Water Treatment Lecture 4 EENV
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Transcript of Water Treatment Lecture 4 EENV
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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.SedimentationinwaterTreatment
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.SedimentationinwaterTreatment
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
DistributionDisinfection StorageFiltration
Figure2:FiltrationTreatmentPlant
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GroundWaterWater
RapidMixing Flocculation Sedimentation Recarbonation
DistributionDisinfection StorageFiltration
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
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10Figure6:RectangularsedimentationTank
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11Figure7:CircularsedimentationTank
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12Figure8:CircularsedimentationTank
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Figure9:CircularsedimentationTankSolidcontacttype
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Figure10:CircularsedimentationTankSolid contact type
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Solidcontacttype
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Figure11:RectangularsedimentationTanksludge collection system
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sludgecollectionsystem
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Figure12:RectangularsedimentationTankl d ll ti t
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sludgecollectionsystem
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Figure13:RectangularsedimentationTankl d ll ti t
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sludgecollectionsystem
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Figure14:RectangularsedimentationTankl d ll ti t
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sludgecollectionsystem
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4.SedimentationinwaterTreatment
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
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4.SedimentationinwaterTreatment
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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4.SedimentationinwaterTreatment
Type2
i) Flocculate during sedimentationi) Flocculateduringsedimentation
ii) Sizeconstantlychanging
iii) Settlingvelocityischanging
iv) Settlingvelocityincreasewithdepthandextentofflocculation.
v) Occursduring:
i) Alumorironcoagulation
vi) Concentration:low
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4.SedimentationinwaterTreatment
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.SedimentationinwaterTreatment
4.4Sedimentationtheory
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4.SedimentationinwaterTreatment
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
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4.SedimentationinwaterTreatment
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.SedimentationinwaterTreatment
4.5.1PlainSedimentation:
Dragcoefficientiscalculatedas:
Re
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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
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4.SedimentationinwaterTreatment
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
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4.SedimentationinwaterTreatment
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
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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
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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
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4.SedimentationinwaterTreatment
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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