Fuel 110 (2013) 114–123

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Fuel

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Kinetics of water disinfection using UV-C radiation

Andreza B. Silva, Nelson M. Lima Filho, Maria A.P.F. Palha, Sandra M. Sarmento ⇑Universidade Federal de Pernambuco, Departamento de Engenharia Química, Av. Prof. Arthur de Sá, SN, Cidade Universitária, 50740-521, Recife, PE, Brazil

h i g h l i g h t s

" The current work is concerns with modelling of UV-C disinfection process." Radiation field properties were modelled according to the incidence model." Kinetic model related quantum yield, absorbed energy and growth rate." 6 Log inactivation was achieved in all conditions." Model developed fits well the data (to 95%).

a r t i c l e i n f o

Article history:Received 8 May 2012Received in revised form 31 October 2012Accepted 5 November 2012Available online 21 December 2012

Keywords:Water disinfectionUV-C radiationAnnular photoreactorEscherichia coliUV dose

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.11.026

⇑ Corresponding author. Tel.: +55 81 21267251; faxE-mail address: sarmento@ufpe.br (S.M. Sarmento

a b s t r a c t

The current work is concerned with the development of a kinetic model for UV disinfection process at254 nm. The micro-organism was Escherichia coli (E. coli). The inactivation of E. coli active cells, suspendedin sterile double-distilled water, free-of solids, nutrient and any trace of broth, was carried out in a bench-scale annular photoreactor, designed specifically for developing kinetic studies. The UV-C light sourceused was a tubular low pressure Philips lamp, model TUV-36W. The reactor model was based on the con-servation principles. The radiation field was modelled according to the radial incidence model. The inci-dent radiation energy at the photoreactor optical entrance was measured by homogeneous potassiumferrioxalte actinometry (1.4 � 10�9 Einstein cm�2 s�1). The Naperian absorption coefficient of the E. coliactive cells was obtained (1.05 � 10�8 cm2 MPN�1). The kinetic model proposed was a simple phenome-nological equation relating the reaction rate to both the concentration of microbial active cells and theradiation energy absorbed by these cells. A term related to the rate of growth of microbial cells was addedto the kinetic model. The experimental runs were carried out for conditions where the initial concentra-tion of E. coli active cells varied from 2.0 � 104 MPN cm�3 to 2.0 � 106 MPN cm�3, pH7 and temperature25 �C. Disinfection efficacies of 4.3-Log and 6.3-Log were achieved for both conditions and the respectiverequired UV doses were 23.4 mW cm�2 and 31.2 mW s cm�2. The kinetic model fitted the data well (to95%). The kinetic parameters, n and K, were estimated in 0.5 and 2.9 � 10�2 s�1(cm3 Einstein�1)0.5,respectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction disadvantages led to the development of UV disinfection technol-

Water has been recognised as a remarkable agent for spreadingillness since John Snow’s research [1] on the Broad Street, London,cholera outbreak (in 1854). Many diseases currently endemic indeveloping countries are due to microbial contamination of water,so disinfection of public water supply is a primary concern in thosecounties [2,3].

Chlorination has been the preferred disinfection technology [4].Nevertheless, it has disadvantages: (a) Ineffectiveness against pro-tozoa, such as Cryptosporidium and Giardia; (b) Production of haz-ardous by-products such as THMs and HAAs [4,5]. These

ll rights reserved.

: +55 81 21267278.).

ogy (UV-DIS) and other Advanced Disinfection Technologies(ADTs). The UV-DIS is effective against all waterborne pathogens,including these protozoa. Hazardous by-products are not formedif low-pressure lamps are used. These lamps emit 85–95% of theiroutput power at 254 nm [5]. UV-DIS is a rapid process, so little con-tact time is needed (seconds rather than minutes) and UV devicesoccupy less space than that required for chlorination [2,4]. In addi-tion, photoreactor design and lamp technology have improved con-siderably [6]. These facts led to recognition by US EnvironmentalProtection Agency (USEPA) that UV-DIS is the best current disinfec-tion technology [5]. Since 2000, more than 400 UV disinfectionunits worldwide have been treating drinking water, typically withflow-rates of less than 3.8 � 103 m3 per day. Two larger UV unitswere built in United State of America to treat daily water volumesof 6.9 � 105 m3 and 8.4 � 106 m3 [7]..

A.B. Silva et al. / Fuel 110 (2013) 114–123 115

The efficiency of UV-DIS depends on factors related to: (a) themicro-organisms – physiological state (pre-culturing and growthphase), strain diversity, repair mechanisms and particle association[5]; (b) the radiation field properties – spectral incident radiant en-ergy distribution, absorption and scattering of UV light; lamp agingand fouling [8].

UV disinfection relies on the sensitivity of the micro-organismto UV radiation (microbial response to UV light). This is uniqueto each micro-organism and is determined by its ability to absorbat 200–280 nm (germicidal wavelength range), so inactivatingtheir active cells through UV-induced damages such as the forma-tion of pyrimidine dimers in their DNA. UV-induced damages dis-rupt the DNA structure, so that, if a critical number of dimers isformed, the DNA cannot replicate [5]. Thymine-dimers Th iT aremore readily formed, as the thymine absorptive is greater thanthe cytosine one, and the quantum yield for the formation ofTh iT is greater than for the formation of other dimers [5,9].Therefore, micro-organisms with DNA rich in thymine tend to bemore sensitive to UV disinfection.

Microbial response is a function of wavelength. UV action peaksat or near 260 nm, has a local minimum near 230 nm, and drops tozero near 300 nm, so that the maximum germicidal effectiveness isaround 260 nm [2,5]. However, UV light at 260 nm is producedless-efficiently than at 254 nm.

The UV-induced mutations in the DNA of micro-organisms, un-like chlorination, do not inactivate their metabolic function, suchas respiration and enzymatic activities, nor kill the micro-organ-isms. Many micro-organisms have enzyme systems that repairUV-induced damages. Two types of repair have been described:dark repair and photo-repair. Even though microbial repair can oc-cur, neither photo-repair nor dark repair is anticipated to affect theperformance of drinking water UV disinfection. Dark repair doesnot require light and has been demonstrated with almost all bacte-ria. Photo-repair occurs in conditions of prolonged exposure toradiation emitted in the range of 310–490 nm, and targets pyrim-idine dimers specifically.

The UV sensibility of a micro-organism is described by the inac-tivation kinetic parameters, so that kinetics can be used to obtainthe disinfection efficacy or MIC (log) of a full-scale system as wellas to assess the UV dose (UVDose) requirement to obtain a certainMIC [5]. A UV-sensitive micro-organism has a high k�-value and re-quires a low UVDose for inactivation, as described by the first-or-der disinfection Model of Chick–Watson, amended by Haas [8] toattend UV disinfection, log CX

Cox

� �¼ �k�, where CX and Co

X are theconcentration of microbial active cells at a given process timeand initial one, respectively. The Fluence, or UVDose, is definedas the energy required to inactivate a micro-organism, or, the UVenergy per unit area incident on a surface and currently obtainedfrom the product of the UV fluence rate (UVF-rate) and the expo-sure, or irradiation time [9].

According to USEPA [9], the design and management of disin-fection systems require knowledge of removal kinetics of patho-genic micro-organisms. It seeks to determine the influence ofUVDose on disinfection kinetics. The typical shape of the micro-organism response curve to UV light (UVD-response) generally isnot linear as predicted by the first-order disinfection Chick–WatsonModel, but has three regions: (a) Shoulder phase – at low UVDosethe inactivate rate (RX is zero; this is associated with DNA repairmechanism, or it may arise from the requirement of several hits,DNA damages, before the cells become inactivated; (b) semi-logphase – RX increases with UVDose, obeying the First-order Model;(c) saturation phase – at high UVDose RX decreases with an increas-ing in UVDose, which is attributed to UV resistance, sub-populationof micro-organisms and/or presence of particulates and clumpedcells [2]. Several mathematical models have been proposed todescribe the microbial response to UV radiation, varying from the

simplest empirical model, which is the First-order model ofChick–Watson, to complex mechanistic ones, such as the Series-event Model [10] and Modified-series-event Model [11,12].

The Series-event Model assumes that, when a bacterium is irra-diated with UV-C light, its DNA suffers successive and cumulativediscrete damages (events) until a threshold limit (m) that producesinactivation as a final outcome. Each step is represented by first-or-der kinetics with respect to the average intensity of UV light withinthe system [10,13]. Severin [10] applied this model to the inactiva-tion process of the Escherichia coli. The kinetic data obtained in thebatch system were not suitable to predict the performance of acontinuous reactor [11], as that model did not describe the intrin-sic kinetic of the process.

Labas et al. [11] used the Series-event Model as a background tothe Modified-series-event Model. However, it was assumed thatthe inactivation rate had a linear dependence with respect to theconcentration of active cells, and was also proportional to the LocalVolumetric Rate of Photon Absorption by those cells (LVRPA) risingto an unknown order (n). The LVRPA is a process parameter depen-dent on the position and time [6,14]. Therefore, the RadiativeTransfer Equation (RTE) was solved inside the reactor in order toquantify the LVRPA by the active cells in each time and position in-side of the photoreactor [11,12]. This model was applied to inacti-vate the E. coli suspended in both distilled water and distilledwater with nutrients. The resulted models described the intrinsickinetic of those processes [11,12].

The Labas’ Modified-series-event Model was applied to disin-fection process of the E. coli by heterogeneous photocatalysis withsimilar success [15]. In fact, the kinetic model proposed for theinactivation of E. coli was based on a rigorous reaction mechanismcomprised of hydroxyl radical generation reaction scheme [16] andseries-events disinfection mechanism [15]. The catalyst used wasEvonik-Degussa P25 TiO2 (reactor operating mode: slurry mode;range of catalyst load: 2.0 � 10�5 g cm�3 to 2.0 � 10�4 g cm�3).The model accounted explicitly for the LVRPA (lamp emissionmodel: superficial diffuse emission model) [6,15]. The resultingmodel describes the intrinsic kinetic of the E. coli inactivation byheterogeneous photocatalysis.

The current work is concerned with the modelling of the kinet-ics of micro-organism inactivation by UV-C radiation and modelvalidation. The kinetic model proposed is a simple phenomenolog-ical equation relating the inactivate rate to both the concentrationof microbial active cells and radiation energy absorbed by thesecells. A term related to the rate of growth of microbial cells wasadded to the kinetic model. The radiation field property modelsare based on the radial incidence model. The UVDose is estimatedbased on the effective irradiation time during which the activecells are exposed, which depends on the number of recycle circlesthe cells undergo rather than the mean residence time that is sin-gle value. The micro-organism chosen was E. coli, an indicatororganism of drinking water feacal contamination, and suggestedby USEPA [9] as an alternative model micro-organism for moresusceptible bacteria and the protozoa Cryptosporidium and Giardia.

2. Mathematical models

Developing models of photochemical processes is complex,since solutions are required of the moment, radiation and multi-component mass conservation equations. Mainly, the complexityin modelling lies in the fact that mass and radiation balances arecoupled by reaction terms, due to the effect of radiation-absorb-ing-reactant concentration on reacting medium radiation absorp-tion, which, as the former variable, varies dynamically with thereaction extent [6]. To state and solve those conservation equationsone must take into account the features of the experimental set-up

116 A.B. Silva et al. / Fuel 110 (2013) 114–123

and the operating condition. In this work they are: (a) photoreactoris operated in a continuous mode inside of a loop of batch recircu-lation system under isothermal conditions and with a high recircu-lation flow-rate; (b) volume of tank (VT)� volume of reactor (VR)(c) there is no radiation emission in the reaction section of the pho-toreactor; (d) photon transport is highly superior in the radialdirection, r, of the reaction section [16–18]; (e) photoreactor andrecirculation tank (R-tank) are supposed to behaved as a differen-tial reactor and a well-mixed tank [19]; (f) photon scattering doesnot occur in the reaction section as the water-model is solid-free,the microbial cells (X) are not crumpled and their diameters aresmall; (g) microbial active cells are inactivated in the photoreactor.

2.1. Kinetic model

The microbial active cell inactivation reaction by UV radiationat 254 nm is a photochemical reaction. Hence, the inactivation rate,Rx(t), can be formulated by means of a simple phenomenologicalequation that relates the concentration of the active cells and theLocal Volumetric Rate of Photon Absorption, LVRPA or ea

kðr; tÞ. Tothis model is added a term to account for the microbial growthrate, RG(t) [11,20,21].

RXðtÞVR¼ �KCXðtÞh½ea

kðr; tÞ�niVRþ hRGðtÞiVR

ð1Þ

where hiVR represents the average of a process property P takenover the reactor volume [20]; K, n (reaction order in respect toLVRPA) are the kinetic parameters to be estimated, t, r and k arethe process time, radius and wavelength (254 nm), respectively.

2.2. Inactivation rate

The rate of a disinfection process carried out in a continuousphotoreactor inside a loop of a batch recirculation system can beobtained by means of a material balance [20,21]. When hypothesesa, b, e and g are fulfilled, the resulting model is:

RXðtÞVR¼ VT

VR

dCXðtÞdt

ð2Þ

Initial condition for Eq. (2):

t ¼ 0! Cxð0Þ ¼ Cox ð3Þ

2.3. Volumetric rate of photon absorption

The photon transport in a given propagation direction (X), in apseudo-homogeneous medium with no emission, is obtained byapplying the RTE to the reaction section of the photoreactor[6,22]. The photoreactor used was designed to provide significantlysuperior radiation transport along the radial direction [18] (Featured), so the transport of radiation can be represented adequately bythe radial model [17] and the radiation field modelled as one-dimentional [6]. The resulting equation, in terms of the spectralspecific intensity of radiation at 254 nm and propagation directionX, IX,k (r,t), is:

rðIX;kðr; tÞÞ ¼ �jTðtÞIX;kðr; tÞ ð4Þ

where jT is the total volumetric absorption Naperian coefficient at254 nm (this coefficient accounts for the absorbing active cells,broths and any other absorbing species, i, that might be presentin the reacting mixture [6,11]).

Conceptually, the spectral specific intensity of radiation andspectral incident radiant energy at 254 nm, Gk(r, t), are related as[6]:

Gkðr; tÞ ¼Z

XIkðr;XÞdX ð5Þ

The resulting model for the profile of the spectral incident radiantenergy in an annular photoreactor with the design Features c, dand submitted to operating conditions of this study (Feature f), is gi-ven by:

Gkðr; tÞ ¼rq

rGw exp½�jTðtÞðr � rqÞ� ð6Þ

where rq is the external radius of the inner tube; Gw is the incidentradiant energy at the optical entrance of the photoreactor (bound-ary condition of Eq. (4)), determined experimentally.

The spectral incident radiant energy is related conceptually toLVRPA by means of [6]:

eakðr; tÞ ¼ jxðtÞGkðr; tÞ ð7Þ

where jX is the volumetric absorption Naperian coefficient of themicro-organism active cells at 254 nm [11].

The resulting model for the LVRPA is:

eakðr; tÞ ¼

rq

rjxðtÞGw exp½�jTðtÞðr � rqÞ� ð8Þ

Eq. (7) and Eq. (8) must be integrated over the reactor volume to ob-tain the corresponding hGkðr; tÞiVR

and heakðr; tÞiVR

or, VRPA (Volu-metric Rate of Photon Absorption), as these process parametersare not uniform [6].

Gkðr; tÞVR¼ 2Gw

1jTðtÞ

rq

r2p � r2

q

" #f1� exp �jTðtÞðrp � rqÞ

� �g ð9Þ

eakðr; tÞVR

¼ 2GwjxðtÞjTðtÞ

rq

r2p � r2

q

h i f1� exp½�jTðtÞðrp � rqÞ�g ð10Þ

where rp is the inner radius of the external tube.

2.4. Incident radiation energy at the photoreactor optical entrance

The value of the incident radiation energy at the photoreactoroptical entrance at 254 nm is obtained precisely by homogeneouspotassium ferrioxalate actinometry [23] associated with a suitedmaterial balance taken over the actinometer (potassium ferriox-alte) [24,25]. The overall photochemical reaction is:

2Fe3þ þ C2O2�4 !

hm2Fe2þ þ 2CO2 ð11Þ

The volume-averaged actinometer reaction rate, hRFe2þ ;kðr; tÞiVR, is

described by a phenomenological approach [6,24,25].

hRFe2þ ;kðtÞiVR¼ /Fe2þ ;khea

kðr; tÞiVRð12Þ

The value of the Fe2+ overall quantum yield at 254 nm, /Fe2þk is1.25 mol/Einstein [23].

Fe3+ and Fe2+ absorb radiation considerably at 254 nm. TheirNaperian absorption coefficients at 254 nm, aFe3þ and aFe2þare4992 M�1 and 2560 M�1, respectively [23]. Hence, the model forVRPA takes the following form:

eakðr; tÞVR

¼ 2jFe3þ ðtÞGw

jFe3þ ðtÞ þ jFe2þ ðtÞ� � rq

r2p � r2

q

h i� 1� exp �½jFe3þ ðtÞ þ jFe2þ ðtÞ�ðrp � rqÞ

� �� �ð13Þ

where jFe3þ ðtÞ and jFe3þ ðtÞ are the volumetric absorption Naperiancoefficients of Fe3+ and Fe2+, respectively, at 254 nm at a given pro-cess time [24,25].

Nevertheless, at the beginning of the reaction (t ? 0), conver-sion of Fe3+ into Fe2+ is very low jFe2þ � 0 and the radiation is ab-sorbed mainly by the Fe3+ ðjFe2þ � 1Þ [24,25]. This fact leads to thefollowing simplification of Eq. (13):

A.B. Silva et al. / Fuel 110 (2013) 114–123 117

eakðr; tÞVR

¼ AI

VIGw ð14Þ

where AI and VI or VR are the irradiated area and irradiated volume,respectively.

The model developed for Gw was obtained by substituting Eq.(14) into Eq. (12), when t ? 0, and re-arranging algebraically theterms of the resulting equation:

Gw ¼1

/Fe2þ

VR

AI

DCFe2þ

Dt

� t!0

ð15Þ

2.5. UV dose

Labas et al. [11] have commented on the concept of UVDose andhow inappropriately it has been calculated. They proposed a mod-ified-dose model based on LVRPA but also claimed that UVDosecalculation based on the average incident radiation energy takenover the reactor volume, hGkðr; tÞiVR

, is an improvement as, doingso, one is taking into account that the latter parameter is a functionof the time and Cartesian coordination. In the current work, theUVDose is based on hGkðr; tÞiVR

in order to make comparison withliterature results. UVDose is defined for batch and continuous sys-tems based on the exposure time (effective irradiation time), tEI(t),and residence time, s, respectively [11]:

Batch system : UVDoseðtÞ ¼ hGkðr; tÞiVRtEIðtÞ ð16Þ

Continuous flow system : UNDoseðtÞ ¼ hGkðr; tÞiVRs ð17Þ

where hGkðr; tÞiVRis given by Eq. (9).

In the current study, the UV disinfection was carried out in acontinuous photoreactor inside a batch recycling system. Then,the UVDose was defined according to Eq. (17). Nevertheless, theresidence time was substituted by the effective irradiation timethat is bases on the number of recycling circle, nc, residual timeinterval (DTR = time of one recirculation cycle – sampling timeincrement) and mean residence times of the active cells in the pho-toreactor, sP, and R-tank, sT.

TEIðtÞ ¼ ncðtÞ½DTR þ sP þ sT � ð18Þ

where the number of recycling circle is defined as nCðtÞ ¼Sampling timeðtÞ

Time of one recirculation circle; DTR, in these studies the time of one recircu-

lation circle and were 140 s and 20 s, respectively.The UVDose pro-posed model for the UV disinfection process in a continuous annularphotoreactor inside of batch recycling system is following:

UVDoseðtÞ ¼ 2Gw

jTðtÞrq

r2p � r2

q

" #f1� exp½�jTðtÞðrp � rqÞ�gTEIðtÞ ð19Þ

Fig. 1. Experimental set-up: (a) Schematic diagram. (b) Photography – 1. Photo-reactor; 2. Lamp; 3. Mechanical stirrer; 4. Recirculating tank; 5. Sample port; 6.Circulating valve; 7. Drain; 8. Pump; 9. Heat exchanger).

3. Material and methods

3.1. Micro-organism, maintenance broth and reagents

The organism (E. coli ATCC 25922, DAUFPE: 224) was providedby the Departamento de Antibióticos, Universidade Federal de Per-nambuco (Brazil). The culture was maintained on Mueller–HintonAgar (Difco™) at 5 �C, and subcultured for every 2 months. All re-agents used were of Analytic Grade. All aqueous solutions wereprepared with distilled water.

The E. coli cells were grown in a 250 cm3 Erlenmeyer flask con-taining 90 cm3 of Lactose Broth (Merck�) shaking for 20 h at 37 �C.The E. coli culture was centrifuged at 5000 rpm for 20 min. The mi-cro-organism cell pellets were rinsed twice and re-suspended in100 cm3 of sterile solid-free double-distilled water in order to pre-pare the stock suspension.

3.2. Experimental set-up

Water disinfection using UV-C radiation was carried out in anannular photoreactor designed especially for kinetic studies(Fig. 1). The reactor is part of a closed-circuit recycling systemcomprised of a recirculation tank (working volume, VT: 8000 cm3)mechanically stirred (Fisaton, model 715), a pump (Bomax, modelMaxMag MD-10L) and a heat exchanger connected to a thermo-static bath. The whole system was operated as a closed loop sys-tem. The sample port was located precisely at the R-tank existpipe, 3 cm far from its bottom. The germicidal lamp used was aPhillips low pressure mercury tubular lamp, model TUV-36W. Ta-

Table 1Experimental set-up and UV lamp features.

Item Parameter Value

Reactor External radius of the quartz tube (rq) 2.2 cmInner radius of the Pyrex tube (rp) 3.0 cmCross section for flow 13.1 cm2

Length (L) 48.0 cmVolume (VR) 610.0 cm3

R-tank Volume 12,000.0 cm3

Work volume (VT) 8000.0 cm3

UV lamp Nominal power 36.0 WOutput power at 254 nm (Ew) 9.0 WLength (LL) 119.9 cm

0 5 10 15 20 25 300.0

0.8

1.6

2.4

3.2

4.0

E x

10

(s-1

)

Time (s)

Fig. 2. Photoreactor: E-curve for 48.3 cm3 s�1 flow rate.

118 A.B. Silva et al. / Fuel 110 (2013) 114–123

ble 1 lists information about the reacting system and germicidallamp.

3.3. Residence time distribution

The photoreactor and R-Tank residence times were obtained bypulse method [26]. The tracer was 0.1 M aqueous potassium chlo-ride. The tracer concentration at the exit of the device was mea-sured by an on-line Mettler-Toledo MC-226 conductivimeter. Theruns were carried out at 25 �C and the flow-rate was 48 cm3 s�1.The stirring rates were 0 rpm, 250 rpm and1000 rpm.

3.4. Incident radiant energy at the photoreactor optical entrance

The value of Gw was determined by potassium ferrioxalate acti-nometry according to Romero et al. [24,25]. Analytical details ofthis technique are given in Murov et al. [23]. Operation conditions:working volume: 8000 cm3; flow-rate: 48 cm3 s�1; stirring-rate:1000 rpm; temperature: 25 �C; pH7; actinometer (potassium ferriox-alate) concentration: 6 mM. Sample were collected during the reac-tion period of 720 s, at regular interval of 120 s, starting from t = 0 s.

3.5. E. coli active cell concentration

The concentration of the E. coli active cells, expressed in MostProbable Number of micro-organism (MPN) present in a 1 cm3

sample, was determined by multiple-tube fermentation technique[27,28]. 10�2–10�6 dilution series of the sample were prepared,and portions of 0.5 cm3 were inoculated into five tubes containing4.5 cm3 of EC broth (Merck�). These were incubated at 44.5 �C for24 h. The number of organisms was assessed by counting the num-ber of positives in the last three dilutions showing growth and thendetermining the MPN by following the directions on Hoskins’ table[28,29]. In this work, the detection limit of the enumeration meth-odology was 0.01 MPN cm�3 (1 MPN/100 cm3).

3.6. Naperian absorptivity

The Naperian absorption coefficient of the E. coli active cells at254 nm (ax) was obtained using a UV–Vis spectrophotometer(Spectronic, model Genesys 2PC).

3.7. Disinfection runs

The equipment was sterilized beforehand by a sodium hypo-chlorite solution (1:3 v/v) circulating for 30 min. At the end of thisperiod, the set-up was drained and rinsed four times with steriledouble-distilled water. The lamp was turned on for 1 h in orderto stabilise its emission and the system temperature. Meanwhile,the R-tank was fed aseptically with sterile double-distilled water(8000 cm3, ph7 and 25 �C), which was inoculating with E. coli byadding an adequate amount of the stock suspension to adjust the

initial concentration of active cells of 2.0 � 104 MPN cm�3 and2.0 � 106 MPN cm�3. Then, the target water was stirred for60 min to become a homogeneous microbial suspension and to al-low the bacteria to adjust to the environmental conditions beforeexposure to UV-C radiation. When the lamp emission and systemtemperature reached a steady-state, the 0-s sample was takenaseptically, the pump was turned on and the water suspensionstarted circulating through the system (flow-rate: 48 cm3 s�1);then was irradiated. 5 cm3 samples were taken aseptically at spe-cific time intervals of 120 s for measurement of both the suspen-sion absorbance at 254 nm and concentration of E. coli active cells.

4. Results and discussion

4.1. Residence time and dynamic behaviour

Fig. 2 shows the E-curve of the tracer (KCl) in the photoreactorat flow-rate of 48 cm3 s�1. As it can be observed, there was a shortdelay (13.8 s) during which little or no tracer was detected at theexit of the photoreator, accompanied by a sharp rise in tracer con-centration at 18.0 s, then a decrease with a short-circuiting flow(by-passing), at 22.8 s, then a steady period. An E-curve with suchcharacteristics is not typical for a differential reactor [26]. The va-lue found for the photoreactor residence time of the tracer, sP, was8.4 s. The variance, r2, and reduced variance, r2

h , were 0.11 s2 and0.13, respectively. The Tanks-in-Series model was used to evaluatethe non-ideal behaviour of the photoreactor, leading to N (numberof tanks-in-Series) equal to 7.6. The Closed-Vessel-Dispersion mod-el was used to investigate the mixing level in the photoreactor. Thedispersion number was 0.007, hence, for the flow-rate of 48 cm3 -s�1, the photoreactor behaved as a non-ideal plug flow reactor withlow axial dispersion.

0 60 120 180 240 300 3600.000

0.002

0.004

0.006

0.008

0.010

E (

s-1)

Time (s)

0 rpm

250 rpm

1000 rpm

Fig. 3. Recirculation tank: E-curve for 48.3 cm3 s�1 flow rate of and various stirringrates.

12.0

15.0

M)

A.B. Silva et al. / Fuel 110 (2013) 114–123 119

Fig. 3 shows the E-curve obtained for the R-tank at a flow-rateof 48 cm3 s�1 and stirring- rates of 0 rpm, 250 rpm and 1000 rpm.These curves show that the tracer was detected as soon as it wasinjected into the R-tank, and also that its concentration becamemaximal around 19.9 s, followed by a decrease with a short-cir-cuiting flow. In all cases, the short-circuiting flow intensity washigh during the first 180 s, then, decreasing to a certain commonlevel. This type of E-curves is typical for a non-ideal well-mixedtank with dispersion [26].

The residence time, variance, reduced variance and number ofTanks-in-Series for the R-tank are given in Table 2. The additionof stirring at 250 rpm in the R-tank, with a flow-rate of 48 cm3 s�1,increased the residence time by 9.3%; but further stirring rate in-creases did not change the residence time. Conversely, the axialdispersion level was an indirect function of the stirring rate, asthe reduced variance increased to 11% and 44% when the stirringrate was increased from 0 rpm to 250 rpm and 0 rpm to1000 rpm. The R-tank behaved as a non-ideal well-mixed tankand N could be considered equal to 1 in each condition. It is impor-tant to point out that the mixing condition in the R-tank was en-hanced considerably by stirring. At 0 rpm, the axial dispersionwas maximal at the beginning (290 s) of the process; after thistime interval the dispersion was reduced. At 250 rpm and1000 rpm, the respective time intervals where intense axial disper-sion took place were 125.2 s and 63.3 s; after that the behaviour ofthe R-tank approached closely to being the well-mixed with verylow axial dispersion.

Therefore, the optimal condition for carrying out the UV disin-fection studies, when recirculation flow-rate is 48 cm3 s�1 (maxi-mum capacity), is that with a stirring rate of 1000 rpm, whenbetter homogenisation of the microbial suspension is achieved inthe R-tank. This assumes that samples withdrawn at the exit pipeof the R-tank represent the condition in the photoreactor with a

Table 2Residence time and variance (R-tank).

Parameter Stirring rate (rpm)

0 250 1000

st � 10�2 (s) 1.1 1.3 1.3r2 � 10�3 (s2) 6.4 7.8 8.7r2

h0.9 1.0 1.3

N 1.2 1.0 0.8 ? 1.0

good margin of reliability provided the sampling was done att P 63.3 s.

4.2. Incident radiant energy at the photoreactor optical entrance

The potassium ferrioxalate actinometry associated to a suitablemass balance (Item 2.4) was used to quantify the boundary condi-tion of Eq. (4), Gw [23,25], which gives precise values of Gw for anannular photoreactor in a batch recycling system, providing thedynamic profile of the concentration of Fe2+ is linear. For this, con-version must be smaller than 12% [23].

Figs. 4 and 5 show both the dynamic profiles of the concentra-tion and conversion of Fe2+ to be linear. Maximal conversion was

less than 12%. The Eq. (14)DC

Fe2þDt

h iterm was obtained from Fig. 4

as 1.9 � 10�9 mol cm�3 s�1. Hence, Gw was 1.4 � 10�9 Ein-stein cm�2 s�1 (6.4 � 10�1 mW cm�2).

The GW value precision was obtained by comparing the lampoutput power, EW, (measured at the lamp envelope by the manu-facturer) with EW,Ac, germicide power, measured at the quartz tubesurface, obtained in the current study by actinometry [25].

EW ;Ac ¼VT

/Fe2þ

DCFe2þ

DT

� t!0

ð20Þ

The Gw and the lamp power (EW,Ac) are related by:

Gw ¼1AI

VI

VTEW ;Ac ð21Þ

The discrepancy in the values of Ew and EW,Ac was 3.9% (Table 3).Mainly, this discrepancy was due to the scattering and refractionof UV light by quartz glass with a refractive index of 1.52. The valueof GW obtained from Eq. (21) was 6.36 � 10�1 mW cm�2, thus, thevalue obtained for GW of 6.4 � 10�1 mW cm�2 (1.4 � 10�9 Ein-stein cm�2 s�1) by the potassium ferrioxalate actinometry washighly accurate.

4.3. Naperian absorption coefficient

Fig. 6 presents the Naperian absorbance of E. coli at 254 nm as afunction of concentration of these bacterium active cells, whichranged from 4.0 � 101 to 2.0 � 106 MPN cm�3. The obtained valuefor the Naperian absorption coefficient of the E. coli active cellswas 1.05 � 10�8 cm2 MPN�1. Labas et al. [11] claimed for this coef-ficient a value of 1.38 � 10�9 cm2 CFU�1 (E. coli, strain ATCC 8739,suspended in clear water).

0 150 300 450 600 7500.0

3.0

6.0

9.0

Time (s)

CFe

2+ x

107

(

Fig. 4. Dynamic profile of the concentration of Fe2+ ions.

0 140 280 420 560 7000

2

4

6

8

10

12

XFe

2+ (

%)

Time (s)

Fig. 5. Dynamic profile of the conversion of Fe2+ ions.

Table 3Comparison between the lamp power at 254 nm obtained in the current work and theclaimed by manufacturer (www.phillips.com).

Manufacturer Current work

LL (cm) Nominal Ew (W) Ew (W) LL�Ea (cm) Ew (W) Ew,Ac (W)

119.9 36.0 14.6 48.0 5.8 5.6

a Effective length of the lamp.

0 200 400 600 800 100010-7

10-6

10-5

10-4

10-3

10-2

10-1

100

2.0 x 106 MPN.cm-3

2.0 x 104 MPN.cm-3

Cx/

Co

Process time (s)

Fig. 7. Dynamic profile of normalised concentration of E. coli active cells.

120 A.B. Silva et al. / Fuel 110 (2013) 114–123

4.4. Disinfection process

Fig. 7 depicts the dynamic profile of the normalised concentra-tion of E. coli active cells for two typical runs with distinct initialconcentrations of 2.0 � 104 MPN cm�3 and 2.0 � 106 MPN cm�3

but under similar operation conditions (GW = 1.4 � 10�9 Ein-stein cm�2 s�1, recirculation flow-rate: 48 cm3 s�1; stirring rate:1000 rpm, pH7 and temperature: 25 �C). The E. coli response-curveto UV-C did not follow a simple exponential decay. The inactivationprocess of the E. coli by UV-C was highly efficient as the concentra-tion of E. coli was reduced from 2.0 � 106 MPN cm�3 and2.0 � 104 MPN cm�3 to a non-detectable level (detection limit:0.01 MPN cm�3). For both conditions, the inactivation occurredaccording to two kinetic regimes: (a) Fast Regime, when thenumber of active cells reduced steeply (extension: 480 s; finalconcentrations: 1.0 � 103 MPN cm�3 and 4.0 � 101 MPN cm�3; log-

101

102

103

104

105

106

1.0

2.0

3.0

4.0

5.0

6.0

Linear regression

Ax1

02

CX (MPN.cm-3)

Fig. 6. Naperian absorbance versus concentration of E. coli active cells.

arithmic inactivation: �4.2-Log and 2.7-Log; initial inactivationrate hRXðtÞit!0: 3.7 � 104 MPN cm�3 s�1 and 2.4 � 102 MPN cm�3 -s�1); (b) Slow Regime (tailing phase), when the inactivation ratedecreased (tailing) followed by a sudden reduction of the numbersof actives cells to a non-countable level (Extension: 480–840 s and480–600 s; Final concentration of tailing phase (both conditions):2.0 � 101 MPN cm�3; Logarithmic inactivation: 5-Log and 3-Log;inactivation rates: 3.3 MPN cm�3 s�1 and 2.2 MPN cm�3 s�1). Tail-ing phenomenon was observed in E. coli inactivation by UV-C radi-ation and is characterised by the slowing of microbial inactivationrate with increase in UVDose or irradiation time, with the last ac-tive cells being most difficult to inactivate. Tailing started, nor-mally, after at least 99% of the initial available micro-organismswere inactivated (2-log inactivation) [5], in agreement with theactual results. Tailing might be caused by the clumping of micro-organisms or by a resistant subpopulation.

The E. coli response to UVDose in a continuous annular photore-actor loop system is given in Fig. 8. The E. coli UVDose–responsedid not follow the First-order Model, or presented a shoulderphase, but rather a saturation phase followed of a sudden increasein log-inactivation from 5-Log to 6.3-Log (2.0 � 106 MPN cm�3)and 3-Log to 4.3-Log (2.0 � 104 MPN cm�3), which led to inactiva-tion levels of 99.9999% and 99.99%, respectively.

UVDose appeared to be a direct function of the initial concen-tration of the E. coli active cells, as a 102 increasing of this param-

0.0 8.0 16.0 24.0 32.0 40.00.0

1.5

3.0

4.5

6.0

7.5

2.0 x 106 MPN.cm-3

2.0 x 104 MPN.cm-3

log

Inac

tivat

ion

UVDose (mW.cm-2)

Fig. 8. UVDose response in a continuous annular photoreactor In a batch loopsystem.

Table 4Inactivation efficiency and required UVDose without photo-reactivation in a continuous annular photoreactor in a batch loop system.

CoX (MPN cm�3) 2-Log inactivationa 4-Log inactivationa

Tp (s) TH (s) UVDose (mW s cm�2) TP (s) TE (s) UVDose (mW s cm�2)

2.0 � 104 100.0 6.0 3.3 692.3 41.6 12.32.0 � 106 457.6 27.5 5.8 511.4 30.7 22.6

a Literature dose values for E. coli ATCC 25928 based on batch experiments without photo-reactivation: 2-Log: 6.5 mW s cm�2 and 4-Log: 8.0 mW s cm�2 [33].

A.B. Silva et al. / Fuel 110 (2013) 114–123 121

eter increased the UVDose value to 75.6% and 83.4% for conditionsof 2-Log and 4-Log units of inactivation, respectively. Table 4 givesinformation about the E. coli inactivation efficiency and requiredUVDose. A direct dependence of E. coli initial concentration on UV-Dose was expected, as its estimation was based on the averagedincident radiation energy over the reactor volume that is a dy-namic function of the absorption coefficient, which itself dependsdynamically on the concentration of E. coli active cells. The respec-tive UVDose required for achieving 6.3-Log and 4.3-Log units ofinactivation (inactivation levels of 99.9999% and 99.99%), were31.3 mW s cm�2 and 23.5mW s cm�2, respectively. The processtimes required for that were 960 s and 720 s, corresponding toeffective irradiation times of 57.7 s and 43.3 s and to 6.8 and 5.1accomplished recycling circles. The UVDose values obtained inthese studies were concerned with the tailing phase, which ex-plains their values. The literature claims UVDose values for theE. coli ATCC 25922 of 8 mW s cm�2 and 10 mW s cm�2 for 4- and6-Log units of inactivation, respectively [30], based on batch exper-iments with no provision for non-uniformity of the radiation field.

An improved E. coli UVDose value (�10 mW s cm�2 for 3-Logunits of inactivation) was obtained by applying the Dose-distribu-tion Model, considering both the dependence of spatial coordinateson the spectral specific intensity and the non-uniformity of theradiation field, using the PSS Emission Model for Lamp output toestimate the boundary condition of the balance equation [4]. Thecurrent study found a UVDose of 8.6 mW s cm�2 for 3-Log unitsof inactivation (initial concentration: 2.0 � 106 MPN cm�3). Thisresult is in agreement with that reported by Chiu et al. [4]. How-ever, for the initial concentration of 2.0 � 104 MPN cm�3, a 127%greater UVDose was found (19.5 mW s cm�2) for 3-Log units ofinactivation. With these conditions, the inactivation of the respec-tive active cells was carried out at both Fast and Slow Kinetic Re-gimes. With the latter regime, tailing took place; hence, theactive cells might have developed self-protection against UV-Cradiation, leading to higher UVDose requirement for inactivation.

The quantum efficiency of a disinfection process can be evalu-ated by means of the pseudo-quantum yield, Ux [12], which corre-sponds to the proportion of the absorbed photons that results indimer formation: the other photons are emitted as photons (fluo-rescence) or dissipated as thermal energy [31]. This process param-eter is defined by Eq. (22).

/x ¼Inactivation rate

Volumetric rate of photon absorption by the bacteriað22Þ

Due to some characteristics of photochemical processes [6], in gen-eral, their quantum efficiency are calculated when t ? 0, with theinitial rates, Eq. (23) [12].

/t!0x ¼

hRXðtÞit!0VR

heakðr; tÞi

t!0VR

ð23Þ

where hRXðtÞit!0VR

and heakðr; tÞi

t!0VR

are the volume-averaged values ofthe inactivation rate, caused by UV-C damages at the microbial ac-tive cells, and volumetric rate of photon absorption by the E. coli ac-tive cells at t ? 0, respectively.

Disinfection efficiency is affected by water quality [5]. In thecurrent work, the target waters were prepared with double-dis-tilled water and stirred vigorously (1000 rpm) for 60 min beforeexposure (dark step of the process). Then, it is reasonable to claimthat, during the process illuminated step, the mechanical and os-motic effects on the cell viability were negligible; and the E. coliinactivation efficiency achieved was due to the UV-C radiation ef-fect on the active cells. The main reasons for that is that upon sud-den changes in osmotic pressure (or environmental osmolarity),microbial cells undergo an osmotic shock (shock response). How-ever, these cells are able to adapt themselves to this new environ-ment by adjusting their metabolic flow (stress response) [32,33].This hypothesis has been supported by the results reported byPaleologou et al. [34], who studied the disinfection of E. coli byphotocatalysis, sonolysis and UV-C irradiation. They claimed thatduring preliminary studies (Condition: E. coli suspended in steriledeionised water under continuous stirring, in dark, without thepresence of TiO2 and ultrasound) the concentration of E. coli wasunchanged after 120 min, showing that the microbial cells werestable at those conditions and that cell damages due to osmoticstress and stirring were negligible.

Table 5 shows the values of pseudo-quantum yield at t ? 0 forinitial concentrations of 2.0 � 104 MPN cm�3 and 2.0 � 106 -MPN cm�3. As expected, these results suggest that an increase inthe concentration of the active cells has a direct and significant ef-fect on the pseudo-quantum yield. The process quantum efficiencyat t ? 0 is very low for both concentrations of E. coli active cells, asthe values of the pseudo-quantum yield at t ? 0 were very small(<<<1). This result suggests that a single photon is unlikely to inac-tivate a cell [7]. In fact, only 65 thymine-dimers are produced perevery 107 nucleotides in the DNA of E. coli for every 1 mW s cm�2

of UV-C irradiated. In addition, only 30% of UV radiation at254 nm are available to be absorbed or/and scattered [31]. Labaset al. [12] claimed a pseudo-quantum yield value of 3.6 � 10�9 -UFC quanta�1 for the inactivation of E. coli (stain ATCC 8739) car-ried out under the following conditions: Initial concentration ofE. coli active cells: 8.3 � 104 CFU cm�3; diluted media concentra-tion: 5.0 � 10�6 g cm�3; Gw: 5.9 � 10�9 Einstein cm�2 s�1 andVRPA in t ? 0 of 2.2 � 1012 quanta cm�3 s�1. The discrepancy be-tween the result obtained for concentration of 2.0 � 104 MPN cm�3

and that claimed by Labas et al. [12] can be explained by the factthat the pseudo-quantum yield is a highly process-dependent va-lue and it does not have the quality of an intrinsic kinetic property[6]. Distinct photoreactors were used in these studies, so distinctprocess efficiencies might be expected.

4.5. Kinetic parameter estimation

A simple phenomenological model that relates the inactivationrate to volumetric rate of photon absorption and growth rate, Eq.(1), is proposed in this study. The proposed model considers thecomplete inactivation process, as it occurred in a single stage:

Active Cell!hm Inactivated Cell

hRXðtÞiVR¼ �Kh½CXðtÞ�iVR

h½eakðr; tÞ�

niVRþ hRGðr; tÞiVR

ð1Þ

Table 5Process parameter when t ? 0 E. coli inactivation by UV radiation at 254 nm.

CoX ðMPN cm�3 hRxit!0 ðMPN cm�3 s�1Þ heait!0 ðquanta cm�3 s�1Þ /x (MPN quanta�1)

2.0 � 104 2.4 � 102 3.0 � 1011 7.9 � 10�10

2.0 � 106 3.7 � 104 9.0 � 1012 4.1 � 10�9

122 A.B. Silva et al. / Fuel 110 (2013) 114–123

Given that the microbial growth rate, hRGðr; tÞiVRcan be considered

equal to zero when the micro-organism cells are suspended in ster-ile nutrient-free water [11,35] and the model developed for theVRPA, Eq. (10), the resulting model for the inactivation rate is asfollows:

hRXðtÞiVR¼ �KCXðtÞ 2Gw

kxðtÞKTðtÞ

rq

½r2p � r2

q �f1� exp½�KTðtÞðrp � rqÞ�g

( )n

ð26Þ

The kinetic parameters, n and K, were estimated numerically byfeeding the complete data to a multi-parameter non-linear regres-sion algorithm coupled with the Box optimisation routine, givingestimated values of n = 0.5 and K = 2.9 � 102 s�1 (cm3 s Einst-ien�1)0.5. The final model is described by Eq. (27).

hRXðtÞiVR¼ �2:9� 102CXðtÞh½ea

kðr; tÞ�0:5iVR

ð27Þ

Fig. 9 shows the comparison made between the theoretical andempirical dynamic profiles of the normalised concentration of theE. coli active cells. The model fits the data with an accuracy of 95%.

Labas et al. [11], who were the first to relate the VRPA to theinactivation rate and thus amending the Series-event model pro-posed by Severin [10], reported that E. coli inactivation by UV-Cradiation, in water free of nutrient and suspended solids, occursas two events. The values claimed by them for the parametersn and K are: 0.205 ± 0.015 and 1.3142 � 102 ± 5.21 s�1

(cm3 s Einstiein�1)0.205.The simple model used in the current work led to kinetic

parameter n and K values of similar order of magnitude to thoseobtained by Labas et al. [11]. In addition, the model suggests, likeLabas et al. [11], the rate of E. coli inactivation process by UV-C isaffected moderately by the VRPA, or by the incident energy or irra-diation power as these parameters are correlated directly. Hencethe efficiency of the disinfection process is more likely to dependstrongly on irradiation time. This fact was expected, as only 30%of the UV-C radiation incident is ready to be absorbed and/or scat-tered in a process of inactivation of E. coli [31].

0 200 400 600 8000.0

0.2

0.4

0.6

0.8

1.0

CX/C

o

Time (s)

Model

Data: 2x106

MPN.cm-3

Data: 2x104 MPN.cm-3

Fig. 9. Comparison between the theoretical and empirical dynamic profiles ofnormalised concentration of the E. coli active cells.

5. Conclusion

A study of the kinetic of E. coli inactivation by UV radiation at254 nm was performed in water free of nutrients and suspend sol-ids. The process was carried out in an annular photoreactor de-signed specially to provide significant photon transport radially.Simple kinetic phenomenological model, which takes into accountvolumetric rate of photon absorption, the growth rate of E. coli andsingle stage inactivation, was proposed. This model fits the datawith accuracy of 95%.

Acknowledgements

The authors would like to thanks the CNPQ for the financialsupport and CAPES for Mrs. A.B. Silva’s MSc grant. Special thanksare given to the Departamento de Engenharia Química and Pós-grad-uação em Engenharia Química da Universidade Federal de Pernam-buco for their financial support that enabled the divulgation ofthis work, and to Mr. G. Titmus for the English proof reading.

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