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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Unsteady

Source

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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Crowe, Sommerfield and Tusji (1998)

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 (STAR-CCM+)– Flow Science (FLOW-3D)– COMSOL AB (COMSOL Multiphysics)– CHAM (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

Velocity 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 oor 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– Computational Fluid Dynamics (CFD): What is Good CFD-Modeling Practice and What Can Be the

Added Value of CFD Models to WWTP Modeling? 2012 WEFTEC– A protocol for the use of computational fluid dynamics as a supportive tool for wastewater treatment

plant modelling, 2012WST– Computational Fluid Dynamics: an important modelling tool for the water sector 2012 IWC

Conference– Good Modelling Practice in Applying Computational Fluid Dynamics for WWTP Modelling 2016 WST

• Submitted Papers– CFD for Wastewater: An Overview (in review) 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.

Thank you!Questions?

Web: http://rsamstag.com/

Phone: +1 (206) 851-0094

Email: randal.samstag@rsamstag.com