LAUSR

Abstract Recent, partial nitrification/anammox process has received increasing attention in water nitrogen removal. And denitrification process might also play an indispensable role to enhance nitrogen removal performance. In this study, the symbiosis of heterotrophic denitrification bacteria (HDB) and anaerobic ammonium oxidation bacteria (AAOB) was observed in a size-fractioned single-stage partial nitrification/anammox reactor to explore their individual contribution. The reactor was operated in three phases, with different aeration rates (0.6, 0.4, and 0.5 L·min−1) for 73 days. In the third phase, a maximum nitrogen removal efficiency (NRE) of 92.0% was achieved and the nitrogen removal rate (NRR) was 0.498 kg N·m-3·d−1. A high special anammox activity (SAA) of 227.4 mg N·g−1VSS·d−1 was observed in the granules. The specific denitrification activity (SAD) was 82.6 mg N·g−1VSS·d−1 in the flocs, but was only 10.4 mg N·g−1VSS·d−1 in the granules. High-throughput sequencing analysis showed that the AAOB were enriched in granules, whereas the HDB were dominant in the flocs. The results indicated that most of the nitrogen was removed by the granules through partial nitrification/anammox process, and the remaining nitrate was removed by the flocs through heterotrophic denitrification.