softening - tu delft opencourseware · 2.2 water quality and softening 2.3 softening processes ......

Softening

WATER TREATM

ENT

WATERTREATMENT

diameter 0.5 - 4 m

heig

ht +

/- 6

met

ers

AB

C

E

D

supply of hard watersupply of lyeperiodic dosing of sand grains (0.1-0.4 mm)forming pelletsoutlet for softened waterperiodic outlet of pellets (2 mm)

ABCDEF

in rest

in progress

F

hardness reduction [Ca2+] (mmol/l)

0

6

0

HC

O3- r

aw w

ater

(mm

ol/l)

5

1

2

3

4

5

1 2 3 4

NaOHNa2CO3

Ca(OH)2NaOHNa2CO3

Na2CO3

groundwater NaOHsurface water NaOH

groundwater Ca(OH)2surface water Na2CO3

FrameworkThismoduleexplainssoftening.

ContentsThismodulehasthefollowingcontents:

1 Introduction2 Principle 2.1 Whysoftening? 2.2 Waterqualityandsoftening 2.3 Softeningprocesses 2.4 Pelletreactor 2.6 Softeninginatreatmentplant3 Theory 3.1 Equilibrium 3.2 Kinetics 3.3 Massbalance 3.4 Hydraulics 3.6 Influenceofparameters4 Practice 4.1 Splittreatment 4.2 Choiceofchemicals 4.3 Constructionalternativeforreactors 4.4 Seedingmaterial 4.5 Pelletstorage

154

water treatmentsoftening

1 Introduction

Groundwaternormallyremainsinthesubsoilformanyyearsbeforeitispumpeduporflowsoutintothesurfacewater.Duetothelongresidencetimeinthesubsoil,groundwaterisinchemicalequilibrium(i.e.,calciumcarbonateequilibrium).Groundwater comes in contact with the atmos-pherewhen it ispumpedupordischarged intosurfacewater.Whencarbondioxidedisappearsfromthewater,itisnotincalciumcarbonateequi-libriumanymore.Also, when water is heated the equilibrium ischanging,theCa2+andHCO3

--ionswillprecipi-tate in the form of calcium carbonate (CaCO3).EspeciallyhighconcentrationsofCa2+andHCO3

--ionswillleadtoinconveniencesforthecustomersbecauseof thecalciumcarbonatescaling (e.g.,depositsinwaterboilers).Topreventprecipitationofcalciumcarbonateatthe customers’ taps, calcium ions are partiallyremovedfromthewaterbydrinkingwatercom-panies.Thisiscalledsoftening.

2 Principle

2.1 Whysoftening?Thefinancialbenefitsofsofteningaregreaterthanthecosts.Amsterdam Water Supply calculated that thebenefitsofsoftenedwaterforonesinglehouseholdcomes toasavingofapproximately45eurosayear(mainlyasaresultoftheirdecreaseduseofdetergent,lessmaintenanceonwashingmachinesandboilers,andlowerenergycosts),whereasthesofteningcostsforahouseholdareapproximately10eurosayear.

Inadditiontodecreasingthehardnessofwater,another important reason for softening is the

reducedreleaseofheavymetals.OtherreasonswhysofteningisusedaregiveninTable1.

2.2 WaterqualityandsofteningThehardnessofwaterisclassifiedfromverysofttoveryhard(Table2).Anumberofwaterqualityparametersare influ-encedasa resultof thesofteningprocess.Forthese parameters, standards are included inthe Dutch National Drinking Water Standards(Waterleidingbesluit).Inadditiontothesestandards,guidelinevaluesweredevelopedbyVEWIN.

Publichealth-decreasedreleaseofheavymetalsfromdis-

tributionnetwork-nouseofhouseholdsofteningdevices

Ethics-preventionofstains-user’scomfort

Environment-reductionofheavymetalsinsludgeWWTP-reductioninuseofdetergentanddecreasedphosphatecontentinwastewater-reductionofconcentratedischargeofhouseholdsofteningdevices

Economy-reductioninusageofdetergent-reductionofscalingandcorrosionofhouse-holdequipment-reductionofenergyconsumptionofheatingdevices-reductionindamagetoclothes

Table 1- Reasons why softening is applied

Table 2 - Classification of hardness

unit verysoft soft fairlysoft fairlyhard hard veryhardmmol/l <0.5 0.5-1.0 1.0-1.8 1.8-2.5 2.5-5.0 >5.0eq/m3 <1 1-2 2-3.5 3.5-5 5-10 >10*D <3 3-6 6-10 10-15 15-25 >25

155

softeningwater treatment

Theproducedwateralwaysneedstocomplywiththestandards.Theguidelineisatargetthatthewatercompaniessetthemselves.

The most important water quality parametersinfluencedbysofteningareacidity,hardness,bicarbonate,sodium,andthesolubilitypotentialformetalslikecopperandlead.

Acidity(pH)- standard=7.0<pH<9.5- guideline=8.0<pH<8.3

Directlyafterthesofteningprocess,acidityofthewaterishigherthantheabove-mentionedguide-line.BymeansofpH-correction(aciddosing),pHisdecreasedtothedesiredvalue.

Hardness- standard=1.5mmol/lattheminimum- guideline=1.5 <hardness<2.5mmol/l

Hardnessisdefinedasthesumoftheconcentra-tionofdissolvedcalciumandmagnesiumions.Thehardnessissometimesexpressedin°D(Germandegrees),orequivalents,perm3.Table2showstheconversionofmmol/ltoGermandegreesanditsequivalentsperm3.

Bicarbonateconcentration- standard =nostandard- guideline >2mmol/l

Thebicarbonateconcentrationshouldbehigherthan2.0mmol/l,resultinginwaterwithsufficientbufferingcapacity(pHstability);

Sodiumconcentration- standard =120mg/l- guideline =aslowaspossible

Becausesodium influencesbloodpressureandtherefore,indirectly,heartandvasculardiseases,theconcentrationshouldnotbehigherthan120mg/l.

SolubilitypotentialSoftening also works to reduce the solubility ofmetalsfrompipematerial.Fordrinkingwaterthemost important metals are lead (Pb) and cop-per (Cu), because these metals have a healthimpact.- standard Cu2+<3mg/l- guideline Cu2+<2mg/l- standard Pb2+<0.2mg/l- guideline Pb2+<0.01mg/l

Thevaluesforcopperandleadsolubilityaredeter-minedbyapipetest.Thepipetestisperformedinstagnantwaterandtakes16hours.Empiricalrelationshipshavebeenderived togivea rapidindicationabouttherelease:

2max 4Cu 0.52 TAC -1.37 pH 2 SO 10.2− = ⋅ ⋅ + ⋅ +

maxPb -141 pH 12 T 1135= ⋅ + ⋅ +

Figure 1- Scaled heating element

156


inwhich:Cumax =copperdissolvingcapacity (mg/l)TAC =TotalAnorganicCarbon (mmol/l)Pbmax =leaddissolvingcapacity (mg/l)SO4

2-=sulfateconcentration (mmol/l)T =temperature (oC)

TheTACconcentrationcanbeinfluencedbythesofteningprocess(mainlycausedbythedecreaseinHCO3

-).

2.3 SofteningprocessesThehardnessofrawwaterintheNetherlandsvariesbetween0.5and5mmol/l.Groundwaterextractedfromcalcareoussubsoils,especially,canhaveahighdegreeofhardness.Water extracted from deep sand layers of theVeluweis,ontheotherhand,fairlysoft(approxi-mately0.5mmol/l).Thehardnessofsurfacewater isnormally from2.0toamaximumof3.0mmol/l.

The hardness of water can be decreased bymeansofdifferentprocesses:- dosingofabase- ionexchange- membranefiltration

Dosingofbase(NaOH,Ca(OH)2orNa2CO3)Becauseofashift in thecalciumcarbonicacidequilibrium,spontaneouscrystallizationoccurs.By dosing the base in a reactor with seedinggrains, crystallization will occur on the surfaceoftheseedinggrains,forminglimestonepellets.Thisprocessiscalledsofteninginapelletreactorandwillbedescribedfurtherinsection2.4.Theprocessofsofteningbymeansofapelletreactorwas developed in the early 70s byAmsterdamWaterSupply.

IonexchangeThe ions (for this, calcium and/or magnesium)areexchangedwithother ions (sodium isusedthemost).

Membrane filtrationDependingonthetypeofmembrane,thehardnessispartly(nanofiltration)orfully(reverseosmosis)removed.

2.4 PelletreactorThe principle of the pellet reactor is shown inFigure3.Thepelletreactorconsistsofacylindricalvesselpartiallyfilledwithseedingmaterial.Thediameteroftheseedingmaterialisapproximately0.2-0.6mmandithasalargecrystallizedsurface.Waterispumpedinanupwarddirectionthroughthereactoratavelocityvaryingbetween60and100m/h.Atthesevelocitiesthesandbedisinafluidizedcondition.

Rawwaterandchemical(base)areinjectedintothebottomofthereactorbyseparatenozzles.Waterandchemicalsarewell-distributedoverthecross-sectionofthereactor(plugflow)oncesuffi-cientflowresistanceisrealizedoverthenozzles.

Figure 2 - Pellet reactor used for softening of drinking water

157


Theprocessconditions(suchaschemicaldosesand flow velocity) need to be selected so thatthe solubility product of calcium carbonate isexceeded.Asaresult,calciumcarbonatewillbeformed(quickreaction)andprecipitateontotheseedingmaterial.

Theseedinggrains’diameterwill increaseasaresult of the calcium carbonate deposit (forma-tionofpellets).Thepelletswillbecomeheavierand settle to the bottom of the reactor. Finally,thepellets(at a diameter of 1.0 - 1.2 mm) will be(atadiameterof1.0-1.2mm) will bewillberemovedfromthereactorandnewseedinggrainswillbebroughtin.Thepelletscanbereusedintheindustry.Softeningwithapelletreactordoesnotgeneratewasteproducts.

2.5 SofteninginatreatmentplantTo incorporate a softening installation (includ-ingposttreatment)intoanexistinggroundwatertreatmentprocess,thefollowingpossibilitiesareconsidered:- softeningofrawwater- softeningofaeratedwater- softeningafterrapidfiltration.

Softeningofrawwaterisdonedirectlyafteritispumpedup.If ironandmanganesearepresent indissolvedform in thewater (anaerobicwater), thesesub-stanceswillbetrappedintheCaCO3grains.Theadvantageofthisisthattheloadingonthesandfiltersisreduced.AdisadvantageisthattheCaCO3grainsbecomelesspure,affectingthegrowthofcrystals,result-inginfluffypellets.Another disadvantage is that the base dosageishighdue to thehighconcentrationof carbondioxideinrawwater.Beforethesofteningreactionstarts,the carbon dioxide needs to be convertedthecarbondioxideneedstobeconvertedtoHCO3

-andCO32-..

When softening takes place after an aerationphase,a lowerchemicaldosewillbesufficient,becausesomeofthecarbondioxideisremovedduringaeration.Anadditional(possible)costadvantageofsoften-ing (aerated) rawwater is that, inmany cases,existingfilters thathavebeenusedfor ironandmanganeseremovaluptillthispoint,canalsobeappliedas‘carry-over’filters.

diameter 0.5 - 4 m

heig

ht +

/- 6

met

ers

AB

C

E

D

supply of hard watersupply of lyeperiodic dosing of sand grains (0.1-0.4 mm)forming pelletsoutlet for softened waterperiodic outlet of pellets (2 mm)

ABCDEF

in rest

in progress

F

Figure 3 - Schematic representation of a pellet reactor

Figure 4 - Limestone pellets

158


Whensofteningafterfiltrationisapplied,thepur-estpelletsareformed.Ironandmanganeseareremovedbythefilters.A disadvantage is that after softening a new(expensive)rapidfiltrationstepmustbeaddedtotheprocesstoremovethe‘carry-over.’

3 Theory

3.1 EquilibriumThe calcium carbonic acid equilibrium deter-mines whether calcium carbonate precipitates.For an extensive explanation on this subject,youarereferredtolecturenotesfromthecourse‘Introduction inSanitaryEngineering (CT3420),’specifically the chapter onwater quality.Onlythe most important formulas are mentionedhere(answersforT=10ºC):

[ ]3 3 7

12

H O HCOK 3.44 10

CO

+ −−

⋅ = = ⋅

23 3 11

23

H O COK 3.25 10

HCO

+ −−

−

⋅ = = ⋅

2 2 9s 3K Ca CO 4.4 10+ − − = ⋅ = ⋅

5s 1a

2

K KK 4.6 10

K−⋅

= = ⋅

( )s

3 2 s

SI pH pH

2 log HCO pK pK log 2−

= −

= − ⋅ + + +

( )23 2 2 3 aCaCO CO H O Ca 2 HCO K+ −+ + ↔ + ⋅

In groundwater abstracted in South Limburg, ahigh concentration of calcium ions is present.GiventhatthereisnoCO3

2-inthewater(pHis6),calciumdoesnotprecipitatebutremains indis-solvedforminthewater,resultinginwaterwithahighdegreeofhardness.

Bysoftening,thepHofthewaterisincreasedasaresultofdosingabase.Whencausticsodaisused,thefollowingreactionswilloccur:

NaOHOH- + HCO3

-

CO32- + Ca

NaOH + Ca2+ + HCO3

-

Na+ + OH-

CO32- + H2O

CaCO3

CaCO3 + H2O + Na+

→

→

→

→

Theabove-mentionedreactionsareirreversible;inreality,equilibriumwillbeset.AlsoforthebasesCa(OH)2andNa2CO3,similarreactionequationscanbeformulated.Bydosingabase,thecarbonicacidequilibriumshiftstotheleft,formingcalciumcarbonate.TheSaturationIndexexceeds1.AtasimilarSI,crys-tallization of calcium carbonate occurs, forminga deposit on the seeding grains present in thereactors.

3.2 KineticsExperimental research shows that the kineticequationforprecipitationofcalciumcarbonatecanbedescribedwiththefollowingequation:

( )2

2 2t 3 s

d Ca- k S Ca CO - K

dt

++ −

= ⋅ ⋅ ⋅

inwhich:kt =reactionconstant (..)S =specificarea (..)(…) =supersaturationordrivingforce

Thereactionconstantktisafunctionoftempera-tureandisgivenbythenextequation:

Kt=0.0255.1.053(T-20)

Thespecificareainfluidizedreactorsisdefinedas:

( )1 pS 6

d−

= ⋅

159


inwhich:p = porosity (-)d = diameterofthepellets (m)

Asmallerdiameterofpellets results ina largerspecificareaand,thus,afastersofteningreaction.Asmallerporosityresultsinahigherspecificareaandafasterreaction.Supersaturationisthechemicaldrivingforceforthecrystallizationreaction.Thehigherthisdrivingforce,thefasterthereactionproceeds.

3.3 MassbalanceIn a pellet reactor calcium carbonate forms adepositontheseedinggrainsaddedtothereac-tors.Adecreaseincalciumconcentrationresultsinanincreaseinpelletdiameter.Thisincreaseisafunctionofthecalciumconcentrationdecrease:

( )d f c∆ = ∆

Thetotalequationbecomes:

( ) [ ] [ ]( )3 3k 2 1 p 1 2

N d d Ca Ca M Q6π⋅ ⋅ − ⋅ ρ = − ⋅ ⋅

inwhich:Ca1 = calciumconcentrationbeforereaction

(mol/m3)Ca2 = calciumconcentrationafterreaction

(mol/m3)M = molecularweightofcalciumcarbonate

(100g/mol)Q = flow (m3/s)Nk = numberofpelletsinthereactorpertime unit (-)d1 = diameterofseedingmaterial (m)d2 = diameterofpelletswhentheyarere- movedfromthereactor (m)ρp = calciumcarbonatedensity(=2840)(kg/m3)

Theseedingmaterialwithasmalldiameterwillbelocatedatthetopofthereactor.Slowly,calciumcarbonatestartstodepositontheseedingmate-rial,andthepelletsgrowandsettle.

Eventually,thepelletsarelocatedatthebottomofthereactorandaredischarged.

3.4 HydraulicsTodesignapelletreactor,onemustunderstandthehydraulicsofafluidizedbed.With the hydraulic formulas, the porosity andheight of the expanded(fluidized) bed can bedetermined.

Thehydraulicsofpelletreactorsarethesameasbackflushingrapidfilters.Water flows inanupwarddirection through thebottomof the reactor and, becauseof thehighvelocity, thebedfluidizesandexpands. Insandfiltrationtheexpansionwillextendamaximumof20%;inthesofteningprocesstheexpansioncanreach200%.

Themaximumresistanceisgivenbytheweightofthegrainsunderwater,or:

( ) p wmax

w

H 1 p Lρ − ρ

= − ⋅ ⋅ρ

inwhich:Hmax =maximumresistance (m)ρw =densitywater (kg/m3)ρp =densitypellets (kg/m3)

The velocity at maximum resistance is calledvmin.At a higher velocity than vmin, the resistanceremainsconstantandthebedexpands.

Theexpansioncanbecalculatedwiththeequa-tion:

e o

o e

L 1 pE

L 1 p−

= =−

inwhich:Le =heightofexpandedbed (m)Lo =heightoffixedbed (m)pe =porosityofexpandedbed (-)po =porosityoffixedbed (-)

160


The porosity of an expanded bed at a certainupwardvelocityiscalculatedusing:

( )3 0.8 1.2

e w0.8 1.83 p we

p v v130g d1 p

ρ= ⋅ ⋅ ⋅

ρ − ρ−

The height of the expanded bed can be calcu-latedwhentheupwardvelocityandtheporosityareknown:

oe o

e

1 pL L

1 p−

= ⋅−

In Figures 5 through 7 the influence of somehydraulic parameters as a function of upwardvelocityisgiven.Aparticlewithadiameterof0.3mmisseedingmaterial;aparticlewithadiameterof1.5mmisthedischargedpellet.

Figure5 implies thatwithan increasingupwardvelocity,porosityinthereactorincreases.Besidesthat,itisobviousthatwithalargerdiam-eter(forexample,causedbydepositsofcalciumcarbonateonseedingmaterialformingpellets)theporositydecreases.

The specific surface area for crystallizationdecreases in the reactor from top to bottom.A

direct consequence of the increase in porosityat a higher upward velocity is the greater bedexpansion.

Thespecificareadecreasesatahigherupwardvelocityduetohigherporosity.For particles with a diameter of 0.3 mm, theincreaseintheexpansionathigherupwardveloci-tiesisrelativelylarge.Thesesmallparticlescanbeflushedout.

0.35

0.45

0.55

0.65

0.75

0.85

0.95

50 60 70 80 90 100 110 120

Poro

sity

(-)

velocity (m/h)

d = 0.3 mmd = 0.6 mmd = 1.0 mmd = 1.5 mm

Figure 5 - Porosity as a function of pellet diameter and upward velocity

Figure 7 - Specific area as a function of pellet diameter and upward velocity

1500

2000

2500

3000

3500

4000

50 60 70 80 90 100 110 120

spec

ific

surfa

ce a

rea

S (m

2 /m3 )

velocity (m/h)

d = 0.3 mmd = 0.6 mmd = 1.0 mmd = 1.5 mm

0.8

1.6

2.4

3.2

4

50 60 70 80 90 100 110 120

bed

expa

nsio

n E

(-)

velocity (m/h)d = 0.3 mmd = 0.6 mmd = 1.0 mmd = 1.5 mm

Figure 6 - Bed expansion as a function of pellet dia-meter and upward velocity

161


3.5 InfluenceofparametersWiththebasicequationsforsofteningreactionsandthehydraulicequationsforanexpandedbed,itispossibletodesignapelletreactorandshowthevaluesofsomecharacteristicparametersasafunctionofheight.Because the solutionof theequation is difficult(defining the reactor in terms of a number ofdiscrete intervals, dx, and solving the equationperdx),computerprogramsareusedtodesigninstallations.

Table 3 shows some input data of a referencecalculation,Figure8showstheresultsofacalcu-lationgraphically.Themostimportantparameterforthedesignofapelletreactoristheheightoftheexpandedbed.Thisdetermines theheightof thepellet reactorandthebuildingwherethepelletreactorwillbeplaced.UsingtheinputdatafromTable3itfollowsthat,aftercalculating,theheightoftheexpandedbedis5.43m.

Table4showstheconsequenceofvaryinginputparameters.Thetablestill includesanunknownparameter,dCa.Thecomputerprogramcalculatesa theoretical basedosage to reach theeffluentconcentrationCa2.Thisvalueisonlyreachedwhenthereactorisinfinitelyhigh,whentheequilibriumiscompletelyreached.However,aninfinitelyhighreactorisneitherpracticalnorfeasible;therefore,a

supersaturationofcalciumcarbonateisaccepted,andleadstoalowerreactor.Toreachtheefflu-entconcentrationCa2,ahigherdosingofabaseshouldtakeplace.InpracticeavalueofdCato0.10mmol/lisacceptable.

Notonlytheexpandedbedheightchangeswithchangesinoneoftheinputdata,butotherpara-metersalsochange.Table 5 gives an overviewof the influences ofvaryingoneinputdatapoint.

Figure9givessomeparametersasafunctionofthelevelinthereactor.

L

pd

d2

d1

v

Ca1

Ca2

Ca1

Ca2

Figure 8 - Resistance as a function of upward velocity and grain diameter

Rawwater

composition

Ca1

TAC

HCO3-

T

(mmol/l)

(mmol/l)

(mmol/l)

(oC)

3.5

5.0

4.25

10

Softenedwater

composition

Ca2

dCa

(mmol/l)

(mmol/l)

1.5

0.06

Pelletreactor

characteristics

v

d1

ρ1

d2

ρp

(m/h)

(mm)

(kg/m3)

(mm)

(kg/m3)

80

0.3

2650

1.0

2840

Table 3 - Softening with caustic soda in pellet reac-tor Table 4 - Consequences of variation in input data on

expanded bed height, dosing caustic soda

Referenceconditions

Variableconditions

Expan-dedbedheightLe

(m)

T=100C T=50C 6.73

v=80m/h v=120m/h 10.9

d2=1.0mm d2=0.75mm 5.39

d1=0.3mm d1=0.2mm 5.58

ρo=2650kg/m3 ρp=4200kg/m3 4.55

dCa=0.06mmol/l dCa=0.10mmol/l 2.78

Ca2=1.50mmol/l Ca2=1.0mmol/l 3.12

162


Ca: drivingforcecalciumconcentration=softening(supersaturationlargestatthebottomofthe reactor(seeSI):thusmostremovalatthebottomofreactor);d : asaresultofthegrowingofpellets,stratificationwilltakeplace,becausethepelletswithlarge diameterswillsettletothebottom;p : porosityincreasesbecausethediameterdecreasesinheight(oppositeofd);S : specificareahasamaximumatacertainheight.BelowthisheightSdecreasesbecausethen thepelletdiameterisdecisive.AbovethisheightSdecreasesbecauseporosityismoredeci- sive;pH: highestatthebottomofthereactorduetochemicaldosageatthe bottom;SI : seeCa(asaresultofchemicaldosagethesupersaturationincreases).

Ca (mmol/l)

0

6

0 5

1

2

3

4

5

1 2 3 4

L e (m

)

0

6

0 1.50

1

2

3

4

5

0.50 1.00d (mm)

L e (m

)

p (-)

0

6

0.5 1.0

1

2

3

4

5

0.6 0.7 0.8 0.9

L e (m

)

0

6

1500

L e (m

)

2000

1

2

3

4

5

1800 2100 2400 2700S (m2/m3)

0

6

0 3.00SI

1

2

3

4

5

0.50 1.00 1.50 2.00 2.50

L e (m

)

0

6

7 11

1

2

3

4

5

8 9 10pH

L e (m

)

Figure 9 - Some characteristic softening parameters as a function of height

163


4 Practice

4.1 SplittreatmentWhenonlyapartofthewaterflowissoftened,itiscalledsplittreatment.Figure 10 shows the principle. One part of thewater passes through the softening installation(andobtainsalowerhardness)andonepartdoesnotpass thesoftening installation (andhas thesamehardnessastherawwater).Afterwards,thetwoflowswillmix,resultinginanoverallhardnessof1.5mmol/l.

Splittreatmenthasanumberofadvantages.Oneisthattheconsumptionofchemicalsislower.Inraw water an amount of carbon dioxide existswhichneedstobeconvertedintocarbonate.

Inthecaseofsplittreatment,onlycarbondioxideneedstobeconvertedinonepartoftheflow;intheby-pass,nodosageofchemicalstakesplace.

When softened water is mixed with raw waterwhich bypassed the softening installation, thewater has a lower supersaturation after mixing(principleofTillmanscurve).Anotheradvantageofsplit treatment is that theinvestment costs will be lower, because fewerreactorsneedtobebuilt.Thechoiceforsplittreatmentissignificantforlargedesigncapacities(i.e.,largersavings).

Split treatmentwillonlybeusedwhen thecon-centrationofmagnesiumintherawwaterisnottoohigh.Themaximumsofteningdepthisdowntoacalciumconcentrationof0.5mmol/l.Whenthemagnesiumconcentrationishigh(about1mmol/l),thebypasspercentageapproaches0%toreachatotalhard-nessof1.5mmol/l.

4.2. ChoiceofchemicalsSelectionof theproper chemical (caustic soda,lime or soda) is determined by the raw watercompositionanddesiredqualityafter softening.Incaseseveralchemicalsareapplicable,aspectsof operational management will also becomeimportant.Previousstandardsweregiventhatmustbecom-pliedwith.These standards determine, to a considerableextent,whatbasecanbeusedforsoftening.Table6showsthechangeinwaterqualityforthemostimportantparameterswhenbasesaredosedtowater(onthebasisofirreversiblereactions).

The water quality after softening can be easilydeterminedandconclusionscanbedrawnastowhetherthewatermeetsthestandards.Inreal-ity, equilibrium reactions occur but change theconcentrationsonly slightly.However, forafirstestimate,thevaluesinTable6giveanindicationofthechanges.

THsplit treatment

THend

THraw water

(1-R) QQ

R Q

Figure 10 - Principle of split treatment

Table 5 - Influence of changes in input data on para-meters

increaseof influence on

Le dos Nk S G SImax

T << < - > >> <

v >> - - << >> -

d2 > - << << >> -

d1 <> - >> > > -

p1 < - >> > > -

Ca1 << > - - - >

Ca2 <> << << - - <<

Ca(OH)2 >> - - - - <<

164


inbicarbonateconcentration in relation tohard-nessreduction.With the application of caustic soda, for everymmol/lcalciumreductionthehydrogencarbonateconcentrationdecreaseswith1mmol/l(line1:1).With lime per mmol/l hardness reduction, thehydrogencarbonateconcentrationdecreaseswith2mmol/l(line1:2).

Using the previously mentioned computer pro-gram,theexactwaterqualityaftersofteningwithcausticsodaorlimecanbecalculated.Figure12showstheprogressforboththebasesofsomecharacteristicsofteningparametersasafunctionofheightinthereactor.Distinctdifferencescanbenoticed.Incasedifferentchemicalscanbeused,achoicewillneedtobemadethattakesotherqualityparameters(suchasCusolubility,TAC)andopera-tionalmanagementaspectsintoaccount.

CausticsodaCausticsodaisprovidedasa50%solution(=50%causticsodaand50%water)byatankertruck.Since a 50% caustic soda solution crystallizesata temperature lower than12°C,causticsodaisdilutedtoa25%solutionusingdemineralizedwater.This25%solutiononlycrystallizesattem-peratureslowerthan-18°C.

Thecausticsodaispumpedoutofthetankerinastoragetanktruck.

Itshouldbenotedthatwiththeabove-mentionedreactions,the‘consumption’ofbicarbonate(HCO3

-)differs.With the application of caustic soda (NaOH), 1mmol/lHCO3

-isusedfortheremovalof1mmol/lcalcium.Withlime(Ca(OH)2)thatis2mmol/landadosingofsodaash(Na2CO3),noHCO3

-isused.TheextenttowhichHCO3

-isstillpresentinwater,,afterremovalofthedesiredconcentrationofcal-cium, is of importance regarding the buffering is of importance regarding the bufferingcapacityofwaterandseveralotherwaterqualityparameters (copper resolution, corrosion index,etc.).

Inaddition,itshouldbementionedthatwiththeapplicationof lime,twicetheamountofcalciumcarbonateisformedthanwiththeapplicationofcausticsodaorsodaash(thusmorepellets).Whenthesodiumandcalciumconcentrationsofrawwaterarehigh,itwillnotbepossibletosoftenthiswaterwithcausticsoda,becausethesodiumstandardof120mg/lwillbeexceeded.Whenrawwaterhasalowbicarbonateconcentration,soften-ingwithlimewillalsonotbepossible.

IntheNetherlandsseveralsofteninginstallationshavebeenbuiltinwhichdifferentbasesareusedforsoftening.Figure11showswhatbaseisusedinDutchprac-tice.Thefigureuseslinestoindicatethedecrease

hardness reduction [Ca2+] (mmol/l)

0

6

0

HC

O3- r

aw w

ater

(mm

ol/l)

5

1

2

3

4

5

1 2 3 4

NaOHNa2CO3

Ca(OH)2NaOHNa2CO3

Na2CO3

groundwater NaOHsurface water NaOH

groundwater Ca(OH)2surface water Na2CO3

Figure 11 - Application of base as a softening chemi-cal in Dutch practice

NaOH Ca(OH)2 Na2CO3

neutralization

CO2 -1 -2 -1

HCO3- 1 2 2

Ca2+ 0 1 0

Na+ 1 0 2

softening

CO2 0 0 0

HCO3- -1 -2 0

Ca2+ -1 -1 -1

Na+ 1 0 2

Table 6 - Change in water composition (mmol/l) per mmol/l dosage of chemicals

165


Ca (mmol/l)

0

6

0

L (m

)

5

1

2

3

4

5

1 2 3 40

6

0

L (m

)

1.50

1

2

3

4

5

0.50 1.00d (mm)

p (-)

0

6

0.5

L (m

)

1.0

1

2

3

4

5

0.6 0.7 0.8 0.90

6

1500

L (m

)

3000

1

2

3

4

5

1800 2100 2400 2700S (m2/m3)

0

6

7

L (m

)

11

1

2

3

4

5

8 9 10pH (-)

0

6

0

L (m

)

3.00SI

1

2

3

4

5

0.50 1.00 1.50 2.00 2.50

Ca: decreaseincalciumconcentrationisslowerwiththeapplicationoflimethanwithcaustic soda.d : softeningwithlimeiswell-distributedoverthereactorheight.p : porosityisdirectlydependentonthediameteroftheparticles.S : specificareahasamaximumatacertainheight.Withlimethemaximumareaisatahigher point.pH: thepHofwaterincreasesinitiallywithdosinglimeasaresultofdissolvingtheCa(OH)2

particles(limedissolvesfasterthansofteningtakesplace).SI: seeCa(asaresultofchemicaldosage,thesupersaturationincreases)

Figure 12 - Softening with caustic soda (red) or lime (blue)

NaOH

Ca(OH)2

166


Often,morestoragetanksarepresent.Safetyregulationsprescribethatthestoragetanksbeplacedapartfromthesofteninginstallationinareservoirwithavolumeofatleastonestoragetankplus10%ofthetotalstoragecapacity.When caustic soda is diluted with partially sof-tened water, calcium carbonate will be formedandneedstoberemovedfromthestoragetanks.Calciumcarbonatewillnotprecipitateinthestor-agetankswhendemineralizedwaterisusedfordissolution.Thedemineralizedwaterispreparedwithionexchangers.

Fromthestoragetankscausticsodaispumpedintoadosinginstallation.Thisdosinginstallationcanbeonesingledosingpumpperpelletreac-tororacausticsoda ringpipe.Causticsoda isinjectedusinganozzleatthebottomofthepelletreactor.

Anozzleisaspeciallydesigneddosingelementfortheequaldistributionofthechemical.Figure13showstheAmsterdamnozzle,Figure14showstheworkingmechanism.

BecausewaterandcausticsodaflowthroughtheAmsterdamnozzle,afalsebottomdesignisuti-lizedinthereactor.Underthisbottom,thewaterispresent.Betweenthebottomplatescausticsodaispresent,andabovethebottomplatestheactualreactorstarts.Watertobesoftenedflowsthroughthenozzleintothereactor.Atthesametime,butthroughanotherchannel, a concentration of caustic soda flowsthroughthesamenozzleintothereactor.Whentheoutflowvelocityofthewaterissufficient(1-9m/h), agoodmixingof the chemical andwatertakesplace.

Figure 13 - The Amsterdam nozzle

lye chamberin false floor

influent

lye

waterv=1.9 m/s

detail injector (35 per m2)

Figure 14 - Schematic representation of an Amsterdam nozzle

Figure 15 - Dosing system with separate caustic soda and water dosing nozzles in the bottom

167


InadditiontotheAmsterdamnozzle,manymoredosing systems exist.There are systems withseparatedosingpointsinthebottomofthereac-tor, with an inflow ofwater by separatewaternozzles.

An equal distribution of chemical (calcium orcausticsoda)andwateroverthebottomshouldbetakenintoaccountinthedesign.Especiallyfordosingcausticsoda,sufficientdosingpointsneedtoberealized.Foreverym2ofreactorbottom,30-40nozzlesneedtobepresent.Dosingononepointinthereactor,likeatangentialinlet,isexclusivelypossiblewithlime,becausethesofteningreactionismuchslower.

LimeDosageoflimeis,asfarasoperationalmanage-mentisconcerned(necessaryinstallation+main-tenanceactivities),morecomplicatedthandosingcausticsoda.Lime is a suspension that is less soluble thancausticsoda(solubility1.7g/l)anditneedstobeproducedonlocation.There are several options for the production oflime:- quick lime (installation: dosing + hydration

installation,dosing+solutioninstallation,dos-inginstallation)

- hydratedlime(installation:dosing+solutioninstallation,dosinginstallation)

- stablelimewater(installation:dosinginstalla-tion).

Theflowof limewatercanbeupto10%ofthetotalflowthroughthereactor.Limewaterinpowderedformissuppliedbytankertrucks and stored in silos. From the lime silos,powderedlimeistransportedtoaproductiontank(underneaththesilo).Intheproductiontank,limewater is produced in thedesired concentration.Limewaterdosagecantakeplacewithonedosingpumpperreactororbyusingaringpipe.InFigure17alimewaterdosingnozzleisshown,andinFigure18thesupplypipesofthelimedos-ingnozzlesarerepresented.Equalresistanceineverypipeisimportant.Ifthatisnotthecaseanunequaldistributionofchemicals

Figure 16 - Dosing system with separate caustic soda and water dosing nozzles in the bottom

Figure 17 - Lime water dosing nozzle

Figure 18 - Supply pipes of the lime dosing nozzles

168


takesplaceatthebottomofthepelletreactorandthesofteningprocesswillbeaffected.Another aspect of lime as a chemical is thatwater, after leaving the softening reactor, hasan increased suspended matter concentration.Thedepositcontentiscalledcarry-overandisaresultof:- contaminationofcalciumhydroxide- CaCO3fromcalciumhydroxidepreparation- undissolvedcalciumhydroxideparticles- homogeneousnucleation(spontaneouspre-

cipitationnotonseedingmaterial)andpelleterosion.

4.3 Construction alternatives for reac-tors

Differenttypesofreactorscanbeusedforsoften-ing.Therearecylindricalreactorsandreactorswithvaryingdiametersoverheight.IntheNetherlands,mainlycylindricalreactorsareused.TwoDutchversionsarebrieflydiscussed:- cylindricalreactorwithflatbottom(Amsterdam

reactor)- cylindricalreactorwithconicalbottompartand

tangentialinlet.

DuetothecylindricalformoftheAmsterdamreac-tor, homogeneous fluidizationoccurs;mixing inhorizontaldirectionshardlyoccurs.

Pelletreactorsaredischargedseveraltimesaday.To remove limestone grains several dischargepointsareinstalledinthebottom.Thedischargeofgrainscannottakeplaceinonecentralplace,otherwiseaconeshapeoccursinthereactor.Duringthedischargethedosingofcausticsodaisstoppedtopreventlossofcausticsodainthereactor.

Seedingmaterialisbroughtinat about 1 m abovetabout1mabovethebottomofthepelletreactor.

In the cylindrical reactor with a tangential inlet,water isbrought inat thebottomof the reactor(mixingcompartment)usingabaffletodirectthewaterflow.Inthemixingcompartment,thechemi-calisdosedandmixingtakesplace.Limestonegrainsaredischargedthroughapointinthemixingcompartment.Atabout1mabovethemixingcompartment,seedingmaterialisbroughtintothepelletreactor.

Softenedwaterleavesthepelletreactorthroughanoverflowweir.Thefunctionoftheoverflowweiristoprovideauniform abstraction of softened water from thereactorbyavoidingpreferentialflowpaths.Forauniformabstraction,anotchedweircanbeused(Figure22).To prevent seedingmaterial from flushing out,thepelletreactorcanhaveawidenedupperpart.In thiswidenedupperpart, theupwardvelocity

Figure 19 - Different types of reactors

Figure 20 - Tangential flow of raw water

Figure 21 - Several pipes at the bottom of a pellet reac-tor

169


willdecreaseandtheseedingmaterialwillsettleback.

Several pipes are present at the bottom of thereactor:- asupplypipeforraw,notsoftened,water- asupplypipefordosingthechemical- asupplypipeforseedingmaterial- adrainpipeforpellets.

All thesepipesneed tobewellarranged in thetreatment building.Typically, pipes of differentcolorsarechosentopreventmistakes.

Theupperpartofthesofteninginstallationneedstobeconstructedinsuchamannerthat,withapossible unequal inflowofwater, thewaterwillleavethesofteningreactorequally.Therefore,anotchedweirconstructioncanbeusedasshowninFigure22.

4.4 SeedingmaterialSeedingsandstoragetakesplaceinasilo.Seedingsandisdosedfromthesilo(byavibrat-inggully,forexample)intoaseedingsandwasher,wheresmallparticlescanbewashedout.Forthebenefitofdisinfection,itisalsopossibleto dose caustic soda into the washed seedingsand.

Theseedingmaterialisusuallydisinfectedtobesure no bacteriological contamination of waterwill occur.This disinfection of seeding materialtakesplacewithcausticsodaorchlorinebleach-inglye.

Topreventseedingmaterialfromwashingoutofthereactorandaffectingthenexttreatmentproc-esseswithafinefractionofseedingmaterial,theseedingmaterial iswashedbefore itenters thereactor.Here,seedingmaterialisbroughtintoasilowithawashingvelocityhigherthanthevelocityinthepelletreactor.Inthisway,thefinestfractionofseedingmaterialisremoved.

Figure 22 - Notched water weirFigure 23 - Seeding sand storage silo with sand washer

underneath

170


Theuseofseedingsandisminimal.The storage capacity is usually sufficient for anumberofmonths.

4.5 PelletstoragePelletstoragecanbemanagedinsilosandcon-tainers.The size of the pellet storage is dependent onthepelletproductionandfrequencyofpelletcol-lection.With the application of lime, twice the amountofpelletsareproduced thanareproducedwithcausticsoda.Thevolumeofapelletsiloisusuallyequaltotheamountofpelletsproducedinoneweek.

The pellet silos are equipped with a drainagesystemtodrainwaterthatcomeswiththepelletdischarge.

ThepelletsshowninFigure24consistof99.5%calcium carbonate.These pellets are brown incolor.The reason for this brown coloring is thepresenceofonly0.5%ironinthepellets.

Figure 24 - Pellet storage silo

Furtherreading

• Unitprocessesindrinkingwatertreatment,W.Masschelein(1992),(ISBN0824786785)(635pgs)

• Het kalkkoolzuurevenwicht opnieuw bezienDHV(1983),(118pgs)

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softening - tu delft opencourseware · 2.2 water quality and softening 2.3 softening processes ......

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