An Ultraviolet (UV) disinfection system transfers energy from a mercury arc lamp to an organism's genetic material. When UV radiation penetrates the cell wall of an organism, it destroys the cell's ability to reproduce. UV radiation, generated by an electrical discharge through mercury vapor, penetrates the genetic material of microorganisms and retards their ability to reproduce. The effectiveness of a UV disinfection system depends on the characteristics of the wastewater, the intensity of UV radiation, the amount of time the microorganisms are exposed to the radiation, and the reactor configuration. For any one treatment plant, disinfection success is directly related to the concentration of colloidal and particulate constituents in the wastewater.

The main components of a UV disinfection system are mercury arc lamps, a reactor, and ballasts. The source of UV radiation is either the low-pressure or medium-pressure mercury arc lamp with low or high intensities.

Physical methods include ultraviolet rays (UV), a more than 100 years old technique. The wastewater must flow through a chamber where it is exposed to UV light at a wavelength of 200 to 310 nm. The inactivation of the pathogens takes place due to the absorption of UV via proteins and the cells are damaged irreversibly.

The advantages (Cornel & Weber, 2004), no unwanted by-products, good efficiency, the technology is easy to combine with other treatment options.

As disadvantages are known: Lack of depository effect, possible regrowth of pathogens, the wastewater must be free of suspended solids, the fouling on protection pipes an lack of practicable dosage measurement.

A very important issue with UV treatment is the fact that the wastewater must be very well treated and nearly free of turbidity and suspended solids which can be realised by high efficient biological treatment, best followed by a sand filtration step.