The invention relates to a plant that comprises a vat (100) for separating suspended matter in the form of sludge. The cylindrical vat (100) is provided with a bottom (102) and comprises, from the bottom to the top, a sludge deposition area (Z1) having a flocculation area (Z2) provided on the top thereof, which in turns has an effluent outlet area (Z3, Z4) provided on the top. The bottom (102) includes a passageway (103) for the sludge. An effluent inlet (550) for supplying the vat (100) is formed through the wall (101) thereof in the flocculation area (Z2). The effluent circulation device (400) in the vat (100) includes mobile upright partitions (410) that define, together with the wall (101) and the bottom (102) of the vat (100), effluent compartments (CE) opened at the top and the bottom. The partitions (410) do not extend into the upper portion (Z4) of the vat (100) in order to define the outlet area (104) of the purified effluents (SN). A driving unit (420) displaces the partitions (410) at low speed relative to the wall (101) and the bottom (102) of the vat (100).