A　pilot-scale　study　on　start-up　and　stable　operation　of　mainstream　partial　nitrification-anammox　biofilter　(PN/AF)　system　was　conducted.　The　partial　nitrification　system　was　controlled　based　on　online　pH-dissolved　oxygen　(DO)　linkage　control　strategy　for　nitrite　oxidizing　bacteria　(NOB)　out-selection.　The　immobilized　anammox　system　was　an　upload　biofilter　reactor　where　anammox　bacteria　were　immobilized　on　the　volcanic　carriers.　The　results　showed　that　the　nitrite　accumulation　ratio　(NAR)　was　maintained　above　90%,　and　the　NO2-N/NH4+-N　value　was　kept　at　0.96　+/-　0.18　in　the　effluent　of　the　partial　nitrification　system;　the　mean　ammonia　concentration　of　the　effluent　was　below　5　mg/L,　and　total　inorganic　nitrogen　(TIN)　removal　loading　could　achieve　0.11　kg/m(3).d　after　the　stable　operation　in　the　whole　system.　The　results　of　high-throughout　sequencing　showed　that　the　NOB　(mainly　Nitrospira,　Nitrotoga)　was　out　of　the　partial　nitrification　system　after　30-day　operation,　the　relative　abundance　of　which　is　less　than　1%　observed　from　Krona　maps,　and　ammonia　oxidizing　bacteria　(AOB,　mainly　Nitrosomonas)　became　the　dominant　species　of　nitrobacteria,　the　8%　of　AOB　stabilized　finally.　Dominant　anammox　bacteria　was　identified　as　genus　Candidatus　Brocadia,　and　one　kind　of　rare　anammox　named　Asahi　BRW_2　was　found　to　be　at　the　downside　of　the　volcanic　biofilter　reactor.　It　could　be　found　the　online　pH-DO　linkage　control　strategy　allowed　out-selection　of　NOB　from　the　mainstream　SBR　system,　and　realized　a　stable　partial　nitrification.　In　addition,　high　DO　(2.5　+/-　0.5　mg/L)　condition　was　found　to　be　favorable　for　NOB　out-selection　and　the　stabilization　of　nitritation.　The　anammox　bacteria　could　be　well　immobilized　in　volcanic　carriers,　with　an　ability　of　low　ammonia　resistance.　The　study　was　helpful　for　engineering　application　of　mainstream　anammox　process.