Ozone Contactor Design Improvements using CFD Modeling

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Copyright © 2012 International Ozone Association
Proceedings of the International Ozone Association - Pan American Group Annual Conference, September 23-26, 2012, Milwaukee, Wisconsin
Tuesday AM – Session 16 – Ozone Systems - 2
Ozone Contactor Design Improvements using Computational Fluid
Dynamic Modeling
Justin Bartels1
, Michael A. Oneby 2
, Thomas Bell-Games3
1. MWH Americas, 175 W. Jackson Blvd., Suite 1900, Chicago, IL 91007, USA
2. MWH Americas, 789 N. Water St, Suite 430, Milwaukee, WI 53202-3558, USA
3. Burgess & Niple, 5085 Reed Rd., Columbus, OH 43220, USA
Abstract
A Computational Fluid Dynamic (CFD) analysis performed during the design of four ozone
contactors identified improvements to increase performance of an intermediate ozone process at
a municipal water treatment plant within the design constraints of existing basin dimensions and
an existing hydraulic profile. The Hap Cremean Water Plant (HCWP), one of three water
treatment plants owned and operated by the City of Columbus (City), is being upgraded and
improved. Improvements include the addition of an intermediate ozone process. The existing
treatment train includes coagulation, flocculation, and sedimentation, followed by lime softening,
recarbonation, filtration and disinfection. One existing sedimentation basin will be converted to
four parallel recarbonation basins in series with four ozone contactors. Specific objectives of the
CFD analysis were to assess and improve the flow and velocity distribution patterns, contact
basin design parameters and minimize the head loss through the basins. The analysis utilized the
Flow-3Dv10 computer program developed by Flow Science, Inc. Ozone dissolution is
performed by four parallel sidestream injection systems, each dedicated to a single contactor.
Each sidestream injection system consists of an injection pump, injector, and a nozzle manifold.
The ozonated sidestream is reintroduced into the bulk flow of settled, recarbonated water through
the nozzle manifold located in a submerged box conduit at the front end of each ozone contactor.
Baffling within each contactor and in the influent channel was adjusted during the model-based
design process which allowed for improvement in flow distribution and minimization of head
loss through the contactor. Improvements to flow distribution included the minimization of
recirculation (dead) zones and improved transition between the injection channel and the
serpentine-baffled portion of the contactor. The MRT fell within 6% of the HRT at both
flowrates modeled. The θ10, θ90, BF and MI performance indicators all fell within the “excellent”
range. Headloss through the entire basin (Recarbonation Basin and Ozone Contactor) was
reduced to a total of 27 mm (0.09 ft) at a flow rate of 3,200 m3
/h (20 mgd) and 280 mm (0.92 ft)
at a flow rate of 9,900 m3
/h (63 mgd). The CFD analysis employed during the design process
allowed modifications to improve hydraulic performance with minimal headloss. The expected
result of the work on the complete HCWP improvements is more effective use of the transferred
ozone dose and minimal use of ozone quench at the outlet of the contactors to eliminate residual.
Key Words: Ozone, Ozonation, Ozone Contacting, Sidestream Injection, Numerical Modeling,
Computational Fluid Dynamics, Hydraulic Efficiency

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Introduction
The existing Hap Cremean Water Plant (HCWP) is located east of I-270 off Morse Road in
Columbus, Ohio. It is currently the largest water treatment facility in the City of Columbus
(City), serving a population of more than 500,000 people. The treatment facility consists of two
parallel treatment trains, Plant A and Plant B, beginning at a raw water influent screening station
and continuing through all treatment processes including mechanical screening, low‐service
pumping, alum coagulation/ flocculation/sedimentation, lime softening, recarbonation, dual-
media rapid sand filtration, and chlorine disinfection. Flows for Plants A and B each have an
approved rated capacity of 236.6 cubic meters per hour (m3
/h) (62.5 million gallons per day
[mgd]) and are typically segregated although flow streams can be periodically joined and then re‐
divided during routine maintenance activities.
In April 2010 the City authorized a capital improvement project that included an upgrade of the
existing recarbonation system, addition of intermediate ozonation following recarbonation, and
conversion/rehabilitation of the existing sand filters for operation as biologically active filters.
Figure 1 shows the operational flow diagram of the plant with the location of the proposed
upgrades. The new treatment plant improvements were designed to ensure compliance with
upcoming water quality regulations, specifically addressing the Stage 2 Disinfection By-Products
Rule. The new ozonation system is the focus of this paper.
Due to the location and nature of the modifications within the existing system (i.e. converting
existing settling basins to ozone contactors), the main focus of the HCWP design became its
ability to maximize the efficiency of the ozonation process and limiting head loss trough the
plant.
To confirm that the new ozonation system would meet performance requirements, the design
team determined that Computational Fluid Dynamic (CFD) modeling was required in order to
develop the final locations and configurations of the new contactors, injectors, gates, and baffling
system(s). This conclusion was based on the site constraints at the plant and considered the
potential for the City to pursue post-construction treatment credits.
The CFD analysis was performed on the new ozonation system for Plant A. This plant was
chosen due to the symmetrical layout of the two plants and the lower degree of available
freeboard versus Plant B. The key issues to be resolved through the numerical modeling
approach included: 1) Developing necessary modifications to the contactor geometry and gate
and baffling system(s) to minimize short-circuiting and areas of recirculation/stagnation; 2)
Creating uniform flow distribution within the ozone influent and post-ozonation channels to
maximize dosing efficiency; 3) Determining the resident time distribution (RTD), mean resident
time (MRT) and the hydraulic efficiency of the new ozone contactors; and 4) Minimizing head
loss through the recarbonation and ozonation systems to limit structural modifications to the
upstream portion of the plants.

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Figure 1. HCWP Operational Flow Diagram and Plant Upgrade Locations
Numerical Modeling
The three-dimensional CFD modeling of Plant A was constructed using the Flow-3Dv10
computer program developed by Flow Science, Inc. Flow-3D is based on the use of structured,
free-form rectangular gridding based on the fractional area volume obstacle representation
method and the original and true form of the volume of fluid technique. General information
related to the Flow-3D software package can be found at www.flow3d.com.
The CFD models included a portion of the softened water inflow channel, the recarbonation
influent and influent distribution channels, the recarbonation basins, the ozone influent
distribution channel, the ozone contactors, the post‐ozone and post-ozone distribution channels
and the filter influent channel. The CFD models also included the carbonic acid and ozone
injectors, diversion pumps and piping and all proposed flow control devices (i.e. gates and
baffling). Two configurations were tested as part of the analysis. The first configuration is
referred to as the “original” configuration was based on the 25% design submittal. The second
configuration is referred to as the “proposed” option and is representative of the 95% design that

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was developed during the original configuration CFD model run based on observations of the
steady-state flow and velocity patterns established within the ozone contactors. Renderings of
the two models are presented in Figure 2.
Figure 2. CFD Model Configurations for Plant A
Each of the CFD models was comprised of a series of two simulations (a base simulation and a
restart simulation) each consisting of several linked, multi-block and nested meshes. The coarser,
less-defined base models run faster and were used to get the flow within the plant moving
towards a steady-state condition. The finer, more-defined restart models were then run in order
to provide the necessary resolution to adequately define the geometry of the system components.
The modeling approach utilized Cartesian (i.e. x-, y-, z-) coordinates with finer mesh sizes in the
vicinity of the injectors, diversion pumps and piping, gates and over/under baffling and coarser
meshing extending into the recarbonation basins and ozone contact tanks. The mesh spacing for
the restart simulation ranged between 76 mm and 305 mm (0.25- and 1.00-foot).
The CFD models were run under a one-fluid, free surface condition utilizing a combination of
the gravity and viscosity and turbulence options available within the program. Turbulence was
modeled using the renormalized group theory model. Pressure calculations were made within
each model simulation using the generalized minimum residual implicit pressure-velocity solver

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option.
The boundary conditions used in the numerical analysis included user-specific volumetric flow
rates and pressure boundaries at the inlet and outlet to the plant, respectively.
Flow into the plant via the softened water inflow channel ranged between 3,200 m3
/h (20 mgd)
and 9,900 m3
/h (63 mgd) and assumed an even flow distribution between Plants A and B and that
all of the contactors within Plant A were in operation. The 3,200 m3
/h (20 mgd) flow condition
was tested on both the original and proposed design configurations. The 9,900 m3
/h (63 mgd)
flow scenario was only tested on the proposed design.
The downstream water surface elevations within the filter influent channel were 251.97 m
(826.68 ft) and 252.03 m (826.86 ft) for the minimum and maximum daily demands, respectively.
These elevations were based on hydraulic grade within the existing plant.
In order to help visualize the flow patterns within the ozone contactors, massless tracking
particles were added to the models immediately upstream of the ozone injectors after steady-state
conditions were achieved. These particles were tracked through the model simulations and
helped identify areas impacting the hydraulic efficiency of the contactors such as short-
circuiting, channeling and regions of recirculation or stagnation. In addition to flow
visualization the addition and tracking of the particles through the contactors provided a means
to quantitatively determine the MRT, RTD and hydraulic efficiency of the contactors for each
CFD model scenario tested.
It should be noted that the analysis assumed isothermal conditions. It is recognized that even
small temperature differences within the new contactors, basins and channels leading up to the
contactors can cause the water to stratify (especially during winter and summer months where
outside ambient temperatures can exacerbate the effect) leading to lower disinfection efficiencies.
However, as the ozone contactors and other portions of both Plants A and B will be fully covered,
the heat transfer into or out of this portion of the facility should occur slow enough to maintain
thermal equilibrium during any given month, season or year. This, along with proper gate and
baffle design (i.e. to promote localized turbulence/mixing and vertical movement of flow within
the plant during normal operation) should help minimize the impacts that differential temperature
may have on the performance of the new ozone system.
Model Results
Flow and Velocity Distribution
Graphical (i.e. qualitative) results showing the flow and velocity patterns within the proposed
recarbonation and ozonation facilities for Plant A are presented in Figures 3 through 5 for each
of the model scenarios tested. All plots use velocity units of meters per second (m/s) and feet per
second (ft/s) and a universal color scale ranging between 0.0 m/s (0.0 ft/s) and 0.5 or 0.9 m/s (1.5
or 3.0 ft/s) for the minimum and maximum plant flows, respectively.

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Figure 3. Flow and Velocity Results – Original Baffle Configuration
Plant A - 3,200 m3
/h (20 mgd)

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Figure 4. Flow and Velocity Results – Proposed Baffle Configuration
Plant A - 3,200 m3
/h (20 mgd)

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Figure 5. Flow and Velocity Results – Proposed Baffle Configuration
Plant A - 9,900 m3
/h (63 mgd)
Figure 6 provides snap shots of the particle tracking results over the course of both design
configuration model runs at the minimum plant flow of 3,200 m3
/h (20 mgd). The plot provides

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a side-by-side comparison of the particle progression through the contactors for both design
configurations.
Figure 6. Particle Tracking Comparison - Original versus Proposed Design Configurations
Plant A - 3,200 m3
/h (20 mgd)

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As seen in the figures, the original baffle design produced a non-uniform flow distribution within
the contactors and large dead zones immediately downstream of the ozone injectors that
extended nearly half the length of each contactor. These zones (seen as darker blue patches
along the free surface on Figure 3 and as concentrations of particles along the free surface
immediately downstream of the ozone injectors on Figure 6) are indicators of poor hydraulic
efficiency.
The addition of the serpentine channels and additional over/under baffling to the ozone
contactors as seen on Figures 4 through 6 showed a marked improvement in the hydraulic
conditions within the ozonation system. Although smaller recirculation zones were still evident
within the contactors and near the central underflow, the flow conditions were more uniformly
distributed both spatially and vertically within the contactors over the course of the model
simulations.
RTD Analysis
The RTD and hydraulic efficiency of contact tanks are typically determined using stimulus-
response tracer studies to detect design flaws such as short circuiting and regions of recirculation
or stagnation (i.e. dead zones). As these studies need to be performed after a contactor design is
implemented and are often impractical or prohibitively expensive to conduct in the field, an
alternative means of determining the RTD can be made using CFD modeling as it accounts for
the complete flow dynamics within any given contactor.
The RTD analysis for this study adopted the use of particle tracking (i.e. the particle tracking
method). The initial block/pulse of particles used in the analysis totaled approximately 66,000 in
number and was split evenly between the two Plant A ozone contactors (east and west). The
particles were added immediately upstream of the ozone injectors after steady-state conditions
were achieved and were tracked over time through the contactors while observing the particle
response at the outlet. The total simulation time for the two 3,200 m3
/h (20 mgd) scenarios was
9,200 seconds (~2.6 hours). The total simulation time for the 9,900 m3
/h (63 mgd) scenario was
3,700 seconds (~1.0 hour).
Figures 7 through 9 show the normalized RTD and cumulative particle concentration curves (i.e.
F-curves) generated for the three scenarios tested. Using the F–curves it is possible to calculate
the percentage of particles recovered at the outlet of the contactor for various times from the area
under the curve. For example, in Figure 7 it can be seen that the percentage of particles
recovered after 3,600 sec (1 hour) within the west contactor was 58%, meaning that 58% of the
particles spent an hour or less within the contactor.
As shown on Figure 7 the normalized RTD curves for the original design configuration contain
multiple early peaks and the MRT calculated from the RTD curves is over 60% higher than the
theoretical hydraulic residence time (HRT). Furthermore, even after running the model over 2.1
times the HRT, nearly 20 to 25% of the particles introduced to the contactors remained within the
model domain with most being caught within the large recirculation zones identified in the
previous section.

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The installation of the proposed design improvements significantly improved the system
performance as the multiple peaks were removed from the RTD curves and the MRT fell to
within 4% of the HRT for both plant flows (see Figures 8 and 9). The shapes of the F-Curves
were also steeper than what was observed under the original design which demonstrates the
benefits the serpentine channels and additional over/under baffling have on the hydraulic
performance of the contactors.
Figure 7. Normalized RTD and Cumulative Distribution Curves – Original Baffle
Configuration – Plant A - 3,200 m3
/h (20 mgd)

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Figure 8. Normalized RTD and Cumulative Distribution Curves – Proposed Baffle
/h (20 mgd)

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Figure 9. Normalized RTD and Cumulative Distribution Curves – Proposed Baffle
/h (63 mgd)
After the RTD for the contactors were developed several performance indicators were pulled
from the results to assess the hydraulic efficiency of the disinfection system and benchmark
performance. Although these performance indicators focus on hydraulic efficiency, they also
reflect the expected water quality within the contactors. These indicators are the Effective
Volume Factor (EVF) or θ10, θ90, and the Morril Index (MI).

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The EVF and θ90 values are dimensionless time ratios taken as the first 10% and 90% of the
particles reach the outlet, respectively compared to the HRT. A contactor with significant short
circuiting, channeling and/or dead zones will have a lower EVF and a higher θ90 than a contactor
that exhibits plug flow characteristics.
The MI measures the degree of diffusion within a disinfection system. The MI compares the
times the first 10% and 90% of the particles have passed through the contactor and determines
the spread between t10 and t90 or the relative tightness of the RTD curve. The tighter the
distribution and the higher the MI ratio, the closer to plug flow the system is.
These three hydraulic performance indicators were broken down into a four-tier rating system as
listed in Table I to aid in the evaluation of the new HCWP ozonation system. The ranges
highlighted in the table are typical within the water industry. Results for each of the three CFD
operating scenarios based on this rating system are provided in Table II.
Table I
HYDRAULIC PERFORMANCE INDICATORS
Hydraulic Efficiency Indicator EVF or θ10 θ90 MI
Poor <0.2 >2.3 >10.0
Compromising 0.2 to 0.5 2.0 to 2.3 5.0 to 10.0
Acceptable 0.5 to 0.7 1.5 to 2.0 2.5 to 5.0
Excellent >0.7 <1.5 <2.5
As expected the results of the RTD analysis for the original design showed the performance
indicators lying mostly within the “compromising” and “poor” ranges of the four-tier rating
system based on qualitative results presented in the previous sections. Incorporation of the
serpentine channels and additional baffling within the proposed design demonstrated a much
higher degree of hydraulic efficiency within the contactors under both minimum and maximum
plant flows. The performance indicators for these two runs were shown to lie almost exclusively
within the “excellent” range of the four-tier rating system used in the analysis.

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Table II
HYDRAULIC PERFORMANCE RESULTS – PLANT A
Contactor Contactor Contactor
Hydraulic Efficiency Indicator West East West East West East
Design Configuration Original Proposed Proposed
Contactor Volume “V” (Ml / mg)1
4.28/ 1.13 3.14 / 0.83 3.26 / 0.86
Plant A Flow “Q” (mgd / m3
/h) 3,200 / 20 3,200 / 20 9,900 / 63
t10 (sec)2
2,302 2,437 2,603 2,591 867 847
t90 (sec) 21,799 23,095 4,870 5,188 1,681 1,786
HRT (sec)3
4,868 3,587 1,184
MRT (sec)4
7,815 7,789 3,471 3,562 1,191 1,236
HRT/MRT 0.62 0.63 1.03 1.01 0.99 0.96
EVF or θ10
5
0.47 0.50 0.73 0.72 0.73 0.72
θ90
6
4.43 4.70 1.36 1.45 1.42 1.51
MI 9.47 9.48 1.87 2.00 1.94 2.11
Notes:
(1) The water levels used to generate the operational contactor volume and the HRT for the
minimum and maximum (plant flows were taken within the post-ozonation distribution
channel from the results of the CFD analysis.
(2) The minimum recommended industry standard for ozone contactors is 10 minutes (600
sec).
(3) HRT = V/Q where V represents the contactor volume and Q represents the average
volumetric flow rate through the contactor. The minimum HRT for the project is 15 minutes
(900 sec).
(4) MRT =
∗ ∗
∗
where ti represents the time of the ith
measurement, Ci represents the
number of particles passing through the outlet between times ti and ti-1, and dt represents the
time between measurements.
(5) θ10 =
(6) θ90 =

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Head Loss Analysis
The estimated head loss within the modified portion of Plant A for each model scenario, based
on the pressure, velocity and water surface elevations taken from the CFD models are shown in
Table III. The downstream point used in the head loss analysis was located within the filter
influent channel channel approximately 6.1 m (20.0 ft) downstream of the post ozone
distribution channel.
TABLE III
PLANT A HEAD LOSS DETERMINATION
Design
Configuration
Plant A
Flow
(m3
/h / mgd)
Head Loss
(mm / ft)
Downstream of the
Carbonic Acid Diffuser
Upstream of the
Original 3,200 / 20 6.1 / 0.02 24.4 / 0.08
Proposed 3,200 / 20 12.2 / 0.04 27.4 / 0.09
Proposed 9,900 / 63 128.0 / 0.42 280.0 / 0.92
As seen in the table, the average head loss within the facility is expected to vary between 6.1 mm
(0.02 ft) and 280.0 mm (0.92 ft) depending on the plant flow rate and design configuration.
These losses exceed the available headloss for the proposed design and would require
modification of existing treatment systems to account for elevated water levels.
Sensitivity Analysis
As seen in Table III over half of the head loss through the new recarbonation and ozonation
system for Plant A occurs within the recarbonation influent channel (RI2) upstream of the
carbonic acid diffuser underflow baffle. In order to reduce the head loss at this location a
sensitivity analysis was performed on this area of the plant to investigate various modifications
to the proposed design. The analysis tested the following modifications as shown in Figure 10
and described below:
1. Modification 1 – Replace the proposed vertical underflow baffle located immediately
upstream of the carbonic acid diffuser with a 45° slanted underflow baffle;
2. Modification 2 – Move the carbonic acid diffuser and vertical underflow baffle located
within RI2 downstream 29.72 m (97.5 ft) to align with the proposed design ozone
contactor underflow baffling;
3. Modification 3 – Remove the proposed vertical underflow baffle and shift the carbonic
acid diffuser upstream to the entrance to RI2;

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4. Modification 4 – Remove the proposed vertical underflow baffle, shift the carbonic acid
diffuser upstream to the entrance to RI2 and shift the position of gate RI2-03 to the east
side of the existing channel;
diffuser upstream to the entrance to RI2 and remove gate RI2-03 and the wall containing
gate RI2-03; and
diffuser upstream to the entrance to RI2, extend RI2 to the south by installing a new wall
along the west side of the existing channel and realign gate RI2-03 to run north-south
along a new wall located at the west end of the softened water inflow channel.
The revised CFD models for this portion of the facility included approximately 67.1 m (220.0 ft)
of the recarbonation influent channel and all gates and walls located within the softened water
and recarbonation influent channels. Boundary conditions used for each of the sensitivity runs
included an upstream volumetric inflow of 9,900 m3
/h (63 mgd) and a downstream water surface
elevation set at 252.07 m (827.00 ft) within the recarbonation influent channel. All other CFD
model parameters used in the analysis were the same as discussed in the previous sections.
Graphical results showing the flow patterns within this portion of the facility for the various
design modifications are presented in Figure 11. The plot has the same velocity units and scale
used in Figures 3 through 6.
Head loss estimates for each of the proposed RI2 modifications are presented in Table IV.
As seen in the Figure 11, the results of the sensitivity analysis shows how the proposed
underflow baffle added for the carbonic acid injection creates a high velocity zone immediately
upstream of the baffling and a highly turbulent recirculation zone immediately downstream of
the baffling. Shifting or changing the orientation of the proposed underflow baffle helps to
reduce the flow velocities and degree of turbulence at the upstream end of RI2, however,
removing the baffle and shifting gate RI2-03 to the east (Modification No. 4) or eliminating it
completely (Modification No. 5) proved the best means to smooth out the flow between the
softened water inflow channel and RI2 as it helps reduce or even eliminate the size of the flow
separation areas at the entrance to RI2 and provide more uniform vertical and/or spatial
distribution of the flow within the channel.

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Figure 10. Proposed RI2 Channel Modifications
Plant A - 9,900 m3
/h (63 mgd)

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Figure 11. Flow and Velocity Results - Proposed RI2 Channel Modifications
Plant A - 9,900 m3
/h (63 mgd)
As seen in the table below, the average head loss within this portion of the facility could be
reduced by over 70% with the adoption of Modification Nos. 4, 5 or 6. These modifications

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would bring the proposed head loss within allowable limits and prevent structural modifications
to the upstream treatment system(s).
Table IV
Head loss Determination - RI2 Modifications
Plant A
Flow
(m3
/h / mgd)
Modification
ID
Modification
Description
Head Loss
(mm / ft)
Upstream of the
Underflow Baffle
9,900 / 63
-
Proposed Vertical Underflow
Baffle
137.2 / 0.45
1
Replace Vertical Underflow
Baffle with 45° Slanted
Underflow Baffle
106.7 / 0.35
2
Move Vertical Underflow
Baffle 29.72 m (97.5ft)
Downstream
100.6 / 0.33
3 Remove Underflow Baffle 79.3 / 0.26
4
Remove Underflow Baffle and
Move Gate RI2-03 to East
Side of Channel
39.6 / 0.13
5
Remove Underflow Baffle and
Remove Gate RI2-03 and
Wall Containing Gate RI2-03
39.6 / 0.13
6
Remove Underflow Baffle,
Extend RI2 to South and
Move Gate RI2-03 to
Softened Water A Inflow
Channel
39.6 / 0.13
DISCUSSION AND CONCLUSIONS
The numerical modeling effort proved to be very efficient at sorting through some of the major
design constraints and components for the new ozonation system by allowing the design team to
quickly and economically test a variety of contactor, injector, gate, and baffling systems designs.
In particular, the numerical modeling approach proved to be very beneficial in:
 Estimating the hydraulic performance of the new recarbonation and ozonation systems;
 Increasing the design efficiency of the contactors through revisions to the original design
configuration;

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 Minimizing the head loss through this portion of the plant;
 Reducing construction and design costs;
 Minimizing the operational complexity; and
 Limiting the City’s risk/exposure to poor system performance that would likely lead to an
increase in the amount of ozone and chemical quenching (and thus operating costs)
required to operate the plant.
Lastly, the modeling effort provided the City with a high degree of confidence in pursuing post-
construction treatment credits, if ever required.
The decision to utilize a numerical modeling approach for any project should be made on a case-
by-case basis. Careful consideration needs be taken by both owners and designers early on and
throughout the design process to weigh the schedule and financial impacts these analyses entail.
Informed decisions can then be made about how and when these types of analyses could best be
used (or not used) to protect public and private interests and improve the overall design.
REFERENCES
1. “Recommended Standards for Water Works - Policies for the Review and Approval of
Plans and Specifications for Public Water Supplies”. 2007 Edition. n.d. Web. 28 March.
2012. http://10statesstandards.com/waterstandards.htML.

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