In　partial　nitritation　process,　the　sludge　concentration　was　higher　than　4000　mg·L-1,　DO　was　less　than　0.2　mg·L-1,　the　temperature　of　15-29℃,　the　reactor　HRT　of　4.6　h,　and　the　circulation　ratio　was　75%,　not　only　the　microbial　biomass　reached　the　highest　value　but　also　the　ratio　of　nitrite　to　ammonium　of　the　effluent　could　be　controlled　at　approximate　1.0　by　adjusting　the　dissolved　oxygen　concentration.　A　quantitative　PCR　and　the　16S　rRNA　genes　clone　library　results　demonstrated　that　ammonia-oxidizing　bacteria　were　the　dominant　among　nitrite-oxidizing　bacteria　at　low　dissolve　oxygen　concentration,　that　the　bacteria　promoted　the　running　of　the　partial　nitritation.　Compared　with　conventional　nitrification　systems,　in　partial　nitritation　process,　Nitrobacter　and　Nitrospira　could　not　be　detected　among　the　system,　and　abundance　of　Planctomycetes　was　higher　in　partial　nitritation　process　than　that　of　conventional　nitrification　systems.　Phylogenetic　analysis　of　bacterial　amoA　gene　sequences　suggested　that　ammonia-oxidizing　bacteria　of　the　partial　nitritation　system　was　dominated　by　Nitrosomonas　sequences,　so　partial　nitritation　is　to　be　feasible　technically　and　economically.　©,　2015,　Science　Press.　All　right　reserved.