Apparatus for opaque fluid disinfection by means of ultraviolet emission. The apparatus comprises a reactor body in which a cylindrical reactor chamber is defined that is closed at the longitudinal ends by a first and a second terminal, respectively provided with an inlet port from which a fluid to be treated enters and with an outlet port from which the treated fluid exits from the reactor chamber, at least one ultraviolet light source housed in a tube longitudinally extended inside the reactor chamber. The apparatus comprises, inside the reactor chamber, a tube holder baffle comprising a sealing system in the area adjacent to the internal walls of the reactor body and a tubular jacket supported by said tube holder baffle, said jacket being coaxial with the tube and having larger size than the latter in a manner so as to define an annular interspace between the tube and the jacket such to allow the passage of the fluid to be treated through the tube holder baffle.
