A method of operating a centrifugal separator is provided. The method includes supplying a liquid mixture to be separated to the inlet of the centrifugal separator, separating the liquid mixture into at least one separated liquid component and a separated sludge component, discharging at least one separated liquid component from at least one liquid outlet of the centrifuge rotor, removing gas from a space surrounding a centrifuge rotor of the separator, to obtain a sub-atmospheric pressure in the space, and discharging a separated sludge component from at least one sludge outlet arranged at a periphery of the centrifuge rotor to the space which is delimited by a frame, wherein the discharging is performed when the sub-atmospheric pressure in the space surrounding the centrifuge rotor is within a preset pressure interval ΔP.