The present invention generally relates to a system for treating a fluid and specifically to a treatment system (200) configured for selectively activating a first (206) and a second UV light source (104). The aim of the invention is to reduce the effective energy consumption of a system for treating a fluid with UV light. The invention especially concerns to overcome the drawback with mercury light sources, which do not turn on immediately. Only the second UV light source (104) is an UV mercury based light source and the electrical power supply is configured to selectively deactivate the first UV light source (206) based on a predetermined condition based on a warm-up period for the second light source.
