The proposed model of solid-state digestion assumes the development of a distinct zone of depleted waste around each viable seed particle. Biodegradation is envisaged to occur chiefly in thin shells at the interface between depleted and raw waste, at a rate proportional to the interfacial area. These shells are estimated to expand at a constant 5 cm/year. The reaction thus accelerates following a square law until neighbouring shells meet. Deceleration is then rapid as the shells merge. The proposed model agrees with available lysimeter data; first-order decay kinetics do not.