Water Research 36 (2002) 1043–1055

Wastewater disinfection by ozone: main parameters forprocess design

Pei Xua, Marie-Laure Janexb, Philippe Savoyeb, Arnaud Cockxc,Valentina Lazarovab,*

aLaboratoire Hydrosciences, MSE, UMR no. 5569, Universit !e Montpellier II, 34095 Montpellier cedex 5, FrancebOndeo Services-CIRSEE, 38 Rue du Pr !esident Wilson, 78230 Le Pecq, France

cLaboratoire GPI, INSA, 135 avenue de Rangueil, 31077 Toulouse cedex 4, France

Received 1 March 2000; accepted 30 May 2001

Abstract

Wastewater disinfection by ozone was investigated at pilot scale on different wastewater effluents. Variations inoperating conditions showed that a very low hydraulic retention time (2min) was sufficient for efficient fecal coliforminactivation, provided a sufficient ozone dose was transferred to the effluent. Therefore, the transferred ozone dose

appeared to be the critical parameter for the design of wastewater disinfection. As a consequence, the ‘‘Ct’’ approachcommonly applied in drinking water treatment should not be used for wastewater ozonation. Design parameters ofozonation were proposed for two types of regulations, and for effluents of different qualities. It was demonstrated that

only with an efficient filtration step one can meet stringent standards such as the California Title 22 criteria. In all cases,viruses were totally inactivated; consequently, viruses do not constitute a limiting factor in wastewater disinfection byozone.

The standard drinking water model failed to match the experimental data obtained on real wastewater effluents.A modified approach was successfully developed, based on the simultaneous consumption of ozone by themicroorganisms and the organic matrix. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Wastewater disinfection; Ozone; Process design; Water quality; Wastewater reuse; Disinfection modelling

1. Introduction

Wastewater reuse has become an attractive option forprotecting the environment and extending available

water resources. In the last few years, there has been asignificant diversification of water reuse practices, suchas green space and crop irrigation, recreational im-

poundment, various urban uses including toilet flushing,industrial applications and water supply augmentationthrough groundwater or reservoir recharge [1,2]. Thesafe operation of water reuse systems depends on the

reliability of wastewater disinfection, which is the most

important treatment process for public health protec-tion. The health-related microbiological regulations [3]and the more recent impetus of producing virus-free

effluents [4] require the development of highly effectiveadvanced disinfection processes. Chlorination is still themost widely used means to inactivate pathogenic

microorganisms in water and wastewater, but alterna-tive technologies have to be evaluated because ofincreasing concern over undesirable byproducts afterchlorination and its inefficiency in eliminating some

epidemic microorganisms at low chlorine doses [5,6].Ozone has been proved to be one of the most effective

disinfectants and is widely used to inactivate pathogens

in drinking water, especially in Europe [7,8]. Designengineers in the US began to evaluate ozone for

*Corresponding author. Tel.: +33-134-802-251; fax: +33-

130-536-207.

E-mail address: valentina.lazarova@lyonnaise-des-eaux.fr

(V. Lazarova).

0043-1354/02/$ -see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 0 4 3 - 1 3 5 4 ( 0 1 ) 0 0 2 9 8 - 6

wastewater disinfection in the early 1970s. However,because of operational and maintenance problems that

appeared in the first generation of facilities, it has beenconsidered to be a less attractive alternative to chlorinethan UV disinfection. Also, many researchers initially

sought to achieve a measurable level of dissolved ozoneresidual in treated wastewater, which resulted in highozone dosages that were not economically feasible [9].Earlier studies pointed out the need for a thorough

investigation of wastewater ozone treatment in order topredict disinfection performance and design the disin-fection system for wastewater disinfection [10–12].

The present study investigates the main factors relatedto ozone disinfection performance, for the purpose offacilitating its design and application to wastewater

disinfection.

2. Materials and methods

Experiments were performed in a continuous-flowpilot plant with different types of effluents to evaluate

ozone disinfection performance on different targetmicroorganisms.

2.1. Experimental set-up: continuous-flow ozone pilotplant

Ozonation tests were conducted in two different pilotsdesigned as bubble diffuser columns (Fig. 1), which wereoperated in continuous counter-current mode. Ozone

between 2% and 6% was generated from oxygen (95–98% purity), with generators provided by Ozonia. Thespecifications of the ozone generators and contactors

used and the operating conditions are given in Table 1.Tracer test studies were performed by impulse

injections of sodium chloride in the different operating

conditions on both pilots. This made it possible tocharacterize the pilots as a series of two to five CSTRs.Salt recovery showed that dead zones were between 5%

and 12% for the smaller hydraulic retention times(HRT), and up to 25–30% for the highest HRT (10minin pilot 1, and 15min in pilot 2). This was taken into

Fig. 1. Schematic diagram of the continuous-flow ozone pilot plant.

P. Xu et al. / Water Research 36 (2002) 1043–10551044

account to calculate real contact times when data were

expressed as a function of ‘‘Ct’’.The ozone concentrations in the influent gas and off-

gas were measured by UV absorbance monitors.Residual ozone concentration in water was analysed

by the indigo carmine method of [13], using HACH DR/2000 spectrophotometer. The mass transfer efficiency inthe columns was found to be consistently 55% and 30–

50%, respectively, in the pilots 1 and 2. In order tocompare the data, the transferred ozone dose (TOD)was used as a descriptive parameter throughout the

study. It is defined as follows:

TOD ¼ Qgas=Qliqð½O3�g in � ½O3�g outÞ; ð1Þ

where Qgas and Qliq are gas and water flow rates,

respectively, [O3]g in is the ozone concentration in thefeed gas to the column, [O3]g out is the ozone concentra-tion in the off-gas leaving the column. For a given set of

operating conditions, a time interval of three to fourtimes the mean hydraulic retention time was allowed toreach steady state and take samples. In order to study

the effect of post-contact without additional ozoneintroduction, water samples were taken from the outletof the column and were kept in a brown bottle withoutlight or air, to be analysed after a controlled contact

time.

2.2. Determination of ozone demand

The immediate ozone demand of the effluents, X ; wasevaluated from the plots of the ozone residual vs. thetransferred ozone dose during disinfection tests, accord-

ing to the method proposed by Roustan et al. [14]. Xrepresents the minimum dose to be transferred to getmeasurable ozone residual in the water. Mathematically,

the concept is represented by the following equations,where the ‘‘instantaneous’’ demand would correspond to

an infinite rate constant

d½O3�dt

¼KLað½O3�* � ½O3�Þ � kD½O3� if TOD > X ;

½O3� ¼ 0 andd½O3�dt

¼ 0 if TODoX ; ð2Þ

where KLa is the mass transfer coefficient; [O3]* is theequilibrium (maximum) concentration of dissolvedozone, [O3], corresponding to Henry’s law; kD is the

first order decay constant, in min�1. TOD is thetransferred ozone dose calculated by

TOD ¼Z t

0

KLað½O3�* � ½O3�Þ dt: ð3Þ

Eq. (2) can easily be solved in an open, completely mixed

reactor (CSTR with a contact time t) to estimate theozone concentration

½O3� ¼TOD� X

1þ kDt: ð4Þ

2.3. Wastewater characterisation

Effluents from three different wastewater treatmentplants (WWTP) were used for the studyFtwo second-ary effluents and one tertiary effluent. The secondarytreatment trains in Evry, France (48,000m3 d�1) and

Washington, UK (90,000m3 d�1) are similar: pretreat-ment, primary clarification, activated sludge (extendedaeration and high rate activated sludge, respectively),

secondary clarification and discharge. The tertiarytreatment train in Indianapolis, USA (300,000m3 d�1)consists of dual media filtration and chlorination after

primary clarification, and coupled bio-roughing andactivated sludge nitrification facilities. The effluent forpilot testing was taken after the tertiary filtration. The

main characteristics of the effluents during the tests aregiven in Table 2.

Table 1

Characteristics of the ozone pilots and operating conditions used in the study

Parameter Pilot 1 Pilot 2

(Evry, France/Washington, UK) (Indianapolis, USA)

Column height, m 2.6 3.6

Column diameter, m 0.15 0.30

Porous plate porosity, mm 100 50

Hydraulic retention time, min 2–10 3–15

Number of CSTRs in seriesa 2–5 2

Applied ozone dose, mgL�1 3–16/4–50 1–35

Transferred ozone dose, mgL�1 2–13/4–30 0.5–12

aObtained by tracer tests in the different operating conditions.

P. Xu et al. / Water Research 36 (2002) 1043–1055 1045

Fecal Coliforms and E. coli were chosen as standard

fecal indicators in the study, because they are usuallyregulated for wastewater discharge or reuse. Othermicroorganisms were also studied in the case of the

Washington effluent: enterococci Clostridium (as surro-gates for more resistant organisms), Salmonellae, en-teroviruses and F-specific bacteriophages (considered a

good model for virus disinfection [15]). The microbio-logical methods used were based on the French and UKIndustry Standard Methods [16] with additional dilutionof the samples to appropriate levels. The analytical

methods are detailed in Table 3.

A number of physico-chemical parameters were

monitored on the effluents before and after ozonationusing Standard methods for pH, TOC, Turbidity, SS,total and filtered COD, TOC, BOD5, UV 254 abs. (total

and after filtration), N–NO2, alkalinity, Mictotox testsfor toxicity and particle size distribution. To investigatethe effect of ozonation on colour abatement, absorbance

at 400 nm was used to characterise apparent colour.Water appears to be coloured when dissolved matterabsorbs visible light or when suspended particles scatterlight (Rayleigh scattering). Finally, to stop the effect of

ozone residual on microorganism concentration after

Table 2

Wastewater characteristics, average (min–max)

Parameter Tertiary effluent Secondary effluent

Indianapolis (USA) Evry (France) Washington (UK)

Suspended solids (mgL�1) 2.3 (o1–4) 5 (3–6) 18 (7–33)

COD, (mg O2 L�1) 30 (24–38) 36 (26–56) 71 (41–150)

TOC (mgL�1) 8 (5.5–10.2) o10 (o10–14) 26 (o11–30)

UV 254 abs (m�1) 15.5 (12.5–20.8) 22.2 (17.4–20.8) 34.9 (26.0–50.9)

pH 7 (6.9–7.2) 7.3 (7.3–7.4) 7.5 (7.4–8.0)

Fecal coliforms (log CFU per 100mL) F 3.6–4.5 4.3–6.5

E. coli (log CFU per 100mL) 2.7–4.3 F FClostridium (log CFU per 100mL) F 3.0–4.5 3.6–5.5

Table 3

Analytical methods for microbiological parameters

Parameters Analytical methods

E. coli (Indianapolis/Evry) Standard method 9222 O-M/Enterolert, Idexx (CIRSEE)

Fecal coliforms (Indianapolis/Washington) Standard method 9222 D-M/membrane filtration: incubation on 0.45 mm membrane

lauryl sulphate Broth for 4 h at 371C then 14 h at 441C. Enumeration of presumptive

fecal coliforms and confirmation by subculture into lactose peptone water at 371C in

conjunction with an oxidase test

Enterococci Membrane filtration: incubation on Slanetz and Bartley Agar for 4 h at 371C

followed by 44 h at 441C. Count all maroon colonies, confirmation on bile aesculin

azide agar

Clostridium Membrane filtration: heat-treat the sample at 751C for 10min. Serially dilute and

vacuum filter appropriate dilutions/volumes through 0.45mm membrane. Incubate

on Perfringens OPSP medium anaerobically at 371C for 48 h. Count all black

colonies and confirm in crossley milk

Enterovirus Suspended cell plaque assay: adsorption onto a cellulose nitrate membrane at pH 3.5;

elution by a positively charged protein solution; flocculation of the protein solution

and centrifugation; virus numeration by tissue culture assay (recovery quoted>20%)

F-specific RNA bacteriophages Incubation with a host strain: direct plating using a semi-solid overlay technique,

with Salmonella typhimurium WG49 as host bacterium (MS2 bacteriophage used as

positive control)

Salmonellae Filter appropriate volumes of sample through a 0.45mm filter (using filter-aid if

turbid). Pre-enrich the filter in buffered peptone water for 24 h at 371C. Enrich a

0.1ml portion of the culture in RVS broth for 48 h at 411C subculturing onto XLD

Agar and Brilliant Green Agar after 24 and 48 h. Presumptive Salmonellae are

confirmed serologically and biochemically

P. Xu et al. / Water Research 36 (2002) 1043–10551046

sampling, 4% (wt/vol) sodium thiosulfate was added tothe treated effluent samples.

To investigate the influence of particles on disinfectionperformances, additional tests were run with the Evryeffluent after filtration (75 mm Arkal prefilter, followed

by a 10mm canvas filter), to lower the suspended solidsconcentration below 2mgL�1.

3. Results and discussion

3.1. Inactivation of fecal coliforms: impact ofoperating conditions and wastewater quality

3.1.1. Influence of wastewater quality on ozone demandFig. 2 illustrates the determination of ozone demand

in the Washington secondary effluent for different values

of the hydraulic retention time. The measured values of7.4–9.6mgL�1 are significantly higher than those in the

other two effluents, as expected from the water qualitydata (see Table 2). The ozone demand of the Evrysecondary effluent (extended aeration with nitrification)

of 3.1–4.2mgL�1 is similar to the values measured in theIndianapolis tertiary effluent of 2.5–5.3mgL�1. Theseresults are in agreement with an earlier study performedon other effluents, which showed organic content to be a

much more influential parameter than suspended solidson the ozone demand [12]. These values are usedhereafter when discussing the disinfection performances.

3.1.2. Influence of operating conditions on coliform

inactivationFig. 3 summarises the results from all the experiments,

where the residual concentration of bacteria after

Fig. 2. Determination of the immediate ozone demand according to the classical approach used for drinking water (effluent from

Washington, UK).

Fig. 3. Performances of ozone for FC inactivation on three different effluents: comparison of concentration level after ozonation with

reuse standards.

P. Xu et al. / Water Research 36 (2002) 1043–1055 1047

ozonation is plotted as a function of the transferredozone dose (TOD). One important observation is that a

significant 1–3 log inactivation is already reached whenTOD approximates the immediate ozone demand, i.e.with no measurable residual of ozone in solution. This is

in agreement with earlier data on wastewater ozonation[12]. More precisely, the higher the immediate ozonedemand, the higher the inactivation level reached at thatdose. This phenomenon can be explained by the fact that

bacteria themselves participate in the immediate ozonedemand, because of their high kinetic rate with ozone.One important consequence from these results is that

design and control of ozonation for fecal coliformdisinfection in wastewater should not be based on thestandard parameter of residual ozone or ‘‘Ct’’ factor. In

agreement with this, Rakness et al. [10] reported thatdirect measurement of ozone residual within the full-scale contactors at Indianapolis had been unsuccessful

although good disinfection had occurred. The ‘‘Ct’’approach is applied to drinking water because moreresistant microorganisms like Giardia are targeted.Data from Fig. 3 show that the hydraulic retention

time (HRT) has no impact on the performances of fecalcoliform (FC) or E. coli disinfectionFfor a given TOD,a 2min HRT provides the same inactivation as 10min

HRT. These results have major consequence for thedesign of ozone wastewater disinfection, demonstratingthat mass transfer is the critical step that has to be

optimised, and that no long-contact-time chamber isnecessary.Fig. 4 illustrates this statement, showing the distribu-

tion of ozone residual and FC inactivation along the

ozonation column and after 2, 4 and 6min additional

contact in the post-contactor chamber. Residual ozonedecreased significantly in the post-contactor after 2min

and no significant increase in FC inactivation isobserved. The quick decay of ozone may be explainedby the wastewater matrix-consuming ozone; therefore,

no further inactivation can be expected from a post-contactor without additional ozone injection. As aconsequence, no credit of additional inactivation canbe attributed to a storage reservoir or outfall that would

be used for discharge of ozonated wastewater.

3.1.3. Effect of wastewater quality on ozone

disinfection performancesDespite similar values of immediate ozone demand

and not very different initial concentrations of bacteria,the secondary and tertiary effluents of Evry and

Indianapolis display different inactivation performances(see Fig. 3). It appears that only the tertiary effluent isable to meet stringent standards for almost total bacteria

inactivation like the Californian Title 22 criteria. Thisresult led to a more in-depth investigation of theinfluence of particles on disinfection.

Comparative tests were performed with the effluentfrom Evry, and with the same effluent after a filtrationstep. These tests were performed simultaneously in order

to minimise any fluctuation in water quality or operatingconditions. The ozone demand of the effluent was foundto be exactly the same. The effect of filtration ondisinfection is shown in Fig. 5: an additional 1 log

inactivation was obtained by filtration. It must bestressed that for a given bacteria concentration (logN0),a lower level of contamination was obtained after

filtration with the same ozone dose. This conclusion

Fig. 4. FC inactivation and residual ozone distribution vs. contact time in and after the ozone column (Washington secondary effluent,

HRT 4min, TOD 13.1mgL�1).

P. Xu et al. / Water Research 36 (2002) 1043–10551048

explains the higher inactivation level obtained with thetertiary effluent from Indianapolis, with its very low

suspended solids content.

3.2. Ozone disinfection of other microorganisms

In order to evaluate the disinfection efficiency of

ozone on other microorganisms, fecal streptococci (FS),Clostridium, Salmonellae, Enterovirus, and F+-specificbacteriophages were chosen as target microorganisms

for additional tests with the Washington secondaryeffluent. The inactivation efficiencies are presented inTable 4. It should be noted that the initial concentration

of microorganisms limited the maximum inactivationvalues that could be reached. The resistance of fecalcoliforms and enterococci to ozonation was similar, inagreement with previous results [17,18]. It is important

to stress also that a relatively low ozone dose of8.6mgL�1 totally inactivates Salmonellae.A higher efficiency of ozonation was observed for

virus inactivation: a transferred dose of 4.8mgL�1 with4min HRT was enough for total inactivation ofenteroviruses (>2.9 log inactivation). Total inactivation

of F+ coliphages (>2.2 log inactivation) was reachedwith a slightly higher transferred dose of 8.6mgL�1.The strong virucidal power of ozone for wastewater

disinfection confirms its well-known performance indrinking water [7,18]. Indigenous enteric viruses isolatedfrom wastewater effluents have been shown to be muchmore resistant [18]. In comparison, coliphages have been

found to be very sensitive to ozone, which puts somedoubt on the validity of these coliphages as surrogatesfor enteric viruses [19,20].

This study shows that the ozone dose required tosatisfy WHO regulations (1000FC per 100mL) alsoprovides total inactivation of indigenous enteric viruses.

This indicates that ozone would be highly recommendedfor the production of virus free water, required in several

countries for landscape (Australia, 1 pfu/50L) or agri-cultural irrigation (Arisona and Hawaii, USA, 1 pfu/

40L).Compared to all other microorganisms, the higher

resistance of Clostridium was confirmed by the experi-

mental data. A thorough investigation was made withthe effluents from Evry and Washington with thatindicator (Fig. 6). With a TOD approximately equal to

the immediate ozone demand of the effluents (3–5mgL�1 for Evry and 8–10mgL�1 for Washington),less than 0.5 log inactivation of Clostridium wasachieved. The maximum inactivation level was less than

2 log for high TOD of 33mgL�1 (HRT 9.6min,Washington secondary effluent).

3.3. Impact of ozonation on effluent water quality

Due to the high oxidative potential of ozone,ozonation has a beneficial effect on effluent quality,which argues in favour of its application for wastewater

reuse (Table 5). The most significant effect of ozone wason UV-254 absorbance and colour (Figs. 7a and b).With an increase in transferred ozone dose from 2 to

13mgL�1 at HRT 4min, the variation of UV absor-bance in the Evry secondary effluent increased from28% to 55%. These results indicate that the ozone reacts

and oxidises the organic matter, in particular, thecompounds having double bonds and/or an aromaticstructure that determine the value of the absorbance at254 nm. The UV absorbance abatement was higher in

the Evry secondary effluent than in Washington,indicating the presence of refractory contaminants.Finally, no significant difference was observed between

different contact times in the reactor (not shown),revealing the fast kinetics of the reaction between ozoneand unsaturated and aromatic compounds. The critical

factor for water quality improvement is also the ozonedose transferred into the water.

Fig. 5. Impact of a 10 mm pre-filtration on the inactivation of total coliforms by ozone (secondary effluent in Evry, France).

P. Xu et al. / Water Research 36 (2002) 1043–1055 1049

Total BOD5 increased up to 20% after ozonation ofthe highly polluted secondary effluent in Washington,

UK. This is typical with ozone, which can oxidiserecalcitrant compounds and thereby increase effluentbiodegradability. The total COD was not significantly

influenced by ozonation, while an increase of dissolvedCOD was observed. The difference between total anddissolved COD behaviours could be related to the globaldecrease of turbidity observed during the tests (Fig. 7-

c)Fsome particles and high weight organic compoundswould be destroyed by ozone and converted intodissolved compounds.

Toxicity was detected neither in the untreatedsecondary effluent nor in the ozonated effluents. Theseresults are in compliance with previous studies on urban

wastewater disinfection by ozone [17]. It is important tostress that the presence of toxicity after ozonationreported in the literature is usually related to the

presence of industrial wastewater [21–24].

3.4. Design of wastewater ozonation for given regulations

Ozone disinfection results were compared for twodifferent water reuse standards: (1) WHO stringent

guidelines for irrigation, fecal coliformso1000 cfu per100mL, and (2) Californian Title 22 standards, totalcoliformso2.2 cfu per 100mL (see Fig. 3). The TOD

required to meet WHO guidelines are 2, 4 and15mgL�1, respectively, for the tertiary affluent and thetwo secondary effluents in Evry and Washington for an

HRT of 2min and an additional beneficial effect of 30%reduction in UV absorbance. Compliance with thestringent Title 22 criteria of virtually total removal offecal coliforms can be reached only after tertiary

filtration in the Indianapolis effluent and with a TODof 8mgL�1 for 2min HRT. A very low suspended solidsconcentration (o5mgL�1) emerges as the most im-

portant design requirement to meet very stringentdisinfection requirements.In both scenarios, total inactivation of viruses is

achieved, which may be important if viruses are alsoincluded in regulations.

3.5. Modelling approach of wastewater disinfectionby ozone

Disinfection is standardly described, for drinkingwater, by the Chick–Watson model:

d N½ �dt

¼ �kN ½N�½O3�: ð5Þ

In the case of a CSTR, Eq. (5) enables the evaluation ofthe number of microorganisms, N:

½N�½N0�

¼1

1þ kN ½O3�t; ð6Þ

Table

4

Comparisonofozonedisinfectionondifferentmicroorganismsin

thesecondary

effluentin

Washington,UK

a

HRTmin

TOD

(mgL�1)Fecalcoliform

(Cfu

per

100mL)

Clostridium

(/100mL)

Enterococci(/100mL)

Salmonella

Enterovirus(pfu/10L)

F-coliphage(/mL)

logN

0N

Red

logN

0N

Red

logN

0N

Red

N0

NN

0N

Red

N0

NRed

44.8

5.00

4600

0.34

3.56

6200

F4.73

7100

0.88

FF

775

0>

2.89

96

21.68

48.6

4.94

1320

1.82

3.67

4800

04.63

980

1.64

Prst

Abs

544

12

1.66

144

0>

2.16

415.2

4.95

300

2.48

3.81

3100

0.31

4.74

173

2.50

Prst

Abs

650

0>

2.81

122

0>

2.09

9.6

11.0

5.45

840

2.52

4.53

21000

0.21

4.86

400

2.26

Prst

Abs

654

0>

2.82

9.6

24.8

5.92

14

4.42

4.46

2000

1.16

4.71

14

3.56

Prst

Abs

774

0>

2.89

9.6

29.5

5.20

14

4.06

4.46

550

1.72

4.53

16

3.33

Prst

Abs

774

0>

2.89

aNote:RedF

logðN

0=N

Þ;PrstF

present;AbsF

absent.

P. Xu et al. / Water Research 36 (2002) 1043–10551050

where N is the density of viable microorganisms (N0 att ¼ 0), t is the hydraulic retention time in the reactor,and kN is the inactivation rate constant.

In fact, the use of the Chick–Watson model impliesthe presence of ozone residual to achieve inactivation ofmicroorganisms; there would be no disinfection before

the ozone demand is met, i.e. for transferred ozone doseslower than 8mgL�1, the immediate ozone demand ofwastewater.

In order to account for the 1–3 log inactivationobserved without measurable residual in wastewaterozonation, a modified approach was therefore devel-oped, considering the consumption of ozone by the

organic matter as a combination of a rapid and a slow

kinetics, characterised by coefficient rates kX and kY[25]:

d½O3�dt

¼ KLað½O3�* � ½O3�Þ � kX ½X �½O3� � kY ½Y �½O3�;

ð7Þ

d Y½ �dt

¼ �kY ½Y �½O3�; ð8Þ

d X½ �dt

¼ �kX ½X �½O3�: ð9Þ

The comparison with the previous equations used fordrinking water shows that we can obtain the same model

Fig. 6. Performances of ozone for Clostridium inactivation. (secondary effluents of Washington, UK and Evry, France).

Table 5

Impact of ozonation on water quality in Washington, UKa

Parameters HRT 4.0min HRT 2.0min HRT 9.6min HRT 4.0min

TOD 9.2mgL�1 TOD 12.3mgL�1 TOD 15mgL�1 TOD 21.1mgL�1

C0 9 24 17 10

TBOD C 8 30 21 12

(mgL�1) ðC � C0Þ=C0 �11% +25% +24% +20%

C0 73 94 93 70

TCOD C 66 92 94 69

(mgL�1) ðC � C0Þ=C0 �10% �2% +1% �1%

C0 42 43 47 38

DCOD C 42 51 63 47

(mgL�1) ðC � C0Þ=C0 0% +19% +34% +24%

Total C0 0.44 0.48 0.51 0.43

UV Abs C 0.36 0.4 0.39 0.27

(Cm�1) ðC � C0Þ=C0 �20% �17% �24% �38%

Dissolved C0 0.22 0.23 0.23 0.22

UV Abs C 0.17 0.17 0.17 0.12

(Cm�1) ðC � C0Þ=C0 �25% �24% �27% �44%

aC0 is the initial concentration of secondary effluent; C is the concentration of ozonated effluent.

P. Xu et al. / Water Research 36 (2002) 1043–1055 1051

by simplification if

kD ¼ kY ½Y �;

kX ¼ N:

Eqs. (7–9) can easily be solved in an open completelymixed reactor (CSTR with a contact time t) to estimate

ozone concentration and also concentrations of thedifferent species X and Y (rapidly consumed and the

slowly consumed organic matter):

½O3� ¼DOT

1þ kX ½X�tþ kY ½Y �tð10Þ

½X � ¼½Xo�

1þ kX ½O3�t; ð11Þ

½Y � ¼½Yo�

1þ kY ½O3�t: ð12Þ

The use of these equations should be interpreted as avalid model for quantification but not necessarily as an

accurate representation of the more complex masstransfer steps and heterogeneous reactions taking placeduring the disinfection process. In particular, radical

mechanisms are ignored for simplification. Besides, in afirst step, hydraulic aspects are ignored in the inter-pretation of the data and we assume that the bubble

column can be represented as a CSTR, which is closeenough to the experimental data. The focus is on thekinetic modelling, which can be completed afterwardswith hydrodynamic information. The exact value of the

fitting parameters will then be slightly altered, but nottheir order of magnitude. A very similar approach wastaken by Hunt and Mari *nas [26] on the inactivation of

E. coli with ozone in synthetic waters.This modified approach was applied to the experi-

mental data from Washington WWTP, with residual

ozone as a function of TOD. It led to the followingfitting parameters (Fig. 8):

kX ¼ 10 Lmg�1 min�1;

kY ¼ 0:01 Lmg�1 min�1;

½Xo� ¼ 8 mg L�1;

½Yo� ¼ 60 mg L�1:

This represents a first improvement, but the majordifference between both models appears when consider-

ing the inactivation of fecal coliforms. For transferredozone doses lower than the fast ozone demand,integrated exposure to ozone in terms of Ct is equal tozero for the classical approach and reaches

0.40mgminL�1 with the modified model for a TODof 8mgL�1. When applying the Chick–Watson modelgiven by Eq. (5), this slight difference in the Ct makes it

possible to account for the inactivation of fecal coli-forms during X organic matter consumption. Fig. 9shows the strong impact of this low Ct in wastewater,

which inactivates 2 log of E. coli. The correspondinginactivation rate constant kN is equal to100Lmg�1min�1 and the fitting curves are depicted in

Figs. 9a and b. Besides, the model properly accounts forthe fact that the contact time has no effect on

Fig. 7. Influence of ozonation on wastewater quality. (a) Total

and dissolved UV-254 absorbance removal vs TOD (4min

HRT, secondary effluents of Washington, UK and Evry,

France), (b) colour removal vs TOD at different contact times

(Evry secondary effluent) and (c) turbidity change versus

transferred ozone dose (Washington secondary effluent).

P. Xu et al. / Water Research 36 (2002) 1043–10551052

inactivation performances. Therefore, a significant im-provement is observed when the apparent ozone

decomposition is represented as a mixed second orderrate expression depending on the fast ozone demand X(Eq. (11)).

4. Conclusions

The experimental results obtained at pilot scale ondifferent wastewater effluents confirm the efficiency ofozone for wastewater disinfection. Depending on the

quality of the effluent, a TOD of between 2 and15mgL�1 was necessary to meet the WHO standardfor irrigation (1000FC per 100mL). Such a dose was

shown to provide total elimination of enteroviruses inthe worst quality secondary effluent, which can be a

major advantage of ozone for regulations that includevirus removal. In agreement with previous data,

bacteriophages were found very sensitive to ozone,laying doubt on the pertinence of such microorganismsas indicators for ozone treatment. By contrast, thehigher resistance of Clostridium confirms that they are

good candidates for resistant microorganism indicator.Ozonation also provides a significant reduction of UVabsorbance and colour, which can be an advantage for

some reuse applications. More stringent regulations likeTitle 22 require the implementation of an efficienttertiary filtration step.

From an operational viewpoint, transfer of ozonefrom the gas phase to the water was found to be thecritical step for fecal coliform inactivation with ozone,because of the fast kinetics between ozone and coliform

bacteria. No difference in inactivation was found

Fig. 8. Modelling of the residual ozone with the modified approach developed for wastewater disinfection.

Fig. 9. Inactivation of E. coli with ozone for the classical (a) and modified (b) models applied to disinfection in wastewater effluent.

P. Xu et al. / Water Research 36 (2002) 1043–1055 1053

between 2 and 10min hydraulic retention time, for agiven ozone dose transferred to the effluent. As a

consequence, the ‘‘Ct’’ approach commonly applied indrinking water treatment should not be used for theozonation of wastewater. The new approach to waste-

water ozone contactor design must be based on shortcontact times and enhanced mass transfer. Further, nocredit of inactivation can be attributed to a storagereservoir or outfall that would be used for wastewater

discharge after ozonation, because of high ozone decayin an effluent matrix.Finally, a kinetic model was developed in order to

account for the 1–3 log inactivation of bacteria that wasobserved experimentally without a measurable concen-tration of ozone in the bulk solution. In fact, a

combination of the classic Chick–Watson disinfectionmodel and the ‘‘instantaneous demand’’ model used indrinking water was insufficient. The modified model was

based on the simultaneous consumption of ozone by themicroorganisms and by the organic matrix. Theapparent decomposition rate of dissolved ozone wasrepresented successfully by mixed second-order rate

equations.

Acknowledgements

The authors would like to thank Luc Burtin

(CIRSEE) for technical assistance, Ozonia for theprovision of an ozone generator, Evry and Washingtonwastewater treatment plants staff for field assistance,CIRSEE and Northumbian Water Group of Ondeo

Services for cooperation and lab analysis.

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