mcgill ozone contactor design project

Analysis of an ozone contactor

Analysis of an Ozone Contactor TankPresented by: Nadera Nawabi, Henk Williams & Nick Mead-Fox

Nadera Nawabi Data Analyst

Henk Williams CFD Modeller

Nick Mead-Fox CFD Modeller Meet our team

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 reportProject Overview

ScopeDevelop a 3-D 2-phase model (air & water) that predicts the hydraulic processes of an ozone contactor

ObjectiveMaximize 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

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 microorganismsOzone Disinfection

Source: (Rakness, 2005)

Giardia and Virus Removal and Inactivation Requirements

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Source: (Camp Dresser & McKee, 1994)

CT concept was developed by EPA to quantify disinfection effectiveness

CT Requirements for Various Disinfectants

C is usually defined as the ozone residual concentration at the outlet of a chamber and T is the residence time of microorganisms in the chamber7

Contactor Flow (mgd)Total Air Flow(scfm)Simulated Ozone Dose (mg/l)T10 /THDT(mins)T10 (mins)

20.00000.5210.385.3529.40000.617.074.3145.00000.664.623.0520.001201.40.6610.386.8529.401201.00.697.074.8845.001200.60.714.623.2820.003504.00.6810.387.0629.403502.80.787.075.5145.003501.80.804.623.69

Tracer Results from Report

T10 is the residence time of the first 10% of the water to travel from the contactor inlet to outlet, to ensure a minimum exposure time for 90% of the water and microorganisms entering a disinfection contactor.

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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

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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?

http://www.civil.uwaterloo.ca/watertreatment/facilities/full.asp

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Hence, a more purified, safe and clean water!

Source: (Water Liberty Research Center)

http://www.waterliberty.com/presentation-dd.php11

Design Approach

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Timeline

CFD TheoryNumerical Models and Considerations

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

Continuity equations being solved for mass, momentum, and energy

Energy is the critical parameter in a turbulent system, requiring a more complicated energy equation

Turbulence ModelsTwo 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 easilyInaccurate when simulating rotating flow, or flow with strong curvature

Transferred to omega once had working models in k-epsilon

omega - w specific energy dissipationIncreases accuracy rotating flow, but is less stable, more dependent on initial conditions.

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 t is turbulent viscosity- actual term used to fit continuity

Prototypes with ANSYS Fluent Modelling and Data Presetation

The Importance of Pipe Prototypes

Refining Data PresentationSteady vs. Transient ModellingVelocity ProfilingPhase ProfilingUniformity IndicesResidence Time

Boundary Condition Properties

Multiphase Modelling

The Importance of Pipe PrototypesIdentifying Boundary Condition PropertiesInletPressure InletsVelocity InletsMass Flow InletsInlet VentIntake-FanOutletPressure outletsOutflowOutlet VentsDegassersVelocity OutletsExhaust Fan

Multiphase Models - VOFFor two immiscible fluids; uses a single set of momentum equations and the volume fraction in each cell is tracked.

Applications

Stratified FlowsFree Surface FlowsFilling, SloshingLarge BubblesTracking Interfaces

Multiphase Models - MixtureFor 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 FlowsSedimentationCyclone Separators

Multiphase Models - EulerianEulerian - Most complex multiphase model.

Solves a set of n momentum and continuity equations for each phase.

Applications

Bubble columns

Risers

Particle suspension

Fluidized beds

Ensuring Model Convergence

Incompatible Boundary Conditions

Turbulence Errors

Boundary Backflow

Vertical Outlets

Mass Balance

Multiphase Mass BalanceTracer 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/sOutlet Area = 3.23 m^2, volume fraction = 0.25, effective outlet area = 2.42 m^2Outlet Flow = Q = 0.9858m^3/s , outlet velocity = -0.4069Air vent Area = 14.6612, Qair = 350 SCFM = 0.1652 m^3/s, Vair = 4 m/sEffective area = Q/v = 0.0413Volume fraction = 0.0413/14.6612 = 0.002817

Diffuser Modelling

Velocity, Area, and Flow: The problems with surface outletsSquare Inlets: Not representativeStriped Inlets: Successful, but cant be placed adjacent to wallsVolume fraction more appropriate and versatile than re-modelling area changes.In all Cases: Inlet Area >>> Mesh Size

The Final Product

Particle Pathlines: 350 SCFM

Final Contactor Geometry

Depth = 6.55mLength = 25.6 mWidth = 3.81 m

Final Mesh

Phase Modelling

Velocity Profiling

Final Phase Distribution

Trace 1: Design Flows

Inlet Area = 1.52 m^2, inlet velocity = 0.6485526 m/s, Q = 0.9858m^3/sOutlet Area = 3.23 m^2, volume fraction = 0.25, effective outlet area = 2.42 m^2Q = 0.9858m^3/s , outlet velocity = -0.4069Air vent Area = 14.6612, Qair = 350 SCFM = 0.1652 m^3/s, Vair = 4 m/sEffective area = Q/v = 0.0413volume fraction = 0.0413/14.6612 = 0.002817

Tracer 1: Qw = 45 MGD = 1.972 m^3/s, Qa = 350 SCFM = 0.16518

Air Flow vs. Phase Profiles

Qair = 350 SCFMQair = 700 SCFM

Qair = 1400 SCFM

t = 1000 Seconds

Pathlines of Residence Time

Qair = 700 SCFM

Qair = 1400 SCFM

Tracer Tests and Residence Times

ScenarioWater inlet velocity(m/s)Ozone Outlet Area (m^2)Ozone injection velocity (m/s)Average residence time (s)Tracer Residence TimeControl0.6485003.953.05Trace 1 (350 SCFM)0.64850.04134 9.33.69Air 2(700 SCFM)0.64850.08264 11NA

Air Flow vs Velocity Profiles

Scale: 0 - 1.24 m/s Scale: 0 4m/s

Qair = 0 SCFM

Qair = 350 SCFM

Qair = 700 SCFM

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.

A Reference for Further AnalysisOzone 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

Acknowledgements

Paul Rodrigue, PE Environmental Engineer at CDM Smith

Shawn McCollum, McGill IT Services

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



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_


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.htmlRakness, 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

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