Enhanced nitrogen removal assisted by mainstream partial-anammox from real sewage in a continuous flow A2/O reactor.

• Anammox (Ca.Brocadia , 0.69%) were enriched in biofilms, markedly higher than flocs. • 15N-isotope tracer tests showed partial denitrification is a prerequisite for anammox. • Anammox contributed to 68% of the nitrogen loss via anoxic biofilms in batch tests. • Nitrogen removal efficiency improved 16.9% by combining denitrification and anammox. • Mainstream partial-anammox might has great potential of engineering application. Introducing anammox technology into low-ammonia sewage treatment offers sustainable nitrogen removal, but there remain challenges to its future applications. Here, anammox bacteria were enriched by integrating into partial denitrification (NO 3 −-N to NO 2 −-N) in the anaerobic/anoxic/oxic (A2/O) reactor with low COD/N ratios (2.7 ± 0.4) sewage. This significantly enhanced nitrogen removal through partial anammox process over stable operation of 300 days. Microbial community analysis showed that the anammox bacteria were enriched in anoxic-carrier biofilms (Ca. Brocadia , 0.69%), and their abundance was significantly higher than the flocculent sludge (16 s rRNA sequencing: P < 0.001; qPCR: P < 0.001). Further activity testing via 15N-isotope tracing revealed that partial denitrification is a prerequisite for in situ occurrence of anammox reaction. Anammox contributed to 68% of the total nitrogen loss via anoxic-carrier biofilms in batch tests. Combining denitrification and anammox, the nitrogen removal efficiency of the A2/O process improved by approximately 16.9% for low COD/N ratios real sewage. [ABSTRACT FROM AUTHOR]