A　system　consisting　of　a　two-stage　UASB　and　anoxic-oxic　reactor　was　used　to　enhance　COD　and　nitrogen　removal　from　landfill　leachate.　To　improve　denitrification　efficiency,　the　raw　leachate　with　recycled　final　effluent　was　pumped　into　the　first-stage　UASB　(UASB1)　to　carry　out　simultaneous　denitrification　and　methanogenesis.　The　results　over　180　d　show　that　the　maximum　organic　removal　rate　in　UASB1　and　UASB2　was　12.5　and　8.5　kg　COD　m(-3)　d(-1)　in　the　oxic　zone　of　the　A/O　reactor,　respectively.　The　COD　and　biochemical　oxygen　demand　(BODS)　removal　efficiency　of　the　system　was　80-92%　and　about　99%,　respectively.　Without　controlling　temperature　(17-30　degrees　C)　and　dissolved　oxygen　(0.5-4.0mgL(-1)),　the　maximum　NH4+-N　removal　rate　was　0.68kg　NH4+-N　m(-3)　d(-1),　and　about　99%　of　NH4-N　removal　was　obtained　by　nearly　complete　nitritation.　The　81-93%　total　nitrogen　removal　was　obtained　by　complete　denitrification　in　the　UASBI　and　in　the　anoxic　zone　of　the　A/O　reactor.　Fluorescence　in　situ　hybridization　(FISH)　analysis　of　the　sludge　samples　from　NO　reactor　showed　that　ammonia　oxidizing　bacteria　(AOB)　consisted　4%　of　the　eubacterium,　while　nitrite　oxidizing　bacteria　(NOB)　counted　less　than　0.2%　of　that.　The　study　shows　that　the　main　factors　achieving　and　maintaining　nitritation　are　a　proper　range　of　free　ammonia　concentration　obtained　by　dilution　recycled　final　effluent　that　inhibits　NOB　but　not　AOB;　effective　control　on　aeration　time　by　indication　of　＂ammonia　valley＂　on　pH　profile;　and　highly　efficient　denitrification　and　its　reproducing　alkalinity　to　result　in　pH　above　8.5.　(c)　2007　Elsevier　Ltd.　All　rights　reserved.