NOVELTY - A denitrification/methane fermentation apparatus, is new. The apparatus (1) has a transfer paths (9a,9b) for transferring supernatants from an acid fermenter (6) and a solid-liquid separator (8) into a water-treatment assembly (3). The process liquid from the water-treatment assembly is circulated to a equalizing tank (4), solubilization tank (5) and the acid fermenter through the respective circulation paths (14c-14e).    USE - The apparatus is useful for processing organic waste such as agricultural waste material, livestock waste material and foodstuffs waste material with biogas production (claimed).    ADVANTAGE - The process liquid is circulated from water-treatment assembly to equalizing tank, solubilized tank and acid fermenter, reducing inhibition of the methane-fermentation microbes by ammoniacal nitrogen, and increasing the efficiency of methane fermentation.    DETAILED DESCRIPTION - A denitrification/methane fermentation apparatus (1) has a transfer paths (9a,9b) for transferring supernatant portions from an acid fermenter (6) and a solid-liquid separator (8) into a water-treatment assembly (3). The process liquid obtained from the water-treatment assembly is circulated to a equalizing tank (4), solubilization tank (5) and the acid fermenter through the respective circulation paths (14c-14e). The biogas production tank (2) is positioned above the water-treatment assembly, and perform processing at more than 50 degrees C. The carbon/nitrogen ratio of the material being processed is controlled to maintain the optimum ratio for methane production. The water-treatment assembly has a nitrification tank for carrying out nitrification reaction in aerobic conditions, and a denitrification tank for carrying out denitrification process in anaerobic conditions.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the denitrification and methane-fermentation apparatus. (Drawing includes non-English language text).    denitrification and methane-fermentation apparatus (1)    biogas production tank (2)    water-treatment assembly (3)    equalizing tank (4)    solubilization tank (5)    acid fermenter (6)    solid-liquid separator (8)    transfer paths (9a,9b)    circulation paths (14c-14e)