NOVELTY - A DNA fragment of nitrous acid reductase (NiR K) gene, comprising sequence chosen from any one of 16 fully defined sequences of 515 base pairs (S1) as given in the specification, where (S1) derived from NiR K gene of uncultured bacterium clone U13, M17, M27 or M58 close-related bacteria, Mesorhizobium sp. 4FB11 close-related bacteria or Pseudomonas mendocina close-related bacteria, is new.    USE - For detecting microorganisms associated with nitrogen removal, where the microorganisms are used in water treatment process.    ADVANTAGE - Enables efficient detection of microorganisms associated with nitrogen removal, in wastewater.    DETAILED DESCRIPTION - A DNA fragment of nitrous acid reductase (NiR K) gene, comprises (a) a sequence chosen from any one of 16 fully defined sequences of 515 base pairs (S1) as given in the specification, where (S1) is derived from NiR K gene of uncultured bacterium clone U13, M17, M27 or M58 close-related bacteria, Mesorhizobium sp. 4FB11 close-related bacteria, Alcaligenes sp. STC1 close-related bacteria, A.xylosoxidans close-related bacteria or Pseudomonas mendocina close-related bacteria, (b) a fully defined sequence of 514 base pairs (S2) as given in the specification, where (S2) is derived from NiR K gene of Mesorhizobium sp. 4FB11 close-related bacteria, (c) a fully defined sequence of 128 base pairs (S3) as given in the specification, where (S3) is derived from NiR K gene of Paracoccus denitrificans Pd1222 close-related bacteria, (d) a fully defined sequence of 164 base pairs (S4) as given in the specification, where (S4) is derived from NiR K gene of uncultured bacterium clone HNIS-6 close-related bacteria, or (e) a sequence chosen from any one of 19 fully defined sequences of 165 base pairs (S5) as given in the specification, where (S5) is derived from NiR K gene of Paracoccus denitrificans Pd1222 close-related bacteria, Thauera aromatica AR1 strain close-related bacteria, uncultured bacterium clone HNIS-6, nirSAK4, nirSAK2 or ANIS-76 close-related bacteria, A.eutrophus close-related bacteria, Azoarcus toluvorans close-related bacteria, Paracoccus pantotrophus close-related bacteria, Thauera linaloolentis close-related bacteria, Thauera terpenica 58Eu strain close-related bacteria or Pseudomonas aeruginosa Mi11 strain close-related bacteria, where the above mentioned bacteria are obtained from activated sludge useful in activated sludge process for carrying out nitrogen reduction in water treatment plant.    INDEPENDENT CLAIMS are included for the following:    (1) microorganisms such as uncultured bacterium clone U13, M17, M27 or M58 close-related bacteria, Mesorhizobium sp. 4FB11 close-related bacteria, Alcaligenes sp. STC1 close-related bacteria, A.xylosoxidans close-related bacteria, Pseudomonas mendocina close-related bacteria, Paracoccus denitrificans Pd1222 close-related bacteria, Thauera aromatica AR1 strain close-related bacteria, uncultured bacterium clone HNIS-6, nirSAK4, nirSAK2 or ANIS-76 close-related bacteria, A.eutrophus close-related bacteria, Azoarcus toluvorans close-related bacteria, Paracoccus pantotrophus close-related bacteria, Thauera linaloolentis close-related bacteria, Thauera terpenica 58Eu strain close-related bacteria, or Pseudomonas aeruginosa Mi11 strain close-related bacteria, having the sequences as mentioned above, obtained from activated sludge, useful in digestive-fluid circular-type denitrifying activated sludge process for carrying out nitrogen reduction in water treatment plant;    (2) activated sludge comprising one or more of the microorganisms as mentioned above, used for digestive-fluid circular-type denitrifying activated sludge process for carrying out nitrogen reduction in water treatment plant; and    (3) water treatment method for reducing nitrogen, using the activated sludge.