IslamicUniversityofGazaEnvironmentalEngineeringDepartment

Water TreatmentWaterTreatmentEENV4331

Lecture5:Filtration

Dr.Fahid Rabah

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5.FiltrationinwaterTreatment

5.1 Definition of Sedimentation:Filtration is a solid liquid separation process in which theliquid passes through a porous medium to remove asmuch fine suspended solids as possible

5.2 Locations of filtration tanks in water treatment:Filtration tanks are used in all types of water treatmentplants except for disinfection treatment plants. SeeFigures 5.1 through 5.4 illustrating the location of filtrationtanks.

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River

Screen

RiverWater

PreSedimentation

Screen

Coagulation FlocculationSedimentation

DistributionDisinfection StorageFiltration

Figure5.1:FiltrationTreatmentPlant (RiverWater)

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Screen

Surfacewater

S di t tiCoagulation Flocculation

Sedimentation

DistributionDisinfection StorageFiltration

Figure5.2:FiltrationTreatmentPlant

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GroundWaterWater

RapidMixing Flocculation Sedimentation Recarbonation

DistributionDisinfection StorageFiltration

Figure5.3:SofteningTreatmentPlantSinglestagesofteningsoftening

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GroundWaterwell

FiltrationAeration

DistributionDisinfection Storage

Figure5.4:AerationTreatmentPlant( iron and manganese removal plant)(ironandmanganese removalplant)

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5.FiltrationinwaterTreatment5 3 Need for filtration:

Settlingisnotsufficienttoremoveallparticlesand5.3 Need for filtration:

flocsfromwater. FiltrationNeededforfineparticlesnotremovedbydi t tisedimentation.

FilterscanalsocaptureGiardia cysts, viruses,andasbestos fibersasbestosfibersTypicaloverflowqualitiesfromsedimentationtanksrangefrom1to10NTU.Filtration,usuallyrapidsandfiltration,isthenemployedforfurtherpolishing,i.e.togettheturbidity to lower than 0 5 NTU (as required byturbiditytolowerthan0.5NTU(asrequiredbylegislation).Rapid sand filtration after prior sedimentation is the

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Rapidsandfiltrationafterpriorsedimentationisthemostcommonconfigurationworldwide

5.FiltrationinwaterTreatment

5.4 Types of filters used in water treatment:Granular material filters are the most used types of filtersin water treatment. Usually sand, anthracite, and Garnet.y

There are three types of granular filters:1. Single medium filters :

one type of media is used: either sand or anthraciteone type of media is used: either sand or anthracite

2. Dualmedia filters: two types of media is used usually sandand anthraciteand anthracite

3. Multimedia filters: three types of media are used usuallysand , anthracite , and Garnet

Most famous filters in water treatment are Rapid Sand Filters.Most famous filters in water treatment are Rapid Sand Filters.

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5.FiltrationinwaterTreatment

5.5 Geometry and components of Rapid Sand Filter:y p p

Rapid sand filters are always rectangular tanks.

Figures 5.5 to 5.10 show typical Rapid sand filters used inwater treatment.

Main components of Rapid sand filter are: Main components of Rapid sand filter are:1. A concrete tank2. Filter media3 Under drain system3. Under drain system4. Backwash system: pressurized water and air lines

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Figure5.5:Rapidsandfiltercomponents

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Figure5.6a:Rapidsandfiltercomponents: withgraveland

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p p gperforatedpipesunderdrainsystem

Fi 5 6b

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Figure5.6b:Rapidsandfiltercomponents: withgravelandperforatedpipesunderdrainsystem

Figure5.7:Rapidsandfiltercomponents: withgraveland

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p p gperforatedpipesunderdrainsystem

Fi 5 8

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Figure5.8:Rapidsandfiltercomponents:withductsunder drainsystem

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Figure5.9:Rapidsandfiltercomponents:withnozzleunder drainsystem

5.FiltrationinwaterTreatment

Figure 5.10 :Figure5.10:Rapidsandfilterperforatedslabandnozzleunderdrain

system

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5.FiltrationinwaterTreatment

Figure5.11:Nozzle used in Rapid sand filterNozzleusedinRapidsandfilterunderdrainsystem

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5.FiltrationinwaterTreatment

5.6 Operation of Rapid Sand Filter:

There are two modes of operation of Rapid sand filterThere are two modes of operation of Rapid sand filter

Filtration mode ( see Figure 5.12 )

Backwashing mode ( see Figure 5.13 )

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5.FiltrationinwaterTreatment

5 7 Filtration mode:

Waterflowsdownwardthroughabed5.7Filtrationmode:

ofsandandgravel Particles are captured on and between Particlesarecapturedonandbetweensandgrains

Filteredwateriscollectedintheunderdrain, sent to disinfectiondrain,senttodisinfection

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5.FiltrationinwaterTreatment

5 8 Backwash mode:

Sand is backwashed when

5.8Backwashmode:

Sandisbackwashedwhen Itbecomesclogged,or Turbidityoffilteredwatergetstoohigh

During backwash, water is pumpedDuringbackwash,waterispumpedupwardsthroughthesandbed

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5.FiltrationinwaterTreatment

Sandbecomesfluidized,andparticlesareflushedfromthesand

Dirtybackwashwaterispumpedintoasettling pond and eithersettlingpond,andeither Recycledbackintoplant,orDi d Disposed

Backwashingcanconsume1%to5%ofaplantsproduction

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22Figure5.12:Rapidsandfilterduring

filtration

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Figure5.13:Rapidsandfilterduringbackwashing

5.FiltrationinwaterTreatment

5.9Filtermediaproperties

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Figure5.14:filtermediagraindistribution

5.FiltrationinwaterTreatment

h f l d d h d h lThesefiltersusesandandcrushedanthracitecoalonagradedgravelbase.

Medialayersarearrangedinacoursetofinegradationinthedirectionofflow,whichallowsgreaterdepthofpenetrationoffloc particles.p

Multimediafiltersareselectedwithspecificgravitiessothatmoderate intermixing between media layers occurs duringmoderateintermixingbetweenmedialayersoccursduringbackwashing.

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5.FiltrationinwaterTreatment5.10FiltermediapropertiesThefiltermediaischaracterizedbytwomainparameters: the effective size and the uniformity

Effectivesizeofthefiltermedia

parameters:theeffectivesizeandtheuniformitycoefficient.

Th ff ti i f th di i th di tTheeffectivesizeofthemediaisthediameterthat10%ofthefiltermediaislessthanitsizeandisdenotedasd10.

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ddU

Uniformitycoefficientofthefiltermedia

10dU=Uniformitycoefficientd60 =sievesizethatpasses60%byweightd10 =sievesizethatpasses10%byweight

d60 andd10 arefoundbysieveanalysisofthemediatobeusedinthefilter.

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Anotherimportantsievesizeisd90 thatisusedtocalculatethebackwashrate.

5.FiltrationinwaterTreatment

27Figure5.15:Rapidsandmedialayers

Figure5.16:Rapidsandmedia layersmedialayers

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5.FiltrationinwaterTreatmentTable 5.1Table5.1

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5.FiltrationinwaterTreatmentTable 5 2Table5.2

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5.FiltrationinwaterTreatmentTable 5.3Table5.3

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5.FiltrationinwaterTreatment

Figure5.17:Headlossandeffluentturbidityincreasewithtimeduringfiltration

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y g

5.FiltrationinwaterTreatment

Figure5.14:Headlossinrapidsandfilter during filtration cycle

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filterduringfiltrationcycle

Figure5.14:Headlossinrapidsandfilter

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g p

Headlossinacleanfilter

CarmenKozeny equation:

2 vAV

gk

Lh 2

3

2)1(

dAV

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sAQv

Where,k=dimensionlesscoefficient,5forsand,6foranthracite;v filtration rate m3/m2 d or filtration velocity m/d

s

v=filtrationratem3/m2.d,orfiltrationvelocitym/d.A=thegrainsurfacearea;As=surfaceareaofthesandfilter;V= the grain volume;V=thegrainvolume;=filterporosity;around0.40forsandfilter=shapefactor;1forsphericalparticles,0.70forsand;=dynamic viscosity; N s/m2

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=dynamicviscosity;N.s/m=waterdensity;kg/m3

h=headlossincleanfilter,m

Example5.1:

Adualmediafilteriscomposedof0.30manthracite(meanparticlesize0.20mm)thatisplacedovera0.60mlayerofsand(meanparticlesize0.70mm)

/withafiltrationrateof9.78m/h.Assumethegrainsphericity =0.75andporosity()=0.40forboth.Estimatetheheadlossinthecleanfilterat150C.

A H d l i th th it lA.Headlossintheanthracitelayer:

mh 0508.000272.0*0020*750

6400*1000*819

)40.01(*00113.0*6*30.02

3

2

B.Headlossinthesandlayer:

002.0*75.040.0*1000*81.9 3

2

mh 6918.000272.0*007.0*75.0

640.0*1000*81.9

)40.01(*00113.0*5*60.02

3

2

B.Headlossinthesandlayer:y

mhtotal 743.06918.00508.0

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Headlossduringfiltration(None clean filter)(Nonecleanfilter)

Where

filterdtl bVavh Where,v=filtrationratem3/m2.d,orfiltrationvelocitym/d.a,b=coefficientsdependingonthefiltermediaproperties;Vfilt d = filtered volume per unit area of filter since last backwash; m3/m2Vfiltered filteredvolumeperunitareaoffiltersincelastbackwash;m /m(hl)t =headlossatanytime(t),m

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Example5.2:

Afilterhasaheadlossof0.30mwhenclean(newlywashed),and1.30mafter24hrsoffiltrationatarateof1.5L/s.m2 .Estimatetheheadlossbothimmediatelyafterbackwashand10hrslater,ifthefiltrationrateischangedto2L/s.m2 .

A.Estimatethevaluesofaandb:

5.15.1 0*

10005.130.0 ba

3600*24*1000

5.1*1000

5.130.1 ba

Bysolvingthe2equationssimultaneously,a=200,b=5.14

B.Calculateheadlossforthenewflowrate:

2 mbH 40.00*20010002

0

mH 88.13600*10*10002*14.5200

10002

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1000100010

FiltrationhydraulicsCalculations

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FiltrationhydraulicsCalculations

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FiltrationhydraulicsCalculations

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Calculationsoffilterbackwashrate

Thebackwashflowrateiscalculatedusingthefollowingequations:

90

5.0

90

7.330408.069.1135d

Gd

v nb

3d 2

390

gsd

Gn

Where,b k h t 3/ 2 d

bdesignb vv 3.1vb =backwashratem3/m2.d,d90 = sievesizethatpasses90%byweight=dynamicviscosity;N.s/m2

= water density; kg/m3 = waterdensity;kg/m3

s = filterparticlesdensity;kg/m3

Gn =Galileonumber,dimensionlessg = gravitational acceleration m/s2

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g= gravitationalacceleration,m/s

Calculationsoffilterexpansion

Theexpansionduringbackwashiscalculatedusingthefollowingequations:q

ee LL

11 22.0

s

designbe v

v s

Where,L= beddepthduringfiltration,mL = expanded bed depth mLe =expandedbeddepth,me =expandedbedporosity,dimensionless=bedporosityduringfiltration,dimensionlessv = settling velocity of the filter particles m/svs=settlingvelocityofthefilterparticles,m/s

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Calculationsofheadlossduringfilterbackwash

Headlossduringbackwashingiscalculatedusingthefollowingequation:q

seeLh 1

Where,Le =expandedbeddepth,me =expandedbedporosity,dimensionless

d k / 3 =waterdensity;kg/m3

s =filterparticlesdensity;kg/m3

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BackwashhydraulicsCalculations

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BackwashhydraulicsCalculations

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BackwashhydraulicsCalculations

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BackwashhydraulicsCalculations

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FiltrationhydraulicsCalculations

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FiltrationhydraulicsCalculations

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FiltrationhydraulicsCalculations

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