Exogeneous hydrogen acceptors may be used by baterial systems under anaerobic conditions for the complete removal of carbon from solution without methane fermentation. The metabolic processes involved are completely analogous to aerobic metabolism. When exogeneous hydrogen acceptors are present in mixed culture anaerobic systems, such as anaerobic digestors, the traditional concept of two stage fermentation is an incorrect model. Biological systems are more accurately characterized by the reactions occurring within the systems and the products produced by the systems than by terms such as aerobic or anaerobic.