Abstract: In an apparatus for making organic fertilizer from waste organic materials by aerobic fermentation, the interior of an inclined slowly rotating digester cylinder is provided with a series of knives which gradually cut and shred the softer components of the raw material and a series of heavy chains located downstream from said knives, which said chains have overlapping loops which crush and grind hard and brittle components of the raw material, all while the material is in process of aerobic fermentation. Materials which resist the comminuting action of both the knives and chains are discharged with the digested organic materials and are then separated therefrom. The apparatus is capable of accepting and treating municipal garbage and refuse without prior separation of unwanted material, and without preliminary grinding of the material prior to its introduction into the digester.