Thickening is a mechanical method for creating more concentrated solid portions faster than simple gravity settling. Look in the reactor section for what a thickener can look like, schematically. Basically, all it does is remove the very bottom sediment from the batch, as well as removing the clear fluid from the very top, and thus there will be a continual creation of new packed sediment as well as removal of clear liquid. Eventually, if you are running this as a batch process, all that will be left is that sludge.