Autotrophic nitrogen removal discovered in suspended nitritation system.

A lab-scale sequencing batch reactor was operated in batch mode to treat ammonium-rich synthetic wastewater, and minor N loss (about 9%) was observed without seeding anaerobic ammonium oxidation bacteria (AnAOB). Around 4 wk after the dissolved oxygen concentration was decreased (from 0.9+/-0.1 to 0.4+/-0.1 mg L(-1)) and the hydraulic retention time was elevated (from 11.0 to 14.7 h), 80% of total nitrogen removal efficiency and 0.49 kg N m(-3) d(-1) of maximum nitrogen removal capacity were obtained. During a single cycle, a peak of NO2(-) concentration was observed at 60 min, and the pH first increased from 8.02 to 8.17 and then returned to 8.04. Fluorescence in situ hybridization analysis revealed that the aerobic ammonium-oxidizing genus Nitrosomonas dominated the community, while AnAOB and very small amounts of Nitrospira spp. were also detected. The ratio of nitrate produced to ammonia consumed (0.10+/-0.02) was lower than that achieved with the completely autotrophic nitrogen removal over nitrite process. This study revealed that nitritation, Anammox and autotrophic denitrification were responsible for the nitrogen removal. The nitrate production was caused by the co-action of nitrite-oxidizing bacteria and AnAOB.