The　application　of　mainstream　anammox　is　highly　desirable　for　municipal　wastewater　treatment.　However,　enrichment　of　anammox　bacteria　(AnAOB)　is　challenging,　particularly　given　the　vicious　competition　from　denitrifying　bacteria　(DB).　Here,　suspended　sludge　biomass　management,　a　novel　operational　strategy　for　hybrid　process　(suspended　sludge/biofilm),　was　investigated　for　570　days　based　on　a　modified　anaerobic–anoxic–oxic　system　treating　municipal　wastewater.　By　successively　decreasing　the　suspended　sludge　concentration,　the　traditional　hybrid　process　was　successfully　upgraded　to　a　pure　biofilm　anammox　process.　During　this　process,　both　the　nitrogen　removal　efficiency　(NRE)　and　rate　(NRR)　were　significantly　improved　(P　＜　0.001),　from　62.1　±　4.5%　to　79.2　±　3.9%　and　from　48.7　±　9.7　to　62.3　±　9.0　g　N/(m3·d),　respectively.　Mainstream　anammox　was　improved　in　the　following:　Candidatus　Brocadia　was　enriched　from　0.70%　to　5.99%　in　anoxic　biofilms　[from　(9.94　±　0.99)　×　108　to　(1.16　±　0.01)　×　1010　copies/g　VSS,　P　＜　0.001];　the　in　situ　anammox　reaction　rate　increased　from　8.8　±　1.9　to　45.5　±　3.2　g　N/(m3·d)　(P　＜　0.001);　the　anammox　contribution　to　nitrogen　removal　rose　from　9.2　±　2.8%　to　67.1　±　8.3%　(P　＜　0.001).　Core　bacterial　microbiome　analysis,　functional　gene　quantification,　and　a　series　of　ex　situ　batch　experiments　demonstrated　that　the　stepwise　decreases　in　suspended　sludge　concentration　effectively　mitigated　the　vicious　competition　of　DB　against　AnAOB,　enabling　high-level　AnAOB　enrichment.　This　study　presents　a　straightforward　and　effective　strategy　for　enriching　AnAOB　in　municipal　wastewater,　shedding　fresh　light　on　the　application　and　upgradation　of　mainstream　anammox.　©　2023