An ultra violet steriliser is an electrical device that produces ultra violet energy that is used to destroy microorganisms, without chemicals. Water passes through the steriliser unit, where micro-organisms are exposed to a lethal dose of ultra violet light in the 254 nanometer range. Ultra violet light in this wave length inactivates a wide range of micro-organisms including bacteria, viruses, algae, protozoa, parasites and mold spores.

This inactivation occurs as the ultra violet light scrambles the organism's DNA structure, making reproduction impossible. The intensity of the ultra violet light and the micro-organism's exposure time to the ultra violet light are factors that influence which micro-organisms are inactivated. This is referred to as the "kill dose", which is simply the intensity multiplied by the exposure time.

Intensity is measured in microwatts per square centimetre while the exposure time is measured in seconds. The kill dose is therefore measured in microwatt-seconds per square centimetre (µW- sec/cm²). The relationship between kill % and dose rate for the inactivation of E.Coli is show in the table below.

E.Coli - Relationship Between Dose and Kill

Kill %		Dose (µW- sec/cm²)
90		5.4
99		10.8
99.9		16.2
99.99		21.6
99.999		27

The flow rate of the water through the steriliser is also critical; if the flow rate is too fast, the ultra violet light will not have the exposure time necessary to inactivate the organisms, limiting the effectiveness of the steriliser unit.