UV light is a most effective killer of bacteria and viruses when used at a wavelength of 253.7nm (nano-meters). To guarantee total water purity, the dosage of UV light
must be equal to 40,000 uW-sec/cm2 (40 mJ/cm2).
This dosage factor is a calculation of the intensity of light and the contact time with the treated water and is the fundamental requirement for claiming Class A purification.
Class A systems are those designed to disinfect and/or remove microorganisms, including bacteria and viruses, from contaminated water to a safe level.
But not water with an obvious contamination source. The systems are intended to be installed on visually clear water (not coloured, cloudy, or turbid).