A novel continuous-flow three-stage tandem system based on partial nitrification/Anammox granular sludge and partial denitrification-Anammox biofilm (PN/A-PD-A) for advanced nitrogen removal from mature landfill leachate

A novel continuous-flow system coupling partial nitrification/Anammox (PN/A), partial denitrification (PD), and Anammox (Amx) biofilm reactors was developed (PN/A-PD-A) to treat mature landfill leachate (MLL). To maximize synergy, the NH₄+-N removal in the PN/A reactor was regulated based on the NO₂–-N accumulation ratio (NAR) in the PD stage, ensuring optimal substrate stoichiometry for the final Amx polishing step. Over 174 days of operation, the system achieved a superior total nitrogen removal efficiency (TNRE) of 98.30 ± 0.14% (effluent TN: 21.80 ± 1.71 mg/L). The PN/A granular sludge, enriched with Candidatus_Kuenenia (5.87%) and Nitrosomonas (9.73%), demonstrated high adaptability to MLL characteristics and contributed to 83.51% of the TN removal. In the PD stage, the dominant genus Thauera (43.91%) facilitated efficient NAR (82.86 ± 1.61%) at a limited COD/NO₃–-N ratio of 2.32 ± 0.02. The Anammox biofilm (Candidatus_Kuenenia, 27.80%) in the Amx reactor contributed to 13.10% of TN removal, ensuring to meet the MLL discharge standard. Kinetic and metagenomic analyses confirmed that distinct shifts from complete to partial nitrification (and denitrification) in enzymes activity and gene abundance under chronic MLL stress underpinned the robust NO₂–-N accumulation in both PN/A and PD reactors. Notably, compared to conventional nitrification–denitrification process, the PN/A-PD-A system significantly reduced oxygen demand (60.18%), exogenous organic carbon consumption (91.61%), sludge yield (83.72%), and CO₂ emission (94.66%), demonstrating a sustainable pathway for low-carbon nitrogen removal from high-strength wastewater.