Abstract: The present invention on relates to a method and a system for conversion of organic waste into biogas, i.e. a methane containing gas, with an improved efficiency and economy. The system comprises a reactor (3) for holding organic waste for production of biogas by digestion and having an output for digested waste, and an anaerobic tank (6) that is connected to the reactor (3) output for anaerobic hydrolysis of the digested waste and having an output for hydrolysed material that is connected to an input of the reactor for adding hydrolysed material to the content of the reactor. The anaerobic hydrolysis process makes the energy content of material that has not been digested in the reactor available for bacterial digestion and thus, the hydrolysed material is fed back into the reactor for further bacterial conversion into biogas.