Light-driven algal-bacterial granular sludge enables high-rate anammox for mature landfill leachate treatment.

Mature landfill leachate with high NH₄⁺-N, low C/N and a need for deep nitrogen removal requires cost-effective solutions. In this study, an algal-bacterial granular sludge (ABGS) system was developed to address the limitations associated with slow-growing anaerobic ammonium-oxidising bacteria (AnAOB) and establish a functionally diverse microbial consortium under low C/N, sulfate-containing conditions. After 150 days of operation with synthetic wastewater simulating mature landfill leachate, stable ABGS was obtained, comprising Chlorella, ammonia-oxidising bacteria (AOB), AnAOB, heterotrophic denitrifying bacteria (HDB), sulfate-reducing bacteria (SRB) and sulfur-oxidising bacteria (SOB). Without mechanical aeration, Chlorella-derived photosynthetic oxygen supported AOB-driven partial nitrification, enabling PN/A-dominated nitrogen removal accompanied by nitrate-to-nitrite recycling and sulfur transformation. The system achieved 98.27% total inorganic nitrogen removal efficiency (TINRE) and a nitrogen removal rate (NRR) of 393.08 mg N/(L·d). AnAOB reached a total abundance of 2.8%, including Candidatus Brocadia, Candidatus Kuenenia and Candidatus Anammoxoglobus, with Nitrosomonas as the principal AOB genus and Thiobacillus as an enriched sulfur-related genus. A subsequent 10-day real-leachate shock test demonstrated short-term resilience, with average NH₄⁺-N and TIN removal efficiencies of 92.3% and 88.9%, respectively. This work provides a new approach to anammox-based nitrogen removal and simultaneous organic carbon reduction in mature landfill leachate treatment.