WEF Webcast: Optimizing Clarifier Design and Performance CFD Modeling as a Tool for Clarifier Design Outline of Presentation What is CFD? Typical Applications Activated Sludge Example Projects Benefits of CFD 1
What is CFD? Log Jam Courtesy of Flow Science, Inc. Computational Fluid Dynamics (CFD) requires the use of a computer to study the dynamic behavior of liquids and gases. How does it Work? NEORSD Junction Box ARCADIS Using the Navier-Stokes equations, and an expression of continuity, CFD programs calculate flow through thousands of control volumes that make up a model. 2
Typical Applications Straight Hydraulics Rating Curves Flow Splits Headloss Mixing Disinfection Solids Settling Grit Settlement Activated Sludge NEORSD CSOTF - URS Headloss and Mixing 100 90 Strong Mixing occurs downstream of first overflow baffle and in drop shaft 80 70 Frequency 60 50 40 30 20 10 End of Mixing Chamber End of Pipe Additional mixing (conditioning) occurs in pipe that connects to the distribution box 0-0.2-0.15-0.1-0.05 0 0.05 0.1 0.15 Percent Deviation from Perfect Mixing Mattabassett Wastewater Treatment Plant Wright Pierce 3
Disinfection Contact Time T m Time when maximum concentration of tracer exits contact basin (measured from time when a slug test is initiated) Calumet Water Reclamation Plant CH2M Hill / ARCADIS Chlorine contact tank design and upgrade Solids Settling Intake Design - ARCADIS Transport and settlement of particulate matter 4
Activated Sludge To simulate clarifier flows, relative motions from sludge settling are computed from a balance between buoyancy and viscous forces (i.e., a drift-flux velocity). An unstable density distribution is initialized - the heavier material sinks and separates. Activated Sludge 21 16 (a) 11 (a) Density Distribution Circular Clarifier (red is high density) (b) Depth Varying Velocity Distribution (r = ½ R) 6 1-1.00E+00-5.00E-01 0.00E+00 5.00E-01 1.00E+00 (b) CFD simulations can effectively predict flow patterns caused by density differences within settling tanks. 5
Example 1 Parametric Design Case Number D L P t 1 36.0 37.0 45 26.2 3.0 2 36.0 52.4 30 26.2 3.0 3 12.0 37.0 45 26.2 3.0 4 36.0 48.0 45 33.9 3.0 5 36.0 24.0 45 17.0 3.0 6 60.0 37.0 45 26.2 3.0 7 36.0 52.4 60 26.2 3.0 Density Current Baffle Design - NEFCO Comparative Analysis 6
Baseline Condition No Baffle Solids Distribution (colored by solids concentration, red is high) Case 1 - Density Current Baffle Solids Distribution (colored by solids concentration, red is high) 7
Example 2 Bespoke Clarifiers Downriver Wastewater Treatment Facility - Tetratech Daukss, P.R., Richardson, J.E. et al. (2010), Integrated Approach to Secondary Clarifier Improvements Based on CFD Modeling and Proven Best Practices, Water Environment Federation. Drogue Data 8
Operating Conditions Secondary Clarifier System Design Flows and Loadings Secondary Clarifier Tanks Number 6 Size 165-foot diameter Side Water Depth 13.5 feet Design Flow Conditions Peak Design Flow Rate Average Annual Flow Rate Average Dry Weather Flow Rate Minimum Design Flow Rate Maximum Firm Capacity Overflow Rate Average Annual Overflow Rate Average Dry Weather Overflow Rate Solids Loading Conditions Maximum Solids Loading Rate Maximum Return Activated Sludge 125 MGD 65 MGD 50 MGD 25 MGD 1,170 GPD/SF for 5 clarifiers in service 608 GPD/SF for 5 clarifiers in service 468 GPD/SF for 5 clarifiers in service 50 lb\sf\day at 4000 Mg\L, 50% RAS 30% at 125 MGD, 37.5 MGD Influent Flow (Detail) Baseline CFD Results (red surface shows position of sludge blanket, blue cross hatching shows approximate effluent launder locations) 9
Short-Circuiting Flow Pattern in Vertical Plane near Gooseneck (colored by speed, red is fast) Comparative Analysis Scenario Number Description Effluent Solids Concentration (% of Benchmark) 1 Benchmark N/A Alt. 1 Single Launder Modification, 80% Existing Perimeter Feed Alt. 2 Single Launder, Modified Perimeter Feed Design, Short Skirt Baffle 75% Alt. 3 Alt. 4 Alt. 5 Single Launder, Modified Perimeter Feed Design, Long Skirt Baffle and One Target Baffle Single Launder, Modified Perimeter Feed Design, Long Skirt Baffle and Two Target Baffles Single Launder, Modified Perimeter Feed Design, Intermediate Skirt Baffle and One Target Baffle 60% 60% 63% 10
Modified Design Target Baffle Skirt Baffle Target Baffle and Skirt Baffle New Peripheral Feed Channel and Single Weir Effluent Launder Benefits of CFD Flow Visualization Like-for-Like Analysis 1 Easy to Carryout What-If Studies Takes full Advantage of Field Data 1 Should not be relied upon to provide predictions of actual operational performance in the future. 11