A sedimenting assembly for sedimenting debris contained in a processed waste liquid in a sedimentation tank includes a plurality of partition plates extending vertically across a direction in which the processed waste liquid flows and disposed at spaced intervals in the direction, thereby defining a zigzag channel through which the processed waste liquid flows horizontally. The zigzag channel ensures a time during which to sediment the debris against a time during which the processed waste liquid flows through the zigzag channel, allowing the debris to settle reliably.