Indian Journal of Chemical Technology Vol. 1 0, November 2003, pp. 680-683

A rticles

Modelling of sterilization and air exchange rates for indoor environment: A n application to effect of ozone concentration on room bacteria

J J Gaware, P V Joshi & V Abrol*

Environmental Assessment Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 1 May 2002; revised received 14 August 2003; accepted 2 September 2003

Under the assumption of w�� 1tiixed indoor air, tlie effect of ozone generated from an ozonizer on the room bacteria is mathematic8ny oumeUed. · "Ti).(r�.Qde}. computiltioDs give the effective air exchange rate, sterilization rate, ozone destruction rate and the ... el�e:rateioroiQne for ozoriiter.

The sterilization of air is of immense necessity in certain working areas such as wards and operation theatres in the hospitals. For sterilization of air, processes like heating, irradiation and chemical treatment are used. The above processes have their own limitations when the room to be sterilized is occupied by persons. Most of the chemicals used for sterilization are harmful to the health. Though ozone is a harmful gas, it has been confirmed that the low dose of ozone can be used for sterilization of the room airl . Maximum permissible level of ozone as specified by EPA (USA)2, epeB (IDL) is 1 57 J.!gm-3 for 8 h of continuous inhalation. When ozone was generated continuously from an ozonizer for 76 h and was emitted in the room, it was observed that room air ozone concentration did not increase beyond 25 J.!gm-3, so that it was reported that the controlled emission of ozone could be safely used for room air steri lization even while occupancy in the rooml .

Based on these experiments, a model that could be an useful mathematical tool for assessing the performance of ozonizer as air sterilizer is presented. Efficiency of the sterilizer, in terms of sterilization rate and air exchange rate can successfully be assessed using the proposed model. In addition, it can be used for estimating ozone destruction rate and release rate for ozone. These would be useful parameters in designing the experiments i .e. maintaining the safe room air ozone levels for required low air bacteria concentrations.

Experimental Procedure Silent electric discharge based ozonizer was used

for the generation of ozone. The generated ozone diffuses in the room and mixes with room air. Ozone

*For correspondence (E-mail: vinoda@apsara.barc.emet.in)

monitor based on chemiluminescent reaction of ozone with ethylene was used for ozone measurement) . The

measurement of air bacteria was done using Dynastat air bacteria sampler. The sampler collects the air at a rate of 40 Lpm and bacteria are separated by impaction on nutrient strip placed in the sampler. The culture medium used in the nutrient strip was

tripticase soya agar. On collecting the sample strips.

they were kept for growth in an incubator at 37°e for 24 h. The concentration of air bacteria was estimated by counting the number of colonies formed on the strip in terms of colony forming unit (cfu), a special unit used for air bacteria concentration4• Each colony represents a single bacterium. Further, the estimated concentration was converted to cfum-3 of air. The details of the procedure are given elsewhere5 .

The experiments were performed in a room of size 54 m3 having occupancy of one person with a movement of 2-3 persons. The room is air­

conditioned maintaining the temperature at 22°e and relative humidity at 60%. Ozone and air bacteria were monitored at a distance of 2 m from ozonizer at 1 .5 m (roughly breathing height) from the ground level. whereas ozonizer was placed at a height of 1 .75 III above the ground. Before starting the experiment. levels of ozone inside the room were monitored for 24 h, which varied in the range of 1 -2 J.!gm-3, during this period. Air bacteria samples were collected the day after starting the ozonizer and the time was marked as 00:00 h. During first 4 h, eight samples were col lected and later the samples were collected at 24. 48 and 76 h .

Results and Discussion Due to the emission of ozone, ozone levels inside

the room were increased, but only up to a concentration of 25 J.!gm-3. The temporal variations i n

Gaware et al.: Modelling of sterilization and air exchange rates for indoor environment A rticles

bacterial concentration are presented in Table 1 . The sterilization criterion was fixed at 1 5 cfum-3• Table 2 presents the build-up of bacterial concentration after putting off the ozonizer, whereas data presented in Fig. 3 give temporal variations in ozone concentration after putting off the ozonizer. It was observed that the ozonizer takes 2 h for room air sterilization, whereas, commercial ozonizers claim a period of 60-90 min. for the sterilization.

Air exchange rate and sterilization rate The mass balance of bacteria concentration flow

into and out of indoor volume V including indoor sources Qi is given by the well-known equation6-8, whose solution is given by method of integrating factor,

c = aCout + Ae-(a+k)t In (a + k) . . . ( 1 )

where Cn and Cout are indoor and outdoor concentrations, k is bacteria sterilization rate, a is effective air exchange rate and indoor bacteria source Qi=O.

a At t = 0, Cin = 875 => A = 875 - Cout (a + k)

:. C = __ a_C {1 _ e-(a+k)t } + 875e-(a+k) t

I n (a + k) out

a -3 as t � 00, Cin = Cout = 1 5 cfum (a + k )

:. C = 1 5 (1 - e-(a+k) t ) + 875e-(a+k) t m .

=> C - 1 5 = 860e-(a+k)1 m

Using bacterial concentration data from Table I , and solving by regression analysis, for exponential decay, the values obtained are,

� (a + k) = 0.07 1 min-I with a correlation coefficient R=0.998. The fit is shown in Fig. 1 .

Using solution (2) as buildup of bacteria with no ozonizer action; k = O.

Table I -Temporal variations in the concentrations of bacteriu and ozone after ozonizer is put on

Time after ozonizer is put on Bacteria conc.

'7 E .2

1000

800

� 600 c: u c 0 u 400 III .;:: � III co 200

o

h: min

00:00 00:02 00:05 00: 1 0 00: 1 5 00:20 00:30 00:45 0 1 :00 02:00 04:00

o 1 '0

cfum-3

875

434

275 1 5 8 1 00 55 1 5 1 5

R=O.998

20 30 4'0 lime (min)

Ozone com:. _ 1

)..tgm ·

I 2 1 2 1 8 22 23 24 24 24 24 24

50 6'0

Fig. I -Variation in bacterial concentration with t ime when ozonizer was on

Cin = Cout + A e -at

at t = 0, Cn = 15 cfum-3

:. A = 1 5 - Cout

. . . ( 2 )

Substituting the above value of A In Eg. (2), the relation obtained is,

therefore, Cin= Cout + 15 e -at - Cout e -at

at t � 00 Cn � Cout = 875 m-3

therefore Cin = 875 - 860 e-at . . . (3)

Using the data in Table 2 and solving by regression for exponential growth, a can be estimated as a = 0.0545 min-t with R=0.988. The fit is shown in Fig. 2.

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Articles Indian J. Chern. Techno!., November 20m

Table 2-Build up of bacterial concentration with time, after putting off the ozonizer

Time (min)

Bacterial cone. (cfum-3)

Since (a+k) = 0.07 1 min-I

therefore, k = 0.0 165 min-I

1 0

1 5

20

30

30

70

a, the effective air exchange rate is estimated to be 3.27 change per hour and k, the sterilization rate is estimated to be -1 changes per hour.

Kulkarni et al. I had estimated the sterilization rate during their study as 0.058 min-I which is more than 3 times the value estimated here (0.0165 min- I). This is because of the fact, that, the effect of air exchange rate has not been considered. It seems, that, inclusion of effect of air exchange reduces the value of k considerably. The effective air exchange rate a is related to the ventilation flow rate f and mixing factor (non-dimensional) m, varying between 0. 1 to 0.4, given by,

a = mJlv

f varies from 29.43 m:/h to 7 .63 m3/h by this relation for a room, at volume v of 6x3x3 m dimensions.

Ozone destruction rate Mass balance equation for ozone concentration

when ozonizer is put off can be given as6,

. . . (4)

where 8 is ozone destruction rate and Cbk IS background concentration of ozone.

From Eq. (3), it can be derived that,

C e(a+5)1 = a C e(a+c5)1 + f3 o (a + 8 ) bk aCbk f3 -(a+5 )1 so that C = + e

, 0 (a + 8 ) aC

At l = 0, Co= 24 )lgm-3 � f3 = 24 - (a +b�) th t C = aCbk (1 - -(a+5 ) 1 ) + 24e-(a+c5 )t so a ,

0 ( a + 8 ) e

682

40 50 60 70 RO 1 25 320 620 875 X7.'i

1 000

800 � "? R=O.988 .E 2 £ 600 • c: 0 c: 0 0

.� 400 � I1l III

200 •

0 • 0 10 20 30 40 50 60 70

Time (min)

Fig. 2-Build-up of bacteria with time after putting off ozonizer

25 .

� "? E 20 :2 £ c: 0 c: 15 8 I1l .� t) 10 I1l III

5

•

o 2

R=O.99B

•

4 6 Time (min)

8 1 0

Fig. 3-Decay o f ozone with time after putting off ozonizer

aC At t � 00 C = bk = 1 )lgm-3 (Fig. 3)

, 0 (a + 8 ) (a + 8 )

so that, Cbk = �--.!... a Substituting the value of Cbk

. . . (5 )

Gaware et al.: Modelling of sterilization and air exchange rates for indoor environment Articles

Using measured ozone concentration values shown in Fig. 3 and solving by regression for exponential decay,

(a +8 ) = 0. 10 15 min-1 with R=0.998. The fit is

shown in Fig. 3.

but a = 0.0545 min-1

so that ozone destruction rate 8 = 0.047 min-1 = 2.82 changes per hour

Ozonizer release rate It is very difficult to estimate ozonizer release rate9

experimentally for the present open type ozonizer. However, an idea about ozonizer release rate can be obtained by analytical method.

When ozonizer is on, using all the terms in the standard equation6,

C = aCbk (1 - e -(a+8 )1 ) o (a + 8 ) + [Q / 54 (a + 8 ) ]{1 _ e-(a+8 ) 1 } +C e-(a+8 ) 1 bk . . . (6)

Solving at t ---7 00, where Co = Cmax = 24 Ilgm-3,

therefore, Q, the source strength = 54(24 - 1 )(a+8)

= 1 26.06 Ilgmin-1 = 2. 1 Ilgs-1

Conclusion Apparently the method discussed above could be suitably used for the estimation of the rate constants associated with room air cleaning by ozone sterilization technique. For the present .experimental conditions, under the assumption of well mixed indoor air, the estimations are as follows,

(i) a, the effective air exchange rate: 0.0545 min- I i .e. 3 .27 changes per h.

(ii) k, sterilization rate:0.0165 min-I i.e. - I change per h.

(iii) 8, ozone destruction rate: 0.047 min-I i .e. 2.82 changes per h.

(iv) Q, release rate for ozonizer: 126.06 Ilgmin- I i .e . 2. 1 Ilgs-1

In the absence of a verified and val idated computational fluid dynamics (CFD) code 10 for internal flows in a room, the present model generates reasonable magnitudes of the quantities ( i ) to ( i \' ) above, in the simplified treatment of the problem.

Acknowledgement The authors are thankful to Shri V. D. Puranik.

Head, Environmental Assessment Division (EAD) , Dr. S . Sadasivan, Ex-Head, EAD and Dr. T. N. Mahadevan, Head, Environmental Monitoring and Analysis Section, EAD for their encouragement and useful comments.

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2 EPA web address http://www.epa.gov. 3 Arora P K, Kulkarni P B & Yohra K G, Indian J Techllol. 1 5

( 1 977) 528. 4 Parat S, Fricker-Hidalgo H, Pedrix A, Berner D, Pel l i ssier N

& Grillot R, Am Ind Hyg Assoc J, 57 ( 1 996) 996 5 Joshi P V & Jamuna K, IASTA Bulletin, I O ( 1 997) 29 6 Patil R S, Indoor Aerosols alld Exposure Asses.wl('lII. ill

Aerosols Generations and Role in Medicine. !1Il!U.\·(IT & Environment, edited by Nambi K S & Sapra B K (A l l ied Publications Ltd., New Delhi), 1998, 267

7 Terje Grontoft, Almos Environ, 36 (2002) 5661 8 Kulkarni M M & Patil R S, Almos Environ, 36 (2002 ) 4777 9 Berner D, Leder M T, Regnier R, Hecht G & Gerbert J M.

Anll Occup Hyg, 46 (2002) 347 IO Lee E, Feigley C E & Khan J, Ann Occup Hyg. 36 ( 2002 )

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