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Application of CFD toWastewater Process Engineering

Randal W. Samstag, P.E., B.C.E.EPrincipal Technologist

Ed Wicklein, P.E.Senior Technologist

Carollo Engineers

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“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 Levenspiel(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.

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CFD can calculate the velocity fields.

Hydraulic Model

– Continuity (massconservation)

– Momentumtransport

– k-epsilon turbulencemodel

– Control volumesolution scheme

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3D Solids Transport ModelIndividual Control Volume

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3D transport models can be implementedby user defined functions (UDF) in Fluentor other commercial software.

Solids transport UDF

– Solids Transport

– Vesilind settling

– Density couple

– Viscosity impact

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3D transport models can be implementedby user defined functions (UDF) in Fluentor other commercial software.

Biokinetic Models

– ASM Models

– Advanced oxidationModels

– Disinfection models

Sobremisana, Ducoste, de los Reyes III(2011)

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CFD is well established for analysis ofhydraulic components.

Often flows are split between paralleltreatment components as well

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Pump Stations – Optimize IntakeHydraulics

Adverse Hydraulics:

Vortices

Pre-rotation

Turbulence

Velocity Distribution

Lead to:

Decreased capacity

Cavitation

Excessive wear

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Screening / Headworks

CFD can optimize design

Screen channel flowbalance

Screen flow distribution

Infer grit deposition fromvelocity profiles

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Grit Management

Vortex grit systemefficiency is a functionof approach and exitvelocity

Aerated grit tanks requireproper sizing andbaffling to preventshort circuiting

Grit deposition can beinferred from velocityprofiles and neutraldensity particletracking

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Flow Splitting

Flow Splitting is critical tooptimize the capacity ofparallel treatmentcomponents

CFD analysis can be rigidlid or free surface

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Head losses through complex systems with non-uniform approach conditions can be investigated.

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But CFD can also be used for analysis oftransport processes.

Often flows are split between paralleltreatment components as well

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Primary Clarification

CFD can be used toinvestigate geometricinfluences on solidsremoval.

CFD can be used toinvestigate sludgeconsolidation problems.

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Aeration Tanks

Multiphase modeling canbe used to investigatewater-air flows.

Dissolved air transfermodels can beincorporated.

Solids transport andbiokinetic models can beincorporated.

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Secondary Sedimentation

Clarifiers require effectiveinlet energy dissipation.

Baffles can aid insedimentation.

Density currents dominateflow field, therefore acustom transport model isrequired.

CFD analysis of activatedsludge sedimentation isvery well established.

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Disinfection - UV

Hydraulics are critical:

Flow split betweentrains

Flow distribution

Head losses

Dose models can beincorporated whendeveloping newdesigns.

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Mixing Systems

Mixing Used When:

Combining FluidStreams

Chemical Additions

Minimizing Stagnation

Typical Mixing Systems

Natural Diffusion

Passive Baffles

Aeration

Mechanical

Pumped Jets

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DigestersGood mixing is critical to performance

Improved kinetics more gas productionReduces foamingEfficient mixing saves power

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Case Studies Using Transport Modeling

Comparison of UDF transport model to out-of-the-box commercial multiphase model

Activated sludge lamella clarifiers

Use of UDF model to evaluate sedimentation inlets

Use of UDF models to evaluate activated sludgemixing

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Model Comparison Case Study:Transport Modeling versus Mixture Model*

Transport modeling treats concentrations as apassive scalar quantity that react and aretransported through the fluid grid.

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* Wicklein and Samstag (2009)

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In transport modeling the solids settlingvelocity is treated empirically.

kCs eVV 0

(Vesilind Equation)

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In the Fluent mixture model settling isdependent on particle size.

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g

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sdrag

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(from Stokes Law)

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Comparison of Field Data to MixtureModel Results for a Rectangular Tank

Field Data

Mixture model

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Comparison of Field and TransportModel Results for Rectangular Tank

Field Data

TransportModel

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Comparison of Field to Mixture Model forRadial Flow (Circular) Tank.

Field Data

MixtureModel

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Comparison of Field to UDF TransportModel (Radial Flow Tank)

Field Data

UDF Model

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Transport versus MixtureModelingConclusions

Either transport modeling or mixture modeling canreasonably predict solids transport, if properlycalibrated.

We have a lot of data to calibrate transport modelsusing empirical settling velocity.

We have very little data to know the appropriateparticle size to use in a mixture model for activatedsludge.

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Sedimentation Case Study:

Activated Sludge Lamellas*

*Samstag, Wicklein, Lee (2012)

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The Boycott Effect has been used as thebasis for the PNK theory.

Boycott (1920) observed adifference in apparentsettling rate of blood inslanted tubes.

PNK theory: Settling isenhanced by the ratio ofthe projected area ofinclined plates or tubes.

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Does this apply to flow-throughactivated sludge lamella clarifiers?

Coarse Grid Custom Model 2D and 3D Commercial Models

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Results: No difference between tankswith and without lamella plates!

With lamella plates Without lamella plates

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Sedimentation Case Study:Clarifier Inlet ComparisonExisting Configuration

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Clarifier Inlet ComparisonExisting Configuration

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Alternative Inlet Configurations

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Alternative Velocity Vector Plans

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Alternative Velocity Plans

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Alternative Velocity ProfilesC

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Comparison Solids Profiles

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Mixing Case Study:Jet mixing and aeration in a sequencing batchreactor (SBR)*

415,350 mixed tetrahedralcells

2,108,308 nodes

Inlet flow into jet nozzles

Outlet flow to pump suction

Air added as second phase

Solids transport, settling,and density impactmodeled by UDF

*Samstag, Wicklein, et al. (2012)

Mesh projected onto modelsurfaces.

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Velocity profiles for pumped mixing andaeration

Simulated Pumped Mixing Profile Simulated Aeration Profile

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Comparison of pumped mix velocityprofiles for increasing jet velocities

Existing(2.5

m/secJet)

3.0m/sec

Jet

3.5m/sec

Jet

4.0m/sec

Jet

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Comparison of solids profiles forincreasing jet velocities

Existing(2.5

m/secJet)

3.0m/sec

Jet

3.5m/sec

Jet

4.0m/sec

Jet

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Min / Max Deviations from Average

Layer

Average TSS Concentration (mg/L)

2.5 m/sec 3.0 m/sec 3.5 m/sec 4.0 m/sec

Top 1,208 1,404 2,102 2,155

2 2,385 2,331 2,280 2,285

3 2,519 2,374 2,322 2,308

4 2,538 2,422 2,448 2,387

5 2,554 2,518 2,526 2,443

6 2,604 2,620 2,511 2,456

Bottom 3,008 2,806 2,559 2,500

Average 2,402 2,353 2,392 2,362Max DeviationFrom Average

(%) 50% 40% 12% 9%

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Comparison of Power Levels at DifferentJet Velocities

Jet Velocity Mix Criterion

PowerLevel

(hp/MG)

PowerLevel

(W/m3)

2.5 m/sec jetvelocity

50% MaxDeviation 39 7.7

3.0 m/sec jetvelocity

40% MaxDeviation 66 13.0

3.5 m/sec jetvelocity

12% MaxDeviation 105 20.7

4.0 m/sec jetvelocity

< 10% MaxDeviation 156 30.8

To meet a 10 percent deviation criterion would require fourtimes more power than currently installed.

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Comparison of Power Levels to OtherMixing Devices

Mixer Reference Mix Criterion

PowerLevel

(hp/MG)

PowerLevel

(W/m3)

4.0 m/sec jet This study< 10% MaxDeviation

156 30.8

Large Propeller inRacetrack

Carollo FieldVisit

Little MLSSseparation

5 ~1

Surface MixingImpeller

CarolloWitnessed Test

0.6 m/sec (2fps) bottomvelocity

39 7.6

Hydrofoil mixerOtun et al.(2009)

< 30% MaxDeviation

39 7.6

Hyperboloidmixer

Otun et al.(2009) < 11% Max

Deviation20 4.0

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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.

It 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.

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Comparison of density-coupled and neutraldensity simulationsDensity-coupled

Solids transport modelcalculates the local solidsconcentration based onflow regime.

The influence of the localsolids concentration onthe local density is theniteratively calculated.

This approach was verifiedby the field solids profiletest data.

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Comparison of density-coupled and neutraldensity simulationsNeutral Density

Solids transport modelcalculates the local solidsconcentration based onflow regime.

Influence of the local solidsconcentration on the localdensity was turned off.

This approach over-predicted measured solidsmixing.

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Comparison of density-coupled andneutral density simulations

Density-coupled Neutral density

Neutral density simulation dramatically over-predicts thedegree of mixing.

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CFD and Process EngineeringConclusions

CFD is well established and important for analysisof hydraulic components.

There 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.