The invention relates to a drainage device for a filter unit for microbiological testing of liquids, said drainage device including a mechanical support (10), which is adapted to receive a filter unit (16), and an suction pump (22) connected to said mechanical support to aspirate a liquid substance contained in said filter unit. The device comprises: a weight sensor (18) on which said mechanical support is mounted; a user interface (26) for entering data relating to drainage and/or said liquid substance; and a control unit (24) connected to said suction pump (22), the weight sensor (18) and said user interface (26), said control unit (24) being adapted to determine, from said data, at least a first weight corresponding to a first representative signal; and, when said weight sensor (18) supplies a signal corresponding to said first signal, said control unit (24) starts operation of said suction pump (22) so as to aspirate said liquid substance contained in said filter unit (16).