NOVELTY - Biogas system comprises a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber to be filled with a digestive mixture, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber.    USE - The biogas system is useful for maintaining methane gas pressure substantially equal, and for industrial and commercial applications.    ADVANTAGE - The biogas system is highly reliable.    DETAILED DESCRIPTION - Biogas system comprises a cylindrical digestion vessel having a bottom wall, a first circumferential wall and a top opening to define a digestion chamber to be filled with a digestive mixture, and a cover having a top side, a second circumferential wall and a bottom opening to define a gas storage chamber, where the cover is inserted in the digestion vessel via the top opening with its bottom opening directed towards the bottom wall to capture gas that is released inside the digestion chamber, where the first circumferential wall comprises first cylindrical wall sections that merge into first elongated guides, which first elongated guides extend in the longitudinal direction of the first circumferential wall and project inwardly with respect to the first cylindrical wall sections, and where the second circumferential wall comprises second cylindrical wall sections that merge into second elongated guides, which second elongated guides extend in the longitudinal direction of the first circumferential wall and are recessed inwardly with respect to the second cylindrical wall sections to provide an elongated insertion space that is accessible from outside, where the first elongated guides are at least partly inserted into the second elongated guides. An INDEPENDENT CLAIM is a method for manufacturing the biogas system by means of rotation molding of a thermoplastic resin, comprising forming a hollow body comprising the digestion vessel and the cover inside the casting mold, releasing the hollow body from the casting mold, and making at least one cut to separate the cover from the digestion vessel to enable the cover to be inserted in the digestion vessel.    DESCRIPTION OF DRAWING(S) - The drawing shows a longitudinal section of a biogas system.    Biogas system (1)    Thin walled digestion vessel (10)    Bottom wall (12)    First guides (21)    Straight end wall (22)    Inlet pipe (30)    Upper part (31)    Lower part (32)    Inlet funnel (33)    Outlet pipe (35)    Funnel shaped discharge extension (36)    Lowered discharge edge (37)    Tube connections (38)    Thin walled connection (40)    Gas storage chamber (47)    Hose connection (48)    Hose (49)