ss
`
`E
`
`American Society
`of Civil Engineers
`
`March 2003
`
`Volume 129, Number3
`
`ISSN 0733-9372
`
`CODEN: JOEEDU
`
`Journalof
`Sel
`
`Seatile
`
`contents continue on back cover
`
`Editorial
`
`189
`
`191
`
`Clash of Engineering Scientists and Environmental Engineers
`Teresa B. Culver
`
`Editor’s Note
`Mark Rood
`
`Technical Papers
`192
`Electrochemical Reduction of 2,4-Dinitrotoluene in a Continuous Flow
`Laboratory Scale Reactor
`Rajesh B. Doppalapudi, George A. Sorial, and Stephen W. Maloney
`Airborne Bacteria Control Under Chamber and Test-Home Conditions
`Meng-Hui Lai, Demetrios J. Moschandreas, and Krishna R. Pagilla
`
`Standardization of Methods for Fluence (UV Dose) Determination in
`Bench-Scale UV Experiments
`James R. Bolton and Karl G. Linden
`
`Comparative Study of Two Bioassays for Applications in Influent
`Wastewater Toxicity Monitoring
`S. Ren and P. D. Frymier
`
`Sphere Drag and Settling Velocity Revisited
`Phillip P. Brown and Desmond F. Lawler
`
`Field and Laboratory Evaluation of the Impact of Tall Fescue on
`Polyaromatic Hydrocarbon Degradation in an Aged
`Creosote-Contaminated Surface Soil
`Sandra L. Robinson, John T. Novak, Mark A. Widdowson,
`Scott B. Crosswell, and Glendon J. Fetterolf
`
`Investigation of Cadmium Desorption from Different-Sized Sediments
`Sui Liang Huang
`
`Natural Stabilization of Stored Industrial Sludges
`Hazim Tugun, Raymond C. Loehr, and Xuijin Qui
`
`1
`
`EXHIBIT 1022
`
`
`
`Environmental and Water
`ResourcesInstitute
`
`1
`
`EXHIBIT 1022
`
`

`

`contents continued from front cover
`
`258
`
`Thermal Treatment for Incinerator Ash: Evaporation and Leaching
`Rates of Metals
`Zhen-Shu Liu, Ming-Yen Wey, and Shu-Jen Lu
`
`267
`
`Uncertainty of Weekly Nitrate-Nitrogen Forecasts Using Artificial Neural
`Networks
`Momcilo Markus, Christina W.-S. Tsai, and Misganaw Demissie
`Technical Notes
`
`275
`
`Stabilization of Electrical Arc Furnace Dust with Low-Grade MgO Prior
`to Landfill
`Ana |. Fernandez, Josep M. Chimenos, Neus Raventos,
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`Lourdes Miralles, and Ferran Espiell ASCE
`
`of Civil Engineers 1801 Alexander Bell Drive
`
`RESTON, VA 20191-4400
`
`TUL0733-9372(200303)129'3'1-8
`
`American Society
`
`2
`
`

`

`aSCeE, Journalot
`~
`Environmental
`Engineering
`
`Editor
`Mark J. Rood
`University of Illinois,
`Urbana-Champaign
`
`.
`
`Associate Editors
`Robert G. Arnold
`University ofArizona
`Margaret Katherine Banks
`Purdue University
`Morton Barlaz
`North Carolina State University
`Stuart A. Batterman
`University of Michigan
`Carl F. Cerco
`U.S. Army Engineer Research
`and Development Center
`Jiayang (Jay) Cheng
`North Carolina State University
`Teresa B. Culver
`University of Virginia
`Bruce A. DeVantier
`Southern Illinois University
`Dionysios D. Dionysiou
`University of Cincinnati
`Wendell P. Ela
`University of Arizona
`
`
`
`Environmental and Water
`ResourcesInstitute
`Governing Board
`Cecil Lue-Hing, President
`Philip H. Burgi
`C. Dale Jacobson
`Donald M. Phelps
`-Michael A. Ports
`Thomas M. Rachford
`
`Kyle E. Schilling
`Robert C. Williams
`
`Raymond A. Ferrara
`Omni Environmental Corporation
`Joseph R. V. Flora
`University of South Carolina
`Hilary I. Inyang
`-
`University of North Carolina, Charlotte
`Bruce Emest Logan
`The Pennsylvania State. University
`Victor S. Magar
`Battelle Memorial Instituté
`Spyros G. Pavlostathis
`Georgia Institute of Technology
`Eric A. Seagren
`University of Maryland
`George A.Sorial
`University of Cincinnati
`Joo Hwa Tay
`Nanyang Technological University
`Yi-Tin Wang
`University of Kentucky
`
`-
`
`“RDA HAZy
`
`
`
`
`FEB 2 8 2003
`
`LNANGY
`
`
`
`3
`
`

`

`GeneralInformation
`Journal of Environmental Engineering _
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`
`a
`of
`centerpiece
`the
`are
`digesters
`comprehensive, Brown
`and Caldwell-
`designed
`solids-processing
`system ‘that
`meets
`new, more
`stringent Class A
`pathogen-control requirements.
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`Journal of
`Environmental
`Engineering
`
`VOLUME 129 / NUMBER3
`
`MARCH 2003
`
`Editorial
`
`189
`
`191
`
`Clash of Engineering Scientists and Environmental Engineers
`Teresa B. Culver
`
`Editor’s Note
`Mark Rood
`
`Technical Papers
`192
`
`202
`
`209
`
`"216
`
`222
`
`232
`
`241
`
`248
`
`258
`
`267
`
`Electrochemical Reduction of 2,4-Dinitrotoluene in a Continuous Flow
`Laboratory Scale Reactor
`Rajesh B. Doppalapudi, George A. Sorial, and Stephen W. Maloney
`Airborne Bacteria Control Under Chamber and Test-Home Conditions
`Meng-Hui Lai, Demetrios J. Moschandreas, and Krishna R. Pagilla
`
`Standardization of Methods for Fluence (UV Dose) Determination in
`Bench-Scale UV Experiments
`James R. Bolton and Karl G. Linden
`
`Comparative Study of Two Bioassays for Applications in Influent Wastewater
`Toxicity Monitoring
`S. Ren and P. D, Frymier
`
`Sphere Drag and Settling Velocity Revisited
`Phillip P. Brown and Desmond F. Lawler
`
`Field and Laboratory Evaluation of the Impact of Tall Fescue on Polyaromatic
`Hydrocarbon Degradation in an Aged Creosote-Contaminated Surface Soil
`Sandra L. Robinson, John T. Novak, Mark A. Widdowson, Scott B. Crosswell,
`and Glendon J. Fetterolf
`
`Investigation of Cadmium Desorption from Different-Sized Sediments
`Sui Liang Huang
`
`Natural Stabilization of Stored Industrial Sludges
`Hazim Tugun, Raymond C. Loehr, and Xuijin Qui
`
`Thermal Treatment for Incinerator Ash: Evaporation and Leaching Rates of
`Metals
`Zhen-Shu Liu, Ming-Yen Wey, and Shu-Jen Lu
`
`Uncertainty of Weekly Nitrate-Nitrogen Forecasts Using Artificial Neural
`Networks
`Momcilo Markus, Christina W.-S. Tsai, and Misganaw Demissie
`
`Technical Notes
`
`275
`
`Stabilization of Electrical Arc Furnace Dust with Low-Grade MgO Prior to
`Landfill
`Ana I. Fernandez, Josep M. Chimenos, Neus Raventés, Lourdes Miralles, and
`Ferran Espiell
`
`5
`
`5
`
`

`

`Standardization of Methods for Fluence (UV Dose)
`Determination in Bench-Scale UV Experiments
`
`JamesR. Bolton! and Karl G. Linden, M.ASCE?
`
`Abstract: Ultraviolet (UV) disinfection is now an accepted technology for inactivation of a variety of waterborne pathogens in waste-
`water and drinking water. However, the techniques used in much of the previous research aimed at providing information on UV
`effectiveness have not yet been standardized. Thus in many peer reviewed published literature,it is not clear how the UV irradiations were
`carried out, nor how the average fluence (or UV dose) given to the microorganisms has been determined. A detailed protocol for the
`determinationof the fluence (UV dose) in a bench scale UV apparatus containing UV lamps emitting either monochromatic or broadband
`UV light was developed. This protocol includes specifications for the construction of a bench scale UV testing apparatus, methods for
`determination of the average irradiance in the water, details on UV radiometry, and considerations for microbiological testing. Use of this
`protocol will aid in standardization of bench scale UV testing and provide increased confidence in data generated during such testing.
`
`DOI: 10.1061/(ASCE)0733-9372(2003)129:3(209)
`CE Database keywords: Ultraviolet radiation; Standardization.
`
`Introduction
`
`Ultraviolet light has been shown to be very effective for the dis-
`infection of drinking water and wastewater (Meulemans 1987;
`von Sonntag and Schuchmann 1992; Jacangelo et al. 1995;
`Clancy et al. 2000). The inactivation mechanism involves absorp-
`tion of ultraviolet light by DNA or RNA pyrimidine bases (thym-
`ine or cytosine in DNA and uracil or cytosine in RNA) causing a
`photochemical reaction in which a chemical dimer is formed be-
`tween the two bases. The dimer inhibits the formation of new
`DNA(or RNA)chainsin the processofcell replication (mytosis)
`thus resulting in the inactivation (inability to replicate) of affected
`microorganisms by ultraviolet light.
`In most studies of the UV inactivation of microorganisms, a
`low pressure UV lamp has been utilized, which emits nearly
`monochromatic light at 253.7 nm, almost at the peak of germi-
`cidal effectiveness for E. coli and Cryptosporidium (Gates 1930;
`Linden et al. 2001). For this reason, such lamps are often called
`“germicidal” lamps. More recently, medium pressure UV lamps
`have been used because of their much higher germicidal UV
`power per unit length. Medium pressure UV lamps emit over a
`broad range of wavelengths, including germicidal wavelengths
`from 200 to 300 nm.
`In studies of the UV inactivation of microorganisms, it is nec-
`essary to determine the UV response of a given microorganism in
`
`Ipresident, Bolton Photosciences Inc., 628 Cheriton Cres., NW,
`Edmonton AB, Canada T6R 2M5.
`2Professor, Duke Univ., Dept. of Civil and Environmental Engineer-
`ing, P.O. Box 90287, 121 Hudson Hall, Durham, NC 27708-0287.
`E-mail: kglinden@duke.edu
`Note. Associate Editor: George A. Sorial. Discussion open unti! Au-
`gust 1, 2003. Separate discussions must be submitted for individual pa-
`pers. To extend the closing date by one month, a written request must be
`filed with the ASCE Managing Editor. The manuscript for this paper was
`submitted for review and possible publication on January 23, 2002; ap-
`proved on April 29, 2002. This paper is part of the Journal of Environ-
`mental Engineering, Vol. 129, No. 3, March 1, 2003. OASCE, ISSN
`0733-9372/2003/3-209-215/$18.00.
`
`the water matrix in which the organism is foundoris spiked. The
`UV response is usually determined in a bench scale apparatus
`(often referred to as a “collimated beam”’), in which part of the
`output of a UV lampis directed onto a horizontal surface, either
`down a long “collimator,” consisting of a cylindrical tube or
`through successive apertures,
`(The beam is never truly colli-
`mated, since there remains some dispersion in the beam. This
`dispersion has to be considered if long water path lengths are
`used.) The cell suspension to be irradiated is placed on the hori-
`zontal surface below the bottom of the collimator as illustrated in
`Fig. 1. Various workers have used a variety of procedures and
`types of collimated beam apparati. In much of the peer reviewed
`published literature; it is not clear how the UV irradiations were
`carried out, nor how the averagefluence (or UV dose—see below
`for terms and definitions) given to the microorganisms has been
`determined. Thus the quality of the data in the literature needs to
`be assessed with this fact in mind.
`There are many uses of a bench scale (collimated beam) ap-
`paratus for investigations in UV disinfection. Among these are:
`1. Development
`of
`standardized
`fluence
`(UV dose)—
`inactivation response relationships for use in biologicalacti-
`nometry (biodosimetry) testing;
`2. Generation of fundamental fluence (UV dose)—inactivation
`response data for different pathogens to determine compara-
`tive UV susceptibility; and
`Investigation of the photochemical degradation of contami-
`nants.
`
`3.
`
`In all these applications, proper use of the collimated beam testing
`equipmentis essential to obtain accurate and reproducible results.
`
`This paper aims to lay out a detailed step-by-step procedure by
`which fluences (UV doses) can be determined reliably and repro-
`ducibly in a bench-scale collimated beam apparatus for both
`monochromatic and broadband UV lamps.
`Irradiance and fluence rate are closely related, but often mis-
`understood, concepts. The terminology reported herein adheres to
`the recent recommendations of the International Union of Pure
`and Applied Chemistry Working Party on Ultraviolet Disinfection
`
`JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / MARCH 2003 / 209
`
`6
`
`

`

`
`
`
`
`
`Enclosure
`
`
`
`Gpagiey
`
`
`
`
`UV lamp (4)
`housing
`
`Aperture
`2 or 4” baffle
`
`Sliding
`drawer
`
`
`
`
`
`“_
`
`Irradiation
`
`Fan
`
`Collimated
`Beam
`Device
`
`Stirrer and
`
`
`
`pettaish CS]
`
`LP/MPUV lamp
`
`transparent
`window and
`shutter
`
`UV
`power
`supply
`
`O4--3
`
`Fig. 1. Examples of bench scale devices for conducting UV experiments
`
`(Bolton 2000). Although details can be found elsewhere, three
`essential points of nomenclature need to be discussed in relation
`to proper experimentation with UV sources.
`First, although in pastliterature the terms “intensity” and “‘ir-
`radiance”’ have been used,it is important to realize that fluence
`rate is the appropriate term for UV disinfection, since UV can
`impinge on the microorganism from any direction. On the other
`hand, the radiometer that is used with a collimated beam appara-
`tus measuresthe irradiance. Fortunately, in a well designed bench
`setup, the fluence rate and the irradiance are virtually the same.
`The irradiance is defined as the total radiant power incident
`from all upward directions on an infinitesimal element of surface
`of area dA containing the point under consideration divided by
`dA. Irradianceis the appropriate term whena surface (e.g., in UV
`curing)is being irradiated by UV light coming from all directions
`above the surface.
`The fluence rate is defined as the total radiant powerincident
`from all directions onto an infinitesimally small sphere of cross-
`sectional area dA, divided by dA. Fluence rate is the appropriate
`term when, for example, a microorganism is being irradiated by
`UV light emanating from many different directions (e.g.,
`in a
`multilamp array).
`The fluence rate or irradiance should be expressed in the In-
`ternational System of Units Wm*;
`however,
`the
`unit
`mW cm?(= 10 W m“”) isstill quite commonin UVdisinfection
`literature.
`
`Second, the term “UV dose” is utilized almost universally in
`UV disinfection literature. However, for situations in which the
`irradiance or the fluence rate is constant (e.g.,
`in a collimated
`
`beam), multiplication by the exposure time (in seconds) gives the
`corresponding terms radiant exposure or fluence. The term “‘flu-
`ence” has most commonly been called the UV dose; however,
`“dose” is a term that, in other contexts, is used to describe the
`total absorbed energy (e.g., UV dose required to induce sun burn-
`ing on the human skin). In the case of microorganisms, almostall
`of the incidentultraviolet light passes through the organism with
`only a few percent being absorbed. The term fluence is thus more
`appropriate, since it relates to the “incident” UV energy, rather
`than “absorbed” UV energy.
`Third, the apparatus with which many researchers perform UV
`disinfection experiments on the bench scale is named a “‘colli-
`mated beam.” However, this term has a specific meaning in phys-
`ics and optics, in which a light beam has truly parallel rays. This
`is not the case in the present context. Thus, use of the term col-
`limated beam is a misnomerbut has become commonlanguageto
`describe bench scale testing in UV studies. However, its use has
`becomepart of the jargon of UV research and the term should be
`well understood before utilizing it. An alternative term, “quasi-
`parallel beam” has been suggested (Sommeret al. 2001) to better
`describe the type of experimental apparatus utilized by most re-
`searchers.
`
`Background
`
`The use of a bench scale (collimated beam) apparatusin applica-
`tions to UV disinfection wasfirst reported by Qualls and Johnson
`
`210 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / MARCH 2003
`
`7
`
`

`

`(1983). Their original apparatus consisted of low-pressure UV
`lamps housed in a cardboard box with a 2-in.-diam, 72-cm-long
`tube extending from a cut-out hole in the middle of the lamp arc
`length. A reflection correction (4%) was made for light reflected
`from the water surface, and corrected for UV absorption when the
`absorption by the sample was “significant.” Since this first report,
`the design of collimated beam testing equipment has been some-
`what of an art form based on utility and budget. Some designs of
`collimated beam systemspresented in the literature are illustrated
`in Fig. 1. Blatchley (1997) mathematically and experimentally
`evaluated designs of collimated beam systems as well as typical
`building materials. He concluded that the sample to be irradiated
`should be at least 20 cm from the UV lampand that an apparatus
`made of unpainted wood provided surfaces with minimal reflec-
`tions.
`The diversity of approaches to bench scale UV testing is evi-
`dent in the literature. Sommeret al. (1995) compared fluence (UV
`dose)-response curves for B. subtilis among three different labo-
`ratories and apparati. They concluded that to avoid “edge” ef-
`fects, the sample should notbestirred and that only a small vol-
`ume of cell suspension near the center of the dish should be used
`for analysis of the degree of inactivation. They also found that
`corrections should be made for divergence of the UV beam as it
`passes through the cell suspension. This study forms the basis of
`the current German protocol for UV testing. However, otherstud-
`ies utilize stirring during batch experiments and account for non-
`homogeneity of the irradiation field mathematically in the dose
`calculations (e.g., Bukari et al. 1999; Mofidi et al. 2001; Craik
`et al. 2001).
`The appropriate use of a radiometer for measuring fluence rate
`was investigated by Severin and Roessler (1998), who studied
`radiometer readings as a function of the distance from a UV lamp
`versus calculations of the fluence rate. They found that the radi-
`ometer considerably underestimated the fluence rate near the UV
`lamp. These discrepancies illustrate the fact that a radiometer de-
`tector measures irradiance, not fluence rate and that the radiom-
`eter detector has a limited “viewing” angle (Ryer 1997).
`Howlight travels through water containing particles was in-
`vestigated by Qualls et al. (1983), who considered the effect of
`absorbing particles on the fluence (UV dose). They found that
`conventional spectrophotometry considerably overestimated the
`absorbance of the wastewater sample and recommended the use
`of an “opalescent plate’’ method to obtain true absorbancesin the
`case of samples containing suspended solids. This work was later
`corroborated by Scheible et al. (1986) and Linden and Darby
`(1998).
`Proper measurement of UV fluence in a bench apparatus is
`often based on the work by Morowitz (1950), who derived the
`expressions for calculating average UV fluence rate in a com-
`pletely mixed batch reactor based on the Beer-Lambert Law. The
`definition of fluence rate for polychromatic UV sources was dis-
`cussed by Meulemans (1987) and later Linden and Darby (1997)
`* through the concept of “germicidal effectiveness” and they rec-
`ommended a ‘germicidal weighting factor’ to account for the
`observed different response of microorganisms at different wave-
`lengths. Fluence measurementutilizing actinometry as an alterna-
`tive to physical probes, such as a radiometer, has been used by
`numerous researchers (Harris 1987; Kryschi et al. 1988; von
`Sonntag and Schuchmann 1992; Rahn 1997; Linden and Darby
`1997, 1998) and Hoyer et al. (1992) introduced the concept of
`using an actinometer solution to calibrate a radiometer.
`
`Bench Scale Apparatus
`
`The actual design of a bench scale (collimated beam) apparatus
`does not need to be standardized absolutely. There are many de-
`signs that are efficient in the deliverance of UV energy, and modi-
`fications are necessary for each specific application. However, a
`number of basic attributes and guidelines for the design of a
`bench scale UV system must be recognized to ensure comparable
`and reproducible results. Fig.
`1
`is a diagram of two possible
`benchscale testing designs.
`In general, there are a number of components that should be
`considered essential
`in the design and construction of a bench
`scale UV testing device. These include:
`1. Shutter: shutters are a means by which to regulate the time
`of exposure factor in the fluence (UV dose) calculation. Histori-
`cally, shutter design has ranged from manually using a piece of
`cardboard to a pneumatically or electronically driven mechanism
`to block or allow passage of UV energy to a stage. During short
`irradiation times, the accuracy of a shutter system becomes im-
`portant for delivering a repeatable dose.
`2. Window: The lamp enclosure should be thermally stable,
`since the output of many UV lampsis quite temperature sensitive.
`It is often useful to employ a quartz window to assure that no
`change in air drafts occur when a shutter is used. This is impor-
`tant for medium pressure UV lamps that run quite hot (400-
`600°C) as the absolute lamp output and its spectral distribution
`are affected by changes in the temperature of the lamp. The out-
`put of low pressure lamps is also quite sensitive to temperature.
`3. Power supply: it is very important to maintain a constant
`emission from the UV lamp over exposures that may be as long as
`an houror two.If the electrical supply is subject to fluctuations,it
`may be necessary to use a constant voltage power source.
`4, Collimating tube: The objective of a collimated beam appa-
`ratus is to provide a spatially homogeneousirradiation field on a
`given surface area. Therefore, it is important to note that many
`materials (glass, plastic, etc.) highly reflect UV when the inci-
`dence angle is very low. Thusthe inner surface of the collimating
`tube should be “roughened”and painted with a “flat black” paint
`to preventreflection from the sidewalls of the collimating tube. In
`some designs, a collimating tube is not used and the beam is
`defined by apertures placed at a few distances from the lamp to
`create quasi-paralle] radiation. Whatever the design, the end result
`must be a beam that is reasonably uniform over the Petri dish (we
`use the term “Petri dish,” although, in practice, any kind of dish
`or beaker may be used) to be irradiated. Also, the divergence of
`the beam must be small enough, such that the radiometer detector
`can measure the irradiance in the beam accurately (see. ‘““Back-
`ground” section). A method to verify the extent of irradiance
`homogeneity achieved by the collimating device is presented
`below.
`5. Platform: The platform on which the Petri dish and stirring
`motor is placed for UV exposure should be thermally and physi-
`cally stable and easily raised or lowered. The need for easy and
`reproducible vertical adjustmentis that the calibrated plane of the
`radiometer detector must be placed at exactly the same height as
`that of the top of the water during UV exposure for proper irra-
`diance measurement. In some designs, there is a place for the
`radiometer detector at the side of the Petri dish, so that the rela-
`tive output of the UV lamp can be monitored over the exposure
`time.
`6. Stirring: In order to assure equal fluence (UV dose) forall
`microorganisms in the suspension,it is important to maintain ad-
`equate stirring during the UV exposure. The derivation of the
`
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`as National Institute of Standards and Technology, a division of
`the United States Department of Commerce. Through the transfer
`of standards technique, the output of a detector is compared to a
`standard undertightly controlled laboratory conditions. The cali-
`bration factor is computed and programmed into the radiometer,
`allowing direct readings in the optical units desired. Detectors
`should be re-calibrated at least once per year.
`Chemical actinometry can be a useful
`tool for periodically
`checking thecalibration of the detector, If a baseline assessment
`of the irradiance is made using actinometry to calibrate the detec-
`tor, drift of the calibration can be detected through comparison to
`periodic actinometric measurements. Useful actinometers and
`protocols for use have been presented in the literature (Kuhn
`1989; Mark et al. 1990; von Sonntag and Schuchmann 1992;
`Murovet al. 1993; Rahn 1997).
`
`Acceptance Angle
`The radiometer detector head is designed to measure irradiance
`under conditions where the incident UV light is normal (or near
`normal) to the surface of the detector head. The manufacturer of
`the radiometer should specify the acceptance angle, which is de-
`fined as the total angle (sum of the left and right divergence
`angles) of the cone through which the detector can properly mea-
`sure the irradiance. The acceptance angle is usually quite limited
`(10—15°), hence a radiometer can give significant errors if used to
`measure the irradiance near a UV lamp where the beam is diver-
`gent. If the beam is somewhatdivergent, a “diffuser” head should
`be used on the detector to improve the acceptance of off-angle
`light (Ryer 1997).
`
`Spectral Sensitivity of Detector
`The sensitivity of the detector is wavelength dependent, and thus,
`for polychromatic sources, the radiometer reading will not accu-
`rately measure the true irradiance. If the spectral emission of the
`UV lampis known,a “sensor factor’’ correction can be made (see
`below).
`
`Corrections Necessary When Using Low Pressure UV
`Lamp
`
`The radiometer detector only provides a measure of the irradiance
`incident on the water at the center of the beam. Several correc-
`tions are required to obtain the average irradiance in the water.
`This latter value is most important, since this provides an estimate
`of the average fluence rate to which each microorganism is ex-
`posed and is the basis on which the delivered fluence (UV dose)
`to a sample can be calculated.
`
`Reflection Factor
`Whenever a beam of light passes from one medium to another,
`where the refractive index changes, a small f