The invention is a decanter centrifuge for separating a first substance and a second substance with different densities, comprising a bowl rotating in use around a horizontal axis of rotation in a direction of rotation, said axis of rotation extending in a longitudinal direction of said bowl, a radial direction extending perpendicular to the longitudinal direction, a base plate provided at one longitudinal end of said bowl, said base plate having an internal side and an external side, an outlet opening provided in said base plate for discharge of one of said substances. A slide valve body is adapted for covering an adjustable part of the outlet opening to delimit an effective area of the outlet opening.