A system and method for continuous decomposition of organic material to produce biogas and compost in which successive reactor zones in a primary reactor are used in turn to receive organic material which is decomposed in situ anaerobically followed by aerobic decomposition and excavation of raw compost from the same reactor zone where it is received into the organic treatment facility. Organic rich liquor recovered during anaerobic decomposition is further decomposed in a secondary reactor from which spent liquor is returned to the primary reactor while biogas harnessed from both the primary and secondary reactors is treated, and optionally used to fuel the operation of the organic treatment facility or for sale as fuel or electricity.