ENVE 301 Environmental Engineering Unit Operations
ENVE 301Environmental Engineering Unit OperationsLecture 14Filtration - ISPRING 2014Assist. Prof. A. Evren Tugtas
Filtration Settled water has a turbidity in the range from 1 to 10NTU (typically 2 NTU)Level of turbidity interferes with the disinfectionprocessTurbidity must be reduced further after thesedimentation process.EPA requires treated water turbidity level to be below0.3 NTU (1 NTU is regulatory)Filtration is used to remove turbidity to required levels2
Filtration Filtration is a solid-liquid separation techniqueLiquid passes through a porous medium toremove as much fine suspended solids aspossible.Particulates removed may be those alreadypresent in the source water or generated duringthe treatment processes3
Filtration – what can be removed. SiltClayMicroorganisms (bacteria, viruses, protozoan cycsts)Colloidal or precipitated humic substancesOther natural organic matters from the decay ofvegetationsPrecipitates of aluminum or iron used in coagulationCalcium carbonate, magnesium hydroxide precipitates fromlime softeningIron and/or manganese precipitates4
FiltrationApplications in Water treatment Filtration is applied to chemically coagulated andsettled waters In case of low-turbidity waters, Direct filtration isapplied to remove turbidity5
FiltrationApplications in Wastewater treatment Untreated secondary effluents Chemically treated secondary effluents Chemically treated raw wastewaters6
FILTERTYPESFiltrationGranular Bed Filters1. Single-medium filters(sand or crushedanthracite coal)2. Dual-media Filters(sand and crushedanhracite)3. Multimedia Filters(sand, crushedantracite, garnetPreCoat FiltersMembrane FiltersThin layer ofvery finemedium (e.g.diatomaceusearth)7
Granular Bed FiltrationRef: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 19998
Pre-coat FilterRef: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 19999
FiltrationFilter types according to hydraulic arrangement1. Gravity Filters: Open to atmosphere, flowthrough the medium is achieved through gravity2. Pressure Filters: Utilize a pressure vessel tocontain the filter medium. Water is derived tovessel under pressure and leaves the at slightlyreduced pressure.10
Filtration Filters can also be classified by the rate offiltration (flow rate per unit area)Rapid granular bed filters: higher filtration ratescompared to slow sand filtersSlow sand filters etc.11
Filtration Filtration can also be classified as Depth filtration: solids are removed within thegranular material (e.g. Rapid granular filters)Cake filtration: solids are removed on the enteringface of the granular material (e.g. Pre-coat ormembrane filters)Slow sand filters utilize both depth and cakefiltration12
Filtration After a period of operation (filter cycle), filtersbecome clogged and must be cleanedRapid sand filters are cleaned by backwashing upright high rate flow of waterSlow sand filters are cleaned by scraping the dirtfrom the surface13
Granular-Medium Filtration Filtration mechanism is essentially the same forall the filtersCleaning (backwashing) phase is quite differentdepending on whether the filter operation is ofthe semicontinuous or continuous one.14
Granular-Medium FiltrationParticle removal mechanisms The principal mechanisms of removal of material within thegranular bed filtration are1. Straining2. Sedimentation3. Impaction4. Interception5. Adhesion6. Chemical adsorption7. Physical Adsorption8. Flocculation9. Biological growth15
Granular-Medium FiltrationParticle removal mechanisms1. Straining Mechanical :Particles larger than the pore space of thefiltering medium are strained out mechanically Chance contact: Particles smaller than the pore spaceare trapped within the filter by chance contactRef: Davis M.L. Water and WastewaterTreatment: Design Principles and Practice.2010. McGrawHill16
Granular-Medium FiltrationParticle removal mechanisms2. Sedimentation: Particlessettle on the filteringmedium within the filter3. Impaction: Heavyparticles will not followthe flow streamlinesRef: Davis M.L. Water and WastewaterTreatment: Design Principles and Practice.2010. McGrawHill17
Granular-Medium FiltrationParticle removal mechanisms4. Interception: Many particles are removed whenthey come in contact with the surface of thefiltering mediumRef: Davis M.L. Water and WastewaterTreatment: Design Principles and Practice.2010. McGrawHill18
Granular-Medium FiltrationParticle removal mechanisms5. Adhesion: Flocculant particles become attached to thesurface of the filtering medium Some material is shreared away and pushed deeper into thefilter bed because of the waterRef: Metcalf & Eddy, Inc. (2003). Wastewater Engineering-Treatment and Reuse, 4th ed.,McGraw-Hill, New York, NY.19
Granular-Medium FiltrationParticle removal mechanisms6. Chemical adsorption BondingChemical interaction7. Physical adsorption Electrostatic forcesElectrokinetic forcesVan der Waals forces20
Granular-Medium FiltrationParticle removal mechanisms8. Flocculation: Large particles overtake smallerparticles. These particles are then removed byone of the removal mechanismsRef: Davis M.L. Water and WastewaterTreatment: Design Principles and Practice.2010. McGrawHill21
Granular-Medium FiltrationParticle removal mechanisms9. Biological growth: Biological growth will reduce thepore volume and may enhance the removal ofparticles with any of the removal mechanisms. Substances collected on the surface of filter required nutrients biological growth (slimy layer) Layer known as “schmutzdecke”http://www.slowsandfilter.org/f valve.html22
Ref: American Water WorksAssociation. Water Quality andTreatment: A handbook ofcommunity water supplies. 5th ed.McGraw Hill, 199923
Ref: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in EnvironmentalEngineering. 2nd ed. Boston, MA: PWS Publishing Company, 1996.24
Ref: Reynolds, T. D., and P. A. Richards. Unit Operations and Processes in EnvironmentalEngineering. 2nd ed. Boston, MA: PWS Publishing Company, 1996.25
Removal of Microorganisms by Filtration Giardia lambia and Cryptosporidium parvum arehighly resistant to disinfection – waterborneoutbreaks in North AmericaRapid filtration, slow sand filtration, diatomaceusearth filtration and membrane filtration iseffective in removing pathogenic microorganisms26
Removal of Microorganisms by FiltrationRef: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 199927
Filter Media Common types of filter media are Silica sandAnthracite coalGarnetNaturally occuring high density mineralsIlmeniteOther types of media Granular activated carbon odor removal (filtrationand adsorption)28
Filter Media – Granular Bed Garnet generic term for reffering to severaldifferent minerals – silicates of iron, aluminum andcalcium mixtures Specific gravity range from 3.6-4.2Ilmenite iron titanium ore Specific gravity range from products/productid/26394/productname/Philippine ilmenite sand pebbles/29
Filter Media - Precoat Diatomaceous earth is composed of fossilizedskeletons of microscopic diatoms (most commonprecoat filter medium)Deposits of these materials from ancient lakes oroceans are processed and used in filtrationMean pore sizes of the grades used in watertreatment are 5-17 mRef: iving.com/natural flea killer.html30
Filter Media - Precoat Perlite is the less common precoat filter medium. Itcomes from glassy volcanic rockContains 2-3% water31
Filter Media Filter media has number of properties affectingfiltration performance SizeShapeDensityHardnessPorositySize distribution32
Filter MediaGrain size and size distribution Grain size has an important effect on filtrationefficiency and on backwashing requirements Grain size affects Clean water headlossBuild-up of head loss during the filter cycle33
Filter Media As the size of the granular media gets smaller, poreopenings will be smaller filtration efficiencywould increase.However, as the size of pores decrease, headlossthrough the medium would increaseAs the size of the granular media increase, pore sizewould also increase, which would cause headloss todecreaseHowever, small particles may pass through theselarge pores34
Filter Media Uniform Media:Uniformly graded deep bed filters are relatively coarsemedia ranging from 0.5 mm to 6.0 mmUC is typically 1.2-1.3Greater media depth is substituted for the lack of finemediaDepths of 1.2 – 1.8 are common (some cases 2.4 m)Filters of this type are not expanded during backwashThis filters designed for air and air/water backwash35
Grain Size - Sieve AnalysisRef: http://enginemechanics.tpub.com/14080/css/14080 127.htm36
Filter Media – Size gradation Size gradation of filter media is described by twoparameters Effective size (ES)Uniformity coefficient (UC)ES is that size for which 10% of the grains aresmaller by weight (d10)10% passing point of the curve for sieve analysisUC is the measure of the size range of the media37
Filter Media – Size gradation UC is the ratio of d60 to d10 .(d60) size is also read from the sieve analysiscurve, it is the size for which 60% of the grains aresmalled by weightES d10UC d60/ d1038
Filter Media – Size gradation Required backwash rate of the filter can becalculated by d90 (90% of the grains are smaller byweight)39
Ref: Reynolds, T. D., and P. A. Richards. UnitOperations and Processes in EnvironmentalEngineering. 2nd ed. Boston, MA: PWSPublishing Company, 1996.40
Filter MediaLecture notes of Assist. Prof. Bilge Alpaslan Kocamemi41
Sieve AnalysisRef: American WaterWorks Association.Water Quality andTreatment: Ahandbook ofcommunity watersupplies. 5th ed.McGraw Hill, 199942
Grain Shape and Roundness The shape and roundness of the filter grains isimportant because they affect the backwash flow requirements of the mediumthe fixed bed porositythe headloss for flow through the mediumthe filtration efficiencythe ease of sievingDifferent measures of grain shape have evolved inthe literature – sphericity is an accepted terminology43
SphericityLecture notes of Assist. Prof. Bilge Alpaslan Kocamemi44
Grain Density and Porosity Grain density: Mass per unit grain volumeGrain density is important because it affects thebackwash flow requirements for a filter medium.High density grains require higher wash ratesFixed bed porosity: Ratio of void volume to totalbed volume, expressed as percentage𝑉𝑜𝑖𝑑 ��𝑖𝑡𝑦 𝑇𝑜𝑡𝑎𝑙 𝑏𝑒𝑑 𝑣𝑜𝑙𝑢𝑚𝑒 𝑉𝑇45
Grain Density and Porosity Porosity affects Backwash flow rateFixed-bed head lossSolids holding capacity of the mediumFixed bed porosity is affected by grain sphericitLess spherical particles have higher fixed-bed porosityLow UC no effect on porosityHigh UC nesting of small grains in pores maydecrease the porosity46
Grain Density and Porosity Fixed bed porosity is determined by Placing a sample of known mass and density in a transperenttube with known diameterThe depth of the filter medium is measured to calculate thebed volume𝐺𝑟𝑎𝑖𝑛 𝑉𝑜𝑙𝑢𝑚𝑒 𝑇𝑜𝑡𝑎𝑙 𝑚𝑎𝑠𝑠 𝑖𝑛 ��𝑉𝑜𝑖𝑑 𝑣𝑜𝑙𝑢𝑚𝑒 𝐵𝑒𝑑 𝑣𝑜𝑙𝑢𝑚𝑒 𝑔𝑟𝑎𝑖𝑛 𝑣𝑜𝑙𝑢𝑚𝑒47
Grain Density and Porosity Fixed bed porosity: Affected by the extent ofcompaction in the mediumLoose-bed porosity: Bed is agitated by inversionand allowed to settle freely (no compaction),highest porosity will be obtained48
Ref: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 199949
Rapid Sand FiltrationLecture notes of Assist. Prof. Bilge Alpaslan Kocamemi50
Ref: Metcalf & Eddy, Inc. (2003). Wastewater Engineering-Treatment and Reuse, 4th ed.,McGraw-Hill, New York, NY.51
Desing ParametersRef: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996.52
Ref: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996.53
Ref: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996.54
Ref: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996.55
Ref: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996.56
Desing ParametersLecture notes of Assist. Prof. Bilge Alpaslan Kocamemi57
Lecture notes of Assist. Prof. Bilge Alpaslan Kocamemi58
Lecture notes of Assist. Prof. Bilge Alpaslan Kocamemi59
Lecture notes of Assist. Prof. Bilge Alpaslan Kocamemi60
Lecture notes of Assist. Prof. Bilge Alpaslan Kocamemi61
Underdrain System Underdrain systems have two purposes To collect water that passes through the filter mediaTo distribute the backwash water uniformlySupport gravel is required when openings of theunderdrain system are larger than the filtermedium directly above itUneven distribution of washwater can displacesupport gravel62
Underdrain System There are four basic types of underdrain systems Pipe LateralsBlocksFalse BottomPorous Bottom63
Underdrain SystemPipe LateralsRef: Reynolds, T. D., and P. A.Richards. Unit Operations andProcesses in EnvironmentalEngineering. 2nd ed. Boston, MA:PWS Publishing Company, 1996. Pipe laterals were very popular due to low cost and theiradaptability to be used in pressure filtersProblems: High head loss, poor washwater distributionNot used very often anymore.64
Underdrain SystemPipe Laterals Pipe underdrain systems containcentrally located manifold pipeSmall, equally spaced laterals areattachedLateral pipes have one or two rowsof perforations on their bottom (6 –19 mm).Orifices OR laterals spaced at 8 to30 cmApproximately 45 cm of support Ref: American Water WorksAssociation. Water Treatment Plangravel is usedDesing 4th ed. McGraw Hill, 199865
Underdrain SystemBlocks Vitrified clay blocks arecommonly used6 mm dispersion orificesare located at the top ofeach blockSupport gravel is usedRef: American Water Works Association. Water TreatmentPlan Desing 4th ed. McGraw Hill, 199866
Underdrain SystemBlocksRef: American WaterWorks Association. WaterTreatment Plan Desing4th ed. McGraw Hill,199867
Underdrain SystemFalse Bottoms This underdrain system containsuniformly spaced invertedpyramidal depressionsUnglazed porcelains spheres areplaced at the depressionsSome false bottom underdrainsystems have nozzlesRef: American Water Works Association. Water Treatment Plan Desing 4thed. McGraw Hill, 199868
Lecture notes of Assist. Prof.Bilge Alpaslan Kocamemi69
Lecture notesof Assist. Prof.Bilge AlpaslanKocamemi70
Gravity Filter PipingRef: American Water Works Association. Water Treatment Plan Desing 4thed. McGraw Hill, 199871
Backwash of Rapid Filters In between the filter cycles, filters need to bebackwashed to remove deposited suspendedmaterialBackwashing is required when Headloss across the filter increasesFilter water quality deteriotesMaximum time limit has been exceeded72
Ref: American Water Works Association. Water Treatment Plan Desing 4thed. McGraw Hill, 199873
Backwash of Rapid Filters Backwashing methods Upflow water backwash with full fluidization Surface wash fluidized bed wash Air scour assisted backwashRef: American WaterWorks Association.Water Quality andTreatment: Ahandbook ofcommunity watersupplies. 5th ed.McGraw Hill, 199974
Lecture notes of Assist. Prof. Bilge Alpaslan Kocamemi75
Upflow Water Backwash with full fluidization Washwater is introduced from the bottom of thebed through the underdrain systemBackwash water should be turned on graduallyover a 30 s time interval to prevent beddisturbanceFilter medium gradually assumes the fluidized stateThe backwash flow is continued until the wastewashwater is reasonable clear ( 10NTU)76
Surface wash fluidized bed wash Surface wash has been used extensivelyJets of water from orifices is injected Orificeslocated 2.5 to 5 cm above the surface of the fixedbed.Orifice dia. 2-3 mmSurface jets are operated 1 to 2 min before theupflow washSurface wash is terminated 2-3 min before the endof upflow wash77
Air scour assisted backwash Air scour system supply air to the full filter areafrom orifices located under the filter mediumImproves the effectiveness of the backwash systemIf air scour is used during overflow, there is therish of loosing mediaAir scour system can be used with fine sand, dualmedia and triple media78
Air scour alone before the backwash Water level should be lowered to 15 cm belowthe edge of the backwash overflowAir scour alone should be turned on for 1-2 minAir scour is turned offWaswater is turned-on at a low rate to expel mostof the air from the bed before overflow occursWater wash rate is increased to fluidize andrestratify the bed, and filter is cleanLecture notes of Assist. Prof. Bilge Alpaslan Kocamemi79
Simultaneous air scour and backwash Lower the water level to just above the surface of thefilter medium.Turn on air scour for 1 to 2 minutes.Add low-rate water wash at below half the minimumfluidization velocity as water level rises.Shut off air scour about 15 cm below overflow levelwhile water wash continues. Most air will be expelledbefore overflow.After overflow occurs, increase the water wash rate tofluidize and restratify the bed, and wash until clean.80
Ref: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 199981
Ref: American Water Works Association. Water Quality and Treatment: A handbook ofcommunity water supplies. 5th ed. McGraw Hill, 199982
Lecture 14 Filtration - I SPRING 2014 Assist. Prof. A. Evren Tugtas ENVE 301 Environmental Engineering Unit Operations
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