1. Analysis of an Ozone
Contactor Tank
Presented by: Nadera Nawabi, Henk Williams & Nick Mead-Fox

2.  Nadera Nawabi – Data Analyst
 Henk Williams – CFD Modeller
 Nick Mead-Fox – CFD Modeller
Meet our team…

3.  Determine geometry of the ozone contactor tank at the San
Andreas Water Treatment Plant (SAWTP)
 Develop a computational fluid dynamics (CFD) model of the
ozone contactor to determine flow characteristics
 Compare CFD simulations to the tracer test results obtained
from the SAWTP report
Project Overview

4. Scope
 Develop a 3-D 2-phase model (air & water) that predicts the
hydraulic processes of an ozone contactor
Objective
 Maximize ozone contact time in SAWTP ozone contactors
 Qualitative: analyze “dead spots” in velocity contours before and
after the addition of gas bubblers
 Quantitative: use particle tracking to calculate the average retention
time of particles in the system
Scope & Objective

5.  Ozone has been used for water treatments for almost 100 years
 It is a very strong oxidizing agent and a powerful disinfectant
 Ozone is very effective against almost all microorganisms
Ozone Disinfection

6. Source: (Rakness, 2005)
Giardia and Virus Removal and
Inactivation Requirements

7. Source: (Camp Dresser & McKee, 1994)
CT concept was developed by EPA to quantify disinfection effectiveness
CT Requirements for Various
Disinfectants

8. Contactor
Flow
(mgd)
Total
Air Flow
(scfm)
Simulated
Ozone
Dose
(mg/l)
T10 /T HDT
(mins)
T10
(mins)
20.00 0 0 0.52 10.38 5.35
29.40 0 0 0.61 7.07 4.31
45.00 0 0 0.66 4.62 3.05
20.00 120 1.4 0.66 10.38 6.85
29.40 120 1.0 0.69 7.07 4.88
45.00 120 0.6 0.71 4.62 3.28
20.00 350 4.0 0.68 10.38 7.06
29.40 350 2.8 0.78 7.07 5.51
45.00 350 1.8 0.80 4.62 3.69
Tracer Results from Report

9.  ‘San Francisco Water Department: San Andreas Water Treatment
Plant Ozone Contactor Tracer Tests’
→ used to determine the dimensions of the tank and
compare simulation results
Source of Data

10.  Reducing dead zone regions (areas with very low velocity) in the
ozone contactor tank will improve the disinfection efficiency of the
contactor
Source: (University of Waterloo, 2014)
Why improve hydraulics of an ozone
contactor?

11. Hence, a more purified, safe and
clean water!
Source: (Water Liberty Research Center)

12. Learn
Software
•Complete ANSYS Fluent tutorials
2D, 1 Phase
Prototype
•Achieve proper flow through system – water only
•Extract velocity profile and learning about basic boundary conditions
Geometry of
Tank
•Determine dimensions of ozone contactor using fluid flow relationships and basic geometry
2D, 2 Phase
Tank
Prototype
•Visualize flow in filled container
3D, 2 Phase
Tank
Prototype
•Replicate 2D results
•Experiment with bubblers – full tank bottom vs discrete inlet- mass balance
3D, 2 Phase
Tank Real
Design
•Remove air pocket include ozone bubbler
•Model inlets and outlets
•Achieve steady state
•Extract particle data, and velocity/phase animations
Design Approach

13. Timeline
Dates Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Week 9 Week 10 Week 11 Week 12 Week 13 Week 14
Tasks
Design Team Formation
Assignment of Project
Meet with Advisor
Design Brief
Team Presentation
Build Simple 2D Figure in
Ansys
Build 1-Phase of Model
Build Simple 2D structure
with 2 Phase
Build 2D of Model with 2
Phase
Build 3D Model with 2 Phase
Mid-Term Presentation
Remove Air Pocket in 3D
Model and Include Ozone
Bubbler
Model Inlets and Outlets
Achieve Steady State
Extract Particle Data and
Velocity/Phase Animations
Final Presentation
Written Report

14. CFD Theory
Numerical Models and Considerations

15. Continuity Equations
 1st order upwind scheme - Finite Differencing Scheme
→ Tracks changes by using the mesh element directly upstream of the point
being calculated, solves continuity equations relatively stable and has good
convergence properties, loses some accuracy due to numerical diffusion
 Other schemes: QUICK, 2nd order, WENO more accurate, but greatly increases
computation time of simulatins and increases divergence probability

16.  Continuity equations being solved for mass, momentum, and energy
 Energy is the critical parameter in a turbulent system, requiring a more
complicated energy equation

17. Turbulence Models
Two primary models were used:
k-epsilon
 Tracks changes in k: the turbulent kinetic energy
 Tracks changes in e: the rate of energy dissipation, or change in kinetic energy
(turbulence)
 Relatively stable and converges easily
 Inaccurate when simulating rotating flow, or flow with strong curvature
→ Transferred to omega once had working models in k-epsilon
omega - w
 specific energy dissipation
 Increases accuracy rotating flow, but is less stable, more dependent
on initial conditions.

18. Turbulence Model: k-omega
k:
→ change with time, change with distance (convection) = velocity change with
(shear and viscous elements), current energy, change with dissipation
w:
similar to above
then
where µt is turbulent viscosity- actual term used to fit continuity

19. Prototypes with ANSYS Fluent
Modelling and Data Presetation

20. The Importance of
Pipe Prototypes
Refining Data Presentation
 Steady vs. Transient Modelling
 Velocity Profiling
 Phase Profiling
 Uniformity Indices
 Residence Time
Boundary Condition Properties
Multiphase Modelling

21. The Importance of Pipe Prototypes
Identifying Boundary Condition Properties
Inlet
 Pressure Inlets
 Velocity Inlets
 Mass Flow Inlets
 Inlet Vent
 Intake-Fan
Outlet
 Pressure outlets
 Outflow
 Outlet Vents
 Degassers
 Velocity Outlets
 Exhaust Fan

22. Multiphase Models - VOF
For two immiscible fluids; uses a single set of momentum equations and
the volume fraction in each cell is tracked.
Applications
 Stratified Flows
 Free Surface Flows
 Filling, Sloshing
 Large Bubbles
 Tracking Interfaces

23. Multiphase Models - Mixture
For two or more phases; phases treated as interpenetrating continua.
Solves for the mixture momentum equations, prescribes relative velocities to
dispersed phases.
Applications
 Low Load Particle-laden Flows
 Bubbly Flows
 Sedimentation
 Cyclone Separators

24. Multiphase Models - Eulerian
Eulerian - Most complex multiphase model.
Solves a set of n momentum and continuity equations for each phase.
Applications
 Bubble columns
 Risers
 Particle suspension
 Fluidized beds

25. Ensuring Model Convergence
 Incompatible Boundary Conditions
 Turbulence Errors
 Boundary Backflow
 Vertical Outlets
 Mass Balance

26. Multiphase Mass Balance
Tracer 1: Qw = 45 MGD = 1.972 m^3/s, Qa = 350 SCFM =
Inlet Area = 1.52 m^2, inlet velocity = 0.6485526 m/s, Q = 0.9858m^3/s
Outlet Area = 3.23 m^2, volume fraction = 0.25, effective outlet area = 2.42 m^2
Outlet Flow = Q = 0.9858m^3/s , outlet velocity = -0.4069
Air vent Area = 14.6612, Qair = 350 SCFM = 0.1652 m^3/s, Vair = 4 m/s
Effective area = Q/v = 0.0413
Volume fraction = 0.0413/14.6612 = 0.002817

27. Diffuser Modelling
 Velocity, Area, and Flow: The problems with surface outlets
 Square Inlets: Not representative
 Striped Inlets: Successful, but can’t be placed adjacent to walls
 Volume fraction more appropriate and versatile than re-modelling area changes.
 In all Cases: Inlet Area >>> Mesh Size

28. The Final Product
Particle Pathlines: 350 SCFM

29. Final Contactor Geometry
Depth = 6.55m
Length = 25.6 m
Width = 3.81 m

30. Final Mesh

31. Phase Modelling

32. Velocity Profiling

33. Final Phase Distribution

34. Trace 1: Design Flows
Inlet Area = 1.52 m^2, inlet velocity = 0.6485526 m/s, Q = 0.9858m^3/s
Outlet Area = 3.23 m^2, volume fraction = 0.25, effective outlet area = 2.42 m^2
Q = 0.9858m^3/s , outlet velocity = -0.4069
Air vent Area = 14.6612, Qair = 350 SCFM = 0.1652 m^3/s, Vair = 4 m/s
Effective area = Q/v = 0.0413
volume fraction = 0.0413/14.6612 = 0.002817
Tracer 1: Qw = 45 MGD = 1.972 m^3/s, Qa = 350 SCFM = 0.16518

35. Air Flow vs. Phase Profiles
Qair = 350 SCFM Qair = 700 SCFM
Qair = 1400 SCFM
t = 1000 Seconds

36. Pathlines of Residence Time
Qair = 700 SCFM
Qair = 1400 SCFM

37. Tracer Tests and Residence Times
Scenario Water inlet
velocity
(m/s)
Ozone Outlet
Area (m^2)
Ozone injection
velocity (m/s)
Average
residence
time (s)
Tracer
Residence
Time
Control 0.6485 0 0 3.95 3.05
Trace 1
(350 SCFM)
0.6485 0.0413 4 9.3 3.69
Air 2
(700 SCFM)
0.6485 0.0826 4 11 NA

38. Air Flow vs Velocity Profiles

 Scale: 0 - 1.24 m/s Scale: 0 – 4m/s
 Qair = 0 SCFM
 Qair = 350 SCFM
 Qair = 700 SCFM

39. Qualitative Conclusions
 The relationship between air flow, residence time and disinfection
capacity is nonlinear and poorly understood.
 Air flows required for disinfection and appropriate residence time are
too low to induce turbulence and decrease the presence of hydraulic
dead zones within the contactor.
 The disinfection process is far from homogenous.
 The calculation of CT-values has a significant margin of error.
→ Calculated vs. “True” contact times.
→ Any amount of air flow increases contactor residence time, but does
not necessarily improve the contactors disinfection capacity.

40. A Reference for Further Analysis
 Ozone contactor performance optimization.
 Simulating disinfection scenarios: Injector surface area and
velocity, flow composition, interior surface effects, and gas
extraction methods.
 Dual media injectors - liquid water injection with high ozone
concentrations to mix water and eliminate dead zones.
 The chemistry of ozone disinfection by incorporating CFD-based
CT-calculations

41. Acknowledgements
 Paul Rodrigue, PE
Environmental Engineer at CDM Smith
 Shawn McCollum, McGill IT Services

42. Works Cited
 Stenmark, E. (2013, November 1). On Multiphase Flow Models in ANSYS CFD Software. Retrieved November 27, 2014, from
http://publications.lib.chalmers.se/records/fulltext/182902/182902.pdf
 24.4.1. Discrete Phase Boundary Condition Types. (n.d.). Retrieved November 27, 2014, from
http://www.arc.vt.edu/ansys_help/flu_ug/flu_ug_sec_discrete_bctypes.html
 24.4.1. Discrete Phase Boundary Condition Types. (n.d.). Retrieved November 27, 2014, from
http://www.arc.vt.edu/ansys_help/flu_ug/flu_ug_sec_discrete_bctypes.html
 24.4.1. Discrete Phase Boundary Condition Types. (n.d.). Retrieved November 27, 2014, from
http://www.arc.vt.edu/ansys_help/flu_ug/flu_ug_sec_discrete_bctypes.html
 Ma, J., & Srinivasa, M. (2008, January 1). Particulate modeling in ANSYS CFD. Retrieved November 27, 2014, from
http://www.ansys.com/staticassets/ANSYS/staticassets/resourcelibrary/confpaper/2008-Int-ANSYS-Conf-particulate-modeling-in-ansys-cfd.pdf
 25.3.2. Modeling Open Channel Flows. (n.d.). Retrieved November 27, 2014, from http://www.arc.vt.edu/ansys_help/flu_ug/flu_
 24.4.1. Discrete Phase Boundary Condition Types. (n.d.). Retrieved November 27, 2014, from
http://www.arc.vt.edu/ansys_help/flu_ug/flu_ug_sec_discrete_bctypes.html
 17.2.1. Approaches to Multiphase Modeling. (n.d.). Retrieved November 27, 2014, from
http://www.arc.vt.edu/ansys_help/flu_th/flu_th_sec_mphase_approaches.html
 Rakness, K. L., Ozone in Drinking Water Treatment - Process Design, Operation, and Optimization (1st Edition). American Water Works Association
(AWWA): 2005.
 Camp Dresser & McKee. San Francisco Water Department: San Andreas Water Treatment Plant Ozone Contactor Tracer Tests. 1994.
 WaterLiberty.com - Ancient Water Purification System - Black Mica. (2013, January 1). Retrieved November 27, 2014, from
http://www.waterliberty.com/presentation-dd.php
 Full-Scale Water Treatment Facilities. (2014, January 1). Retrieved November 27, 2014, from
http://www.civil.uwaterloo.ca/watertreatment/facilities/full.asp

43. THANK YOU!
QUESTIONS?