Abstract: The device includes a vertical sheath (11) extending in a part of the height of the tank (1), a funnel (14) located around the upper part of the sheath (11), a vertical shaft (19) driving the propeller (20) in rotation, anti-vortex blades (24) located inside the sheath (11), below the propeller (20), a pipe (33) for injecting a gas under pressure in the sheath (11), below the anti-vortex blades (24), and uniquely static stirring obstacles (31, 32) for stirring the liquid (3) by turbulence inside the sheath (11), these obstacles being located below the anti-vortex blades (24). The invention is applicable for example to the treatment of fatty waste products extracted from town sewage effluents or agricultural food industries.