With the ultraviolet device, disinfection is achieved by the formation an electric arc in a glass tube containing a special gas, the evaporation and ionization of mercury by the heat produced, and the resultant ultraviolet beams. The UV beam provides 99,8% sterilization by penetrating the cell walls of the microorganisms in the water and breaking their DNA. The wave length range that allows for maximum disinfection effect in the ray spectrum is between 240 – 280 nm. This range is within the spectrum of rays known as UV-C. The most effective disinfection is achieved with rays at 254 nm wave length. Lamps used in UV units contain mercury vapour and have low or medium pressure. When a certain amount of voltage is applied at the anode-cathode ends of the lamp, mercury vapour starts to radiate by the resulting ignition. Main advantages of UV Disinfection: