UVT is defined as the percentage of incident light that passes through the sample.1 It will be decreased by particulate material and by dissolved compounds that absorb at 254 nm. For our purposes the main thing to be concerned with is dissolved organic compounds. Other absorbing compounds include nitrate, nitrite, bromide, thiosulfate, ferrous iron, chloramines, chlorates, chlorites and ozone.
For purposes of monitoring life support systems all of these compounds are undesirable in excess so a reduced UVT would be undesirable no matter which of these compounds is the cause. In systems using bromine as a disinfectant bromide levels may prevent use of this technique.
UVT or UV Transmittance is the measure of UV energy at a particular wavelength or frequency which is actually transmitted through water from the UV lamp; The higher the UVT, usually expressed as a percentage (%), the more energy is transmitted through the water, and therefore the more effective the deactivation of pathogens. Put another way, if there is low UVT, the amount of UV energy that reaches pathogens in the water may be too low to deactivate them, making the UV system ineffective.
UV light effectiveness as a sterilizer is determined primarily by the combined effects of UV light intensity, the exposure time of the system (defined by the flow rate of the system) and the UVT of the water. The first two variables, flow rate and lamp dosage, are determined by the selection of the correct UV treatment system. The third, UVT, is dependent on the specific water source being treated and it can significantly challenge the effectiveness of the UV disinfection system.
