The control device (5) has a manual-control button (7) which, by means of a lever (23), moves a tie (9) to open the discharge valve and return to the rest position following discharge. To alter the operating mode of the control device (5) to interrupt drainage of the tank in advance, the tie (9), even if already installed on the tank, may be fitted selectively and removably with a member (37) having a semicylindrical wall (39) fittable to the tie (9) and connectable in articulated manner to the lever (23). When pressed with the tie (9) activated, the button (7) engages the member (37) to restore the tie (9) immediately to the rest position.