NOVELTY - A reactor (4) comprises a reaction chamber (6) feedable with a mixture comprising pre-treated wet waste and adapted to house a biochemical reaction in anaerobic environment. The biochemical reaction transforms the mixture into biogas and into a reject comprising heavy and light fractions. The reactor comprises a removal device (35) having a collection element (36) for light fraction fluidically connectable to a storage station (60); and a mobile conveying element (38) for conveying light fraction towards the collection element.    USE - A reactor for anaerobic production of biogas from pre-treated wet waste.    ADVANTAGE - The reactor allows to automatically and efficiency remove the light fraction of digestate without needing to employ an operator and to contemplate a manhole for allowing such operator to reach the free surface of the wet waste and water mixture. The removal device removes such light fraction of digestate efficiently, continuously and repeatedly in time. The reactor has a simple construction. The need to create a tube specifically dedicated to connecting a basin to the discharge tank is avoided. The reactor limits the entity of the leakage of biogas during the step of conveying the light fraction of the digestate to the discharge tank, thus limiting the danger of sparks and fires.    DETAILED DESCRIPTION - A reactor (4) comprises a reaction chamber (6) feedable with a mixture comprising pre-treated wet waste and adapted to house a biochemical reaction in anaerobic environment. The biochemical reaction transforms the mixture into biogas and into a reject comprising a heavy fraction meant to precipitate in the chamber, and a light fraction meant to float on a free surface (61) of the mixture. The reactor comprises a removal unit (35) accommodated inside the chamber and containing a collection element (36) for light fraction fluidically connectable to a storage station (60) of light fraction; and a mobile conveying element (38) adapted to interact with light fraction at the free surface to convey the light fraction towards the collection element.    DESCRIPTION OF DRAWING(S) - The drawing is a sectional view of the reactor of the anaerobic digestion plant.    Reactor (4)    Reaction chamber (6)    Removal device (35)    Collection element (36)    Mobile conveying element (38)    Storage station (60)    Free surface (61)