softening - tu delft opencourseware · 2.2 water quality and softening 2.3 softening processes ......
TRANSCRIPT
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
water treatmentsoftening
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
softeningwater treatment
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
water treatmentsoftening
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
softeningwater treatment
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
water treatmentsoftening
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
softeningwater treatment
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
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water treatmentsoftening
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
softeningwater treatment
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
water treatmentsoftening
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
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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
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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
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softeningwater treatment
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
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water treatmentsoftening
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
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softeningwater treatment
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
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water treatmentsoftening
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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water treatmentsoftening