h= 0.75 mh= 1.5 mh= 2.25 mh= 3.0 m

h/t [m/h]

cum

ulat

ive

freq

uenc

y di

strib

utio

n [%

] 100

80

60

40

20

01 2 3 4 50

Sedimentation

WATER TREATM

ENT

WATER TREATMENT

1

2

3

4

5

t = 0 t = t = 2

tube

constantwater temperature

sample of the solution

silt

trap

100

80

60

40

20

00 0.5 1 1.5 2 2.5 3

distance under water surface [m]

susp

ende

d so

lids

cont

ent

[%] time [s]

600

5400

900120018002700

7200

3600

FrameworkThis module represents sedimentation.

ContentsThis module has the following contents:

1. Introduction2. Theory 2.1 Sedimentation of discrete particles 2.2 Horizontalflowsettlingtanksinpractice 2.3 Settlingefficiencyofasuspension3.Influencesonsettlinginahorizontalflowtank 3.1 Influenceofturbulence 3.2 Influenceofstability 3.3 Influenceofbottomscour3.3 Influenceofflocculantsettling4. Practice 4.1 Determinationofthedimensionsofanidealsettlingtank 4.2 Inlet constructions 4.3 Outlet constructions5. Settlingtankalternatives 5.1 Verticalflowsettlingtank 5.2 Flocblanketclarifier 5.3 Traysettlingtanks 5.4 Tilted plate settling

52

sedimentation water treatment

1 Introduction

Sedimentation is a treatment process in which suspendedparticles,likeflocs,sandandclayarere-movedfromthewater.Sedimentationcantakeplacenaturallyinreser-voirsorincompactsettlinginstallations.Examples of settling installations are the horizon-talflowsettlingtanks,thetiltedplatesettlersandtheflocblanketinstallations.

Sedimentation is frequently used in surface water treatment toavoid rapidcloggingofsandfiltersaftercoagulationandflocformation(Figure1).Sedimentation is applied in groundwater treat-mentinstallationsforbackwashwatertreatment.

Inhorizontalflowsettlingtanks(Figure2)wateris uniformly distributed over the cross-sectionalareaofthetankintheinletzone.Astable,non-turbulent,flowinthesettlingzonetakescareofthesettlingofsuspendedmatterinthe settling zone. Thesludgeaccumulatesonthebottomoriscon-tinuouslyremoved.Intheoutletzonethesettledsludgemustbepre-ventedfrombeingre-suspendedandwashedoutwiththeeffluent.

Sedimentationoccursbecauseofthedifferenceindensitybetweensuspendedparticlesandwa-ter.Thefollowingfactorsinfluencethesedimentationprocess: density and size of suspended particles, water temperature, turbulence, stability of flow,bottomscourandflocculation:- density the greater the density of the par-

ticles, the faster the particles set-tle

- size the larger the particles are, the faster they settle

- temperature the lower the temperature of the wateris,thehighertheviscosity,so the slower the particles settle

-turbulence themoreturbulenttheflowis,theslower the particles settle

-stability instabilitycanresultinashort-cir-cuit flow, influencing the settlingof particles

-bottomscourduring bottom scour, settledparticles are re-suspended and washedoutwiththeeffluent

-flocculation flocculationresultsinlargerparti-cles,increasingthesettlingveloc-ity.

2 Theory

2.1 SedimentationofdiscreteparticlesDiscrete particles do not change their size, shape orweightduringthesettlingprocess(andthusdonotformaggregates).Adiscreteparticle inafluidwillsettleundertheinfluence of gravity. The particlewill accelerate

Reservoir

Fe (III)

by sedimentation

Cl2/ClO2

Floc formation

Floc removal

Ozonation

Filtration

Activated carbon filtration

Clear water reservoir

Figure 1 - Process scheme of a surface water treat-ment plant

sedimentation zone L

Q

Q

Q

Q

B

H

V0

V0

inletzone

outletzone

slib zone

Figure 2 - Horizontal flow settling tank

53

water treatment sedimentation

until the frictional drag force of the fluid equalsthe value of the gravitational force, after whichthevertical(settling)velocityoftheparticlewillbeconstant(Figure3).

The upward directed force on the particle, caused by the frictionaldragof thefluid, canbecalcu-latedby:

2wup D sF c v A

2ρ

= ⋅ ⋅ ⋅

in which:Fup = upwarddirectedforcebyfriction[N]cD = dragcoefficient[-]ρw= densityofwater[kg/m3]vs = settlingvelocity[m/s]A = projectedareaoftheparticle[m2]

Thedownwarddirectedforce,causedbythedif-ference indensitybetween theparticleand thewater,canbecalculatedby:

( )down s wF g V= ρ − ρ ⋅ ⋅

in which:Fdown=downwarddirectedflowbygravity[N]ρs =specificdensityofparticle[kg/m3]g =gravityconstant[m/s2]V =volumeofparticle[m3]

Equalityofbothforces,assumingasphericalpar-ticle,givesasthesettlingvelocity:

s ws

D w

4v g d3 c

ρ − ρ= ⋅ ⋅ ⋅

⋅ ρ

in which:d=diameterofsphericalparticle[m]

Thesettlingvelocityisthusdependenton:- densityofparticleandfluid- diameter(size)ofparticle- flowpatternaroundparticle.

Theflowpatternaroundtheparticleisincorporat-edinthedragcoefficient.Thevalueofthedragcoefficient is not constant, but depends on themagnitudeoftheReynoldsnumberforsettling.For spherical particles theReynolds number isgivenby:

sv dRe

⋅=

ν

in which:ν=kinematicviscosity[m2/s]

Indrinkingwatertreatmentpractice,laminarset-tlingnormallyoccurs.TheReynoldsnumber forlaminar settling of spheres is Re<1, resulting in thefollowingrelationshipbetweentheReynoldsnumberandthedragcoefficient:

SubstitutionofthisrelationshipintheequationforthesettlingvelocitygivestheStokes’equation:

2s ws

w

1 gv d18 v

ρ − ρ= ⋅ ⋅ ⋅

ρ

Thesettlingvelocityisthusdependentonthevis-cosityofthefluidandalsothetemperature.

Fup [N]

Fdown [N]

sedimentation speedVs [m/s]

Figure 3 - Forces on a settling particle

1,000

100

10

1

0.10.1 1 10 100 1,000 10,000 100,000

Reynolds number [-]

resi

stan

ce c

oeff

icie

nt c

D

observed relationship

cD=Re

+ Re

+ 0.34 24 3

cD=Rena

Figure 4 - Relationship between Reynolds number and drag coefficient

54

sedimentation water treatment

Therelationshipbetweenkinematicviscosityandtemperature is:

( )

6

1.5497 10vT 42.5

−⋅=

+in which:

T=temperature[oC]

WhentheReynoldsnumberRe>1600,settlingis turbulentandwhen1<Re<1600,settling is intransitionbetweenlaminarandturbulent.InFigure4therelationshipbetweenthedragco-efficientandtheReynoldsnumber isrepresent-ed.

InFigure5 thesettlingvelocityasa functionofparticle size and density is shown.

2.2 Horizontalflowsettlingtanksinpractice

In practice, settling occurs in flowingwater.Anidealhorizontalflowsettlingtankhasthefollow-ing characteristics: - at the inlet the suspension has a uniform com-

positionoverthecross-sectionofthetank- the horizontal velocity vo is the same in all

partsofthetank- aparticlethatreachesthebottomisdefinitive-

lyremovedfromtheprocess.

Theflowvelocityinahorizontalsettlingtankis:

oQv

B H=

⋅

in which:vo = horizontalflowvelocity[m/h]Q = flow[m3/h]B = widthofthetank[m]H = heightofthetank[m]

The surface loading of a settling tank is deter-minedby:

QqB L

=⋅

in which:q = surfaceloading[m3/(m2•h)]L = lengthofthetank[m]

In Figure 6 the trajectory of a particle is repre-sented. After t1 the water leaves the tank andafter t2 the particle is settled. The particles will settle, therefore, when t2 <t1. Thevelocityoftheparticleisdividedintohorizon-talandverticalcomponentsandthesettlingtimescanbewrittenas:

2 1 ss 0 s s

H L H B H L 1 1t t v qv v v Q v q

⋅ ⋅≤ ⇒ ≤ ⇒ ≤ ⇒ ≤ ⇒ ≥

2 1 ss 0 s s

H L H B H L 1 1t t v qv v v Q v q

⋅ ⋅≤ ⇒ ≤ ⇒ ≤ ⇒ ≤ ⇒ ≥

Inspecialcases,whenthesettlingvelocityequalsthe surface loading, the particle reaches the end ofthetank.Thissettlingvelocityiscalledthecriti-calvelocityvso.It can be concluded that a particlewill only beremovedifthesettlingvelocityisgreaterthanorequaltothecriticalsettlingvelocity(Figure7).

Figure 5 - Settling velocity of discrete spherical parti-cles

T=10oc10,000

100

1

0.01

0.00010.0001 0.001 0.01 0.1 1 10 100

settl

ing

velo

city

[m

m/s

]

diameter [mm]

5000

500

505

1

ρs - ρw =

Figure 6 - Settling in a horizontal flow settling tank

H, t2 v0

L, t1

q

55

water treatment sedimentation

Afterdeterminingthesettlingvelocityofaparticleduring a settling test, the surface loading and thus thedimensionsofthetankcanbedetermined.Itisremarkablethat,intheory,settlinginahori-zontalflowsettlingtankisonlydeterminedbytheflowandthesurfaceareaofthetankandisinde-pendentontheheightofthetank.

Thefractionoftheparticlesthatsettleincasevs <vsois(Figure7):

s s

so so

v T vhH v T v

⋅= =

⋅

in which:T=residencetimeofwaterinthesettlingtank[s]

Theresidencetimeofwaterinthesettlingtankisexpressed as T and equals t1 from Figure 6.

2.3 SettlingefficiencyofasuspensionIn a suspension the fraction of particles with a settlingvelocityhigher than thesurface loadingsettle completely. The fraction with a lower set-tlingvelocitysettlespartly.Theefficiencyisdeter-minedfromthecumulativefrequencydistributionofsettlingvelocitiesobtainedfromasettlingtest.

The settling test is executed in a cylindrical con-tainer(column)filledwithahomogeneoussam-pleofthesuspensiontobetested(Figure8).Atdifferenttimeintervalssamplesaretakenatdif-ferent depths and analyzed for suspended solids, turbidityoranyotherindexthatcanbereducedbysettling.Thedepthismeasuredwiththewatersurfaceasreference.InTable1theanalysesofasettling column test at depth h=1.0 m are repre-sented(Figure8).

Figure 7 - Settling of a suspension in a horizontal flow

1

2

3

4

5

t = 0 t = t = 2

tube

constantwater temperature

sample of the solution

silt

trap

Figure 8 - Settling column and representation of different settling velocities

vs > vso - all particles settle completely

vs = vso - all particles settle completely

vs < vso - some of the particles settle completely

v0vs

v0Hvs

v0h

L

vs

56

sedimentation water treatment

InFigure9thecumulativefrequencydistributionofthesettlingvelocitiesisrepresented.Theratioof samplingdepthand time isgivenasa func-tionoftherelativesolidsconcentration.Thesol-idswiththelowestsettlingvelocitydeterminetheresidence time of a settling system.

Theparticleswithasettlingvelocityhigherthanthecriticalsettlingvelocityvsoareremovedcom-pletely.ThisisrepresentedinFigure9bytheredarrow.Expressing the relativesolidsconcentra-tionforasettlingvelocityofvso as po,thefirstpartofthesettlingefficiencyis:

1 or 1 p= −

in which:r1 = part of the efficiency caused by complete

settling[-]po = relative solids concentration at surface

loading so[%]

From theparticleswitha lower settling velocitythanvso,onlytheparticlesthatenterthetankatareducedheightwillberemoved.Fromthefractionofparticlesdpwithsettlingve-locity vs, only the fraction h/H or vs/vso will beremoved. This part of the efficiency (partial re-moval)canbedescribedby:

o op ps

2 sso so0 0

v 1r dp v dpv v

= =∫ ∫

in which:r2 = partoftheefficiencycausedbypartialset-

tling[-]

The efficiency caused by partial settling is rep-resentedbythebluesurfaceinFigure9dividedby the critical settling velocity. Graphically, thispartofthetotalefficiencycanbedeterminedasshown in Figure 10.

p [%

] 100

80

60

40

20

00 0.5 1 1.5

po

vso vs [10-3 m/s]

vs dp ∫po

0

1vso

equal-sizedsurfaces

Figure 10 - Efficiency of partial settling

p [%

] 100

80

60

40

20

00 0.5 1 1.5

po

vs dppo

0

dp

vso vs [10-3 m/s]

1-po

Figure 9 - Cumulative frequency distribution of settling velocities

Table 1 - Particle concentration and relative particle concentration from a settling test at a depth of h = 1.0 m

t(s) 0 666 900 1800 2700 3600 5400 7200c(ppm) 86 84 79 57 41 29 7 3

p=c/co(%) 100 98 92 66 48 34 8 4

57

water treatment sedimentation

The equation of the total settling efficiency be-comes:

( )op

o sso 0

1r 1 p v dpv

= − + ∫

Fordifferentvaluesofvsotheefficiencyiscalcu-lated and the results are represented in Figure 11. Itcanbeconcludedthatwithincreasingsurfaceloadingof thesettling tank(by increasingflow),thesettlingefficiencydecreases.

3 Influences on settling in a hori-zontalflowtank

In the preceding paragraphs an ideal flow anddiscrete settling were assumed. Inpractice,however,theidealsituationdoesnotexistandtheefficiencyisinfluencedby:- turbulenceofflow- instabilityofflow- bottomscour- flocculation.

3.1 InfluenceofturbulenceInlaminarflowinahorizontalflowtank,aparticlefollows a straight line. Inturbulentflow,eddieswilltransportparticlesinarandomdirection,influencingthesettlingoftheparticles (somesettle faster andothers slower)(Figure12).

WiththeReynoldsnumbertheflowcharacteris-ticscanbedetermined:- laminarflow: Re<2000- turbulentflow: Re>2000.

TheReynoldsnumber forflowinatankcanbecalculated with:

ov RRe

⋅=

ν

in which:R=hydraulicradiusofasettlingtank[m]

Thehydraulicradiusofarectangulartankcanbecalculated with:

B HRB 2 H

⋅=

+ ⋅

With the expression vo=Q/(B•H) the Reynolds

0 1 2 3 4 50

20

40

60

80

100

vso [m/h]

effic

ienc

y r

[%]

Figure 11 - Removal efficiency in a horizontal flow set-tling tank

Figure 12 - Influence of turbulence on settling in a hori-zontal flow settling tank

2 3 4 6 8 2 3 4 6 8 2 3 4 6 81.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 0.001 0.01 0.1 1

2.0

1.5

1.21.11.00.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

VsVso

VsVo

effic

ienc

y [-

]Figure 13 - Influence of turbulence on the efficiency of

settling

58

sedimentation water treatment

numbercanberewrittenas:

Q 1ReB 2 H

= ⋅ν + ⋅

In Figure 13 the settling efficiency for turbulentflow is represented as a function of vs/vso and vs/vo.

Inpracticeturbulenceisnotalwaysadisadvan-tagebecause, ingeneral,flocculantsettlingoc-curs(section3.4).Turbulenceincreasesthecol-lision frequency of particles, thus increasing the efficiencyoftheflocculantsettling.

3.2 InfluenceofstabilityFlow iscalledstablewhenshortcircuitingdoesnot occur. In Figure 14 an example of a short-circuit flowcaused by wind effects is illustrated. The windcreatesadead zone (or eddy) in the corner ofthe settling tank.Thewater flowcan then flow,locally, in the opposite direction from the general flowthroughthetank.

Stability of flow is characterized by the Campnumbercp:

2o

pv

cg R

=⋅

SubstitutingtheequationsforvoandRforarec-tangular tank, the Camp number becomes:

2

p 3 3Q B 2 Hcg B H

+ ⋅= ⋅

⋅

cp>1• 10-5 stableflowcp < 1• 10-5 unstableflow

InFigure15theminimalresidencetime(Ti)andtheaverageresidencetime(Ta)ofwaterdropletsare represented in comparison with the theoreti-calresidencetime(To)fordifferentvaluesoftheCampnumber.

FromFigure15itcanbeconcludedthatthelowertheCampnumberis(andthusmoreshort-circuitflowoccurs),theshortertheminimalandaverageresidence timesbecome.This isdue to thede-crease in theeffective cross-sectionof the set-tling tankand, therefore, toan increase in flowvelocity.Theefficiencyofasettlingtank,therefore,willbelowerthanisthecaseinastableflowcondition.

3.3 InfluenceofbottomscourIn theory, a particle is removed from thewaterwhenitreachesthebottomofthesettlingtank.Inpractice,however,itispossiblethatresuspen-sion of already settled particles occurs.

InFigure16theforcesonparticlesatthebottomofthetankareshown.The shear force of water on a spherical particle is:

lt = × r × 2w scv

8

down

Figure 14 - Short-circuit flow caused by wind

1.0

0.8

0.6

0.4

0.2

010-7 10-6 10-5 10-4 10-3 10-2 10-1

Cp

T0

TTa

T0

Ti

T0

Figure 15 - Short-circuit flow

59

water treatment sedimentation

in which:τ = hydraulicshear[N/m2]λ = hydraulicfrictionfactor(λ=0.03)[-]vsc= criticalscourvelocity[m/s]

Theshear forceofparticlesat thebottom (me-chanicalfriction)isproportionaltothesubmergedweight of the sludge layer:

f = β . (ρs - ρw ) . g . d

in which:f = mechanicalshear[N/m2]β = mechanicalshearfactor(b=0.05)[-]

Inequilibriumthehydraulicshearequalstheme-chanicalshearandthecriticalscourvelocitycanbecalculated:

s wsc

w

40v g d3

ρ − ρ= ⋅ ⋅ ⋅

ρ

Whentheflowvelocityinasettlingtankislowerthanthescourvelocity,bottomscourwillnotoc-cur:

v0<=vsc nobottomscour

Giventhesurfaceloading,thewidthanddepthofasettlingtankcanbedeterminedbasedonthiscriterion.

3.4 InfluenceofflocculantsettlingDuring settling, aggregates are formed as a result of collisions between particles, and settling ve-locities will increase. This phenomenon is called flocculantsettling(Figure17).

InTable2theresultsofasettlingtestofafloc-culant suspension are shown..

In Figure18 the cumulative frequency distribu-tionofsettlingvelocitiesisgivenatdifferenttankdepths.Fromthefactthatthedistributionsdiffer

d

f

N

v0

sediment

bottom of tank

Figure 16 - Bottom scour

h=0.075m h=1.5m h=2.25m h=3.0mt = 0 s 100 100 100 100t = 600 s 93 96 98 99t = 1200 s 81 86 88.5 89.5t = 1800 s 70.5 77.5 81 83t = 2700 s 28 38 46.5 53t = 3600 s 13.5 22 31 40t = 5400 s 3 8 13.5 20t = 7200 s 1.5 3 6 9.5

Table 2 - Relative particle concentration from a settling test

t=0 t= t=2 t=3

Figure 17 - Flocculant settling

60

sedimentation water treatment

h= 0.75 mh= 1.5 mh= 2.25 mh= 3.0 m

h/t [m/h]

cum

ulat

ive

freq

uenc

y di

strib

utio

n [%

] 100

80

60

40

20

01 2 3 4 50

Figure 18 - Cumulative frequency distribution of set-tling velocities at different tank depths

ExampleAhorizontalflowsettlingtankhasaheightof2m, a width of 20 m and a length of 45 m. The flowthroughthetankis0.5m3/sandthewatertemperature is 10 0C

Check if the tankmeets thehydraulic require-ments.

Thehorizontalflowvelocityandthecriticalset-tlingvelocityare:

3o

Q 0.5v 12.5 10 m/ sB H 20 2

−= = = ⋅⋅ ⋅

3SO

Q 0.5v 0.56 10 m/ sB L 20 45

−= = = ⋅⋅ ⋅

Thehydraulicradiusof thetankcanbecalcu-lated with:

B H 20 2R 1.67B 2 H 20 2 2

⋅ ⋅= = =

+ ⋅ + ⋅

TheReynolds number and theCampnumberare:

3o

6v R 12.5 10 1.67Re 15935

1.31 10

−

−⋅ ⋅ ⋅

= = =ν ⋅

( )2325o

p12.5 10v

c 0.954 10g R 9.81 1.67

−−

⋅= = = ⋅

⋅ ⋅

TheReynoldsnumberishigherthan2000andtheflowwillbeturbulent.TheCampnumberisabout1•10-5andnoshort-circuitflowwilloccur.

Determinetheefficiencyofthesettlingtankcon-sidering the suspension from the settling test in Table2.

In Figure 19 the suspended solids content is represented as a function of water depth at the differentsamplingtimesfromTable2.Between0 and 0.75 meters the progress of the graph is estimated(dottedline).Theresidencetimeis:

B H L 20 2 45 3600sQ 0.5⋅ ⋅ ⋅ ⋅

τ = = =

From Figure 19 the efficiency can be deter-mined, assuming a residence time of 3600 sec-ondsandaheightof2meters.Thebluesurfaceabove the line indicates the amount of solidsthat are settled. The green surface below theline indicates the amount of solids that are still in suspension after 3600 seconds. It can beconcluded after measuring the surfaces that the settlingefficiencyis80%.

100

80

60

40

20

00 0.5 1 1.5 2 2.5 3

distance under water surface [m]

susp

ended

sol

ids

conte

nt

[%] time [s]

600

5400

900120018002700

7200

3600

Figure 19 - The relative particle concentration

61

water treatment sedimentation

overtheheightofthetank, itcanbeconcludedthatflocculantsettlingoccurs.

FromFigure19itcanalsobeconcludedthattheefficiencyincreaseswiththeincreasingdepth.Forflocculantsettling,incontrasttodiscreteset-tling,theheightofthetankisofimportancetothesettlingefficiency.

4 Practice

4.1 Determinationofthedimensionsofanidealsettlingtank

Inidealsettlingtankstheflowisstable(cp>10-5)withoutturbulence(Re<2000).At a temperature of 10oC these conditions are metwithahorizontalflowvelocityandahydrau-lic radius of: vo = 6.4•10-3m/sR < 0.41 m

Tanksthatmeettheseconditionsareshort,wideand shallow or long, narrow and deep (Figure20).These constructions, however, are expensive

due to the amount of space they occupy. Inpractice,atankwillbeacompromisebetweenthe Reynolds and Camp numbers, on the onehand, and the construction costs, on the other, limitingthelength/width/depthratios.

4.2 InletconstructionsIn the preceding paragraphs it was assumed that thewaterisuniformlydistributedoverthecross-sectionofthetank,but inpracticethisassump-tion is not totally accurate. For an even distribution of the water over thewidth(anddepth)ofthetank,inletconstructionsare introduced. In Figure 21 an example of an inlet construction is represented.The inlet velocity is reducedbyintroducing several inlet channels, followed byadiffuserwallthatdistributesthewaterovertheentirecross-sectionofthetank.

Adiffuserwall (Figure22)hasopenings todis-tributethewateroverthewidth(anddepth)ofthetank.At theendof thewall, theflowvelocity inthe inlet channel is zero and so is the velocityhead.Theheadlosscausedbyfriction,however,is lower than thedecrease invelocityhead, re-sultinginanincreaseinthepiezometriclevel.Thewater level at the end of the inlet channelis, thus, higher than the level at the beginning.The result is that at the end of the inlet channel morewaterentersthetankthanatthebeginningoftheinletchannel.Toavoidthisunevendistribu-tion,theheadlossovertheopeningsinthedif-fuserwallmustbe larger than thedifference inpiezometriclevelinducedbythedecreaseinflow

Vo

Vo

≈ 0.82

≈ 0.41≈

Figure 20 - Settling tanks with laminar and stable flows

Q

Q4

diffuser wall

sedimentation zone

Figure 21 - Inlet construction

h

v02

2g

Q

0

z h z+

Q

vvi

energy line

Figure 22 - Diffuser wall

62

sedimentation water treatment

velocity.

Inpractices,moreaswellasalternativeinletcon-structionsexist,liketheCliffordandtheStuttgar-terinlet(Figure23).

4.3 OutletconstructionsThe outlet construction is situated at the end of the settling tank and generally consists of anoverflowweir.At the outlet construction, re-suspension of set-tled solidsmust be prevented and the flow ve-locity inanupwarddirectionwill thusbe limited(Figure24).

Theflowvelocityinanupwarddirectionis:

H so1 Qv v5 B H

= ⋅ <⋅

in which:vH=outflowvelocityinanupwarddirection[m/s]

Resulting in:

L 5H<

MosthorizontalflowsettlingtankhaveanL/H>5and thus:

soQ 5 H v

n B< ⋅ ⋅

⋅

Therefore, the lengthof theoverflowweirmustbeseveraltimesthewidthofthetank.Tocreatesufficient length for theoverflowweir,several troughs are placed parallel to eachother(Figure25).

4.4 SludgezoneandremovalIn the sludge zone the solids are accumulated. Theremovalofthesludgecanbedonehydrauli-cally and mechanically.

Hydraulic sludge removal isdoneat regular in-tervalsbydewatering the tankandflushing thesludgewithpressuredwater(fromhydrants)toahopperatthebottomofthetankfromwhereitisremovedbygravityorbypumping.

Figure 23 - Clifford and Stuttgarter inlet

Clifford inlet Stuttgarter inlet

H Vo

L

Vso VH

Figure 24 - Upward velocity to overflow weir

Figure 25 - Overflow weir for effluent discharge

63

water treatment sedimentation

Mechanicalsludgeremovalisfrequentlyappliedwhensludgevolumesarelargeorthesludgeisunstable, resulting in anaerobic decompositionduring storage in the sludge zone. Mechanicalsludgeremovalconsistsofscrapersthat transport the sludge to a hopper in the mid-dleofaroundsettlingtankorneartheinletofarectangulartank.Fromthehopper,thesludgeisremoved.

5 Settlingtankalternatives

5.1 VerticalflowsettlingtankInverticalflowsettlingtankstheinletofthewa-tertobetreatedissituatedatthebottomofthetankandthewaterflowsinanupwarddirection(Figure26).Theflowvelocityequals,inthiscase,thesurfaceloading:

o oQv s

B L= =

⋅

Theresultisthatonlyparticleswithasettlingve-locity higher than the upflow velocity will settleandotherswillbewashedout:

s>=s0 settles completely

s < s0 does not settle

Thesettlingefficiency is entirely determinedbythe particles that settle completely (see Figure9):

or 1 p= −

Thesettlingefficiencyofdiscreteparticlesinver-ticalflowsettlingtanksislowerthaninhorizontalflowtanks,andverticalflowtanksare thereforenotusedfordiscrete,totallyflocculated,suspen-sions. In the case of flocculant settling, vertical flowtanksareused(e.g., in the formofflocblanketclarifiers).

5.2 FlocblanketclarifierThe floc blanket clarifier consists of a (conical)verticalflowtank(Figure27).Coagulantisdosedattheinletoftheclarifierandfloc formation occurs in the installation. Small,lightflocswithasettlingvelocity lower than theupflow velocity are transported with the waterflowinanupwarddirectionandcollidewithlarg-er,heavierflocs.Afterattachment,thesettlingve-locityincreasesuntiltheyreachthebottomofthe

Q

H

V0

Vs

Figure 26 - Vertical flow settling

Q/A

sludgedischarge

floc blanket

sludgedischarge

A

heavy sludge disposal possibility

Q

Figure 27 - Floc blanket installation

64

sedimentation water treatment

tank,whereasludgeremovaldeviceisinstalled.

Flocs that do not form aggregates are transport-ed to the top of the installation where the surface area is the largest and the upflow velocity thelowest,andaflocblanketisformed.Theflocblanketismanipulatedbyadrainattherequiredthickness.Theflocblankethasafilter-ing effect for even small flocs. Therefore, highefficienciescanbe realizedwith relativelyshortresidence times.

The plant at Berenplaat in Rotterdam is where one of the floc blanket clarifiers in the Nether-landsislocated(Figure28).Theinletofthewaterisat the topof the installation.Astirringdevicecreatesturbulenceintheflocformationchambertoincreasethecollisionfrequency.Afterleavingthemixingchamber,theflocsformablanketandtheeffluentwaterisdrainedbytroughs.

5.3 TraysettlingtanksThe efficiency of discrete settling can be in-creased by applying horizontal baffles (falsefloorsortrays)(Figure29).Insection2.2itisshownthatthesettlingefficiencyfor discrete particles is independent of the height ofthesettlingtank.Theapplicationofhorizontalbafflesgivesadouble surfaceareaandhalf ofthe surface loading, resulting in an increase in ef-ficiency.Horizontalbafflesalsoimprovetheflowpattern.

TheReynoldsnumberdecreasesandtheCampnumberincreases.Theflowbecomeslessturbu-lentandmorestable.

5.4 TiltedplatesettlingIntiltedplatesettlingtanks,waterpassesbafflesthat are placed at a steep angle.

InFigure30anexample isgivenof a counter-currenttiltedplatesettlingtank.Thewaterflowsin upward direction and the sludge settles on the plates and slides down. The angle of the plates must be about 55o to 60o to secure sludge re-moval.InFigure 31 the flow in a counter-current tiltedplatesettlingtankispictured.Using geometry the following surface loading can bederived:

o ow ts ' s

H cos w+

= ⋅⋅ α +

in which:so’= verticalsurfaceloading[m/h]w = distancebetweenplates[m]t = thicknessofplates[m]H = heightofplates[m]α = angleofplateswiththehorizontal[o]

In a similar way the surface loading for a co-cur-rentflowcanbederived:

f

e

dc

b

a

a raw water feedb stirring mechanismc blending spaced floc blankete clear water dischargef sludge discharge

Figure 28 - Floc blanket installation at Berenplaat

Vo

V‘so = Vso/2

Vo

V‘so = Vso/2

Figure 29 - Tray settling

Q

sedimentation zone, surface A

Figure 30 - Counter-current tilted plate settling

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water treatment sedimentation

o ow ts ' s

H cos w+

= ⋅⋅ α −

In Table 3 the design parameters of Figure 31thatareappliedinpracticearegiven.The angle of the plates in co-current systems can begentlerthanincounter-currentsystemswith-outdeterioratingthesludgeremoval.SubstitutingthevaluesofTable3intotheequa-tionsforsurfaceloadingforbothco-currentandcounter-current systems results in:

oo

ss '

20≈

Thespaceoccupiedbytiltedplatesettlingtanksis thus a factor 20 smaller than is needed for hori-zontalflowtanks.

BoththeCampnumberandtheReynoldsnumberdepend on the hydraulic radius and the horizontal flowvelocity.

InFigure32thestabilityboundary,cp>10-5, and theturbulenceboundary,Re<2000,aregiven.Inaddition,thecombinationsofhydraulicradiusandhorizontalflowvelocityofhorizontalflowandtiltedplatetanksappliedinpracticeareshown.From the graph it can be derived that the flowinhorizontalflowtanksisturbulentandinsomecasesinstable(andshort-circuitflowcanoccur).Theflowintiltedplatetanks,however,isfavora-ble.TheReynoldsnumberisalwayssmallerthan2000, resulting in laminar flow; and the Campnumberisalwayshigherthan10-5, resulting in a stableflowwithoutshort-circuiting.

Designparameters Valuecounter-current 550 - 600

co-current 300 - 400

H 1 - 3 mw 3.4 - 8 cm

t 5 mm

Table 3 - Design parameters

100.000

10.000

1.000

0.100

0.010

0.0010.0001 0.001 0.01 10.1 10

horizontal flow sedimentation tanks

tilted platesedimentation

Cp >

10

-5

vs <0.

05 m

/s

Re < 2,000

hydr

aulic

rad

ius

[m]

horizontal flow speed v0 [m/s]

Figure 32 - Hydraulic conditions for optimal settling

q

H

Vo

V‘so

L

t

w

w sin L cos

Figure 31 - Flow through a tilted plate settler

Furtherreading

Sedimentationandflotation,TU-Delft(2004)Water treatment: Principles and design, MWH(2005),(ISBN0471110183)(1948pgs)

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sedimentation water treatment