lecture 6 softening

8/13/2019 Lecture 6 Softening

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IslamicUniversityofGaza EnvironmentalEngineeringDepartment

EENV4331

Lecture6: Softening

Dr.

Fahid Rabah

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6.HardnessandSofteninginwaterTreatment

6.1 Definition of hardness:

A. Hardnessis

the

term

often

used

to

characterize

a

Dosenotformafoamorlatherwellandcause

soa scum

Ca2++(Soap)Ca(Soap)2 (s) (soap

scum)

Thisincreasetheamountofsoapneededfor

washing

causescalingonpipesandhotboilers.

calciumcarbonatecrystals

leavestainson lumbin fixtures

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3

. Scalingduetocalcium andmagnesium

precipitation


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6.HardnessandSofting inwaterTreatment

B. Hardnessisdefinedasthesumofthedivalentmetalcations(inconsistentunits)existingin

watersuch

as

:

Ca2+ ,Mg2+ ,Fe2+ ,Mn 2+

c c y

m

u

,

g

ions(the

predominant

minerals

in

natural

waters)

+ +.

wheretheconcentrationofeachionisin

3,meq/L.

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.

Precipitation

Topsoil

Subsoil

2 2 2

Limestone CaCO3(s) + H2CO3 Ca(HCO3)2g 3(s) + 2 3 g 3 2

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F.CarbonateandnoncarbonateHardnessCarbonateHardness(CH) Ca2+,

Mg2+ associated

with

HCO3

,CO32

enca e emporary ar ness ecauseheatingthewaterwillremoveit. Whenthewaterisheated,theinsolublecarbonateswill

precipitateand

tend

to

form

bottom

deposits

in

waterheaters.

Ca2+,Mg2+ associatedwithotherions,Cl,NO3,

SO42

NCH=TH CH

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6.HardnessandSofteninginwaterTreatmentF.HardnessUnits

TotalHardness= (Ca2+ +Mg2+ )Thehardnessunitisthesameasthatconsistentunit usedforbothoftheabove

ions.Themostusedunitsare:

CaCOofEW 3

speciesofEW3

Where,

EW= equivalent

weight

pec es= onor a ca or ompoun

Radical:suchas CO32,SO4

2

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MWEW

For radicals and

MWEW

argeec r ca

For

ions

MW=molecularweightoftheSpecies

Example3.1: a)FindtheEWofMg2+ thathasaconcentrationof10mg/Las

theioninwater.b)FindtheEWofCaCO3

c)findconcentrationofMg + as mg L

CaCO3.

2+

mmole

mg24.3MW

mole

g24.3MW

mole

(n)Valance

mmole

(n)Valance

]1

[.mg24.3

EW mg

12.15EW MW

EW

9

a ance


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mg100MW g100MW

eq2(n)chargeElectrical

mmole

meq2chargeElectrical

MW

EW ]1

[.mg100

EW mg

50EW

D)FindingtheMg2+ concentrationas CaCO3:

CaCOofEW

speciesofEWspec esasmgaasmg 3

CaCOasm /L1.154 mg/meq50

2Mgasmg/L10CaCOasmg/Lin)(Mg2 3

Mg + conc.

10

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6.2 Relation between Alkalinity and hardness:

A. DefinitionofAlkalinity

relatedverycloselytohardnessandsofteningprocess.The

followingequationisusedtomeasurethealkalinityinwater:

a n y= 3 + 3

+

Wheretheconcentrationsareinmeq/Lor mg/LasCaCO3 + .

Therelationbetweenthealkalinityspeciesisgivenbythefollowingthree

equations:

[HCO3-]

]][CO[H10

2

310.33-

OHH10-14

pH10H , ,

12Wheretheconcentrationsaremoles/Linthesethreeequations


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VariousFormsofalkalinityandCO2relativetopHinwaterat25oC.

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Examp e6.1:

.

followingconcentrationsofions2+

Mg2+ 10mg/L

Na+ 11.8mg/LK+ 7.0 m LHCO3

110mg/LSO4

2 67.2mg/L

Cl 11m /L

Constructabarchartoftheionsintermof mg/LCaCO3 CalculatetheTH,CH,NCH,Alkalinity

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.

Ion Conc. M.W. n Eq. Wt. Conc. Conc.

CaCO3Ca

2+40.0 40.1 2 20.05 1.995 99.8

Mg2+

10.0 24.3 2 12.15 .823 41.2Na+ 11.8 23.0 1 23.0 .51 25.7

+. . . . .

HCO3-

110.0 61.0 1 61.0 1.80 90.22- . . . . .

Cl-

11.0 35.5 1 35.5 .031 15.5

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(cations) = (anions)

175.6

=

175.6

mg/Las

CaCO3 O.K

. = . meq .

Note: (errorintherangeof10%isaccepted)

Note:onecheckisenough(either as{mg/LasCaCO3}oras {meq/L} )

TotalHardness=

(Ca2+)

+(Mg2+)=

99.8

+41.2

TH=141mg/LasCaCO3or TH=1.995+0.823= 2.818 meq/L

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Alkalinity=(HCO3)+(CO3

2)+(OH)(H+)

a. SincepH=7.2 [H+]=10pH =107.2 mole/L=107.2 g/L

g/eq1

/LHg10eq/L

-7.2

eq/L10H -7.2

OHH10- OH0110- .-

=b.

g/L10*17mole/L10OH -6.8-6.8 /LOHg10*17eq/L-6.8

eq/L10OH -6.8-

mole/L10OH - .

C. (HCO3)=1.80 meq/L=1.80*103 eq/Lfromthisexamplecalculations

asinthetable. = * = * * 3 = * 3

[HCO3]=(109.8*103 g/L)/(61g/mole)=1.80*103 mole/L

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theportionofthehardnessassociatedwithcarbonateorbicarbonate

Alkalinity=90.1mg/LasCaCO3

TH=141mg/LasCaCO3

CH=90.2

mg/L

as

CaCO3

NoncarbonateHardness:

NCH=THCH

=141

90.1=

50.9

mg/L

as

CaCO3

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TH

Na+

. . .

Ca2+ Mg2+ K+

HCO3 SO4

2 Cl

.

90.2 160.10.0 175.6CH NCH

CaSO4

MgSO4 Na2SO4

NaCl

KCl

Note:thechemicalsatthelowerlineof thebargraphiscalledthehypothetical

combinationofpositiveandnegativeionsinthewatersample

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6.HardnessandSofteninginwaterTreatment6.3Softenin definition:a) Softeningisthechemicalprocessesinwhichhardness

causing

ions

(

Ca2+

,

Mg2+

)

are

removed

from

water

ether

.

b) SofteningmaybeachiviedbychemicalprecipitationusingtheLimeSodaAsh methodorbyionexchange.

c) In

the

chemical

precipitation

method

the

objective

is

to

produceCaCO3 andMg(OH)2:2+ 2

s

Mg2++2OH Mg(OH)2(s)

additionofLime[Ca(OH)2]andSodaash[

21

.


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6.HardnessandSofteninginwaterTreatmentf) Acommonwatertreatmentgoalistoprovidea

waterwithahardnessintherangeof75to120mg/L

asCaCO3

g) ToprecipitateCaCO3 andMg(OH)2weneedtoraisethepHto10.3bytheadditionofLime[Ca(OH)2].TheadditionoftheOHw conver

3

o3

h) ToprecipitateMg(OH)2weneedtoraisethepHto11bytheadditiono 2 3 . 3

neededtoreactwiththeremainingCa2+

2

removeCO2whichisnecessarytoraisethepH.

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6.HardnessandSofteninginwaterTreatment6.4ChemistryofLimeSodaAshSoftening

softeningreactions Neutralization

of

carbonic

acid

[ToraisethepHweneedfirsttoneutralizeanyacidinthewater]CO2+Ca(OH)2 CaCO3(s)+H2O

PrecipitationofCHduetocalcium:[ToraisethepHto10.3alltheHCO

3

isconvertedtoCO3

2]Ca2+ +

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

[ToraisethepHto11addmorelime]

Mg2+ +2HCO3

+Ca(OH)2

Mg2+ +CO3

2+CaCO3(s)

+2H2

O+

3 2 2(s) 3(s)

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Removalof

NCH

due

to

calcium

Ca2+ +SO42+ Na2CO3 CaCO3(s)+2Na

+ +SO42

Ca2+ +2Cl1+ Na2CO3 CaCO3(s)+2Na+ +2Cl1

Removalof

NCH

due

to

magnesium

2+ 2+ s

Ca2++Na2CO3 CaCO3(s)+2Na+

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So teningprocess imitations:

1. LimeSodasofteningcannotproduceawatercompletelyfreeofhardness

becauseof

the

solubility

of

CaCO3and

Mg(OH)2,

limitations

of

mixing

andreactiontime.

2. Thus,theminimumcalciumhardnessthatcanbeachievedis

30mg/LasCaCO ,theminimumMagnesium hardnessthatcanbeachieved

is 10

mg/L

as

CaCO3,

this

gives

a

minimum

hardness

of

40

mg/L

as

CaCO3.3. However,normallythewateristreatedtogiveahardnessintherangeof75to

120 m L as CaCO .

4. AnExcesslimebeyondthestoichiometricamountisusuallyaddedto

removeMg2+ hardness .Theminimumexcesslimeisusually20mg/L

3, . 3 . .

5. Mg2+ inexcessof40mg/LasCaCO3isnotdesiredasitformsscaleinwaterheaters.Mg2+ isexpensivetoremove,soweonlyremoveMg2+

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nexcesso mg as a 3.


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C emica srequirements:Accordingtothesofteningchemicalreactions:

Lime is added at the ratio of 1:1 for each carbonate hardness

compoundexceptforMg(HCO3)theratiois 2:1[i.e.2limeforeach 1Mg(HCO3)].

Limeisalsoaddedattheratioof1:1foreachMa nesiumNCH

compound

such

as

MgSO4Sodaash isaddedattheratioof1:1foreachMagnesiumor

CalciumNCHcom oundsuchasM S O ,andCaSO .

CO2neededis1:1ratiowiththeexcesslimeadded,and1:1ratiowithCaCO3remainingaftersoftening,2:1ratiowith

Mg(OH) remainingafter

softening.

Theunitsofthechemicalsareeitherinmeq/Lormg/LCaCO3 Example6.2illustratesthechemicalrequirements

calculations.

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6.HardnessandSofteninginwaterTreatment6.5ExcessLime Softening example

Example:6.2

waterw t t e on cc aracter st css own e ow sto eso tene tot e

minimumpossiblehardnessbylimesodaashexcesslimeprocess.Calculate

therequiredchemicalquantitiesinmeq/L.Drawabardiagramofthesoftened

water. AssumethataresidualofCaCO3

of0.60 meq/LandaresidualofMg(OH)2of0.20 meq/Lwillremaininthesoftenedwater.

7.43.4 6.4

Na2+

CO2

Ca2+

HCO3

Mg2+

4.9 7.4

SO42

..

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: . ..

Solution:me = . + . + . . + . . +excess me

=8.5+1.25=9.75meq/L

SodaAsh=6.44.9=1.5meq/L (toremoveNCH,MgSO42)

Na

+ :

TheadditionofsodaashaddstothewateranequalamountofNa+.

Sinceweadded1.5meq/Lsodaash,thiswilladd1.5meq/LNa+

The

original

concentration

existing

in

the

raw

water

is

1.0

meq/L .

Total Na+ =1+1.5=2.5meq/LNa+.

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: . ..

CO32

Mg2+ Na2+Ca2+

..

SO42

..

HO

0.6 0.8 3.3

Bargraphofthesoftenedwater

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lecture 6 softening

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