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Water Research 36 (2002) 1043–1055

Wastewater disinfection by ozone: main parameters for

process design

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

aLaboratoire Hydrosciences, MSE, UMR no. 5569, Uni versit!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 in

operating conditions showed that a very low hydraulic retention time (2 min) was sufficient for efficient fecal coliform

inactivation, 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’’ approach

commonly applied in drinking water treatment should not be used for wastewater ozonation. Design parameters of

ozonation 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 by

ozone.

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 the

microorganisms 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 for

protecting the environment and extending availablewater resources. In the last few years, there has been a

significant diversification of water reuse practices, such

as green space and crop irrigation, recreational im-

poundment, various urban uses including toilet flushing,

industrial applications and water supply augmentation

through groundwater or reservoir recharge [1,2]. The

safe 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-freeeffluents [4] require the development of highly effective

advanced disinfection processes. Chlorination is still the

most widely used means to inactivate pathogenic

microorganisms in water and wastewater, but alterna-

tive technologies have to be evaluated because of

increasing concern over undesirable byproducts after

chlorination 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]. Design

engineers in the US began to evaluate ozone for

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

130-536-207.

E-mail address: [email protected]

(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

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wastewater disinfection in the early 1970s. However,

because of operational and maintenance problems that

appeared in the first generation of facilities, it has been

considered to be a less attractive alternative to chlorine

than UV disinfection. Also, many researchers initially

sought to achieve a measurable level of dissolved ozone

residual 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 to

predict disinfection performance and design the disin-

fection system for wastewater disinfection [10–12].

The present study investigates the main factors related

to ozone disinfection performance, for the purpose of

facilitating its design and application to wastewater

disinfection.

2. Materials and methods

Experiments were performed in a continuous-flow

pilot plant with different types of effluents to evaluate

ozone disinfection performance on different target

microorganisms.

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

plant

Ozonation tests were conducted in two different pilots

designed as bubble diffuser columns (Fig. 1), which were

operated in continuous counter-current mode. Ozone

between 2% and 6% was generated from oxygen (95–

98% purity), with generators provided by Ozonia. The

specifications 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 to

characterize 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 (10 min

in pilot 1, and 15 min 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

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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 in

the columns was found to be consistently 55% and 30–

50%, respectively, in the pilots 1 and 2. In order to

compare the data, the transferred ozone dose (TOD)

was used as a descriptive parameter throughout the

study. It is defined as follows:

TOD ¼ Qgas

=Qliq

ð½O3

g in

½O3

g out

Þ; ð1Þ

where Qgas and Qliq are gas and water flow rates,

respectively, [O3]g in is the ozone concentration in the

feed 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 four

times the mean hydraulic retention time was allowed to

reach steady state and take samples. In order to study

the effect of post-contact without additional ozone

introduction, water samples were taken from the outlet

of the column and were kept in a brown bottle without

light 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. the

transferred ozone dose during disinfection tests, accord-

ing to the method proposed by Roustan et al. [14]. X

represents the minimum dose to be transferred to get

measurable 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 ¼ K Lað½O3 * ½O3Þ kD½O3 if TOD > X ;

½O3 ¼ 0 and d½O3

dt ¼ 0 if TODoX ; ð2Þ

where K La is the mass transfer coefficient; [O3]* is the

equilibrium (maximum) concentration of dissolved

ozone, [O3], corresponding to Henry’s law; kD is the

first order decay constant, in min1. TOD is the

transferred ozone dose calculated by

TOD ¼ Z t

0

K Lað½O3 * ½O3Þ dt: ð3Þ

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

reactor (CSTR with a contact time t) to estimate the

ozone concentration

½O3 ¼ TOD X

1 þ kDt: ð4Þ

2.3. Wastewater characterisation

Effluents from three different wastewater treatment

plants (WWTP) were used for the studyFtwo second-

ary effluents and one tertiary effluent. The secondarytreatment trains in Evry, France (48,000m3 d1) and

Washington, UK (90,000m3 d1) are similar: pretreat-

ment, primary clarification, activated sludge (extended

aeration and high rate activated sludge, respectively),

secondary clarification and discharge. The tertiary

treatment train in Indianapolis, USA (300,000 m3 d1)

consists of dual media filtration and chlorination after

primary clarification, and coupled bio-roughing and

activated sludge nitrification facilities. The effluent for

pilot testing was taken after the tertiary filtration. The

main characteristics of the effluents during the tests are

given 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, mg L1 3–16/4–50 1–35

Transferred ozone dose, mg L1 2–13/4–30 0.5–12

aObtained by tracer tests in the different operating conditions.

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Fecal Coliforms and E. coli were chosen as standard

fecal indicators in the study, because they are usually

regulated for wastewater discharge or reuse. Other

microorganisms 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 UK

Industry Standard Methods [16] with additional dilution

of 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 ozonation

using Standard methods for pH, TOC, Turbidity, SS,

total and filtered COD, TOC, BOD5, UV 254 abs. (total

and after filtration), N–NO2, alkalinity, Mictotox tests

for toxicity and particle size distribution. To investigate

the effect of ozonation on colour abatement, absorbance

at 400 nm was used to characterise apparent colour.

Water appears to be coloured when dissolved matter

absorbs visible light or when suspended particles scatter

light (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 (mg L

1

) 2.3 (o

1–4) 5 (3–6) 18 (7–33)COD, (mg O2 L1) 30 (24–38) 36 (26–56) 71 (41–150)

TOC (mgL1) 8 (5.5–10.2) o10 (o10–14) 26 (o11–30)

UV 254 abs (m1) 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 100 mL) F 3.6–4.5 4.3–6.5

E. coli (log CFU per 100 mL) 2.7–4.3 F F

Clostridium (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 0.45mm membrane

lauryl sulphate Broth for 4 h at 371C then 14h 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 10 min. Serially dilute and

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

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

0.1 ml 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


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sampling, 4% (wt/vol) sodium thiosulfate was added to

the treated effluent samples.

To investigate the influence of particles on disinfection

performances, additional tests were run with the Evry

effluent after filtration (75 mm Arkal prefilter, followed

by a 10 mm canvas filter), to lower the suspended solids

concentration below 2 mg L1.

3. Results and discussion

3.1. Inacti vation of fecal coliforms: impact of

operating conditions and wastewater quality

3.1.1. Influence of wastewater quality on ozone demand

Fig. 2 illustrates the determination of ozone demand

in the Washington secondary effluent for different values

of the hydraulic retention time. The measured values of

7.4–9.6 mg L1 are significantly higher than those in the

other two effluents, as expected from the water quality

data (see Table 2). The ozone demand of the Evry

secondary effluent (extended aeration with nitrification)

of 3.1–4.2 mg L1 is similar to the values measured in the

Indianapolis tertiary effluent of 2.5–5.3 mg L1. Theseresults are in agreement with an earlier study performed

on other effluents, which showed organic content to be a

much more influential parameter than suspended solids

on the ozone demand [12]. These values are used

hereafter when discussing the disinfection performances.

3.1.2. Influence of operating conditions on coliform

inacti vation

Fig. 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 fromWashington, UK).

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

reuse standards.


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ozonation is plotted as a function of the transferred

ozone dose (TOD). One important observation is that a

significant 1–3 log inactivation is already reached when

TOD 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 that

dose. This phenomenon can be explained by the fact that

bacteria themselves participate in the immediate ozone

demand, because of their high kinetic rate with ozone.

One important consequence from these results is that

design and control of ozonation for fecal coliform

disinfection in wastewater should not be based on the

standard parameter of residual ozone or ‘‘Ct’’ factor. In

agreement with this, Rakness et al. [10] reported that

direct 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 more

resistant microorganisms like Giardia are targeted.

Data from Fig. 3 show that the hydraulic retention

time (HRT) has no impact on the performances of fecal

coliform (FC) or E. coli disinfectionFfor a given TOD,

a 2 min HRT provides the same inactivation as 10 min

HRT. These results have major consequence for the

design of ozone wastewater disinfection, demonstrating

that mass transfer is the critical step that has to be

optimised, and that no long-contact-time chamber is

necessary.

Fig. 4 illustrates this statement, showing the distribu-

tion of ozone residual and FC inactivation along the

ozonation column and after 2, 4 and 6 min additional

contact in the post-contactor chamber. Residual ozone

decreased significantly in the post-contactor after 2 min

and no significant increase in FC inactivation is

observed. The quick decay of ozone may be explained

by 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 can

be 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 performances

Despite 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 is

able to meet stringent standards for almost total bacteriainactivation like the Californian Title 22 criteria. This

result led to a more in-depth investigation of the

influence of particles on disinfection.

Comparative tests were performed with the effluent

from Evry, and with the same effluent after a filtration

step. These tests were performed simultaneously in order

to minimise any fluctuation in water quality or operating

conditions. The ozone demand of the effluent was found

to be exactly the same. The effect of filtration on

disinfection is shown in Fig. 5: an additional 1 log

inactivation was obtained by filtration. It must be

stressed that for a given bacteria concentration (log N 0),

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 13.1 mg L

1

).


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explains the higher inactivation level obtained with the

tertiary 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+-specific

bacteriophages were chosen as target microorganisms

for additional tests with the Washington secondary

effluent. The inactivation efficiencies are presented in

Table 4. It should be noted that the initial concentration

of microorganisms limited the maximum inactivation

values that could be reached. The resistance of fecal

coliforms and enterococci to ozonation was similar, in

agreement with previous results [17,18]. It is important

to stress also that a relatively low ozone dose of

8.6mgL1 totally inactivates Salmonellae.

A higher efficiency of ozonation was observed for

virus inactivation: a transferred dose of 4.8 mg L1 with

4 min HRT was enough for total inactivation of

enteroviruses (>2.9 log inactivation). Total inactivation

of F+ coliphages (>2.2 log inactivation) was reached

with a slightly higher transferred dose of 8.6mg L1.

The strong virucidal power of ozone for wastewaterdisinfection confirms its well-known performance in

drinking water [7,18]. Indigenous enteric viruses isolated

from wastewater effluents have been shown to be much

more resistant [18]. In comparison, coliphages have been

found to be very sensitive to ozone, which puts some

doubt on the validity of these coliphages as surrogates

for enteric viruses [19,20].

This study shows that the ozone dose required to

satisfy WHO regulations (1000FC per 100mL) also

provides total inactivation of indigenous enteric viruses.

This indicates that ozone would be highly recommended

for the production of virus free water, required in several

countries for landscape (Australia, 1 pfu/50 L) or agri-

cultural irrigation (Arisona and Hawaii, USA, 1 pfu/

40 L).

Compared to all other microorganisms, the higher

resistance of Clostridium was confirmed by the experi-

mental data. A thorough investigation was made with

the effluents from Evry and Washington with that

indicator (Fig. 6). With a TOD approximately equal to

the immediate ozone demand of the effluents (3–

5 m g L1 for Evry and 8–10mg L1 for Washington),

less than 0.5 log inactivation of Clostridium was

achieved. The maximum inactivation level was less than

2 log for high TOD of 33 mgL1 (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 was

on UV-254 absorbance and colour (Figs. 7a and b).

With an increase in transferred ozone dose from 2 to

13mgL1 at HRT 4 min, the variation of UV absor-

bance in the Evry secondary effluent increased from

28% to 55%. These results indicate that the ozone reactsand oxidises the organic matter, in particular, the

compounds having double bonds and/or an aromatic

structure that determine the value of the absorbance at

254 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 ozone

and unsaturated and aromatic compounds. The critical

factor for water quality improvement is also the ozone

dose 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).


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Total BOD5 increased up to 20% after ozonation of

the highly polluted secondary effluent in Washington,

UK. This is typical with ozone, which can oxidise

recalcitrant compounds and thereby increase effluent

biodegradability. The total COD was not significantly

influenced by ozonation, while an increase of dissolved

COD was observed. The difference between total anddissolved COD behaviours could be related to the global

decrease of turbidity observed during the tests (Fig. 7-

c)Fsome particles and high weight organic compounds

would be destroyed by ozone and converted into

dissolved compounds.

Toxicity was detected neither in the untreated

secondary effluent nor in the ozonated effluents. These

results are in compliance with previous studies on urban

wastewater disinfection by ozone [17]. It is important to

stress that the presence of toxicity after ozonation

reported in the literature is usually related to the

presence of industrial wastewater [21–24].

3.4. Desi gn of wastewater ozonation for gi ven regulations

Ozone disinfection results were compared for two

different water reuse standards: (1) WHO stringent

guidelines for irrigation, fecal coliformso1000 cfu per

100 mL, and (2) Californian Title 22 standards, total

coliformso2.2 cfu per 100 mL (see Fig. 3). The TOD

required to meet WHO guidelines are 2, 4 and

15mgL1, respectively, for the tertiary affluent and the

two secondary effluents in Evry and Washington for an

HRT of 2 min and an additional beneficial effect of 30%reduction in UV absorbance. Compliance with the

stringent Title 22 criteria of virtually total removal of

fecal coliforms can be reached only after tertiary

filtration in the Indianapolis effluent and with a TOD

of 8mg L1 for 2 min HRT. A very low suspended solids

concentration (o5 m g L1) emerges as the most im-

portant design requirement to meet very stringent

disinfection requirements.

In both scenarios, total inactivation of viruses is

achieved, which may be important if viruses are also

included in regulations.

3.5. Modelling approach of wastewater disinfection

by ozone

Disinfection is standardly described, for drinking

water, by the Chick–Watson model:

d N ½

dt ¼ kN ½N ½O3: ð5Þ

In the case of a CSTR, Eq. (5) enables the evaluation of

the number of microorganisms, N :

½N

½N 0

¼

1

1 þ kN

½O3

t; ð6Þ

T a b l e 4

C o m p a r i s o n o f o z o n e d i s i n f e c t i o n o n d i ff e r e n t m i c r o o r g a n i s m s i n t h e s e c o n d a r y

e ffl u e n t i n W a s h i n g t o n ,

U K a

H R T m i n T O D ( m g L

1 ) F e c a l c o l i f o r m

( C f u p e r 1 0 0 m L )

C l o s t r i d i u m

( / 1 0

0 m L )

E n t e r o c o c c i ( / 1 0 0 m L )

S a

l m o n e l l a

E n t e r o v i r u s ( p f u / 1 0 L )

F

- c o l i p h a g e ( / m L )

l o g N 0

N

R e d

l o g N 0

N

R e d

l o g N 0

N

R e d

N 0

N

N 0

N

R e d

N

0

N

R e d

4

4 . 8

5 . 0

0

4 6 0 0

0 . 3

4

3 . 5

6

6 2 0 0

F

4 . 7

3

7 1 0 0

0 . 8

8

F

F

7 7 5

0

> 2 . 8

9

9 6

2

1 . 6

8

4

8 . 6

4 . 9

4

1 3 2 0

1 . 8

2

3 . 6

7

4 8 0 0

0

4 . 6

3

9 8 0

1 . 6

4

P r s t

A b s

5 4 4

1 2

1 . 6

6

1 4 4

0

> 2 . 1

6

4

1 5 . 2

4 . 9

5

3 0 0

2 . 4

8

3 . 8

1

3 1 0 0

0 . 3

1

4 . 7

4

1 7 3

2 . 5

0

P r s t

A b s

6 5 0

0

> 2 . 8

1

1 2 2

0

> 2 . 0

9

9 . 6

1 1 . 0

5 . 4

5

8 4 0

2 . 5

2

4 . 5

3

2 1 0 0 0

0 . 2

1

4 . 8

6

4 0 0

2 . 2

6

P r s t

A b s

6 5 4

0

> 2 . 8

2

9 . 6

2 4 . 8

5 . 9

2

1 4

4 . 4

2

4 . 4

6

2 0 0 0

1 . 1

6

4 . 7

1

1 4

3 . 5

6

P r s t

A b s

7 7 4

0

> 2 . 8

9

9 . 6

2 9 . 5

5 . 2

0

1 4

4 . 0

6

4 . 4

6

5 5 0

1 . 7

2

4 . 5

3

1 6

3 . 3

3

P r s t

A b s

7 7 4

0

> 2 . 8

9

a

N o t e : R e d F l o g ð N 0

= N Þ ;

P r s t F p r e s e

n t ; A b s F a b s e n t .


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where N is the density of viable microorganisms (N 0 at

t 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 implies

the presence of ozone residual to achieve inactivation of

microorganisms; there would be no disinfection before

the ozone demand is met, i.e. for transferred ozone doses

lower than 8 mgL1, the immediate ozone demand of

wastewater.

In order to account for the 1–3 log inactivation

observed without measurable residual in wastewater

ozonation, 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 K Lað½O3 * ½O3Þ kX ½X ½O3 kY ½Y ½O3;

ð7Þ

d Y ½

dt ¼ kY ½Y ½O3; ð8Þ

d X ½

dt ¼ kX ½X ½O3: ð9Þ

The comparison with the previous equations used for

drinking 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 4.0 min HRT 2.0 min HRT 9.6 min HRT 4.0 min

TOD 9.2mgL1 TOD 12.3mg L1 TOD 15 mgL1 TOD 21.1mg L1

C 0 9 24 17 10

TBOD C 8 30 21 12

(mgL1) ðC C 0Þ=C 0 11% +25% +24% +20%C 0 73 94 93 70

TCOD C 66 92 94 69

(mgL1) ðC C 0Þ=C 0 10% 2% +1% 1%

C 0 42 43 47 38DCOD C 42 51 63 47

(mgL1) ðC C 0Þ=C 0 0% +19% +34% +24%Total C 0 0.44 0.48 0.51 0.43

UV Abs C 0.36 0.4 0.39 0.27

(Cm1) ðC C 0Þ=C 0 20% 17% 24% 38%Dissolved C 0 0.22 0.23 0.23 0.22

UV Abs C 0.17 0.17 0.17 0.12

(Cm1) ðC C 0Þ=C 0 25% 24% 27% 44%

aC 0 is the initial concentration of secondary effluent; C is the concentration of ozonated effluent.


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by simplification if

kD ¼ kY ½Y ;

kX ¼ N:

Eqs. (7–9) can easily be solved in an open completely

mixed reactor (CSTR with a contact time t) to estimate

ozone concentration and also concentrations of the

different species X and Y (rapidly consumed and the

slowly consumed organic matter):

½O3 ¼ DOT

1 þ kX ½X t þ kY ½Y tð10Þ

½X ¼ ½X o

1 þ kX ½O3t; ð11Þ

½Y ¼ ½Y o

1 þ kY ½O3t: ð12Þ

The use of these equations should be interpreted as a

valid model for quantification but not necessarily as an

accurate representation of the more complex mass

transfer steps and heterogeneous reactions taking place

during the disinfection process. In particular, radical

mechanisms are ignored for simplification. Besides, in a

first 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 close

enough to the experimental data. The focus is on the

kinetic modelling, which can be completed afterwards

with hydrodynamic information. The exact value of the

fitting parameters will then be slightly altered, but not

their order of magnitude. A very similar approach was

taken 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 following

fitting parameters (Fig. 8):

kX ¼ 10 L mg1 min1;

kY ¼ 0:0 1 L m g1 min1;

½X o ¼ 8 mg L1;

½Y o ¼ 6 0 mg L1:

This represents a first improvement, but the major

difference between both models appears when consider-

ing the inactivation of fecal coliforms. For transferred

ozone doses lower than the fast ozone demand,

integrated exposure to ozone in terms of Ct is equal tozero for the classical approach and reaches

0.40mgminL1 with the modified model for a TOD

of 8 mg L1. When applying the Chick–Watson model

given 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. 9

shows the strong impact of this low Ct in wastewater,

which inactivates 2 log of E. coli . The corresponding

inactivation rate constant kN is equal to

100Lmg1 min1 and the fitting curves are depicted in

Figs. 9a and b. Besides, the model properly accounts for

the 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 (4 min

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).


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inactivation performances. Therefore, a significant im-

provement is observed when the apparent ozone

decomposition is represented as a mixed second order

rate expression depending on the fast ozone demand X

(Eq. (11)).

4. Conclusions

The experimental results obtained at pilot scale on

different wastewater effluents confirm the efficiency of

ozone for wastewater disinfection. Depending on the

quality of the effluent, a TOD of between 2 and

15mgL1 was necessary to meet the WHO standard

for irrigation (1000 FC per 100 mL). Such a dose was

shown to provide total elimination of enteroviruses in

the worst quality secondary effluent, which can be a

major advantage of ozone for regulations that include

virus removal. In agreement with previous data,

bacteriophages were found very sensitive to ozone,

laying doubt on the pertinence of such microorganisms

as 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 UV

absorbance and colour, which can be an advantage for

some reuse applications. More stringent regulations like

Title 22 require the implementation of an efficient

tertiary filtration step.

From an operational viewpoint, transfer of ozone

from the gas phase to the water was found to be the

critical 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.


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between 2 and 10 min hydraulic retention time, for a

given ozone dose transferred to the effluent. As a

consequence, the ‘‘Ct’’ approach commonly applied in

drinking water treatment should not be used for the

ozonation of wastewater. The new approach to waste-

water ozone contactor design must be based on short

contact times and enhanced mass transfer. Further, nocredit of inactivation can be attributed to a storage

reservoir or outfall that would be used for wastewater

discharge after ozonation, because of high ozone decay

in an effluent matrix.

Finally, a kinetic model was developed in order to

account for the 1–3 log inactivation of bacteria that was

observed experimentally without a measurable concen-

tration of ozone in the bulk solution. In fact, a

combination of the classic Chick–Watson disinfection

model and the ‘‘instantaneous demand’’ model used in

drinking water was insufficient. The modified model was

based on the simultaneous consumption of ozone by themicroorganisms and by the organic matrix. The

apparent decomposition rate of dissolved ozone was

represented successfully by mixed second-order rate

equations.

Acknowledgements

The authors would like to thank Luc Burtin

(CIRSEE) for technical assistance, Ozonia for the

provision of an ozone generator, Evry and Washington

wastewater treatment plants staff for field assistance,

CIRSEE and Northumbian Water Group of Ondeo

Services for cooperation and lab analysis.

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