Improving WWTP Design And Operations Through 3D CFD

Improving WWTP Design andOperations Through 3D CFDModeling(Application of CFD to WastewaterProcess Engineering)Randal W. Samstag, P.E., B.C.E.EPrincipal TechnologistCarolloTemplateWaterWave.pptxEd Wicklein, P.E.Senior TechnologistCarollo Engineers

CarolloTemplateWaterWave.pptx“If we know what is happening withinthe vessel, then we are able to predictthe behavior of the vessel as a reactor.Though fine in principle, the attendantcomplexities make it impractical to usethis approach.” – Octave LevenspielChemical Reaction Engineering (1972)Computational fluid dynamics (CFD) changesthis picture. Using CFD, we can computethree-dimensional velocity fields and followinteractions of reactants and productsthrough a tank. We can use this informationto optimize tank geometry.2

CFD can calculate the velocity fields inprocess tanks and channels.Hydraulic ModelCarolloTemplateWaterWave.pptx– Continuity (massconservation) ρ ρ U i 0 t X j3

CFD can calculate the velocity fields inprocess tanks and channels.Hydraulic ModelCarolloTemplateWaterWave.pptx– Continuity (massconservation)– Momentumtransport ρ ρ U i 0 t X j U i U i P U iρ ρU j (ν t) Fi t X j X i X j X j4

CFD can calculate the velocity fields inprocess tanks and channels.Hydraulic Model– Continuity (massconservation)– Momentumtransport– k-epsilon turbulencemodel ρ ρ U i 0 t X j U i U i P U iρ ρU j (ν t) Fi t X j X i X j X jν t CµCarolloTemplateWaterWave.pptx ( ρk ) ( ρkU i ) t X i X j ( ρε ) ( ρεU i ) t X i X jk2ε µt k µ Pk Pb ρε YM Skσ k X j µt ε εε2 µ C1ε ( Pk C3ε Pb ) C2ε ρ Sεσ Xkk k j 5

CFD can calculate the velocity fields inprocess tanks and channels.Hydraulic Model ρ ρ U i 0 t X j– Continuity (massconservation) U i U i P U i– Momentumρ ρU j (ν t) Fitransport t X j X i X j X j– k-epsilon turbulencek2modelν t Cµε– Control volume µt k solution scheme ( ρk ) ( ρkU i ) µ Pk Pb ρε YM SkCarolloTemplateWaterWave.pptx t X i ( ρε ) ( ρεU i ) t X i X j X j σ k X j µt ε εε2 µ C1ε ( Pk C3ε Pb ) C2ε ρ Sεσ Xkk k j 6

But CFD can also be used as to tracksolids flow and reactions in the tank.φinCarolloTemplateWaterWave.pptxφout7

But CFD can also be used as to tracksolids flow and reactions in the tank.φinCarolloTemplateWaterWave.pptxφoutA typical transport model φ U i φ ν t φ () Si t X i X i σ s X i8

3D Solids Transport ModelCarolloTemplateWaterWave.pptxIndividual Control Volume9

3D transport models can be implementedby user defined functions (UDF) in Fluentor other commercial software.The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and theninsert it again.Solids transport UDFCarolloTemplateWaterWave.pptx– Solids Transport C U i C ν t C C () Vs t X i X i σ s X i z10

3D transport models can be implementedby user defined functions (UDF) in Fluentor other commercial software.The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and theninsert it again.Solids transport UDFCarolloTemplateWaterWave.pptx– Solids Transport– Vesilind settling C U i C ν t C C () Vs t X i X i σ s X i zVs Vo * exp( k * C )11

3D transport models can be implementedby user defined functions (UDF) in Fluentor other commercial software.The image cannot be displayed. Your computer may not have enough memory to open the image, or the image may have been corrupted. Restart your computer, and then open the file again. If the red x still appears, you may have to delete the image and theninsert it again.Solids transport UDF– Solids Transport– Vesilind settling– Density couple C U i C ν t C C () Vs t X i X i σ s X i zVs Vo * exp( k * C )CarolloTemplateWaterWave.pptxρ ρ w / (1 - C * (1 - ρ /ρ w ))12

3D transport models can also beimplemented for reactions in the fluid.Biokinetic ModelsCarolloTemplateWaterWave.pptx– IWA Activated SludgeModels (ASM)– Advanced oxidationmodels– Disinfection modelsSobremisana, Ducoste, de los Reyes III(2011)13

CFD is well established for analysis ofhydraulic components.CarolloTemplateWaterWave.pptxOften flows are split between paralleltreatment components. Proper flow split canhave important process consequences.14

Pump Stations – Optimize IntakeHydraulicsAdverse Hydraulics:VorticesPre-rotationTurbulenceVelocity DistributionLead to:Decreased essive wear15

Screening / HeadworksCFD can optimize designScreen channel flow balanceScreen flow distributionCarolloTemplateWaterWave.pptxInfer grit deposition fromvelocity profiles16

Grit ManagementVortex grit systemefficiency is a functionof approach and exitvelocityAerated grit tanks requireproper sizing andbaffling to preventshort circuitingCarolloTemplateWaterWave.pptxGrit deposition can beinferred from velocityprofiles and neutraldensity particletracking17

Flow SplittingFlow Splitting is critical tooptimize the capacity ofparallel D analysis can be rigidlid or free surface18

CarolloTemplateWaterWave.pptxHead losses through complex systems with nonuniform approach conditions can be investigated.19

But CFD can also be used for analysis oftransport processes.CarolloTemplateWaterWave.pptxThese are processes where reactions happenas well as hydraulics.20

Primary ClarificationCFD can be used toinvestigate geometricinfluences on solidsremoval.CarolloTemplateWaterWave.pptxCFD can be used toinvestigate sludgeconsolidation problems.21

Aeration TanksMultiphase modeling canbe used to investigatewater-air flows.Dissolved air transfermodels can beincorporated.CarolloTemplateWaterWave.pptxSolids transport andbiokinetic models can beincorporated.22

Secondary SedimentationClarifiers require effectiveinlet energy dissipation.Baffles can aid ty currents dominateflow field, therefore acustom transport model isrequired.CFD analysis of activatedsludge sedimentation isvery well established.23

Disinfection - UVHydraulics are critical:Flow split between trainsFlow distributionHead lossesCarolloTemplateWaterWave.pptxDose models can beincorporated whendeveloping newdesigns.24

Mixing SystemsMixing Used When:Combining Fluid StreamsChemical AdditionsMinimizing StagnationTypical Mixing SystemsNatural DiffusionPassive icalPumped Jets25

DigestersCarolloTemplateWaterWave.pptxGood mixing is critical to performanceImproved kinetics more gas productionReduces foamingEfficient mixing saves power26

Case Studies Using Transport ModelingActivated sludge lamella clarifiersUse of UDF model to evaluate sedimentation inletsCarolloTemplateWaterWave.pptxUse of UDF models to evaluate activated sludgemixing27

Sedimentation Case Study:CarolloTemplateWaterWave.pptxActivated Sludge Lamellas**Samstag,Wicklein, Lee (2012)28

The Boycott Effect has been used as thebasis for the PNK theory.Boycott (1920) observed adifference in apparentbatch settling rate of bloodin slanted tubes.CarolloTemplateWaterWave.pptxPNK theory: Settling isenhanced by the ratio ofthe projected area ofinclined plates or tubes.29

Does this apply to flow-throughactivated sludge lamella clarifiers?CarolloTemplateWaterWave.pptxCoarse Grid Custom Model2D and 3D Commercial Models30

Results: No difference between tankswith and without lamella plates!CarolloTemplateWaterWave.pptxWith lamella platesWithout lamella plates31

Sedimentation Case Study:Clarifier Inlet ComparisonCarolloTemplateWaterWave.pptxExisting Configuration32

Clarifier Inlet ComparisonCarolloTemplateWaterWave.pptxExisting Configuration33

CarolloTemplateWaterWave.pptxAlternative Inlet Configurations34

CarolloTemplateWaterWave.pptxAlternative Velocity Vector Plans35

CarolloTemplateWaterWave.pptxAlternative Velocity Plans36

CarolloTemplateWaterWave.pptxAlternative Velocity Profiles37

CarolloTemplateWaterWave.pptxComparison Solids Profiles38

Mixing Case Study:Jet mixing and aeration in a sequencing batchreactor (SBR)*415,350 mixed tetrahedralcells2,108,308 nodesInlet flow into jet nozzlesOutlet flow to pump suctionCarolloTemplateWaterWave.pptxAir added as second phaseSolids transport, settling,and density impactmodeled by UDF*Samstag,Mesh projected onto modelsurfaces.Wicklein, et al. (2012)39

Velocity profiles for pumped mixing andaerationCarolloTemplateWaterWave.pptxSimulated Pumped Mixing ProfileSimulated Aeration Profile40

Comparison of pumped mix velocityprofiles for increasing jet velocities3.0 m/sec Jet3.5 m/sec Jet4.0 m/sec JetCarolloTemplateWaterWave.pptxExisting(2.5 m/sec Jet)41

Comparison of solids profiles forincreasing jet velocities3.0 m/sec Jet3.5 m/sec Jet4.0 m/sec JetCarolloTemplateWaterWave.pptxExisting(2.5 m/sec Jet)42

Min / Max Deviations from AverageCarolloTemplateWaterWave.pptxAverage TSS Concentration (mg/L)Layer2.5 m/sec3.0 m/sec3.5 m/sec4.0 ,5592,500AverageMax DeviationFrom Average(%)2,4022,3532,3922,36250%40%12%9%43

Comparison of Power Levels at DifferentJet VelocitiesCarolloTemplateWaterWave.pptxJet Velocity2.5 m/sec jetvelocity3.0 m/sec jetvelocity3.5 m/sec jetvelocity4.0 m/sec jetvelocityMix Criterion50% MaxDeviation40% MaxDeviation12% MaxDeviation 10% MaxDeviationPower Level Power Level(hp/MG)(W/m3)397.76613.010520.715630.8To meet a 10 percent deviation criterion would require fourtimes more power than currently installed.44

Comparison of Power Levels to OtherMixing DevicesMix Criterion 10% Max4.0 m/sec jetThis studyDeviationLarge Propeller in Carollo FieldLittle MLSSRacetrackVisitseparationSurface MixingCarollo0.6 m/sec (2 fps)ImpellerWitnessed Test bottom velocityOtun et al. 30% MaxHydrofoil mixer(2009)DeviationOtun et al.Hyperboloid 11% txMixerReferencePowerPowerLevel (hp/ Level (W/MG)m3)15630.85 1397.6397.6204.045

Frequent CFD practice for mixing is toassume neutral density.The influence of solids on thevelocity pattern is ignored.A velocity profile is thencalculated assuming clearwater.CarolloTemplateWaterWave.pptxIt is then assumed that a givenminimum velocity (2.5 ft/min)will be sufficient to providemixing.But it is SOLIDS that we are tryingto mix. They aren’t typicallymodeled.46

Comparison of density-coupled and neutraldensity simulationsDensity-coupledSolids transport modelcalculates the local solidsconcentration based onflow regime.CarolloTemplateWaterWave.pptxThe influence of the localsolids concentration onthe local density is theniteratively calculated.This approach was verifiedby the field solids profiletest data.47

Comparison of density-coupled and neutraldensity simulationsNeutral DensitySolids transport modelcalculates the local solidsconcentration based onflow regime.CarolloTemplateWaterWave.pptxInfluence of the local solidsconcentration on the localdensity was turned off.This approach overpredicted measured solidsmixing.48

Comparison of density-coupled andneutral density upledNeutral densityNeutral density simulation dramatically over-predicts thedegree of mixing.49

CFD and Process EngineeringConclusionsCFD is well established and important for analysisof hydraulic components.CarolloTemplateWaterWave.pptxThere is growing appreciation that CFD can be apowerful tool for analysis of the impact ofgeometry and hydrodynamics on processperformance.Results from studies of lamella settlers, radial flowclarifier inlets, and activated sludge mixing showthat CFD can establish important conclusions forprocess engineering.50

CFD and Process Engineering Conclusions CFD is well established and important for analysis of hydraulic components. There is growing appreciation that CFD can be a powerful tool for analysis of the imp