The　sustainable　recovery　and　utilization　of　sludge　bioenergy　within　a　circular　economy　context　has　drawn　increasing　attention,　but　there　is　currently　a　shortage　of　reliable　technology.　This　study　presents　an　innovative　biotechnology　based　on　free　nitrous　acid　(FNA)　to　realize　sustainable　organics　recovery　from　waste　activated　sludge　(WAS)　in-situ,　driving　efficient　nitrogen　removal　from　ammonia　rich　mature　landfill　leachate　by　integrating　partial　nitrification,　fermentation,　and　denitrification　process　(PN/DN–F/DN).　First,　ammonia　((1708.5　±　142.9)　mg·L−1)　in　mature　landfill　leachate　is　oxidized　to　nitrite　in　the　aerobic　stage　of　a　partial　nitrification　coupling　denitrification　(PN/DN)　sequencing　batch　reactor　(SBR),　with　nitrite　accumulation　ratio　of　95.4%　±　2.5%.　Then,　intermediate　effluent　(NO2−–N　=　(1196.9　±　184.　2)　mg·L−1)　of　the　PN/DN-SBR,　along　with　concentrated　WAS　(volatile　solids　(VSs)　=　(15　119.8　±　2　484.2)　mg·L−1),　is　fed　into　an　anoxic　reactor　for　fermentation　coupling　denitrification　process　(F/DN).　FNA,　the　protonated　form　of　nitrite,　degrades　organics　in　the　WAS　to　the　soluble　fraction　by　the　biocidal　effect,　and　the　released　organics　are　utilized　by　denitrifiers　to　drive　NOx−　reduction.　An　ultra-fast　sludge　reduction　rate　of　4.89　kg·m−3·d−1　and　nitrogen　removal　rate　of　0.46　kg·m−3·d−1　were　realized　in　the　process.　Finally,　F/DN-SBR　effluent　containing　organics　is　refluxed　to　PN/DN-SBR　for　secondary　denitrification　in　the　post　anoxic　stage.　After　175　d　operation,　an　average　of　19　350.6　mg　chemical　oxygen　demand　organics　were　recovered　per　operational　cycle,　with　95.2%　nitrogen　removal　and　53.4%　sludge　reduction.　PN/DN–F/DN　is　of　great　significance　for　promoting　a　paradigm　transformation　from　energy　consumption　to　energy　neutral,　specifically,　the　total　benefit　in　equivalent　terms　of　energy　was　291.8　kW·h·t−1　total　solid.　©　2023　Chinese　Academy　of　Engineering