The　partial　nitritation/anammox　(PN/A)　process　has　been　applied　to　ammonium-rich　wastewater　treatment,　but　the　operational　boundary　has　not　been　well　determined　for　long-term　stability.　This　pilot-scale　study　was　targeted　at　a　single-stage　PN/A　process　using　a　sequencing　batch　reactor　(SBR)　(volume:　53　m(3))　and　granulated　activated　sludge.　The　maximum　nitrogen　removal　rate　reached　0.83　kg　N/(m(3).d).　Microbial　analysis　suggested　that　ammonium　oxidizing　bacteria　were　mainly　present　in　small　sludge　flocs　while　anammox　bacteria　were　prone　to　grow　in　large　sludge　granules.　The　PN/A　performance　was　enhanced　when　dissolved　oxygen　(DO)　was　increased　from　0.25　to　0.76　mg/L,　and　deteriorated　at　DO　higher　than　1.15　mg/L.　The　PN/A　was　inhibited　at　free　ammonia　(FA)　over　77.0　mg/L.　High　DO　or　FA　concentrations　inhibited　anammox　activity　and　further　induced　high　and　inhibitory　nitrite　concentrations.　Therefore,　appropriate　DO　and　FA　concentrations　should　be　controlled　to　achieve　single-stage　PN/A　in　SBRs.