A fluid treatment device (10, 100) and method comprising;- a container (20, 120) comprising an inlet (11, 111) and an outlet (12, 112), wherein the container (20, 120) is configured to contain a fluid comprising organisms, and wherein the fluid has a flow direction from the inlet to the outlet,- a first treatment element (1, 101) inside the container (20, 120), wherein the first treatment element (1, 101) comprises a treatment material and the treatment device is configured to move the treatment material substantially perpendicular to the flow direction.
