An anaerobic purification device comprising: - a reactor tank; - a fluid collector located in the upper section of the reactor tank; - at least one gas-collecting system for, in operation, collecting gas from the fluid contained in the reactor tank, the at least one gas-collecting system arranged at a level below the fluid collector; - at least one gas-liquid separation device; - at least one riser pipe connected to the at least one gas-collecting system and discharging into a gas-liquid riser pipe of the at least one gas-liquid separation device; and - a downer pipe connected to a liquid outlet of the at least one gas-liquid separation device and discharging into the bottom of the reactor tank, wherein the gas-liquid separation device comprises: the gas-liquid riser pipe; a separation pipe attached to the gas-liquid riser pipe, the separation pipe defining an angle with a plane perpendicular to the gas-liquid riser pipe between -45 degrees and +45 degrees, the separation pipe configured to, in operation, receive fluid from the gas-liquid riser pipe; at least one pipe gas outlet located, when in operation, in a surface along the separation pipe facing away from the ground, the at least one pipe gas outlet configured to, in operation, lead at least a portion of the gas in the separation pipe outside the gas-liquid separation device; a hydraulic cyclone attached to the separation pipe, the hydraulic cyclone configured to, in operation, receive fluid from the separation pipe; at least one cyclone gas outlet configured to, in operation, lead the gas entering the hydraulic cyclone outside the hydraulic cyclone;; and the liquid outlet configured to, in operation, guide degassed fluid outside the hydraulic cyclone, wherein the gas-liquid separation device and the reactor tank are assembled inside one and the same closed structure. - a reactor tank; - a fluid collector located in the upper section of the reactor tank; - at least one gas-collecting system for, in operation, collecting gas from the fluid contained in the reactor tank, the at least one gas-collecting system arranged at a level below the fluid collector; - at least one gas-liquid separation device; - at least one riser pipe connected to the at least one gas-collecting system and discharging into a gas-liquid riser pipe of the at least one gas-liquid separation device; and - a downer pipe connected to a liquid outlet of the at least one gas-liquid separation device and discharging into the bottom of the reactor tank, wherein the gas-liquid separation device comprises: the gas-liquid riser pipe; a separation pipe attached to the gas-liquid riser pipe, the separation pipe defining an angle with a plane perpendicular to the gas-liquid riser pipe between -45 degrees and +45 degrees, the separation pipe configured to, in operation, receive fluid from the gas-liquid riser pipe; at least one pipe gas outlet located, when in operation, in a surface along the separation pipe facing away from the ground, the at least one pipe gas outlet configured to, in operation, lead at least a portion of the gas in the separation pipe outside the gas-liquid separation device; a hydraulic cyclone attached to the separation pipe, the hydraulic cyclone configured to, in operation, receive fluid from the separation pipe; at least one cyclone gas outlet configured to, in operation, lead the gas entering the hydraulic cyclone outside the hydraulic cyclone;; and the liquid outlet configured to, in operation, guide degassed fluid outside the hydraulic cyclone, wherein the gas-liquid separation device and the reactor tank are assembled inside one and the same closed structure. the gas-liquid riser pipe; a separation pipe attached to the gas-liquid riser pipe, the separation pipe defining an angle with a plane perpendicular to the gas-liquid riser pipe between -45 degrees and +45 degrees, the separation pipe configured to, in operation, receive fluid from the gas-liquid riser pipe; at least one pipe gas outlet located, when in operation, in a surface along the separation pipe facing away from the ground, the at least one pipe gas outlet configured to, in operation, lead at least a portion of the gas in the separation pipe outside the gas-liquid separation device; a hydraulic cyclone attached to the separation pipe, the hydraulic cyclone configured to, in operation, receive fluid from the separation pipe; at least one cyclone gas outlet configured to, in operation, lead the gas entering the hydraulic cyclone outside the hydraulic cyclone;; and the liquid outlet configured to, in operation, guide degassed fluid outside the hydraulic cyclone, wherein the gas-liquid separation device and the reactor tank are assembled inside one and the same closed structure.