TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The biofilm comprises at least one of an aerobic microorganism chosen from a strain of aerobic microorganism capable of producing cellulases for the decomposition of cellulose or hemicellulose to fermentable sugars, a strain of aerobic microorganism capable of producing amylases for the decomposition of starch to fermentable sugars, and a strain of aerobic microorganisms capable of producing lignin-modifying enzymes for the decomposition of lignin. The method further involves (d) removing the fermentation product from the fermentation compartment, particularly by transporting the fermentation products through the oxygen permeable membrane into the oxygen supply compartment. In the reactor, oxygen is transported from the oxygen supply compartment to the fermentation compartment through the oxygen permeable membrane and an oxygen zone is formed in the fermentation compartment adjacent to the oxygen permeable membrane and the aerobic microorganism of the biofilm are located adjacent to the oxygen permeable membrane in the aerobic zone and an oxygen gradient with decreasing oxygen content with increasing distance from the oxygen permeable membrane is formed within the biofilm located on the oxygen permeable membrane. The biofilm comprises consortium of at least two strains of the aerobic and anaerobic microorganisms. The anaerobic microorganism in the biofilm is located on the oxygen permeable membrane in the bio-film in an oxygen depleted region, preferably anaerobic zone and optionally also in the base material mixture which contains organic feedstock and which is supplied to the fermentation compartment, and the base material mixture is brought in contact with the biofilm. The method involves controlling supply of oxygen through the oxygen permeable membrane, such that the oxygen content within the biofilm or within a zone in the base material mixture located next to the oxygen permeable membrane and containing the biofilm decreases with increasing distance from the oxygen permeable membrane to a level at which anaerobic condition are formed, preferably decreases to approximately zero and the conditions in the base material mixture in the fermentation compartment beyond this zone are anaerobic. The fermentation products are removed from the oxygen supply compartment and separated from a gas or liquid phase e.g. by condensation, adsorption or distillation. The oxygen and/or the fermentation products are transported through the oxygen permeable membrane by solution diffusion process. Preferred Reactor: The reactor comprises a unit for continuously or discontinuously supplying gaseous oxygen or oxygen dissolved in a liquid into the oxygen supply compartment, and a unit for continuously or discontinuously supplying a base material mixture containing organic feedstock into the fermentation compartment. The surface of the oxygen permeable membrane facing the fermentation compartment and thus facing the base material mixture is covered with the biofilm. The oxygen permeable membrane is designed in such a way that the fermentation products are removable from the fermentation compartment to the oxygen supply compartment by transportation through the oxygen permeable membrane and the reactor comprises a unit for removing the fermentation products from the oxygen supply compartment. The oxygen permeable membrane is a tubular structure and the oxygen supply compartment lies outside the tubular oxygen permeable membrane and the fermentation compartment lies in the tube-like space within the tubular oxygen permeable membrane or vice versa. EXAMPLE - No suitable example given.