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 sequencing batch reactor (PN/DN-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) = (15119.8 ± 2484.2) mg·L⁻¹), is fed into an anoxic reactor for fermentation coupling denitrification process (F/DN-SBR). 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 NO_x⁻ 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 19350.6 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.