• • The two-stage A/O–MBBR system achieved average NH4+-N and COD removal efficiencies of 97.7% and 66.8%, respectively, under influent NH4+-N up to 1800 mg·L−1, demonstrating robust resistance to shock loads and suitability for high-strength leachate.
• • Coupling Anammox with partial nitrification-denitrification contributed 13–16% to nitrogen removal, elevating total nitrogen (TN) removal to 93.9%, significantly outperforming conventional biological processes in energy and carbon source savings.
• • High-throughput sequencing confirmed Proteobacteria as dominant (>50%), with denitrifying genera Azoarcus and Thauera enriched; Planctomycetota increased from 0.6% to 3.7%, and Candidatus Kuenenia was detected, evidencing successful Anammox biofilm colonization.
• • The multi-stage spatial configuration and biofilm microenvironment enabled simultaneous organic oxidation, nitrification, denitrification, and Anammox, overcoming limitations of traditional processes for high C/N and toxic leachate, offering a practical low-carbon treatment pathway.