municipal wastewater; mainstream Anammox; A²/O-IFAS process; carrier;  font-size: 10pt; ">simultaneous nitrification and denitrification

A full-scale wastewater treatment plant (WWTP) located in central China was upgraded based on the original inverted A²/O process, and the A²/O-IFAS process was formed by introducing carriers in the anaerobic and anoxic tanks. After more than four and a half years of stable operation, the system showed excellent nitrogen removal efficiency, and the reddish biofilm appeared on the carriers in anaerobic and anoxic tanks. The biofilm on the carriers in the anaerobic and anoxic tanks hadAnammox activity. The microbial community composition of biofilm and suspended sludge was analyzed by high�瞭hroughput sequencing method.Candidatus Kuenenia,the dominant Anammox bacteria (AMX) at the genus level,was concentrated in the biofilm in the anoxic tank, and its relative abundance was significantly higher than that in the biofilm in the anaerobic tank and suspended sludge. Local aeration under the screens in the anaerobic and anoxic tanks was used to prevent blocking of the carriers and promote fluidization, which made the anaerobic/anoxic system always in alternate change of low dissolved oxygen (DO).It also promoted the simultaneous nitrification and denitrification (SND) process, and provided substrates for AMX. Coexistence, symbiosis and natural competition of traditional nitrifying bacteria and denitrifying bacteria in the suspended sludge and AMX in the biofilm removed nitrogen synergically at low DO. The low DO formed by local aeration and addition of carriers in the anaerobic and anoxic tanks promoted the emergence of mainstream Anammox phenomenon and became one of the main reasons for stable operation of the system.