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NOTE / NOTE
Sensor factor correction for collimated beam experiments using a medium pressure ultraviolet lamp Mei-Ting Guo, Hong-Ying Hu, and James R. Bolton
Abstract: A detailed protocol is presented documenting how a ‘sensor factor’ correction should be calculated when undertaking quasi collimated beam experiments using a polychromatic medium pressure ultraviolet lamp. The correction factor was determined as 0.778 through Pyrex filtration test. This new protocol is particularly important if the radiometer sensor has significant sensitivity at wavelengths longer than 300 nm. Key words: collimated beam, ultraviolet, UV, disinfection, radiometer. Re´sume´ : Cet article pre´sente un protocole de´taille´ documentant comment calculer la correction d’un « coefficient de capteur » lors d’expe´riences sur des faisceaux quasi collimate´s utilisant une lampe ultraviolette polychromatique a` pression moyenne. Le coefficient de correction a e´te´ de´termine´ a` 0,778 par l’essai de filtration avec Pyrex. Ce nouveau protocole est particulie`rement important si le capteur radiome´trique est tre`s sensible aux longueurs d’ondes supe´rieures a` 300 nm. Mots-cle´s : faisceau collimate´, ultraviolet, UV, de´sinfection, radiome`tre. [Traduit par la Re´daction]
Introduction Linden and Darby (1997) first pointed out that radiometer readings, when using medium pressure (MP) UV lamps, must be corrected for the wavelength sensitivity of the radiometer and the germicidal effectiveness of the microorganisms. Subsequently, Bolton and Linden (2003) published a detailed protocol for the determination of the UV dose (fluence) in a Petri dish for the purpose of determining the UV dose (fluence) – response curve for a microorganism in a quasi collimated beam apparatus. If the ultraviolet (UV) lamp in the housing of the quasi collimated beam apparatus is a polychromatic medium pressure (MP) UV lamp, a sensor factor correction must be made to allow the determination of the true irradiance at the surface of the aqueous solution in the Petri dish. The Bolton–Linden Protocol has Received 16 January 2008. Revision accepted 24 September 2008. Published on the NRC Research Press Web site at jees.nrc.ca on 7 November 2008.
now become a virtual ‘standard’ in such measurements. Convenient Excel spreadsheets are available from the International Ultraviolet Association (Member Zone) (www.iuva. org) or from Prof. James Bolton (
[email protected]). Bolton and Linden (2003) gave the following procedure for the determination of the ‘sensor factor’ in the case where a polychromatic MP UV lamp is used: ‘‘Since the incident UV beam contains wavelengths over the full range of 200–300 nm, allowance has to be made to account for the variation of the sensitivity of the detector over this band. The sensor factor is the sensitivity of the detector at 254 nm divided by the weighted average (weighted by the photon emission from the UV lamp as it impinges on the Petri dish) sensitivity of the detector over the 200–300 nm band. The sensor factor is given by
S254 Sensor factor ¼ X N li S li
½4
i
M. Guo and H. Hu. Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China. J.R. Bolton.1 Department of Civil and Environmental Engineering, 3-133 Markin/CNRL Natural Resources Engineering Facility, University of Alberta, Edmonton, AB T6G 2W2, Canada. Written discussion of this note is welcomed and will be received by the Editor until 31 January 2009. 1Corresponding
author (e-mail:
[email protected]).
J. Environ. Eng. Sci. 7: 677–679 (2008)
where S254 = the detector sensitivity at 254 nm; and Nli and Sli = relative lamp emission (normalized to unity) and the detector sensitivity in a narrow wavelength band centered at the wavelength li. The summation is taken over a finite number of narrow wavelength bands (e.g., 5 nm) over the germicidal range (e.g., 200–300 nm). Once this factor is determined, it is fixed as long as the same UV lamp is used and minimal decay of the UV
doi:10.1139/S08-044
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J. Environ. Eng. Sci. Vol. 7, 2008 lamp output has occurred. The sensor factor is almost always greater than unity, reflecting the fact that the detector is generally less sensitive to wavelengths above or below 254 nm.’’
Fig. 1. UV dose (fluence) – response curve for E. coli, where the germicidal UV dose (fluence) has been determined using the Bolton–Linden protocol.
Note that the above Protocol assumes that the radiometer detector has zero sensitivity above 300 nm. However, if the radiometer detector has significant sensitivity above 300 nm, then the radiometer reading, as used in the calculation of the fluence (UV dose) in MP collimated beam experiments, will be too high. During a recent series of experiments involving a study of photoreactivation in E. coli and in the total coliform in wastewater (Guo et al. 2008), it was found necessary to modify the Bolton–Linden protocol for the determination of the sensor factor to account for sensitivity of the radiometer detector above 300 nm.
Experimental Experiments were conducted using a Calgon Carbon Model PS1-1-120 quasi collimated beam apparatus. Alternately, a 15 W low pressure (LP) UV lamp or a 1000 W MP UV lamp can be placed in the housing of the apparatus. In the case of the LP UV lamp, the Petri dish containing a suspension of E. coli was 30 cm from the UV lamp. In the case of the MP lamp, the Petri dish was 130 cm from the UV lamp. All experiments were conducted using a 6 cm OD Pyrex Petri1 dish. Irradiance measurements were made using a Model 1L1400A Radiometer (International Light) with a Model SEL240/QNDS2/TD UV detector calibrated by the manufacturer at 254 nm. The radiometer and detector were calibrated by International Light over the wavelength range 200– 400 nm. A 50 mL sample of a conserved culture of E. coli CGMCC 1.3373 was incubated in LB broth at 37 8C overnight until the stationary phase was reached. The cells were collected by centrifugation (10 000 r/min, 10 min, 4 8C), washed twice with a sterilized saline solution (0.9%), and subsequently suspended in the sterilized saline solution to achieve a concentration of approximately 105 CFU/mL. After irradiation, 500 mL of the irradiated samples was removed, serially diluted, and then plated in triplicate on nutrient agar to determine the viable organism levels following exposure. The plates were incubated at 37 8C for 24 h. All samples were processed by the standard count technique.
The UV dose (fluence) – response curve for E. coli The Bolton–Linden Protocol is designed to allow the determination of the germicidal UV dose (fluence) for a polychromatic medium pressure UV lamp. Since the UV dose (fluence) for a low pressure UV lamp is (by definition) a germicidal UV dose (fluence), if experiments are conducted using both a low pressure and a medium pressure UV lamp, the UV dose (fluence) – response curves should come out the same. Figure 1 shows such a comparison for E. coli. Obviously the two UV dose (fluence) response curves diverge, when they should be essentially the same (within experimental error). Clearly, something is wrong.
Fig. 2. Relative sensitivity of the radiometer detector (–––––) and the relative lamp emission (- - - - -) as a function of wavelength.
Radiometer detector sensitivity versus wavelength Figure 2 shows the radiometer detector sensitivity as a function of wavelength and the relative emission spectrum of the medium pressure UV lamp. Clearly, the detector has a significant sensitivity at wavelengths beyond 300 nm. From a multiplication of the relative detector sensitivity by the relative lamp emission for each wavelength, it was determined that 22.2% of the radiometer reading arises from wavelengths from 300 to 400 nm. Since the Bolton– Linden protocol is designed only for the wavelength range 200–300 nm, the radiometer reading should be multiplied by the factor 0.778 before being used in the Excel spreadsheets to determine the UV dose (fluence). The result is that the UV doses (fluences) in Fig. 1 were overestimated. #
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Guo et al. Fig. 3. UV dose (fluence) – response curve for E. coli, where the germicidal UV dose (fluence) has been determined using the Bolton–Linden protocol by multiplying the UV doses (fluences) for the MP lamp by a factor of 0.778.
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pared to a computer value of 0.18. This clearly validates the mathematical analysis carried out above.
Corrected UV dose (fluence) – response curve for E. coli Figure 3 shows the UV dose (fluence) – response curves for E. coli after the above correction has been imposed. Now the UV dose (fluence) – response curves for the LP and MP lamps agree within experimental error.
Conclusion In cases where a radiometer detector is used for measurement of the irradiance from a MP UV light source, where the detector has significant sensitivity at wavelength beyond 300 nm, the germicidal irradiance will be overestimated and an appropriate correction should be made.
References
Verification using a Pyrex filter As a verification that part of the radiometer reading arises from light with wavelengths above 300 nm, the radiometer reading was recorded with and without a Pyrex1 filter placed on top of the detector. The ratio of the reading with the Pyrex1 filter to that without was 0.12 ± 0.02, as com-
Bolton, J.R., and Linden, K.G. 2003. Standardization of methods for fluence (UV Dose) determination in bench-scale UV experiments. J. Environ. Eng. 129: 209–216. doi:10.1061/(ASCE) 0733-9372(2003)129:3(209). Guo, M.T., Hu, H.Y., and Bolton, J.R. 2008. Comparison of lowand medium-pressure ultraviolet lamps: Photoreactivation of E. coli and total coliform in secondary effluent of municipal wastewater treatment plants. Water Res. In press. Linden, K.G., and Darby, J.L. 1997. Estimating effective germicidal dose from medium pressure lamps. J. Environ. Eng. 123(11): 1142–1149. doi:10.1061/(ASCE)0733-9372(1997) 123:11(1142).
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