11/11/2017 1 Steve Saunders IBIS Group Inc. CFD simulation has become the preferred approach for modeling contact basins • Through–flowphysicalmodelsareexpensivepluswater/wastewater utilitiesmaytakeexceptiontothelargedischargeratesrequiredtorun them • O verthepast20+years,validationcomparisonsagainstlabscaleand onsitetestsshowexcellentagreementbetweenCFDandreal-world contactbasinperformance.
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Steve Saunders IBIS Group Inc. · PDF file02.06.2015 · 11/11/2017 1 Steve Saunders IBIS Group Inc. CFD simulation has become the preferred approach for modeling contact basins •
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11/11/2017
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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
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
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0.8
0.9
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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
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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.
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