UV rays are harmful to organisms, and we are reminded of this every summer at the latest. But what exactly is that? UV rays or ultraviolet rays have a shorter wavelength (30-400 nm) than the light that humans perceive, so they are not visible to us, but are very dangerous for skin and eyes. In water treatment, this electromagnetic radiation is used specifically to neutralize organic compounds in the water. This is because UV rays trigger a photochemical reaction which leads to the oxidation of matter (release of electrons, absorption of oxygen), ideally to CO2. During such UV OXIDATION, wavelengths in the range of 185 to 254 nm and special low-pressure mercury lamps are used.