A fluidised bed filter comprises a drying pan, and a filter assembly having a membrane over the pan, and a plurality of openings in the membrane. A filter bag is secured to each aperture by means of a clamping ring attached to the rim of the respective aperture, and cooperating with a clamping band within the cuff of the bag. The material of the bag is clamped between the ring and the band. The ring is covered in a cloth strip, which is folded over to produce a flap, which is stitched between the layers of a double fabric forming the membrane. The band is retained in a tunnel formed by a fold in a double fabric forming the filter bag by stitching.