High-loaded　ammonia　and　low-strength　organics　mature　landfill　leachate　is　not　effectively　treated　by　conventional　biological　processes.　Herein,　an　innovative　solution　was　proposed　using　a　three-stage　Simultaneous　Ammonium　oxidation　Denitrifying　(SAD)　process.　Firstly,　ammonia　(1760　+/-　126　mg　N/L)　in　wastewater　was　oxidized　to　nitrite　in　a　partial　nitrification　sequencing　batch　reactor　(PN-SBR).　Next,　93%　PN-SBR　effluent　and　concentrated　external　waste　activated　sludge　(WAS;　MISS　=　23057　+/-　6014　mg/L)　were　introduced　to　an　anoxic　reactor　for　integrated　fermentation　and　denitrification　(IFD-SBR).　Finally,　ammonia　(101.4　+/-　13.8　mg　N/L)　released　by　fermentation　in　the　IFD-SBR　and　residual　7%　nitrite　in　the　PNSBR　were　removed　through　the　anaerobic　ammonium　oxidation　(anammox)　process　in　the　SAD　up-flow　anaerobic　sludge　bed　(SAD-UASB).　In　addition,　NO3--N　generation　during　the　anammox　process　could　be　reduced　to　nitrite　by　partial　denitrification　(PD)　and　reused　as　substrate　for　anammox.　A　satisfactory　total　nitrogen　(TN)　removal　efficiency　(98.3%),　external　sludge　reduction　rate　(2.5　kg/m(3)　d)　and　effluent　TN　concentration　(16.7　mg/L)　were　achieved　after　long-term　operation　(280　days).　The　IFD-SBR　and　SADUASB　contributed　to　81.9%　and　123%　nitrogen　removal,　respectively.　Microbial　analysis　showed　that　anammox　bacteria　(1.5%　Candidatus　Brocadia)　cooperated　well　with　partial　denitrifying　bacteria　(4.3%　Thauera)　in　SAD-UASB,　and　average　nitrogen　removal　contribution　were　83.1%　during　significant　stability　of　anammox　and　9.2%　during　the　denitrification　process,　respectively.　The　three-stage　SAD　process　provides　an　environmental　and　economic　approach　for　landfill　leachate　treatment　since　it　has　the　advantage　of　25.4%　less　oxygen,　100%　organic　matter　savings　and　47.9%　less　external　sludge　than　traditional　biological　processes.　(C)　2019　Elsevier　Ltd.　All　rights　reserved.