NOVELTY - A nitrous-acid-type nitrification process of introducing organic natural water containing ammonia nitrogen into nitrite-formation tank and oxidizing portion of ammonia nitrogen to nitrite nitrogen by action of ammonium oxidizing bacteria, and anaerobic ammonia oxidation process of introducing effluent from nitrite-formation tank in to anaerobic ammonia oxidation reaction tank, making nitrite nitrogen into electron acceptor and generating nitrogen gas by effect of autotrophic microorganism is carried out. Thus, denitrifying method of organic natural water is performed.    USE - Denitrifying method of organic natural water.    ADVANTAGE - The method is carried out precisely with improved denitrifying rate and reduced usage amount of oxygen, and without generation of ammonia nitrogen residue in treated water.    DETAILED DESCRIPTION - A nitrous-acid-type nitrification process of introducing organic natural water containing ammonia nitrogen into a nitrite-formation tank and oxidizing a portion of ammonia nitrogen to nitrite nitrogen by the action of ammonium oxidizing bacteria, and anaerobic ammonia oxidation process of introducing effluent from nitrite-formation tank in to anaerobic ammonia oxidation reaction tank (3), making nitrite nitrogen into electron acceptor and generating nitrogen gas by the effect of autotrophic microorganism by making ammonia nitrogen in to electron donor is carried out. Thus, denitrifying method of organic natural water is performed. The total-nitrogen concentration and ammonia-nitrogen concentration in the effluent from nitrite-formation tank is measured. The nitrous-acid-type nitrification process is controlled so that the ratio of concentration of nitrite nitrogen computed based on the measured value and ammonia nitrogen becomes target value.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of apparatus for denitrifying method of organic natural water. (Drawing includes non-English language text)    Sub-nitrification tank (1)    Anaerobic ammonia oxidation reaction tank (3)    Substrate-nitrification tank effluent (4)    Support (5)    Aeration apparatus (6)    Control apparatus (7)