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 Configuration – Plant A - 3,200 m3/h (20 mgd)

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Figure 9. Normalized RTD and Cumulative Distribution Curves – Proposed Baffle Configuration – Plant A - 9,900 m3/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 θ105 0.47 0.50 0.73 0.72 0.73 0.72

θ906 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 Carbonic Acid Diffuser

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;

5. Modification 5 – Remove the proposed vertical underflow baffle, shift the carbonic acid diffuser upstream to the entrance to RI2 and remove gate RI2-03 and the wall containing gate RI2-03; and

6. Modification 6 – Remove the proposed vertical underflow baffle, shift the carbonic acid 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 Carbonic Acid Diffuser

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

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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.

ozone contactor design improvements using cfd modeling

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