This　study　proposes　a　novel　strategy　of　stably　enriching　anammox　in　mainstream,　based　on　the　competitive　difference　to　NO2-　between　anoxic　biofilms　and　suspended　sludge.　A　modified　anaerobic-anoxic-oxic　(A2/O)　process　run　for　500　days　with　actual　municipal　wastewater.　Microbial　analysis　revealed　that　anoxic-carrier　biofilms　had　a　significantly　higher　abundance　of　anammox　(qPCR:　0.74%　-　4.34%)　than　suspended　sludge　(P＜　0.001).　Batch　tests　showed　that　anammox　within　anoxic-carrier　biofilms　contributed　to　significant　nitrogen　removal,　coupled　with　partial-denitrification　(NO3- → NO2-).　The　anammox　genus,　Ca.　Brocadia,　was　highly　enriched　when　suspended　sludge　was　accidentally　lost.　Further　batch　tests　found　that　reducing　suspended　biomass　helped　anammox　enrichment　in　anoxic-carrier　biofilms,　because　the　suspended　sludge　had　strong　NO2-　competition　(NO2- → N2)　with　anammox　(increased　nirK).　Metagenomic　sequencing　revealed　that　Ca.　Brocadia　dominates　in　the　anoxic-carrier　biofilms,　and　is　the　most　important　narG　contributor　to　NO3- → NO2-,　which　could　have　promoted　the　competition　of　NO2-　with　heterotrophic　bacteria.　For　this　A2/O　process,　the　low　effluent　total　nitrogen　(8.9　mg　±　1.0　mg　N/L)　was　attributed　to　partial-denitrification　coupling　with　anammox,　demonstrating　that　this　process　is　applicable　to　the　general　influent　N-concentration　range　(30　mg　-　50　mg　NH4+-N/L)　of　municipal　wastewater　treatment　plants　(WWTPs).　Based　on　the　special　competitive　preference　of　anammox　for　NO2-,　this　study　provides　a　promising　and　practical　alternative　for　enriching　anammox　bacteria　in　municipal　WWTPs.