UV Transmittance
The most important factor describing the water quality in UV-C treatment applications is transmittance (UVT). The UV transmittance is affected by dissolved organic matter (DOC) and suspended solids (TSS) which absorb or scatter the germicidal UV radiation.
How to measure UV Transmittance
The transmittance value of water indicates the permeability of UV-C radiation through water. The UVT value is measured at a wavelength of 254 nm (which is the dominant emission line of a low pressure UV lamp and is close to the maximum absorption wavelength of DNA and RNA) at a specified water pathlength. The measurement is a relative measurement at which the water sample is compared with pure water which is used to calibrate the instrument. The result is expressed in percent [%].
The path length of the UV radiation through the water during the transmittance measurement is determined by the quartz cuvette. A 10 mm cuvette is commonly used but also cuvettes of 40, 50 and 100 mm are being used. With the transmittance value publication also the path length must be specified. For instance such as: UVT254 nm @ 10 mm = 95 %. The effect of the transmittance value of the water can be very strong. The graph above illustrates this.
Impact of UV transmittance in practice
For standard drinking water with a UVT of 95 % – 90 % the graph shows that after 5 cm of water resp. 23 % – 41 % energy is lost – or in other words, disinfection power lost. In case of a waste water application with a UVT of 60 % – 40 % the reduction of disinfection power after 5 cm of water will be resp. 92 % – 99 %. This means that the water quality, UV transmittance, is a very important factor in the design of a UV treatment system and for the selection of the right UV system for your application.
The quality of the water, the permeability (transmittance) for UV-C radiation can also be expressed as “absorption, Abs”, “spectral absorption coefficient, SAC” or “spectral attenuation coefficient, SAtC”. For the correct dimensioning of a UV system, the SAtC value must be used because the SAC value is measured with filtered water. (SAtC = SAC + scattering coefficient). There are mathematical relations between transmittance, absorbance and SatC.
The design of the treatment chamber (shape, size, number of lamps) determines for which type of water quality (read UV-C transmittance) the system can be used.