Abstract: A waste digestion system including a primary digester and one or more first subsequent digesters for processing non-liquid organic waste material without the addition of external heat using thermophilic aerobic bacteria to produce a stable product, said digesters being configured to contain contents including the waste material, each of said digesters further including mixing means configured to mix the contents of that digester with feed air drawn through the contents during processing in such a way as to maintain essentially aerobic conditions within the digester; such that the waste material is first processed in the primary digester before a proportion of the contents of the primary digester are transferred to at least one first subsequent digester where further processing occurs; if the first subsequent digester completes the processing then it is a final subsequent digester.