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Steve Saunders

IBIS Group Inc.

CFD simulation has become the preferred approach for modelingcontact basins

• T hrough– flow physicalm odelsareexpensiveplusw ater/w astew aterutilitiesm ay takeexceptiontothelargedischargeratesrequiredtorunthem

• O verthepast20+ years,validationcom parisonsagainstlabscaleandonsitetestsshow excellentagreem entbetw eenCFD andreal-w orldcontactbasinperform ance.

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Three aspects to evaluate in contact basin simulations:

• S ystem Hydraulicefficiency ofthebasingeom etry

• M ixingefficiency ofthedisinfectantchem ical

• Kineticsofthedisinfectantchem ical• Chem icaldecay

• M icroorganism deactivation

U singatim edependentCFD m odel,tracerisintroduced atthebasininlet. T heconcentrationofthetracerism onitoredovertim easitem ergesfrom thebasinoutlet.

Datagathered atthem odeloutletareusedtoplotaresidencetim edistribution(R T D)curve. T heslopeandinflectionpointsoftheR T D curveareindicativeofthehydraulicperform anceofthecontactsystem . DatapointsT 10,T T DT andT 90

areinputsusedforevaluationprotocol.

T 10: tim eatw hichtracerconcentrationhasreached 10% ofthetargetvalue

T T DT : T heoreticalDetentiontim e (Voltank/Q effluent )

T 90: tim eatw hichtracerconcentrationhasreached90 % ofthetargetvalue

BF: BaffleFactor – T 10/T T DT Valuesnear1.0 indicategoodplugflow. Valueslow erthan0.3 indicatesom eshortcircuitingistakingplace.

M I: M orrillIndex -T 90/T 10 Valuesgreaterthan5.0 indicatesom eflow isgettinghungupinrecirculationzones.

S ystem HydraulicEfficiency

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0 0.5 1.0 1.5 2.0

tracerfractionatexit

norm alizedtim e(t/T T DT )

R esidenceT im eDistribution

TTDT

T10

T90

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1 hour

10 min

T = 0

5 min

Tracerfraction

S ystem HydraulicEfficiency Exam ple

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.0 20.0 40.0 60.0 80.0 100.0 120.0

norm alizedconcentrationat

exit

T im e(m inutes)

R esidenceT im eDistribution-N orthBasin

TTDT

T10

T90

BF: T10/TTDT = 0.44

MI: T90/T10 = 4.82

deviation from targetdisinfectant concentration(0.0 is perfectly mixed.)

M ixingEfficiency oftheDisinfectantChem ical

Cdev (%)

U singasteadystateCFD m odel,tracerrepresentingthedisinfectantisintroducedatthedisinfectantinjectionpoint.

R esultsw illshow w hatthedisinfectantdispersionlevelsw ouldlooklikeifthesystem hadbeenrunningw ithconstantoperatingconditionsfordays.

Contourplotsoftracerconcentrationshow qualitatively how w ellthedisinfectantism ixingasitm ovesthroughthebasin.

Deviationfrom targetconcentrationcanbeusedtoquantify theoutputandisexpressedasapercentage.

w hereCi isthelocalconcentration

T hedatasetcanalsobequeriedtodeterm inecoefficientofvariation,CoV,foram easurem entplane.

w here s isthestandarddeviationofthesam pledataave

oV

C

sC

target

targeti

dev

C

CCC

)(*100

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DisinfectantDecay

T heefficacy ofthecontactsystem isdependentonthetim etheactivem icroorganism sintheeffluentareexposedtodisinfectantatsufficientconcentrationtoneutralizethem .

Disinfectantscurrently inuselikesodium hypochloriteorperaceticacid begintodecay im m ediately uponintroductiontotheeffluentstream . T heirlevelsofconcentrationarem odeledby areacting tracer w hosebehaviorisspecifiedby auserdefinedfunction(U DF)basedonpublisheddata.

A com m only usedfirstorderapproxim ationis:

W here:Ct isthelocalconcentrationofdisinfectantattim etC0 isinitialfully m ixed(target)concentrationk isakineticconstantinm in-1 Itvariesw ithdisinfectanttypeandw aterqualityt istim einm inutes

Contours of disinfectant concentration normalized against targetconcentration level. Contour level 1.0 denotes the target concentration.

KineticsofDisinfectantChem ical

kt0t eCC

KineticsofDisinfectantChem ical

DisinfectantDecay (P AA)

R esearchershavefoundthataP AA m ixturew henintroducedtow astew aterm ay experiencew hatiscalled initial oxidativeconsumption. T hisisincluded intheequationasaconstant,D.

W here:D isinm g/LT hem agnitudeofD isafunctionofeffluentquality andisinfluencedinparticularby effluentturbidity andorganiccontent.

ktt eDCC )( 0

Graphic from Rossi, et. al. 2007

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Log(Nt/N0)

N eutralizationofM icroorganism s

T heratesatw hichm icroorganism sareneutralized arespecifictothedisinfectantinuseandthetargetm icroorganism . Forexam ple,differentcoliform sw illhavedifferingsensitivitiestothedisinfectantinuse. T heirneutralizationissim ulated usingasecondtracerinputw hosereactiontothedisinfectanttracerisdefinedby U DF’sbasedonpublisheddata.

Forthem icrobialinactivation,Hom ’sm odeliscom m only used:

W here:Nt islocalcolony form ingunitnum berofm icroorganism sattim etN0 iscolony form ingunitcountofm icroorganism senteringthebasink isthedisinfectionrateconstantCt isthelocaldisinfectantconcentration attim etn istheHom dilutioncoefficientM istheHom tim eexponentt istim einm inutes

Disinfectant tracerintroduced atinjector point

KineticsofDisinfectantChem ical

mnt

0

t

tkCN

Nlog

Contours of microorganism population. Nt isthe local population and N0 is the populationin the effluent prior to exposure to thedisinfectant.

Microorganismtracer introduced atmodel inlet

KineticsofDisinfectantChem ical

N eutralizationofM icroorganism s

T heresultsreportedfrom m icroorganism testing arereportedasL ogbase10 oftheratioN t/N 0 T henum bersontheverticalscalerepresentordersofm agnitudechangeinthepopulationofviablem icroorganism s.

Graphic from Rossi, et. al. 2007

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KineticsofDisinfectantChem ical

T ip: Developandcalibratethedecay andm icroorganism U DF’sw ithaplugflow reactorm odel.

By setting wall boundary conditions tofrictionless, the velocity profile of the flowpassing through the model remains uniformthus allowing perfect plug flow.

P ublications

T heuseofCFD forcontacttankevaluationisw elldocum ented. S om epapersthatcanbedow nloadedforfree…

Angeloudis,A. S toesser,T . Falconer,A. Predicting the Disinfection Efficiency Range in Chlorine Tanks Through a CFD Based Approach,2014

Greene,D. N um ericalS im ulationofChlorineDisinfectionP rocessesinN on-IdealR eactors,2002

Haas,C. Joffe,J. Disinfection Under Dynamic Conditions: Modification of Hom’s Model for Decay,1994

Khan,L .W ickleinE. T eixeira,E. Validation of a Three Dimensional Computational Fluid Dynamics Model of a Contact Tank,2006

R auen,W . Angeloudis,A. Falconer,A. Appraisal of Chlorine Contact Tank Modelling Practices,2012

R ossi,S . Antonelli,M . M ezzonotte,V. N urizzo,C. Peracetic Acid Disinfection: A Feasible Alternative to Wastewater Chlorination,2007

S antoro,D.Bartrand,T . L iberti,L . N otarnicola,M . Haas,C. CFD Modeling of Municipal Wastewater Disinfection by Peracetic Acid (PAA) in ContinuousSerpentine Reactors,2007

W ilson,J. Venayagam oorthy,S . Evaluation of Hydraulic Efficiency of Disinfection Systems Based on Residence Time Distributions Curves,2010

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Q uestionsorCom m ents?

S teveS aunders

ibisgroup@ bellsouth.net

https://ibisgroupcfd.com /

Ed Wicklein, PE

Principal Technologist, Carollo Engineers

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• Some organisms are resistant to chemical disinfection, but sensitiveto UV light

• Reduced chemicals

• Smaller footprint

• Measured by Dose:

Dose = UV Intensity x Contact Time

Watts/m2 x seconds

and reported in mJ/cm2

• Lamp output: low pressure, low pressure high output, medium pressure

• Open vs closed vessel

• Lamp orientation: horizontal, slanted, vertical

http://www.xylem.com/treatment/kr/products/duron-uv-disinfection-system

http://www.calgoncarbon.com/wp-content/uploads/2015/02/Wastewater-Disinfection__UVT-2015-ENG-WW01.pdf

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• Flow Split – systems are often parallel and assume even flow split

• Flow Distribution – all flow paths need similar exposure time

• Water Level Control – open channel systems often have multiplebanks in series which require similar submergence for range of flows

• Head Losses – often retrofit into fixed hydraulic grade

S ym m etricalsplitisunusually betterw ithsim ilarlossesoneachflow path

DividingFlow M anifoldsareHardtoS plitatL ow HeadL oss

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T urningintochannelsleadstoflow separationcausingpoordistributionthroughfirstbank

Geom etricchangeshelpturnflowandim provesystem efficiency

• Discrete ordinance radiation model

• Lagrangian particle tracking collects exposure

• Dose model converts data to target inactivation and RED

Hydraulic Model Radiation Model Dose Collection Dose Distribution

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• UVT – can incorporate processvariability

• Lamp Output - can adjust forknown properties of lamptypes

• Surface reflection andabsorbance can be adjusted fordifferent materials

• 3 trains, 2 banks each

• High head loss leads toeven flow split

• Some variability in flowdistribution through lamps

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• Used for post processing todevelop log inactivation of targetspecies based on known kinetics

• Can identify severity of hydraulicshort circuiting

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Ed Wicklein

Phone: +1 (206) 538-5166

Email: ewicklein@carollo.com

Additional CFD UV modeling resources:

http://www.sandia.gov/cfd-water/uvdisinfection.htm