Inactivation of Cryptosporidium Oocysts in a Pilot-Scale Ozone

e reactorgas phaser the masson with fastand viable,

Inactivation of Cryptosporidium Oocysts in a Pilot-ScaleOzone Bubble-Diffuser Contactor. I: Model Development

Jae-Hong Kim1; Robert B. Tomiak2; and Benito J. Marinas3

Abstract: A mathematical model was developed to simulate the performance of a pilot-scale ozone bubble-diffuser column. Thhydrodynamics was represented with the axial dispersion reactor model. An analytical solution was developed for the liquid andozone mass balances in which dissolved ozone decomposes by first-order kinetics. Numerical approximations were provided fobalances for viable microorganisms and the more general case of dissolved ozone decomposition through a second-order reactiozone demand in natural organic matter. Model components required to predict the liquid and gas phase ozone concentrationmicroorganism number density profiles throughout the bubble-diffuser column included input parameters~liquid and gas flow ratesinfluent gas and dissolved ozone concentrations, temperature, and countercurrent or cocurrent operation mode!, empirical correlations~dispersion number, volumetric mass transfer coefficient, Henry’s law constant!, and batch or semibatch kinetic information~ozonedecomposition rate constants and fast-ozone demand, and microorganism inactivation lag phase and rate constant!. A sample model runfor the case of first-order ozone decomposition revealed that the analytical and numerical solutions were practically identical.

DOI: 10.1061/~ASCE!0733-9372~2002!128:6~514!

CE Database keywords: Ozone; Bubble; Disinfection; Dispersion; Models.

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Introduction

Ozone has been found to be an effective disinfectant for conling Cryptosporidium parvumoocysts~Gyurek et al. 1999; Ren-necker et al. 1999, 2000; Driedger et al. 2000, 2001a!. In con-trast, the more common disinfectants such as free or combchlorine are practically ineffective for such purpose~Gyurek et al.1997; Rennecker et al. 2000; Driedger et al. 2000, 2001a!. As aresult, ozonation has been identified as a viable alternativmore conventional disinfection of drinking water with freecombined chlorine, primarily for systems treating water with lobromate formation potential~Driedger et al. 2001b!.

A common reactor design currently in use for ozone disinftion of drinking water throughout the U.S. is the multichambbubble-diffuser contactor~Bellamy et al. 1991!. One or more ofthe upstream chambers in these units are transfer chambewhich ozone gas is introduced through a system of ceramistainless-steel diffusers installed at the bottom of the unit.remaining downstream chambers are used as reactive chamfor additional contact with residual dissolved ozone. A transchamber can be designed for operation in either countercuflow ~water and gas flowing in the downward and upward dir

1Dept. of Civil and Environmental Engineering, Univ. of Illinois aUrbana-Champaign, Urbana, IL 61801.

2Naval Mobile Construction Battalion FIVE, United States Navy.3Dept. of Civil and Environmental Engineering, Univ. of Illinois a

Urbana-Champaign, Urbana, IL 61801~corresponding author!. E-mail:[email protected]

Note. Associate Editor: Robert G. Arnold. Discussion open until Nvember 1, 2002. Separate discussions must be submitted for indivpapers. To extend the closing date by one month, a written requestbe filed with the ASCE Managing Editor. The manuscript for this pawas submitted for review and possible publication on March 9, 20approved on October 8, 2001. This paper is part of theJournal of Envi-ronmental Engineering, Vol. 128, No. 6, June 1, 2002. ©ASCE, ISS0733-9372/2002/6-514–521/$8.001$.50 per page.

514 / JOURNAL OF ENVIRONMENTAL ENGINEERING / JUNE 2002

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tions, respectively! or cocurrent flow~both water and gas flowingin the upward direction! configuration. The transfer portion ofbubble diffuser contactor can consist of countercurrent and corent flow chambers of similar size connected in series~Coffeyet al. 1995!, or just countercurrent flow chambers connectedsmall reactive up-flow chambers~Marinas et al. 1999!.

Process design of an ozone bubble-diffuser contactor requmaking decisions such as selecting the number and distributiotransfer and reactive chambers, water column height, chamcross-sectional area, number and type of diffusers, gas flowand ozone gas concentration. The choice of contactor configtion and operation conditions can affect the hydrodynamicsmass transfer, which in turn can impact the inactivation efficienof C. parvumoocysts. In addition, the performance of the contator is also affected by water quality parameters, primarily thoaffecting the kinetics of ozone decomposition, such as tempture, pH, natural organic matter~NOM! content, and alkalinity.

Mathematical models present an attractive tool for simuneous consideration of the effects of contactor configuration,erating conditions, and water quality parameters on the disintion efficiency achieved in flow-through ozone bubble-diffuscontactors. The distribution of dissolved ozone in laboratory apilot scale bubble-diffuser columns has been represented arately with the axial dispersion reactor~ADR! model ~Lev andRegli 1992a; Marinãs et al. 1993; Zhou et al. 1994; Chen 199Singer and Hull 2000!. The use of the ADR model to represethe disinfection efficiency achieved in bubble-diffuser columhas also been proposed~Lev and Regli 1992a; Smith and Zho1994!. However, with the exception of two preliminary experments with Giardia muris cysts modeled by Chen~1998!, thevalidity of the ADR model to represent disinfection databubble-diffuser columns remains to be demonstrated experimtally.

The overall objective of this study was to expand the applition of the ADR model for simultaneous representation of ozoconcentration profiles and inactivation data. The study has

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distinct parts. Part I presents the development of ADR moequations for two different cases of ozone decomposition kineCase I assumes first-order decomposition kinetics, and Caconsiders that dissolved ozone also decomposes accordingsecond-order reaction with NOM having fast ozone demand.resulting ADR model expressions are integrated by analytand/or numerical methods. Part II presented in a subsequentlication ~Kim et al. 2002! focuses on validating the ADR modewith experimental data obtained for the inactivation ofC. parvumand C. murisoocysts in a pilot-scale ozone-bubble diffuser cumn ~Owens et al. 1994; Miltner et al. 1997; Owens et al. 200!.The ADR model is also applied in Part II for the simulationozone contactor performance under a broad range of selecteerating conditions and water quality characteristics.

Model Development

Ozone Decomposition Kinetics

Ozone decomposes in water through a reaction mechanismincludes initiation by hydroxide ion attack followed by propagtion and termination steps involving various radicals as intermdiate species~Staehelin and Hoigne´ 1982; Buhler et al. 1984;Staehelin et al. 1984!. Consistent with this mechanism, for watecontaining relatively high concentrations of radical scavengthe overall rate of ozone decomposition kinetics is approximafirst order~Singer and Hull 2000!

dCL

dt52kDCL (1)

in which t5time ~T!; CL5dissolved ozone concentration at timet~ML23!; CL,05initial dissolved ozone concentration~ML23!; andkD5first-order ozone decomposition rate constant~T21!.

Lev and Regli~1992b! and Chen~1998! modeled the ozoneconsumption in natural waters with more general expressionsing into account an additional second-order rate term for theaction between ozone and NOM with fast ozone demand

dCL

dt52kDCL2kRDCL (2)

dD

dt52kRDCL (3)

in which kR5second-order rate constant~L3 M21 T21!; andD5concentration of the NOM fraction with fast ozone dema~ML23!. The system of Eqs.~2! and~3! converges to Eq.~1! whenthe initial fast ozone demandD0 becomes negligible. Two separate models will be developed in this study for the cases of ozdecomposition without~Case I! and with ~Case II! fast ozonedemand.

The effect of temperature on ozone decomposition rate cstants can be represented with the Arrhenius expressions

kD5AD expS 2ED,a

RT D (4)

kR5AR expS 2ER,a

RT D (5)

in which AD ,AR5frequency factors for ozone decomposition ractions ~T21!, ~L3 M21 T21!; ED,a ,ER,a5activation energy forozone decomposition reactions~J/mol!; T5absolute temperature~K!; andR58.314 J/~mol K)5ideal gas constant.

.IIa

-

p-

t

-

-

Disinfection Kinetics

The inactivation kinetics of microorganisms by a chemical disfectant has often been expressed with an empirical rate expreswith the form of that for a second-order elementary chemireaction, or

dN

dt52kNCLN (6)

Eq. ~6! can be integrated to obtain

lnS N

N0D52kNE

0

t

CLdt (7)

in which N andN05number density of viable microorganismsrespective timest and 0~L23!; N/N05survival ratio~dimension-less!; and kN5second-order inactivation rate consta~L3 M21 T21!. Notice that, in the case of constant disinfectaconcentration, Eq.~7! results in the classic pseudofirst-ordChick–Watson model expression.

The inactivation kinetics of some microorganisms such asEs-cherichia coli with ozone has been found to be consistent wEq. ~7! ~Hunt and Marinas 1997!. In contrast, ozone inactivationcurves forC. parvumoocysts can be characterized by an initlag phase during which little or no inactivation occurs followeby a pseudofirst-order rate of inactivation~Rennecker et al. 1999!.The postlag phase portion of such curves can be modeled withdifferential form of the delayed pseudofirst-order Chick–Watsmodel proposed by Wickramanayake and Sproul~1988!

dN

dt52

N1

N0kNCLN (8)

in which N1 /N05 intercept of the extrapolated pseudofirst-ordportion of the inactivation curve with the ordinate~dimension-less!. For modeling purposes, the integrated form of Eq.~8! canbe extended to the lag phase by converting resultingN/N0 valuesgreater than 1 to a value of unity~Rennecker et al. 1999!.

The effect of temperature on the inactivation rate constantbe represented with an Arrhenius expression

kN5AN expS 2EN,a

RT D (9)

in which AN5 frequency factor for the inactivation reactio~L3 M21 T21!; and EN,a5 activation energy for the inactivationreaction~J/mol!.

Axial Dispersion Reactor Model

Case I. First-Order Ozone Decomposition ReactionMass balances can be performed in the infinitesimal control vume of the bubble-diffuser column shown in Fig. 1 to obtain tADR model expressions representing the steady-state dissoand gas-phase ozone concentrations for the case of first-oozone decomposition kinetics~Marinas et al. 1993!

dd2CL

dz2 7dCL

dz1NLS CG

m2CLD2NDCL50 (10)

dS CG

m Ddz

2NL

S S CG

m2CLD50 (11)

in which CG5 gas-phase ozone concentrations~ML23!; z5x/L5normalized downward distance from the top of the contactor

JOURNAL OF ENVIRONMENTAL ENGINEERING / JUNE 2002 / 515

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the axial direction regardless of liquid flow direction~dimension-less!; d5EL /(LUL)5 dispersion number or inverse of the Pećletnumber~dimen-sionless!; NL5(kLaL)/UL5Stanton number~di-mensionless!; ND5(kDL)/UL5Damkohler number~dimension-less!; S5(mUG)/UL5stripping factor ~dimensionless!;x5downward distance in the axial direction~L!; L5depth of thewater column ~L!; EL5liquid phase dispersion coefficient~L2 T21!; UL , UG5liquid and gas phase approach velociti~L T21!; kLa5volumetric mass transfer coefficient~T21!; andm5Henry’s law constant~dimensionless!. The effect of tempera-ture on the ozone Henry’s law constant can be approximatedthe empirical expression obtained by Marinãs et al.~1993! for thedata reported by Perry and Chilton~1973!

logm5H 3.252840

Tfor 278 K<T<288 K

6.2021687

Tfor 288 K<T<303 K

(12)

Eqs. ~10! and ~11! were developed after neglecting the effeof pressure, and assuming that the gas holdup, gas-phase dsion, and gas-phase ozone decomposition were negligible.assumption of negligible pressure effect, perhaps the most aable, was considered acceptable because the water column opilot-scale bubble-diffuser unit used in Part II of this study wrelatively short~i.e., L52.65 m!. The sign of the advection termin Eq. ~10! was negative for countercurrent or positive for cocrent operation mode.

The ADR model can now be extended to represent the numdensity of microorganisms with the additional steady-statepression:

dd2N

dz2 7dN

dz2NNNCL50 (13)

in which NN5(kNL)/UL(L3 M21). As for Eq. ~10!, the sign ofthe advection term in Eq.~13! is negative for countercurrent opositive for cocurrent operation mode.

The boundary conditions required to integrate Eqs.~10!, ~11!,and~13! can be approximated with those for an ideal closed vsel ~Levenspiel 1999! or

CLuz505CL,01ddCL

dz Uz50

(14)

Fig. 1. Schematic of ozone bubble-diffuser column with control vume used for mass balances


r-e-e

r

dCL

dz Uz51

50 (15)

CGuz515CG,0 (16)

Nuz505N11ddN

dzUz50

(17)

dN

dzUz51

50 (18)

for countercurrent operation mode, or

dCL

dz Uz50

50 (19)

CLuz515CL,02ddCL

dz Uz51

(20)

CGuz515CG,0 (21)

dN

dzUz50

50 (22)

Nuz515N12ddN

dzUz51

(23)

for cocurrent operation mode, in whichCL,05dissolved ozoneconcentration entering the reactor~ML23!; and CG,05gas phaseozone concentration entering the reactor~ML23!. Note thatN1 isused in Eqs.~17! and~23! instead of the actual number densityviable microorganisms entering the reactorN0 in order to accountfor the lag phase of the inactivation curve. Such a conditionresult inN/N0 values being greater than unity inside the bubbcolumn, indicating that the microorganisms are still in the lphase and therefore the trueN/N051. This effect is further dis-cussed in Part II of this study~Kim et al. 2002!.

Case II. Simultaneous First-Order and Second-Order OzoneDecomposition ReactionsThe system of equations for Case II is similar to that presentedCase I except that the mass balance for dissolved ozone shinclude an additional reaction term to account for the secoorder reaction with the NOM fraction with fast ozone demand

dd2CL

dz2 7dCL

dz1NLS CG

m2CLD2NDCL2NRDCL50 (24)

and an additonal mass balance is needed for the NOM fracfast ozone demand

dd2D

dz2 7dD

dz2NRDCL50 (25)

in which NR5(kRL)/UL(L3 M21). As also specified before foEqs. ~10! and ~13!, the sign of the advection terms in Eqs.~24!and ~25! is negative for countercurrent or positive for cocurreoperation mode. The boundary conditions for Eq.~24! are thesame as those for Eq.~10!, and for Eq.~25! are

Duz505D01ddD

dzUz50

(26)

dD

dzUz51

50 (27)

for countercurrent operation, and

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n

dD

dzUz50

50 (28)

Duz515D02ddD

dzUz51

(29)

for cocurrent operation.

Model Parameters

Mass Transfer Coefficient

The volumetric mass transfer coefficientkLa can be determinedby multiplying the mass transfer coefficientkL (L T21) and inter-facial transfer areaa (L21) obtained with the expressions

kL5ShDL

dB(30)

a56UG

~VB7UL!3dB(31)

in which Sh5Sherwood number~dimensionless!. The sign in thedenominator of Eq.~31! is negative for countercurrent or positivfor cocurrent operation mode. The Sherwood number can beresented by the empirical correlation developed by Hughm~1967!

Sh5210.0187S RG0.484

•ScL0.339FdBg1/3

DL2/3 G0.072D 1.61

(32)

with RG5(dBVB)/nL5gas-phase Reynolds number~dimension-less!; ScL5nL /DL5aqueous-phase Schmidt number~dimension-less!; g5gravitational constant~L T22!; DL5molecular diffusiv-ity of ozone in water~L2 T21!; nL5kinematic viscosity of water~L2 T21!; and dB ,VB5average bubble diameter~L! and rise ve-locity ~L T21! given by the expressions~Marinas et al. 1993!

dB5dB,010.21UG (33)

VB55210dB

1.004

mL

for 0.05 cm<dB<0.18 cm

~20133.8e24.88dB!1.004

mL

for 0.18 cm<dB<0.40 cm

(34)

with mL5absolute viscosity of water ~ML21 T21! anddB,05average bubble diameter extrapolated toUG50 ~L!. Thespecific units for the variables in Eqs.~33! and~34! are cm fordB

anddB,0 , cm/s forVB andUG , and cP formL . Eq. ~33! is validonly for UG<0.6 cm/s, and a bubble column with inner diameof approximately 0.15 m~6 in.!, and a spherical fine diffuser witha diameter of 2.54 cm~1 in.!.

Dispersion Number

The effect of operating conditions on the dispersion numberbe represented with the expression developed by Marinãs et al.~1993! for a pilot-scale ozone bubble-diffuser contactor similarthe unit used in Part II of the present study, i.e., a bubble coluwith an inner diameter of approximately 0.15 m~6 in.!, and aspherical fine diffuser with a diameter of 2.54 cm~1 in.!

-

d51.84S 0.0018519.7UG

1/2

UL5/3 S nL

dBD 7/6D (35)

The factor of 1.84, or 4.88/2.65, multiplying the original expresion developed by Marinãs et al.~1993!, inside the parentheses iEq. ~35!, is a correction factor to account for the differencewater column height for the reactor used by Marinãs et al.~1993!,L54.88 m, and that modeled in Part II of this study,L52.65 m~Kim et al. 2002!.

Integration of Axial Dispersion Reactor ModelEquations

Analytical Solution

The mass balances for gaseous and aqueous ozone for the cfirst-order ozone decomposition kinetics~Case I! can be solvedanalytically. The differential equations, Eqs.~10! and ~11! withcorresponding boundary conditions Eqs.~14!–~16! for counter-current operation mode, or Eqs.~19!–~21! for cocurrent operationmode will be solved in this section. The first step is to rearranEq. ~11! such that

CL5CG

m2

S

NL

dS CG

m Ddz

(36)

Eq. ~36! was then substituted into Eq.~10! to obtain the followinghomogeneous linear differential equation with constant coecients:

2dS

NL

d3S CG

m Ddz3 1S d6

S

NLD d2S CG

m Ddz2

1S NDS

NL711SD dS CG

m Ddz

2ND

CG

m50 (37)

The top and bottom signs for terms with two signs in Eq.~37!correspond to countercurrent and cocurrent operation modesspectively. The general solution for Eq.~37! is

CG~z!

m5a1el1z1a2el2z1a3el3z (38)

in which l1 , l2 , andl35the roots of the corresponding charateristic equation:

S 2dS

NLDl31S d6

S

NLDl21S NDS

NL711SDl2ND50 (39)

Once again, the top and bottom signs for terms with two signEq. ~39! correspond to countercurrent and cocurrent operamodes, respectively. The integration constants,a1 , a2 , anda3 ,can be obtained by substitution of Eq.~38! into the three bound-ary conditions Eqs.~14!–~16! for the countercurrent operatiomode

S 12l1

S

NLD ~12l1d!a11S 12l2

S

NLD ~12l2d!a2

1S 12l3

S

NLD ~12l3d!a35CL, in (40)


sub:

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l1S 12l1

S

NLDel1a11l2S 12l2

S

NLDel2a2

1l3S 12l3

S

NLDel3a350 (41)

el1a11el2a21el3a35CG, in

m(42)

or Eqs.~19!–~21! for cocurrent operation mode

l1S 12l1

S

NLDa11l2S 12l2

S

NLDa21l3S 12l3

S

NLDa350

(43)

S 12l1

S

NLD ~11l1d!el1a11S 12l2

S

NLD ~11l2d!el2a2

1S 12l3

S

NLD ~11l3d!el3a35CL, in (44)

el1a11el2a21el3a35CG, in

m(45)

The aqueous ozone concentration profile can be obtained bystitution of Eq.~38! into Eq.~36! giving the following expression

CL~z!5S 12l1

S

NLDel1za11S 12l2

S

NLDel2za2

1S 12l3

S

NLDel3za3 (46)

An important consideration is that this analytical solution is prtically the same whether or not microorganisms such asC. par-vumoocysts are present inside the bubble-diffuser contactor.reason for the lack of this effect is that the relatively low numdensity of microorganisms typically found in natural waters orthe seeded water used in pilot-scale disinfection experimentsnot produce measurable ozone demand.

The distribution of viable microorganisms can be obtainedintegrating Eq.~13! after substitution of Eq.~46! with Eqs.~17!,


-

s

~18!, or ~22!, ~23! as the boundary conditions for respective coutercurrent or cocurrent operation mode. However, an analytsolution for Eq.~13! coupled with Eq.~46! was not available andtherefore the system of equations is to be solved by numerapproximation as described in the following section.

Numerical Solutions

Numerical solutions were developed for both Cases I and II. Tfirst step was to replace each second-order ordinary differenequation with a set of two first-order ordinary differential equtions. Eq.~10! was replaced with

dCL

dz5CL* (47)

ddCL*

dz7CL* 1NLS CG

m2CLD2NDCL50 (48)

Eq. ~24! with Eq. ~47! and

ddCL*

dz7CL* 1NLS CG

m2CLD2NDCL2NRDCL50 (49)

Eq. ~25! with

dD

dz5D* (50)

ddD*dz

7D* 2NRDCL50 (51)

and Eq.~13! with

dN

dz5N* (52)

ddN*dz

7N* 2NNNCL50 (53)

in which CL* , D* , andN* 5auxiliary variables defined by Eqs~47!, ~50!, and ~52!. The resulting system of five first-order differential equations for Case I, or seven first-order differen

Fig. 2. Diagram of computer model used for numerical solution of mass balances

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equations for Case II, and corresponding boundary conditionsreplaced by a corresponding set of finite-difference expresson a mesh of points that covered the range of integration.iterative calculation was performed until the sum of the diffences between successive calculations at each mesh point, sby the magnitude of each variable, was less than 10210. The cal-culation algorithm was adopted from Press et al.~1992! and pro-grammed in C11 language.

The structure of the program including input, internal calcution, and output is depicted in Fig. 2. The model input includspecific operating conditions, source water characteristics,column operating mode. The model parameters were calculwith the empirical correlations described in the preceding sectand incorporated into the corresponding mass balances.model output was the profiles of viable microorganisms, aaqueous and gaseous ozone throughout the water column ithe bubble-diffuser contactor.

Preliminary Model Application

Although a more comprehensive evaluation and application ofmodel will be presented in Part II~Kim et al. 2002!, a samplecalculation is presented in this section to examine the accuracthe numerical solution. The model is applied to represent theformance of a pilot-scale bubble-diffuser contactor with a wacolumn height of 2.65 m and an inner diameter of 0.15 m. Theis introduced through a spherical fine bubble diffuser 2.54 cmdiameter. The configuration of this contactor is the same as ththe unit modeled in Part II. The model input for both analyticand numerical solutions wereCG,0530 mg/L, QG51.5 L/min,QL515 L/min, kD51023 s21, D050, T520°C, and countercurrent operation mode. The target microorganism isC. muris oo-cysts with an ozone inactivation rate consistent with pseudofiorder Chick–Watson kinetics~i.e., no lag phase! as given in PartII. The analytical and numerical solutions for the dissolved ozoconcentration profile, and numerical solution for the survival ra

Fig. 3. PreliminaryC. murisoocysts survival ratio profile obtaineby numerical solution, and aqueous ozone concentration profiletained by both analytical and numerical solution of axial dispersreactor model equations for case I~first-order ozone decompositiokinetics!

ss

ed

dd

e

e

f-

f

of C. murisoocysts, are shown in Fig. 3. As depicted in Fig. 3, tanalytical and numerical solutions for the dissolved ozone ccentration profile were found to be practically identical, thus vadating the numerical approximation method. It should be nothat, as discussed in a preceding section, the presence of thcroorganisms had no effect on the ozone concentration profil

Conclusions

An ADR model was developed to simulate the performance opilot-scale bubble-diffuser ozone contactor. The accuracy ofapproximation method used to integrate the model equationschecked by demonstrating that the numerical solution wmatched by an analytical solution developed to represent ozconcentration profiles for the case of first-order ozone decomsition kinetics. The model was validated with experimental dand used to develop simulations in Part II.

Acknowledgments

The writers would like to acknowledge the Civil EngineerinCorps, U.S. Navy, for financial support provided to the secowriter ~R.B.T.! during graduate work at the University of IllinoisThe writers also appreciate the review of this paper by JamOwens and Richard Miltner at the National Risk ManagemResearch Laboratory, U.S. Environmental Protection AgenCincinnati.

Notation

The following symbols are used in this paper:AD 5 frequency factor for first-order ozone decomposition

reaction~T21!;AN 5 frequency factor for inactivation reaction

~L3 M21 T21!;AR 5 frequency factor for second-order ozone decompo-

sition reaction~L3 M21 T21!;a 5 interfacial transfer area~L21!;

CG 5 gas phase ozone concentration~ML23!;CG,0 5 initial or influent gas phase ozone concentration

~ML23!;CL 5 liquid phase ozone concentration~ML23!;CL* 5 auxiliary variable for the liquid phase ozone con-

centration~ML23!;CL,0 5 initial or influent liquid phase ozone concentration

~ML23!;D 5 concentration of natural organic matter fraction

with fast ozone demand~ML23!;D* 5 auxiliary variable for concentration of natural or-

ganic matter fraction with fast ozone demand~ML23!;

DL 5 molecular diffusivity of ozone in water~L2 T21!;D0 5 initial or influent concentration of natural organic

matter fraction with fast ozone demand~ML23!;d 5 dispersion number~inverse of Pećlet number!

~dimensionless!;dB 5 average bubble diameter~L!;

dB,0 5 average bubble diameter extrapolated toUG50 (L);

-


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est

n-

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nd

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R.,

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ED,a 5 activation energy for first-order ozone decomposi-tion reactions~J/mol!;

EL 5 liquid phase dispersion coefficient~L2 T21!;EN,a 5 activation energy for inactivation reaction~J/mol!;ER,a 5 activation energy for second-order ozone decom-

position reactions~J/mol!;g 5 gravitational constant~L T22!;

kD 5 first-order ozone decomposition rate constant~T21!;

kL 5 mass transfer coefficient~L T21!;kLa 5 volumetric mass transfer coefficient~T21!;kN 5 second-order inactivation rate constant

~L3 M21 T21!;kR 5 second-order ozone decomposition rate constant

~L3 M21 T21!;L 5 depth of the water column in bubble-diffuser con-

tactor ~L!;m 5 Henry’s law constant~dimensionless!;N 5 number density of viable microorganisms~L23!;

N* 5 auxiliary variable for number density of viablemicroorganisms~L23!;

ND 5 normalized first-order ozone decomposition rateconstant~Damkohler number! ~dimensionless!;

NL 5 normalized mass transfer coefficient~Stantonnumber! ~dimensionless!;

NN 5 normalized inactivation rate constant~L3 M21 T21!;

NR 5 normalized second-order ozone decomposition rateconstants~L3 M21 T21!;

N0 5 number density of viable microorganisms enteringcontactor~L23!;

N1 5 number density of viable microorganisms resultingfrom extrapolating postlag phase first-order por-tion of an inactivation curve~L23!;

QG 5 gas flow rate~L3 T21!;QL 5 water flow rate~L3 T21!;

R 5 ideal gas constant~J/mol K!;RG 5 gas phase Reynolds number~dimensionless!;

S 5 stripping factor~dimensionless!;ScL 5 liquid phase Schmidt number~dimensionless!;Sh 5 Sherwood number~dimensionless!;

T 5 temperature~K!;t 5 time ~T!;

UG 5 approach gas velocity~L T21!;UL 5 approach liquid velocity~L T21!;VB 5 bubble rise velocity~L T21!;

x 5 downward distance in axial direction~L!;z 5 normalized downward distance in axial direction

~dimensionless!;a i 5 integration constant~ML23!;l 5 characteristic equation variable~dimensionless!;

l i 5 root of characteristic equation~dimensionless!;mL 5 absolute viscosity of liquid phase~ML21 T21!; andnL 5 kinematic viscosity of liquid phase~L2 T21!.

References

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Driedger, A. M., Rennecker, J. L., and Marinãs, B. J.~2000!. ‘‘Sequentialinactivation ofCryptosporidium parvumoocysts with ozone and freechlorine.’’ Water Res.,34~14!, 3591–3597.

Driedger, A. M., Rennecker, J. L., and Marinãs, B. J.~2001a!. ‘‘Inactiva-tion of Cryptosporidium parvum oocysts with ozone andmonochloramine at low temperature.’’Water Res.,35~1!, 41–48.

Driedger, A. M., Staub, E., Pinkernell, U., Marinãs, B. J., Ko¨ster, W., andvon Gunten, U.~2001b!. ‘‘Inactivation of Bacillus subtilisspores andformation of bromate during ozonation.’’Water Res.,35~12!, 2950–2960.

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Kim, J.-H., Tomiak, R. B., Rennecker, J. L., Marinãs, B. J., Miltner, R. J.,and Owens, J. H.~2002!. ‘‘Inactivation of Cryptosporidiumoocysts ina pilot-scale ozone bubble-diffuser contactor. II: Model validation aapplication.’’J. Environ. Eng.,128~6!, 522–532.

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Marinas, B. J., Liang, S., and Aieta, M. E.~1993!. ‘‘Modeling hydrody-namics and ozone residual distribution in a pilot-scale ozone bubdiffuser contactor.’’J. Am. Water Works Assoc.,85~3!, 90–99.

Marinas, B. J., Rennecker, J. L., Teefy, S., and Rice, E. W.~1999!. ‘‘As-sessment of ozone disinfection efficiency with fluorescent-dyed pstyrene microspheres: Full-scale demonstration.’’J. Am. Water WorksAssoc.,91~9!, 79–89.

Miltner, R. J., Shukairy, H. M., Rice, E. W., Owens, J. H., Schaefer, F.III, and Dahling, D. R.~1997!. ‘‘Comparative ozone inactivation ofCryptosporidiumand other microorganisms.’’Proc., 1997 Int. Sympo-sium on Waterborne Cryptosporidium, American Water Works Asso-ciation, Denver, 229–241.

Owens, J. H., Miltner, R. J., Rice, E. W., Johnson, C. H., Dahling, D.Schaefer, III, F. W., and Shukairy, H. M.~2000!. ‘‘Pilot-scale ozoneinactivation ofCryptosporidiumand other microorganisms in naturawater.’’ Ozone. Sci. Eng.,22, 501–517.

Owens, J. H., Miltner, R. J., Schaefer III, F. W., and Rice, E. W.~1994!.‘‘Pilot-scale inactivation ofCryptosporidiumand Giardia.’’ Proc.,1994 American Water Works Association Water Quality TechnolConf., American Water Works Association, Denver, 1319–1328.

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Rennecker, J. L., Driedger, A. M., Rubin, S. A., and Marinãs, B. J.

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~2000!. ‘‘Synergy in sequential inactivation ofCryptosporidium par-vum with ozone/free chlorine and ozone/monochloramine.’’WaterRes.,34~17!, 4121–4130.

Rennecker, J. L., Marinãs, B. J., Rice, E. W., and Owens, J. H.~1999!.‘‘Inactivation of Cryptosporidium parvumoocyst with ozone.’’WaterRes.,33~9!, 2481–2488.

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Staehelin, J., Bu¨hler, R. E., and Hoigne´, J.~1984!. ‘‘Ozone decompositionin water studied by pulse radiolysis 2.OH and HO4 as chain interme-diate.’’ J. Phys. Chem.,88~22!, 5999–6004.

Staehelin, J., and Hoigne´, J. ~1982!. ‘‘Decomposition of ozone in water:Rate of initiation by hydroxide ions and hydrogen peroxide.’’Environ.Sci. Technol.,16~10!, 676–681.

Wickramanayake, G. B., and Sproul, O. J.~1988!. ‘‘Ozone concentrationand temperature effects on disinfection kinetics.’’Ozone. Sci. Eng.,10, 125–135.

Zhou, H., Smith, D. W., and Stanley, S. J.~1994!. ‘‘Modeling of dissolvedozone concentration profiles in bubble columns.’’J. Environ. Eng.,120~4!, 821–840.


l dataated

hio Riversimulate

rsed recom-ments for

Inactivation of Cryptosporidium Oocysts in a Pilot-ScaleOzone Bubble-Diffuser Contactor. II: Model Validation

and ApplicationJae-Hong Kim1; Jason L. Rennecker2; Robert B. Tomiak3; Benito J. Marinas4; Richard J. Miltner5;

and James H. Owens6

Abstract: The axial dispersion reactor~ADR! model developed in Part I of this study was successfully validated with experimentaobtained for the inactivation ofC. parvumandC. murisoocysts with a pilot-scale ozone-bubble diffuser contactor operated with treOhio River water. Kinetic parameters, required to model the effect of temperature on the decomposition of ozone in treated Owater and oocyst inactivation, were determined from batch and semibatch ozonation experiments. The ADR model was used tothe effects of operating conditions~feed-gas ozone concentration, liquid flow rate, and gas flow rate!, and water quality related paramete~fast ozone demand, first and second order ozone decomposition rate constants, and temperature! on the performance of the pilot-scalcontactor. The model simulation provided valuable insight into understanding the performance of ozone disinfection systems anmendations for ozone contactor design and optimization. For example, the simulation revealed that meeting inactivation requireC. parvumoocysts would be more challenging at relatively lower temperatures.

DOI: 10.1061/~ASCE!0733-9372~2002!128:6~522!

CE Database keywords: Ozone; Bubbles; Simulation; Disinfection.

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Introduction

The overall objective of this two-part study was to apply the axdispersion reactor~ADR! model for the simultaneous representtion of ozone concentration profiles and inactivation data. Axdispersion reactor model equations for two different casesozone decomposition kinetics were developed in Part I~Kim et al.2002!. Case I assumed first-order decomposition kinetics,Case II considered that dissolved ozone also decomposed acing to a second-order reaction with the fraction of the natuorganic matter~NOM! having fast ozone demand. The resultimass balance expressions were integrated by an approximmethod, and the accuracy of the numerical solution to repre



3Naval Mobile Construction Battalion FIVE, United States Navy.4Dept. of Civil and Environmental Engineering, Univ. of Illinois a

Urbana-Champaign, Urbana, IL 61801~corresponding author!. E-mail:[email protected]

5National Risk Management Research Laboratory, United Statesvironmental Protection Agency, Cincinnati, OH 45268.

6National Risk Management Research Laboratory, United Statesvironmental Protection Agency, Cincinnati, OH 45268.

Note. Associate Editor: Robert G. Arnold. Discussion open until Nvember 1, 2002. Separate discussions must be submitted for indivpapers. To extend the closing date by one month, a written requestbe filed with the ASCE Managing Editor. The manuscript for this pawas submitted for review and possible publication on March 9, 20approved on October 8, 2001. This paper is part of theJournal of Envi-ronmental Engineering, Vol. 128, No. 6, June 1, 2002. ©ASCE, ISS0733-9372/2002/6-522–532/$8.001$.50 per page.


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the distribution of dissolved ozone throughout the water coluinside the bubble diffuser contactor was checked with an anacal solution for Case I.

The main objective of Part II of this study is the validationthe ADR model with experimental data obtained for the inactivtion of C. parvumandC. murisoocysts with a pilot-scale ozonebubble diffuser contactor operated with treated Ohio River wa~Owens et al. 1994; Miltner et al. 1997; Owens et al. 2000!. Therate ofC. parvumoocyst inactivation with ozone was representwith the kinetic constants developed by Rennecker et al.~1999!.In addition, batch and semibatch tests were also performed asof this study to obtain the kinetic parameters required to mothe decomposition of ozone in Ohio River water, and the inavation of C. murisoocysts, which may serve as biological surrgate indicators forC. parvumoocysts~Owens et al. 1994; Miltneret al. 1997; Owens et al. 2000!, with ozone as a function of temperature.

Additional objectives of Part II of this study are to providADR model simulations of contactor performance under a brorange of operating conditions and water quality characteristand to use these results to develop recommendations for ocontactor chamber design and operation.

Material and Methods

Water Quality

Pilot-scale ozonation tests were performed with treated ORiver water from the Cincinnati Water Works treatment plaCincinnati ~Owens et al. 1994; Miltner et al. 1997; Owens et2000!. Water treatment processes in this plant include alumagulation, sedimentation, pH adjustment, and sand filtrat

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The water had a pH between 8.0 and 8.5, total organic carconcentration of 1.7260.42 mg/L, and alkalinity of 59615 mg/Las CaCO3 .

Cryptosporidium oocysts

Experiments were performed withC. muris~strain RN66! andC.parvum~Iowa strain! oocysts.C. murisoocysts were propagatein female CF-1 mice inoculated with approximately 23105 oo-cysts. Fecal matter was collected and mixed with 0.01%~v/v!Tween 20~ICN Biomedicals Inc., Ohio!. The resulting slurry wassequentially passed through sieves with mesh sizes of 10, 20and 100. Subsequently, the oocysts were separated from thematter suspension by a sucrose gradient centrifugation me~Owens et al. 2000!. C. parvumoocysts were propagated in5-day-old Holstein bull calf. Fecal material containing the oocywas sieved and separated by sequential discontinuous sugradients and isopycnic Percoll gradient centrifugation~Arro-wood and Donaldson 1996!. Purified C. muris and C. parvumoocysts were stored in phosphate buffered saline with penic~100 units/mL! and streptomycin~100 mg/mL! at 4°C. Owenset al. ~2000! provided additional details about oocyst preparatmethods.

Batch Ozone Decomposition Experiments

The kinetics of ozone decomposition in treated Ohio River wawas determined by performing batch ozonation experiments wthe apparatus shown in Fig. 1. The water was vacuum filtethrough a membrane with a nominal pore size of 0.45mm ~Micro-Wynd II Filter Cartridge, Cuno Inc., Meriden, Conn.! prior to use.Ozone gas, produced from pure oxygen with an ozone geneModel GL-1 ~PCI Ozone and Control Systems, W. CaldweN.J.!, was bubbled through distilled-de-ionized water in a gwashing cylinder. The cylinder was immersed in an ice bathproduce a stock solution of approximately 31 mg/L. An ozodecomposition test was initiated by transferring a predetermivolume of the stock solution into a 100-mL gas-tight syrin~SGE, Ringwood, Australia! containing treated Ohio River wateNo headspace was allowed inside the syringe to avoid ozlosses by volatilization. Dilution of the experimental wateraddition of ozone stock solution was estimated at 9.7%, andinitial ozone dose after dilution was 3.060.1 mg/L. Samples werecollected with a gas-tight syringe at 5-min intervals for a toreaction time of approximately 30 min. Precautions were takeminimize ozone volatilization during sample collection asample processing. The experimental temperature was contrat target values in the range of 5–30°C by immersing the syri

Fig. 1. Schematic of batch reactor apparatus

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batch reactor in a water bath. All tubing and reactor materwere glass, Teflon, or stainless steel 316.

Semibatch Inactivation Experiments

The inactivation kinetics ofC. muris oocysts with ozone wasdetermined with a semibatch reactor apparatus following the pcedures described by Rennecker et al.~1999! for C. parvumoo-cysts. The reactor consisted of a 250-mL gas-washing cylincontaining 200 mL of 0.01 M phosphate-buffered distillede-ionized water at pH 7.0. The temperature was maintainetarget values within the range of 5–30°C by immersing the retor in a water bath. The dissolved ozone concentration wasconstant at 0.560.1 mg/L by continuous bubbling of ozonated gproduced from pure oxygen in the ozone generator. Appromately 1.253106 oocysts were injected into the reactor and suject to ozonation for various contact times corresponding to taCT ~product of dissolved ozone concentration and contact tim!values. When the targetCT value was reached, the diffuser waremoved from the reactor and dissolved ozone was quicquenched with sodium thiosulfate. The oocysts were thenlected on a Poretics polycarbonate track-etched filter with a nonal pore size of 1.0mm ~Fisher Scientific, Itasca, Ill!, and storedin 0.01%~v/v! Tween 20 solution. Viability was assessed with48 h following the procedures described in a subsequent sec

Pilot-Scale Experiments

Flow-through experiments were performed with the pilot-scozone bubble-diffuser contactor system shown in Fig. 2. The ctactor was a glass column with an internal diameter of 0.15 m~6.0in.! and a height of 2.74 m~9.0 ft!. Samples could be withdrawnfrom the influent and effluent lines, and from four additional poequally spaced at 0.46 m~1.5 ft.! apart throughout the water column. The water column height inside the contactor was set atm for all experiments. Treated Ohio River water was first pasthrough a cartridge filter with a nominal pore size of 1.0mm~Cuno Inc., Meriden, Conn.! to minimize the interference fromparticles during oocyst viability determination by in vitro excstation. The stock oocyst suspension (;53109 oocysts/L) wasinjected into the experimental water at a rate of 2 mL/min juahead of the contactor inlet. The resulting seeded experimewater was fed to the contactor operated in a countercurrent m~liquid flowing in the downward direction, and gas flowing in thupward direction! at a mean flow rate of 6.4 L/min, correspondinto a hydraulic residence time of approximately 7.5 min. Ozogas, generated from pure oxygen with the ozone generator,bubbled into the water through a spherical fine diffuser withdiameter of 2.54 cm~1 in.! installed at the bottom of the contactor. The gas to liquid flow ratio was kept approximately constat approximately 0.1 and the feed-gas ozone concentrationvaried within the range of 10.4–52.6 mg/L by adjusting the voage of the generator. All experiments were performed at rotemperature~22–26°C!. Samples were collected for viability assessment from the influent and effluent sampling ports for allexperiments, and from intermediate sampling taps for seleexperiments withC. muris oocysts. The dissolved ozone waquickly quenched by collecting the samples in a flask containsodium thiosulfate and Tween 20. The sample was then store4°C for viability assessment as described subsequently. Samwere also collected for liquid and gas phase ozone analyses


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Ozone Concentration Determination

The dissolved ozone concentration was measured by indigo crimetry at 600 nm~Bader and Hoigne´ 1981!. A molar absorptivityof 23,150 M21 cm21 ~Chiou et al. 1995! for indigo reagent wasused in the computation of dissolved ozone concentrations fobatch, semibatch, and pilot-scale experiments. The gas-pozone concentrations were determined by ultraviolet absorptio254 nm. A Model HC detector measured the gas applied tocontactor, and a Model HC detector, modified by the manufturer to operate in the appropriate concentration range, measthe off gas from the contactor~PCI Ozone and Control SystemW. Caldwell, N.J.!.

Tracer Test

A tracer test was performed with the pilot-scale ozone-bubdiffuser contactor under identical operating conditions as thused for ozonation experiments except that the ozone genewas run with the voltage turned off. Sodium chloride~NaCl!, usedas the tracer compound, was introduced as a step input at aof 100 mg/L. Samples were collected at the liquid outlet andtotal dissolved solids concentration was determined with ameter Model 532 T1~Myron L Co., Encinitas, Calif.!. The sam-pling was continued for a period of time approximately thrtimes the hydraulic residence time of the contactor.

Fig. 2. Schematic of pilot-scale ozone bubble-diffuser contactor stem


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Viability Assessment

The viability of C. murisoocysts in samples from semibatch eperiments was determined by an in vitro excystation assOocysts were separated from the samples by centrifugatio1,1203g for 10 min. The resulting pellet was resuspended inmL of prewarmed~37°C! Hanks Balanced Salt Solution and incubated for 1 h at37°C. After incubation, the oocysts were agaseparated by centrifugation at 1,1203g for 10 min followed byaspiration of supernatant. The pellet was vortexed and the reing concentrated oocyst suspension was then examined usinOlympus BX-60 phase contrast microscope~Leco Corporation,Chicago! at a magnification of 1,0003. The number of excystedoocysts, sporozoites, and intact oocysts were enumeratedthat the total number was at least 800 to ensure statistical sigcance up to 99.5% inactivation efficiency. Viability was then cculated according to the approach previously used by Renneet al. ~1999! for C. parvumoocysts.

Viability of C. muris oocyst samples from pilot-scale testinwas also assessed by in vitro excystation as described by Owet al. ~2000!. ConcentratedC. muris oocyst samples were incubated after dilution with an equal volume of RPMI 1640 soluti~Sigma Chemical, St. Louis! for 1 h at37°C. The samples wereexamined with an upright phase-contrast microscope~Axiophot,Zeiss! at 4003 magnification. The number of excysted oocyspartially excysted oocysts, and intact oocysts were countedviability was calculated using the equation provided by Oweet al. ~2000!.

The inactivation efficiency ofC. parvumoocysts in samplesobtained from pilot-scale experiments was determined by themal infectivity assay described by Owens et al.~1994!. Serial-diluted treated or control samples were administered via intution to neonatal BALB/c mice~caged in litters of 5–6 with onedam!. All samples were performed in duplicate. The 50% infetious dose (ID50) for each set of ozonated or control oocysts wdetermined using the Spearman–Karber method~Finny 1978!.

Results and Discussion

Ozone Decomposition Kinetics

Results obtained for the experiments performed to assess thcomposition kinetics of ozone in treated Ohio River water withozone doseCL,053.060.1 mg/L at temperatures of 5–30°C apresented in Fig. 3. Each kinetic curve was characterized brelatively fast initial ozone demand followed by first-order dcomposition at a slower rate. The first-order rate constants wdetermined from the corresponding slopes shown in the figand the initial fast ozone demand from the intercept of each linregression. The initial fast ozone demand was found to beproximately constant atD050.5260.11 mg/L for all tempera-tures investigated. The rate constantkR for the reaction betweenozone and NOM with fast ozone demand could not be measudue to sampling procedure limitations of the batch reactor apratus. For modeling purposes, the value ofkR53.2060.16 L/~mg s), determined by Hunt and Marinãs~1999! for thefast initial reaction of ozone with Aldrich humic acid at 20°C, wused at all temperatures~i.e.,ER,a50!. Although the applicabilityof suchkR value for Ohio River water could not be confirmeexperimentally, its use was found to be acceptable for modepurposes. Conversion of the second-order reaction, estimatedthe expression developed by Hunt and Marinãs~1999! for a plugflow reactor~i.e., d50!, was found to exceed 99% in less tha

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0.01 min of reaction time, or practically 100% conversion befothe first sample was taken at the reaction time of 1 min. Thiconsistent with the first data point in each curve of Fig. 3deviating appreciably from the corresponding linear regressThe use of thiskR value corresponded to modeling the ozodecomposition kinetics as a practically instantaneous initial daccording toD0 followed by much slower first-order decay.

The first-order rate constantskD obtained from the linear plotsin Fig. 3 were plotted according to the Arrhenius law in Fig.The activation energyED,a and frequency factorAD were deter-mined to be (7.2560.26)3104 J/mol and 10(10.160.5) s21, respec-tively.

Semibatch Inactivation Kinetics

Results obtained for the semibatch ozone disinfection expments withC. murisoocysts are presented in Fig. 5. Notice ththe CT axis is broken for better depiction of the data at lotemperatures. The inactivation kinetics ofC. murisoocysts withozone was consistent with pseudofirst-order Chick–Watson ki

Fig. 3. Effect of temperature on decomposition kinetics of ozonetreated Ohio River water. Experiments performed with batch reaat ozone dose of 3.060.1 mg/L.

Fig. 4. Temperature dependence of first-order ozone decomposrate constant

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ics for the entire temperature range of 5–30°C investigated.inactivation rate constantskN , obtained from the slopes of thregression lines in Fig. 5, are plotted according to Arrheniusin Fig. 6. A linear regression of the plot resulted in an activatienergy EN,a5(9.2861.01)3104 J/mol, and a frequency factoAN510(14.761.8) L/~mg s).

The ozone disinfection kinetics, reported by Rennecker e~1999! for the same Iowa strainC. parvumoocysts used in thepilot experiments of the present study, were employed for meling purposes. It is important to note that results obtained bymodified in vitro excystation method in Rennecker et al.~1999!have been shown to be consistent with animal infectivity datathe inactivation ofC. parvumoocysts with ozone~Renneckeret al. 1999, 2000!, chlorine dioxide~Owens et al. 1999; Ruffellet al. 2000!, and monochloramine after ozone pretreatment~Ren-necker et al. 2000!. Consistent with the delayed Chick–Watsomodel presented in Part I of this study~Kim et al. 2002!, the C.

Fig. 5. Effect of temperature on inactivation kinetics ofC. murisoocysts with ozone in synthetic phosphate buffer solution. Expments performed with semibatch reactor.

Fig. 6. Temperature dependence ofC. muris inactivation rate con-stant


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parvum inactivation curves were characterized by an initial lphase,CTlag, during which little inactivation occurred followedby a subsequent pseudofirst-order decrease in viability. The incept of the extrapolated pseudofirst-order portion of the inacttion curves with the ordinate@see Eq.~8! in Part I of this study#was N1 /N052.0, a constant value for the temperature range5–30°C~Rennecker et al. 1999!. The effect of temperature on thpostlag phase rate constants was characterized by an activenergy EN,a5(8.1260.59)3104 J/mol, and a frequency factoAN510(12.761.1) L/~mg s) ~Rennecker et al. 1999!.

Tracer Test

An experimental tracer curve obtained with the pilot-scale ozbubble-diffuser contactor is shown in Fig. 7. Eqs.~33! and ~35!from Part I of this study~Kim et al. 2002! with dB,050.1 cm~Marinas et al. 1993! were used to estimate the dispersion numd50.58. A predicted tracer curve was developed using thispersion number with the ADR model. The resulting partial diffeential equation was integrated by a finite differences method~Ma-rinas et al. 1993!. As depicted in Fig. 7, there was gooagreement between experimental and predicted tracer curvessupporting the validity of the ADR model to represent the hyddynamics of the pilot-scale ozone contactor.

Model Validation

Results obtained from experiments performed for the simuneous characterization of the distributions of dissolved ozoneviableC. murisoocysts throughout the water column of the piloscale bubble-diffuser contactor are presented in Fig. 8. Threeperiments, each in duplicate, were performed at different avefeed-gas ozone concentrations (CG,0) of 14.1, 23.5, and 37.5mg/L. The average temperature for these experiments was61.6°C.

The dispersion number and the kinetic parameters for thecomposition of ozone and the inactivation ofC. muris and C.parvumoocysts obtained in preceding sections, together withmass transfer correlation, Eqs.~30!–~34! presented in Part I ofthis study~Kim et al. 2002!, were used to predict the performanof the pilot-scale bubble-diffuser contactor with the ADR modThe resulting predicted liquid and gas phase ozone concentraprofiles andC. murisoocyst survival ratios are compared to thexperimental data in Fig. 8. As depicted in Fig. 8, there wagenerally good agreement between experimental data and prtions. Some of the discrepancies observed might have res

Fig. 7. Comparison of tracer test curves determined experimentand predicted with axial dispersion reactor model


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from a combination of analytical and experimental errors, vaability in the water quality of the treated Ohio River water, adeviations from ADR model assumptions, such as occurrencehigher degree of mixing near the diffuser~Le Sauze et al. 1992!.

Additional pilot-scale experiments were performed with boC. murisandC. parvumoocysts but with microbial samples takeonly from the contactor influent and effluent lines. The dissolvozone concentration profiles for these experiments are presein Fig. 9. The respective average feed-gas ozone concentrawere CG,0510.4, 29.4, and 48.4 mg/L forC. muris, and CG,0

518.7, 37.8, and 43.8 mg/L forC. parvum. The average tempera

Fig. 8. Comparison of ozone concentration andC. muris oocystsurvival ratio profiles measured experimentally and predicted waxial dispersion reactor model throughout water column heightpilot-scale bubble-diffuser ozone contactor:~a! dissolved ozone con-centration;~b! gas-phase ozone concentration;~c! C. muris oocystsurvival ratio.

Fig. 9. Comparison of ozone concentration profiles measured expmentally and predicted with axial dispersion reactor model throuout water column height of pilot-scale bubble-diffuser ozone conttor: ~a! dissolved ozone concentration forC. murisexperiments;~b!dissolved ozone concentration forC. parvumexperiments.

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ture for these experiments was 23.661.6°C for C. muris, and24.561.6°C forC. parvum. The observed inactivation efficiencyexpressed in terms of overallN/N0 values computed from theviability of contactor influent and effluent samples for all expements of which ozone concentration profiles are shown in Figand 9, are plotted against the corresponding average appliedphase ozone concentrations in Fig. 10. Corresponding ozonetransfer efficiencies, expressed as 1003(CG,02CG,1)/CG,0 ~withCG,15ozone concentration in off gas, or atz5x/L51! for allexperiments are shown in Fig. 11.

Fig. 10. Comparison of overallC. murisandC. parvumoocyst sur-vival ratios measured experimentally and predicted with axial dispsion reactor model for tests performed with pilot-scale bubbdiffuser ozone contactor

Fig. 11. Comparison of transfer efficiencies measured experimtally and predicted with axial dispersion reactor model for tests pformed with pilot-scale bubble-diffuser ozone contactor

ss

Axial dispersion reactor model predictions for the dissolvozone concentration profiles, overall survival ratios, and tranefficiencies are also shown in Figs. 9, 10, and 11, respectivelydepicted in the figures, the model predicted the experimentasults generally well. Deviations in the predictions of survival rtios lower than 0.01 might have resulted, at least in part, franalytical variability associated with the viability assessmmethods at relatively high inactivation efficiencies. Reinforcithe recommendation by Owens et al.~2000!, the similarity in bothexperimental and predicted results shown in Fig. 10 for theCryptosporidiumspecies supports the use ofC. murisoocysts asbiological surrogate indicators forC. parvumoocysts under therange of experimental conditions investigated.

The results presented in this section indicate that the Amodel proposed in Part I of this study~Kim et al. 2002! is apromising tool to predict the inactivation ofC. parvumand C.murisoocysts in pilot-scale bubble columns. The model will nebe applied to simulate the performance of a bubble-diffuser ozcontactor under a broader range of operating conditions and wquality characteristics.

Model Application

The performance of the pilot-scale ozone bubble-diffuser contor was simulated with the ADR model for a wide range of opating conditions~feed-gas ozone concentration, liquid flow ratand gas flow rate!, and water quality related parameters~fastozone demand, first- and second-order ozone decompositionconstants, and temperature!. The baseline conditions selected fmodel simulations were: target microorganism5C. parvumoo-cyst; water quality5treated Ohio River water; temperatur520°C; water flow rate56.4 L/min; gas flow rate50.64 L/min;feed-gas ozone concentration530 mg/L.

Fig. 12. Axial dispersion reactor model simulation of pilot-scabubble-diffuser ozone contactor performance at various feedozone concentrations:~a! dissolved ozone concentration;~b! C. murisandC. parvumoocyst survival ratio with contactor operated in coutercurrent mode;~c! C. muris and C. parvumoocyst survival ratiowith contactor operated in cocurrent mode~numbers in plots areCG,0

values!.


re

30,andtion

.in afor

byt

dis-disfore/L.

atfo

nselti-be

n thode

r-n of

inthe-

con-gasgas

h ofed tobblenserallne

end.ra-ere

ina-lting

sur-

Effect of Operating ConditionsModel simulations similar to the predictions shown in Fig. 8 wedeveloped to assess the inactivation efficiency ofC. parvumandC. muris oocysts for feed-gas ozone concentrations of 15,and 50 mg/L with the reactor operated in both countercurrentcocurrent modes. The resulting dissolved ozone concentraprofiles for both operating modes are presented in Fig. 12~a!, andthe corresponding inactivation profiles are shown in Fig. 12~b! forcountercurrent and in Fig. 12~c! for the cocurrent operating modeAs depicted in Fig. 12, the cocurrent operation mode resultedhigher level of overall inactivation compared to that obtainedthe countercurrent mode. This observation, also predictedSmith and Zhou~1994! with a back-flow cell model, is consistenwith the occurrence of a higher average concentration ofsolved ozone in the cocurrent contactor. Predicted averagesolved ozone concentrations were 65, 36, and 29% highercocurrent operation than for countercurrent operation at thespective feed-gas ozone concentrations of 15, 30, and 50 mg

As also depicted in Fig. 12~a!, the dissolved ozone residualthe contactor effluent was higher for countercurrent mode thancocurrent mode for the same feed-gas concentration. Coquently, if these profiles were for the first chamber of a muchamber contactor, then the inactivation efficiency that wouldachieved in subsequent reactive chambers would be greater icase of the first chamber being operated in a countercurrent m


-r-

r-

e.

The viableC. parvumprofile corresponding to the countercurent operating mode at the lowest feed gas ozone concentratio15 mg/L @Fig. 12~b!# revealed that no inactivation took placethe top 30% of the contactor. This finding was consistent withinactivation kinetics ofC. parvumoocysts with ozone being characterized by an initial lag phase~Rennecker et al. 1999!. Thelower inactivation efficiency observed forC. parvumcompared tothat for C. muris was also due primarily to theC. parvumlagphase.

The ozone dose applied in an ozone contactor operated atstant water flow rate can be varied by changing the feed-ozone concentration at constant gas flow rate, by altering theflow rate at constant feed-gas concentration, or by varying botthese operating parameters. The operating conditions selectapply a given dose can affect the performance of an ozone budiffuser contactor as depicted with the ADR model simulatiopresented in Fig. 13. Both transferred ozone dose and ovinactivation efficiency increased with increasing applied ozodose. In contrast, the transfer efficiency had the opposite trThe overall inactivation efficiency was higher in cocurrent opetion, while the transferred ozone dose and transfer efficiency wfound to always be higher in countercurrent operation. Combtions of feed-gas ozone concentration and gas flow rate resuin constant transferred ozone doses of 2–8 mg/L@Fig. 13~a!# arerepresented by isodose lines along the surface of the mesh

s ozoneins lines

Fig. 13. Axial dispersion reactor model simulation of pilot-scale bubble-diffuser ozone contactor performance as function of feed-gaconcentration and gas flow rate:~a! transferred dose;~b! transfer efficiency;~c! C. parvumoocyst survival ratio with contactor operatedcountercurrent mode;~d! C. parvumoocyst survival ratio with contactor operated in cocurrent mode. White mesh surfaces and continuoucorrespond to countercurrent operation, and gray mesh surfaces and dashed lines correspond to cocurrent operation~numbers in plots aretransferred ozone dose values!.

gas floweses

Fig. 14. Axial dispersion reactor model simulation of pilot-scale bubble-diffuser ozone contactor performance as function of water andrates:~a! dispersion number;~b! transferred dose;~c! C. parvumoocyst survival ratio;~d! transfer efficiency. White mesh surfaces and lincorrespond to countercurrent operation, and gray mesh surfaces correspond to cocurrent operation~numbers in plots are transferred ozone dosin mg/L, or dispersion numbers!.

fertes. Ther

tes

the

e in

il-er-

as-ndin-amtes0.4g/

pre-loting

flownemnans-

le

e inflow

niccanlly,on-

ndffi-

en-mn

all-rentthe

faces in Figs. 13~b–d!. These lines predicted that higher transefficiencies would be achieved when using lower gas flow raand the corresponding higher feed-gas ozone concentrationscorresponding inactivation efficiencies would be slightly lowfor the countercurrent operating mode@Fig. 13~c!#, but slightlyhigher for the cocurrent operating mode, at lower gas flow raand the corresponding higher feed-gas ozone [email protected]~d!#. For example, according to the 4 mg/L isodose line forcountercurrent contactor in Fig. 13~c!, N/N050.0352 at QG

50.5 L/min, andN/N050.0343 atQG52.0 L/min, and accord-ing to the corresponding line for the cocurrent operating modFig. 13~d!, N/N050.0117 atQG50.5 L/min, andN/N050.0215at QG52.0 L/min.

The effects of liquid and gas flow rates on dispersion arelustrated in Fig. 14~a!. The corresponding transferred dose, ovall inactivation, and transfer efficiency are shown in Figs. 14~b–d!, respectively. Consistent with Eq.~35! in Part I of this study~Kim et al. 2002!, the dispersion number increased with decreing water flow rate and increasing gas flow rate. The correspoing transferred dose and overall inactivation efficiency alsocreased, while the transfer efficiency decreased, with the strends in flow rates. Combinations of water and gas flow raresulting in constant dispersion number in the range of 0.1–and constant transferred ozone dose in the range of 2–4 mwere determined from Figs. 14~a and b!, respectively, for the caseof the countercurrent operating mode. The inactivation levelsdicted for the resulting isodispersion and isodose lines were pted in Fig. 14~c!. For the same transferred ozone dose, increas

,e

-

e

,L

-

the dispersion number by decreasing both the water and gasrates resulted in higher inactivation efficiency. Dissolved ozoconcentration and inactivation profiles along the reactor colufor dispersion numbers of 0.1, 0.2, and 0.3 and a constant trferred ozone dose of 3 mg/L corresponding to PointsA, B andCin Fig. 14~c! were shown in Fig. 15. The main factor responsibfor the increase in inactivation efficiency from PointA to PointC,despite the increase in dispersion number, was the increasmean residence time resulting from the decrease in waterrate.

Effect of Source Water CharacteristicsWater quality parameters such as pH, alkalinity, natural orgamatter, and various other organic and inorganic constituentsaffect the kinetics of ozone decomposition, or more specificathe fast ozone demandD0 and subsequent ozone decompositirate constantskR andkD . Axial dispersion reactor model simulations of the effects thatkD and D0 could have on inactivationefficiency were shown in Fig. 16 for both countercurrent acocurrent operating modes. In general, lower inactivation eciencies were observed at higher values of bothkD andD0 . Thiswas primarily a direct result of the corresponding lower conctrations of dissolved ozone present throughout the water coluinside the contactor.

An examination of the effect of temperature on the overinactivation efficiency ofC. parvumoocysts achieved in the pilotscale ozone bubble diffuser contactor operated in countercurmode was presented in Fig. 17. Various parameters affecting


ooming

effi-ntraeraingcti-a-

inhelity

de-ted

ivert,

tiva-dtheer

nac-

d

con-ntra

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-

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torratepera-

inactivation efficiency such as ozone decomposition ratekD , in-activation ratekN , Henry’s law constantm, and overall volumet-ric mass transfer coefficientkLa were normalized with respect ttheir values at 20°C and plotted in the same figure. Ozone decposition rate and Henry’s law constant increased with increastemperature, thus having negative effects on the inactivationciency due to the occurrence of lower dissolved ozone concetions. On the other hand, the inactivation rate constant and ovvolumetric mass transfer coefficient increased with increastemperature, thus exerting a positive effect on the overall inavation efficiency. Overall, the effect of temperature on inactiv

Fig. 15. Axial dispersion reactor model simulation of dissolveozone concentration andC. parvumoocyst survival ratio profiles inpilot-scale ozone contactor at various dispersion numbers andstant ozone transferred dose of 3 mg/L~Cavg values are average dissolved ozone concentrations calculated by integrating the concetion profiles along the normalized water column height; PointsA, BandC are indicated in Fig. 14.

Fig. 16. Axial dispersion reactor model simulation ofC. parvumoocyst survival ratio in pilot-scale bubble-diffuser ozone contactoa function of initial fast ozone demand and first-order ozone decposition rate constant. White mesh surface corresponds to counterent operation, and gray mesh surface corresponds to cocurreneration.


-

-ll

tion rate overpowered all counteracting effects, thus resultinggreater overall inactivation efficiency at higher temperature. Tsame trends were observed for waters with varying water qua~and corresponding different decomposition rate constant! as de-picted in Fig. 18. The temperature dependence of the ozonecomposition rate in the various hypothetical waters was evaluausing the same activation energy found for the treated Ohio Rwater used in this study withD050. This analysis suggested thain general, a higher temperature would lead to a greater inaction efficiency ofC. parvumoocysts for both countercurrent ancocurrent modes. The effect of increasing temperature onoverall inactivation efficiency became greater for relatively lowozone decomposition rate constants~at 20°C! as depicted in Fig.18. These observations for the effect of temperature on the i

-

-

r--

Fig. 17. Axial dispersion reactor model simulation of effect of temperature on first-order ozone decomposition andC. parvuminactiva-tion rate, Henry’s law, and volumetric mass transfer rate constaand overall inactivation efficiency achieved in the pilot-scale bubbdiffuser ozone contactor.

Fig. 18. Axial dispersion reactor model simulation ofC. parvumoocyst survival ratio in a pilot-scale bubble-diffuser ozone contacas function of temperature and first-order ozone decompositionconstant. White mesh surface corresponds to countercurrent otion, and gray mesh surface corresponds to cocurrent operation.

i-nac

lot-teds

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athatate

herw-er-deperliedneomt thncesin

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tivation efficiency ofC. parvumoocysts~also generally valid forC. murisoocysts! should not be generally extended to other mcroorganisms because the temperature dependence of the ivation rate constant might be weaker.

Conclusions

The ADR model presented in Part I of this study~Kim et al.2002! was evaluated with experimental data obtained with a piscale ozone bubble-diffuser contactor. The ADR model predicviable C. murisandC. parvumoocyst, and ozone concentrationgenerally well. The model was further utilized to simulate tperformance of a pilot-scale ozone contactor, providing valuarecommendations for ozone contactor design and operation.example, the simulation revealed that meeting inactivationquirements forC. parvumoocysts would be more challengingrelatively lower temperatures. It is important to emphasize tthe trends observed for the effects of various operating and wquality parameters on the inactivation efficiency ofC. parvumand C. murisoocysts should not be generally extended to otmicroorganisms, or to contactors with a different design. Hoever, the ADR model could be a useful tool to simulate the pformance of any pilot-scale ozone bubble-diffuser contactor una broad range of operating and water quality conditions if proinformation is provided. For example, this model can be appto predict any microorganism inactivation in any water if ozodecomposition and ozone inactivation kinetics are available frbench-scale tests. In addition, the model can be used to predicperformance of ozone contactors with other configurations, obubble sizes and the dispersion number can be estimated uthe correlations such as Eqs.~33! and ~35! from Part I of thisstudy ~Kim et al. 2002!.

Acknowledgments

This research was partially funded by the U.S. EnvironmenProtection Agency. The writers would like to acknowledge tCivil Engineering Corps, U. S. Navy, for financial support prvided to the third author~R.B.T.! during graduate work at theUniversity of Illinois. The writers would also like to extend thegratitude to Dr. Marilyn Marshall, Department of Veterinary Scence, University of Arizona for providing information about ocysts, and the Cincinnati Water Works for providing water for tstudy. Bench-scale disinfection experiments and modeling tawere performed at the William H. Richardson Memorial Disinfetion Laboratory of the University of Illinois.

Notation

The following symbols are used in this paper:AD 5 frequency factor for first-order ozone decomposition

reaction (T21);AN 5 frequency factor for inactivation reaction (T21);

Cavg 5 average dissolved ozone concentration calculatedby integrating concentration profile along normal-ized water column heightz (ML23);

CG 5 gas-phase ozone concentration (ML23);CG,0 5 initial or influent gas-phase ozone concentration

(ML23);CG,1 5 ozone concentration in off-gas, or atz5x/L

51 (ML23);

ti-

r

r

r

e

g

s

CL 5 liquid phase ozone concentration (ML23);CL,0 5 initial or influent liquid phase ozone concentration

(ML23);CT 5 product of dissolved ozone concentration and con-

tact time (ML23 T);CTlag 5 lag phaseCT (ML23 T);

D0 5 initial or influent concentration of natural organicmatter fraction with fast ozone demand (ML23);

d 5 dispersion number~inverse of Pećlet number! ~di-mensionless!;

dB,0 5 average bubble diameter extrapolated toUG50 (L);

ED,a 5 activation energy for first-order ozone decomposi-tion reactions~J/mol!;

EN,a 5 activation energy for inactivation reaction~J/mol!;ER,a 5 activation energy for second-order ozone decom-

position reactions~J/mol!;g 5 gravitational constant (L T22);

kD 5 first-order ozone decomposition rate constant(T21);

kLa 5 volumetric mass transfer coefficient (T21);kN 5 second-order inactivation rate constant

(L3 M21 T21);kR 5 second-order ozone decomposition rate constant

(L3 M21 T21);L 5 depth of water column in bubble-diffuser contac-

tor ~L!;m 5 Henry’s law constant~dimensionless!;N 5 number density of viable microorganisms (L23);

N0 5 number density of viable microorganisms enteringcontactor (L23);

N1 5 number density of viable microorganisms resultingfrom extrapolating postlag phase first-order por-tion of inactivation curve (L23);

QG 5 gas flow rate (L3 T21);QL 5 water flow rate (L3 T21);

T 5 temperature~K!;t 5 time ~T!;x 5 downward distance in axial direction~L!; andz 5 normalized downward distance in axial direction

~dimensionless!.

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Documents

Inactivation of Cryptosporidium Oocysts in a Pilot-Scale Ozone