Abstract: A first thermally insulated hollow cylinder is mounted for rotation about its longitudinal axis and a second hollow cylinder is mounted within the first cylinder for conjoint rotation therewith. The ends of the cylinders are closed to define a first annular chamber therebetween and a second chamber within the second cylinder. Inlet means for waste is provided communicating with the annular chamber adjacent one axial end thereof and a portion of said second cylinder adjacent the other axial end of said annular chamber is formed as a sieve permitting transfer of material from said annular chamber to the second chamber. Discharge means are provided communicating with the chamber within said second cylinder at the same axial end as the inlet, for the discharge of composted material.