aeration and gas stripping w ater treatment - tu · pdf fileframework. this module explains...

Aeration and gas stripping

WATER TREATM

ENT

WATERTREATMENT

Qw, cw,0

Qa,ca,0

Qa, ca,e

Qw, cw,e

k5k4k3

k2

k1

0.001 0.01 0.1 1 10

1

0.8

0.6

0.4

0.2

0

k1k2k3k4k5

RQ

k2tkDT

K (-

)

= = =

1.610.03910oC

FrameworkThismoduleexplainsaerationandgasstripping.

ContentsThismodulehasthefollowingcontents:

1. Introduction2. Theoryofgastransfer 2.1 Equilibrium 2.2 Kinetics 2.3 Massbalance 2.4 Solutionsforthebasicequations3. Practice 3.1 Cascade 3.2 Toweraerator 3.3 Plateaerator 3.4 Sprayaerator 3.5 Alternativeaerationsystems

aeration and gas transfer water treatment

136

the water falls over a weir into a lower placedtrough.Whenthefallingstreamentersthewaterbody,airisentrappedintheformofbubbles,pro-vidingforamixtureofwaterandairinwhichgastransferwilloccur.

Thetoweraerator(Figure2)consistsofacylin-dricalvesselofsteelorsyntheticmaterialthatisfilled with packing material, usually consisting of elementsofsyntheticmaterial.Waterfallsdownandairisblowninaco-currentorcounter-currentdirection.Aplateaerator (Figure3) is ahorizontal perfo-rated plate. Water flows over the plate and air is blown through the orifices, creating a bubble bed ofairandwaterabovetheplate.Sprayers (Figure 4) are typically used becauseof their simple implementation in existing treat-mentplants.Byspraying,acontactsurfacebe-tween the air and water is created for the gasexchange.2 Theory of gas transfer

1 Introduction Aeration(gasaddition)andgasstripping(gasre-moval) are normally the first treatment steps dur-ing the production of drinking water from ground-water or riverbank water. This artificially induced gastransferaimsattheadditionofoxygen(O2)andtheremovalofcarbondioxide(CO2),meth-ane (CH4), hydrogen sulfide (H2S), and othervolatileorganiccompounds(forexample1.2Di-chloropropane(1.2DCP),Trichloroethene(TRI),Tetrachloroethene (PER) and Trichloromethane(chloroform)).

Gastransferisseldomappliedinthetreatmentofsurfacewater becausesurfacewater hasbeenincontactwithairforaprolongedperiod.Conse-quently, surface water contains sufficient oxygen, and other gases, like methane and hydrogen sul-fide, are absent. Theadditionofoxygenisrequiredfortheoxida-tion of bivalent iron (Fe2+), manganese (Mn2+)and ammonium (NH4

+). These substances arepresentindissolvedformingroundwater.Duetochemical and biological oxidation, the substan-ces can be removed by following a filtration step. Thiswillbediscussedinthechapterongranularfiltration. ReducingthecarbondioxideconcentrationleadstoariseinpHandareductionofaggressivecar-bondioxidethatisabletodisintegrate(concrete)pipes.Methane should be removed because its pres-ence has negative influences on the filtration pro-cesses.Hydrogen sulfide has an annoying odor (rotting eggs)and thereforeneeds tobe removed fromthewater.Volatile organic compounds are usually toxic;someofthemareevencarcinogenic.Obviously,these compounds are not allowed in drinking wa-ter.

To achieve gas transfer a number of systemshavebeendevelopedovertheyears.Oneoftheoldestsystemsisthecascade(Figure1).Thewaterfallsinseveralsteps.Ineachstep,

Figure 1 - Cascade aeration

Figure 2 - Tower aeration

water treatment aeration and gas transfer

137

2.1 Equilibrium

Henry’s lawWatercontainsdissolvedgases.Inaclosedves-selcontainingbothgas(e.g.,air)andwater,theconcentrationofavolatilecomponentinthegas-phasewillbe inequilibriumwith theconcentra-tioninthewaterphase,accordingtoHenry’slaw.TheequilibriumconcentrationcanbecalculatedusingthefollowingformofHenry’slaw:

= ⋅w H gc k c

inwhich:cw=equilibriumconcentrationofagasinwater

[g/m3]kH = Henry’s constant or distribution coefficient

[-]cg=concentrationofthegasinair[g/m3]

The distribution coefficient kH depends on thetypeofgas,andthetemperature.Inaddition,pollutionand impurities in thewaterinfluence the equilibrium concentration. This is-suewillnotbediscussedhere.

Inliterature,manydifferentformsofHenry’slawarefound.Oftenpartialpressureisusedinsteadofthegasconcentrationinair,and/ormolarconcentrationinthewaterinsteadofweightconcentration.Con-sequentlythisresultsinadifferentunitforthedis-tribution coefficient, or Henry’s law constant (ie. [mol/(m3Pa)]or[mol/l/atm]).For gas stripping, often the volatility is given insteadofthesolubilityofagas.Inthiscase,thedistribution coefficient is inverted (gas/water, in steadofwater/gas).

Distribution coefficientInTable1foranumberofgasesalistofvaluesis given of the distribution coefficient at different watertemperatures,(intermediatevaluescanbeobtainedwithlinearinterpolation).Inthetableitisshownthatnitrogen,oxygenandmethane have low kH-values. This means thatthese gases hardly dissolve in water and theycan,therefore,beeasilyremoved.The other gases have high kH–values and dis-solve easily, which makes it difficult to remove themfromthewateroreasytoaddthemtowa-ter.

Gas concentration in airThegasconcentrationintheaircg must be known beforetheequilibrium(orsaturation)concentra-tioncanbecalculated.Thisconcentrationcanbedeterminedusingtheuniversalgaslaw:

p V n R T⋅ = ⋅ ⋅

inwhich:p =partialpressureofgasingasphase[Pa]V =totalgasvolume[m3]n =numberofmolesofagas[mol]R =universalgasconstant=8.3142[J/(K.mol)]T =(air)temperature[K]

Figure 4 - Spray aeration

Figure 3 - Plate aeration


138

Thegasconcentrationcanbecalculatedbymulti-plyingthemolargasconcentrationinair[mol/m3]withthemoleculeweightoftheconsideredgas:

gn pc MW MWv R T

= ⋅ = ⋅⋅

inwhich:MW=molecularweightofagas[g/mol]

Partial pressureThepartial pressureofa certaingas ispropor-tionaltothevolumefractionofthatgasinair:

o fp p V= ⋅

inwhich:po = standardpressureatsealevel(=101,325)

[Pa]Vf = volumefraction[-]

InTable2thevolumefractionsofdifferentgasesthatoccurinairaregiven.Thesevaluesarevalid fordryairwithastand-ardpressureof101,325Pa.Withthesevolumefractionsthepartialpressuresofallgasesinaircanbecalculated.Gasesthatdonotoccurinairhaveapartialpressureequaltozeroandthusacgequaltozeroandalsoacwequaltozero(forexample,methane).

InFigure5theequilibrium(saturation)concentra-tionofoxygenisgivenasafunctionofwatertem-perature.Withanincreaseinwatertemperature,thesaturationconcentrationdecreasesbecauselessoxygencanbedissolvedinwarmwater.

Thesaturationconcentrationcwislinearlydepen-dent on pressure. The saturation concentrationforoxygenat thestandardpressureof101,325Pais11.3g/m3.Ataheightof8,000meters(forexample,MountEverest),theairpressureisonly10,000Pawhichmeansthatthesaturationconcentrationforoxy-genis1.1g/m3.In theseaatadepthof100metersbelowsealevel,thepressureis1,100,000Pa.Thisresultsin a saturation concentration for oxygen of 113g/m3.

2.2 KineticsAs soon as water and air are in contact, gas

GasDistribution coefficient (kH)

T=0oC T=10oC T=20oC Molecularweight(MW)[g/mol]

Nitrogen(N2) 0.023 0.019 0.016 28

Oxygen(O2) 0.049 0.041 0.033 32

Methane(CH4) 0.055 0.043 0.034 16

Carbondioxide(CO2) 1.71 1.23 0.942 44

Hydrogen sulfide (H2S) 4.69 3.65 2.87 34

Tetrachloroethelene(C2HCl4) -1 3.20 1.21 167

Tetrachloroethene(C2HCl3) -1 3.90 2.43 131.5

Chloroform(CHCl3) -1 9.0 7.87 119.5

Ammonia(NH3) 5000 2900 1800 171Thesesubstancesarestillintheliquidphaseatatemperatureof00C and therefore the kH is not known

Table 1 - Distribution coefficient for gases and the molecule weight

GasVolume fraction1

[%]

Saturation concentration2

cw [g/m3]Nitrogen(N2) 78.084 17.9

Oxygen(O2) 20.948 11.3

Argon(Ar) 0.934 -

Carbondioxide(CO2) 0.032 0.79

Othergases 0.02 -1Indryairatastandardpressureof101,325Pa2Waterandairtemperatureof100C

Table 2 - Volume fractions of gases


139

molecules will be exchanged continuously. Thedirectionofthenetgastransportdependsonthegasconcentrationinthewater(cw)andtheequi-libriumconcentrationce.InFigure6thegasconcentrationinthewaterattimet=0issmallerthantheequilibriumconcen-tration. This means that more gas can be dis-solved in thewater than ispresentat time t=0.Anetgastransport fromair towateroccurs,asindicated by the arrow in the figure. The net gas transport continues until time t=infinite and the gas concentration in the water is equal to theequilibrium (or saturation) concentration. Then,thegastransportfromwatertoairandviceversaare equal. Hence, no net gas transport occursandthegasconcentrationinthewaterandairdonotchange.Inthatcase,adynamicequilibriumisestablished.

Thevelocityofgastransferisdeterminedbythekinetic equation:

w2 s w

dck (c c )

dt= ⋅ −

inwhich:cw = concentrationofagasinwater[g/m3]k2 = gas transfer coefficient [s-1]The time-dependent gas concentration changeinwater isrepresentedbythetermdcw/dt.Thechangesinconcentrationaredeterminedbythemagnitude of the gas transfer coefficient k2andthedrivingforce(cs–cw).

The gas transfer coefficient k2isadevice-depen-dent parameter. The larger the contact surfaceareabetweentheairandwaterandtherenewalof this surfacearea, thebetter thegas transferand the higher the gas transfer coefficient. The driving force is defined by the amount of gas thatcanmaximallybedissolved inavolumeofwater, the saturation concentration cs, and theamountofgasthatispresentinavolumeofwa-ter, theconcentrationcw.The larger thedrivingforce,thefasterthegastransfer.TheincreaseintheoxygenconcentrationintimeisshowninFigure7foraconstantcs(10mg/l)andaninitialoxygenconcentrationof0mg/l.Inthebeginning,whenthedifferencebetweenthecsandthecwisthelargest,thegastransferoc-curs at maximum velocity.As time passes, thegas concentration in water increases and thedriving forcedecreases,whichgradually resultsin a lower gas transfer rate. For t=infinite the oxy-genconcentrationinwaterequalsthesaturationconcentrationcs.

Forabatchreactorthedifferentialequationcanbesolvedbyintegration,withcw=cw,0attimet=0,taking into account that cs is constant:

2( k t)w s s w,0c c (c c ) e − ⋅= − − ⋅

water temperature (oC)

c w (

g/m

3 )

15.0

12.5

10.0

7.5

5.0

2.5

00 5 10 15 20 25 30 35

air: 21% oxygenpressure: 101325 Pa

Figure 5 - Saturation concentration of oxygen as a function of the water temperature

cg

air interface water

t=infinite

c1

c0

t=2

t=1

t=0

con

cen

trat

ion

Figure 6 - Gas transport from air to water


140

or:

2( k t)s w

s w,0

c ce

c c− ⋅−

=−

2.3 Mass balance Intheprecedingparagraphitisassumedthattheoxygen concentration in air is constant. This isa simplification that is not always applicable. For situationsinwhichthegasconcentrationchang-esinairareimportant,amassbalanceneedstobeformulated.

InFigure8amassbalanceforagastransfersys-temisschematicallypresented.A water flow (Qw),withagasconcentrationinthewater phase (cw,0), and an airflow (Qa), with agas concentration (ca,0), enter the system.Thesame water flow (Qw),withagasconcentrationin thewaterphase (cw,e), and the same airflow (Qa), with a gas concentration (ca,e), leave thesystem.

Forthegastransfersystem,thelawofcontinuityisvalid: thetotalamountofgasthatentersandleavesthesystemmustbeequalandamassbal-ancecanbesetup:

w w,0 a a,0 w w,e a a,eQ c Q c Q c Q c⋅ + ⋅ = ⋅ + ⋅

Byusing themassbalance, thegasconcentra-tions in the air and water are linked and can be applied in thegas transferequationspresentedbelow.

The RQ is the relationship between the airflow and the water flow. Using the mass balance RQ, that relationship can be defined as follows:

w,e w,0a

w a,0 a,e

c cQRQ

Q c c−

= =−

2.4 Solutions for the basic equations Forgastransfersystemsthreeequationsarede-rived:- equilibriumequation- kinetic equation - massbalance

With these equations it is possible to calculatethechanges in thegasconcentrations inwaterandair.

Combining the equilibrium equation and themassbalanceresults in twoequationswith twounknown variables, cwandca.Withdifferentini-tialconditions,differentsolutionsfortheseequa-tionscanbeobtained.In the following section a number of equationsarepresentedthatformthebasisforthecalcula-tion of gas concentrations in water for differentgastransfersystems.

Ifthevariationinthegasconcentrationintheaircannot be neglected, the mass balance needsto be taken into account. The efficiency of a gas transfersystemcanbecalculatedbydividingthe

Qw, cw,0

Qa,ca,0

Qa, ca,e

Qw, cw,e

Figure 8 - Gas transfer system with in- and outflow of water and air

0

4

8

12

0 200 400 600 800 1000 1200 1400

time (s)

conc

entr

atio

n O

2 (g

/m3)

saturation concentration

driving force

co = 0 g/m3

cs = 10 g/m3

k2 = 0.00193 s-1

Figure 7 - Oxygen concentration in water as a func-tion of contact time


141

realizedgastransferbythemaximumachievablegastransfer:

w,e w,0

s w,0

c cK

c c−

=−

The following basic systems can be distin-guished:- plug flow with a constant gas concentration in

air- complete mixed system with a constant gas

concentrationinair- plug flow, co-current flow and a variable gas

concentrationinair- plug flow, counter-current flow and a variable

gasconcentrationinair- complete mixed system with a variable gas

concentrationinair

Plug flow with a constant gas concentration in airA characteristic of a plug flow is that the water is supposed to flow as a “frozen volume” through thegastransfersystem.Thus,allwaterparticlesinthesystemwillhavethesameretentiontime.The efficiency equation, then, can be written into thefollowingequation:

− ⋅= − 2( k t )1K 1 e

An example of a plug flow where the gas con-centration in air and thus cs is supposed to beconstantisafallingdropletfromasprayaeratorintoalargeopenspace.Thechangeinthegas

concentrationinairasaresultofgastransfercanthenbeneglected.

Complete mixed system with a constant gas concentration in the airThe opposite of a plug flow is a complete mixed system.Insuchagastransfersystemthewaterdropsaremixedextensively.Consequently, theretentiontimeofthewaterdropsisvariable.Somewaterdropsleavethesystemdirectly(short-cir-cuit flow) and others stay for a prolonged period of time in the system (eddy formation).The ef-ficiency is calculated with:

⋅

=+

2

2 1k t

1K1

Plug flow, co-current flow and a variable gas concentration in airThe equation for co-current flow can be found withthefollowinginitialconditions:cw=cw,0 attimet=0;ca =ca,0 attimet=0

Thefollowingsolutioncanbederived:

d2

d

k( k t( 1 ) )

RQ

3 kRQ

1 eK

1

− ⋅ +−

=+

Plug flow with counter-current flow and vari-able gas concentration in the airThe equation for counter-current flow can be foundwiththefollowinginitialconditions:

cw=cw,e attimet=te;ca =ca,e attimet=te.

Thefollowingsolutioncanbederived:− ⋅ ⋅ −

− ⋅ ⋅ −

−=

− ⋅

d2

d2d

k( k t (1 ))RQ

4 k( k t (1 ))k RQRQ

1 eK

1 e

System RQApplication drinking water

Application wastewater

Cascade 0.4 O2,CH4-

Toweraerator 5-100 CO2 CHCl3Plateaerator 20-60 CH4,CO2,O2

-

Sprayaerator 0.5 O2,CO2-

Deepwellaerator 0.1-0.4 O2 O2

Coneaerator >5 - O2

Table 3 - Air/water ratio for different gas transfer sys-tems and the gases that can be removed by the system


142

Complete mixed system with variable gas concentration in airThefollowingsolutioncanbederived:

⋅

=+ + d

2

5 k1k t RQ

1K1

In Figure 9 the efficiencies for oxygen (kH=0.039atT=10°C)forthe5basicequationsareplottedagainst the RQ with a k2tof1.61.The lines forK1andK2areobviouslyconstant,because, in this case, RQ is not of importance. The lines forK3,K4andK5climbat increasingvalues of RQ. When RQ approaches infinity, the lines for the different plug flow systems K1, K3andK4andforthemixedsystemsK2andK5co-incide.It can be concluded that a counter-current flow reactor has a higher efficiency than a co-current flow reactor, and plug flow reactors have a higher efficiency than a complete mixed system.

The RQ is an important factor for the gas transfer systems.During thedesignofagas transfersystem, theRQ value must be chosen. This depends on the required efficiency and the type of gas that needs toberemoved(Example1).The example to the right shows that the RQ nec-essary for a 90% removal efficiency of chloro-form is 200 times greater than the value of RQ formethane.Thismeansthatforthesamewaterflow the airflow through the system and the ca-pacityoftheventilatormusteachbeatleast200timesgreater.

A general rule that is applicable for the influence of the type of gas on the efficiency is: the higher the value of kH, the more air is needed for re-moval, resulting in an increased RQ. Different gastransfersystemshavedifferentcharacteris-tics with respect to RQ.

A cascade, for example, has an RQ of approxi-mately 0.4 and is therefore suitable for the re-movalofmethaneandtheadditionofoxygen,butisnotusedfortheremovalofchloroform.

Tower aerators are operated under different RQ valuesandcanbeusedforgasesthatareeithereasy or difficult to remove, like tetra- and trichlo-roethene.Deepwellaeratorshavethesamecharacteristicsascascades.

Example 1: The effect of RQ on the effi-ciencyCalculateforagastransfersystem,thatcanberepresentedbyacompletemixedsystem,the RQ that is necessary for a gas removal efficiency of 90% for methane, carbon dioxide andchloroform.Assumethatthecontacttimein the reactor is infinite and that the water tem-peratureis100C. The efficiency for a complete mixedsystemcanbecalculatedwiththefol-lowingequation:

⋅

=+ + d

2

5 k1k t RQ

1K1

The contact time is infinite, so 1/k2t=0.Theabove equation can be simplified as:

=+ d

5 kRQ

1K1

Gas Efficiency [%]

K5 [-] KD [-] RQ

Methane 90 0.90 0.043 0.39

Carbondioxide 90 0.90 1.23 11.1

Chloroform 90 0.90 9.62 86.6

k5k4k3

k2

k1

0.001 0.01 0.1 1 10

1

0.8

0.6

0.4

0.2

0

k1k2k3k4k5

RQ

k2tkDT

K (-

)

= = =

1.610.03910oC

Figure 9 - Efficiencies of the different basic equations


143

3 Practice

3.1 CascadeThe water in a cascade is falling onto severalsteps. Each step contains an overflow weir and a receivinggutter.Whenwaterpassesoveraweir,an interface between air and water is created.Whenthejetsubmergesintothereceivingbodyof water, significant amounts of air are entrained. Theentrainedairisthendispersedintheformofbubblesthroughoutthereceivingbodyofwater,whichleadstoanexcessivetransferofgases.The gas transfer takes place at the interface be-tweenthewaterandtheairbubbles(Figure10).Becausetheamountofairthatisentrainedislim-ited, the RQ is also limited. According to practical measurements and model investigations, the RQ ofcascadesisapproximately0.4.Theenergyconsumptionofacascade is10-30Wh/m3.

EfficiencyAn estimate of the efficiency for a cascade can be made, assuming that there is a relationshipbetween the measured fall height and the effi-ciency. The efficiency of a cascade depends on thefallheightofeachcascadestepandthenum-

berofsteps:

w,e w,0 n

s w,0

c cK 1 ( 1 k)

c c

−= = − −

−

inwhich:k = efficiency for each step [-]n = numberofsteps

In Table 4 the efficiency is given for oxygen, car-bondioxideandmethaneasafunctionofthefallheightofastep.WiththedatafromTable4andthe equation mentioned above, the efficiency of a cascadewithnstepscanbecalculated.Inpractice,thetotalfallheightofallthecascadestepstogethervariesbetween2and7meters.

From Table 4 it can be seen that oxygen andmethane efficiencies increase with an increase in fall height, but that the carbon dioxide efficiency remains constant. This is a result of the low RQ valueforcascades.Carbondioxideremovalre-quires a higher value of RQ. The interface be-tweenair andwatergets saturated rapidlywithcarbondioxide, regardlessof the retention timeofairbubbles in thewater,which isdependentonthefallheight.Thegreaterthefallheight,thedeeperthepenetrationinthetrough,andthelon-gertheretentiontime.

Weir loadingWeirloadingistheamountofwaterpermeterperhour that flows over the weir. The weir loading can be calculated by dividingthe flow by the net weir length (Figure 11):

= ww

nett

Qq

L

inwhich:

Figure 10 - Scheme of a cascade

K [%] h = 0.2 h = 0.4 h = 0.6 h = 0.8 h = 1.0 h = 1.2O2 14 25 36 46 51 55

CO2 14 14 15 15 15 15

CH4 14 27 37 48 56 62

Table 4 - Efficency coefficient k of different gases as a function of the weir height


144

qw = weirloading[m3/m•h]Lnett = totalweirlength[m]

From various experiments it can be concludedthat the efficiency of a cascade is almost inde-pendent of the weir loading. The advantage ofthisisthatthegastransferisstillsatisfactoryatproduction flows that are lower than the design flow. Withcascadestheweir loading isgenerallybe-tween50and100m3/(m•h).

Trough depthThetroughdepthofacascadeischoseninsuchawaythatthefallingwaterjetwillnotreachthebottom.Airbubblesaredragged toamaximumdepthand this results inamaximumcontactorretentiontimeandamaximumgastransfertime.Asaruleofthumb,thetraydepthmustbemorethantwo-thirdsofthefallheight.

Trough widthThetroughwidthmustbelargeenoughtoreceivethefallingwaterjet(Figure12).

The fall timeof thewater jet canbecalculatedwiththefollowingequation:

= ⋅ ⋅ 21h g t2

or

⋅=

2 htg

Thedistancexcanbecalculatedwhenthewater

velocityvo is known. To calculate the velocity, the equation of the complete overflow is used:

23 w

net

Qdg L

=×

and

wo

net

QvL d

=×

inwhich:Qw=discharge[m3/s]d = thickness of the falling water jet [m]vo=velocityofthefallingwaterjet[m/s]

The distance can be calculated with the equa-tion:

= ⋅ox v t

Withthedistancexthetroughwidthcanbecal-culated.Asa ruleof thumb, the troughwidth isat leasttwicethedistancex:

= ⋅B 2 x

Itisobviousthatthetroughwidthmustbecalcu-

X

h

H

B

Figure 12 - Scheme of the width of a cascade trough

80 mm 80 mm 80 mm + + +(...) = Lnet

40 mm

Lgross

Figure 11 - Weir loading of a cascade aerator


145

lated using the maximum flow that is discharged overtheweir.

ConfigurationsThecascadetroughscanbeplacedintwodiffer-entways.Theycanbeplacednexttoeachotherorontopofeachother(Figure13).Placingthemnexttoeachotherisadvantageousbecause it looks attractive. The advantage of putting them on top of eachother is that lessspace isused.Thedisadvan-tage, however, is that this makes maintenance more difficult.

3.2 Tower aerator Atoweraeratorconsistsofacylinderofsteelorsynthetic material that is filled with a packing me-dium.Packing media can consist of stacked slats or tubes, or specially designed packing material like thePall-ringandtheBerl-saddle.

Inthetopsectionofthetowerthewaterisdividedover the packing medium and flows down over the medium surface. As a result of the flow of wa-ter over the packing medium, a large contact sur-facebetweentheairandwateriscreatedforgastransfer.Inaddition,thewaterfallsindropsfromone packing element to the other, continuously forming new drops thus renewing the air-waterinterface.Theaircanberenewedbynaturalventilationorwiththehelpofaventilator.Incaseaventilatorisused,theaircanhaveaco-orcounter-currentflow in the tower. In Figure 14 a tower aerator with counter-current flow is represented.

In Figure 15 different types of packing material are represented. The packing material can be producedfromsyntheticmaterial,metal,carbonorceramicmaterial.Thedimensionsoftheindividualpiecesvaryfrom6 mm to 75 mm. In practice, installations usedfor purifying drinking water use mostly synthetic packing material with a dimension of 25-50 mm.

Figure 13 - Cascades beside each other and on top of each other

A

B

C

D

E

A influentB packing materialC air supplyD effluentE air discharge

Figure 14 - Representation of a counter-current tower aerator


146

Surface loadingThe surface loading (flow divided by surface area)thatinpracticeisusedintoweraeratorsis40to100m3/(m2•h).The applied packing height, that determines the retention timeof thewater in the toweraerator,variesbetween3and5meters.

Efficiency With tower aerators, removal efficiencies can be ashighas95%.The applied RQ depends on the gases that need toberemoved.

InFigure16theresultsofapilotexperimentus-ingatoweraeratorarerepresented.It can be concluded that the efficiency hardly changeswhenthesurfaceloadingisincreased.This is considered remarkable. In most gas trans-fer systems ,a larger flow results in a greater flow rate,resulting inashorterretentiontimefor thewater, and a lower efficiency. This insensitivity to the surface loading with atowercascadecanbeexplainedbythefactthattheretentiontimeinatoweraeratorispracticallyindependent of the water flow. The water falls un-der the influence of gravity, so the retention time is mainly determined by the type of packing ma-terialusedandtheheightofthebed.Itisindiffer-ent ifmoreorlesswaterfallsthroughthetowerbecausetheretentiontimeremainsunchanged.

In Figure 17 more results from the removal ef-ficiency experiments are given.

For all points in the graph, with the combina-tion of packing height and RQ, an efficiency of 99%isreached.Fromthisgraphitcanbecon-cluded that, at a certain point, an increasing RQ value does not lead to a reduction of the packing height.Atthatpointtheamountofair isnotde-cisivebuttheminimumnecessaryretentiontimeforremovalof99%isreached.

CloggingA disadvantage of the tower aerator is that thesystem issensitive toclogging. If iron (Fe2+) ispresentingroundwater,itwilloxidizeinthetoweraerator(Fe3+) and remain on the packing material (Fe(OH)3). Because the oxidized iron influences thegastransfernegatively,itwillbenecessarytoback flush the tower aerator. Water with a high velocity,oracombinationofwaterandair,isthenflushed through the tower aerator, removing the iron contamination from the packing material. In addition to flushing, it will be necessary to pe-riodically clean periodically the packing mate-rial chemically. In this case, the packing material mustberemovedfromthetoweraerator.

Co- or counter-current flowAtoweraeratorcanbeoperatedinbothco-cur-rent flow and counter-current flow (Figure 18).

80

85

90

95

100

0 20 40 60 80 100

effic

ienc

y (%

)

RQ (-)

18 m3/(m2*h)36 m3/(m2*h)

trichloro ethenepacking material: hy-pack steel 30mmheight packing material 3mtemperature: 11 oC

54 m3/(m2*h)72 m3/(m2*h)

Figure 16 - Removal efficiency of a tower aerator as a function of RQ at different surface loadings

Figure 15 - Different types of packing material


147

Intheparagraphontheoryitwasexplainedthatcounter-current flow results in a higher efficiency than co-current flow. Still, co-current flow is ap-plied.Thereasonsforthisare:- toavoidhighcarbondioxideremovalswhich

will cause limestone scaling. Using a co-cur-rent aerator with low values of RQ, the addi-tion of oxygen and the removal of methaneare sufficient while carbon dioxide removal willbelimited.

- to apply needed high surface loadings. Us-ing counter-current flow, “flooding” can occur. Thismeansthatawaterlayeriscreatedinthecolumnbecauseofthebuoyancyofair,whichcan even result in the tower aerator filling up withwater.

3.3 Plate aeratorAplateaeratorconsistsofahorizontalperforatedplate. Water flows over the plate and air is blown through its orifices, creating a bubble bed of air

andwaterabovetheplate(Figure19).

This results in intense contact between the airandthewater.The combination of horizontal water flow and ver-tical airflow (i.e., the flows are perpendicular), is called cross-flow aeration. The height of the bubble bed is determined byadjustingtheheightoftheweirattheendoftheplate.Thediameteroftheholesintheperforatedplateisusually1-1.5mm.Theopensurfaceareavar-ies from1.5% to3%of the total plate surfacearea.Theenergyconsumptionofaplateaeratoris30-40Wh/m3.

Duetothereducedconstructionheightandheadloss,thistechniqueoffersgoodpossibilitiesforin-corporatingitinexistingtreatmentplants.Some-timesitispossibletoplacetheplateaeratorsinthe filter building directly above the filters.

Efficiency The efficiency of plate aerators is mainly deter-mined by the applied RQ and the retention time ofthewaterontheplate.There isnoanalyticalequation for calculating the efficiency, unlike the co- and counter-current flows. In practice, the applied RQs vary from 20 to 60 andtheappliedsurfaceloadingvariesfrom30to

0

5

10

15

0 10 20 30 40

heig

ht p

acki

ng m

ater

ial (

m)

RQ (-)

18 m3/(m2*h)36 m3/(m2*h)

trichloro ethenepacking material: hy-pack steel 30mmefficiency: 99%temperature: 11 oC

54 m3/(m2*h)72 m3/(m2*h)

Figure 17 - Required packing height and RQ to achieve an efficiency of 99% at different surface loadings

co-current flow counter-current flow

airwater water

air

Figure 18 - Design alternatives for tower aerators

air

water

Figure 19 - Representation of a plate aerator


148

tween air and water is saturated. Because thedropletremainsintactduringthefall,theinterfaceisnotrenewedandthegastransferstops.

Energy consumptionSprayaeratorsneedacertainpressuretoguar-anteeanequallydistributedspray.Forsprayersthat produce fine droplets (mist), the pressure is thegreatest,abouta10-meterwatercolumn.Theenergyconsumptionofthesehighpressuresprayaeratorsis,therefore,thelargest.

CloggingAdisadvantageofsprayersistheirhighsensitiv-itytoclogging.

Alternatives in practice

40m3/(m2.h).

CloggingPlateaeratorsaresensitivetocloggingbecauseof the small orifices in the plate. Iron deposits found on the plate can block the orifices and af-fect the flow through the plate. Short-circuit flows can occur, influencing nega-tivelythegastransfer.Dependingonthe iron loading, theplatehas tobe cleaned once a month or once every othermonth. It might also be necessary to clean theplatechemicallyonceortwiceayear.

3.4 Spray aerator Sprayaeratorsdividewater intosmalldroplets,whichresultsinalargeair-waterinterface(Figure20).Theenergyconsumptionofsprayaeratorsis10-50Wh/m3,dependingonthetypeofaerator.

Anadvantageofsprayaeratorsistheeaseofin-corporation intoexisting installations.Thesprayaerators can be placed directly above the filters.

EfficiencyWhen the air is intensively renewed, the efficien-cyofsprayaeratorscanbecalculatedwith thefollowingequation:

− ⋅− ⋅= − = −

22

2h( k )g( k t)K 1 e 1 e

The efficiency for the addition of oxygen can vary from65to80%,for thecarbondioxideremovalthe efficiency varies from 60 to 80%.

In Figure 21 the efficiency of the Dresden-nozzle for carbondioxide removalasa functionof thefallheightisshown.It is remarkable that after a certain fall height the efficiency remains more or less constant. The reason is thataftersometime the interfacebe-

Figure 20 - Spraying small droplets of water

2

1

00 0.25 0.5 0.75 1

KCO2 [-]

h [m

]

Figure 21 - Efficiency Dresden-nozzle as a function of the fall height


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Sprayaerators canbedivided into twogroups:

upward-anddownward-directedsprayaerators.An example of the first type is the ‘Amsterdam’ spray aerator (Figure 22). In this type of sprayaerator, two jets are directed perpendicular toeachother,dispersing thewater.Thisresults inmanydropletsintheair.Duringthefallofthewa-ter droplets, the gas transfer takes place. AnexampleofthesecondtypeofsprayeristheDresdensprayer(Figure23),ortheplatespray-er. Here, the water flows through a plastic tube and strikes a disc (plate), shaping the water like an umbrella, and eventually disintegrating intodroplets.

3.5 Alternative aeration systems

Vacuum gas transfer systemAvacuumgastransfersystemisusuallyexecut-

ed as a tower aerator filled with a packing mate-rialinwhichthepressureisloweredbyavacuumpump(Figure24).Duetothevacuumpump,gas isremovedfromthe tower, resulting in lowergasconcentrationsand a decreased pressure there. Because thegasconcentrationsinthetowerarelowerthanintheatmosphere,thesaturationconcentrationsinthetowerarealsolower.Becauseofthelowsat-uration concentrations, it is possible to removehigher levelsofgasfromthewaterthanispos-sible under atmospheric conditions. This makes a vacuum gas transfer system ideal for remov-ingdissolvednitrogenandoxygenfromthewaterand is frequently applied before the denitrification process.

The efficiency of the vacuum gas transfer system depends on the vacuum pressure that is main-tained in the tower. In the absence of an air flow, the RQ equals zero. Since oxygen is not brought into the system, oxidation of iron cannot occur.Thisallowsthewater tobepumpedtothenexttreatment process, contrary to a cascade. Ina cascade oxidation of iron does occur, which,whenthewaterispumpedtothenexttreatment

Figure 22 - Amsterdam sprayer

Figure 23 - Dresden sprayer

A

B

D

E

A influentB packing materialC air supplyD effluentE air discharge

E pump

pump

Figure 24 - Representation of a vacuum liquid-gas exchange


150

process, causes the iron flocs to break up making them harder to remove in the filter.

Like the tower aeration system, the vacuum sys-temisnotverysensitivetosurfaceloading.Theapplied surface loading varies from 50 to 100m3/(m2.h).

Agreatdisadvantageofthevacuumgastransfersystemisitshighenergyconsumption,requiringapproximately1,600Wh/m3tomaintainit.

Deep-well aeratorWater flows through the deep well, entraining air byaventuri(Figure25right),orairissuppliedatthebottomofthewell(Figure25left).Due to the high water pressure at the bottomofthewell,anincreaseinairpressureisestab-lished,whichresultsinahigheroxygenconcen-tration. With a higher saturation concentration,more oxygen can be dissolved into the waterthanatatmosphericconditions.

Deepwellaeratorsaremainlyusedinthetreat-ment of wastewater, because the oxygen con-sumptionofwastewaterisnormallyhigh.

Theadvantageofadeepwellaeratoristhatlargeamountsofwatercanbetreatedagainstrelative-lylowenergycosts.Theenergyconsumptionforthedeepwellaeratorisapproximately5Wh/m3.

Venturi aeratorTheventuriaeratorconsistsofatubewithare-ducedcross-sectionalarea,wheretheincreasedwater velocity occurs. At the place where thewater velocity is the highest (through orifices in the tube), air is entrained.Due to thestrongturbulence,an intensivemixingof theentrainedair with the water leads to the dispersion of fine bubbles.Since the amount of air that can be entrainedis relatively small, the RQ of a venturi aerator is rathersmall,varyingbetween0.2to0.4.The efficiency for oxygen addition ranges from 80

to95%.

The advantage of the venturi aerator is that itrequires little space and the system is not ex-pensive. A disadvantage is that only limited flow variationscanbeallowedforanoptimaleffect.Theenergyconsumptionisapproximately20-30Wh/m3.

Bubble aerationThetransferofgasbymeansofabubbleaera-tor is accomplished by injecting compressedair through orifices of various sizes into the water(Figure27).Airisdistributedbyperforatedpipes at the bottom of a tank. During the rise of the formed bubbles, gas transfer takes place. Thissystemismainlyusedinwastewatertreat-ment. The principle of gas transfer by bubbleaerationisthesameasincascades.

Cone aeratorAconeaeratorisusedasagastransfersystemforthetreatmentofwastewater.The cone aerator consists of a large rotating

h

H

2 rows of air pipes

inflowingwater

outflowing water

supply of compressedair

dischargeaeratedwater

supply of raw water

dischargeof sludge

Figure 25 - Design alternatives for a deep-well aerator

air filter

air supply

raw water supplyaerated water evacuation

Figure 26 - Representation of a venturi aerator


151

blade in the formofacone,situated inabasinon the water’s surface(Figure 28).Through theblade, water is abstracted from underneath theconeandsprayedlaterallyoverthewater’ssur-face.Becausewaterdropletsareformedandairisentrained,gastransfercanbeachieved.As a result of the suction of water from under-neathandthehorizontaldistributionofthewater,a circular flow is created and the water in the ba-sinisaerated.

Figure 27 - Bubble aeration system Figure 28 - Cone aerator

Further reading

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

• Modellering van intensieve gasuitwisselings-gasuitwisselings-systemen(inDutch),A.W.C.vandeHelm(MScthesis)


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aeration and gas stripping w ater treatment - tu · pdf fileframework. this module explains...

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