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CFD for Wastewater: What Can it Do?
Randal W. Samstag, MS, PE, BCEE
Civil and Sanitary Engineer
Bainbridge Island, WA US
Presentation to the Department of Civil andEnvironmental EngineeringUniversity of Washington
7 April 2016
CFD for Wastewater: What Can it Do?
• What is it?
• How is it done?
• What is it good for?
• Who is doing it?
“If we know what is happening within the vessel,then we are able to predict the behavior of thevessel as a reactor. Though fine in principle, theattendant complexities make it impractical to usethis approach.” – Octave Levenspiel (1972)
Computational fluid dynamics (CFD) changesthis picture. Using CFD, we can compute three-dimensional velocity fields and followinteractions of reactants and products througha tank. We can use this information to optimizetank geometry.
Types of Models
Black Box Models Grey Box Models
Glass Box Models
CFD: What is it?CFD Solves the Reynolds Averaged Navier-Stokes
(RANS) Equations by Numerical Schemes
• Continuity Equation: Law ofMass Conservation
• Momentum Equations:Newton’s Second Law(Incompressible Flow )
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Breakdown of the MomentumEquations
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Unsteady Term
Advective TermPressure Term
Diffusion TermSource Term
The Momentum Equations are a SpecialCase of the Generalized Transport Equation
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Discretization Techniques
• Finite difference
• Method of weighted residuals (finite element)
• Finite volume formulation
• Grid-less methods
All of these have been used in CFD for wastewater. Finitedifference was the first technique used. Finite element has beenused for clarifier modeling. Finite volume formulation is themost common commercial CFD software approach. Grid-lessmethods have not been much used, but may be promising.
How to eliminate pressure?
• Transform the governing equations:
– Vorticity / stream function method
• Use iteration and convergence
– SIMPLE
Both of these methods have been used in CFD for wastewater.
Turbulence Modeling• Simplest model: Prandtl’s Mixing Length
Hypothesis (Plane mixing layer, width δ)
• Two Equation Models: k – epsilon
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3D transport models can be coupled to the velocitycalculations to simulate sedimentation and mixing.
• Solids Transport
• Vesilind settling
• Density couple
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Samstag, et al. (1992)
3D transport models can be implemented forwastewater quality parameters as well.
Biokinetic Models
– ASM Models
– Advanced oxidationModels
– Disinfection models
Sobremisana, Ducoste and de los Reyes III (2011)
Multi-phase models can simulatemotion of water and air.
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How to Do CFD?Good Modelling Practice
Wicklein et al. (2016)
How to do CFD?Software
• Hand Coded– Fortran– C++
• Commercial platforms (Examples)– ANSYS (Fluent and CFX)– Cd-adapco– Flow-3D– Comsol– PHOENICS
• Open source platforms– OpenFOAM
Hand Coded InterfaceTANKXZ
Fluent Interface
Standard OpenFOAM Interface
Visual CFD Interface for OpenFOAM
2015 OpenFOAM WorkshopSurfers Paradise
• Presentations by IWA WGmembers
• Teaching by NelsonMarques on OpenFOAM– Modeling– Meshing– Simulation– Post-Processing
• Example cases– Parshall Flume– Flow Splitter– Clarifier– UV Channel
OpenFOAM WorkshopCase Study: Splitter Box
OpenFOAM WorkshopCase Study: Activated Sludge Clarifier
What can be done with CFD?Wastewater Treatment Examples
Case Study: WWT Hydraulics
• Adverse Hydraulics
Vortices
Pre-rotation
Turbulence
Velocity Distribution
• Leading to
Decreased capacity
Poor flow distribution
Cavitation
Excessive wear
Provided by Jim Wicks, PhDThe Fluid Group
Case Study: WWT HydraulicsDrop Shafts
To transfer flood waterand sewage rapidlydown into a main sewerwithout blockage
Multiphase flow
Velocity dependent gritdeposition
Provided by Jim Wicks, PhD
Case Study: WWT HydraulicsFlow Splits
Equal transfer ofwastewater to multipledownstream units(ensuring solidscomponent accounted forcorrectly)
Velocity dependent gritdeposition
Multiphase flow
Provided by Jim Wicks, PhD
Case Study: BiokineticsEindhoven WWTP Facility
(The Netherlands)
From Rehman et al. (2014)
Configuration of the Bioreactor
Geometry
Diffuserssimplification
Vector Plots
• Bad mixing zones dueto recirculation
• Averaged out insystemic modeling
• Change in aeration leads to different flow patterns
Vertical cross section inthe aerated region
Dissolved oxygen and ammonia conc. (mg/L)
Oxygen concentration at 3.45m depth Ammonia concentration at 3.45mdepth
Case Study: Jet Mixing in a sequencingbatch reactor (SBR)
Case Study: MixingCFD of Jet mixing and aeration
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, anddensity impact modeled byUDF
Samstag et al. (2012)
Mesh projected onto modelsurfaces.
Case Study: MixingVelocity profiles for pumped mixing and aeration
Simulated Pumped Mixing Profile Simulated Aeration Profile
Case Study: MixingComparison of pumped mix velocity profiles for increasing jet velocities
Existing(2.5m/secJet)
3.0m/secJet
3.5m/sec Jet
4.0 m/secJet
Case Study: MixingComparison of solids profiles for increasing jet velocities
Existing(2.5m/secJet)
3.0m/secJet
3.5m/sec Jet
4.0 m/secJet
Case Study: MixingComparison of density-coupled and neutral density simulations
Density-coupled
Solids transport modelcalculates the local solidsconcentration based onflow regime.
The influence of the localsolids concentration on thelocal density is theniteratively calculated.
This approach was verified bythe field solids profile testdata.
Case Study: MixingComparison of density-coupled and neutral density simulations
Neutral Density
Solids transport model calculatesthe local solids concentrationbased on flow regime.
Influence of the local solidsconcentration on the localdensity was turned off.
This approach over-predictedmeasured solids mixing.
Case Study: Activated SludgeClarification (Calibration)
Samstag et al. (2010)
Case Study: Activated Sludge Clarifier
• Radial flow clarifier
• Questions:
– Optimum Depth?
– Optimum Inlet ?
– Optimum Feedwell?
– Optimum Effluent Zone?
Samstag and Wicklein(2014)
Radial Flow Clarifier CFD Model
• 3D Fluent CFD
• 1,100,000 hexahedralcells
• K-epsilon turbulencemodel
• User defined functions(UDF) to implement– Solids settling and
transport
– Density coupling
Case Study: Activated Sludge ClarifierCalibration CFD
Case Study: Activated Sludge ClarifierAlternative Inlet Configurations
Case Study: Activated Sludge ClarifierAlternative Velocity Vector Plans
Case Study: Activated Sludge ClarifierAlternative Velocity Plans
Case Study: Activated Sludge ClarifierAlternative Velocity Profiles
Case Study: Activated Sludge ClarifierComparison Solids Profiles
Case Studies: Disinfection
• Dye, disinfectant, andmicroorganismtransport in a chemicaldisinfection system
Provided by StevenSaunders, P.E.
Case Study: Dissolved Air FlotationDAF
Seawater DAF –desalination use
Prediction of absolutemaximum capacity(typically 10-20% aboveabove design max)
Comparison of nozzledesign and airintroduction
Provided by Jim Wicks,PhD
Case Study: DigestersDigesters
Non-Newtonian flow above3% DS
Rheology for all digestatesdifficult to measure.Typically we use of a libraryof values for various drysolids contents (and forvarious sludge types)
Useful for optimisingmethane gas production;reducing foaming; limitingsolids deposition andavoiding ‘dead’ volumes.
Provided by Jim Wicks,PhD
Who is Doing CFD?
• IWA CFD Working Group
“The WG intends to solve shortcomings arising fromlack of knowledge of CFD in the water andwastewater community in the short term byproducing papers and books as well as hands-ontraining for the IWA MIA members (and beyond).Furthermore, a book dedicated for training newpeople in the water/wastewater field will beproduced.”
IWA CFD Working GroupManagement Team
IWA CFD Working GroupWork Products
• Published Papers– Good Modelling Practice in Applying Computational Fluid Dynamics for WWTP Modelling, WEFTEC
2012– A protocol for the use of computational fluid dynamics as a supportive tool for wastewater treatment
plant modelling, WST– Computational Fluid Dynamics: an important modelling tool for the water sector, IWC Conference– Good Modelling Practice in Applying Computational Fluid Dynamics for WWTP Modelling, WST
• Submitted Papers– CFD for Wastewater: An Overview, WST
• Workshops– WEFTEC 2012 (Fluent)– WWTMod 2012– Watermatex 2015 (OpenFOAM)– WEFTEC 2016 (Flow-3D)– IWA Biennium 2016 (Proposed)
• Book Plans– IWA Scientific and Technical Report– CFD for Water Book for Students and Practitioners
There is also a LinkedIn GroupCFD for Wastewater
Further Reading
• Levenspiel 1972 Chemical Reaction Engineering.• Rodi 1980 Turbulence Models and Their Application in Hydraulics: A state-of-the art review.• Samstag, R. W., McCorquodale, J. A. and Zhou, S. P. 1992 Prospects for transport modeling of process tanks, Water
Science and Technology. 26(5/6), 1401-1410.• Sobremisana, Ducoste, and de los Reyes III 2011 Combining CFD, floc dynamics, and biological reaction kinetics to
model carbon and nitrogen removal in an activated sludge system. Water Environment Conference, WEFTECConference.
• Crowe, Sommerfield, and Tusji (1998) Multiphase Flows with Droplets and Particles.• Rehman U., Maere T., Vesvikar M., Amerlinck Y. and Nopens I. 2014. Hydrodynamic-biokinetic model integration
applied to a full-scale WWTP. In: IWA World Water Congress & Exhibition 2014, Sep 21-26, Lisbon, Portugal.• Wicklein, E., Batstone, D.J., Ducoste, J., Laurent, J., Griborio, A., Wicks, J., Saunders, S., Samstag, R., Potier, O. and
Nopens, I. 2016 Good modelling practice in applying computational fluid dynamics for WWTP modelling. WaterScience and Technology. 73 (5) 969-982.
• Samstag, R.W., Wicklein, E.A., Reardon, R. D., Leetch, R. J., Parks, R. M. and Groff, C. D. 2012 Field and CFD Analysisof Jet Aeration and Mixing, Proceedings of the Water Environment Federation, WEFTEC: Session 51 throughSession 60, pp. 4113-4139(27).
• Samstag, R.W., Zhou, S., Chan, R.L., Royer, C. and Brown, K. 2010 Comprehensive Evaluation of SecondarySedimentation Performance, Water Environment Federation, WEFTEC Conference.
• Samstag, R. W. and Wicklein, E. A. 2014 Clarifier Design Optimization, Carollo Engineers In-house report.• Amato, T. and Wicks, J. 2009 The practical application of computational fluid dynamics to dissolved air flotation,
water treatment plant operation, design and development'. J. Wat. Supply: Research & Technology - AQUA, 58(1),pp 65-73.
