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±2484.2)mg·L-1),is　fed　into　an　anoxic　reactor　for　fermentation　coupling　denitrification　pro-cess(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　sec-ondary　denitrification　in　the　post　anoxic　stage.After　175　d　operation,an　average　of　19　350.6　mg　chem-ical　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.