Abstract:Nitrite oxidizing bacteria (NOB) was cultured mixedly in whether nitrification sludge or partial nitrification sludge system, as a result, its characteristics are not so clear to the researchers. In this paper, NOB, which was inoculated by the excess activated sludge from the pilot-scale SBR in the same lab, was enriched with the way of increasing NO2--N concentration gradually, and combining high concentration of Free nitrite acid (FNA) and DO. After 65days, FISH analysis showed that more than 80% of the bacteria population was NOB (Nitrobacter: 80%; Nitrospira: 5%), which demonstrated the successful enrichment of NOB. Furthermore, the activated sludge granulated naturally under the average MLSS concentration, MLVSS/MLSS and SVI of 700mg/L, 0.278and 6mL/g, respectively. The reason about the extremely low SVI was probably that the sludge had high fraction of inorganics, which could be used as crystal nucleus to form the granules. Results of the research indicated that even with the wastewater whose NO2--N concentration was up to 1000mg/L, this sludge could play a good performance with specific nitrifying rate and specific OUR of 131.03mg/(g MLVSS·h) and 169.5mg O2/(g MLVSS·h), respectively.