The invention concerns a plant comprising, on a vertical axis, a homogenizing vessel (1) followed by a settling vessel (2) and a flocculating vat (3). The homogenizing vessel (1) is fed by tangential intake (92) of the effluent and the input (611) and the output (612) of the upper tube (61) comprise a helical guide (613, 614) for driving the effluent in rotation. The lower tube (62) of larger diameter than that of the upper tube (61) is terminated by an outlet in the form of an upside-down cone (622) with reduced cross-section (621). The lower portion (31) of the flocculation vat (3) is truncated, with an apex angle, smaller than that of the upper portion (32), said lower portion (31) being parallel to the cone of the outlet of the lower tube (62), and anti-backflow cones (7, 8) surround the conduit (6) in the settling vessel (2).