Experimental Assessment of Photon Fluence Rate Distributions in a

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Experimental assessment of photon fluence rate distributions in a medium-pressure UV photoreactor Mengkai Li, Zhimin Qiang, Chen Wang, James R. Bolton, and Ernest R. Blatchley Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06298 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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Environmental Science & Technology

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Experimental assessment of photon fluence rate distributions

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in a medium-pressure UV photoreactor

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Mengkai Li,†,‡ Zhimin Qiang,‡ Chen Wang,‡ James R. Bolton,§

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Ernest R. Blatchley III*,†ǁ

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†

Lyles School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907,

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United States

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‡

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Eco-Environmental Sciences, University of Chinese Academy of Sciences, Chinese

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Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China

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§

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Edmonton, AB T6G 2W2, Canada.

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ǁ

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Lafayette, Indiana 47907, United States

Key Laboratory of Drinking Water Science and Technology, Research Center for

Department of Civil and Environmental Engineering, University of Alberta,

Division of Environmental & Ecological Engineering, Purdue University, West

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*Corresponding author: ph. 1-765-494-0316; fax 1-765-4940395; email

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[email protected]; Lyles School of Civil Engineering, Purdue University, 550

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Stadium Mall Drive, West Lafayette, IN 47907 USA.

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ACS Paragon Plus Environment


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ABSTRACT: The performance of a medium-pressure (MP) mercury lamp

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photoreactor is strongly influenced by the spatial photon fluence rate (PFR)

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distributions which are wavelength-dependent. To address this issue, PFR

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distributions in an MP lamp photoreactor were measured using a 360-degree response

27

micro-fluorescent silica detector (MFSD). To accurately express the optical behavior

28

in an MP photoreactor, PFR, MFSD response PFR (PFRMFSD), and effective

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germicidal PFR (PFRGER) were defined and compared. The measured axial and radial

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PFRMFSD values agreed well with the corresponding results from a simulation model

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(UVCalc®). The PFR and PFRGER were obtained from the measured PFRMFSD by

32

using correction factors calculated by the UVCalc®. Under identical UV transmittance

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(254 nm) conditions (75% and 85%), the weighted average PFRGER values were

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13.3−18.7% lower than the corresponding PFR values, indicating that PFRGER, rather

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than PFR should be used in MP photoreactor design to meet disinfection standards.

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Based on measured lamp output, medium absorption spectrum, MFSD response, and

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microbial deoxyribonucleic acid response spectrum, the detailed relationships

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between the PFR, PFRMFSD, and PFRGER were elucidated. This work proposes a new

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method for the accurate description of wavelength-dependent PFR distributions in MP

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photoreactors, thus providing an important tool for the optimal design of these

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systems.

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Key words: photon fluence rate distribution; medium-pressure lamp; UV

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photoreactor; micro-fluorescent silica detector (MFSD)

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■ INTRODUCTION

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Ultraviolet (UV) photoreactors are commonly used for disinfection and advanced

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oxidation processes in water and wastewater treatment.1,2 Medium-pressure (MP)

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mercury (Hg) lamps are often preferentially chosen for large-scale applications

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because their high output power leads to a relatively small photoreactor size.3-6

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Moreover, their polychromatic output can be advantageous, as compared with

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(essentially) monochromatic low-pressure (LP) Hg lamps, if the lamp output spectrum

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correlates well with the absorption spectrum of the target microorganism or chemical

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pollutant. Examples of applications in which these advantages have been

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demonstrated include UV disinfection,5,7 UV photolysis of nitrosodimethylamine,4

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and UV/H2O2 advanced oxidation process.4

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Fluence is the master variable that governs the performance of UV photoreactors.

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An essential prerequisite for fluence calculations or measurements is the accurate

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quantification of fluence rate (FR) distributions. The FR distributions in LP

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photoreactors have been well described. For these applications, numerical simulation

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models have been developed, including the point source summation, line source

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integration, extensive source with volumetric emission, and UVCalc® models.8-15

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Moreover, several measurement technologies, such as the spherical actinometry,16,17

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SIC detector18,19 and micro-fluorescent silica detector (MFSD),20,21 have been

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demonstrated for the determination of FR distributions in LP photoreactors.

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For MP photoreactors, the FR distributions are more complex than those of LP

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lamp reactor in several respects: 1) the FR at a test point is attributable to photons of

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all wavelengths represented in the MP lamp output spectrum, while only a single

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wavelength (i.e., 254 nm) should be considered for LP lamps; 2) because of the

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absorption spectra (or UV transmittance (UVT) spectra) of the media through which

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UV radiation is transmitted, the FR contribution of each wavelength will demonstrate

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a spatial variation; 3) the target microorganism or chemical pollutant has also a

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characteristic absorption spectrum (or quantum yield), which makes the reaction rate

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wavelength-dependent; and 4) a detector (or sensor) has inevitably a unique response

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spectrum, and therefore does not respond uniformly to photons of all wavelengths in

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the output spectrum of an MP lamp.

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Because of the complex optical behavior described above, experimental

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measurements of polychromatic FR distributions are beneficial, not only for a

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validation of numerical FR distribution models, but also for a direct determination of

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FR distributions that can be used to assess the performance of MP photoreactors.

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However, to date, no such measurements have been reported. Important barriers to

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this approach include a lack of an appropriate detector, one that requires a 360-degree

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UV response (FR detector) and that can function properly when subjected to high

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fluence rates, as observed in MP photoreactors.

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In our previous work, an MFSD was developed for in-situ measurements of FR

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distributions in an LP photoreactor.21,22 The key features of this 360-degree response

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detector, such as fast response, water-resistance and small volume (0.07 mm3), are

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particularly suitable for FR distribution measurements in aqueous solutions. In

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addition, Ge-doped silica, the sensitive window of MFSD, has a high resistance to UV

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irradiation, and its response spectrum spans a wavelength range of 200−275 nm.

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Therefore, it was hypothesized that the MFSD would also function well for FR

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distribution measurements in an MP photoreactor.

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Note that for a photochemical or photobiological reaction, the local reaction rate

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is proportional to the local absorbed photon flux.23 Moreover, the MFSD, a

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photoelectric detector, should obey the photoelectric effect, indicating that the electric

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current induced by the photoelectric effect is proportional only to the (local) photon

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flux incident on the sensitive window. Therefore, for purposes of this study, it is more

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appropriate to use photon fluence rate (i.e., PFR, units of einstein m−2 s−1) rather than

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energy-based FR (units of mW cm−2) for description of the spatial distributions of

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radiant energy.

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In this study, experiments were conducted to measure the in-situ PFR

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distributions in an MP photoreactor by use of the MFSD. Three aqueous media,

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namely deionized (DI) water and aqueous solutions of copper sulfate (CS) and

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potassium acid phthalate (PP), were used to examine the effect of medium absorption

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spectrum. The experimental results were compared with a PFR simulation model

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(UVCalc®). In addition, numerical analyses were conducted to elucidate the detailed

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relationships between PFR, MFSD response PFR (PFRMFSD), and effective germicidal

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PFR (PFRGER) in an MP photoreactor based on measured lamp output, MFSD

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response, and microbial deoxyribonucleic acid (DNA) response spectrum. This work

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will improve our understanding of the complex optical behavior that characterizes MP

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photoreactors, and thus is useful for the optimal design of polychromatic UV

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photoreactors.

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■ EXPERIMENTAL SECTION

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Measurement platform. Figure 1 represents a schematic diagram of the

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experimental platform for measuring the PFR distributions in a single-lamp MP

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photoreactor. A cylindrical quartz-wall UV photoreactor (inner diameter = 190 mm

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and inner length = 1000 mm) wrapped in a black cotton cloth was used; UV

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reflectance from the air/quartz/air interfaces of this photoreactor has previously been

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estimated at approximately 8%.24 An MP Hg lamp (arc length = 100 mm, outside

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diameter = 11 mm; Foshan Comwin Light and Electricity Co., Ltd., China) with a

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quartz sleeve (outside diameter = 23 mm) was housed in the center of the cylindrical

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quartz photoreactor. The total input power of the lamp was 500 W with a UVC

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(200−300 nm) efficiency of 17.5%.

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Figure 1. Schematic diagram of the experimental setup for measuring photon fluence

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rate (PFR) distributions.

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An MFSD (1.0 mm length × 0.3 mm outside diameter), whose fluorescence was

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transmitted by an optical fiber housed in a long, thin and hollow stainless-steel tube

129

and then amplified by a multimeter, was fixed onto a two-dimensional guideway with

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its vertical axis oriented parallel to the lamp axis. This configuration allowed the

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detector to move either parallel (displacement precision of 1 mm) or perpendicular

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(displacement precision of 0.01 mm) to the lamp axis. The MFSD was calibrated at

133

254 nm as described previously.21 Because of high PFR and high temperature in the

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gap between the MP lamp and the sleeve, a reference detector for monitoring the lamp

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output fluctuation was not used in this study.20, 25

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Measurement process. Test solutions included DI water (UVT254 = 100%) and

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aqueous solutions of CuSO4 (CS) and potassium acid phthalate (PP) with

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concentrations adjusted to yield UVT254 values of 85% and 75%. The use of DI water

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as a test medium represents a simplified condition, in which the effect of medium

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absorption spectrum could be eliminated; as such, the difference in the PFRMFSD

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distributions between MP and LP photoreactors was attributable only to the

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polychromatic output of the MP lamp and the MFSD response spectrum. For a fixed

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UVT254 value, measurement data in the two test solutions (i.e., CS and PP) could be

144

used to investigate the effect of medium absorption spectrum on the MP photoreactor

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performance.

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In the central cross-section of the photoreactor, the detector was moved

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perpendicular to the lamp axis (i.e., from the sleeve to the reactor wall) to measure the

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radial PFR distributions at preselected test points in various test media. Then, to

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determine the axial PFR distributions, the detector was moved parallel to the lamp

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axis at radial distances of 30.0 and 60.0 mm away from the lamp surface. The detector

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readings were collected and stored at a 2-second interval by a data acquisition switch

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unit (34972A, Agilent, USA). At each test point, the detector remained in place for 0.5

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min and a total of 15 detector readings were collected. The last 10 stable readings

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were then averaged as the reported PFR value.

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Chemicals, analytical methods, and PFR simulation model. CS and PP

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reagents were of analytical grade and purchased from Sigma-Aldrich (St. Louis, MO,

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USA). UVT spectra were measured by a spectrophotometer (2600, Shimadzu, Japan)

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in a 1.0 cm path-length quartz cell. A PFR model (UVCalc® version 1B, Bolton

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Photosciences Inc., Edmonton, Alberta, Canada) was employed to calculate the PFR

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distributions at each wavelength under each UVT condition. This model employs the

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multiple segment source summation (MSSS) approximation and accounts for

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refraction and reflection at the air/quartz/water interfaces.26 With a known relative

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output spectrum of the MP lamp and a UVT spectrum of the test medium, the PFR

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distributions in the MP photoreactor could be obtained. The model-simulated PFR

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distributions were compared with the MFSD measured data, and then further used to

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calculate the correction factors for the conversions among PFR, PFRGER and PFRMFSD.

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Details of the calculations are presented below.

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The relationships between PFR, PFRMFSD, PFRGER. To accurately quantify the

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performance of an MP photoreactor, it is necessary to clarify the relationships among

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three key variables, namely PFR, PFRMFSD, and PFRGER as defined previously. An

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illustration of these relationships is presented in Figure 2. MP lamps emit

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polychromatic UV and visible radiation at wavelengths ranging from 200 to 500 nm.

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The PFR at a test point in an MP photoreactor is usually defined as the unweighted

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sum of the PFR values contributed by all photons of across the most effective

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wavelength range of 200−300 nm for UV disinfection and UV-based AOPs. For

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purposes of model simulations, this wavelength range (i.e., 200−300 nm) was divided

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into 20 bands (i.e., the band number n = 20). Hence, band1 represented 200−205 nm,

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and band20 represented 296−300 nm. A vector-based analysis with multiple vector

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components representing the behaviors of various wavelength bands was performed in

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this study, which is applicable to polychromatic lamp calculations.

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Figure 2. Illustration of the relationships between PFR, PFRMFSD, and PFRGER.

184 185

[P], P, and [pn] were defined as the vector lamp power, scalar lamp power, and

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fraction of the lamp power from every wavelength band, respectively (see eq 1).

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[PFR], A and [an] were defined as the vector PFR, scalar PFR, and fraction of the PFR

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contributed by each wavelength band (within 200−300 nm) at a test point in an MP

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photoreactor, respectively (see eq 1). Note that A and [an] will be different at each test

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point in the photoreactor. By using UVCalc®, [PFR] can be obtained with a known [P]

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and a determined medium absorption spectrum:

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UVCalc [ P] = P[ pn ]  →[PFR] = A[an ] = A[a1 , a2 , a3......an ] UVT,200-300nm

(1)

20

193

∑ [a ] = 1

(2)

n

n =1

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Likewise, [PFRMFSD], B and [bn]T were defined as the vector PFRMFSD, scalar

195

PFRMFSD, and fraction of PFRMFSD from each wavelength band (i.e., eq 3), 10


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respectively. Note that the relative response spectrum of MFSD was normalized to

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1.00 at 254 nm (i.e., b11 = 1), and the sum of bn ( ∑ [bn ] ) was greater than 1.

20

n =1

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 b1  b   2  [bn ] =  b3    ......  bn 

(3)

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Moreover, [PFRGER], C and [cn]T were defined as the vector PFRGER, scalar

200

PFRGER, and fraction of PFRGER from each wavelength band (i.e., eq 4), respectively.

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Note that the relative response fraction of microorganism (DNA) was normalized to

202

1.00 at 254 nm (i.e., c11 = 1), and the sum of cn ( ∑ [cn ] ) was greater than 1.

20

n =1

203

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 c1  c   2  [cn ] =  c3    ......  cn 

(4)

As a result, the [PFRMFSD] can be calculated as follows:

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[PFR MFSD ] = [PFR][bn ] = A[ an ][bn ] = A[ a1 , a2 , a3 ......an ]    b1  b   2   b3  ......    bn 

(5)

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[PFR MFSD ] = A[ a1b1 , a 2 b2 , a 3b3 ,......a n bn ]

(6)

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Because the MFSD reading (i.e., B) is linearly dependent on the photons received and

208

the MFSD response spectrum, one has:

209 210

B = A(a1b1 + a2b2 + a3b3 + ......anbn ) By algebra: 11


(7)


211

A 1 = = CFPFR / MFSD B (a1b1 + a2b2 + a3b3 + ......anbn )

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(8)

212

Since the correction factor, PFR/PFRMFSD (or CFPFR/MFSD), can be obtained by

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combining numerical simulations (e.g., UVCalc®) with a known MFSD response

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spectrum and the MFSD reading, the PFR value (i.e., A) can then be calculated using

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eq 8. The scalar PFRGER can be calculated analogously as follows:

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A 1 = = CFGER / MFSD C (a1c1 + a2c2 + a3c3 + ......an cn )

(9)

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In addition, the vector [PFR] and [PFRGER] are equal to the products between A and

218

[an] and between C and [cn], respectively.

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■ RESULTS AND DISCSSSION

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Spectra. Figure 3a shows the UVT spectra of the CS and PP solutions. The

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concentrations of both solutions were adjusted to yield UVT254 values of 75% or 85%,

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as indicated above. Note that in the short-wavelength region of 200−250 nm, the UVT

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values of the PP solution were greater than those of the CS solution; while in the

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long-wavelength region of 250−300 nm, the opposite was true.

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1.0

(a)

UVT

0.8 0.6 0.4 PP, UVT254 = 85% CS, UVT254 = 85% PP, UVT254 = 75% CS, UVT254 = 75%

0.2 0.0

0.05

226 227 228

DNA MFSD

1.2

Relative response

225

(b)

1.0

MP lamp 0.04

Normalized at 254 nm

0.8

0.03 0.02

0.6 0.4

229

0.2

230

0.0

0.01

Relative lamp output

1.4

0.00 200

225

250

275

300

325

350

231

Wavelength (nm) 232 233

Figure 3. UVT spectra of copper sulfate (CS) and potassium acid phthalate (PP)

234

solutions (a) and MFSD response, DNA response, and MP lamp output spectra

235

(Foshan Comwin) (b).

236 237

The MFSD response, DNA response, and MP output spectra are shown in Figure

238

3b. These results indicate that the MFSD does not respond uniformly across all 13



239

wavelengths to output radiation from the MP lamp. Measureable responses by the

240

MFSD are evident only for the range of 200−275 nm, with peak response at 248 nm.

241

For the DNA response spectra, peaks were evident at approximately 210 nm and 260

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nm. Wavelengths above 300 nm are not effectively absorbed by DNA and are

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generally inefficient for disinfection. The MP lamp output spectrum was obtained

244

from the manufacturer and measured using a spectroradiometer. The principal

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emission wavelength is greater than 350 nm; output in the 200−300 nm accounted for

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only 17.5% of the total output for this lamp. In fact, this low efficiency of UV output

247

has been regarded as a main demerit of MP lamps as a source of UVC radiation.

248

PFRMFSD distributions in DI water. Because the MFSD does not respond

249

uniformly to all wavelengths emitted from an MP lamp, it can only measure the

250

PFRMFSD distributions in the MP photoreactor. Figure 4a illustrates the measured and

251

modeled radial PFRMFSD distributions in the central cross-section of the photoreactor

252

filled with DI water. When the detector was moved from 10 to 70 mm from the sleeve

253

surface, the PFRMFSD decreased from 12 × 10−5 to 2.6 × 10−5 einstein m−2 s−1. The

254

modeled results agreed well with the measurement results across this range of radial

255

distances.

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(a)

Modeled PFRMFSD Measured PFRMFSD

20

-5

-2

-1

PFRMFSD (x10 einstain m s )

25

15

10

5

0 0

20

40

60

80

Radial distance from the sleeve surface (mm) 256

10

259

-5

260 261 262 263

Modeled PFRMFSD (30 mm rad. dist.) Measured PFRMFSD (30 mm rad. dist.)

(b)

-1 -2

258

PFRMFSD (x10 einstein m s )

257

8

Modeled PFRMFSD (60 mm rad. dist.) Measured PFRMFSD (60 mm rad. dist.)

6

4

2

0 264

0

10

20

30

40

50

60

70

80

Axial distance from the lamp center (mm) 265 266

Figure. 4. Measured and model simulated radial (a) and axial (b) PFRMFSD

267

distributions in DI water.

268

The measured and modeled axial PFRMFSD distributions in the MP photoreactor

269

filled with DI water are illustrated in Figure 4b. The detector was moved from the 15



270

lamp center to the lamp end along two lines parallel to the lamp axis, at radial

271

distances of 30.0 mm and 60.0 mm from the lamp surface. The measured and

272

modeled PFR values decreased when approaching the lamp end. The model

273

simulations generally agreed with the measured data; however, for the test points

274

along the vertical line 30.0 mm away from the lamp surface, the model

275

underestimated measured PFRMFSD values in the near lamp-end region (i.e., 30−70

276

mm from the lamp center). One possible explanation for this behavior was reflection

277

at the water/air interface at the top liquid level in the photoreactor, as indicated by

278

Fresnel’s Law. This would result in an increased measured PFRMFSD in the near

279

lamp-end region, which was not been considered by the model. Smaller effects would

280

be found far from the liquid level and near wall region (i.e., the 60.0 mm radial

281

distance to the lamp surface). Similar effects have been reported in an LP photoreactor,

282

and disappeared in an air medium.21

283

PFRMFSD distributions in different media. Figure 5 shows PFRMFSD

284

distributions in the photoreactor filled with CS and PP solutions. Because of the

285

additional effect of UVT spectra, the PFRMFSD simulation was more complex than that

286

in DI water. Model simulation results again agreed well with the measured results in

287

both media for both UVT conditions, indicating robustness of the MP PFR model

288

simulation. Despite the same UVT254 value, the PFR distributions in CS and PP were

289

different, which can yield differences in photoreactor performance. At present, most

290

UV facilities equipped with either LP or MP lamps use UVT254 as the principal

291

control parameter. These results indicate that for polychromatic systems, including

16


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MP photoreactors, the UVT spectrum of the treated water and its potential variation

293

range should also be considered in UV photoreactor design.

25 Modeled PFRMFSD, UVT254 = 85% Measured PFRMFSD, UVT254 = 85%

20

Modeled PFRMFSD, UVT254 = 75% Measured PFRMFSD, UVT254 = 75%

-2

-1

PFRMFSD (einstein m s )

(a) 15 10 5 0

294 295 296 297 298 299

PFRMFSD (x10-5 einstein m-2 s-1)

25

(b) 20 15 10 5 0 0

20

40

60

80

300

Radial distance from the sleeve surface (mm) 301 302 303

Figure 5. Measured and model simulated radial PFRMFSD distributions in PP (a) and

304

CS (b) solutions.

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305

Relationships between PFR, PFRMFSD, and PFRGER: Impact of MFSD and

306

DNA response spectra. PFRMFSD distributions provide relevant, but incomplete

307

information regarding photoreactor performance, including disinfection.

308

considering disinfection applications, PFR and PFRGER distributions in an MP

309

photoreactor may be more relevant than PFRMFSD distributions. Among these three

310

parameters, differences in MFSD and DNA spectra play the critical roles (see Figure

311

3b). In the discussion above, the scalar PFR (i.e., A) and PFRGER (i.e, C) value were

312

calculated by the CFPFR/MFSD and CFGER/MFSD based on the measured PFRMFSD (i.e.,

313

eqs 8 and 9). CFPFR/MFSD and CFGER/MFSD values in various media were calculated and

314

are shown in Figures S1a and S1b, respectively. It was found that the CFPFR/MFSD was

315

equal to 1.34 in DI water because the MFSD only responded across the wavelength

316

range 200−275 nm. The CFPFR/MFSD did not change when the test point moved from

317

the sleeve surface to the reactor wall, indicating that the PFR and PFRMFSD

318

demonstrated the same ratio at all points in the photoreactor. Likewise, CFGER/MFSD

319

was independent of the position of the test point. It was equal to 1.23 in DI water. The

320

calculated PFR and PFRGER distributions, as well as the PFRMFSD distribution in DI

321

water, are shown in Figure 6a. These results indicate that PFR values were highest

322

among the three parameters. Because of a greater response of DNA compared to that

323

of MFSD, the PFRGER values were slightly higher than PFRMFSD in all test points.

18


When

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18

(a)

16

PFR PFRGER PFRMFSD

14 12 10 8 6

324 325 326 327 328

PFR ( x 10-5 einstein m-2 s-1)

4 2 10

(b)

PFR (PP) PFRGER (PP) PFRMFSD (PP)

6 4 2 0 200

329 330

PFR (CS) PFRGER (CS) PFRMFSD (CS)

8

(c)

Band5 (CS) Band5 (PP) Band11 (CS) and (PP) Band16 (CS) Band16 (PP)

160 80

331 60

332

40

333

20

334

0 0

335

10

20

30

40

50

60

70

Radial distance from the sleeve surface (mm)

336

Figure 6. Radial PFR, PFRMFSD, and PFRGER distributions in DI water (a) and CS and

337

PP solutions (UVT254 = 75%) (b), and radial PFR distribution of various wavelength

338

bands in PP and CS solutions (UVT254 = 75%) (c).

19



339

For the CS and PP solutions, the CFPFR/MFSD increased with increasing distance

340

from the sleeve surface to the reactor wall. This is because the MFSD only responded

341

to radiation with wavelengths of 200−275 nm. Moreover, because the UVTs of both

342

CS and PP in wavelength range of 200−275 nm were lower than that of 275−300 nm

343

(see Figure 3a), the fraction of PFR in the wavelength range of 200−275 nm would

344

represent a smaller fraction of the PFR value (i.e., 200−300 nm) in the far-lamp region

345

than that in near-lamp region. Therefore, a higher CFPFR/MFSD was observed in the

346

far-lamp region than that in near-lamp region. In addition, CFGER/MFSD values

347

decreased first and increased with increasing distance from the sleeve surface. This

348

behavior was attributable to the complex response spectra of the MFSD and DNA.

349

The corrected PFR and PFRGER distributions, as well as the PFRMFSD distribution in

350

media with UVT254 of 85% and 75% are illustrated in Figures S2 and 6b, respectively.

351

Relationships between PFR, PFRMFSD, and PFRGER: Impact of medium

352

absorption spectrum. In the section above, differences among the PFR, PFRMFSD,

353

and PFRGER were discussed, which were impacted by the MFSD and DNA response

354

spectra, principally. However, differences between two media conditions were also

355

apparent. For solutions with UVT254 of 85% (i.e., Figure S2), all three PFR

356

parameters in PP were larger than those in CS. The differences between the CS and PP

357

for all three PFR parameters were greater in the near-lamp region than in the far-lamp

358

region. For the condition of UVT254 equal to 75% (i.e., 7c), all three PFR parameters

359

in PP in near-lamp region were greater than those in CS in near-lamp region, while

360

they are smaller in far-lamp region.

20


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361

To explore the results in greater detail, a short-wavelength region (200−250 nm)

362

and a long-wavelength region (250−300 nm) were defined. PP had higher UVT values

363

in short-wavelength region and CS had higher UVT values in long-wavelength region

364

(Figure 3a). The PFR contributions of three representative wavelength bands, namely

365

220−225 nm (band5), 250−255 nm (band11), and 275−280 nm (band16) are

366

illustrated in Figure 6c. The PFRs for band11 of CS and PP were indistinguishable,

367

because the UVTs of both solutions were adjusted to 75% at 254 nm. The PFRMFSD

368

and PFRGER from band11 in CS and PP were the same too (see Figures S3 and S4),

369

because the MFSD and DNA responses were both normalized to 1 at 254 nm. In the

370

near-lamp region, because of the short path-length and higher UVT values in

371

short-wavelength region, the PFR of band5 in PP was higher than that for CS,

372

although it decreased rapidly with increasing path length. In the far-lamp region, the

373

PFRs of band5 in CS and PP were both zero, while the PFR of band16 in PP

374

decreased more quickly than in CS, which induced a higher PFR value in CS than that

375

in PP in far-lamp region. Similar results were observed with PFRMFSD and PFRGER

376

distributions (see Figure 6b). These results demonstrate that PFR (or PFRMFSD and

377

PFRGER) spatial distributions in various solutions (i.e., various UVT spectra) were

378

complex and would be impacted by path-lengths of various locations in the

379

photoreactor and transmittance ability of each wavelength.

380

Weighted average PFR, PFRMFSD and PFRGER. Table 1 shows the weighted

381

average PFR, PFRMFSD, and PFRGER values for various UVT254 values. The weighted

382

average values of all three PFR parameters for the 100% UVT case were higher than

21



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383

those in other UVT cases, and the weighted average PFRGER in 100%, 85% CS, 85%

384

PP, 75% CS and 75% PP solutions were 18.7%, 14.0%, 13.7%, 14.2%, and 13.3%

385

lower than the corresponding values of PFR. Hence, it is important to emphasize the

386

application of PFRGER in UV disinfection, rather than PFR for MP photoreactors. For

387

identical values of UVT254, the weighted average PFRGER were different among the

388

various media, indicating that disinfection performance will depend on the absorption

389

spectra of the media, rather than only UVT254 as commonly used in LP photoreactors.

390

In addition, the weighted average values of all three parameters in CS solution with

391

85% UVT254 were lower than that in PP solution. However, they were higher than

392

those in PP solution with 75% UVT254. This is because at 75% UVT254, the PP

393

solution had higher PFR values in the near-lamp region and lower PFR values in PP in

394

far-lamp region than the CS solution (see Figure 6b), yet the PFR in the far-lamp

395

region was assigned a greater weight in the weighted average PFR calculation for a

396

larger radius (details of this calculation have been described previously24).

397

Table 1. Weighted average PFR, PFRGER, and PFRMFSD (einstein m−2 s−1) in

398

various media. Parameter

DI water (100%)

CS (85%)

PP (85%)

CS (75%)

PP (75%)

PFR

4.87

2.30

2.70

2.04

1.58

PFRGER

3.96

2.00

2.33

1.75

1.37

PFRMFSD

3.64

1.08

1.46

0.74

0.76

399

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400

Potential applications. In recent years, many novel UV sources have been

401

developed and their potential applications in water treatment have been

402

investigated.27-32 An important attribute of UV sources is their emission spectrum.

403

Different wavelengths can induce different photochemical effects, including

404

disinfection. However, the traditional concept of PFR covering the UV range of

405

200−300 nm cannot completely describe process performance in polychromatic

406

systems. Therefore, it is necessary to account for the wavelength-dependent behavior

407

of these systems. The use the PFRGER in UV disinfection, which has been

408

demonstrated in this study, based on the DNA response spectrum (normalized 1 at 254

409

nm) for MP lamp or other non-LP UV lamps, represents such an approach. Ideally, the

410

same PFRGER from MP and LP lamps would induce the same inactivation

411

performance relative to a given target organism, while the same PFR would not. In

412

addition, the required UV disinfection fluence (or dose) of current disinfection

413

standards are based on the bench-scale quasi-collimated beam apparatus equipped

414

with an LP lamp (i.e., monochromatic exposure at 254 nm). By comparison, the

415

PFRGER may represent a more comprehensive description of fluence delivery to

416

photochemical targets than PFR in non-LP lamp when the DNA response spectrum is

417

normalized 1 at 254 nm.

418

This same logic could be useful for the UV-based photolysis or advanced

419

oxidation process when polychromatic lamps are applied. For example, since H2O2,

420

chlorine, and other chemicals excited by UV demonstrate unique absorption spectra,

421

the required fluence (product of PFR and exposure time) for target pollutant

23



422

degradation should take account of the spectra of the UV sources. A definition of an

423

effective PFR (or fluence), such as the PFRGER (or germicidal fluence), could be

424

applied which would be obtained from eq 8 based on the absorption spectrum of the

425

chemical solution (e.g., H2O2 and chlorine). This would be more clearly to represent

426

UV performance for a given application than traditional methods that do not account

427

for wavelength-dependent effects. Hofman-Caris has also introduced a similar fluence

428

definition for the polychromatic lamp application.33

429

This study also involved application of a novel UV sensor (i.e., MFSD) in an MP

430

photoreactor. Because each detector has a unique response spectrum, calibration is

431

crucial for its application. Some UV detectors (or sensors) used for MP photoreactors

432

are calibrated at 365 nm because this wavelength is close to the centroid of the lamp

433

emission spectrum. In this study, the MFSD was calibrated at 254 nm. Meanwhile,

434

both MFSD and DNA response spectra were normalized 1 at 254 nm. Hence, the

435

PFRMFSD could be readily transferred to PFRGER. This method may lead to a new

436

detector calibration protocol for MP lamps.

437

The UV detector (or sensor) is also important for long-term monitoring of UV

438

fluence for an operating UV photoreactor. The results in Figure 6b indicate that due to

439

the different path-lengths and absorption spectra of the media (i.e., water to be

440

disinfected), the PFR constitution from various wavelengths changed among locations.

441

An online UV detector for long-term monitoring is usually installed on the reactor

442

wall or other position, which has a long path-length. When the water quality (medium

443

absorption spectrum) changes, the monitored PFR would be different from the real

24


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444

PFR or PFRGER, inducing an error on fluence monitoring. Therefore, this error should

445

be considered in long-term monitoring of polychromatic photoreactors.

446

■ ASSOCIATED CONTENT

447

Supporting Information. Four figures are provided. This material is available free of

448

charge via the Internet at http://pubs.acs.org.

449

(Figure S1) Radial CFPFR/MFSD and CFGER/MFSD distributions; (Figure S2) Radial PFR,

450

PFRMFSD, and PFRGER distributions with UVT254 of 85%; (Figure S3) Radial PFRMFSD

451

distribution of the various wavelength bands with UVT254 of 75%, and (Figure S4)

452

Radial PFRGER distribution of the various wavelength bands with UVT254 of 75%.

453

■ AUTHOR INFORMATION

454

Corresponding Author

455

*

456

Notes

457

The authors declare no competing financial interest.

458

■ ACKNOWLEDGEMENTS

Phone: 1-765-494-0316; e-mail: [email protected].

459

The authors gratefully acknowledge financial support from the Edward M. Curtis

460

Visiting Professorship of Purdue University, the National Natural Science Foundation

461

of China (21590814, 51525806, 51408592), and the National Geographic Air and

462

Water Conservation Fund (GERC23-15).

463

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Experimental Assessment of Photon Fluence Rate Distributions in a

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