Enhanced anammox performance under lower nitrite accumulation in modified partial nitritation-anammox (PN/A) process.

[Display omitted] • Ultra-high nitrogen removal efficiency (NRE) of 95.5% at C/N of 2.7 was achieved. • Lower NAR (range 40 %∼70 %) instead resulted in better anammox contribution and NRE. • High-efficiency EPDA with less NO X - loss dominated in ammonium removal than PN/A. • Enriching NOB and GAOs in PN/A process instead favored co-metabolism stability. • The nitrate introduction effectively attenuated the competition of ED for nitrite. Higher nitrite accumulation, which is challenging to achieve reliably, is always sought to obtain better nitrogen removal performance in traditional partial nitritation-anammox (PN/A) process. This study developed a modified PN/A process by introducing nitrite-oxidizing bacteria and endogenous metabolism. Advanced nitrogen removal performance of 95.5 % was achieved at a low C/N ratio of 2.7 under nitrite accumulation ratio (NAR) fluctuations. Higher nitrate accumulation at lower NAR (70 ∼ 40 %) resulted in superior anammox contribution (60 ∼ 75 %) and nitrogen removal performance (93 ∼ 98 %). This was attributed to the higher nitrogen removal efficiency of the post-anoxic endogenous partial denitrification coupling anammox process, although the PN/A process occurring first possessed a faster anammox rate of 2.0 mg NH 4 +-N /(g VSS⋅h). The introduction of nitrate allowed more nitrite flow to anammox, promoting a high enrichment of anammox bacteria (Ca. Brocadia, 0.3 % to 2.8 %). This study provides new insights into the practical application of the PN/A process. [ABSTRACT FROM AUTHOR]