NOVELTY - Methane fermentation of organic waste uses circulatory system of interconnected fermentation and membrane separation tanks.    USE - For methane fermentation of organic waste including night soil, septic tank sludge, agricultural sludge, sewage sludge, farm animal manure, kitchen refuse and food waste.    ADVANTAGE - The concentration of methane bacteria in the fermentation tank is held at a high value to increase fermentation efficiency. Fermentation inhibition due to volatile fatty acids or ammonia nitrogen is reduced.    DETAILED DESCRIPTION - High-concentration organic waste is introduced to a fermentation tank (1). Fermented sludge from the tank is circulated and is filtered by a membrane (3) in a separation tank (2). The concentration of suspended solids, including methane bacteria, in the fermentation tank is held at a value necessary for the maintenance of the methane fermentation. Excess fermented sludge is withdrawn from the bottom (5) of the fermentation tank.    An INDEPENDENT CLAIM relates to a method for methane fermentation of organic waste which includes loading a complexing agent composed of organic acid into the fermentation tank. A circulatory system is made up of the fermentation tank (1), membrane separation tank (2), sludge supply path (10) and a sludge return path (11). The concentration of suspended solids including methane bacteria in the fermentation tank is held at 1 - 10% while holding the viscosity of the fermented sludge at 200 (mPa.s) or less. Dilution water is supplied to the fermentation tank so that the concentrations of volatile fatty acids and ammonia nitrogen as fermentation inhibiting substances are suppressed to below a specified value. The amount of permeate in the membrane separation unit is increased to correspond with the amount of dilution water so that the fermentation inhibiting substances are removed out of the system along with the permeate. Internal gas generated in the tank is blown into the membrane separation tank from a diffuser (33) below the membrane separation unit causing a cross-flow along the filtration membrane to clean its surfaces. The internal pressure of the membrane separation tank is held at a positive pressure above atmospheric.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic of the apparatus.    fermentation tank (1)    membrane separation tank (2)    membrane separation unit (3)    sludge removal line (5)    sludge supply path (10)    sludge return path (11)    diffuser (33)