In　suspended　sludge　system,　efficient　enrichment　and　retention　of　anammox　bacteria　are　crucial　obstacles　in　mainstream　wastewater　treatment　by　anammox　process.　In　this　study,　anammox　bacteria　was　self-enriched　in　a　pilot-scale　suspended　sludge　system　of　two-stage　nitrification–denitrification　process　serving　municipal　wastewater　treatment.　With　the　low　ammonia　(NH4+-N)　of　9.3　mg/L,　nitrate　(NO3–-N)　of　15.6　mg/L　and　COD/NO3–-N　of　2.2　under　extremely　low　nitrogen　loading　rate　of　0.012　kg　N/m3/d,　anammox　activity　bloomed　after　its　abundance　increasing　from　5.9　×　107　to　4.6　×　109　copies/g　dry　sludge.　Significant　NH4+-N　removal　was　occurred　and　maintained　stably　in　the　denitrification　reactor　with　anammox　bacteria　accounting　for　1.13%,　even　under　temperature　decreasing　to　20.0℃.　The　adequately　anoxic　environment,　efficient　retention　with　the　static　settlement,　and　NO2–　production　via　NO3–　reduction　provided　favorable　environment　for　anammox　bacteria.　This　study　demonstrated　the　feasibility　and　great　potential　in　mainstream　anammox　application　without　seeding　specific　sludge.　©　2023