TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred System: In the biogas system, the digestion vessel has a constant wall thickness over the first cylindrical wall sections and the first guides, or over the first circumferential wall and the bottom wall. The first guides have a semicircular or U-formed cross section. The first circumferential wall comprises a slurry section and a gas capture section above the slurry section, where the first guides extend along the gas capture section and comprise an end wall at its lower end that extends at a distance from the bottom wall. The biogas system comprises an inlet pipe that extends inside one of the first guides and passes through the end wall, where the inlet pipe extends fully inside the outer circumference of the first cylindrical wall sections; an inlet funnel that is connected to the inlet pipe, where the inlet funnel comprises an inlet opening that at least partly extends outside the outer circumference of the first cylindrical wall sections; and an outlet pipe that extends inside one of the first guides and passes through the end wall. The outlet pipe extends fully inside the outer circumference of the first cylindrical wall sections. Also, the biogas system comprises a discharge extension that is connected to the outlet pipe, where the discharge extension comprises a discharge edge that at least partly extends outside the outer circumference of the first cylindrical wall sections. The end wall merges into a spout that extends inside the digestion chamber towards the bottom wall, where the inlet pipe and/or the outlet pipe is inserted into the spout, where the spout at the side that faces the bottom wall comprises a connection section that is provided with inwardly directed crush ribs, where the crush ribs engage the inserted pipe. The inlet pipe extends opposite to the outlet pipe in circumferential direction. The inlet pipe extends deeper into the slurry section that the outlet pipe. The inlet opening is positioned higher that the discharge edge. Moreover, the biogas system comprises a canvas reinforcement sleeve that extends around the first circumferential wall sections of the digestion vessel. The cover has a constant wall thickness over the second cylindrical wall sections and the second guides. The second guides have a semicircular or U-shaped cross sections. The top side comprises substantially straight top wall sections that extend substantially transverse to the center line of the second circumferential wall, where the top wall sections merge into a raised reinforcement cross that merges into the second circumferential wall. The reinforcement cross and the top wall sections define accommodation spaces where a ballast body has been inserted or confined. The ballast body comprises a lower protrusion having a shape that is complementary to the shape of the accommodation space. The ballast body is a hollow body with a closable filling hole so as to be filled with a liquid or sand, where the hollow body has a constant wall thickness. The ballast body comprises or is made of solid cement or concrete. The digestion vessel and the cover are made of a thermoplastic resin, preferably polyethylene. Preferred Method: In manufacturing the biogas system, the casting mold comprises a closed inner space that forms the outer shape of the digestion vessel that on its top is continued into the cover via an additional bridge, where the additional bridge bridges the difference in diameter of the digestion vessel and the cover at the additional bridge, where the method comprises forming a hollow body comprising the digestion vessel, the additional bridge and the cover inside the casting mold, releasing the hollow body from the casting mold, making a first cut to form the bottom opening and to separate the cover from the additional bridge, and making a second cut to form the top opening and to separate the digestion vessel from the additional bridge. The casting mold comprises basic mold parts and insert molds that are inserted between the basic mold parts, where the basic mold parts form the base of the digestion vessel, the continuation of the digestion vessel into the cover and the top side of the cover, and where the insert molds provide additional height to the digestion vessel and the cover.