An apparatus for performing an operation on a fluid material to separate liquid from solid matter within the fluid material The apparatus comprises a belt structure movable along a path. The belt structure comprises a belt portion adapted to be assembled into a movable tubular structure within which at least part of the operation is to be performed. The tubular structure is permeable to liquid for separation of liquid from solid matter within the fluid material. The tubular structure is continuously assembled at one end thereof and continuously disassembled at another end thereof during movement of the belt structure. The path includes a descending portion along which the assembled tubular structure passes, the descending portion being inclined whereby at least some of the solid matter within fluid material in the tubular structure is caused to move downwardly along the descending portion under the influence of gravity to facilitate cleaning of the permeable tubular structure.