NOVELTY - The reactor comprises: a gas-tight container (2) including a bottom wall and rigid side walls, where an interior of the container is made of a complex containing a wooden frame (7) and a plastic sheet (8) covering the frame to protect the frame from methanation of gases and to ensure sealing of the container; a connection (3) for collecting biogas; and an installation (9) for draining a seepage liquor and recirculating the drained seepage liquor. A bottom of the container is constituted by a concrete basin to contain a guard seepage liquid. The basin forms the bottom wall of the container.    USE - The reactor is useful for methanizing biomass (claimed).    ADVANTAGE - The reactor is cost effective and can be rapidly erected in on-site.    DETAILED DESCRIPTION - The reactor comprises: a gas-tight container (2) including a bottom wall and rigid side walls, where an interior of the container is made of a complex containing a wooden frame (7) and a plastic sheet (8) covering the wooden frame to protect the wooden frame from methanation of gases and to ensure sealing of the container; a connection (3) for collecting biogas; and an installation (9) for draining a seepage liquor and recirculating the drained seepage liquor. A bottom of the container is constituted by a concrete basin to contain a guard seepage liquid. The basin forms the bottom wall of the container. The side walls are arranged partially over the height of the container, and extend from the basin. The reactor further comprises an installation for heating the biomass inside the container. The wooden frame includes wooden support beams, and wooden boards attached with the support beams and joined between the support beams by its longitudinal edges. The support beams are vertical posts. Lower parts of the wooden frame and the plastic sheet are embedded in the basin. The container is built in a shape of a garage of a vehicle, where an access to the container is allowed by a watertight door conceived for the admission of the biomass. The door is made of the complex. The plastic sheet covers the edges of the wooden frame of the door, is elastically deformed by inflation to ensure the sealing of the fluid between the door and the wooden frame. A circuit of air inlet is internally arranged along peripheral edges of the wooden frame of the door, and is coupled with a compressor of air to allow the inflation. The door comprises a peripheral shoulder intended to simultaneously press on a portion and an interior wall of the wooden frame, where the door opens inside the container. An upper wall of the container includes a flexible cover fixed in a watertight manner via each of its side by an inflatable cover roll. The inflatable roll is longitudinally introduced into a blocking groove formed on an upper edge of an upper beam of the wooden frame. The groove is closely protected by the plastic sheet.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a reactor.    Gas-tight container (2)    Connection for collecting biogas (3)    Wooden frame (7)    Plastic sheet (8)    Installation for draining a seepage liquor and recirculating the drained seepage liquor. (9)