The present invention relates to a device for cleaning a fluid, wherein the device comprises a vessel, a filter bed with granular filter material, such as sand, wherein the filter bed is provided in the vessel; a transport system arranged to agitate filter material of the filter bed during operation; and a sensor system. The sensor system comprises a contact body that comes in contact with the filter material in the filter bed; and a vibration sensor connected to the contact body for sensing vibrations of said contact body. The vibration sensor is arranged for producing a signal which is dependent on the vibrations detected by the sensor. The contact body comprises the wall of the vessel and/or a rod extending into the filter bed. The invention relates further to a method for determining the degree of movement of a filter bed with granular filter material, wherein a contact body is used which comes into contact with the filter material in the filter bed, and wherein the degree of movement of the filter material is determined by sensing the vibrations caused by the movement of the granular filter material of said contact body.