Aeration and Gas Stripping.pdf

7/28/2019 Aeration and Gas Stripping.pdf

1/18

Aeration and

gas stripping

WAT

ERTREATME

NT

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

k1

k2

k3

k4

k5

RQ

k2tkDT

K

(-)

===

1.610.03910oC


2/18

Framework

Thismoduleexplainsaerationandgasstripping.

Contents

Thismodulehasthefollowingcontents:

1. Introduction

2. Theoryofgastransfer

2.1 Equilibrium

2.2 Kinetics

2.3 Massbalance

2.4 Solutionsforthebasicequations

3. Practice

3.1 Cascade

3.2 Toweraerator 3.3 Plateaerator

3.4 Sprayaerator

3.5 Alternativeaerationsystems

aerationandgastransfer watertreatment

136


3/18

the water falls over a weir into a lower placed

trough.Whenthefallingstreamentersthewater

body,airisentrappedintheformofbubbles,pro-

vidingforamixtureofwaterandairinwhichgastransferwilloccur.

Thetoweraerator(Figure2)consistsofacylin-

dricalvesselofsteelorsyntheticmaterialthatis

lled with packing material, usually consisting of

elementsofsyntheticmaterial.Waterfallsdown

andairisblowninaco-currentorcounter-current

direction.

Aplateaerator(Figure3) is a horizontalperfo-

rated plate. Water ows over the plate and air is

blown through the orices, creating a bubble bed

ofairandwaterabovetheplate.

Sprayers (Figure 4)aretypically used because

of their simple implementation in existing treat-

mentplants.Byspraying,acontactsurfacebe-

tween the air and water is created for the gas

exchange.

2 Theory of gas transfer

1 Introduction

Aeration(gasaddition)andgasstripping(gasre-

moval) are normally the rst treatment steps dur-

ing the production of drinking water from ground-

water or riverbank water. This articially induced

gastransferaimsattheadditionofoxygen(O 2)

andtheremovalofcarbondioxide(CO 2),meth-

ane (CH4), hydrogen sulde (H2S), and other

volatileorganiccompounds(forexample1.2Di-

chloropropane(1.2DCP),Trichloroethene(TRI),

Tetrachloroethene (PER) and Trichloromethane

(chloroform)).

Gastransferisseldomappliedinthetreatmentofsurfacewaterbecausesurface water hasbeen

incontactwithairforaprolongedperiod.Conse-

quently, surface water contains sufcient oxygen,

and other gases, like methane and hydrogen sul-

de, are absent.

Theadditionofoxygenisrequiredfortheoxida-

tion of bivalent iron (Fe2+), manganese (Mn2+)

and ammonium (NH4+). These substances are

presentindissolvedformingroundwater.Dueto

chemical and biological oxidation, the substan-

ces can be removed by following a ltration step.

Thiswillbediscussedinthechapterongranular

ltration.

Reducingthecarbondioxideconcentrationleads

toariseinpHandareductionofaggressivecar-

bondioxidethatisabletodisintegrate(concrete)

pipes.

Methaneshould be removed becauseits pres-

ence has negative inuences on the ltration pro-

cesses.

Hydrogen sulde has an annoying odor (rottingeggs)andthereforeneedstoberemovedfrom

thewater.

Volatile organic compounds are usually toxic;

someofthemareevencarcinogenic.Obviously,

these compounds are not allowed in drinking wa-

ter.

To achieve gas transfer a number of systems

havebeendevelopedovertheyears.

Oneoftheoldestsystemsisthecascade(Figure

1).Thewaterfallsinseveralsteps.Ineachstep,

Figure 1 - Cascade aeration

Figure 2 - Tower aeration

watertreatment aerationandgastransfer

137


4/18

2.1 Equilibrium

Henrys law

Watercontainsdissolvedgases.Inaclosedves-

selcontainingbothgas(e.g.,air)andwater,the

concentrationofavolatilecomponentinthegas-

phasewillbeinequilibriumwiththeconcentra-

tioninthewaterphase,accordingtoHenryslaw.

Theequilibriumconcentrationcanbecalculated

usingthefollowingformofHenryslaw:

= w H gc k c

inwhich:

cw=equilibriumconcentrationofagasinwater

[g/m3]

kH = Henrys constant or distribution coefcient

[-]

cg=concentrationofthegasinair[g/m3]

The distribution coefcient kH depends on the

typeofgas,andthetemperature.

Inaddition,pollutionandimpuritiesinthe water

inuence the equilibrium concentration. This is-

suewillnotbediscussedhere.

Inliterature,manydifferentformsofHenryslaw

arefound.

Oftenpartialpressureisusedinsteadofthegas

concentrationinair,and/ormolarconcentrationin

thewaterinsteadofweightconcentration.Con-

sequentlythisresultsinadifferentunitforthedis-

tribution coefcient, or Henrys law constant (ie.

[mol/(m3Pa)]or[mol/l/atm]).

Forgas stripping, often thevolatility is given insteadofthesolubilityofagas.Inthiscase,the

distribution coefcient is inverted (gas/water, in

steadofwater/gas).

Distribution coefcient

InTable1foranumberofgasesalistofvalues

is given of the distribution coefcient at different

watertemperatures,(intermediatevaluescanbe

obtainedwithlinearinterpolation).

Inthetableitisshownthatnitrogen,oxygenand

methane have low kH-values. This means that

these gases hardly dissolve in water and they

can,therefore,beeasilyremoved.

The other gases have high kHvalues and dis-

solve easily, which makes it difcult to remove

themfromthewateroreasytoaddthemtowa-

ter.

Gas concentration in air

Thegasconcentrationintheairc g must be known

beforetheequilibrium(orsaturation)concentra-tioncanbecalculated.Thisconcentrationcanbe

determinedusingtheuniversalgaslaw:

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


5/18

Thegasconcentrationcanbecalculatedbymulti-

plyingthemolargasconcentrationinair[mol/m 3]

withthemoleculeweightoftheconsideredgas:

gn p

c MW MWv R T

= =

inwhich:

MW=molecularweightofagas[g/mol]

Partial pressure

Thepartialpressureofacertaingasispropor-

tionaltothevolumefractionofthatgasinair:

o fp p V=

inwhich:

po = standardpressureatsealevel(=101,325)

[Pa]

Vf = volumefraction[-]

InTable2thevolumefractionsofdifferentgases

thatoccurinairaregiven.

Thesevaluesarevalidfordryairwithastand-

ardpressureof101,325Pa.Withthesevolume

fractionsthepartialpressuresofallgasesinair

canbecalculated.Gasesthatdonotoccurinair

haveapartialpressureequaltozeroandthusa

cgequaltozeroandalsoac wequaltozero(for

example,methane).

InFigure5theequilibrium(saturation)concentra-

tionofoxygenisgivenasafunctionofwatertem-

perature.Withanincreaseinwatertemperature,

thesaturationconcentrationdecreasesbecause

lessoxygencanbedissolvedinwarmwater.

Thesaturationconcentrationcwislinearlydepen-

dent on pressure. The saturation concentration

foroxygenatthestandardpressureof101,325

Pais11.3g/m3.

Ataheightof8,000meters(forexample,Mount

Everest),theairpressureisonly10,000Pawhich

meansthatthesaturationconcentrationforoxy-

genis1.1g/m3.

Intheseaata depthof100metersbelowsea

level,thepressureis1,100,000Pa.Thisresults

in a saturation concentration foroxygen of 113

g/m3.

2.2 Kinetics

As soon as water and air are in contact, gas

Gas

Distribution coefcient (kH

)

T=0oC T=10oC T=20oCMolecularweight

(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 sulde (H2

S) 4.69 3.65 2.87 34

Tetrachloroethelene(C2

HCl4

) -1 3.20 1.21 167

Tetrachloroethene(C2

HCl3

) -1 3.90 2.43 131.5

Chloroform(CHCl3) -1 9.0 7.87 119.5

Ammonia(NH3

) 5000 2900 1800 17

1Thesesubstancesarestillintheliquidphaseatatemperatureof00C and therefore the kH

is not known

Table 1 - Distribution coefcient for gases and the molecule weight

Gas

Volume

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 -

1Indryairatastandardpressureof101,325Pa

2Waterandairtemperatureof100C

Table 2 - Volume fractions of gases


139


6/18

molecules willbe exchanged continuously. The

directionofthenetgastransportdependsonthegasconcentrationinthewater(cw)andtheequi-

libriumconcentrationce.

InFigure6thegasconcentrationinthewaterat

timet=0issmallerthantheequilibriumconcen-

tration. This means that more gas can be dis-

solvedinthewaterthanispresentattimet=0.

Anetgastransportfromair towateroccurs, as

indicated by the arrow in the gure. The net gas

transport continues until time t=innite and the

gas concentration in the water is equal to the

equilibrium (or saturation) concentration. Then,

thegastransportfromwatertoairandviceversa

are equal. Hence, no net gas transport occurs

andthegasconcentrationinthewaterandairdo

notchange.Inthatcase,adynamicequilibrium

isestablished.

Thevelocityofgastransferisdeterminedbythe

kinetic equation:

w2 s w

dck (c c )

dt=

inwhich:

cw = concentrationofagasinwater[g/m3]

k2 = gas transfer coefcient [s-1]

The time-dependent gas concentration change

inwaterisrepresentedbythetermdcw/dt.The

changesinconcentrationaredeterminedbythe

magnitude of the gas transfer coefcient k2andthedrivingforce(cscw).

The gas transfer coefcient k2isadevice-depen-

dent parameter. The larger the contact surface

areabetweentheairandwaterandtherenewal

ofthissurfacearea,thebetterthegastransfer

and the higher the gas transfer coefcient.

The driving force is dened by the amount of gas

thatcanmaximallybedissolvedinavolumeof

water, the saturation concentration cs, and the

amountofgasthatispresentinavolumeofwa-

ter, theconcentrationcw.Thelargerthedriving

force,thefasterthegastransfer.

Theincreaseintheoxygenconcentrationintime

isshowninFigure7foraconstantcs(10mg/l)

andan initialoxygenconcentrationof0 mg/l.In

thebeginning,whenthedifferencebetweenthe

csandthecwisthelargest,thegastransferoc-

curs at maximum velocity.As time passes, the

gas concentration in water increases and thedrivingforcedecreases,whichgraduallyresults

in a lower gas transfer rate. For t=innite the oxy-

genconcentrationinwaterequalsthesaturation

concentrationcs.

Forabatchreactorthedifferentialequationcan

besolvedbyintegration,withcw=cw,0attimet=0,

taking into account that cs is constant:

2( k t)

w s s w,0

c c (c c ) e =

water temperature (oC)

cw

(g/m3)

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

concentration

Figure 6 - Gas transport from air to water


140


7/18

or:

2( k t)s w

s w,0

c ce

c c

=

2.3 Mass balance

Intheprecedingparagraphitisassumedthatthe

oxygen concentration in air is constant. This is

a simplication that is not always applicable. For

situationsinwhichthegasconcentrationchang-

esinairareimportant,amassbalanceneedsto

beformulated.

InFigure8amassbalanceforagastransfersys-

temisschematicallypresented.

A water ow (Qw),withagasconcentrationinthe

water phase (cw,0), and an airow (Qa), with a

gasconcentration (ca,0), enter thesystem.The

same water ow (Qw),withagasconcentration

inthewaterphase(cw,e), and the same airow(Qa), with a gas concentration (ca,e),leave the

system.

Forthegastransfersystem,thelawofcontinuity

isvalid:thetotalamountofgasthatentersand

leavesthesystemmustbeequalandamassbal-

ancecanbesetup:

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

Byusingthemassbalance,thegasconcentra-

tions in the air and water are linked and can be

applied inthe gastransferequationspresented

below.

The RQ is the relationship between the airow

and the water ow. Using the mass balance RQ,that relationship can be dened 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 calculate

thechangesinthegasconcentrationsinwater

andair.

Combining the equilibrium equation and the

massbalanceresultsintwoequationswithtwo

unknown variables, cwandca.Withdifferentini-tialconditions,differentsolutionsfortheseequa-

tionscanbeobtained.

In the following section a number of equations

arepresentedthatformthebasisforthecalcula-

tion of gasconcentrations in water fordifferent

gastransfersystems.

Ifthevariationinthegasconcentrationintheair

cannot be neglected, the mass balance needs

to be taken into account. The efciency of a gas

transfersystemcanbecalculatedbydividingthe

Qw, cw,0

Qa,ca,0

Qa, ca,e

Qw, cw,e

Figure 8 - Gas transfer system with in- and outow of

water and air

0

4

8

12

0 200 400 600 800 1000 1200 1400

time (s)

concentrationO2

(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


8/18

realizedgastransferbythemaximumachievable

gastransfer:

w,e w,0

s w,0

c cK

c c

=

The following basic systems can be distin-

guished:

- plug ow with a constant gas concentration in

air

- completemixed system with a constantgas

concentrationinair

- plug ow, co-current ow and a variable gas

concentrationinair

- plug ow, counter-current ow and a variablegasconcentrationinair

- complete mixed system with a variable gas

concentrationinair

Plug ow with a constant gas concentration

in air

A characteristic of a plug ow is that the water is

supposed to ow as a frozen volume through

thegastransfersystem.Thus,allwaterparticles

inthesystemwillhavethesameretentiontime.

The efciency equation, then, can be written into

thefollowingequation:

= 2

( k t )K 1 e

An example of a plug ow where the gas con-

centration inairandthuscsissupposedtobe

constantisafallingdropletfromasprayaerator

intoa largeopenspace.Thechangeinthegas

concentrationinairasaresultofgastransfercan

thenbeneglected.

Complete mixed system with a constant gas

concentration in the air

The opposite of a plug ow is a complete mixed

system.Insuchagastransfersystemthewater

dropsare mixedextensively. Consequently, the

retentiontimeofthewaterdropsisvariable.Some

waterdropsleavethesystemdirectly(short-cir-

cuit ow) and others stay for a prolonged period

of time in thesystem(eddy formation).The ef-

ciency is calculated with:

=

+

2

2 1

k t

1K

1

Plug ow, co-current ow and a variable gas

concentration in air

The equation for co-current ow can be found

withthefollowinginitialconditions:

cw=cw,0 attimet=0;

ca =ca,0 attimet=0

Thefollowingsolutioncanbederived:

d2

d

k( k t( 1 ) )

RQ

3 k

RQ

1 eK

1

+

=

+

Plug ow with counter-current ow and vari-

able gas concentration in the air

The equation for counter-current ow can be

foundwiththefollowinginitialconditions:

cw=cw,e attimet=te;

ca =ca,e attimet=te.


=

d2

d2

d

k( k t (1 ))

RQ

4 k( k t (1 ))k RQ

RQ

1 eK

1 e

System RQ

Application

drinking

water

Application

wastewater

Cascade 0.4 O2

,CH4

-

Toweraerator 5-100 CO2

CHCl3

Plateaerator 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


9/18

Complete mixed system with variable gas

concentration in air


=

+ +d

2

5 k1

k t RQ

1K1

In Figure 9 the efciencies for oxygen (kH=0.039

atT=10C)forthe5basicequationsareplotted

against the RQ with a k2tof1.61.

ThelinesforK1andK2areobviouslyconstant,

because, in this case, RQ is not of importance.

ThelinesforK3, K4andK5climbatincreasing

values of RQ. When RQ approaches innity, the

lines for the different plug ow systems K1, K3andK4andforthemixedsystemsK2andK5co-

incide.

It can be concluded that a counter-current ow

reactor has a higher efciency than a co-current

ow reactor, and plug ow reactors have a higher

efciency than a complete mixed system.

The RQ is an important factor for the gas transfer

systems.

Duringthedesignofagastransfersystem,the

RQ value must be chosen. This depends on the

required efciency and the type of gas that needs

toberemoved(Example1).

The example to the right shows that the RQ nec-

essary for a 90% removal efciency of chloro-

form is 200 times greater than the value of RQ

formethane.Thismeansthatforthesamewater

ow the airow through the system and the ca-

pacityoftheventilatormusteachbeatleast200

timesgreater.

A general rule that is applicable for the inuence

of the type of gas on the efciency is: the higher

the value of kH, the moreair 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,but

isnotusedfortheremovalofchloroform.

Tower aerators are operated under different RQ

valuesandcanbeusedforgasesthatareeither

easy or difcult to remove, like tetra- and trichlo-

roethene.

Deepwellaeratorshavethesamecharacteristics

ascascades.

Example 1: The effect of RQ on the ef-

ciency

Calculateforagastransfersystem,thatcan

berepresentedbyacompletemixedsystem,

the RQ that is necessary for a gas removal

efciency of 90% for methane, carbon dioxide

andchloroform.Assumethatthecontacttime

in the reactor is innite and that the water tem-

peratureis100C. The efciency for a complete

mixedsystemcanbecalculatedwiththefol-

lowingequation:

=

+ +d

2

5 k1

k t RQ

1K

1

The contact time is innite, so 1/k2t=0.The

above equation can be simplied as:

=

+d

5 k

RQ

1K

1

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

k1

k2

k3

k4

k5

RQ

k2t

kDT

K

(-)

=

=

=

1.61

0.039

10oC

Figure 9 - Efciencies of the different basic equations


143


10/18

3 Practice

3.1 Cascade

The water in a cascade is falling onto several

steps. Each step contains an overow weir and a

receivinggutter.Whenwaterpassesoveraweir,

an interface between air and water is created.

Whenthejetsubmergesintothereceivingbody

of water, signicant amounts of air are entrained.

Theentrainedairisthendispersedintheformof

bubblesthroughoutthereceivingbodyofwater,

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.

Theenergyconsumptionofacascadeis10-30

Wh/m3.

EfciencyAn estimate of the efciency for a cascade can

be made, assuming thatthere is a relationship

between the measured fall height and the ef-

ciency. The efciency of a cascade depends on

thefallheightofeachcascadestepandthenum-

berofsteps:

w, e w, 0 n

s w, 0

c cK 1 ( 1 k)

c c

= =

inwhich:

k = efciency for each step [-]

n = numberofsteps

In Table 4 the efciency is given for oxygen, car-

bondioxideandmethaneasafunctionofthefall

heightofastep.WiththedatafromTable4and

the equation mentioned above, the efciency of a

cascadewithnstepscanbecalculated.

Inpractice,thetotalfallheightofallthecascadestepstogethervariesbetween2and7meters.

From Table 4 it can be seen that oxygen and

methane efciencies increase with an increase in

fall height, but that the carbon dioxide efciency

remains constant. This is a result of the low RQ

valueforcascades.Carbondioxideremovalre-

quires a higher value of RQ. The interface be-

tweenairandwatergetssaturatedrapidlywith

carbondioxide, regardlessofthe retentiontime

ofairbubblesinthewater,whichisdependent

onthefallheight.Thegreaterthefallheight,the

deeperthepenetrationinthetrough,andthelon-

gertheretentiontime.

Weir loading

Weirloadingistheamountofwaterpermeterper

hour that ows over the weir.

The weir loading canbe calculated by dividing

the ow by the net weir length (Figure 11):

=w

wnett

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 = 1.2

O2 14 25 36 46 51 55

CO2 14 14 15 15 15 15

CH4 14 27 37 48 56 62

Table 4 - Efcency coefcient k of different gases as a function of the weir height


144


11/18

qw = weirloading[m3/mh]

Lnett = totalweirlength[m]

From various experiments it can be concluded

that the efciency of a cascade is almost inde-

pendent of the weir loading. The advantage of

thisisthatthegastransferisstillsatisfactoryat

production ows that are lower than the design

ow.

Withcascadestheweirloadingisgenerallybe-

tween50and100m3/(mh).

Trough depth

Thetroughdepthofacascadeischoseninsuchawaythatthefallingwaterjetwillnotreachthe

bottom.Airbubblesaredraggedtoamaximum

depthandthisresultsinamaximumcontactor

retentiontimeandamaximumgastransfertime.

Asaruleofthumb,thetraydepthmustbemore

thantwo-thirdsofthefallheight.

Trough width

Thetroughwidthmustbelargeenoughtoreceive

thefallingwaterjet(Figure12).

The fall time ofthewater jetcan becalculated

withthefollowingequation:

= 21h g t

2

or

=

2 ht

g

Thedistancexcanbecalculatedwhenthewater

velocityvo is known. To calculate the velocity, the

equation of the complete overow is used:

23 w

net

Qd

g L

=

and

wo

net

Qv

L 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.

As a ruleof thumb,the troughwidth is at least

twicethedistancex:

= 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


12/18

lated using the maximum ow that is discharged

overtheweir.

Congurations

Thecascadetroughscanbeplacedintwodiffer-

entways.Theycanbeplacednexttoeachother

orontopofeachother(Figure13).

Placingthemnexttoeachotherisadvantageous

because it looks attractive.

The advantage of putting them on top of each

otheristhatlessspaceisused.Thedisadvan-

tage, however, is that this makes maintenance

more difcult.

3.2 Tower aerator

Atoweraeratorconsistsofacylinderof steelor

synthetic material that is lled with a packing me-

dium.

Packing media can consist of stacked slats or

tubes, or specially designed packing material like

thePall-ringandtheBerl-saddle.

Inthetopsectionofthetowerthewaterisdivided

over the packing medium and ows down over

the medium surface. As a result of the ow of wa-

ter over the packing medium, a large contact sur-

facebetweentheairandwateriscreatedforgas

transfer.Inaddition,thewaterfallsindropsfrom

one packing element to the other, continuously

forming newdrops thus renewing theair-water

interface.

Theaircanberenewedbynaturalventilationor

withthehelpof aventilator.In casea ventilator

isused,theaircanhaveaco-orcounter-current

ow in the tower. In Figure 14 a tower aerator

with counter-current ow is represented.

In Figure 15 different types of packing material

are represented. The packing material can be

producedfromsyntheticmaterial,metal,carbon

orceramicmaterial.

Thedimensionsoftheindividualpiecesvaryfrom

6 mmto 75 mm. Inpractice,installationsused

for 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


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Surface loading

The surface loading (ow divided by surface

area)thatinpracticeisusedintoweraeratorsis40to100m3/(m2h).

The applied packing height, that determines the

retentiontimeofthewaterinthetoweraerator,

variesbetween3and5meters.

Efciency

With tower aerators, removal efciencies can be

ashighas95%.

The applied RQ depends on the gases that need

toberemoved.

InFigure16theresultsofapilotexperimentus-

ingatoweraeratorarerepresented.

It can be concluded that the efciency hardly

changeswhenthesurfaceloadingisincreased.

This is considered remarkable. In most gas trans-

fer systems ,a larger ow results in a greater ow

rate,resultingina shorterretentiontimeforthe

water, and a lower efciency.

This insensitivity to the surface loading with atowercascadecanbeexplainedbythefactthat

theretentiontimeinatoweraeratorispractically

independent of the water ow. The water falls un-

der the inuence of gravity, so the retention time

is mainly determined by the type of packing ma-

terialusedandtheheightofthebed.Itisindiffer-

entifmoreorlesswaterfallsthroughthetower

becausetheretentiontimeremainsunchanged.

InFigure 17 more results from the removal ef-

ciency experiments are given.

For all points in the graph, with the combina-

tion of packing height and RQ, an efciency of

99%isreached.Fromthisgraphitcanbecon-

cluded that, at a certain point, an increasing RQ

value does not lead to a reduction of the packing

height.Atthatpointtheamountofairisnotde-

cisivebuttheminimumnecessaryretentiontime

forremovalof99%isreached.

Clogging

Adisadvantage of the tower aeratoris that the

systemissensitivetoclogging.Ifiron(Fe 2+)is

presentingroundwater,itwilloxidizeinthetower

aerator(Fe3+) and remain on the packing material

(Fe(OH)3). Because the oxidized iron inuences

thegastransfernegatively,itwillbenecessaryto

back ush the tower aerator. Water with a high

velocity,oracombinationofwaterandair,isthenushed through the tower aerator, removing the

iron contamination from the packing material. In

addition to ushing, 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 ow

Atoweraeratorcan beoperatedin bothco-cur-

rent ow and counter-current ow (Figure 18).

80

85

90

95

100

0 20 40 60 80 100

effic

iency(%)

RQ (-)

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

trichloro ethene

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

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

Figure 16 - Removal efciency of a tower aerator as a

function of RQ at different surface loadings

Figure 15 - Different types of packing material


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14/18

Intheparagraphontheoryitwasexplainedthat

counter-current ow results in a higher efciency

than co-current ow. Still, co-current ow 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 methane

are sufcient while carbon dioxide removal

willbelimited.

- to apply needed high surface loadings. Us-

ing counter-current ow, ooding can occur.

Thismeansthatawaterlayeriscreatedinthe

columnbecauseofthebuoyancyofair,which

can even result in the tower aerator lling up

withwater.

3.3 Plate aerator

Aplateaeratorconsistsofahorizontalperforated

plate. Water ows over the plate and air is blown

through its orices, creating a bubble bed of air

andwaterabovetheplate(Figure19).

This results in intense contact between the air

andthewater.

The combination of horizontal water ow and ver-

tical airow (i.e., the ows are perpendicular), is

called cross-ow aeration.

The height of thebubblebed is determined by

adjustingtheheightoftheweirattheendofthe

plate.

Thediameteroftheholesintheperforatedplate

isusually1-1.5mm.Theopensurfaceareavar-

iesfrom1.5%to3%ofthetotalplatesurface

area.

Theenergyconsumptionofaplateaeratoris30-

40Wh/m3.

Duetothereducedconstructionheightandhead

loss,thistechniqueoffersgoodpossibilitiesforin-

corporatingitinexistingtreatmentplants.Some-

timesitispossibletoplacetheplateaeratorsinthe lter building directly above the lters.

Efciency

The efciency of plate aerators is mainly deter-

mined by the applied RQ and the retention time

ofthewaterontheplate.Thereisnoanalytical

equation for calculating the efciency, unlike the

co- and counter-current ows.

In practice, the applied RQs vary from 20 to 60

andtheappliedsurfaceloadingvariesfrom30to

0

5

10

15

0 10 20 30 40

heightpackingmaterial(m)

RQ (-)

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

trichloro ethene

packing 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 efciency of 99% at different surface

loadings

co-current flowcounter-current flow

airwater water

air

Figure 18 - Design alternatives for tower aerators

air

water

Figure 19 - Representation of a plate aerator


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15/18

tween air and water is saturated. Because the

dropletremainsintactduringthefall,theinterface

isnotrenewedandthegastransferstops.

Energy consumption

Sprayaeratorsneedacertainpressuretoguar-

anteeanequallydistributedspray.Forsprayers

that produce ne droplets (mist), the pressure is

thegreatest,abouta10-meterwatercolumn.

Theenergyconsumptionofthesehighpressure

sprayaeratorsis,therefore,thelargest.

Clogging

Adisadvantageofsprayersistheirhighsensitiv-

itytoclogging.

Alternatives in practice

40m3/(m2.h).

Clogging

Plateaeratorsaresensitivetocloggingbecause

of the small orices in the plate. Iron deposits

found on the plate can block the orices and af-

fect the ow through the plate.

Short-circuit ows can occur, inuencing nega-

tivelythegastransfer.

Dependingontheironloading,theplatehasto

be cleaned once a month or once every other

month.It might also be necessary to clean the

platechemicallyonceortwiceayear.

3.4 Spray aerator

Sprayaeratorsdividewaterintosmalldroplets,

whichresultsinalargeair-waterinterface(Figure

20).Theenergyconsumptionofsprayaeratorsis

10-50Wh/m3,dependingonthetypeofaerator.

Anadvantageofsprayaeratorsistheeaseofin-

corporationintoexistinginstallations.Thespray

aerators can be placed directly above the lters.

Efciency

When the air is intensively renewed, the efcien-

cyofsprayaeratorscanbecalculatedwiththe

followingequation:

= =

22

2h( k )

g( k t)K 1 e 1 e

The efciency for the addition of oxygen can vary

from65to80%,forthecarbondioxideremoval

the efciency varies from 60 to 80%.

In Figure 21 the efciency of the Dresden-nozzle

forcarbondioxideremovalasafunctionofthe

fallheightisshown.

It is remarkable that after a certain fall height the

efciency remains more or less constant. The

reasonisthataftersometimetheinterfacebe-

Figure 20 - Spraying small droplets of water

2

1

0

0 0.25 0.5 0.75 1

KCO2[-]

h[m]

Figure 21 - Efciency Dresden-nozzle as a function of

the fall height


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16/18

Sprayaeratorscanbedividedintotwogroups:

upward-anddownward-directedsprayaerators.An example of the rst type is the Amsterdam

sprayaerator (Figure 22).In this type ofspray

aerator, two jets are directed perpendicular to

eachother,dispersingthewater.Thisresultsin

manydropletsintheair.Duringthefallofthewa-

ter droplets, the gas transfer takes place.

Anexampleofthesecondtypeofsprayeristhe

Dresdensprayer(Figure23),ortheplatespray-

er. Here, the water ows through a plastic tube

and strikes a disc (plate), shaping the water like

an umbrella, and eventually disintegrating into

droplets.

3.5 Alternative aeration systems

Vacuum gas transfer system

Avacuumgastransfersystemisusuallyexecut-

ed as a tower aerator lled with a packing mate-

rialinwhichthepressureisloweredbyavacuum

pump(Figure24).

Duetothevacuumpump,gasisremovedfrom

thetower, resulting in lower gasconcentrations

and a decreased pressure there. Because the

gasconcentrationsinthetowerarelowerthanin

theatmosphere,thesaturationconcentrationsin

thetowerarealsolower.Becauseofthelowsat-

uration concentrations, it is possible to remove

higherlevelsofgasfromthewaterthanispos-

sible under atmospheric conditions. This makes

a vacuumgastransfer system ideal forremov-

ingdissolvednitrogenandoxygenfromthewater

and is frequently applied before the denitricationprocess.

The efciency of the vacuum gas transfer system

depends onthevacuumpressure thatis main-

tained in the tower. In the absence of an air ow,

the RQ equals zero. Since oxygen is not brought

into thesystem,oxidation of iron cannot occur.

Thisallowsthewatertobepumpedtothenext

treatment process, contrary to a cascade. In

a cascade oxidation of iron does occur, which,

whenthewaterispumpedtothenexttreatment

Figure 22 - Amsterdam sprayer

Figure 23 - Dresden sprayer

A

B

D

E

A influent

B packing materialC air supply

D effluent

E air discharge

Epump

pump

Figure 24 - Representation of a vacuum liquid-gas

exchange


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17/18


18/18

bladeintheformofacone,situatedinabasin

on thewaters surface(Figure 28). Through the

blade, water is abstracted from underneaththeconeandsprayedlaterallyoverthewaterssur-

face.Becausewaterdropletsareformedandair

isentrained,gastransfercanbeachieved.

As a result of the suction of water from under-

neathandthehorizontaldistributionofthewater,

a circular ow 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(MSc

thesis)


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