wt-chapter-6-2

1/3/2012

1

FILTRATION Water Treatment Course AAiT, Zerihun Alemayehu

AAiT Water Treatment By Zerihun Alemayehu

FILTRATION Filtrationinvolvestheremovalofsuspendedandcolloidalparticlesfromthewaterbypassing it througha layerorbedofaporousgranularmaterial,suchassand.

1/3/2012

2


CLASSIFICATION OF FILTERS

Basedonthefiltermedia Sandfilters,e.g.naturalsilicasand Anthracitefilters,e.g.crushedanthraciticcoal Diatomaceousearthfilters,e.g.diatomaceousearth Metalfabricfilters(microstrainers),e.g.stainlesssteelfabricfilter.


CLASSIFICATION OF FILTERS

Basedonthedepthoffiltermedia Deepgranularfilters,e.g.sand,dualmediaandmultimedia(combinationoftwoormoremedia),granularactivatedcarbon

Precoatfilters,e.g.diatomaceousearth,andpowderedactivatedcarbon,filters

1/3/2012

3


CLASSIFICATION OF FILTERS Basedontherateoffiltration,sandfilterscanbefurtherclassifiedas Gravityfilters Slowsandfilters rapidsandfilters highratesandfilters Pressurefilters


RATE OF FILTRATION Rateoffiltration(loadingrate) istheflowrateofwaterappliedperunitareaofthefilter. It isthevelocityofthewaterapproachingthefaceofthefilter:

whereva=facevelocity,m/d=loadingrate,m3/d.m2Q=flowrateontofiltersurface,m3/dAs=surfaceareoffilter,m2

sa A

Qv

1/3/2012

4


EXAMPLE

A city is to install rapid sand filters downstream of theclarifiers. The design loading rate is selected to be 160m3/(m2d).Thedesigncapacityofthewaterworksis0.35m3/s.Themaximumsurfaceperfilterislimitedto50m2.Design thenumberand size of filtersand calculate thenormalfiltrationrate.


EXAMPLE SOLUTION

1/3/2012

5


MECHANISM OF FILTRATION Thetheoryoffiltrationbasicallyinvolves,transportmechanisms,andattachmentmechanisms.

Thetransportmechanismbringssmallparticlesfromthebulksolutiontothesurfaceofthemedia.a) gravitationalsettling,b) diffusion,c) interceptionandd) hydrodynamics.


MECHANISM OF FILTRATION Theyareaffectedbyphysicalcharacteristicssuchassizeofthefiltermedium,filtrationrate,fluidtemperature,sizeanddensityofsuspendedsolids.

Astheparticlesreachthesurfaceofthefiltermedia,anattachmentmechanismisrequiredtoretainit.Thisoccursdueto (i)electrostaticinteractions (ii)chemicalbridgingorspecificadsorption.

1/3/2012

6



SLOW SAND FILTERS

InSSFwaterisallowedataslowratethroughabedofsand,sothatcoarsesuspendedsolidsareretainedonornearthesurfaceofthebed.

Loadingrateof2.9to7.6m3/d.m2 Therawwaterturbidityhastobe

1/3/2012

7


SLOW SAND FILTERS

Gelatinousslimesofbacterialgrowthcalledschmutzdeckeformonthesurfaceandintheuppersandlayer,consistsofbacteria,fungi,protozoa,rotiferaandarangeofaquaticinsectlarvae.

Theunderlyingsandprovidesthesupportmediumforthisbiologicaltreatmentlayer.

SlowsandfiltersslowlylosetheirperformanceastheSchmutzdeckegrowsandtherebyreducestherateofflowthroughthefilter.requiresrefurbishing


CLEANING SLOW SAND FILTERS

Scrapping:thetopfewmmofsandiscarefullyscrapedoffusingmechanicalplantandthisexposesanewlayerof clean sand. Water is then decanted back into thefilterand recirculated fora fewhours toallowanewSchmutzedecketodevelop.Thefilteristhenfilledtofulldepthandbroughtbackintoservice.

wetharrowing:lowerthewaterleveltojustabovetheSchmutzdecke, stirring the sand and therebysuspending any solids held in that layer and thenrunning thewater towaste.The filter is then filled tofulldepthandbroughtbackintoservice.

1/3/2012

8


TYPICAL SLOW SAND FILTER

Sand filter bed

Gravel

Schmutzecke

Supernatant water

Systemofunderdrains

Weir Raw water

Finished water


TYPICAL SLOW SAND FILTER

1/3/2012

9


TYPICAL SSF CONSTRUCTION DETAILS


ADVANTAGES AND DISADVANTAGES

Advantages Simpletoconstructandsupervise Suitablewheresandisreadilyavailable Effectiveinbacterialremoval Preferableforuniformqualityoftreatedwater

Disadvantages Largeareaisrequired Unsuitablefortreatinghighlyturbidwaters Lessflexibilityinoperationduetoseasonalvariationsinraw

waterquality

1/3/2012

10


DESIGN CRITERIA FOR SSF Parameter Recommended level (UK experience)Design lifePeriod of operationFiltration rate Filter bed areaHeight of filter bed Initial Minimum Effective sizeUniformity coefficient Height of underdrains + gravel layer Height of supernatant water

10-15 year24 h/day0.1 0.2 m/h5-200 m2/filter (minimum of two filters) 0.8-0.9 m0.5-0.6 m0.15-0.3 mm< 30.3-0.5 m1 m


EXAMPLE. SSF DESIGN Designaslowsandfiltertotreataflowof800m3/day. Solution:

assumingafiltrationrateof0.15m/h, Requiredtankarea=(800/24)x(1/0.15)=222m2 Useatank23mlongx10mwide. FromTable6.1,theheightofthetankrequireis:

Systemunderdrain+gravel0.5m Filterbed0.9m Supernatantwater1m

Therefore,totaltankheight=2.4mandtankdimensionbecomes23mlongx10mwidex2.4mhigh

1/3/2012

11


RAPID SAND FILTERS Themostcommontypeoffilterfortreatingmunicipalwatersupplies.

Duringfiltration,thewaterflowsdownwardthroughthebedundertheforceofgravity.

Whenthefilteriswashed,cleanwaterisforcedupward,expandingthefilterbedslightlyandcarryingawaytheaccumulatedimpurities.Thisprocessiscalledbackwashing.


ADVANTAGES AND DISADVANTAGES

Advantages Turbidwatermaybetreated Landrequiredislesscomparedtoslowsandfilter Operationiscontinuous.

Disadvantages Requiresskilledpersonnelforoperationandmaintenance Lesseffectiveinbacteriaremoval Operationaltroubles

1/3/2012

12


TYPICAL GRADATION OF RSF

after backwashing, the larger sand grains settle to the bottom first, leaving the smaller sand grains at the filter surface.

Allows in-depth filtration: provides more storage space for the solids, offer less resistance to flow, and allows longer filter runs.


TYPES OF RSF

RSFbasedonfiltermaterial,threetypes: Singlemediafilters:thesehaveonetypeofmedia,usuallysandorcrushedanthracitecoal

Dualmediafilters:thesehavetwotypesofmedia,usuallycrushedanthracitecoalandsand.

Multimediafilters:thesehavethreetypesofmedia,usuallycrushedanthracitecoal,sand,andgarnet.

1/3/2012

13


RAPID SAND FILTER


OPERATION OF A RSF

Terminal head loss. Constant rate filtration

1/3/2012

14


GRAIN SIZE CHARACTERISTICS Sieveanalysisaplotonsemilogpaperofthecumulativefrequencydistribution, Geometricmean(Xg)and Geometricstandarddeviation(Sg)

Effectivesize,E,or10percentile,P10, E=P10=(Xg/Sg)1.282

Uniformitycoefficient,U,orratioofthe60percentiletothe10percentile,P60/P10. U=P60/P10=(Sg)1.535


RSF FILTER MEDIA TYPICAL PROPERTIES

PROPERTY UNIT GARNET LMENITE SAND ANTHRACITE GACEffective Size, ES mm 0.2 - 0.4 0.2 - 0.4 0.4 - 0.8 0.8 - 2.0 0.8 - 2.0Uniformity Coefficient, UC UC 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 1.7 1.3 - 2.4

Density, g/mL 3.6 - 4.2 4.5 - 5.0 2.65 1.4 - 1.8 1.3 - 1.7

Porosity, % 45 - 58 Not available 40 - 43 47 - 52Not

availableHardness Moh 6.5 -7.5 5.6 7 2 - 3 Low

1/3/2012

15


FILTER HYDRAULICS The loss of pressure (head loss) through a clean stratifiedsandfilterwithuniformporositywasdescribedbyRose:

wherehL=frictionalheadlossthroughthefilter,m va=approachvelocity,m/s D=depthoffiltersand,m CD=dragforcecoefficient f=massfractionofsandparticlesofdiameterd d=diameterofsandgrains,m =shapefactorand= porosity


FILTER HYDRAULICS

1/3/2012

16


FILTER HYDRAULICS Thehydraulicheadlossthatoccursduringbackwashingiscalculatedtodeterminetheplacementofthebackwashtroughsabovethefilterbed.

where De = depth of the expanded bed, m = porosity of the bed and s= porosity of the expanded bed f = mass fraction of sand with expanded porosity

Laminar Turbulent


SETTLING VELOCITY

1/3/2012

17


REYNOLDS NUMBER


EXAMPLE 3

A dual medium filter is composed of 0.3 manthracite (mean size of 2.0 mm) that is placedovera0.6m layerof sand (meansizeof0.7mm)with filtration rateof9.78m/h.Assume thegrainsphericityis=0.75andaporosityforbothis0.40.Estimatetheheadlossofthefilterat15oC.

1/3/2012

18


SOLUTION Calculateheadlossforanthracite

Calculateheadlossforsand


EXAMPLE 4 Estimate the clean filterheadloss for a proposed newsandfilterusingthesand.Usethe following assumptions:loading rate is 216 m3/d.m2 ,specificgravityofsandis2.65,the shape factor is 0.82, thebedporosityis0.45,thewatertemperature is 10oC, and thedepthofsandis0.5m.

Sieve No % retain d(mm) 8-12 7.3 2 12-16 17.1 1.42 16-20 14.6 1 20-30 20.4 0.714 30-40 17.6 0.0505 40-50 11.9 0.0357 50-70 5.9 0.0252 70-100 3.1 0.0178 100-140 0.7 0.0126

1/3/2012

19


SOLUTION


SOLUTION

1/3/2012

20


SOLUTION


EXAMPLE 5

Determine the depth of the expanded sandfilterbedbeingdesignedforExample4.

1/3/2012

21


SOLUTION


Any Questions?

Documents

wt-chapter-6-2