement of thesemipermeable membrane, (iii) easy maintenance andeconomical, as the membrane lifespan is high, (iv) uninterrupted supply of large volume ofwater for industrial or domestic purpose can be obtained. Reverse osmosis is largely usedfor purification of sea water for domestic use.Chemical Analysis of Water

1. Estimation of free chlorine. The residual free chlorine remaining after the municipalprocessing of domestic water is injurious to health and hence estimation of this free chlorine isessential. The principle is the treatment of water containing free chlorine with KI solution.The chlorine present liberates an equivalent amount of iodine which can be estimated withstandard thiosulfate solution using starch as an indicator.

Cl2 + 2KI → 2KCl + I2 I2 + 2Na2S2O3 → 2NaI + Na2S4O6

I2 + Starch → Deep-blue complexMethod: To 50 ml of water sample in a iodine flask, 10 ml of 10% KI solution added,

shaken and waited for some time in the dark and finally titrated with N/50 sodium thiosulfatesolution using starch solution as indicator towards the end. The end point indicates a changefrom deep-blue to just colorless solution.

Calculation:Volume of N/50 Na2S2O3 required = V ml

∴ Strength of free chlorine = V 35.5 102500 1000

6× ×

× ppm.

2. Alkalinity. The alkalinity of water is estimated by estimating (a) OH� and CO32� ions

and (b) HCO3� ions.

(a) OH� + H+ → H2O

CO32�+ H+ → HCO3

�

(b) HCO3� + H+ → H2O + CO2

Fig. 17.8 Reverse osmosis cell.

WATER TREATMENT 367These estimations are done by (a) titrating against standard acid solution using

phenolphthalein as an indicator and (b) using methyl orange as an indicator.Method: 100 ml of water sample is taken in a flask and few drops of phenolphthalein

added and titrated against N/50 H2SO4 solution to colorless end point. The same solution isfurther titrated with 2 to 3 drops of methyl orange indicator.

Calculation: Vol. of acid upto phenolphthalein end point = V1Extra volume of acid added to get methyl orange end point → V2 ml.∴ Phenolphthalein alkalinity (equivalent CaCO3)

P = V 50 1050 100 1000

16× ×

× × ppm

Methyl orange alkalinity (equivalent CaCO3)

M = (V V ) 50 1050 100 10001 2

6+ × ×

× × ppm.

3. Hardness(a) Estimation of temporary hardness. Temporary hardness is due to Ca(HCO3)2 and

Mg(HCO3)2 which cause alkalinity in water. Temporary hardness is determined by finding thealkalinity of water before and after boiling, since temporary hardness is removed on boiling.

Ca(HCO3)2 →Boiling CaCO3 + CO2 + H2O

Mg(HCO3)2 → Mg(OH)2 + CO2Method: Into 50 ml of water sample taken in a conical flask, add 2 to 3 drops of methyl

orange and titrate with N/50 HCl. Let the volume of acid be V1. Now take 100 ml of watersample in a beaker, evaporate to dryness. Add 100 ml of distilled water to the beaker anddissolve the residue left. Then, take 50 ml of this water sample and titrate with N/50 HClusing a few drops of methyl orange as an indicator. Let the volume of acid required be V2 ml.

∴ alkalinity due to temporary hardness in CaCO3 equivalent

= (V V ) 1050 10001 2

6− ×

× ppm

(b) (i) Determination of permanent hardness. The estimation is done by addingexcess of standard Na2CO3 solution to a given volume of boiled water containing permanenthardness. The chlorides and sulfates form insoluble carbonates. The residual Na2CO3 is titratedagainst standard acid and the difference of Na2CO3 equivalent gives permanent hardness.

Method: 50 ml of the water sample is taken in a beaker. 50 ml of N/50 Na2CO3 solutionis added and boiled for 15 minutes. Cool and filter the solution, wash the residue on filterpaper and add it to the filtrate and titrate the N/50 Na2CO3 left in the flask with N/50 HClusing methyl orange as indicator. Let titre vol. be V.

∴ Permanent hardness = (50 V) 1050 1000

6− ×

× ppm.

(ii) Complexometric determination of permanent hardness. Disodium salt ofethylene diamine tetra acetic acid (EDTA) is used as a complexing agent for Ca2+, Mg2+.

–OOC · CH2

–OOC · CH2

N CH CH N2 !! ! 2

CH COO2

–

CH COO2

–


Erichrome black T is used as an indicator and pH is maintained at pH 10 using NH4OH/NH4Cl buffer. Erichrome black T forms unstable complex with Ca2+, Mg2+ giving wine redcolour. When EDTA added and the total Ca2+, Mg2+ forms complex with it and the indicatorbecomes free, the colour of the solution changes from wine red to blue at the end point.

Method: Indicator solution 0.5 gm. Erichrome black T dissolved in 100 ml alcohol.Standard EDTA solution (N/100): 4 gm of EDTA dissolved with 0.1 gm of MgCl2 in

1000 ml water.An aliquot ≈ M

100 Zinc acetate solution (primary standard) is taken, to it 5 ml of NH4Cl�NH4OH buffer and a few drops of EBT indicator are added and titrated to blue end point withEDTA and the strength of EDTA is calculated.

This same procedure is repeated with 50 ml water sample. Let the volume of titrant beV ≈ (f) M/100 EDTA solution.

∴ Permanent hardness = 0.001 V× × ×f 1050

6 ppm.

The EDTA method for the determination is preferred to other methods since the methodhas greater accuracy, simplicity and rapidness.

Buffer solution: A mixture of 142 ml of conc. NH4OH and 17.5 gm NH4Cl is diluted to250 ml distilled water.

Highlights:• Softening methods include:

(i) Hot and cold lime soda process.(ii) Zeolite or permutit process.

(iii) Ion exchange or demineralization.• Micro-organisms from potable water can be removed by:

(i) Boiling for 10 to 15 minutes(ii) Bleaching powder treatment

(iii) Chlorination with Cl2(iv) Chloramine treatment(v) Ozonization.

• Desalination of brackish water done by:(i) Electrodialysis

(ii) Reverse osmosis.• Exhausted ion-exchange resins are regenerated by:

(i) Cation exchangers regenerated by passing strong acid solution through the bed(ii) Anion exchangers regenerated by passing strong base solution through the bed.

• Hardness of water can be estimated by:(i) Permanent hardness determined by titrating with standard Na2CO3 solution.

(ii) Complexometric titration by EDTA• Units of hardness are:

(i) ppm(ii) mg/litre (1 ppm = 1 mg/litre)

(iii) Clark�s degree (1 ppm = 0.07° Cl)(iv) degree French (1 ppm = 0.1° Fr).

(Contd.)

WATER TREATMENT 369

• Hard water does not easily lather with soap, instead it forms a greasy scum.• Water is hard if it contains calcium or magnesium ions.• Scum is a precipitate formed when soap comes in contact with hard water

containing above mentioned ions. Ca2+(aq) + C17H35CO2

�(aq) → Ca(C17H35CO2)2 ↓ insoluble precipitate

• Temporary hard water contains bicarbonates or hydrogen carbonates of calciumor magnesium.

• Temporary hardness is removed by boiling.

Ca(HCO3)2(aq) →boiling

∆ CaCO3 ↓ + H2O(l) + CO2↑.• The solid calcium carbonate is precipitated as scale, which affects heating element

of boilers and gradually blocks the pipes in heating system.• Permanent hardness is not removed by boiling.• The mineral gypsum (CaSO4) is slightly soluble in water and makes water

permanently hard.• Ion exchange resin and other methods of softening of water by removing calcium

and magnesium ions is known as water treatment.

PROBLEMSProblem 1. Calculate temporary hardness and total hardness of a sample of water

containing:Mg(HCO3 )2 = 7.5 mg/l . Ca(HCO3 )2 = 16 mg/l

MgCl2 = 9 mg/l . CaSO4 = 13.6 mg/l

Sol. Temporary hardness = 7.5 16 mg/× + ×LNM

OQP

100146

100162 l

Permanent hardness = 9 10095 13 6 100

136× + ×LNM

OQP

. mg/l.

Problem 2. How many mg. of FeSO4 dissolved per litre gives 200 ppm of hardness?Sol. (56 + 16 + 64)g of FeSO4 ≡ 100 g CaCO3For 100 ppm hardness FeSO4 required per 106 parts of water is 136 parts.

∴ For 200 ppm hardness, FeSO4 required = 136 200100

× ppm = 272 mg/l.

Problem 3. A sample of water has a CO32− concentration of 15.6 ppm. What is the molarity

of CO32− in the sample of water?


Sol. 15.6 ppm = 15.6 × 10�6 g/l of CO32−

∴ CO32� molarity = 15 6 10

606. × −

mol/l.= 2.6 × 10�5 M.

Problem 4. A sample of water has been found to contain the following salts:Ca(HCO3)2 = 10.5 ppm; Mg(HCO3)2 = 12.5 ppm; CaCl2 = 8.2 ppm; MgSO4 = 2.6 ppm;

CaSO4 = 7.5 ppm.

Calculate (i) temporary and permanent hardness and (ii) the vol. of M100 EDTA required

for titration of the 100 ml of the sample to determine the total hardness of the sample.

Sol. (i) Ca(HCO3)2 = 10.5 ppm = 10.5 × 100162 = 6 481. ppm CaCO3

Mg(HCO3) = 12.5 ppm = 12.5 × 100146 = 8.562 ppm CaCO3

CaCl2 = 8.2 ppm = 8.2 × 100111 = 7.387 ppm CaCO3

MgSO4 = 2.6 ppm = 2.6 × 100120 = 2.167 ppm CaCO3

CaSO4 = 7.5 ppm = 7.5 × 100136 = 5.515 ppm CaCO3

∴ Temporary hardness = (6.481 + 8.562) ppm = 15.043 ppm

Permanent hardness = (7.387 + 2.167 + 5.15) ppm = 15.069 ppm∴ Total hardness = (15.043 + 15.069) = 30.112 ppm(ii) The volume of sample taken = 100 ml.

∴ Volume of M/100 EDTA required = 30.112 1001000

× = 3.11 ml.Problem 5. Calculate the amount of lime and soda required for softening 50,000 litres

of hard water containingMgCO3 = 144 ppm; CaCO3 = 25 ppm; MgCl2 = 95 ppm; CaCl2 = 111 ppm; Fe2O3 = 25 ppm; Na2SO4 = 15 ppm.

Sol. MgCO3 = 144 ppm = 144 10084× = 171.4 ppm CaCO3

CaCO3 = 25 ppm.

MgCl2 = 95 ppm = 95 10095× = 100 ppm CaCO3

CaCl2 = 111 ppm = 111 100111× = 100 ppm CaCO3

WATER TREATMENT 371

∴ Lime required = 74100 [2 × MgCO3 + CaCO3 + MgCl2] × volume of water

= 74100 [2 × 171.4 + 2500 + 100.0] × 50,000 mg

= 17,309,000 mg = 17.31 kg.Problem 6. A water sample on analysis gives the following data : Ca2+ = 20 ppm, Mg2+

= 25 ppm, CO2 = 30 ppm, HCO3� = 150 ppm, K+ = 10 ppm. Calculate the lime (87% pure) and

soda (91% pure) required to soften 1 million litres of water sample.

Sol. Ca2+ = 20 ppm = 20 10040× = 50 ppm CaCO3

Mg2+ = 25 ppm = 25 10024× = 104.65 ppm CaCO3

CO2 = 30 ppm = 30 10044× = 68.18 ppm CaCO3

HCO3� = 150 ppm = 150 100

122× = 122.95 ppm CaCO3

∴ Lime requirement = 74100 [Mg2+ + CO2 + HCO3

� ] × vol. of water

= 74100 [104.65 + 68.18 + 122.95] × 106

= 218.9 × 106 mg = 218.9 kg.

Soda requirement = 106100 Ca Mg HCO2 2

3+ + −+ − × vol. of water.

= 106100 50 104 65 122 95 106[ . . ]+ − ×

= 33.6 × 106 mg = 33.6 kg.Problem 7. An exhausted zeolite softener was regenerated by passing 150 litres of NaCl

sol. having a strength of 150 g/l of NaCl. If the hardness of water is 600 ppm, calculate the totalvolume of water that has been softened in the softener.

Sol. 150 litres of NaCl solution contain = 150 × 150 = 22,500 gm NaCl = 22,500 × 100

117 CaCO3 = 1.932 × 104 g CaCO3

If the hardness of water is 600 ppm

∴ Vol. of water = 1932 10 1000600

4. × × litres = 3.129 × 104 litres

Problem 8. 10 ml of 0.85 M100FHG

IKJ Zn (OAC)2 required mol of an EDTA solution for

standardization. 50 ml of a water sample required 20 ml of the above EDTA solution. Calculatethe total hardness of water in ppm.


Sol. Strength of EDTA solution = 10 0.859

× = 0.94 M100FHG

IKJ

1000. ml M100LNM

OQP EDTA solution ≡ 1 g CaCO3

∴ 20 ml of M100FHG

IKJ EDTA solution = 1

1000 g CaCO3× ×20 0 94. = 0.019 g CaCO3.∴ 50 ml of the water sample contains 0.019 g CaCO3

∴ 106 ml of the water sample contains = 0 019 1050

6. × = 377.8 gm CaCO3

∴ Hardness of water = 377.8 ppm.Problem 9. A sample on water on analysis has been found to contain the following :

Ca(HCO3)2 = 10.5 ppm. Mg (HCO3)2 = 12.5 ppm CaSO4 = 7.5 ppm CaCl2 = 8.2 ppm MgSO4 = 2.6 ppm.

Calculate the temporary and permanent hardness in degree French.Sol. Ca(HCO3)2 = 10.5 ppm = 10 5 100

162. × = 6.481 ppm

Mg(HCO3)2 = 12.5 ppm = 12 5 100146

. × = 8.562 ppm

CaSO4 = 7.5 ppm = 7 5 100136

. × = 5.515 ppm

CaCl2 = 8.2 ppm = 8 2 100111

. × = 7.387 ppm

MgSO4 = 2.6 ppm = 2 6 100120

. × = 2.167 ppm∴ Temporary hardness = (6.481 + 8.562) = 15.043 ppm

= 15.043 × 0.1° Fr = 1.504° FrPermanent hardness = (5.515 + 7.387 + 2.167) ppm

= 15.069 ppm = 1.5069° Fr.

SHORT QUESTIONS AND ANSWERS

Q. 1. What is the need for �Desalination of water� programme?????

Ans. Virtually there is population explosion in the modern world. So for drinking,domestic and irrigation purposes river water and other sweet water sources seem to beinsufficient. So for the exploitation of the vast source of sea water, desalination programme isrequired, specially in the sea-side area.

Q. 2. What are the causes of corrosion of boilers?????

Ans. The causes of corrosion are:••••• Oxygen corrosion if boiler water is not properly deaerated.

WATER TREATMENT 373

••••• Electrochemical corrosion, also known as out of service corrosion, when boiler isshut down.

Q. 3. What are the disadvantages of scale formation in a boiler?????

Ans.••••• Scales and sludges are bad conductors of heat, as a result fuel consumption

increases.••••• Tubes and plates are clogged as a result the efficiency of the boiler is decreased.••••• Tubes of the boiler are corroded by the scales deposited and they may crack and

lead to boiler failure.MgCl2 + Fe + H2O = Mg(OH)2 + FeCl2 + H2

Q. 4. Why is demineralisation preferred to zeolite softening?????

Ans. Demineralized water is practically free from all the ions whereas the zeolite softenedwater contains sodium salts which are sources of caustic embrittlement.

Q. 5. Why is chloramine preferred to Cl2 for sterilization of drinking water?????

Ans. It is because chloramine does not impart any repulsive odour to the treated water.Q. 6. What are the criteria for drinking water?????

Ans. It should be devoid of any suspending matter, and harmful dissolved matter oforganic or inorganic matter and pathogenic organisms.

Q. 7. Why is UV radiation superior to other methods of sterilization of water?????

Ans. UV radiation does not impart any unpleasant taste or odour to the water and theprocess is simple.

Q. 8. What is pitting?Ans. Pitting is corrosion of the boiler. Dissolved gases like O2, CO2 , H2S corrode the iron

surface of the boiler particularly near the rivet, bends and leads to leak or boiler failure.Q. 9. What are boiler compounds?????

Ans. When the boiler water is difficult to soften or purify, some chemical substances areadded to the boiler water that makes the scale in the boiler loose and easily removable. Thesecompounds are called boiler compounds and examples of these are flour, kerosene, tannin,agar-agar etc.

Q. 10. What is plumbo solvancy????? How can it be prevented?????

Ans. Water transported through lead pipes are contaminated with Pb2+ due to solubilityof metallic lead in water.

2Pb + 2H2O + O2 = 2Pb(OH)2The hydrolysis of Ca2+ and Mg2+ salts dissolved in water may lead to the formation of

lead salts like PbSO4 and PbCl2.MgSO4 + 2H2O = Mg(OH)2 + H2SO4 2Pb + H2SO4 + O2 = 2PbSO4 + 2H2O MgCl2 + 2H2O = Mg(OH)2 + 2HCl 2Pb + 4HCl + O2 = 2PbCl2 + 2H2O

These lead salts are somewhat soluble in water. This phenomenon is known as plumbosolvancy.

The Pb2+ salts are poisonous and is the source of lead poisoning.Lead poisoning can be prevented by treatment of the water with alkali silicate and

phosphate followed by filtration.


Q. 11. Differentiate between sludge and scale.Ans. Sludge is soft, loose, slimy deposit formed inside the boiler, which is easier to

remove, while scale is a hard, sticky, adherent deposit on the inner surface of the boiler whichis difficult to remove.

Q. 12. What is break-point chlorination?????

Ans. See text page 362.Q. 13. State the harmful effects of silica present in water.Ans. Presence of silica in the water leads to the formation of calcium and magnesium

silicates and these silicates form sticky scales in the boiler which are very difficult to remove.Q. 14. What is the difference between hard water and soft water?????

Ans. See text page 350-355Q. 15. Alkalinity of water cannot be due to the simultaneous presence of OH�,

CO32� and HCO3

�. Why?????

Ans. OH� and HCO3� ions react to form CO3

2� ions: OH� + HCO3� = CO3

2� + H2O.Q. 16. Why do we express hardness of water in terms of CaCO3 equivalent?????

Ans. This mode permits easy addition and subtraction of concentration of hardness-causing constituents, since its mol. wt. is 100. Moreover, it has also been adopted as standardfor expressing hardness.

Q. 17. What are the salts responsible for temporary and permanent hardnessof water?????

Ans. Temporary hardness ⇒ Ca(HCO3)2 and Mg(HCO3)2Permanent hardness ⇒ Chloride and sulphate of Ca2+ and Mg2+.

Q. 18. Mention the units of hardness. Define them.Ans. (i) ppm, (ii) degree.ppm is defined as the parts of hardness salt present in 106 parts of water expressed in

terms of calcium carbonate.Degree hardness is defined as the parts of hardness salt present in 105 parts of water

expressed in terms of calcium carbonate.Q. 19. A water sample contains lithium chloride or zinc acetate as dissolved

solids. Do you think that water will be hard?????

Ans. Yes, as Li+ and Zn2+ both form insoluble soaps.

Q. 20. Calculate the hardness of M1000FHG

IKJ MgSO4 solution.

Ans. MgSO4 = CaCO3 120 g = 100 g

M1000FHG

IKJ MgSO4 solution contains = 120

1000 g, MgSO4 = 1001000 CaCO3 in 1000 ml water.

So, hardness in ppm 0.11000 × 106 = 100 ppm.

Again, hardness in degree = 0.11000 × 105 = 10

WATER TREATMENT 375

Q. 21. The presence of carbon dioxide is avoided in boiler feed water. Explain.Ans. Dissolved carbon dioxide forms carbonic acid and attacks boiler pipes and corrodes.Q. 22. Why does hard water consume a lot of soap?????

Ans. When hard water comes in contact with soap Ca2+ and Mg2+ ions react with soapand scum is formed when the reaction is overconsuming much of soap, lather is then formed.That is why hard water consumes a lot of soap.

Q. 23. Why is calgon conditioning better than phosphate conditioning?????

Ans. Calgon conditioning forms soluble complex saltNa2[Na4(PO3)6] + CaSO4 = Na2[Na2 Ca(PO3)6] + Na2SO4

Calgon Soluble complexwhereas phosphate conditioning i.e., treatment with sodium phosphates, precipitates calciumand magnesium phosphates causing scale in boiler pipes.

Q. 24. Why is water softened before using in boiler?????

Ans. Natural or hard water when fed in boiler produces scale, sludge and causes primingand foaming and boiler corrosion. That is why water is softened before using in boiler.

Q. 25. Why is caustic embrittlement controlled by adding sodium sulphate toboiler-feed water?????

Ans. Sodium sulphate when used blocks hair cracks thus prevents infiltration of causticsoda through the cracks. Thus sodium sulphate prevents embrittlement in boilers.

Q. 26. State two harmful effects of silica in water.Ans. Silica reacts with Ca2+ and Mg2+ ions forming calcium silicate and magnesium

silicate in boiler. This type of scales are difficult to remove.

EXERCISES1. Explain the various steps for the purification of water for municipal supply.2. What is desalination? Name the different methods of desalination and describe any one.3. What are boiler troubles? Why are they caused? What are the methods of their elimination?

4. Distinguish between:

(a) Temporary and permanent hardness.(b) Sludge and Scale.(c) Softening and demineralization.

5. Write notes on: (i) Caustic embrittlement, (ii) Reverse osmosis, (iii) Priming and foaming,(iv) Hot lime-soda process, (v) Boiler corrosion.

6. What is the principle involved in the determination of total hardness of water by EDTA method?

7. What are the functions of lime and soda in hot lime-soda process? Give equations. Why coagulantsare not used in hot lime-soda process?

8. Why does hard water consume more soap?

9. During deionization process, water is first passed through cation exchanger and then throughanion exchanger, why?

10. Describe the methods of disinfection of water. Why is chloramine better than chlorine for steri-lization of water?

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