A biological filter for purification of liquids has primary filtering stages (52a, 52b, 52c) of a particulate material and secondary filtering stages (54, 56, 58) containing a fine-fiber material. The particulate material and fine-fiber material are covered with a biofloc film. The primary filtering stages (52a, 52b, 52c) and the secondary filtering stages (54, 56, 58) form a succession of filtering groups, and liquid is caused to move through the succession of filtering groups. Liquid is aerated during movement through the succession of filtering groups.