About Ultraviolet Transmittance (UVT)
The effectiveness of UV is subject to the clarity of the water treated. Suspended solids in the water act as sequestering agents for pathogens, interfering with light penetration.
Specifically, UVT is a measure of the ratio of light entering and exiting water, usually reported for a path length of 1 cm. As UV absorbance increases, UV transmittance decreases:
%UVT=100 x 10-A.
The UVT of water to be treated is rated by %. Most municipal tap water registers at 85 - 95% UVT. Captured rain water has an unpredictable UVT, which is why filtration technology is invariably specified ahead of UV to ensure adequate transmittance.
Additional relevant water quality parameters include:
Turbidity, a measurement of the scattering of light in water, similar to UVT (ideal value: < 5 NTU).
Hardness, which causes mineral deposits on quartz sleeves, applicable to UV technologies in which the lamps are situated within the water stream (ideal value: < 200mg/L CaCo3).
pH, which affects the solubility of metals and thus compromises water clarity (ideal value: 6.0 - 9.0). The ideal value for Suspended Solids is <10 ppm, for UVT is > 85% @ 254 NM.