This　study　presents　a　biological　system　combined　upflow　anaerobic　sludge　bed　(UASB)　with　sequencing　batch　reactor　(SBR)　to　treat　ammonium-rich　landfill　leachate.　The　start-up　and　operation　of　the　nitritation　at　low　temperatures　were　investigated.　The　synergetic　interaction　of　free　ammonia　(FA)　inhibition　on　nitrite-oxidizing　bacteria　(NOB)　and　process　control　was　used　to　achieve　nitritation　in　the　SBR.　It　is　demonstrated　that　nitritation　was　successfully　started　up　in　the　SBR　at　low　temperatures　(14.0　degrees　C-18.2　degrees　C)　by　using　FA　inhibition　coupled　with　process　control,　and　then　was　maintained　for　482　days　at　normal/low　temperature.　Although　ammonia-oxidizing　bacteria　(AOB)　and　NOB　co-existed　within　bacterial　clusters　in　the　SBR　sludge,　AOB　were　confirmed　to　be　dominant　nitrifying　population　species　by　scanning　electron　microscopic　(SEM)　observation　and　fluorescence　in　situ　hybridization　(FISH)　analysis.　This　confirmation　not　only　emphasized　that　cultivating　the　appropriate　bacteria　is　essential　for　achieving　stable　nitritation　performance,　but　it　also　revealed　that　NOB　activity　was　strongly　inhibited　by　FA　rather　than　being　eliminated　altogether　from　the　system.　(C)　2015　The　Chemical　Industry　and　Engineering　Society　of　China,　and　Chemical　Industry　Press.　All　rights　reserved.