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Using Computational Fluid Dynamics in the Validation of Site-Specific Installations of UV Disinfection Systems

Introduction

This white paper will be useful to engineers working on technical problems related to UV disinfection systems.

Ultraviolet (UV) disinfection of drinking water is receiving rapidly growing acceptance inNorth America and around the world. However, there are a few practical and regulatoryissues facing the UV industry.

UV disinfection does not have residual effects on treated water. Thus, no direct meas-urements can be made on drinking water samples to demonstrate the efficacy of UVdisinfection. Therefore, biodosimetry validation of UV disinfection systems is required.With the establishment of two validation facilities in the United States, off-site valida-tion of UV systems is becoming more widely accepted. However, off-site validationdoes not address site-specific hydraulics (the upstream and downstream piping con-figuration) and water quality issues, or the aging of equipment such as sleeves andlamps. On-site validation can address some of these issues, but often has constraintswith injecting and discharging validation microbes (which must be non-pathogenic tohumans) in water treatment plants. Additionally, there can be issues with sampleinjection and mixing due to physical constraints such as piping configuration. On-sitetesting does not easily allow modifications to the equipment in the case of disinfectionobjectives not being attained. Site-specific validation can be cost prohibitive. Toaddress this issue, this white paper proposes very specific guidelines for the use ofCFD, together with baseline bioassay results, to evaluate reactor performance undersite-specific inlet and outlet hydraulic conditions.

A brief summary of CFD application in the UV industry is presented, as well as comparisons between biodosimetry datagathered for various upstream and downstream hydraulic conditions and CFD modeling results.

Trojan Technologies Inc. and Fluent Inc.

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Flow Modeling and Software Quality Assurance

Computational fluid dynamics (CFD) is the usage of numerical algorithms to solve two and three-dimensional flowfields. While it began as a tool primarily for modeling air flow for aircraft design and weather modeling, it is now usedextensively in a wide variety of industries. CFD modeling can be used as a powerful tool to address some site-specificUV disinfection concerns based on the history and experiences of applying CFD in other industries for regulatory compliance, and research and development of CFD models in the UV industry.

http://www.trojanuv.com

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The use of CFD for determination of UV reactor performance involves the calculation of flow through complex pipingnetworks, as well as the coupling of flow to light irradiation and inactivation kinetics models. The complex piping cancause separated regions of reversed flow, which also leads to high speed regions in which fluid parcels and the accom-panying microbes may absorb less than the required amount of UV irradiation. It is in determining the amount of irra-diation received that CFD plays such a valuable role in this industry.

While flow through straight pipes has been understood for many years through empirical studies, T-junctions and pipebends, especially in quick succession can cause unexpected flow structures which can only be determined throughCFD or experimental methods. CFD has been extensively and favorably compared to experimental data for this classof flows, including U-bends, transition ducts, constricted tubes, and expansions.

The use of CFD in the process industry has been growing substantially in the past ten to fifteen years. This has includednot only validation of basic flow and heat transfer through distributed obstacles, but also multiphase and chemicallyreacting flows, involving equipment ranging from continuously stirred tanks to fluid catalytic cracking (FCC) units. Soit is no surprise that CFD has gained attention for the design, optimization and evaluation of UV reactor performance.

Currently, CFD software packages that are ISO 9001 certified must be rigorously validated for every version of the soft-ware to ensure quality. One method for doing this is suggested by the American Institute of Aeronautics andAstronautics1. For a given CFD application, four levels of tests are suggested: unit tests, benchmark tests, subsystemtests, and a complete system test. Each test levels increases in complexity, with unit tests handling the validation ofa single model or feature, and complete system tests representing a full scale industrial problem.

As an example of how this is handled for commercial software, Fluent Inc. performed a combination of application test-ing and automated matrix testing for the FLUENT 6 product. During the development phase both types of testing takeplace while during production testing only automated testing on all supported platforms is performed. The FLUENT 6test matrix has collected regression tests over a period of ten years and a set of new tests for testing of features innew releases.

How CFD is Used in the UV Industry

CFD modeling of UV disinfection includes the modeling of flow, UV irradiation and microbial inactivation.

Numerical studies of UV reactor fluid dynamics and disinfection can be dated back to the early 1980s, with significantresearch ensuing over the last twenty years. Two approaches to modeling dosage have been taken, one which handlesmicrobial inactivation by using the concentration of live microbes analogous to a chemical species. The alternativeapproach treats microbes as particles traveling through the reactor. Recently AWWARF and WERF commissioned twomajor studies to thoroughly examine CFD modeling of UV reactors and model validation. Both studies showed that CFDmodeling can provide reasonably good predictions of UV reactor disinfection performance2,3.

Starting in the late 1990s, CFD modeling has been increasingly used by UV manufacturers, consulting engineers and utilities,providing many examples of good agreement between CFD predictions and biodosimetry results over an extensive rangeof test conditions, such as flow, UVT and lamp power for a number of commercial medium pressure UV reactors. For man-ufacturers, CFD is commonly used for optimizing reactor design, while engineering services companies have successfullyapplied CFD modeling to municipal projects, including the largest UV drinking water plant in the world in New York City 4.

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The Key to Success When Applying CFD: Validation

1. It is essential to measure accurately the input parameters to CFD, such as lamp UV output power, inactivationkinetics, water absorption spectrum and microbe action spectrum. For example, accuracy in the lamp UV output value(required as input for the CFD model) is critical since any variance of this number can linearly affect the reactor dosedelivery. It is not trivial to measure the lamp UV output power accurately, since doing so requires a goniometric scanof the lamp UV output in three dimensional space.

2. Validation of sub models in CFD is also critical, including the UV fluence rate model and the inactivation kineticsmodel. A number of fluence rate models have been developed over the years, but the cosine emitter model has beenwell validated, and matches the physics of light irradiation most accurately. In addition, there are complicated opticalphenomenal which are sometimes ignored or over-simplified, such as reflection and refraction on the sleeves, lamp tolamp optical interaction (shadowing and absorption), reactor wall reflection, etc. Some of the physics remain to be bet-ter understood, and then properly described mathematically. Therefore, validation against experimental measurementsis the key.

3. Validation of dose distribution generated by CFD is made possible by recent research using microsphere particles5,6.This is a powerful validation tool to ensure confidence in CFD modeling.

4. Biodosimetry validation is the final check of the accuracy of the integral of input parameters, the fluids model, lightirradiation model, inactivation kinetics model and their mathematical processing. As shown in Figure 1, CFD predic-tions are in good agreement with biodosimetry results for five medium pressure UV reactors over a wide range of flowrate, UVT, and three power levels. While there are differences between calculated RED (reduction equivalent dose)and measured RED, it is important to note that biodosimetry, like any other experimental method, has uncertaintiesand errors associated with it, par-ticularly for full-scale biodosimetrytests. With the current biodosime-try technique and protocols, it isexpected to have a +/- 0.2 log ofvariation in microbe log reduction,or approximately +/- 4 mJ/cm2 ofMS2 fluence in the biodosimetryresults. Therefore, validated CFDmodels by biodosimetry do notmean that CFD predictions matchbiodosimetry results perfectly.Validated CFD models need todemonstrate consistent agree-ment with biodosimetry over the entire range of flow rate, UVT and lamp power level tested without a bias against anyof these three operating parameters. There is then reasonable confidence in applying the same CFD model for predictingthe reactor performance within the validated operating range for examining the impact of site-specific conditions, suchas hydraulics and water quality.

While additional physical models and inputs need to be validated as stated above, some of the effort in model validationcan be reduced through the use of CFD software that is ISO-9001 certified, indicating the vendor’s use of an approved QA/QC program.

Figure 1: Comparison between calculated RED vs. measured RED for five MP reactors, flow range: 2.5MGD to 19 MGD, UVT: 75%/cm to 95%/cm, and three lamp power levels.

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Based on the published literature and CFD experiences in various industries, CFD technology has become mature androbust. Compared to some industries like aerospace and power generation which may have to deal with more complicatedphysics such as fluid compressibility, multiphase flow, combustion, etc., CFD modeling of drinking water disinfection inclosed pipes and vessels is more straightforward. CFD modeling of UV disinfection systems can provide reasonably goodagreement with experimental results if the due-diligence discussed above is performed and the modeling is done properly.

Finally, it should be recognized that the value of any tool depends to a varying extent upon the expertise of the user. Theuse of CFD is no longer limited to specialized experts with Ph.D.s in the field, but a good understanding of fluid mechanicsand UV disinfection systems as well as careful model validation are important to a successful modeling effort.

The Application of CFD Modeling for Meeting Regulatory Requirement

After CFD models are validated against biodosimetry over the range of the tests, the same models could be used withinthe operating range in the following areas for meeting regulatory requirement by the EPAUVDGM draft, 20037 to assess:• RED bias safety factor by predicting dose distribution, • the impact of site-specific water quality and its variation,• the impact of lamp and sleeve aging and fouling, • the impact of technology upgrades such as lamp, sleeve and sensor, and• the impact of site-specific hydraulics.

The following example shows how validated CFD modeling is used to assess the impact of site-specific hydraulics on thereactor dose delivery, and how CFD is used effectively to optimize the hydraulic design and ensure public safety.

The Trojan UVSwiftTM 8L24 reactor was vali-dated by third party biodosimetry testingunder a given hydraulic configuration. When amunicipality was considering an installation ofthe reactor, it was found that the proposedsite-specific hydraulic configuration (design 1)did not match the validation configuration dueto the footprint constraint, as shown in Figure2. The local regulatory agency had a questionabout the impact on dose delivery due to thisvariation. A CFD model validated against bio-dosimetry over a wide range of operating con-ditions was used to assess this impact. Boththe site-specific design 1 and the validatedhydraulic configuration were simulated andcompared. As shown in Table 1, design 1resulted in a 9% reduction in dose deliverydue to flow stratification entering into the reac-tor as shown in Figure 3. Upon gaining insightinto the flow behavior using CFD, a newdesign was proposed to incorporate an

Flow Rate

Biodosimetry RED

Piping Configuration [MGD] [mJ/cm2] Hydraulics

Factor 9.0 51.6 1.00

Biodosimetry validated 10.5 47.1 1.00 9.0 N/A 0.91 Site-specific hydraulics design

1: 90Y Bend 10.5 N/A 0.92 9.0 N/A 0.98 Site-specific hydraulics 2:

Reducer-Elbow 10.5 N/A 0.98 Note: Hydraulic factor = RED_CFD_site / RED_CFD_validation

Table 1: Impact of site-specific hydraulic design on dose delivery

Figure 2: Site-specific hydraulics design 1 - piping configuration

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expansion upstream of the elbow (Figure 4) whichresulted in significant improvement in flow distri-bution entering the reactor, shown in Figure 5. Asa result, this new design minimized the impact ondose delivery due to site hydraulic constraints, aswell allowing a simple implementation with only amarginal increase in cost. The alternative of site-specific biodosimetry validation after the installa-tion would have been cost prohibitive and wouldnot have allowed for the opportunity of designoptimization.

Figure 3: Velocity magnitude contours of the site-specific hydraulic design 1

Protocols for Using CFD Modelingfor Assessing the Impact of Site-Specific Hydraulics

The following protocols are proposed for the properuse of CFD modeling for assessing the impact ofUV reactor upstream/downstream hydraulics onreactor performance.

1. Validated CFD models can be used to assessthe impact of site-specific hydraulics, includingelbows, contraction/diffuser, valves and other flowobstructions in the piping upstream and down-stream of UV reactors.

2. The UV reactor with hydraulic piping configu-rations used during biodosimetry validation shouldbe modeled first. The modeled operating condi-tions such as flow rate, UVT, and lamp power levelshould cover the design conditions. Elbows, con-traction/diffuser, valves and other flow obstructionsin the piping within ten diameters upstream of thereactor should be modeled explicitly. Downstream hydraulic configuration is not as critical. Three diameters of down-stream configuration are recommended to be included in the CFD model. Comparison of the CFD results to the physicaltest results should be compared over the entire range of parameters to ensure against bias and model “tuning.”

3. The CFD results should be in agreement with biodosimetry to within at least +/-20%. A CFD calibration factor can becalculated: CFD factor = REDCFD_validation / REDbiodosimetry, where:REDCFD_validation is the reduction equivalent dose predicted by CFD for the validation conditions; andREDbiodosimetry is the reduction equivalent dose measured during biodosimetry validation.

Figure 4: Site-specific hydraulics design 2: with a reducer followed by an elbow in theupstream piping

Figure 5: Velocity magnitude contours of the site-specific hydraulic design 2, with areducer followed by an elbow in upstream piping

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4. The same CFD models employing the same settings and physical models can then be applied to model the performance of the same reactor with site-specific hydraulic conditions. A hydraulic factor can be developed: Hydraulic factor = REDCFD_site / REDCFD_validation,where:REDCFD_site is the reduction equivalent dose predicted by CFD for the site-specific hydraulic conditions.

Thus, REDCFD_site = REDbiodosimetry x CFD factor x Hydraulic factor

Quality control of the work can be managed by ensuring that physical variables are accurate and modeling variablesare robust. The following CFD QA/QC checklist is recommended to ensure the use of the proper model inputs.

Check physical variables:• Physical dimensions of reactor and upstream and downstream hydraulic configuration• Input conditions

- Flow rate- Water physical properties, such as density and viscosity- Water UV transmittance at 254 nm, water absorption spectrum for medium pressure UV reactors- Action spectrum of microbe- Lamp power, spectrum and efficiency (for the given power level)- Sleeve UV transmittance and absorption spectrum for medium pressure UV reactors- Inactivation kinetics

Check modeling variables:• Boundary conditions

- Wall, symmetry, inlet and outlet• CFD model set up

- Mesh quality- Mesh density- Turbulence model selection and parameters- Convergence criteria and mass balance

Choices regarding the items in the above checklist are likely to have some impact on the computational results and,as such, should be well documented and consistent when comparing upstream and downstream hydraulics for differ-ent sites. A sensitivity analysis to input parameters involving significant uncertainty (particularly to grid meshing andturbulence modeling) should be documented.

Summary

• Based on the history and experiences of applying CFD in other industries for regulatory compliance and research and development of CFD models in the UV industry, CFD modeling can be used as a powerful tool to address regulatory issues related to applying UV to drinking water disinfection. Currently some of these concerns, such as reactor performance with site-specific hydraulics do not have a universal, cost-effective alternative.

• CFD models must be validated by biodosimetry and used properly. CFD input parameters must be measured accurately.• Robust CFD QA/QC and a biodosimetry validated model provides confidence in applying CFD for predicting the

relative performance of a UV reactor for site-specific hydraulics.

Further details on using CFD for UV reactor hydraulics can be found in the proceedings of the IUVA Congress, 20058.

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Fluent is a registered trademark of Fluent Inc. All other brand or product names are trademarks orregistered marks of their respective owners. Copyright © 2006 Fluent Inc. All rights reserved.

1 AIAA Guide for the Verification and Validation of Computational Fluid Dynamics Simulations, American Institute of Aeronautics and Astronautics,AIAA-G-077-1998, Reston, VA, 1998.

2 Blatchley III, E.R., Shen, C., Naunovic, Z., Lin, L.S., Lyn, D.A., Robinson, J.P., Ragheb, K., Gregori, G., Bergstrom, D.E., Fang, S., Guan Y.,Jennings, K. and Gunaratna, N., Dyed Microspheres for Quantification of UV Dose Distributions: Photochemical Reactor Characterization byLagrangian Actinometry, Proceedings of Disinfection 2005, Mesa, Arizona USA, February 6-9, 2005.

3 Ducoste, J.J., Liu D., Linden, K.G., Mamane-Gravetz, H. And Bohrerova, Z., Impact of Influent Pipe Configuration on UV Reactor Performance: Isthe Elbow Truly the Worst Case Hydraulic Condition?, Proceedings of Disinfection 2005, Mesa, Arizona USA, February 6-9, 2005.

4 Rokjer, D., Valade, M., Keesler, D. and Borsykowsky, M., Computer Modeling of UV Reactors for Validation Purposes, Proceedings of 2002 WQTCConference, November 10-14, 2002, Seattle, Washington, USA.

5 Blatchley III, E.R., Shen, C., Naunovic, Z., Lin, L.S., Lyn, D.A., Robinson, J.P., Ragheb, K., Gregori, G., Bergstrom, D.E., Fang, S., Guan Y.,Jennings, K. and Gunaratna, N., Dyed Microspheres for Quantification of UV Dose Distributions: Photochemical Reactor Characterization byLagrangian Actinometry, Proceedings of Disinfection 2005, Mesa, Arizona USA, February 6-9, 2005.

6 Anderson, W.A., Zhang, L., Andrews, S.A. and Bolton, J.R., A Technique for Direct Measurement of UV Fluence Distribution, Proceedings of 2003WQTC Conference, November 2-6, 2003, Philadelphia, Pennsylvania, USA.

7 US EPA, 2003, Ultraviolet Disinfection Guidance Manual, Draft, Office of Water, Washington, DC, EPA 815-D-03-007.

8 Mao, T. & Schowalter, D.G., Using Computational Fluid Dynamics in the Validation of Site-Specific Installations of UV Disinfection Systems, ThirdInternational Congress on Ultraviolet Technologies, May 24-27, 2005, Whistler, BC Canada.

Trojan Technologies Inc.As a wholly-owned subsidiary of Danaher Corporation of Washington, DC, Trojan designs, manufactures and sells UV systems for municipal wastewater and drinking water facilities, as well as for the industrial, commercial and residential markets. Trojan also provides UV treatment for the removal of certain chemicals from water. Headquartered in London, Ontario,Canada, the company also has offices in the UK, Germany, Netherlands, Spain, and the US. Trojan’s success is evident inmore than 4,000 municipal UV disinfection facilities operating in over 50 countries – the largest installed base of UV systemsin the world.

For more information: www.trojanuv.com • 888-220-6118

Fluent Inc.Fluent is the world’s largest provider of computational fluid dynamics (CFD) software and consulting services. Fluent’s software is used for simulation, visualization, and analysis of fluid flow, heat and mass transfer, and chemical reactions. It is avital part of the computer-aided engineering (CAE) process for companies around the world and is deployed in nearly everymanufacturing industry. Fluent has a wealth of experience in providing simulation capabilities and services in the wastewaterand drinking water industries, and a well-earned reputation for validated and user-friendly flow modeling tools. Fluent’s corpo-rate headquarters are located in Lebanon, New Hampshire, USA, with offices in Belgium, China, England, France, Germany,India, Italy, Japan and Sweden. Its CFD software is also available around the world through joint ventures, partnerships, anddistributors in Australia, Brazil, China, the Czech Republic, Europe, Korea, the Middle East, and Taiwan.

For more information: www.fluent.com • [email protected] • 800-445-4454 x322

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