the invention concerns a plant comprising , on a vertical axis , a homogenizing vessel followed by a settling vessel and a flocculating vat . the homogenizing vessel is fed by tangential intake of the effluent and the input and the output of the upper tube comprise a helical guide for driving the effluent in rotation . the lower tube of larger diameter than that of the upper tube is terminated by an outlet in the form of an upside - down cone with reduced cross - section . the lower portion of the flocculation vat is truncated , with an apex angle , smaller than that of the upper portion , said lower portion being parallel to the cone of the outlet of the lower tube , and anti - backflow cones surround the conduit in the settling vessel .