These U.V. Systems units have been designed to disinfect potable or process water. The germicidal effect of the UV light inactivates most micro-organisms such as bacteria, viruses and parasites. UV is known to be particularly efficient to inactivate Crypto sporidium Parvum and Guardia Lambkin. The medium pressure lamps have an action on nucleic acid and proteins for micro-organisms inactivation. The UV dose (UV intensity x contact time) defines the treatment efficiency which is provided by the unit. The effective dose applied depends on the UV transmittance of water to be treated as well as the proper hydraulic design of the unit.