A screw conveyor for a centrifugal separator comprises a conveyor hub carrying a helical conveyor flight and providing a helical channel between adjacent turns of helical conveyor flight. The screw conveyor has an up-stream end and a down-stream end. At least two partition walls are arranged in a side-by-side relation to divide at least a radial part of a length of the helical channel into three sub-channels arranged in a side-by-side relation to cause a liquid flowing in the helical channel to flow in an up-stream direction towards the up-stream end in an intermediate sub-channel and in an opposite down-stream direction towards the down-stream end in two adjacent sub-channels on either side of the intermediate sub-channel.