A system for digesting biodigestible feed that preferably includes the steps of comminuting the feed, introducing feed, an oxygen-containing gas, an accelerant, and bacteria into a digestion zone, the bacteria being suitable for digesting the feed under aerobic, anaerobic, and anoxic conditions. The contents of the digestion zone can be changed from aerobic operation to either anoxic or anaerobic operation, or vice versa, without changing the bacteria in the digestion zone.