254 nm - how uv works

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Ultraviolet disinfection technology uses UVC light to target and disable disease causing microorganisms (pathogens).  Over 100 years ago, scientists discovered that if you exposed pathogens to UV light, their reproduction was limited.  The UV light used, resided in the UVC range of the light spectrum.  Specifically, they discovered that light in the 254 nanometer (nm) range was the most effective.  

It was found to be "effective" because quartz material happens to be, by its very composition, excellent at allowing that wavelength.   The most effective germicidal wavelength occurs at a peak of 260 to 265 nm.   254 nm is therefore used to describe quartz based lamp technologies.

When water pathogens are exposed to UV light, their cells become damaged and this damage inhibits reproduction.  The UV light, produced by a special lamp damages the cell’s DNA and RNA and once damaged, they are unable to replicate.  

This physical process renders them harmless.  The amount of damage is a result of the intensity of the UVC output multiplied by the time the water is exposed to the light.  The applied dosage is commonly referred to as microwatts and is often expressed as mJ/cm2.  Doses of 40,000 microwatts (40 mJ) are accepted for water disinfection and doses of 30,000 (30 mJ) are typical for domestic wastewater.