water treatment lecture 4 eenv

IslamicUniversityofGazaEnvironmentalEngineeringDepartment

Water TreatmentWaterTreatmentEENV4331

Lecture4:Sedimentation

Dr.Fahid Rabah

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4.SedimentationinwaterTreatment

4.1 Definition of Sedimentation:It is the process of removing solid particles heavier than water

by gravity force.

Particles that will settle within a reasonable period of timecan be removed using a sedimentation tank (also calledclarifiers).

Sedimentation is used in water treatment at the locationsindicated in Figures 1 through 4.

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4.2 Applications of sedimentation in water treatment:pp

1. Plain settling ( or presedimentation) of river surfacewater .

2. In filtration treatment plants treating surface water toremoves flocculated solids. The sedimentation tank comesafter the flocculation tank.

3. In Softening treatment plants treating hard water toremoves flocculated solids. The sedimentation tank comesft th fl l ti t kafter the flocculation tank.

4. In aeration treatment plant removing iron and manganesefrom ground water

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4.3 Geometry of sedimentation tanks:y

Sedimentation tanks are either rectangular or circulartanks.

Figures 5 to 10 show typical sedimentation tanks used inwater treatment.

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River

Screen

RiverWater

PreSedimentation

Screen

Coagulation FlocculationSedimentation

DistributionDisinfection StorageFiltration

Figure1:FiltrationTreatmentPlant (RiverWater)

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Screen

Surfacewater

S di t tiCoagulation Flocculation

Sedimentation


Figure2:FiltrationTreatmentPlant

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GroundWaterWater

RapidMixing Flocculation Sedimentation Recarbonation


Figure3:SofteningTreatmentPlantSinglestagesoftening

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GroundWaterwell

FiltrationAeration

DistributionDisinfection Storage

Figure4:AerationTreatmentPlant( iron and manganese removal plant)(ironandmanganese removalplant)

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9Figure5:RectangularsedimentationTank

10Figure6:RectangularsedimentationTank

11Figure7:CircularsedimentationTank

12Figure8:CircularsedimentationTank

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Figure9:CircularsedimentationTankSolidcontacttype

Figure10:CircularsedimentationTankSolid contact type

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Solidcontacttype

Figure11:RectangularsedimentationTanksludge collection system

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sludgecollectionsystem

Figure12:RectangularsedimentationTankl d ll ti t

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4.3TheoreticalbackgroundonSedimentation

4 3 1 Type of particles :4.3.1Typeofparticles:

1) Discrete / individual particle

- Size, velocity constant during the settling

D it 2 000 2 200 k / 3- Density 2,000 2,200 kg/m3

2) Flocculent particles

- Size, velocity fluctuates during the settling

- Particles flocculate and grow bigger in size

- Density 1,030 1,070 kg/m3

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Density 1,030 1,070 kg/m


4.3.2Threeclassesofparticlessettling:

Type 1Type1

i) Particlessettlediscretelyataconstantvelocity

ii) Settleasindividualparticlesanddonotflocculate.

iii) E.g. : Sand, grit materialiii) E.g.:Sand,gritmaterial

iv) Occursduring:

i) Presedimentationforsandremoval

ii) Settlingofsandduringrapidsandfiltercleaning

v) Concentration:verylow

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Type2

i) Flocculate during sedimentationi) Flocculateduringsedimentation

ii) Sizeconstantlychanging

iii) Settlingvelocityischanging

iv) Settlingvelocityincreasewithdepthandextentofflocculation.

v) Occursduring:

i) Alumorironcoagulation

vi) Concentration:low

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Type3orZone

i) Settleasmassandformalayer blanket

ii) Concentration high (greater than 1000 mg/L)ii) Concentrationhigh(greaterthan1000mg/L)

iii) Distinctclearzoneandsludgezonearepresent.

iv) Occursduring:

i) Limesofteningsedimentation

ii) Sludgethickenersinwatertreatment.

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4.4Sedimentationtheory

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Settlingvelocity(vs)mustbedeterminedtoassuregoodsedimentationtankdesign.

Overflowrate(vo)mustbesetatsomevalueLESSTHANorEQUALtoVs

tHvs ss A

QHWl

HQVH

tHv

**ts

Q

s

Q

sAQv 0But

WhereQ =flowrate(m3/h)

f ( 2)

svv 0So

As =surfacearea(m2)H =depthofwater,mW=tankwidth,mL = tank length m

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L=tanklength,mt =detentiontime,hr


4.5DesignofSedimentationtanks

4 5 1 Plain Sedimentation:4.5.1PlainSedimentation:

Particlessettleseparately:

s

sC

dgV3

)(4 DC3Vs = settling velocityVs settlingvelocityD=particlediameter=waterdensitys =particledensitys p yCD =dragcoefficients

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4.5.1PlainSedimentation:

Dragcoefficientiscalculatedas:

Re

4.SedimentationinwaterTreatment4 5 D i f S di t ti t k4.5DesignofSedimentationtanks

4.5.1PlainSedimentation:

Atlaminarflow,settlingvelocityequationissimplifiedtoStockslaw:

18)( 2Dgv ss

Vs = settling velocity

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Vs settlingvelocityD=particlediameter=waterdensitys =particledensitys p yCD =dragcoefficients

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4.SedimentationinwaterTreatmentExample 4 1 :Example4.1:Findthesettlingvelocity(vs)forsandparticleswithadiameterof0.020mm. =2650kg/m3,=1.002X103 N.s/m2 at200C, =0.05,=0.03.whatisvsforparticleswith D = 0 5 mm?withD=0.5mm?Solution:AssumefirsttheflowislaminarandcheckforReynoldsnumber:

)10*020)(10002650(809 423

R


so 84.034.01103

11024 DC 110110D

310*50)10002650(819*4 smvs /11.01000*110*3

10*5.0)10002650(81.9*4

SolveagainforRe : 55eR R>1,soitstransitionalflowO.K

SolveagainfirCD :

Solve again for vs :

18.1DCsmvs /10.0Solveagainforvs: s

dmmdmsmvvTake /31/31/10*593 234 29

dmmdmsmvvTake s ./31/31/1059.30


4.5Designtanks

4 5 1 Plain Sedimentation:4.5.1PlainSedimentation:

Example4.2:Designpresedimentationtankstobeusedtoremovegritandsandfromariver

h i d d 20000 3/d d i ki U h fl dwaterthatisusedtoproduce20000m3/ddrinkingwater.Usetheoverflowrateandhorizontalvelocitiescalculatedinexample4.1.Usetwotanks.Solution:

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dmmvvTake s ./3123

0 lwQ

AQv

s *0

*Flow/tank=(Q/2)=20000/2=10000m3/dhAs=10000/31=322.5m2

SelectWidthtolengthratio1:4A W* 4W 322 5 2 W 8 98 L 35 92 (t k W 9 L 36 )A=W*4W=322.5m2W=8.98m,L=35.92(takeW=9m,L=36m)Assumedetentiontime=3hrs

H=t*V0 =(3*31)/24=3.88m3.90mV Q/(W*H) 10000/(24*60*9*3 9) 0 198 m/min

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Vh =Q/(W*H)=10000/(24*60*9*3.9)=0.198m/minTakeweirloadingrate=250m3/m.d:Lweir =Q/Wload =10000/250=40m,Usesuspendedtroughsinsidethetank.

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Figure15:RectangularsedimentationTank

4.SedimentationinwaterTreatmentExample4.3:Repeatexample4.3usingcirculartank.Note:themaximumtankdiameteris40mSolution:

dmmvvTake s ./3123

0 sAQv 0

*Flow/tank=(Q/2)=20000/2=10000m3/dAs=10000/31=322.5m2

As =D2/4=322.5m2D=20.27m20.30m40mO.KAvailable W Q/L 10000/88 23 113 34 m3/m2 d < 250 m3/m d O K

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AvailableWload =Q/L=10000/88.23=113.34m3/m2.d

H=sidewallheight(tankdepth)Din=H

Dtotal

inDout =H+DDtotal=3H+DD=diametercalculatedfrom

DinDout

theOverflowrate D/2

O tl t H

Dtotal

Outletzone=H

Dtotal

D/2 DinDout H

Figure16:Circularsedimentation Tank

inletzone=H

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sedimentationTankDimensionsdefinition

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Figure17:CircularsedimentationTank


4.5DesignofSedimentationtanks

4 5 2 Flocculent Sedimentation ( type 2 and 3):4.5.2FlocculentSedimentation(type2and3):

Thedesignprocedureforsedimentationtanksoftype2and3arethesameastype1.Thedifferenceismainlyintheoverflowrate(v0). Thefollowingtablegivesthedesigncriteriaofthesetwotypes.

Process Settling type Detention Surface Weir loadingProcess Settlingtype Detentiontime(h)

Surfaceloadingrate(m3/m2.d)

Weirloadingrate

(m3/m.d)

Coagulation 2 48 2040 250

softening 3 26 4060 250

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water treatment lecture 4 eenv

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