This　study　investigates　the　effect　of　increased　hydraulic　retention　time　(HRT)　and　dissolved　oxygen　(DO)　on　the　destruction　and　restoration　of　the　performance　of　activated　sludge　and　biofilm　sequencing　batch　reactor　(SBR)　nitritation　after　the　processes　were　simultaneously　started　up　and　stably　operated　at　room　temperature　(25).　The　results　showed　that　was　easier　to　start　nitritation　in　an　activated　sludge　reactor,　but　nitritation　was　destroyed　when　DO　was　2-2.5　mg·L-1.　The　performance　could　be　restored　by　reducing　DO　to　0.5-1　mg·L-1.　The　biofilm　process　was　not　affected　by　DO.　Prolonging　HRT　destroyed　the　stable　operation　of　the　two　processes.　Compared　with　the　method　of　biofilm,　the　activated　sludge　method　had　less　resistance,　but　restoring　performance　was　better　than　for　the　biofilm　process　after　shortening　the　HRT.　Subsequently,　the　temperature　(20,　15,　and　10)　was　continuously　reduced　to　investigate　the　synergistic　effect　of　DO　and　temperature　on　nitritation　stabilization.　The　results　showed　that　the　reduction　of　temperature　destroyed　the　stable　operation　of　nitritation,　but　the　reduction　of　DO　concentration　could　compensate　for　the　adverse　effects　of　temperature　reduction.　In　addition,　when　the　temperature　was　higher　than　20,　the　rate　of　nitritation　in　the　activated　sludge　process　was　better　than　that　in　thebiofilm　method.　At　low　temperature　(below　15),　a　stable　operation　of　nitritation　was　easier　to　achieve　with　the　biofilm　method.　The　capacity　of　the　activated　sludge　process　was　hardly　changed　at　10.　The　above　conclusions　were　confirmed　by　molecular　microbiological　analysis,　and　it　was　found　that　the　stable　operation　of　nitritation　could　be　achieved　without　complete　wash-out　of　the　nitrite-oxidizing　bacteria　(NOB).　©　2017,　Science　Press.　All　right　reserved.