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

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 (NO3--N to NO2--N) in the anaerobic/anoxic/oxic (A(2)/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 15 N-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 A(2)/O process improved by approximately 16.9% for low COD/N ratios real sewage.