A system and a process for producing methane gas by anaerobic digestion of feed materials containing suitable organic materials. The system includes a plurality of digestion modules arranged primarily in series. Each digestion module is positioned with its slurry inlet port at a slightly lower elevation...http://www.google.es/patents/US4252901?utm_source=gb-gplus-sharePatente US4252901 - System and process for anaerobic digestion
