adsorption

Adsorption

WATER TREATM

ENT

WATERTREATMENT

pump

pump atra

zin

e (m

g/l

)

2

1.5

1

0.5

00 10,000 20,000 30,000

bed volumes (m3/m3)influenteffluent

macro pore

micro pores

pesticides

macro pores > 25 nm meso pores 1 - 25 nm micro pores < 1 nm

meso pore

FrameworkThismoduleexplainsadsorption.

ContentsThismodulehasthefollowingcontents:

1. Introduction2. Theory 2.1 Equilibrium 2.2 Kinetics 2.3 Massbalance 2.4 Solutionsforthebasicequations3. Practice 3.1 Pseudo-moving-bedfiltration 3.2 Pressurefiltration 3.3 Powderedactivatedcarbon

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water treatmentadsorption

1 Introduction

Watercontainsdissolvedorganicmatterthatcan-notberemovedwithflocformation,flocremovalorsandfiltration.Thesedissolvedorganiccom-poundsare:- odor-,taste-andcolor-producingcompounds- organicmicropollutants(pesticides,hydrocar-

boncompounds)Activated carbon adsorbs (part of) the organicmatterandismainlyusedtotreatdrinkingwaterproducedfromsurfacewater.

Inthepast,drin�ing water produced from surfacerinkingwaterproducedfromsurfacewaterwouldpassthroughthefollowingsteps:flocformation,flocremoval(sedimentationandfiltra-tion)anddisinfectionwithchlorine.Thiswas sufficient to complywith the drin�ingwaterguidelinesforturbidity,odor,tasteandhygi-enicreliability.

In 1987Amsterdam Water Supply discoveredthepresenceofpesticides(Bentazon)indrinkingwater.Duetothisdiscovery,thetraditionaltreat-ment of surface water was no longer satisfac-toryandanextensionwithactivatedcarbonwasrequired.

Inaddition,chlorinecanreactwithorganicmatter(precursor),andtrihalomethanes(THMs)canbeformed.TheseTHMsaretoxic.Toreducetheconcentration,theformedTHMscanberemovedwithactivatedcarbon.Theproblem,however,isthatcarbonisrapidlysaturatedwithTHMsandhastoberegeneratedfrequently.ItispreferabletopreventtheTHMsfrombeingformed.Thiscanbedonebyreducingtheconcentrationoftheprecursorbeforeaddingchlorine.Precursorscannotbemeasureddirectly,buttheconcentrationoforganicmatter,expressedasTOCorDOC,isanindication.ThebestwaytopreventTHMformationistoavoidthedosingofchlorine,but thisrequiresanothertreatmentsetup.

Activatedcarbonisasubstancewithahighcarbonconcentration(e.g.,pit-coal,turf).

Underhightemperaturesthismaterialbecomescarbonated,meaningthatthecarbonpartlytrans-formsintocarbonmonoxideandwater.Thisishowthecarbongetsitsopenstructure(Figure1).Theinternalsurfaceareaoftheactivatedcarbonisseveraltimeslargerthantheexternalsurfacearea.

macro pore

micro pores

pesticides

macro pores > 25 nm meso pores 1 - 25 nm micro pores < 1 nm

meso pore

Figure 1 - The open structure of activated carbon

Theactivatedcarbonfiltersofthedrin�ingwaterproductionplantatKralingenhavetheobjectivetoimprovethetasteofthewater,toreducetheregrowthofbacteriainthepipednetwor�,andtoremovetoxicsubstancesfromthewater.Thecarbonfiltersareplacedaftertheflocfor-mation,sedimentationandrapidsandfiltrationtoavoidrapidclogging.

Theinstallationisbasedonpressurefilters,sotheconstructionheightcanbelimitedandonlyanextrapumpingphase(middlepressure) isneeded.

Theinstallationhasthefollowingcharacteris-tics:Filterbedheight: 4mFiltrationsurfacearea: 28m2

Numberoffilters: 12Contacttime(EBCT): 12minTypeofcarbon: NoritROW0.95Regenerationfrequency:1.5year

105

adsorptionwater treatment

Hence,amajorityoftheadsorbedsubstancesisadsorbedinsidethecarbon.The dissolved organic matter can be removedfromthewaterbyfiltrationthroughabedofacti-vatedcarbon.

Organicmatterdiffusesfromthewaterphasetothesurfaceofthecarbongrains.Theorganiccom-poundsarefurthertransportedintothecarbontobeattachedinthepores.The adsorption of organicmatter is not finite.Thereisequilibriumbetweentheconcentrationofdissolvedcompoundsinwaterandthequantityofsubstancesthatareadsorbedontothecarbon.When different kinds of organic compoundsarepresent in thewater,competitionwilloccur.Compounds that have beenwell-adsorbedwilloccupy adsorption places that cannot be usedby compounds that are less adsorbable. Largeorganicmolecules can also bloc�micro pores,thus preventing the smaller organic moleculesfromenteringthesemicropores.

Aftersometimetheactivatedcarbonissaturatedwith adsorbed organic matter and the carbonneeds tobecleaned.This isdoneby removingthecarbonfromtheinstallationandheatingitto1000oC.Thisregenerationprocesshastobecarriedoutonceeverycoupleofyears.

Activatedcarbonfiltersoperateinthesamewayasrapidsandfilters.Mostlydownwardflow,openfilters areapplied to prevent fine carbongrainsfromwashingout(Figure2).Sincethecontacttimeisthemostimportantparam-eterforgoodremoval,thefilterisoftendesignedwithhighbedstoreducetheconstructionsurface

area.Therefore,activatedcarbonfilterscannotbeoperatedundergravity,ma�inganextrapumpingphasenecessary.Afterpassingthefilterbed,thewaterreachesthebottomconstructionofthefilterandisthencollectedandtransportedtotheclearwatertankorthenexttreatmentstep.

Whenthefilteriscloggedwithsuspendedmatterorbiomassand the resistance ishigh, thefilterbedisbackwashed.Thebackwashwaterisdrainedbytroughsthatareplacedabovethefilters.Clogging of the filter by suspendedmatter canleadtoahigherregenerationfrequency.Therefore,activated carbon filters are usually placedafterflocformation,flocremovalandrapidsandfiltra-tion(Figure3).

A

A cross-section A-A

Figure 2 - Longitudinal and cross-section of an acti-vated carbon filtration installation

clear water reservoir

activated carbon filtration

rapid filtration

floc removal

reservoir

ozonation

floc formation

Cl2/ClO2Chlorine dosage for transport = 0.3 mg/l

activated carbon filtration:- removal of organic matter- removal of pesticides

ozonation:- desinfection- oxidation of organic matterdosage = 2-3,5 mg/l O3

reservoir:- storage- leveling off- auto purification

Fe(III)

Figure 3 - Treatment process at Kralingen (Evides)

Figure 4 - Activated carbon filtration at Andijk (treatment of surface water)

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Afteractivatedcarbonfiltration,disinfectionta�esplace to prevent biological growth in the pipednetwork.Thisprocesscanconsistofslowsandfiltration,UV-disinfection or dosing chlorine orchlorinedioxide.

2 Theory

2.1 EquilibriumSimilartoaeration,equilibriumisestablisheddur-dur-ingadsorption..The maximum loading (loading capacity qmax)dependsontheconcentrationofadsorbablemat-terinthebulkliquid(water).Thehigherthiscon-centration,thehighertheloadingcapacityis.

Therelationshipbetweentheloadingcapacityandtheconcentrationofadsorbablematterinthebulkliquidiscalledtheadsorption-isotherm.ThebestknownistheFreundlichisotherm:

nmax s

xq K cm

= = ⋅

inwhich:qmax = loadingcapacity (g/kg)cs= equilibriumconcentration (g/m3)x = adsorbedamountofcompound (g)m = massofactivatedcarbon (kg)K = Freundlichconstant ((g/kg).(m3/g)n)n = Freundlichconstant (-)

TheFreundlichconstantsKandnareinfluencedbywatertemperature,pH,typeofcarbonandtheconcentrationofotherorganiccompounds.

UsinglaboratoryexperimentstheFreundlichcon-stantscanbedeterminedforasinglesubstancewithaspecifictypeofactivatedcarbon.InFigure5theresultsofalaboratoryexperimentarerepresented.Whenthesegraphsareplottedonalogarithmicscale (Figure6), theFreundlichconstantKcanbedeterminedfromtheintersectionofthegraphwiththey-axis.TheslopeofthelineisequaltotheFreundlichconstantn.

Thehigher theK-value, the better the adsorp-tion.InTable1thevaluesoftheconstantsKandnaregivenforsomeknownsubstances.

From the structure formulaof a substance, theadsorbability can be derived. In general, non-polarsubstancesarebetteradsorbedthanpolarsubstances. Substances with double bonds willbebetteradsorbed thansubstanceswithsinglebonds.

phenolchloro phenol

dichloro phenol trichloro phenol

350

300

250

200

150

100

50

00 10 20

cequilibrium (mg/m3)

q (m

g/kg

)

Figure 5 - Loading capacity as a function of equilibrium concentration

phenolchloro phenol

dichloro phenol trichloro phenol

150

100

50

5 10 30cequilibrium (mg/m3)

00

q (m

g/kg

)

Figure 6 - Logarithm representation of the Freundlich isotherm

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2.2 KineticsThe kinetics equation (equation of motion) foractivatedcarbonfiltrationisasfollows:

2 0 sdc dcu k (c c )dt dy

= − − ⋅ −

inwhich:k2= masstransfercoefficient (d-1)c0= initialconcentrationoforganiccompound

(mg/l)u = porevelocityofthewater (m/s)cs=equilibriumconcentrationoforganic compoundlinkedtoacertainloadingofthe

activatedcarbon (mg/l)

Thekineticsequationconsistsofaconvectiontermwithwhichtransportofthecompoundthroughthefilterbedcanbedescribed::

dcudy

andaremovalterm:

2 0 sk (c c )−

Therateofmasstransferissimilartoaerationpro-portionaltothedifferencebetweentheprevailingconcentrationandtheequilibriumconcentration.The equilibrium concentration depends on theloadingandisdeterminedbytheFreundlichiso-therm.Thelowertheloadingofthecarbon,theloweristheequilibriumconcentrationandthehigheristhemasstransferrate.Themass transfer coefficient depends on thecompoundtobeadsorbedandthetypeofcarbon(includingthegrainsize).

Inaddition,themasstransfercoefficientcanbeinfluencedby the velocity of thewater passingthecarbongrains.Thehigherthevelocityofthewater, the better themass transfer is betweenliquidandcarbon.

2.3 MassbalanceInFigure7theactivatedcarbonfilterisschema-tizedasacube,inwhich:Q= flow (m3/h)B = widthoffilter (m)L = lengthoffilter (m)

Compound K.((g/kg).(m3/g)n)

n(-)

alkanesCH3Cl 6.2 0.80

CH2Cl2 12.7 12.7

CH2Br 44.4 0.81

CHCl3(chloroform) 95.5 0.67

CHBr3(bromoform) 929 0.66

CH2Cl-CH2Cl(DCEA) 129 0.53

CH2Br-CH2Br(EDB) 888 0.47

CH2Cl-CHCl-CH3(1,2DCP) 313 0.59

CH2Br-CHBr-CH2Cl(DBCP) 6910 0.60

alkenesCCl2=CHCl(TCE) 2000 0.48

CCl2=CCl2(PCE) 4050 0.52

pesticides-organochloridesDieldrin 17884 0.51

Lindane(HCH) 15000 0.43

Heptachlor 16196 0.92

Alachlor 81700 0.26

pesticides-organitrogenesatrazine 38700 0.29

simazine 31300 0.23

pesticides-fenolderivatesdinoseb 30400 0.28

PCP 42600 0.34

pesticides-fenoxycarbonicacid2,4D 10442 0.27

2,4,5TP 15392 0.38

aromatesC6H6(benzene) 1260 0.53

C6H5Cl 9170 0.35

CH5CH3(toluene) 5010 0.43

C6H5NO2(nitrobenzene) 3488 0.43

C6H5COOH 2802 0.42

C6H5OH(phenol) 503 0.54

C10H8(naftalene) 7260 0.42

Table 1 - Freundlich constants K and n of several substances

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dy= heightoffilter (m)c = concentrationoforganiccompound(g/m3)

An organic compound with a concentration c0entersthesystemwithaflowQand leavesthesystemwithaconcentrationc1.Thedifferenceinconcentrationbetweenin-andoutflowisadsorbedontheactivatedcarbonandincreasestheloadingofthecarbon.

Thecontinuityequationormassbalanceis:

dq v dcdt dy

= −ρ

inwhich:v =filtrationrate=Q/BL (m/h)q =loading (g/g)ρ =densityofthecarbon (g/m3)

2.4 SolutionsforthebasicequationsThesystemofequationsisnon-linearandcannotbesolvedanalytically.

When a stationary situation is assumed, andwhentheinfluentconcentrationandtheflowareassumedtobeconstant,theeffluentconcentra-tionoftheactivatedcarbonfiltercanbecalculated

usingtheBohart-Adamsequation.Thisequationisderivedfromthemassbalanceandthekinet-icsequation.

0 02

e

c BV c1 exp � EBCT 1

c q ⋅

= + ⋅ ⋅ − ⋅ ρ

VEBCTQ

=

Q T TBVV EBCT⋅

= =

inwhich:EBCT= emptybedcontacttime(h)BV= filteredwaterperbedvolume(m3/m3)T= filterruntime(h)V= volumefilter(m3)

Figure8showstheprogressoftheorganiccom-poundconcentration in theactivated carbonfil-tration.Waterwithaconcentrationoforganiccompoundc0issupplied.Since,inthebeginning,thecarbonisnotyetloaded,theeffluentconcentrationoftheorganiccompounddropstozero.After some time, the loading of the carbonincreases, the available adsorption places arefilled,andbrea�throughoftheorganiccompoundintheeffluentoccurs.

BV=0

∆ t

2∆ t

3∆ t

H

co

Figure 8 - Progress of the concentration in time and height

Figure 7 - Schematic representation of activated car-bon filter

109


Finally,theactivatedcarbonissaturatedwithoutanyremovaloforganicmatter.

InFigure9 theeffluent concentration is plottedagainst thefilterruntime(expressedinnumberofbedvolumes).Thiscurveiscalledthebreak-throughcurve.Inthebeginningtheeffluentconcentrationis0.After time the effluent concentration increasesuntil the activated carbon is saturated and theeffluentconcentrationisequaltotheinfluentcon-centration.Whentheeffluentconcentrationoftheactivatedcarbonfilternolongermeetsthestandards,thefiltermustberegenerated.

Theruntimeofactivatedcarbonfiltersdependsontheobjective.Thebrea�throughofTHMsoccursrelativelyfast(15,000BV);fortheremovaloftastesubstances,however,longerruntimescanbeapplied(50,000BV)without intermediate regeneration (Figure9).

ContacttimeThecorrelationbetweencontacttimeandfilterruntimedependsontheadsorptioncharacteristicsofthecompoundtoberemoved.Ingeneralthefilterruntimeincreasesexponen-tially with increasing contact time (Figure 10).Hence, per cubicmeter of activated carbon, a

larger volume of water can be treated beforeregenerationisnecessary.Applicationofashortercontacttimeindicatesthatasmallervolumeofactivatedcarbonisneeded.Thisleadstolowerinvestmentcosts.The regeneration costs, on theother hand,willincrease.Aneconomicaloptimumdependsontheadsorp-tionbehaviorofthecompoundtoberemoved.

3 Practice

Tosolve theabove-givenbasicequation,asta-tionarysituationisassumedinwhichtheinfluentconcentrationisconsideredtobeconstant.Inreality,theinfluentconcentrationisnotconstantbutvaries.Hence,thebrea�throughcurvewillnotdescribeaperfect“S”-form(Figure11).Iftheinfluentconcentrationishigh,relativelylargeamountsoforganicmatterareadsorbedbecause

300

filte

r run

tim

e (d

ays)

contact time (min)

250

200

150

100

50

00 10 20 30 40

Figure 10 - Relationship between contact time and filter run time

THM Bentazon taste

0 20,000 40,000 60,000

0.25

0.5

0.75

1

0

bed volumes (m3/m3)

c e/c

0 (-

)

Figure 9 - Breakthrough curve

atra

zine

(mg

/l)

2

1.5

1

0.5

00 10,000 20,000 30,000


Figure 11 - Breakthrough curve measured in practice

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ofalargedrivingforce.When,afterwards,alowinfluentconcentrationoccursandtheloadingofthecarbonisalreadyhigh,thesaturationconcentra-tioncanbealmostequaltotheinfluentconcentra-tionandadsorptionislimited.

Inextremecasesevendesorption(higherconcen-trationsoforganiccompoundsineffluentthanininfluent)canoccur.Thishappens,forexample,afterterminatingtrans-portchlorination.Beforetermination,THMswereremovedinthefiltersandaccumulatedintheacti-vatedcarbon.Aftertermination,THMsnolongerformandtheinfluentconcentrationisnil(Figure12).Intheeffluentoftheactivatedcarbonfilters,THMsarestillpresentbecauseofdesorption.

3.1 Pseudo-moving-bedfiltrationAspecialapplicationforactivatedcarbonfiltrationis the“pseudo-moving-bed”system(Figure13),wheretwofiltersareplacedinaseries.

Afterbrea�throughoccursinthefirstfilter,thesec-ondfilterta�escareofthepolishing,andtheefflu-entofthesecondfilterstillmeetstheguidelines.Themomentthesecondfilterbrea�sthrough,thefirstfilterisregeneratedandthenconnectedafterthesecondfilter.The cleanest filter is thus always the last one(Figure14).

Theadvantageof thissetup is that thestoragecapacityof thefilters isbetterused,resulting inlongerruntimes.Disadvantagesofthissetuparethehighhydraulicloadingsofthefiltersandthecomplexsystemofpipesandvalves.

3.2 Pressurefiltration

chlo

rofo

rm C

HC

l 3 (m

g/l)

00 20,000 30,000


10,000

10

20

30

40

50transport chlorination stopped

Figure 12 - Occurrence of desorption after termination of transport chlorination

t = 0 t = ∆Tfilter 1 is regenerated

t = 2∆T t = 3∆Tfilter 2 is regenerated

Figure 14 - Breakthrough curves for pseudo-moving-bed activated carbon filtration

Figure 15 - Steel pressure filters with activated carbon

pump

pump

Figure 13 - Principle of pseudo-moving-bed filtration

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Whenactivatedcarbonfiltersareplacedinpres-sure vessels, an extra pumping phase can beavoided(Figure15).Thepressureisthensufficientto lead the water through the activated carbonfilterstotheclearwatertan�s.

3.3 Biologicalactivatedcarbonfiltra-tion

Biological activity occurs in all carbon filters.Bacteriathatgrowonthecarbonwilldecomposeorganicmatter.Evenwith pre-chlorination bacteria can grow,becausetheresidualchlorineisadsorbedbytheactivatedcarbon.

Whenorganicmatterispre-oxidizedbyozone(astrongoxidizer),biologicalactivity isstimulated,resultinginincreasedfilterruntimesandincreasedremovaloforganicmatter.Thisiscalled“biologi-callyactivatedcarbonfiltration”(Figure16).Because of the increased biological activity onthe carbon, theorganicmacropollutants (DOC)will occupy fewer adsorption places in the car-bon.Hence,morespaceisleftforthe(persistent)organicmicropollutants.Somepersistentorganicmicropollutantslikepesticidescanevenbe(par-tially)biologicallydecomposedafterozonation.In someseasons (e.g., summer), thebiologicalactivitywillbegreaterthaninotherseasons(e.g.,winter).

Therefore,theadsorptionprocesswillbethedomi-nantprocessinwinter.Theorganicmatterthatisadsorbed inwinterwillbepartiallydecomposedbiologicallyinsummer.Thisphenomenoniscalledbio-regeneration.

Withbiologicallyactivatedcarbonfiltration,quic�lydegradable matter is formed that will stimulatebacterialgrowth.Breakthroughofthisdegradablematter(expressedasAssimilableOrganicCarbon(AOC))must beavoidedtopreventtheregrowthofbacteriainthepipednetwork.Inaddition,bacteriacanbeerodedfromthecar-bonandenter the effluent, thus increasing thecolonycounts.AdisinfectionstepwithUV-radiationorchlorinewillthenbenecessary.

3.4 PowderedactivatedcarbonInadditiontogranularactivatedcarbonfiltration(GAC),powderedactivatedcarbon(PAC)dosingcanbeapplied.

With powdered activated carbon, small carbonparticles(1µm)areaddedtowater.Thesecarbonparticlesaresosmall thatduringtransporttheydonotsettle.Whenthewaterisincontactwiththecarbonpar-ticles(aftersometime),equilibriumbetweentheorganicmatterinthewaterandonthepowderedcarbonwillbeestablished.Theparticleswillberemovedafterwardsbyasandfiltrationstep.

Forapowderedactivatedcarbondosing,amassbalancecanbesetup,schematicallyrepresentedinFigure17:

0 0 e ec V q m c V q m⋅ + ⋅ = ⋅ + ⋅

0 ee

e

c cmq 0 WV q

−= ⇒ = =

inwhich:m= massofactivatedcarbon (g)

run

time

(yea

rs)

00 100

contact time (minutes)

with ozone

without ozone

guideline

2

1.5

0.5

50

1

Figure 16 - Influence of pre-ozonation on required con-

112


A powdered activated carbon dosage has theactivated carbon dosage has thecarbon dosage has theadvantageovergranularactivatedcarbonfiltra-tionthattherearelimitedinvestmentcosts:thereisnoneedforafiltrationinstallation(withanextrapumpingphase);adosingunitforthepowderedactivated carbon together with a mixing tank iscarbon together with a mixing tank issufficient.

Theremovalofpesticideswithpowderedactivatedactivatedcarbon,however,islimitedandtherapidfiltersarequicklyclogged.Thisresultsinalargebackwashwaterloss.

Granular activated carbon filtration hasamoreintensivewater/carbon contact, thus pesticides(andTHMs)areremovedmoreefficiently.

V = volumefilter (m3)W= dosingofactivatedcarbon (g/m3)

Startingwith theFreundlich isotherm, theefflu-ent concentration of an organic compound canbecalculated,orthepowderedactivatedcarbondosagecanbecalculatedforadeterminedefflu-entconcentration.Itisassumedthatequilibriumoccursandthecar-bonismaximallyloaded.The loading capacity is determined by the pre-vailing concentration in the reactor, and in acompletelymixedsystemthisequalstheeffluentconcentration.

Fortwoidealmixersplacedinaseries,theloadingcapacityofthepowderedactivatedcarboninthesecondtan�islowerthaninthefirsttan�becausetheconcentrationinthesecondtankislowerthaninthefirsttan�(Figure18).

Vc0

q0, mceqe, m

Figure 17 - Mass balance of powderd carbon dosage

c1 c0

c1 c0c2

q1

q2

q1

Ideal mixer, 1 - step

2 - step

c1 - c0 = W · q1

c1 - c0 = -W1·q1 -W·q1

(mass balance)

Figure 18 - One and two mixers in series

Figure 19 - Powdered activated carbon dosing unit at Scheveningen

Furtherreading

• Watertreatment:Principlesanddesign,MWH(2005),(ISBN0471110183)(1948pgs)

• Unitprocessesindrin�ingwatertreatment,W.Masschelein(1992),(ISBN0824786785)

(635pgs)

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