The Importance of Water Absorbance/Transmittance on the Efficiency of Ultraviolet Disinfection Reactors James R. Bolton, Mihaela I. Stefan Bolton Photosciences Inc. 92 Main St., Ayr, ON, Canada N0B 1E0 Robert S. Cushing and Erin Mackey Carollo Engineers 12426 West Explorer Drive, Suite #200, Boise, ID, 83713-1531 Introduction It is generally known that the efficiency of ultraviolet disinfection reactors decreases as the absorbance of the water increases or the percent transmittance decreases; however, little is known about the quantitative impact of this parameter, unless extensive computational fluid dynamics (CFD) calculations are undertaken or biodosimetry tests made. In this paper calculations, using a new Multiple Point Source Summation (MPSS) Model, are presented in which the quantitative effect of water transmittance is assessed for several aspects of the performance of a UV reactor. For UV reactors containing low pressure or low pressure-high output UV lamps, the important parameter is the absorbance or percent transmittance at 254 nm. However, for broadband UV lamps, such as medium pressure or pulsed UV lamps, it is important to measure the full scan of absorbance or transmittance in the germicidal region from 200 to 300 nm. Thus, where necessary, measured the molar absorption coefficient spectra have been collected for a variety of compounds that may be present in, or added to, drinking water. This data will be presented in tabular and graphical formats. The Multiple Point Source Summation (MPSS) Model has proven useful in mapping the fluence rate a distributions and in calculating the volume averaged fluence rate in UV reactors. Recently, Bolton 1 presented a new MPSS model that takes account of the reflection and refraction that occurs at the interface between the quartz sleeve and the water in the UV reactor. This model has been used to calculate several characteristics of the fluence rate and fluence, as a function of the percent trasmittance of the water, in an annular UV reactor with a single UV lamp in the center of a quartz sleeve: 1. Volume averaged fluence rate  multiplication of this parameter by the hydraulic residence time (s) gives the maximum fluence a that the UV reactor is capable of delivering for a given flow rate. It is a maximum value because the calculation assumes perfect radial mixing. a In this paper, we are using the terms and definitions recommended in Ref. 2. Most authors have used the terms irradiance or intensity for the quantity here designated as fluence rate and UV dose for the quantity here designated as fluence. See the Section on Terms and Definitions below.