The　direct　integration　of　anammox　process　into　municipal　wastewater　treatment　has　caused　widespread　concern,　but　the　lack　of　anammox　seeds　limited　its　real　application.　This　study　successfully　cultivated　anammox　granules　(322.0　μm)　from　conventional　activated　sludge　treating　municipal　wastewater.　Through　ultra-low　floc　sludge　retention　times　of　8d,　nitrifiers　on　flocs　were　eliminated　and　partial　nitrification　was　realized.　Furthermore,　highly　bacteria-enriched　granules　were　initially　formed,　with　Nitrosomonas　and　Ca.　Competibacter　4-fold　higher　than　that　of　flocs.　Specific　staining　results　revealed　the　microbial　interaction　with　Ca.　Brocadia,　considering　that　Ca.　Competibacter　and　Nitrosomonas　correspondingly　identified　in　the　inner　and　outer　layers　of　granules.　The　percentage　of　Ca.　Brocadia　present　on　the　granules　increased　substantially　from　0.0　%　to　3.0　%,　accompanied　by　a　nitrogen　removal　rate　of　0.3　kg·m−3·d−1.　Our　findings　revealed　a　valuable　reference　for　the　anammox　bacteria　in-situ　enrichment　under　mainstream　conditions,　which　provides　theoretical　guidance　for　anammox-based　processes　practical　application.　©　2024　Elsevier　Ltd