Nitrogen removal from low COD/TIN real municipal sewage by coupling partial denitrification with anammox in mainstream.

• The NRE reached 77.8 ± 4.3% even COD/TIN ratio at 2.9 during 548 days of study. • The effect of PD/A will be enhanced after the added anammox biocarriers adapted. • Anammox bacteria abundance on biocarriers remained stable and was higher than WWTPs. • PD/A contributed about 32–47% nitrogen loss in hypoxic zones in step-feed AAO reactor. The application of partial denitrification coupling with anammox (PD/A) in municipal sewage treatment is one of the most promising research field, and it has just begun. In 548 days of experiment treating real domestic sewage, the continuous flow reactor was started up by adding biocarriers containing anammox bacteria and operated stably after acclimatization. The PD/A was successfully achieved in the step-feed AO system. Furthermore, mass balance of ammonium revealed that anammox could contribute 32–47% to nitrogen loss. Compared with no biocarriers addition phase, the nitrogen removal efficiency (NRE) increased to 77.8 ± 4.3% and the effluent total inorganic nitrogen (TIN) decreased to 11.0 ± 2.1 mg/L at the COD/TIN ratio of 2.9 from day 428 to day 548 under the condition of anammox biocarriers addition and without external carbon source. Quantitative polymerase chain reaction (qPCR) and high-throughput sequencing analysis showed that anammox bacteria on the biological carrier accounted for 3.06% of the total bacteria in the stable phase, which was much higher than the reported <0.003% in wastewater treatment plants (WWTPs). After 247 days of operation with anammox biocarriers addition, the abundance of anammox bacteria on the carriers tended to be stable at 3.36 × 1010 gene copies g−1 dry sludge. The activity of anammox decreased and maintained at 13.14 g NH 4 +-N m−3 d−1 under the biocarriers-only condition, while increased from 11.71 g NH 4 +-N m−3 d−1 to 25.65 g NH 4 +-N m−3 d−1 in the hybrid condition of biocarriers and flocs. This study indicated that PD/A is effective to strengthen nitrogen removal with low COD/TIN ratio. [ABSTRACT FROM AUTHOR]