A column comprising a first end (2a) provided with a column packing means and a second end (2b) provided with a fluid collection means for collecting fluid from over some or all of the internal cross-section of the column, wherein said first and second ends (2a, 2b) are held apart by a column wall. The second end (2b) is provided with blocking means (52, 72, 82, 92) movable between a blocking position where said blocking means (52, 72, 82, 92) substantially obstructs the collection of fluid from a predetermined portion (CR, CR1, CR2) of the cross-section of the column and an non-blocking position where it does not substantially obstruct the collection of fluid from the predetermined portion (CR, CR1, CR2) of the cross-section of the column. This allows the resistance to flow through the collection means to be modified to control where the bed media is deposited over the cross-section of the column.