Impacts of Reflection on UV Dose Delivery – Collimated Beam Apparatus and UV Reactors

Reflected UV light can impact the UV dose delivered by collimated beam apparatus and by UV reactors. With the collimated beam, reflected UV light from the internal walls of the glass vessel containing the microbial sample may increase the UV dose delivered to the sample by as much as 10 percent. Because the standardized UV dose calculation used for the collimated beam apparatus does not account for reflected UV light, the calculation underestimates UV dose causing an error in the measured UV dose response of microbes. The error is eliminated by minimizing the height of the glass vessel above the microbial sample. Reflected UV light may also increase UV dose delivered by UV reactors by as much as 25 percent. If reflection occurs during validation but not at the water treatment plant, the UV dose algorithm developed during validation will over predict UV dose delivered by the reactor. The issue may be addressed by validating the UV reactor with blackened or bead blasted surfaces or by application of a safety factor determined using UV dose models based on CFD.