Clarifying Settling Tank
Capacity: 2D and 3D
Clarifier CFD Modeling
Will Martin, PE
Agenda/Outline
Clarifier modeling background
- What’s the purpose?
- What types of models are available/used?
- What’s the use for each level/type of model?
2D/3D Modeling
- Current state of the industry – historic uses
- Recent developments
Case studies
Processes
in Secondary
Settling Tanks
• Hydrodynamics
• Settling
• Turbulence
• Sludge Rheology
• Flocculation
• Heat Exchange
and Temperature
Settling Tank Models
Empirical Design Criteria
• Surface overflow rate (SOR)
• Solids loading rate (SLR)
Solids Flux Theory
• State point analysis (SPA)
One-dimensional models
Computational Fluid
Dynamics (CFD)
• Two-dimensional (2D) models
• Three-dimensional (3D)
models
2-D Models
• Includes the major process and
factors affecting clarifier
performance and capacity including
hydrodynamics, turbulence,
flocculation, temperature
Concentration (g/L): 0.01 0.02 0.05 0.11 0.23 0.52 1.14 2.50 5.50
8.5 ft14.8 ft 14 ft
Example: 2Dc Model
Limitation: Complex
inlet configurations
3-D Models
Limitation:
• Cost
• Computational
time
Processes in Secondary Settling Tanks
• Hydrodynamics
• Settling
• Turbulence
• Sludge Rheology
• Flocculation
• Heat Exchange
and Temperature
• Empirical design
• Solids flux theory
• 1-D Modeling
• 2-D Modeling
• 3-D Modeling
Case Study #1 – Marion Water Pollution
Control Center (Ohio)
WRRF background
• 10.5 MGD AADF WWTP
• Conventional activated sludge
treatment process
• Primary, secondary, and tertiary
clarifiers
Long-term control plan
and no feasible alternatives
analysis for wet weather flows
• Increase wet weather peak
flows from 28 MGD to 42 MGD
Planned Wet Weather Improvements
CFD Model: Confirm capacity/performance
Biowin Model: Confirm wet weather performance
CFD Model: Evaluate design improvements
Clarifier Design Loadings
Design Conditions Flow (MGD)
Primary Clarifier
SOR (gpd/sf)
Secondary/Tertiary Clarifier SOR (gpd/sf)
4 Clarifiers 6 Clarifiers 8 Clarifiers1
2014-2017 AA Flows 9.8 560 560 370 280
Design AA Flows10.5 600 600 400 300
2014-2017 Maximum
Month 15.3 870 870 580 430
Design Maximum
Month 16.4 930 930 620 460
2014-2017 Peak Day38.5 (PC’s), 28
(SC’s)2,180 1,585 1,060 790
Design Peak Day51 (PC’s), 42 (SC’s)
2,890 2,380 1,585 1,190
Notes:1. Includes the existing four (4) tertiary clarifiers in service as secondary clarifiers.
Secondary Clarifier Field Testing
Sampling Parameters
• Settling properties of the sludge: zone and
compression rates
• Flocculation parameters
• Flocculated suspended solids (FSS) and
dispersed suspended solids (DSS)
• Discrete settling parameters
• Sludge volume index (SVI)
• Mixed liquor suspended solids (MLSS)
• Flow rate
• Return activated sludge (RAS) flow rate
• SC effluent suspended solids (ESS) concentration
• RAS suspended solids (RAS SS) concentration
• Sludge blanket height (SBH)
Sludge
Properties
Clarifier
Loading
Clarifier
Performance
Secondary Clarifier Field Testing
0
200
400
600
800
1000
1200
0
2
4
6
8
10
12
14
16
3/21/2018 3/22/2018 3/23/2018
SO
R (
gpd/s
f)
ES
S (
mg/L
)
Combined ESS Average Individual Clarifier ESS
Clairifer 3 ESS Clarifier 4 ESS
Clarifier SOR (gpd/sf)
Day 1
Normal Conditions
4 Clarifiers
In Service
Day 2
Stress Testing
4 Clarifiers
In Service
0
200
400
600
800
1,000
1,200
0
1
2
3
4
5
6
3/21/18 12:00 AM 3/22/18 12:00 AM 3/23/18 12:00 AM
SO
R (
gpd/s
f)
Heig
ht
(ft)
Sludge Blanket Heights - Average All Clarifiers
Sludge Blanket + Dispersed Heigths - Average All Clarifiers
SOR
Day 1
Normal Conditions
4 Clarifiers
In Service
Day 2
Stress Testing
4 Clarifiers
In Service
Secondary Clarifier Model Calibration
Data Period SOR (gpd/sf) MLSS (mg/L)
ESS (mg/L) RAS TSS (mg/L) Blanket Height (ft)
Field
Measurement
Model
Prediction
Field
Measurement
Model
Prediction
Field
Measurement
Model
Prediction
March 21, 2018 –
Model Calibration560 3,630 7 11 15,550 13,750 1.1 2.3
March 21, 2018 –
Model Validation860 3,630 101 11.5 14,390 13,900 2.7 2.9
Notes:
1. Combined data set for individual clarifier and combined samples.
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
3/22/2018 6:00 3/22/2018 10:00 3/22/2018 14:00 3/22/2018 18:00
ES
S (
mg/L
)
Field Data - ESS Model Results - ESS
Secondary Clarifier Capacity Evaluation
0
500
1,000
1,500
2,000
2,500
3,000
3,500
0
10
20
30
40
50
60
3/30/2014 3/31/2014 4/1/2014 4/2/2014 4/3/2014 4/4/2014 4/5/2014
SO
R (
gpd/s
f)
Flo
w (
MG
D)
5-Year, 24-Hour Storm Hydrograph Flow to Secondary Treatment
Primary Clarifier Loading (gpd/sf) Secondary Clarifier Loading - 4 SC's (gpd/sf)
Secondary Clarifier Loading - 4 SC's (gpd/sf)
Secondary Clarifier Capacity Evaluation
Peak Flow Rate (MGD)
No. of Clarifiers
SOR (gpd/sf) MLSS (mg/L) RAS Rate SVI (mL/g) Polymer? ESS (mg/L) Blanket (ft)
42 8 1,190
2,500 mg/L 40% 100 mL/g Yes <30 mg/L 2-4 ft
2,500 mg/L 40% 135 mL/g Yes<30 mg/L
2-4 ft
2,500 mg/L 40% 100 mL/g Partial<30 mg/L
2-4 ft
2,500 mg/L 40% 135 mL/g Partial<30 mg/L
2-4 ft
2,500 mg/L 40% 100 mL/g No <40 mg/L 2-4 ft
2,500 mg/L 40% 135 mL/g No <40 mg/L 4-6 ft
3,000 mg/L 40% 100 mL/g Yes<30 mg/L
2-4 ft
3,500 mg/L 40% 100 mL/g Yes<30 mg/L
4-6 ft
3,000 mg/L 40% 135 mL/g No >100 mg/L >10 ft
3,500 mg/L 40% 135 mL/g No >100 mg/L >10 ft
Process Model -Contact Stabilization
40% 100 mL/g Yes<30 mg/L
<2 ft
Process Model -Contact Stabilization
40% 135 mL/g Yes<30 mg/L
<2 ft
Process Model -Contact Stabilization
40% 100 mL/g Partial<30 mg/L
<2 ft
Process Model -Contact Stabilization
40% 135 mL/g Partial<30 mg/L
2-4 ft
Process Model -Contact Stabilization
40% 100 mL/g No<40 mg/L
<2
Process Model -Contact Stabilization
40% 135 mL/g No<40 mg/L
2-4 ft
Tertiary Clarifier Design Evaluation
0
2
4
6
8
10
12
14
16
18
20
Mo
del
Pre
dic
ted
ES
S (
mg/L
)
6-foot Launder Spacing 7-foot Launder Spacing 8-foot Launder Spacing
9-foot Launder Spacing 10-foot Launder Spacing
0
2
4
6
8
10
12
14
16
18
20
Mo
del
Pre
dic
ted
ES
S (
mg/L
)
10-foot Center Well Radius
9-foot Center Well Radius
8-foot Center Well Radius
7-foot Center Well Radius
10-foot Center Well Radius, Shallower Depth
Case Study #2 – F. Wayne Hill WRC (Georgia)
• F. Wayne Hill Water Resources Center (FWHWRC)
• 60 mgd advanced wastewater facility
F. Wayne Hill
WRC
Lake Lanier
Discharge
Point
Parameter Units
Monthly
Avg. Limits
Flow mgd 60
TSS mg/L 3
COD mg/L 18
Ammonia-N mg/L 0.4
Total Phosphorus mg/L 0.08
Turbidity Ntu 0.5
Fecal Coliform #/100 2
Historic Primary Clarifier Performance
-100%
-80%
-60%
-40%
-20%
0%
20%
40%
60%
80%
100%
% R
em
ova
l
Primary TSS Removal
Pri TSS Removal from Pri Inf (Load) 30 per. Mov. Avg. (Pri TSS Removal from Pri Inf (Load))
Year
Average
Primary TSS
% Removal
2006 -24%
2007 20%
2008 19%
2009 -27%
Approach used to evaluate the
performance of the primary clarifiers
• Evaluation of historical data and sampling location
• Process and mechanical evaluation
• Special sampling events (2 Phases)
• Two-dimensional (2D) and three-dimensional (3D)
Computational Fluid Dynamics (CFD) modeling.
Revised sampling locations and testing
verified poor primary clarifier performance
Modified Influent, PI, and PE sampling locations and
operating configuration to achieve good sampling
ParameterPI Combined
(mg/L)
PE Combined
(mg/L)% Removal
TSS 327 264 19%
COD 503 437 13%
cBOD 229 208 9%
Jar Testing and Column Testing Results
TSS Removal – 50 to 70%
2Dc Modeling
Solids get trapped
in the scum creating
a thick layer of
solids and scum
that eventually is
carried over the
effluent weirs
3D Modeling
Three dimensional modeling using the ANSYS FLUENT
Results from the existing conditions revealed shortcircuiting from the influent flow into the sludge hopper
Flow Field Inlet Detail
PC Modifications
Before After
Primary Clarifier
% TSS
Removal
Primary 9 – Baffle Installed 48%
Primary 10 – As Is 31%
Primary clarifier performance before
and after proposed modifications
After proposed operational and physical modifications, that included the installation of a canopy baffle, the PSTs showed a significant improvement in performance
0
100
200
300
400
500
600
700
800
900
7/8/09 8/7/09 9/6/09 10/6/09 11/5/09 12/5/09 1/4/10 2/3/10
Co
nc
en
tra
tio
n (
mg
/L)
PE TSS 14 per. Mov. Avg. (PE TSS)
End Phase 1 Sampling Event
Case Study #3 – Passaic Valley
Sewerage Commission (PVSC)
• 330 MGD AADF secondary
treatment capacity
• 400 MGD secondary
treatment peak flow
capacity
• Long-term control plan –
evaluate alternatives to
expand plant wet weather
flow capacity
• Alternatives include step
feed, solids storage, wet
weather treatment
systems, and others
Clarifier Configuration
Modeling Approach
• Build a model of a 3D
representative clarifier
• Refine baseline with operation
and field data
• Sensitivity runs on a ‘slice’ model
(or 2Dc model)
• Confirmation on full 3D model
• Apply 3D model to alternatives
3D Modeling
Secondary
Bypass
Step Feed
3D Modeling Findings
1 RAS rates at peak flow (based on 130 MGD RAS flow): Bypass = 32%, Other simulations =18%
zQuestions?
Will Martin, PE