Effluent purifying plant having a centralised structure, comprising, on a vertical axis, - a homogenisation vessel which is to receive the effluent supplemented with a coagulating or flocculating agent, - a clarification vessel beneath the homogenisation vessel, - a flocculation tank comprising a tapered upper portion extended by a lower portion, which tank is situated inside the clarification vessel and in which sludge flakes are to form, - a descending pipe arranged in the axis and connecting the homogenisation vessel to the lower portion of the flocculation tank through which the effluent is to pass from bottom to top and is to be clarified in contact with the sludge flakes, - the pipe comprising an upper tube, one end of which opens into the homogenisation vessel and the other end of which opens into a lower tube having a larger diameter than that of the upper tube, the lower end thereof opening into the lower portion of the flocculation tank, - the lower portion of the clarification vessel having a tapered portion forming the sludge vessel which is to receive the sludge which is to overflow from the flocculation tank, - an annular vessel which surrounds the clarification vessel and is to receive the filtered water by overflow from the clarification vessel, wherein - the homogenisation vessel comprises a tangential intake for the effluent, characterised in that - the inlet and the outlet of the upper tube comprise a helical guide (613, 614) for setting the effluent in rotation, - the lower tube having a larger diameter than that of the upper tube terminates in an outlet in the form of an inverted cone of reduced cross-section, - the lower portion of the flocculation tank is tapered, with an apex angle smaller than that of its upper portion, the lower portion being parallel to the cone of the outlet of the lower tube, and - at least one anti-backflow cone (7, 8) surrounds the pipe in the clarification vessel. - a homogenisation vessel which is to receive the effluent supplemented with a coagulating or flocculating agent, - a clarification vessel beneath the homogenisation vessel, - a flocculation tank comprising a tapered upper portion extended by a lower portion, which tank is situated inside the clarification vessel and in which sludge flakes are to form, - a descending pipe arranged in the axis and connecting the homogenisation vessel to the lower portion of the flocculation tank through which the effluent is to pass from bottom to top and is to be clarified in contact with the sludge flakes, - the pipe comprising an upper tube, one end of which opens into the homogenisation vessel and the other end of which opens into a lower tube having a larger diameter than that of the upper tube, the lower end thereof opening into the lower portion of the flocculation tank, - the lower portion of the clarification vessel having a tapered portion forming the sludge vessel which is to receive the sludge which is to overflow from the flocculation tank, - an annular vessel which surrounds the clarification vessel and is to receive the filtered water by overflow from the clarification vessel, wherein - the homogenisation vessel comprises a tangential intake for the effluent, characterised in that - the inlet and the outlet of the upper tube comprise a helical guide (613, 614) for setting the effluent in rotation, - the lower tube having a larger diameter than that of the upper tube terminates in an outlet in the form of an inverted cone of reduced cross-section, - the lower portion of the flocculation tank is tapered, with an apex angle smaller than that of its upper portion, the lower portion being parallel to the cone of the outlet of the lower tube, and - at least one anti-backflow cone (7, 8) surrounds the pipe in the clarification vessel.