Fast start-up and stable operation of mainstream anammox without inoculation in an A2/O process treating low COD/N real municipal wastewater

•Fast start-up of mainstream PD/A without inoculation in A2/O process in 60 days.•Rapid in situ enrichment of AnAOB (2.49%) on the virgin carriers in anoxic zone.•NRE improved 16.9% with a stable effluent TIN of 11.6 mg/L under low COD/N ratio.•Anammox contributed to nitrogen loss of 37.9 ± 6.2% treating actual municipal sewage.•Anoxic biofilm performed a better coordination between denitrifiers and AnAOB.