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Maximizing Secondary Clarifier Capacity with Three- dimensional Modeling

Randal Samstag and Ed WickleinCarollo Engineers

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Presentation Outline

• Introduction to the problem• Comparison of models• Case studies:

Center feed circular radial flowCenter feed square radial flowPeripheral feed square countercurrent flowRectangular lamella clarifiers

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The Clarifier

• Used for both primary and secondary separation of solids

• Efficiency depends on Settling characteristicsTank geometry

• The good news:Both settleability and tank geometry can often be improved

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Settleability Can be Improved

• Analysis of the biological populations is crucial

• Selectors encourage populations that settle well

• Depends on:ConfigurationSRT

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Geometry Can Be Improved

Old Geometry New Geometry

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Why do Modeling?

• Thirty years of development using computational fluid dynamics (CFD) for analysis of sedimentation has proven that CFD can 1) Capture the main features of clarifier

behavior2) Model detailed features of hydraulic behavior3) Efficiently predict performance of novel

designs4) Be more cost effective than full-scale

prototypes

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Types of Sedimentation Models

• Solids flux models (state point analysis)• One-dimensional dynamic models

(Biowin, Sedtank, Takacs, Vitasovic, Stenstrom)

• Two-dimensional dynamic models (UNO, TANKXZ, Carollo Fluent UDF)

• Three-dimensional dynamic models (Zhou/McCorquodale, Carollo Fluent UDF)

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State Point Analysis (Clariflux®)• Developed by Vesilind.

Implemented by Carollo Engineers (among others)

• Solves solids flux equations based on measured settling velocity coefficients (or SVI)

• Calculates state point for steady state operation

SOR LineMLSS LineRAS line

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One-dimensional (1D) Dynamic Models

• Developed by Stenstrom, Tracy, Vitasovic, Takacs, Sedtank, Biowin

• Simulate average upward velocity versus downward settling velocity

• Solved dynamically• Layered model• Used for long-term

dynamic simulations

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Two-dimensional (2D) Models

• Incorporate 2D tank hydraulics

Boundary effectsTurbulenceDensity effects

• Used for geometric optimization of symmetrical elements

• Proprietary codes or public domain programs

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Three-dimensional (3D) Models

• Resolution and detail limited only by computing power

• Very detailed grids can be used to capture geometric features as small as several inches

• Crucial for modeling of non-symmetric features

• Implemented in proprietary code or commercial CFD packages with special add-ons

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Each Type of Model Has its Place• State Point Analysis – Steady State

Capacity Analysis• 1D Dynamic Models – Long-term

Dynamic simulations• 2D Models – Simple design evaluations• 3D Models – For design problems that are

not simple

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Examples of 3D Problems

• Analysis of inlet conditionsAlmost all inlet flow is three-dimensional

• Analysis of tank shapes that are not simple

Square radial flow tanksCircular peripheral feed tanksCircular or square peripheral feed and withdrawal tanksTanks with eccentric baffles or effluent troughs

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Case Studies

• Center feed square radial flow• Center feed circular radial flow• Square peripheral feed / withdrawal• Rectangular lamella clarifiers

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Center-feed, Radial-flow Square Clarifiers

• Case study for use of models

State Point Analysis2D Model3D Model

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State Point Comparison

33% RAS 66% RAS

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2D Model – UNO Model

• Developed by J. A. McCorquodale and associates at the University of New Orleans for EPA

• Two-dimensional model based on

Vorticity / stream function model (2D only)Turbulent hydraulicsRadial flow coordinates (axi-symmetric)Solids transportComposite settling modelFlocculation

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2D Model Results Test Calibration Results

FieldUNO Model

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2D Model Results Summary of Model Runs

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3D Model (Zhou CFD)

• Developed by Siping Zhou and J. A. McCorquodale

• Three-dimensional solution based on

Control volume modelTurbulent hydraulicsGeneralized coordinatesSolids settlingSolids transportNo flocculation or compression modeling

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Inlet Comparison

ExistingMultilayer Energy Dissipating Inlet Colum (MEDIC)

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3D Model Results Summary of Model Runs

Clarifier Configuration

Operational Conditions Clarifier Performance

Clarifier Flow (mgd)

SOR (gpd/sf)

RAS Ratio (%)

MLSS (mg/L) SVI (mL/g)

Theoretical RAS Predicted

ESS (mg/L)Predicted

RAS (mg/L)(mg/L)Test Calibration 3.5 714 33 3,600 126 14,509 15 11,000

Existing Clarifier 2.5 510 33 3,250 110 13,000 7.1 10,821

Existing Clarifier 3.5 714 33 3,250 110 13,000 13.1 10,773

Existing Clarifier + Perimeter Effluent Weir

and Baffle

3.5 714 33 3,250 110 13,000 14.5 10,772

Existing Clarifier 4.5 918 33 3,250 110 13,000 83 10,183

Existing Clarifier 3.5 714 66 3,250 190 8,100 428 6,234

Existing Clarifier 3.5 714 100 3,250 190 6,500 1017 5,167

3-Layer MEDIC + Middle Feed Well

2.5 510 33 3,250 110 13,000 5.2 10,943

3-Layer MEDIC + Middle Feed Well

3.5 714 33 3,250 110 13,000 5.7 11,025

3-Layer MEDIC + Middle Feed Well

4.5 918 33 3,250 110 13,000 6.7 10,904

3-Layer MEDIC + Middle Feed Well

3.5 714 33 3,250 190 13,000 10.5 8,482

3-Layer MEDIC + Middle Feed Well

3.5 714 66 3,250 190 8,100 7.9 6,985

3-Layer MEDIC + Middle Feed Well

3.5 714 100 3,250 190 6,500 7.8 6,015

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3D Model Results Inlet Improvements (SVI 110)

Figure 16 Performance comparison between the existing and optimized clarifiers under a peak flow condition (Clarifier flow = 4.5 MGD, RAS = 33.3%, MLSS = 3250 mg/L and SVI = 110)

1) Existing Clarifier

2) Optimized Clarifier

a) Inlet jets entering clarifier [2.45 ft/s (73.4 cm/s)]

a) Inlet jets entering MEDIC (2.45 ft/s) and ones entering clarifier [0.13 ft/s (3.88 cm/s)]

b) Strong turbulence induced by intensive clarifier influent flow

b) Significantly damped turbulence due to substantially reduced clarifier influent flow intensity

c) Dispersed sludge blanket

c) Dispersed sludge blanket

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3D Model Results Inlet Improvements (SVI 190)

Figure 20 Performance comparison between the existing and optimized clarifiers under a poor SVI combined with a low RAS of 33.3% (Clarifier flow = 3.5 MGD, MLSS = 3250 mg/L and SVI = 190)

1) Existing Clarifier

2) Optimized Clarifier

Significant solids inventory due to poor SVI combined with limited RAS capacity

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Conclusions from 3D Modeling

• Optimized inlet would allow increase of safe operating flow from 3.5 to 4.5 mgd per clarifier with good SVI (110 mL/g)

(30% Increase)

• Optimized inlet would allow safe operation at 3.5 mgd per clarifier with poor SVI (190 mL/g) compared to 2.5 mgd with existing inlet

(40% increase)

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Center-feed Circular Radial Flow Tank Comparison of Tangential to Puzzled Inlets

Tangential Inlet Puzzled Inlet

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Carollo Fluent UDF Model• 2D or 3D • Sophisticated grid

generation and visualization tools

• Choice of turbulence models

• User defined functions (UDF) to implement

Solids transportDensity couplingSolids settling velocity

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Calibration of 2D Model to Field TestField Test

Model

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Comparison of Tangential to Puzzled Inlets Inlet Velocities

Tangential Inlet Puzzled Inlet

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Comparison of Tangential to Puzzled Inlets (3D Model)

Inlet Velocity IntensityTangential Inlet Puzzled Inlet

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Optimization of Inlet Inlet Geometry (3D Model)

Existing Inlet Optimized Inlet

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Optimization of Inlet Solids and Velocity Profiles

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Optimization of Inlet Comparison of Inlet Velocity and

EnergyExisting Inlet Optimized Inlet

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Square Peripheral Feed / Withdrawal Tank Overall Geometry and Grid

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Square Peripheral Feed / Withdrawal Overall Solids Profiles

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Square Peripheral Feed / Withdrawal Velocity and Solids Profiles

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Square Peripheral Feed / Withdrawal Sludge Blanket Level Topography

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Rectangular Lamella Clarifier

• Carollo Fluent UDF Model

• 2D and 3D flow in and around the lamella plate modules

• Activated sludge clarifiers

• Two different settling models:

VesilindVesilind with Boycott in lamella zone

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Detailed Grid

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Vesilind Model of Low SVI Condition

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Vesilind Model with Moderate SVI

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Vesilind Model with No Lamellas

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Vesilind/Boycott Model of Moderate SVI

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Vesilind Model of Inlet Baffle

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Conclusions• CFD models are well developed for

evaluation of sedimentation tanks• Each level of model has its place• Several important problems can only be

adequately evaluated using 3D modelsInlet designRadial flow / square shapeNon-symmetrical elements

• Commercial 3D CFD codes can be productively used but only with custom add-ons

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Questions?

Randal W. Samstag(rsamstag@carollo.com)

Ed A. Wicklein(ewicklein@carollo.com)